Light localization is a fascinating physical phenomenon for which the current trends towards the miniaturization of optical devices might be a field of applications [1]. Fabrication of confined structures, where light can be confined over nano or micro scale region is a fantastic challenge for nanoscience based technologies, opens the way to study of new functional artificial materials which allow exploration of new aspects of light-matter interaction [2]. An example concerns planar microcavities, or one-dimensional (1-D) photonic crystals, which are the simplest photonic band-gap (PBG) devices exploitable to manage the spectroscopic properties of luminescent species such as rare earth ions [3]. Moreover the combination of photonic crystals and nonlinear optics due to intense local fields can lead us towards new optical devices [4].
Here, we report on the optical field confinement effects on the enhancement of spectroscopic properties in Er3+ activated dielectric microcavity and the enhancement in the nonlinear absorption in ZnO defect microcavity. Both the microcavities consist of alternating silica and titania films and their corresponding defect layers. In the Er3+ doped dielectric microcavity, the near infrared transmittance spectra confirms the presence of stop band from 1500 nm to 2000 nm with the cavity resonance centered at 1749 nm. The cavity effect on 4I13/2→4I15/2 emission band of Er3+ ions is demonstrated. The later one, concerns ZnO doped 1-D microcavity, fabricated as in the previous example. We have explored the nonlinear optical properties of the ZnO doped microcavity using Z-scan technique in ns regime and observed enhancement in the nonlinear absorption for the ZnO microcavity due to the optical field confinement in the defect layer.

Periodically poled lithium niobate (PPLN) is a highly efficient material for nonlinear frequency conversion including second harmonic generation, difference frequency generation, sum frequency generation, optical parametric oscillation, and other nonlinear processes. Due to the opposite signs of nonlinear optical coefficients in different ferroelectric domains, a nonlinear grating is written in a PPLN, and then the quasi-phase-matching (QPM) could be realized at room temperature over the crystal&’s whole transparent range. In addition to nonlinear optical coefficients, other third-rank tensors such as the piezoelectric and electro-optic coefficients are also modulated along with the periodic domains, results in some interesting properties. Some coupling effects with two or more processes involved are also founded.
In this talk, I will revisit the history of PPLN, especially the key research activities in the past two decades in our group from materials fabrication, domain characterization, physical properties to related applications. We will see how the coupling effects in PPLN induce polariton excitation with an observable negative dielectric constant. The concept of “function integration” is thus proposed based on a PPLN chip, where the electro-optically and acousto-optically controlled nonlinear processes could be realized. Applications of tunable optical diode, low-voltage modulator and polarization insensitive frequency convertor are discussed. Some recent progresses in our group are also introduced, for example, the new patterned poling techniques and frequency conversion of optical vortices. We believe the PPLN would find more and more new applications in this century from solid state lasers to photonic integrated circuits.

Gallium nitride (GaN) and zinc oxide (ZnO) are highly transparent nonlinear optical materials with a wide bandgap of 3.3 eV and 3.37 eV respectively. Both of these semiconductor materials are ideal for UV-Blue light emitting diodes and laser diodes. Due to the built-in strain and defect states in the lattice, these materials are non-centrosymmetric in nature and exhibit strong nonlinear optical effects such as second harmonic generation (SHG) and multiphoton induced emission (MPE). Although GaN has been predicted to possess high second order nonlinear susceptibility similar to LiNbO3, the conversion efficiency is rather low. Similarly strong nonlinear generation observed in ZnO nanowires and microstructures cannot be efficiently harvested. So we propose to generate nonlinear optical effects in selectively grown GaN micropyramids and extract the signal from the tip of the pyramid using ZnO dielectric nano-rods.
Site selective m-polar GaN hexagonal micro-pyramids were prepared using the epitaxial lateral overgrowth (ELOG) technique employing a SiO2 mask and photolithography. ZnO micro-nano rods were subsequently grown on GaN pyramids using vapor transport method. Nonlinear optical microscopy with a tunable femtosecond laser source was used to investigate the competition between the SHG and MPE processes. The excitation wavelengths corresponding to near bandgap, near ZnO exciton and below bandgap energies are used. Optical excitation and collection of signal from a single microstructure was achieved using a multiphoton microscopy and spectroscopy. SHG and multiphoton induced luminescence are observed from both samples. We observed that intensities of both processes are dependent on the vertical axis position; specifically there is a competition between SHG ( chi;(2)) and multi-photon ( chi;(3)) processes as the signal collection plane is scanned through the vertical axis of the micro structure. We also observed that SHG dominates at low optical pump densities, where as MPE dominates as higher pump densities are employed. By controlling the local excitation cross-section within the pyramid or the nano-rod, the nonlinear optical process generated within the hybrid GaN/ZnO structure can be manipulated. These structures can be utilized to generate coherent light in the UV-Blue region due to SHG process or green emission due to MPE process
by controlling the optical excitation density, light-matter interaction length and absorption cross section.

Nonlinear optical absorption in graphene is of direct relevance to both technological applications and fundamental physics involved in interaction between intense light and two-dimensional materials [1-7]. Absorption saturation, one of the nonlinear optical processes in graphene, has enabled graphene to be an effective saturable absorber for generation of ultrafast laser pulses by mode-locking techniques [5] or short laser pulses by Q-switching methods [6]. Graphene&’s absorption saturation is a consequence of Pauli blocking whereby strong interband excitation results in valence-band depletion and conduction-band filling.
It should be pointed out that the previous studies [1, 4-7], either experimental or theoretical, focused on the light intensity dependence of absorption saturation in graphene at wavelengths of 800 nm (or 1.55 eV) and 1550 nm (or 0.8 eV). Little is known about its wavelength dependence, except for the theoretical estimation by Vasko [8]. We previously investigated the spectral dependence of absorption saturation in epitaxial graphene for photon energies between 1.13 and 1.59 eV (or, between 780 and 1100 nm) [2]. Recently, absorption saturation in epitaxial graphene was observed in the spectral range from 0.03 to 0.245 eV (or, from 5,000 to 41,000 nm) and its quadratic photon energy dependence was established in a phenomenological way [3].
In addition, the reports in Ref. [1, 3-7] have overlooked two-photon absorption. In our previous investigation [2], we have shown, both theoretically and experimentally, that two-photon absorption is as important as one-photon interband absorption saturation in AB-stacking bilayer graphene.
Here, we present our unambiguous determination of absorption saturation and two-photon absorption (2PA) in high-quality, CVD-grown, multilayer graphene films over the near-infrared (NIR) and visible spectral range (1.13 - 2.32 eV) using Z-scan technique with femtosecond laser pulses. Our data reveal that the quadratic photon energy dependence of absorption saturation in graphene can be extended to 2.32 eV (or 535 nm) in the visible spectral region. As for 2PA in multilayer graphene, its NIR spectrum is close to the theoretical prediction of light frequency dependence [2], while its visible part follows the spectral profile of the excitonic Fano resonance.
References
[1] G. Xing, et al., Opt. Express 18, 4564 (2010).
[2] H. Yang, et al., Nano Lett. 11, 2622 (2011).
[3] S. Winnerl, et al., J. Phys.: Condens.Matter 25, 054202 (2013).
[4] T. Winzer, et al., Appl. Phys. Lett. 101, 221115 (2012).
[5] Z. P. Sun, et al., ACS Nano 4, 803 (2010).
[6] A. Martinez, et al., Appl. Phys. Lett. 99, 121107 (2011).
[7] H. Zhang, et al., Opt. Lett. 37, 1856 (2012).
[8] F. T. Vasko, Phys. Rev. B 82, 245422 (2010).

In very recent years, nanocarbons, specifically carbon nanotubes and graphene have become the focus of significant research efforts. Characteristics such as potential near-ballistic transport and high mobility make graphene viable as a material for nanoelectronics. Not only this, but its mechanical, electronic and thermal properties are also perfect for mirco- and nanoscale mechanical systems, thin film transistors, and transparent and conductive composites and electrodes. In this work, particular interest was afforded to the 2mu;m wavelength and to the modelocking capabilities of graphene, exploiting its optoelectronic properties to achieve this. Graphene is a prime candidate for several reasons, including its intrinsic broadband operation capabilities due to the gapless linear dispersion of Dirac electrons. Non-linear saturable absorption is required for materials used as a mode locker in lasers to obtain light pulses of very short duration, in the order of femtoseconds.
High yields of graphene were prepared via liquid-phase exfoliation of powdered graphite. This was achieved through the use of methods devised by J. N. Coleman et al.[1] and other groups[2]. These methods rely on the exfoliation and stabilization of graphene using special solvents or surfactants, combined with long sonication times (~170 hours). Unfortunately, commonly used solvents, such as water, have strong absorption peaks at 2mu;m. Therefore, initially potential solvents were tested for their suitability, both for transparency at 2mu;m and for dispersions of graphene. A promising solvent, tetrahydrofuran (THF), was the first to be tested, primarily due to its almost complete transparency at 2mu;m and its ability to dissolve important polymers, such as Poly(methyl methacrylate) (PMMA). Dispersions of graphene with N-Methyl-2-pyrrolidone (NMP) as the solvent were also produced. While it does have slight absorption at 2mu;m, it is proven to both provide efficient dispersions of graphene, and to dissolve polymers such as PMMA. Afterwhich, the dispersions as well as thin films were examined using various apparatus, including UV-Vis-IR spectrometer, raman spectroscopy and z-scan techniques.
Nonlinear optical properties are routinely examined using the so-called z-scan method. This set up consists of a thin sample being moved through the focus of a laser beam to vary the light intensity on the sample. This allows for measurement of the non-linear index n_2 Kerr nonlinearity with the “closed” aperture method, and the non-linear absorption coefficient delta α via the “open” aperture method. Funding from the ISLA project, which aims to develop a set of “building block” components for 2mu;m lasers, is greatly acknowledged.
[1] Khan, U., O'Neill, A., Lotya, M., De, S. and Coleman, J. N., High-Concentration Solvent Exfoliation of Graphene. Small, 6: 864-871 (2010)
[2] Bourlinos, A. B., Georgakilas, V., Zboril, R., Steriotis, T. A. and Stubos, A. K., Small 5, 184 (2009)

