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spring 1997 logo1997 MRS Spring Meeting & Exhibit

March 31 - April 4, 1997 | San Francisco
Meeting Chairs: Linda G. Griffith-Cima, David J. Eaglesham, Alexander H. King

Symposium S—Materials for Optical Limiting II


David Hagan 
Univ of Central Florida 
Orlando, FL 32816-2700

Patrick Hood 
Materials Directorate
Air Force Wright Laboratory
WL/MLPJ Bldg 651
Wright Patterson AFB, OH 45433-7702

Keith Lewis 
Dept of Lasers & Photonics 
DRA Malvern 
Malvern, WR14 3PS United Kingdom 

Ruth Pachter
Materials Directorate
Air Force Wright Laboratory
Wl/MLPJ Bldg 651
Wright Patterson AFB, OH 45433-7701

Joseph Perry 
Jet Propulsion Lab 
California Inst of Technology 
MS 67-201 
Pasadena, CA 91109

Richard Sutherland
Science Applications Intl Corp
513-255-3808 x3157

Symposium Support

*Air Force Wright Laboratory, Materials Directorate
*Defence Research Agency
*Science Applications International Corp.

Proceedings published as Volume 479
of the Materials Research Society
Symposium Proceedings Series.

In sessions below "*" indicates an inviteds paper.

Chair: Ruth Pachter
Monday Morning, March 31, 1997
Salon 11

8:30 AM *S1.1 
MODELLING INTRA-MOLECULAR NON-RADIATIVE PROCESSES, Stephen Till, Defence Research Agency, Dept Lasers and Photonics, Malvern, UNITED KINGDOM.

Existing capabilities for the computational design of new molecular materials for optical limiting allow the calculation of good estimates of excited state energies and of the oscillator strengths and radiative lifetimes associated With electronic transitions between these states. Less well developed is the understanding and capability to predict the magnitude of intra- and inter-molecular non-radiative processes, both of which can play an important role in determining the optical limiting potency of a candidate limiter molecule. This paper will focus on intramolecular processes - that is, internal conversion and inter-system crossing - and will outline theories and models used to understand and quantify them. The applicability of these models to molecules of interest as limiters will be discussed with reference to recent calculations.

9:00 AM S1.2 
THE DESIGN AND SYNTHESIS OF NEW ORGANIC MOLECULES WITH LARGE TWO-PHOTON ABSORPTION CROSS-SECTIONS FOR OPTICAL LIMITING APPLICATIONS, Bruce A. Reinhardt, Air Force Wright Laboratory, WL/MLBP Materials Directorate, Wright Patterson AFB, OH; Lawrence L. Brott, Stephen J. Clarson, Univ of Cincinnati, Dept of MS & E, Cincinnati, OH; Ramamurthi Kannan, Universal Technology Corp, Dayton, OH; Ann G. Dillard, Univ of Dayton, Research Inst, Dayton, OH.

Recently there has been an increase in interest in the investigation of two-photon absorption (TPA) as a mechanism which could lead to optical limiting behavior. To date, systematic investigations of what types of organic structural units lead to large TPA cross-sections at a given wavelength have been lacking. This lack of well defined structure property relationships makes it exceedingly difficult for a synthetic chemist to how where to begin when attempting to synthesize new materials with improved properties. The focus of a recent study in our laboratory has been the design and synthesis of a number of new chromophores with systematic structural variations. This paper describes the details of earlier and ongoing synthetic work in this area and the results of the characterization of these materials at the Photonics Research Laboratory at SUNY Buffalo. The chemistry used to synthesize these matenals offers the molecular design flexibility to incorporate pendant groups for solubility and other functional groups necessary for incorporation of the active two-photon component into polymers. Results to this point seem promising with average TPA cross-sections of selected chromophores measured to have values of 4 times that of current state-of-the-art dyes at 800 nm. Since the characterization is done under experimentally identical conditions, some meaningful structure-property relationships can be formulated to guide future synthetic efforts toward improved materials.

9:15 AM S1.3 
NOVEL TWO-PHOTON ABSORBING ORGANIC CHROMOPHORES FOR OPTICAL LIMITING, J. E. Ehrlich, X. L. Wu, California Inst of Technology, Jet Propulsion Lab, Pasadena, CA; Stephen Edward Rankin, Univ of Minnesota, Chemical Engineering and Mater, Minneapolis, MN; I.-Y. S. Lee, Z.-Y. Hu, H. Rockel, Seth R. Marder, California Inst of Technology, Beckman Inst, Pasadena, CA; Joseph W. Perry, California Inst of Technology, Jet Propulsion Lab, Pasadena, CA.