Au and Ag are two well-known plasmonic materials. However, our consideration shows that the damping of the surface plasmon wave propagating along air-Cr metal interface is negligible for wavelengths from 400 nm to 1200 nm, meaning that Cr is also supposed to exhibit surface plasmon resonance. To survey the plasmon excitation of Cr metal, second harmonic generation (SHG) method seems to be the favorable technique. The SHG intensity I_(2omega;) is proportional to the fourth and second power of the local field factor at the fundamental frequency L_(omega;) and the pump beam intensity I_(omega;), respectively. Therefore, plasmon excitation of Cr metal would emerge strongly from the second order optical standpoint. Our concern in this study is to check whether plasmon excitation of Cr metal emerges in a nonlinear optical phenomenon.
We fabricated V-shaped subwavelength slits formed in a 15nm thick chromium film and detected their second harmonic generation (SHG) response. An array of V-shaped nanoholes was fabricated by electron beam lithography. In this sample, the nanoholes have 150 nm arm-length, 50 nm width, 360 nm periodicity with the angle of apex of 120°. The whole array covered an area of about 100 mu;m². We observed azimuthal angle dependence of the optical SHG from V-shaped chromium nanohole arrays for four different input and output polarization combinations Pin/Pout, Pin/Sout, Sin/Pout and Sin/Sout at the fundamental photon energy of 2.33 eV. As the V shape belongs to C_s symmetry, ten independent nonlinear susceptibility elements chi;₂₂₃⁽²⁾, chi;₁₁₃⁽²⁾,chi;₂₁₂⁽²⁾,chi;₁₂₂⁽²⁾, chi;₁₁₁⁽²⁾, chi;₁₁₃⁽²⁾, chi;₃₁₃⁽²⁾, chi;₃₂₂⁽²⁾, chi;₃₁₁⁽²⁾, and chi;₃₃₃⁽²⁾ were introduced into a fitting program to fit the phenomenological model to experimental data. The theoretical fitting lines were then decomposed to estimate the contributions from nonlinear susceptibility elements.
Phenomenological analysis indicated that the magnitude of the effective nonlinear susceptibility element chi;₃₁₃⁽²⁾ was enhanced due to the broken symmetry in direction 1. Here 1 indicates the direction of the bisector of the V passing through its apex in the substrate plane. Direction 3 is defined as the normal to the surface while 1 and 2 directions are in the sample surface plane and perpendicular to each other. The observed chi;₃₁₃⁽²⁾ element was thus attributed to the strong influence on the gradient operator #9661; ₁ because of the rate of spatial change of the field at the boundary of the nanohole surface. We found that nonlinear susceptibility elements that start with subscript number “3”, also, show a predominant contribution in Pin/Pout configuration. This preferred nonlinear polarization suggests that the induced second order polarization occurs prominently in the direction perpendicular to the metal surface, probably due to enhancement effect by the image dipoles created at the metal-air.

Here we present our results on the synthesis of Aluminum nanoparticles (Al NPs) via pulsed laser ablation of Al target in polar and non-polar organic liquid environment like chloroform, chlorobenzene, toluene, benzene and carbontetrachloride and their nonlinear absorption and scattering properties. A Q-switched, Nd:YAG laser (tau; = 6ns), operating at a repetition rate of 10 Hz and at the fundamental wavelength of 1064 nm was used for laser ablation. Synthesized Al NP colloids of various solvents differ in appearance and their extinction spectra exhibit absorption in the UV region. Growth, aggregation and precipitation mechanisms which influence the optical properties and stability of NPs are found to be related to the dipole moment of the surrounding liquid environment. Owing to the strong surrounding electrical double layer provided by polar molecules, aggregation and precipitation of NPs is prevented making them more stable with a narrow particle size distribution with an average size of 23 nm and 12 nm respectively in chlorobenzene and chloroform. These NPs exhibited maximum optical extinction at 288nm and 244nm, which is assigned to the well-known surface plasmon resonance of Al spherical nanoparticles. In non-polar solvents like carbontetrachloride and benzene, the Al NPs showed maximum optical extinction at 261 nm and 279 nm, respectively, but get precipitated with time due to attractive vander Waals forces. A partially polar solvent like toluene exhibited optical extinction maximum at 285 nm with broadening in the size distribution and precipitation over a week. HRTEM results show the core-shell structure of NPs. Except in carbontetrachloride, particles ablated in remaining solvents have bright region of cavity like core surrounded by a dark region of shell. Cavity formation is tentatively assigned to the release of hydrogen during the solidification of Al NPs due to very sharp dependence of hydrogen dissolution in Al with temperature. Cavities are absent in carbontetrachloride due to lack of hydrogen in the surrounding environment. Nonlinear absorption, scattering and optical limiting behavior of colloidal solutions of Al NPs are studied by performing open aperture Z-scan measurements with 532 nm and the results show that nonlinear scattering is comparable to the two-photon absorption. Two photon absorption and nonlinear scattering coefficients are estimated through theoretical fit of the observed open aperture Z-scan curves. Due to strong two-photon absorption and nonlinear scattering, Al NPs in the polar media showed a normalized transmittance less than 0.3 and lower optical limiting threshold values, indicating the potential use of Al NPs as a versatile optical limiting material.

Imaging complex materials at the single-molecule level reveals spatial and temporal heterogeneities that are lost in ensemble imaging experiments, and an ongoing challenge is the development of luminescent probes with the photostability, brightness, and continuous emission necessary for single-molecule imaging. Lanthanide-doped upconverting nanoparticles (UCNPs) are non-linear probes that overcome problems of photostability and continuous emission, and their upconverted emission can be excited with relatively benign, near-infrared light at peak powers many orders of magnitude lower than those required for conventional multi-photon imaging probes. The brightness of UCNPs, however, has been limited by a poor understanding of energy transfer within the nanocrystal and unavoidable trade-offs between brightness and size. We have developed a novel design paradigm to synthesize UCNPs less than 10 nm in diameter that are over an order of magnitude brighter under single-particle imaging conditions than the brightest existing compositions, allowing us to visualize single upconverting nanoparticles as small as fluorescent proteins. We use a combination of sub-10-nm single-particle characterization and theoretical modeling to find that surface effects become critical at diameters less than 20 nm, and that the fluences used in single-molecule imaging fundamentally change the dominant factors that determine nanocrystal brightness. These results demonstrate that factors known to increase brightness in ensemble experiments are unimportant at higher excitation powers, and that, paradoxically, the brightest probes under single-molecule excitation are barely luminescent at the ensemble level.

Metal nanoparticles are interesting nonlinear optical materials that exhibit ultra-fast response and strong nonlinear optical coefficients chi;(3). The electric field enhancements at the localized surface plasmon (LSP) resonances are known to contribute to the increase of the nonlinear optical coefficients. The specific contributions of the LSP resonances, the particle concentration, and the particle shape to the nonlinear refraction coefficient Re(chi;(3)), and the nonlinear absorption coefficient Im(chi;(3)) are still not well understood. In this work, we measured the nonlinear optical coefficients of suspensions of anisotropic Au nanostars using z-scan characterization. When the resonance of the nanostars moves away from the laser irradiation wavelength, a decrease in the strength of the saturable absorption is observed. We also found that the strength of the saturable absorption increases with the concentration of the nanoparticles, while the nonlinear refraction coefficient shows little dependence.

An organic/inorganic hybrid glass by inserting alkylene-spacers between inorganic oxides was designed and then synthesized for a laser device material. Those alkyl-chains were highly oriented during the sol-gel polymerization to create a new optical property. Subsequently, Cro/CrOX phase was doped into the hybrid glassy matrix. TEM images reveal substantial regions of dark contrast, a highly periodic structure of alkyl-spacers. We believe that the periodic structures are sustained over substantial domains and appear to arise from alkyl chains of the glassy lattice fringes. From the electron diffraction pattern corresponded to the nano-fringe patterns, a lattice space of the structure was calculated about 50 Å from a distance between two diffraction spots in two sets of diffraction patterns. In NLO experiments, the doped hybrid glass showed a new optical property, a generation of huge acoustic wave. Interestingly, the diffraction efficiency (45 %) of the doped hybrid glass was higher than that of methanol (25 %), which meant the compressibility of the hybrid glass was as effective as the liquid; when the laser beam goes through a solid media, the density wave is usually linear. However, the hybrid glass showed a strong ‘acoustic response,&’ which was as strong as liquid. It also showed a huge acoustic wave when the laser beam passed through the glassy medium.

The photophysics, nonlinear absorption characteristics and the influence of the structure on the nature of excited states and reverse saturable absorption of Pt(II) diimine complexes with different carbazolyl-capped acetylide ligands (Pt-1 - Pt-5) were systematically investigated. The photophysical properties of these Pt complexes can be tuned by the different carbazolyl-capped acetylide ligands. Their broadband excited-state absorption and strong nonlinear transmittance performance at 532 nm suggest that these complexes could be promising candidates as broadband nonlinear absorbing materials.

Controlling materials properties is one of the most fundamental goals in the science and engineering of materials. In terms of current trends in designing technological materials, quantifying correlations between variations of physical properties and very small changes of crystal structure can be the key to acquiring knowledge of the physics involved and to allow the efficient design of new materials with improved properties. Methods for monitoring small structural changes are therefore important. Although there are well established methods for probing crystal structures, mostly X-ray diffraction methods, the scope of conditions in which these methods can be applied is limited. For instance, in Materials Science, the X-ray powder diffraction method certainly is the most used one. Its resolution power is limited to intensity measurements, which implies that small changes in the crystal structure are observed only when they produce detectable variation of intensity.
X-ray multiple diffraction through Renninger scanning using synchrotron radiation has been applied to study the interstitial incorporation of Ni2+ ions into ADP (ammonium dihydrogen phosphate, NH4H2PO4) nonlinear optical crystals. The experiment was done at the X-ray energy 6.48keV which is below the nickel's edge absorption. The experimental results show that the intensity profile for the pure and doped samples are different forms and these variations associated with shifts in the structure factor phases, also known as triplet phases. This inversion of asymmetry is caused by the presence of dopant ions into the crystal adduce internal stress and rebalancing of charges in the neighboring atoms. To explain the different forms of intensity profiles related to the fact that in the presence dopent ions in the crystal leads to rotation tetrahedral PO4 anions in the ab plane. This result demonstrates the potential of X-ray multiple diffraction phase measurements to refine atomic coordinate values (of the atoms in the unit cell) and ionic charges, such as those caused by internal stresses in doped crystals. Structural changes in the doped samples accounting for the observed phase shifts are discussed.

Organic compounds [1] and organometallic complexes [2] with dendritic structures can possess large two-photon absorption (2PA) cross-sections. Organometallic complexes in particular offer a range of advantageous properties such as being easily synthetically modified, being thermally and chemically robust, and containing redox-active metal centers, the last-mentioned being potentially important for optical switching.
Systematically-varied octupolar phenyl-cored oligo(phenyleneethynylene)-bridged ruthenium alkynyl complexes have been synthesized via a convergent approach. The nonlinear optical behaviour of these “star-shaped” complexes was explored using the Z-scan technique.[3] The importance of measuring spectral dependencies of the third-order NLO properties has recently been emphasized,[4] so the dispersion of the real and imaginary parts of the second hyperpolarizabilities of the title complexes has been established. Strong NLO responses for the complexes have been observed, with maximal values competitive with those found for higher generation dendrimers,[5,6], outcomes that are maintained when a variety of different scaling schemes [7] are applied. Moreover, instantaneous three-photon absorption (3PA) has been found for some of the title complexes, the magnitude exceeding those of previously-reported organometallic first-generation dendrimers showing record-high 3PA.[5]
[1] M. Drobizhev, A. Karotki, A. Rebane, C. W. Spangler, Opt. Lett. 2001, 26, 1081
[2] M. P. Cifuentes, C. E. Powell, J. P. Morrall, A. M. McDonagh, N. T. Lucas, M. G. Humphrey, M. Samoc, S. Houbrechts, I. Asselberghs, K. Clays, A. Persoons, T. Isoshima, J. Am. Chem. Soc. 2006, 128, 10819.
[3] M. Sheik-Bahae, A. A. Said, T. H. Wei, D. J. Hagan, E. W. Van Stryland, IEEE J. Quant. Electron. 1990, 26, 760.
[4] C. E. Powell, J. P. Morrall, S. A. Ward, M. P. Cifuentes, E. G. A. Notaras, M. Samoc, M. G. Humphrey, J. Am. Chem. Soc. 2004, 126, 12234
[5] M. Samoc, J. P. Morrall, G. T. Dalton, M. P. Cifuentes, M. G. Humphrey, Angew. Chem. Int. Ed. 2007, 46, 731
[6] R. L. Roberts, T. Schwich, T. C. Corkery, M. P. Cifuentes, K. A. Green, J. D. Farmer, P. J. Low, T. B. Marder, M. Samoc, M. G. Humphrey, Adv. Mater. 2009, 21, 2318
[7] T. Schwich, M. P. Cifuentes, P. A. Gugger, M. Samoc, M. G. Humphrey, Adv. Mater. 2011, 23, 1433

The europium (Eu) organometallic complex has attracted great interest because of several of its special properties involving a sharp emission profile and long luminescence lifetime. As a light emitting material, it has a spectrally narrow emission band that provides excellent color purity for use in full color displays and solid-state lighting. However, the long lifetime is a disadvantage in terms of emission efficiency because the luminescent quantum yield of materials is inversely proportional to the lifetime of the excited species [1].
This work focuses on reducing the lifetime of excited species in the Eu³⁺ ion by applying the localized surface plasmon (LSP) effect to the samples using the Eu organometallic complex as an emitting material. The samples were composed of an indium tin oxide (ITO)-coated substrate, a 50 nm thick Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer doped with and without aggregated gold nanoparticles (A-Au NPs), and a 50 nm thick emissive layer mixture composed of 95 wt% poly(N-vinylcarbazole) (PVK) and 5 wt% tris(dibenzoylmethane) mono(1,10-phenanthroline)europium(ΙΙΙ) (Eu(DBM)₃phen). We investigated the SP-coupled lifetime by measuring the time-resolved photoluminescence (TRPL). The lifetimes of the samples with A-Au NPs were significantly reduced compared to the reference sample without A-Au NPs. Moreover, as the A-Au NPs concentration increased, the decay rate went even faster and then became saturated. To obtain a more detailed understanding, we also measured the steady state PL. As the concentration of A-Au NPs increased, the PL intensity of the main peak at around 614 nm remarkably increased and then became saturated. This increasing tendency of the PL intensity is in good agreement with the tendency of reduced lifetimes by the Purcell effect. In addition, the asymmetric ratio (AS) values, which can be solid evidence in detecting LSP-enhanced emission, consistently increased with an increasing A-Au NP concentration, which indicates a stronger resonant coupling between LSP and radiation energy. A Finite Difference Time Domain (FDTD) simulation was additionally carried out to verify the local field enhancement due to metallic nanoparticles. The near-field distribution of the emitting dipole in proximity to the metal dramatically increased. This is noteworthy because the enhanced local field induces asymmetric charge distribution around Eu³⁺ ions, and thus stimulates the electric dipole transition, which eventually increases the decay rate of the emitter.
We demonstrate that the near field coupling between the LSP induced by A-Au NPs and the radiation energy of Eu³⁺ ions reduces the lifetime and thus enhances the luminescent quantum yield of the Eu organometallic complex. These characteristics are expected to be valuable in applications for highly efficient Eu-based organic light emitting diodes.
[1] K. Aslan et al, J. Fluorescence. 15 (2005) 643-654.

Sulphamic acid (HSO3NH2) is a potential material that exhibits excellent non-linear optical properties. Sulfamic acid (SA) single crystals, both pure and doped with 2.5 and 5 mol% Dy, were grown successfully in an aqueous solution by the slow cooling method. A good quality single crystals of pure and Dy doped sulphamic acid were obtained. The grown bulk single crystal of the title compound was subjected to various characterization analyses. Powder X-ray diffraction patterns were recorded to check the variation in the lattice parameters and phase of the crystals. A change in the lattice parameters and volume of the crystals with increasing doping concentration was observed. The optical transparency was found to have a significant effect with doping concentration. The enhancement of percentage of transmission in the recorded UV-vis-NIR region indicates that optical damage can be reduced by Dy3+ doping. The optical band gap was also calculated and results were discussed in detail. Further, the mechanical characterization (nanoindention technique) using standard Oliver-Pharr method suggested a significant increase in the hardness of the grown crystals with increasing doping concentration. In addition various other parameters associated with the mechanical property are explained in detail. The various functional groups were identified from the Fourier transform-Raman method. All the results were compared and discussed in details to get insight of the effect of doping concentration.

Rubidium titanyl phosphate, RbTiOPO4 (RTP), an isostructure in KTP family, has been attracting much attention in recent years due to its potential use in laser generation as a non-linear optical material.[1] Successfully doping the RTP crystals with laser-active ions such as lanthanide (Ln3+) ions enables its use as self-frequency doubling materials in laser technology. And proper co-doping with Nb5+ expands the crystal structure, which increases the distribution coefficient of Ln3+ to a value high enough to obtain efficient fluorescence, while, at the same time, maintaining the SHG properties. [2]
In this study, we characterized defect structures of top seeded solution grown (TSSG) RTP crystals with different levels of doping by Nb and Yb ions using the white beam synchrotron x-ray topography at the National Synchrotron Light Source (NSLS) in Brookhaven National Laboratory (BNL). The crystalline quality of these doped crystals is compared with undoped crystals. Analysis shows that the major defect in non-doped samples is growth sector boundary while in the doped samples, growth striations are observed along with growth sector boundaries. Relatively few linear defects such as dislocations are observed in these samples. It is also notable that doping tends to increase the amount of macroscopic defects such as cracks. Details of defect distributions are compared with the growth process to facilitate high quality growth of doped RTP.
[1] F. C. Zumsteg, J. D. Bierlein, and T. E. Gier, J. Appl. Phys. 47, (1976) 4980.
[2] J.J. Carvajal, R. Sole, Jna. Gavalda, J. Massons, M. Aguilo, F. Diaz, Optical Materials 24,(2003) 425-430

The rapid development of nanoscience and nanotechnology provides many unique opportunities for nonlinear optics. A growing number of nanomaterials, including graphene, carbon nanotubes and their derivatives have been shown to posess remarkable nonlinear optical properties, which promotes the design and fabrication of nano-scale optoelectronic and photonic materials. It is thus of critical importance to explore innovative NLO materials required for the practical applications.[1-3]
Porphyrin covalently functionalized graphene and multi-walled carbon nanotube (MWCNT) nanohybrids have been synthesized via various facile approaches for potential NLO materials. The axial ligands dihydroxotin(IV) tetraphenylporphyrin and the phosphorus-cored porphyrin were covalently linked to the surface of graphene oxide (GO), resulting in the enhanced optical limiting.[4] The fabrication of MWCNT-porphyrin conjugates by using 1,3-dipolar cycloaddition reactions was investigated, which significantly improves the solubility and dispersion stability of MWCNT-based materials in organic solvents;[5] while the porphyrin covalently functionalized reduced-GO was also prepared through 1,3-dipolar cycloaddition for the first time. The conjugated polymers with porphyrin units were introduced onto the surface of MWCNTs through radical polymerization, which may help to improve the solubility of the nanotubes in various solvents.[6] All these nanohybrids exhibit enhanced optical nonlinearities compared to their precursors and the physical blended systems, due to a combination of different nonlinear mechanisms, such as nonlinear scattering, excited-state absorption and photoinduced energy/electron transfer. Porphyrin covalently functionalized graphene and MWCNTs represent a key branch in the field of nano-scale materials and devices for nonlinear optics. Particularly, it is being attached significant importance to the electron/energy transfer from porphyrin moieties to graphene or MWCNTs, which is considered to play an important role in improving optical limiting effect.
References
[1]R. H. Baughman, A. A. Zakhidov, W. A. de Heer, Science 2002, 297, 787.
[2]F. Bonaccorso, Z. Sun, T. Hasan, A. C. Ferrari, Nat. Photonics 2010, 4, 611.
[3]J. Wang, Y. Chen, W. J. Blau, J. Mater. Chem. 2009, 19, 7425.
[4]A. Wang, L. Long, W. Zhao, Y. Song, M. G. Humphrey, M. P. Cifuentes, X. Wu, Y. Fu, D. Zhang, X. Li, C. Zhang, Carbon 2013, 53, 327.
[5]A. Wang, Y. Fang, L. Long, Y. Song, W. Yu, W. Zhao, M. P. Cifuentes, M. G. Humphrey, C. Zhang, Chem. Eur. J. 2013, 19, 14159.
[6]A. Wang, Y. Fang, W. Yu, L. Long, Y. Song, W. Zhao, M. P. Cifuentes, M. G. Humphrey, C. Zhang, Chem. Asian J. 2013, Article in press.

Organic electro-optic materials are at the forefront of current photonics applications that enable high speed data transmission, micro-scale sensors, and terahertz applications. Typically linearly conjugated push-pull type chromophores are the primary molecules of choice for various second and third order non-linear optical (NLO) applications. However, intra-molecular charge transfer responsible for many NLO properties can take place in molecules through other means such as σ-conjugation, cross-conjugation and omniconjugation as well. We report a study of electronic transitions, excited states and first hyperpolarizibility (β) of a series of cross conjugated molecules. In each case, the cross-conjugated bridge was functionalized with a dialkylaminobenzene donor on one end and varying strength acceptors on the other end. Furthermore, a tricyanopyrroline type acceptor was substituted on the cross-conjugated molecules to observe the effects of a strong electron withdrawing group on the ground and excited states of the molecules and consequently β. Through X-ray diffraction (XRD) and hyper-Raleigh scattering (HRS) measurements it was found that certain cross conjugated compounds while adopting a slightly twisted configuration have β values comparable to their fully conjugated control molecules. Based on density functional theory (DFT) results, the likely cause of improved β values is due to larger spatial isolation between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). This reduced overlap of the HOMO and the LUMO in cross-conjugated systems may in fact enable independent modulation of donor and acceptor strengths while minimizing unfavorable effects on electronic transitions and dipole moments.