Molecules exhibiting strong two-photon absorption are currently of considerable interest for a wide range of applications, including optical limiting. We have observed large two-photon absorptivities in a class of diphenylpolyene derivatives with varying lengths of the polyene bridge by using direct nonlinear transmission measurements, and have initiated studies to determine their two-photon absorption spectra and to examine structure/two-photon spectra relationships. Given their large two-photon absorptivities and electronic properties, we have also successfully utilized these materials for optical limiting. Generally, molecular two photon absorptivities are too small for significant optical limiting of nanosecond duration pulses; however, significant optical limiting is observed for several stilbene-like molecules for 5 ns pulses, over a broad wavelength range. The strong optical limiting by two-photon absorption in this material is particularly interesting because the linear transmission is very high () for the samples employed. Thus, these molecules hold potential as broadly transparent two-photon optical limiting materials.

9:30 AM S1.4 
NONLINEAR OPTICAL PROPERTIES OF MODIFIED CARBOCYNINES, Nansheng Tang, Weijie Su, Thomas M. Cooper, Air Force Wright Laboratory, Materials Directorate, Wright Patterson AFB, OH; Daniel G. McLean, Richard L. Sutherland, Science Applications Intl Corp, Dayton, OH.

A number of modified carbocynines have been examined using 100 ps pulses at 532nm in a pump-probe setting. Reverse-saturable absorption is observed in the samples tested. We also find a refractive contribution to the third-order optical nonlinearity that decays much faster than the excited-state lifetime. This refractive nonlinearity is comparable to the nonlinearity from the excited-state absorption. We evaluate the overall optical limiting performance and present our findings.

9:45 AM S1.5 
OPTICAL LIMITING IN PHTHALOCYANINE SOLUTIONS AND IN PURE LIQUID PHTHALOCYANINES, Steven R. Flom, Richard G. S. Pong, James S. Shirk, F. J. Bartoli, Naval Research Laboratory, Optical Sciences Div, Washington, DC; R. F. Cozzens, Michael E. Boyle, Arthur W. Snow, Naval Research Laboratory, Chemistry Div, Washington, DC.

Many optical limiting devices are based on liquid solutions of reverse saturable absorption materials. Solutions have the attractive ability to self heal after exposure to high intensity light but can suffer from low number densities. Using methyl terminated poly(ethyleneglycol) with a number average molecular weight of 350 (MePEG) as a peripheral substituent group, the metal-free and lead substituted tetra MePEG phthalocyanines and napthalocyanines have been synthesized at NRL. The phthalocyanines are viscous liquids at room temperature and hence have the potential for self healing as well as the high number density of a pure material. This paper examines and compares the optical limiting characteristics of a series of these materials. We evaluate the photophysical mechanisms contributing to the optical limiting response through time resolved degenerate four wave mixing transient absorption and Z-scan measurements. Transient absorption studies demonstrate that each of the materials studied are reverse saturable absorbers, i.e., they have excited state absolution cross sections that exceed those of the ground states over a broad region of the visible spectrum. In a lead substituted phthaIocyanine, the excited state lifetime exceeds 6 ns, whereas in a metal-free naphthalocyanine, the observed lifetime is on the order of 0.5 ns, Nanosecond Z-scan measurements allow separation of the absorptive and refractive components of the response that contribute to the limiting. The intrinsic liquid shows both a strong excited state absorption and large thermal refraction. These measurements demonstrate that the pure liquid PbPc retained the photophysics that make other lead phthalocyanines good optical limiting materials and adds the advantages of neat materials.

Chair: Joseph W. Perry
Monday Morning, March 31, 1997
Salon 11

10:30 AM *S2.1 
NONLINEAR OPTICAL CHARACTERIZATION OF NOVEL PORPHYRIN DYE MEDIA, Kenneth J. McEwan, DRA Malvern, Malvern, UNITED KINGDOM; John M. Robertson, Philip K. Milsom, Defence Research Agency, Great Malvern, UNITED KINGDOM; Harry I. Anderson, Univ of Oxford, Dyson Perrins Lab, Oxford, UNITED KINGDOM.

It is important, when developing new reverse saturable absorbing (RSA) dye media, to understand how the molecular structure influences the physical parameters that determine the nonlinear optical response. Using a series of novel porphyrin dye molecules, as example, we will describe the nonlinear optical techniques that we use to characterize materials. The physical parameters we measure include the absorption cross sections of the singlet and triplet excited states from 400 to 700 nm, the singlet and triplet excited state lifetimes and the triplet quantum yield. Together with the linear absorption cross-section, these parameters completely describe the nonlinear response against laser pulses of different wavelength or pulse duration. Measurements which qualitatively and quantitatively verify our predictions will be described. The techniques developed to study RSA media can also be used to characterize optically transparent materials in which the excited state absorption is driven by two photon absorption liquid crystal media in their isotropic phase exhibit this type of response and some results on this class of material will be described. Two photon absorption is an intensity dependent process. This contrasts with RSA media in which the response has a fluence dependence. Some improvements in the nonlinear optical performance are available by combining both types of mechanism in a tandem configuration. Recent results obtained using a liquid crystal in tandem with a RSA dye will be discussed.