For nonlinear optical (NLO) materials to be useful for device applications, the material development should proceed in parallel with innovative device concepts and device fabrication process. Recently, the unique properties of organic NLO materials have also inspired the development of novel nanofabrication methods to replace more traditional techniques and has potential to lead to scalable nanophotonics based on organic NLO materials. Moreover, implementation of a clever designed device structures and fabrication process not only can significantly improve device performance but also reduce energy consumption during operation.
Recent development of highly efficient organic electro-optic (EO) and third order NLO materials and devices has opened the way to promising opportunities to complement and improve current inorganic semiconductor based technologies. For example, organic EO materials can provide a full array of optical functions and can be processed at temperatures that are compatible with CMOS integrated circuits. In respond to exponentially increasing demands for operational bandwidth, structured organic EO materials are expected to play critical role in organic silicon hybrid nanophotonics in the near future. The ability to efficiently guide and control an optical signal in field confined nanoscale integrated devices will significantly boost the performance for structured organic EO materials. Integrated optical circuits based on organic silicon hybrid EO materials will enable low cost, mass production of novel nanophotonic devices for broadband optical signal processing and communications.

The synthesis of novel and efficient frequency conversion materials has resulted in the development of semi-organic materials, which possess large non linearity, high resistance to laser induced damage, low angular sensitivity and good mechanical stability. The present paper deals with the growth of Triglycine Sodium Halides like Triglycine Sodium Chloride(TSC), Triglycine Sodium Bromide(TSB) and Triglycine Sodium Iodide(TSI) by slow evaporation solution growth technique and characterization by powder XRD, UV-vis-NIR, FTIR, SHG and Dielectric studies.
Triglycine Sodium halides were synthesized by dissolving AR grade Glycine and Sodium Halides in the ratio of 3:1 in the triple distilled water by continuous stirring. The grown crystals were optically transparent. Lattice parameters of the grown crystals were determined from Powdered XRD Pattern and are found tobe in good agreement with the literature.
From the Transmission spectrum it is observed that the crystals have a lower cutoff wavelength around 300nm and the crystals has a wide optical window and are good for SHG studies and other related optoelectronic applications. Functional groups present in the samples were identified by FTIR spectral analysis.
The capacitance and the dielectric loss are measured in the frequency range of 100Hz to 5MHz using HIOKI3532-50 LCR HITESTER.. The dielectric constant and Dielectric loss decreases with frequency. The lower values of dielectric constant and dielectric are the suitable parameters for the enhancement of the SHG efficiency.
The second harmonic generation efficiency of the Triglycine Sodium Halide crystals were determined by the Kurtz Powder technique. SHG efficiency of TSC, TSB & TSI crystals are 1.5, 1.2 & 1.4 times that of the standard KDP crystal. Laser damage Threshold frequency was found to be 88.7mJ/pulse for TSIand 54 mJ/pulse for TSC
The above characterization confirms that the grown Triglycine Sodium halide crystals are suitable for the fabrication of various optoelectronic devices.

In recent years all-optical switching and logic devices have received extensive attention due to their potential applications in next generation ultrahigh-speed information processing and optical computing. Kerr nonlinear photonic crystals (NPC) offer a promising route to realize all-optical switching with ultrafast response time and low pump power based on simple and robust physical mechanisms. In this talk we present our extensive investigation on Kerr NPCs made from polystyrene, an organic polymer material with large Kerr nonlinearity and extremely fast response time down to several femtoseconds, and their application to build ultrafast, low power, and high contrast optical switching and logic devices [1-9]. Several relevant issues are discussed and analyzed, including the principal working mechanism of all-optical switching and logic devices in Kerr NPCs, preparation of polystyrene NPCs by means of microfabrication and self-assembly techniques, characterization of optical switching performance by means of femtosecond pump-probe technique, and synthesis of silicon/polystyrene hybrid NPCs by means of nano-imprint technology as a promising route to construct switching, modulating, and logic devices compatible with popular silicon photonics.
References:
1. Y. Liu, F. Qin, F. Zhou, Q. B. Meng, D. Z. Zhang and Z. Y. Li, Front. Phys. China, 2010, 5, 220.
2. Z. Y. Li and Z. M. Meng, J. Mater. Chem. C 2013, in press. Feature Article.
3. X. Y. Hu, Y. H. Liu, J. Tian, B. Y. Cheng and D. Z. Zhang, Appl. Phys. Lett, 2005, 86, 121102.
4. D. Z. Zhang, Y. H. Liu, J. Tian, S. Feng, D. X. Zhang and B. Y. Cheng, Proc. of SPIE, 2006, 6353, 635307.
5. Y. Liu, F. Qin, Z. Y. Wei, Q. B. Meng, D. Z. Zhang and Z.Y. Li, Appl. Phys. Lett., 2009, 95. 131116.
6. F. Qin, Y. Liu and Z. Y. Li, J. Opt, 2010, 12, 035209.
7. F. Qin, Z. M. Meng, X. L. Zhong, Y. Liu and Z. Y. Li, Opt. Express, 2012, 20, 13091.
8. Z. M. Meng, X. L. Zhong, C. Wang and Z. Y. Li, Chin. Opt. Lett, 2012, 10, 112202.
9. Y. Liu, F. Qin, Z. M. Meng, F. Zhou, Q. H. Mao and Z. Y. Li, Opt. Express, 2011, 19, 1945.

In the past few years, our research group has been investigated the suitability of polar oxides containing cations susceptible to second-order Jahn-Teller (SOJT) distortions for nonlinear optical (NLO) and piezoelectric application. Here we report the synthesis, crystal growth, the phase relationship, linear and nonlinear optical properties, stimulated Raman scattering and self -Raman frequency doubling properties of two new nonlinear optical crystals : monoclinic BaTeMo2O9 (β-BTM) and orthorhombic BaTeMo2O9 (a-BTM).

The lack of a dipolar second order susceptibility chi;(2) in silicon due to the centrosymmetry of its diamond lattice usually inhibits efficient second order nonlinear optical processes in the silicon bulk. However , recently the deposition of stressed silicon nitride layers lead to the demonstration of second harmonic generation and electro-optic modulation in silicon waveguides. The predominant cause of this observed second order nonlinearity was nevertheless not completely clear. On one side the breaking of the centryosymmety of the silicon lattice due the extending inhomogeneous strain in the silicon was discussed. On the other side the impact of the SiN-layer and its interface with Si was stated as the possible source of a strong interface-chi;(2).
To clarify this, we prepared several silicon strip waveguides with differently stressed silicon nitride and silicon dioxide coatings. Applying finite-element simulations the inhomogeneous stress/strain- fields inside the silicon waveguides were calculated and an integrated stress gradient was defined as measure for symmetry breaking of the Si-lattice. This theoretical stress/strain field was experimentally verified using x-ray diffraction. Specifically the experimentally observed distortion of the symmetric Si-(111) reflex in the reciprocal space map was accurately described by the calculated diffraction pattern based on the simulated inhomogeneous stress field.
Subsequently second harmonic generation was measured in these waveguides at a pump wavelength of 2200nm. The results show, that indeed an enhancement of the second order nonlinearity occurs for waveguides with increasing integrated stress gradient (larger inhomogeneous strain in the silicon). This strain-related -chi;(2) appears for both, SiNx- and SiO2-covered waveguides and does therefore not depend on the chemical nature of the stressing cover layers. However for SiNx-coated waveguides an additional two-fold enhancement is observed, even if the SiNx-coating is basically unstressed and the inhomogeneous strain in Si is negligible. Capacitance-Voltage measurements on similarly coated metal-insulator-silicon-structures indicate that this SiNx-related effect is caused by fixed positive charges at the Si/SiNx-interface. These charges lead to an electric field on the order of 10^5 V/cm at the interface and can cause electric field induced second harmonic generation (EFISH) in Si. So the particular strong overall four-fold enhancement of a quasi-chi;(2) in SiNx-covered waveguides is a constructive superposition of stress-induced and interface-charge related effects of comparable strength.
Fundamentally, this generation of a sizeable second order nonlinearity in micro- and nanostructures by external means is already significant. In addition the results open new perspectives for the deliberate creation of sizeable nonlinear optical properties in integrated photonic circuits using mechanical strain engineering and static electric fields.

To prepare cholesteric liquid crystalline nonlinear optical materials with ability to be vitrified on cooling and form long time stability cholesteric glasses at room temperature, a series of platinum acetylide complexes modified with cholesterol has been synthesized. The materials synthesized have the formula trans-Pt(PR3)(cholesterol 4-ethynyl benzoate)(1-ethynyl-4-X-benzene), where R = ethyl, butyl or octyl and X = H, F, methoxy and CN. A cholesteric liquid crystal phase was observed in the complexes R = ethyl, and X = F, OCH3 and CN but not in any of the other complexes. When X = CN, a cholesteric glass was observed at room temperature which remained stable up to 130 deg C, then converted to a mixed crystalline/cholesteric phase and completely melted to an isotropic phase at 230 deg C. When X = F or OCH3 the complexes were crystalline at room temperature with conversion to the cholesteric phase upon heating to 190 and 230 deg C, respectively. In the series X = CN, OCH3, F the cholesteric pitch was determined to be 1.7, 3.4 and 9.0 microns, respectively.

Cationic iridium(III) complexes (1-5) with fluoren-2-yl or 7-benzothiazolylfluoren-2-yl substituent on the 2-phenylpyridine (C^N) and/or phenanthroline (N^N) ligands are excellent candidates as nonlinear absorbing materials. These complexes exhibit weak low-energy absorption bands from 1,3MLCT (metal-to-ligand charge transfer)/1,3LLCT (ligand-to-ligand charge transfer) transitions. Extending the π-conjugation on the phenanthroline (N^N ligand) or the 2-phenylpyridine ligands by fluoren-2-yl or 7-benzothiazolylfluoren-2-yl substituents results in the red-shift of the main absorption bands compared to the reference complex that does not contain the fluoren-2-yl or 7-benzothiazolylfluoren-2-yl substituents. However, the extended π-conjugation on the N^N ligand or on the C^N ligands influences the lowest triplet excited state differently. For the complexes with extended π-conjugation only on the phenanthroline ligand, the lowest triplet excited state is predominantly the N^N ligand-centered 3π,π* state in nature. In contrast, the lowest triplet excited states of the complexes with extended π-conjugation only on the C^N ligands are dominated by the 3MLCT/3LLCT states. For the complex with extended π-conjugation on both the phenanthroline and 2-phenylpyridine ligands, the lowest triplet excited state has mixed characters of the 3MLCT/3LLCT and 3π,π* states. All complexes exhibit broadband triplet excited-state absorption in the visible to the near-IR region, which lead to strong reverse saturable absorption (RSA) at 532 nm for ns laser pulses. The strength of RSA is mainly determined by the ratio of the triplet excited-state absorption cross section to that of the ground state. Extending the π-conjugation in the N^N ligand increases the RSA in comparison to extending the π-conjugation in the C^N ligands.

Macromolecular structures containing organic chromophores have received great attention due to their use in optical and electronic applications. Certain molecular assemblies have shown enhanced light harvesting and nonlinear optical properties by virtue of strong excitonic coupling in the multi-chromophore system. New macromolecular geometries have been investigated giving rise to applications in photovoltaic (solar) devices, dielectric effects, as well as for enhanced nonlinear and quantum optical effects. This talk will discuss our recent results utilizing these systems. The excitation mechanism in these systems depends on the nature of the geometrical orientation of covalently attached chromophores, and the extent of delocalization. Through ultra-fast time-resolved and nonlinear spectroscopy, we have characterized the mechanism of energy transport and the relative strength of intra-molecular interactions in the macromolecular systems. For particular assemblies the processes of efficient energy transfer, fast energy re-distribution, and enhanced two-photon absorption cross-sections will be discussed The enhanced effects illustrated in these macromolecular systems provide new opportunities for the use of organic macromolecular materials in photonic applications.

Studies assessing the impact on optical nonlinearity of various molecular structural changes have resulted in the development of structure - nonlinear optical (NLO) property relationships for organometallic complexes - as a consequence, tailoring molecular composition to maximize NLO coefficients is reasonably well understood.
Attention has turned to reversibly modulating the NLO properties. Amongst the possible approaches to effect molecular NLO “switching”, reversible protonation / deprotonation, oxidation/reduction, and photoisomerization protocols have attracted most attention.[1-3] While protonation/deprotonation and photoisomerization sequences have been employed to successfully switch NLO properties in organic molecules, the fully reversible metal centered redox couples at organometallic complexes suggest that these are the compounds of choice for electrochemical switching. We report herein the development of metal alkynyl-based cubic NLO switches that respond to “orthogonal” protic, electrochemical, and photochemical stimuli, affording compounds with several distinct and accessible cubic NLO “states”.
[1] C. E. Powell, M. P. Cifuentes, J. P. L. Morrall, R. Stranger, M. G. Humphrey, M. Samoc, B. Luther-Davies, G. A. Heath, J. Am. Chem. Soc., 2003, 125, 602-610.
[2] G. T. Dalton, M. P. Cifuentes, S. Petrie, R. Stranger, M. G. Humphrey, M. Samoc, J. Am. Chem. Soc., 2007, 129, 11882-11883.
[3] K. A. Green, M. P. Cifuentes, T. C. Corkery, M. Samoc, M. G. Humphrey, Angew. Chem. Int. Ed., 2009, 48, 7867-7870.

High-performance metal-based nonlinear optical (NLO) materials such as metal-organic frameworks[1], metallic clusters[2] and organometallic complexes[3] etc., have attracted considerable attention because they combine the advantages of organic molecules and inorganic materials: large modifiable structures and the presence of many heavy-metal atoms. However, modulation of the NLO properties of these functional metal-based compounds remains largely unexplored. Our previous investigations have demonstrated that changing the connected backbone ligands[4,5] and central metal atoms of heterothiometallic clusters[6] may successfully modulate their optical nonlinearity.
A new synthetic approach employing lanthanide cation templates has recently been developed to modulate the assembly of three unprecedented heterobicluster frameworks (HCFs 1-3) and their NLO properties. Lanthanide cation templates [Nd(DMSO)8]3+, [Nd(DMF)8]3+ and [La(DMSO)9]3+ of the same valent state but of different sizes or geometries may afford W/S/Cu-containing HCFs with the same W/S/Cu building clusters (planar ‘open&’ clusters [WS4Cu4]2+, linear clusters [WS4Cu2] and cyano bridges) but with differing connectivity: a three-dimensional (3D) 4-connected framework in HCF-1, a 3D 5-connected framework HCF-2 and a two-dimensional 3-connected (6,3) network in HCF-3. The observed NLO behaviors of HCFs 1-3 are also quite distinct. Specifically, as the functional position of the linear clusters is varied from double bridge in HCF-1, to single bridge in HCF-2, and terminal position in HCF-3, the hyperpolarizabilities γ steadily decrease. The modulation in the NLO properties of HCFs 1-3 is clearly attributable to the diverse lanthanide cation templates, which are responsible for the differences in bonding and electronic structure, and thereby NLO response.
References:
[1]C. Wang, T. Zhang, W. B. Lin, Chem. Rev. 2012, 112, 1084.
[2]C. Zhang, Y. L. Song, F. E. Kühn, Y. X. Wang, X. Q. Xin, and W. A. Herrmann, Adv. Mater. 2002, 14, 818.
[3]C. E. Powell, M. G. Humphrey, Coord. Chem. Rev. 2004, 248, 725.
[4]C. Zhang, Y. Cao, J. F. Zhang, S. C. Meng, T. Matsumoto, Y. L. Song, J. Ma, Z. X. Chen, K. Tatsumi, M. G. Humphrey, Adv. Mater. 2008, 20, 1870.
[5]C. Zhang, T. Matsumoto, M. Samoc, S. Petrie, S. C. Meng, T. C. Corkery, R. Stranger, J. F. Zhang, M. G. Humphrey, K. Tatsumi, Angew. Chem. Int. Ed. 2010, 49, 4209.
[6]J. F. Zhang, S.C. Meng, Y. L. Song, H. J. Zhao, J. H. Li, G. J. Qu, L. Sun, M. G. Humphrey, C. Zhang, Chem. Eur. J. 2010, 16, 13946.

We have been studying a number of nanosystems that either have potential applications in bioimaging and/or light-activated therapies, or are bioderived. The standard Z-scan technique was routinely used for most of the measurements which were carried out in a wide wavelength range, typically from ~550 to 1600 nm. The range of nanoparticles studied has included colloidal semiconductor nanoparticles (e.g. CdS [1], CdSe [2], CuInS2 [3]), plasmonic nanoparticles [4], metal clusters, lanthanide-doped fluoride [5] and oxide [6] nanocrystals as well as core-shell systems. Among the bioderived systems studied especially interesting one is that of protein amyloid fibers [7].
Many of these materials exhibit nonlinear absorption features due not only to the typical two-photon absorption processes, but also due to multiple-photon absorption taking place, especially at longer wavelengths (e.g. three- four- and five-photon processes). On the other hand, absorption saturation processes may prevail or compete with multi-photon absorption in certain wavelength ranges in some of these materials, especially those characterized by broadband absorption due to surface plasmon excitation.
[1] Szeremeta, J.; Nyk, M.; Wawrzynczyk, D.; Samoc, M. Nanoscale 2013, 5, 2388.
[2] Nyk, M.; Wawrzynczyk, D.; Szeremeta, J.; Samoc, M. Appl. Phys. Lett. 2012, 100, 041102.
[3] Cichy, B.; Wawrzynczyk, D.; Bednarkiewicz, A.; Samoc, M.; Strek, W. Appl. Phys. Lett. 2013, 102, 243702.
[4] Olesiak-Banska, J.; Gordel, M.; Kolkowski, R.; Matczyszyn, K.; Samoc, M. J. Phys. Chem. C 2012, 116, 13731.
[5] Nyk, M.; Wawrzynczyk, D.; Parjaszewski, K.; Samoc, M. J. Phys. Chem. C 2011, 115, 16849.
[6] Wawrzynczyk, D.; Nyk, M.; Samoc, M. J. Mat. Chem. C 2013, 1, 5837.
[7] Hanczyc, P.; Samoc, M.; Norden, B. Nature Photonics 2013, doi:10.1038/nphoton.2013.282

Limitations of light penetration within biological tissue has forced the study of wound healing progression to be limited to 200-300 microns achieved by confocal microscopy. A vast majority of the studies of tumor angiogenesis and wound healing progression have relied on staining (H&E) of tissue sections and fluorescence confocal microscopy analysis of these sections. However the fibrin clot and granulation tissue in wounds is very delicate and is frequently damaged during tissue section preparation. This can lead to distorted, ambiguous images. Deep imaging within tissue (over 300 mu;m) at micrometer resolution has become possible with the advent of two-photon fluorescence microscopy (2PFM). However, the development of 2PFM has frequently overlooked the importance of developing efficient 2PA materials. We report the use of small molecule and block copolymer RGD peptide conjugates for deep ex vivo imaging of tumor vasculature in “whole” excised tumors using 2PFM. The fluorescent probes were administered to mice via tail-vein injection, after which the tumors were excised, fixed, and imaged without further sample preparation. Both RGD conjugates demonstrated specific targeting to tumor blood vessels, and this selectivity imparted excellent contrast in 2PFM micrographs that captured high-resolution 3-D images of the tumor vasculature up to depths of 830 microns in Lewis Lung Carcinoma (LLC) tumors. Additionally, an efficient two-photon absorbing fluorescent probe we developed targeted integrin that was overexpressed in endothelial cells of invading capillaries in a wound healing mouse model, providing an imaging penetration depth up to 1.6 mm.

Two-photon-excited fluorescence (TPEF) has gained widespread popularity in the biology community due to the many advantages it provides in microscopic imaging. Among various probes, semiconductor quantum dots have been shown to provide a particularly powerful approach to luminescent nano-objects for bioimaging including TPEF. However, these inorganic systems suffer from several drawbacks such as toxicity, blinking and (bio)degradability. With this in mind, we have developed innovative approaches towards molecular-based luminescent nanoparticles. These fully organic nano-emitters are obtained from rational and bottom-up routes.
Two alternative and complementary routes will be presented. The first is based on the control of the optical responses via the design of covalent molecular structures of highly confined chromophores in hierarchical architectures (dendrimers) in which interactions between chromophoric subunits are controlled. This led to tunable nano-objects which can outcome semiconductor quantum dots in terms of brightness by orders of magnitude, show cooperative enhancement of nonlinear absorption, and allow fast excitation energy transfer. These tunable, soft organic nanodots (ONDs) have proven of major interest for in vivo bioimaging as well as for sensitive detection of explosives.
The second route is based on the spontaneous assemblies in water of specially engineered water-insoluble dyes that leads to Fluorescent Organic Nanoparticles (FONs), typically 20-40 nm in diameter. Their luminescence color and biocompatibility as well as their colloidal stability can be tuned by the nature of dye building blocks. Moreover, striking luminescence spatial confinement and enhancement can be achieved in core-shell nanoparticles made from dedicated complementary dyes, in relation with the generation of large interfacial electric fields.

The synthesis, linear photophysical, two-photon absorption (2PA), femtosecond transient absorption, and superfluorescence properties of a new symmetrical squaraine derivative (1) are reported. Steady-state linear spectral and photochemical properties, fluorescence lifetimes, and excitation anisotropy of 1 were investigated in various organic solvents. High fluorescence quantum yields (asymp; 0.7) and very high photostability (photodecomposition quantum yields asymp; 10-6-10-8) were observed. An open aperture Z-scan method was used to obtain 2PA spectra of 1 over a broad spectral range (maximum 2PA cross-section asymp; 1000 GM). Excited state absorption (ESA) and gain was observed via fs transient absorption spectroscopy, both reaching a maximum at ca. 500 fs. Importantly, squaraine 1 exhibited efficient superfluorescence. A quantum chemical study of 1 revealed simulated vibronic nature of the 1PA and 2PA spectra that were in good agreement with experimental data, providing potential predictive ability in the design of advanced photonic materials.

Pt(II) diimine complexes bearing benzothiazolylfluorenyl or diphenylaminofluorenyl motifs on the bipyridine (Pt-1 - Pt-3), and benzothiazolylfluorenyl, diphenylaminofluorenyl, or naphthalimidefluorenyl motifs on the acetylide ligands (Pt-4 - Pt-8) were synthesized and their photophysical properties were investigated with the aim of developing ideal nonlinear absorbing materials. The results indicate that these novel V-shape and X-shape Pt(II) complexes exhibit long-lived triplet excited state (tau; up to ~55 mu;s) and broadband triplet excited-state absorption in the visible to the NIR region. Upon ns laser excitationat 532 nm, the transmission of these Pt complexes decreases drastically, clearly demonstrating the occurance of reverse saturable absorption (RSA). The photophysical properties and degree of RSA are influenced significantly by the acetylide ligand substitution and by the nature of the substituent at the fluorenyl component. In addition, DFT calculations were performed to help us understand the nature of the lowest singlet excited state upon terminal substituent variation at the fluorenyl motifs. The photophysical and nonlinear optical studies suggest that some of these Pt(II) complexes with long-lived triplet excited states and broadband excited-state absorptions could potentially be used as broadband nonlinear absorbing materials.

Two C60-(antenna)x analogous compounds having branched hybrid triad C60(>DPAF-C18)(>CPAF-C2M) and tetrad C60(>DPAF-C18)(>CPAF-C2M)2 nanostructures were synthesized and characterized. The structural design was intended to facilitate the ultrafast fs intramolecular energy-transfer from photoexcited C60[>1(DPAF)*-C18](>CPAF-C2M)1or2 or C60(>DPAF-C18)[>1(CPAF)*-C2M]1or2 to the C60> cage moiety upon two-photon pumping at either 780 or 980 nm, respectively. The latter nanostructure showed approximately equal extinction coefficients of optical absorption over 400-550 nm that corresponds to near-IR two-photon based excitation wavelengths at 780-1100 nm for broadband nonlinear optical (NLO) applications. Aside from their enhanced two-photon absorption (2PA) activity at 780 nm, we also demonstrated ultrafast photo-responses at 980 nm showing 2PA cross-section (σ2) values of 995-1100 GM for the hybrid tetrad. These σ2 values were correlated to the observed good efficiency in reducing fs light-transmittance down to 35% at the light intensity of 110 GW/cm2. Accordingly, 2PA characteristics of these nanostructures at multiple NIR wavelengths provided support for their suitability in uses as broadband NLO nanomaterials at 600-1100 nm that includes the 2PA ability of two antenna, DPAF (700-850 nm) and CPAF (850-1100 nm), and the fullerene cage at shorter wavelengths (600-700 nm).

Molecular engineering based on Ru(II) complexes is an efficient way to lead, via a twophoton
absorption process (2PA),¹ to long lived triplet excited-states (³MLCT). This
possibility to irradiate in the near-infrared and the adequate functionnalization of the
coordination complexes may lead to a wide range of applications such as optical power
limiting,^2,3 optical probes, O₂ sensors, biological imaging and/or as photosensitizer (PS) in
photodynamic therapy (PDT). Synthesis and photophysical characterizations of the
compounds will be described (linear, excited-state absorption, and 2PA properties) together
with a strategy of encapsulation and/or grafting of such PS (and more generally,
chromophores) within/to nanoparticles (polymeric, silica, gold). In this domain, presentation
will focus on the two-photon photodynamic therapy (2PA-PDT).⁴ Results and objectives in
bifunctionality therapy/imaging will also be presented^.5
[1] C. Girardot, G. Lemercier, J.-C. Mulatier, J. Chauvin, P. L. Baldeck, C. Andraud Dalton Trans. 2007, 3421;
G. Lemercier, A. Bonne, M. Four, L. M. Lawson-Daku C. R. Chim. 2008, 11, 709; [2] C. Girardot, B. Cao, J.-C.
Mulatier, P. L. Baldeck, J. Chauvin, D. Riehl, J. A. Delaire, C. Andraud, G. Lemercier ChemPhysChem 2008, 9,
1531; [3] M. Four, D. Riehl, O. Mongin, M. Blanchard-Desce, L. M. Lawson-Daku, J. Moreau, J. Chauvin, J. A.
Delaire, G. Lemercier PhysChemChemPhys. 2011, 13, 17304; [4] S. C. Boca, M. Four, A. Bonne, B. van der
Sanden, S. Astilean, P. L. Baldeck, G. Lemercier Chem. Comm., 2009, 4590; [5] S. S. Kelkar, T. M. Reineke
Bioconjugate Chem. 2011, 22, 1879 ; C. Truillet, F. Lux, J. Moreau, M. Four, L. Sancey, S. Chevreux, G. Boeuf,
P. Perriat, C. Frochot, R. Antoine, P. Dugourd, C. Portefaix, C. Hoeffel, M. Barberi-Heyob, C. Terryn, L. Van
Gulick, G. Lemercier, O. Tillement Dalton Trans., 2013, DOI: 10.1039/c3dt50946j.

Polymethine dyes have been shown to have promising molecular third-order nonlinear optical properties but have not shown suitable macroscopic third-order nonlinear susceptibilities and two-photon figures of merit for all-optical signal processing (AOSP). This is largely due to the strong dispersion forces between these molecules, which, at high number density, can lead to aggregation and alterations to the electronic energy level structure that can deleteriously affect the optical nonlinearity and optical loss. We have investigated a design strategy to achieve AOSP that is based on substitution of polymethines with bulky groups that project out of the plane of the thio- or selenopyryllium polymethine which leads to high-number-density films with macroscopic nonlinear optical properties close to those needed for AOSP, along with low linear and nonlinear losses that can result in exceptionally high two-photon figures of merit (Re| chi;(3)|/ Im| chi;(3)|. We show that judicious substitution on the pyryllium group (ends), the central carbon on the polymethine chain (front), and on a central cyclohexenyl group (back) affords a dramatic reduction of aggregation associated with the steric bulk and larger molecular separations which results in near solution like properties in neat molecular films and in 50wt.% blend films with amorphous polycarbonate. These thio- or seleno-pyryllium based polymethines also exhibit a negative Re| chi;(3)| that can be useful in the compensation of self phase modulation (spectral broadening) that is associated with the propagation of strong optical signals in optical fibers. We show that use of a 50 cm liquid core optical fiber filled with a 20 mM solution of a thiopyryllium based polymethine was able to reverse the spectral broadening due to self-phase modulation from propagation of the pulses through a solution of CCl4 and CS2.

Molecular two-photon absorption (TPA) has attracted a lot of interest over the last decade owing to its applications in various fields including spectroscopy, three-dimensional optical data storage, microfabrication, high-resolution three-dimensional imaging of biological systems, and photodynamic therapy. Among these, two-photon-excited fluorescence (TPEF) has gained widespread popularity in the biology community due to the advantages it provides in microscopic imaging.
Among inorganic nanomaterials, semiconductors quantum dots (QDs) have gained a lot of popularity as photonic nano-objects due to their unique photonic and electronic properties. These include recording: excitation cross-sections, luminescence tunability, photostability hellip;. These nanoparticles have thus been heralded as major nanotools for imaging (including bioimaging) purposes as well as for the elaboration of nanostructures for optoelectronics. However, these inorganic systems suffer from several drawbacks such as toxicity, blinking hellip; and raise a number of questions with respect to environmental and safety issues (clearance, biodegradabilityhellip;). In order to address these issues, we have implemented a molecular approach towards biocompatible and eco-friendly soft organic nanodots. These purely organic nanoemitters are obtained from a rational and bottom-up route and differ in both their design and electronic origin of luminescent properties from QDs. Our route is based on the control, at the single nano-object level, of the optical responses via the covalent confinement of optimized chromophores within hierarchical (dendrimeric) architectures in which interactions are controlled. This led to tuneable nano-objects which show record one- and two-photon brightness and have been shown to be a major interest for in vivo imaging.
1. a) O. Mongin, T. R. Krishna, M. H. V. Werts, A.-M. Caminade, J.-P. Majoral, M. Blanchard-Desce, Chem. Commun. 2006, 915-917; b) M. Blanchard-Desce, M. Werts, O. Mongin, J.-P. Majoral, A.-M. Caminade, R. K. Thatavarthy, PCT Int. Appl., 2007, WO 2007080176;. c) O. Mongin, C. Rouxel, J.-M. Vabre, Y. Mir, A. Pla-Quintana, Y. Wei, A.-M. Caminade, J.-P. Majoral, M. Blanchard-Desce, SPIE Proc. NanoScience + Engineering, 2009, 7403, 740303-1 - 740303-12.
T. R. Krishna, M. Parent, M. H. V. Werts, L. Moreaux, S. Gmouh, S. Charpak, A.-M. Caminade, J.-P. Majoral, M. Blanchard-Desce, Angew. Chem., Int. Ed. 2006, 45, 4645-4648; O. Mongin, C. Rouxel, A.-C. Robin, A. Pla-Quintana, Tathavarathy Rama Krishna, G. Recher, F. Tiaho, A. -M. Caminade, J.-P. Majoral, M. Blanchard-Desce, SPIE Proc. NanoScience + Engineering, 2008, 7040, 704006-704017.

Octupolar molecules are of increasing interest in the field of nonlinear optics due to their potentially large two- or three-dimensional quadratic nonlinearities.[1] These molecules are non-dipolar species whose NLO response is related to multidirectional charge-transfer excitations, rather than to dipolar unidirectional excitations. A number of molecules with three-fold rotational symmetry, as well as molecular ions of D3h, C3v or D3 symmetry, have been shown to display promising properties. In order to investigate new molecules that possess this type of structure, we have synthesized a new family of organic 1,3,5-triazine-2,4,6-trione derivatives.[2]
Albeit being known since the late 1890&’s, these simple octupolar structures have never been investigated for their optical properties. Straigthforward syntheses of these derivatives, along with their full spectroscopic characterization (IR, UV-Vis) will be presented. Their linear optical (LO) and nonlinear optical (NLO) properties will then be discussed. In addition to good hyperpolarizabilities and good transparencies, some of these derivatives present also remarkable two photon absorption (TPA) properties. Structural modifications allowing improving this particular property in purely organic derivatives will be presented and their NLO activity will be rationalized with the help of DFT computations.
Organometallic derivatives have also been obtained from precursors possessing pendant alkyne groups (Scheme 1).[3] The syntheses of these organometallic derivatives, along with their spectroscopic characterization (UV-Vis), and including the molecular structure of one example, will be presented. The positive impact of the peripheral organometallic substituents on the NLO performances will finally be analyzed based on additional DFT calculations.
References :
[1]. J. Zyss, I. Ledoux, Chem. Rev. 94 (1994), 77.
[2]. G. Argouarch, R. Veillard, T. Roisnel, A. Amar, H. Meghezzi, A. Boucekkine, V. Hugues, O. Mongin, M. Blanchard-Desce and F. Paul, Chem. Eur. J. 18 (2012), 11811.
[3]. A. Trujillo, R. Veillard, G. Argouarch, T. Roisnel, A. Singh, I. Ledoux and F. Paul Dalton Trans. 41(2012) 7454.

We aim to use organometallic complexes in a novel modular approach based on the coordinating tecton. This more modular approach allows ready testing of structure-property relationships. The conjugated organic part of the tectons provide a coordination site for the incorporation of additional metal centres and controlled assembly of compounds with defined length while mediating charge transfer in the molecule and ultimately the molecular junctions in hybrid devices.
These modular organometallic complexes possess a range of properties that make them promising candidates as new materials for electronics and related applications:
-versatility of structure, size, binding groups and spin state;
-control of redox chemistry and multi-redox states enabling development of device functionality and gated control of electronic function
-control of charge transport mechanisms from short-range superexchange to long-range hopping by moderation of the extent of metal-ligand coupling and/or molecular length.
Our current endeavours in this area will be presented.

Organic molecules are promising candidates for new non-linear optical materials and devices, potentially cheaper and higher performance than inorganic crystals. Design and initial characterization are typically done from the standpoint of isolated molecules, but when the molecules are put into solution or other condensed phases for measurement or application, the nonlinear optical properties are modified by the effect of the environment, making comparisons with the isolated molecule difficult in theory or experiment. To probe these environmental effects, we employ a new theoretical approach for direct calculation of the nonlinear susceptibility for the electric-field-induced second-harmonic generation (EFISH) and hyper-Rayleigh scattering (HRS) experiments in solution. We use atomistic liquid structures from molecular dynamics and apply time-dependent density-functional theory (TDDFT) with the Sternheimer equation. Additionally we show a rigorous definition of local-field factors usable for ab initio calculations, which can differ significantly from those obtained from the simple models typically used. These analyses allow us to make a direct comparison to measurements in condensed phases and to extract molecular properties.

The simultaneous refinement of experimental data and ab initio calculations is shown to afford enriched information about the molecular origins of optical non-linearity. Specifically, non-linear optical (NLO) properties are derived from a combined experimental charge-density study, X-ray constrained wavefunction refinement, and quantum-mechanical calculations.
The case study on the well-known frequency-doubling material, zinc tris(thiourea)sulfate (ZTS), first shows how one can derive solid-state tensorial components of its (hyper)polarizability directly from high-resolution X-ray structural data [1,2]. Then, the molecular dipole moment, mu;i, and (hyper)polarizability coefficients, αij and βijk, are calculated from refinement of experimental diffraction data using the X-ray constrained wavefunction method [3]. When taken alone, the experimental charge-density analysis does not fare well in deriving mu;i, αij and βijk. However, by refining the X-ray data within the constrained wavefunction method, the evaluations of mu;i, αij and βijk,are shown to agree very well with those from ab initio calculations. The small differences observed between ab initio and X-ray constrained wavefunction refinement results can be related directly to gas- versus solid-state phase differences. mu;i is found to be 28.3 Debye (gas-phase) and 29.7 Debye (solid-state) while βijk coefficients are both significant and markedly three-dimensional in form. Accordingly, substantial octupolar as well as dipolar NLO contributions in ZTS are indicated, which challenges the traditional focus on dipolar NLO molecules.
This evaluation of NLO properties and its relation to the molecular structure, using Laplacian bond classifiers, also offers several ways by which ZTS may be more widely functionalized as an NLO material. Specifically, an analysis using Green&’s function can partition all π-contributions to each conjugated bond from all atoms, yielding a ‘global bond-length alternation&’ parameterization of intramolecular charge-transfer.
More generally, this case study on ZTS demonstrates how experimental and computational techniques can be combined to understand NLO structure-property relationships. This is an important tool for the quantum-tailored molecular design of next-generation metalorganic NLO materials.
References:
[1] Cole et al, “Fast calculation of hyperpolarisability of organic non-linear optical molecules”, J. Appl. Phys. 111 (2012) 033512(1-6).
[2] Cole et al, “Molecular origins of non-linear optical activity in zinc tris(thiourea)sulfate revealed by high-resolution X-ray diffraction data and ab initio calculations”, Phys. Rev. B in press (2013).
[3] Cole et al, “Modeling electron density distributions from x-ray diffraction to derive optical properties: constrained wavefunction versus multipole refinement”, J. Chem. Phys. 139 (2013) 064108.

From Mendeleev's periodic table of the elements to the Hertzprung-Russel Diagram that classifies stellar spectra, patterns in relationships between physical quantities often hint at deeper underlying physics. Using these famous examples as a guide, we study patterns in the nonlinear-optical response of a quantum system using Monte Carlo techniques and surveys of measurements. Our aim is to better understand the nature of nonlinear light-matter interactions when the hyperpolarizability is near its limit. This approach seeks to identify the fundamental properties of a quantum system that control and perhaps can be used to optimize the nonlinear-optical response. The understanding that results can be used to unify observations over broad classes of quantum systems and may guide the design of better materials. As a bonus, unexpected new physics may be uncovered.
Our goal is to sample all possible quantum systems. The most general approach is to use Monte Carlo studies, in which the transition moments and energy spectrum are randomly sampled under the constraints of the Thomas Reiche Kuhn sum rules, to determine the properties of the system when the nonlinear response is at the limit. While this approach samples a large configuration space of what is possible, its generality makes specific predictions difficult. However, restricting these results by fixing energy spectra into classes that correspond to real quantum systems partially explains the large gap between the best molecules (where Coulomb potentials are dominant) and the fundamental limit. The reason is found to lie in nature of the Coulomb potential, which has a sub-optimal energy spacing.
We have identified new patterns in 1D potentials that provide insights into the inability of real systems to reach the fundamental limits. Simple systems with power-law potentials, analyzed using semiclassical techniques, reveal singularities with puzzling properties. The nonlinear-optical properties of such singular systems, which have analytical solutions to the Schrodinger Equation, will be discussed and their associated pathologies analyzed.
Other systems amenable to random sampling are quantum graphs, which are essentially quantum wires that are connected into networks to form structures in 3D space. Our present studies focus on their properties in 2D. Undressed quantum graphs are useful model systems because they allow the nonlinear-optical nonlinearly to be calculated using analytical wavefunction as the geometry and topology of the structure is varied through all possible configurations. Furthermore, quantum graphs can be made in the lab by connecting together quantum wires, thus allowing verification of the calculations. We have found that the nonlinear susceptibilities of a quantum graph is profoundly affected by topology and geometry. These results will be placed within the broader context of the key properties that yield the largest nonlinear-optical response.

Computer-aided design of two-photon active media has proven its appreciable role in molecular nonlinear optics. In fact, first-principles quantum chemical calculations can be used to establish (electronic) structure-property relationships which can be further used to design novel materials with large nonlinear response. However, the reliability of the predictions heavily depends on the level of theory, approximations involved as well as the time and length scales of the processes and systems studied. This is particularly true, for instance, in the investigations of environmental effects on the multiphoton absorption spectra of chromophores, which demand the explicit accounting of the interaction between chromophore-media subsystems. The integrated multi-scale approaches have potential to model the properties of such systems with numerous degrees of freedom
with not much demand on computational resources. During the talk, a wide pallete of computational approaches to determine two-photon absorption spectra of organic dyes in solution shall be presented and critically discussed, including static quantum-chemical calculations, rigid- and flexible-body molecular dynamics and hybrid QM/MM molecular dynamics. The assessment of these approaches shall be performed by comparison with experimental data obtained using the Zscan technique. Some illustrative examples shall be presented, including highly solvatochromic betaine dye also known as Reichardt&’s dye [1], 4-dimethylamino-4&’-nitrostilbene (DANS) [2, 3] and Hoechst 33342 dye [4], to name a few.
References
[1] M.Wielgus, R. Zalesny, N. A. Murugan, J. Kongsted, H. Agren, M. Samoc,W. Bartkowiak,
ChemPhysChem 14 (2013) 3731.
[2] M. Wielgus, W. Bartkowiak, M. Samoc, Chem. Phys. Lett. 554 (2012) 113.
[3] N. A. Murugan, J. Kongsted, Z. Rinkevicius, K. Aidas, K.V. Mikkelsen, H. Ågren, Phys.
Chem. Chem. Phys. 13 (2011) 12506.
[4] J. Olesiak-Banska, K. Matczyszyn, R. Zalesny, N. A. Murugan, J. Kongsted, H. Ågren, W.
Bartkowiak, M. Samoc, J. Phys. Chem. B 117 (2013) 12013.

Organic non-linear optical (ONLO) materials offer several key advantages such as intrinsically higher bandwidth on the order of THz and lower power consumption over currently used inorganic materials such as LiNbO3.
Acentric order under an external poling field determines wether a large second-order nonlinear optical response will be obtained from a large molecular hyperpolarizability of an ONLO material at a given loading density. Computational modeling can provide valuable insight into acentric order.
Specifically, coarse-grained Monte Carlo methods with parameters obtained from ab initio quantum mechanical calculations and fully-atomistic force fields are used to investigate the details of molecular interactions that determine the extent of order, thereby guiding the design of novel materials with improved performance.
We will discuss recent theoretical results from simple model systems up to multi-component organic glasses of molecules which have larger dipole moments and larger hyperpolarizabilities. The importance of shape, electrostatics, and pendant groups on resulting order will be illustrated and compared with experimental data.

Multi-photon pumped up-conversion lasing (or stimulated emission) from organic chromophores has attracted increasing attention in the past two decades. Through simultaneous absorption of two (or three, four, five) photons, an infrared laser can be used to generate a visible laser. In comparison with other frequency up-conversion techniques such as second- or third-harmonic generation, multi-photon pumped lasing has the advantages of no phase matching requirement and easy tunability. In this talk, up-conversion lasing induced by simultaneously absorbing as many as five photons will be presented. Spectral properties, lasing efficiencies, spatial, temporal and dynamic properties of multi-photon stimulated emission are going to be covered, as well as the structure/property relationships for the multi-photon lasing materials. Pumped by femto-second lasers, asymmetric behavior between the forward and backward stimulated emission was observed and theoretically explained. Two optical windows provide cavity feedback of the stimulated emission which will affect the lasing threshold, the spectral and temporal properties.
References
[1] Q. D. Zheng, H. M. Zhu, S. C. Chen, C. Q. Tang, E. Ma, X. Y. Chen. Nature Photon., 2013, 7, 234.
[2] G. S. He, C. G. Lu, Q. D. Zheng, A. Baev, M. Samoc, P. N. Prasad, Phys. Rev. A, 2006, 033815.

Measuring nonlinear absorption (NLA) is often easier than measuring its counterpart, nonlinear refraction (NLR). For example, for organic molecules in solution, it can be hard to determine the NLR of the solute because the solvent always has some NLR and this will often dominate the effect of the solute, making characterization of the organic molecule difficult. This rarely happens with NLA because solvents with no NLA can easily be found. To address this, we have developed a dual-arm Z-scan measurement system that simultaneously scans solvent and solution so that the effect of the solute can be directly subtracted out. This differential technique has the effect of eliminating most of the common noise modes in both arms (such as beam pointing instability, etc.) through simultaneous subtraction. We find that this method increases the signal-to-noise by an order of magnitude allowing us to measure NLR of dilute solutions. We can also apply this method to the measurement of thin films on thick substrates.
Determining the dynamics of NLA has been made relatively simple for many years through the use of pump-probe techniques. However these techniques are not easy to implement for the measurement of NLR. To improve upon this situation, we have borrowed from the photothermal deflection technique, which has long been used to measure weak absorption by monitoring the thermally-induced index change. To apply this to NLO measurements, we replace cw lasers with pulsed sources and include a delay arm to allow the determination of the nonlinear dynamics. Advances in bi- and quad-cell detectors with integrated differential amplifiers allow extremely small beam deflections to be measured, and we have a demonstrated sensitivity to induced phase shifts as small as lambda;/20,000. We will show, for example, how the interesting time dynamics of NLR in carbon disulfide can be resolved with this method.

Degenerate two-photon absorption (2PA) is a third-order nonlinear-optical process that involves simultaneous absorption of two photons, where both contribute half of the energy required for the instantaneous transition between initial (ground) and final (excited) state. The rate of the 2PA increases as square of the incident photon flux density - a useful property that is widely implemented in fluorescence microscopy, especially when combined with available ultrafast femtosecond lasers. Because of this and other related technology applications, considerable attention has been devoted over recent decades to the optimization and characterization of the 2PA properties both in synthetic organic dyes as well as in genetically engineered fluorescent protein systems.
It is well known that efficient 2PA occurs due to enhanced ground- and excited state transition dipole moments and/or strong permanent electric dipole moments. Interestingly, the same conditions that are required for enhanced 2PA also lead to inherent sensitivity of the chromophore optical properties with respect to intra- and inter-molecular electrostatic interactions. In particular, it has been shown that the 2PA cross section in the S0 to S1 transition increases in proportion to the square of the difference between the permanent electric dipole moments. Based on this observation, we have recently utilized the 2PA measurement as a new way to determine the instantaneous dipole moment change between the ground- and excited state. To facilitate such measurements we have developed improved experimental techniques for determining the 2PA values in both fluorescing as well as non-fluorescing systems. Furthermore, because of the non-vanishing polarizability of the chromophore, the dipole moment difference itself may be used as a way to determine the strength and the direction of the electric field acting on- and near the molecule.

In our paper we will present recent examples showing that by measuring the 2PA cross sections and -spectra with sufficient accuracy we gain new information about internal electric fields acting on dye molecules in solutions as well as in proteins. We believe that this novel analytical tool opens up an opportunity for detailed study of molecular-level electrostatic interactions including in complex systems such as proteins.

The experimental verification of the optical loss suppression in semiconductors by means of a new method based on the parametric bichromatic irradiation is reported. The physical mechanism responsible for the underlying effect is a result of the two-wave parametric mixing in a conductive (non-ohmic) non-linear medium, which couples the waves. The ratio of the frequency of the probe mode to the frequency of the support mode corresponds to parametric resonance condition: 2:1.The detailed theory of this effect was previously reported in following publications: A.-G. Kussow and A. Akyurtlu, Journal of Nanophotonics, 6, 063506 (2012); A.-G.Kussow and A. Akyurtlu, Phys. Stat. Solidi, (2013), under publication. In experiment, the support mode is generated by the CO2 pulsed laser, and the probe mode was obtained due to the Second Harmonic Generation of the support mode. The transmission measurements from the FTIR device is used to extract the complex refractive index , N=n+ik, of the ZnTe under the condition of bichromatic irradiation. The amplitudes and the phase shift between the probe and the support wave were adjusted until the imaginary part of N becomes zero, k=0. This situation corresponds to the coupled modes with simultaneous lossless propagation of the probe mode through the ZnTe monocrystal. This experimental scheme was realized, with a wealth of data, which supports the theory of the effect.

Two-photon-resonant hyper-Raman spectroscopy is discussed as a method for probing excited electronic states that are both one- and two-photon allowed and therefore contribute to the molecular first hyperpolarizability. Although hyper-Raman scattering is normally a very weak process, a number of electron donor-acceptor substituted conjugated organic molecules that have large first hyperpolarizabilities have been shown to give strong resonance hyper-Raman (RHR) spectra in solution. The quantum mechanical formulation of RHR scattering is reviewed and practical approaches to calculating RHR spectra and excitation profiles are discussed. Results are shown for several representative systems. The potential benefits of a close interplay between experiments, empirical simulations, and electronic structure calculations of ground- and excited-state properties are emphasized.

We investigated hydrogen adsorption and desorption from regular stepped Si(111) surfaces with 9.5° miscut in [-1-12] direction by observing sum frequency generation (SFG) spectroscopy. After hydrogen termination in an UHV chamber, monohydrides were observed on the terrace, while the dihydride existed on each step Si atom and the Si-H bonds lied on a plane perpendicular to the step edge. Depending on the experimental setup and the orientation of molecules with respect to the incident fields, not all vibrational modes of terrace and step might appear in vibrational spectra. However, there was agreement among researchers that four vibrational modes can be observed: a terrace mode A, and step modes C1, C2, and C3. In this paper, the SFG spectra with ppp and ssp polarization combination are reported. In both cases, the terrace mode A (2082 cm-1) and step modes C1 (2094 cm-1) peaks were clearly observed, while C2 and C3 were not. Only one group except ours has observed this kind of step SFG signal, and they found out three vibrational modes of A, C2 and C3 but not C1. Disappearance of the C1 mode in their SFG spectrum was explained to be because the polarizations of both incident IR and visible laser fields were nearly perpendicular to the C1 bond axis. Thus, neither the IR nor the Raman transitions could be efficiently excited, resulting in weak SFG excitation. However, in our SFG spectra, we suggest that the appearance of C1 mode is due to suitable orientation of the step dihydride with respect to the surface plane for SFG excitation. The step dihydride might be tilted by some degrees from the surface normal of the (111) plane. That means that a new configuration of the step dihydride could be formed on the stepped Si surface. This result is a very important contribution to this field.
In the experimental process, the Si surface was flashed at 1200 °C for several times, which resulted in 5 ~ 7 bunched steps and changed the electronic state of step Si atoms. The hydrogen adsorption as the dihydride on each step Si atom might not occur. That is why the step C2 and C3 modes could not be detected on the step bunched 9.5° miscut surface because of the decrease in the step intensities. The validity of this view was confirmed by taking SFG spectra of step bunched Si(111) 5° and 2° (~5 µm terrace length) with the same miscut direction, and not only the C2 and C3 modes but also C1 mode were absent.
The time dependence of the SFG spectra with ppp and ssp polarization combination were taken on the regular step H-Si(111)1x1 surface with 9.5° miscut in [-1-12] direction while the sample was heated at 722K. We found two interesting phenomena: the peak intensity of the terrace mode was reduced faster than that of the step mode and the step mode C1 was shifted by ~2 cm-1 during the heating. I suggest that phenomena may be related to the dipole-dipole interaction among the H-Si species. The theoretical calculation is required to clarify this point.

Phase-matching is critical for coherent nonlinear optical processes such as frequency conversion. Phase-matching allows microscopic nonlinear sources to combine constructively, resulting in more efficient far field emission. Zero index materials, a unique class of metamaterial, have a constant phase throughout the material, allowing nonlinear sources at all positions within the material to add up constructively. These materials provide a route to perfectly phase match nonlinear processes.
We experimentally demonstrate nonlinear propagation without phase mismatch in a bulk fishnet metamaterial with a zero refractive index. The fishnet metamaterial, which was fabricated using focused ion beam milling of a gold and magnesium fluoride multilayer exhibits regimes of positive, zero, and negative refractive indices. The index was characterized by phase measurements over a wavelength range of 1.1-1.65 micrometers using spectrally and spatially resolved interferometry.
Using intrapulse four-wave mixing (FWM), we show that the relative strength of the forward and backward emission correlates with the predicted phase-matching difference. In particular, at the zero crossing of the refractive index, we observe a forward-backward FWM ratio of unity, indicating perfect phase matching for both directions. In contrast to existing phase matching techniques, zero index metamaterials allow perfect phase matching in both the forward and backward directions. This first observation of nonlinear propagation in zero-index materials, which was verified by numerical simulation of the propagation dynamics, could allow efficient bi-directional nonlinear generation, a capability which is lacking in conventional nonlinear materials. The removal of the phase matching requirement may lead to new applications of nonlinear optical metamaterials.

Materials with large third-order nonlinear optical properties and fast response speed are of great interest for various applications such as ultrahigh-bandwidth optical-signal processing and optical limiting. Fast and accurate measurement of the nonlinear refraction index and absorption coefficient changes are of paramount importance for the design and optimization of nonlinear optical materials. Since the emergence of nonlinear optics, the development of nonlinear optical measurement technique play a key role in the advancement of photoelectric and photonic field. Many measurement methods such as the Third harmonic generation (THG), Four-wave mixing (FWM) and Z-scan have been proposed and widely employed today. Despite the tremendous progress in the this area, the drive for novel nonlinear optical nonlinearities measurement technique is still adequate, in part because of the inconsistent and sensitivity of these experimental methods available today. Therefore, there is considerable interest in developing novel nonlinear measurement technique and device with higher sensitivity and smaller footprint to fulfill the demand of academic and industrial field.
In this perspective, we present the fundamentals, developments and applications of a series of third-order nonlinear optical measurement technique with phase object (PO). The discussion will be mainly descriptive, trying to communicate the principles without getting mired in details, either theoretical or experimental. Admittedly, the subjects included reflect the research interests and expertise of the author's group. The feature and versatility of these measurement techniques in materials research for device applications are presented along with engineering approaches for overcoming these hurdles. The paper is organized as follows. Section 2 gives a general introduction on the principle of Zernike phase contrast and the theoretical basis for the optical nonlinearities measurement technique with phase object (PO). In section 3 we address a brief description on various measurement methods with PO including static single-shot measurement techniques and transient nonlinear dynamics measurement methods. The experimental demonstration and procedures are outlined when necessary, and their unique features compared to other techniques are summarized. Finally, section 4 describe the potential applications of this kind of technique.