11:00 AM S2.2 
NONLINEAR OPTICAL PROPERTIES OF THE PENTAAZADENTATE PORPHYRIN-LIKE METAL COMPLEXES AND APPLICATION IN OPTICAL LIMITING, Wenfeng Sun, Shiming Dong, Inst of Photographic Chemistry, Beijing, CHINA; Jinhai Si, Chunfei Li, Harbin Inst of Technology, Dept of Physics, Harbin, CHINA; Duoyuan Wang, Inst of Photographic Chemistry, Academia Sinica, Beijing, CHINA.

Asymmetric pantaazadentate expanded porphyrins are aromatic macrocyclic molecule containing 22 electrons which form coplanar system appearing a more extended optical windows between the Soret and the Q bands in visible region and neat IR region except 1450 nm. It provides a condition of the high transmittance in the ground state. These molecules are easily polarized and a strong optical nonlinearity can be obtained by the excitation from the ground state to the excited state. The dependence of the transmittance of organic molecules on the incident intensity of laser can induce to generate different nonlinear effects in the excited states. here, we will report some results of nonlinear absorption at 532 nm in acetonitrile using 8 ns and 23 ps laser pulse. The experiments showed that the reverse saturable absorption (RSA) was observed in the nanosecond case in the cadmium complex solution and the saturable absorption (sa) was also observed for picosecond pulse at high fluence. A six-level model was presented to explain these nonlinear absorption effects. Almost asymmetrical pentaazadentate porphyrin-like metal complexes exhibited reverse saturable absorption of nanosecond pulse, so that the optical limiting based on RSA was only fit for nanosecond laser, not fit for picosecond laser. However, the optical limitings based on the transverse refraction were all fit for nanosecond, picosecond, and continuous laser. A composite optical limiting based on RSA and self focusing was showed, in which the threshold of the limitation and the largest output power were deduced. A model composite optical limiter was designed.

11:15 AM S2.3 
OPTICAL LIMITING IN MODIFIED MESO-ALKYNYL PORPHYRINS, Nansheng Tang, Weijie Su, Thomas Cooper, Air Force Wright Laboratory, Materials Directorate, Wright Patterson AFB, OH; Mark C. Brant, Donna Brandelik, Richard L. Sutherland, Science Applications Intl Corp, Dayton, OH.

We synthesize several modified meso-alkynyl porphyrins. It is observed that upon metal complexing, the ground state absorption can be modified. Time-resolved fluorescence experiment also confirms the change in the singlet energy levels. Singlet lifetimes in the order of a few nanoseconds have been determined. These lifetimes are sufficient for optical limiting application with nanosecond pulses while retaining fast recovery time. We test the optical limiting performance of these samples with pulses of 100 ps and 7 ns. We will present our results.

11:30 AM S2.4 
PHOTOCONDUCTING LIQUID PHTHALOCYANINE LIMITER MATERIAL, Richard G. S. Pong, James S. Shirk, Steven R. Flom, Naval Research Laboratory, Optical Sciences Div, Washington, DC; Arthur W. Snow, Naval Research Laboratory, Chemistry Div, Washington, DC.

Recently, a class of intrinsic liquid phthalocyanines (PCs) have been synthesized at NRL for optical limiting applications. The pure material forms thin films of high optical quality. In a limiter, these materials have several desirable properties, including a low limiting threshold, a strong thermal contribution to the limiting, and the potential to be self-healing. In this paper, we report that one of these intrinsic liquid materials, a lead tetra(-polyethylene oxide) phthalocyanine [PbPc( PEO)], exhibits a photovoltaic and photoconductive effect. Photoconductivity is common in phthalocyanines. However, it is usually a property of bulk materials. It has not been important in the solutions typically used for optical limiters. Limiters using the new intrinsic liquid PCs do usually contain the pure material. We describe the initial experiments demonstrating the photoelectrical properties and indicate how they might enhance the optical limiting. Experimentally, a sample of PbPc(-PEO) between transparent electrodes was used as the active nonlinear material in an f/5 limiter. The photovoltaic and photoconductive response increased with incident intensity. At higher fluences, a drop in the electrical resistance of the sample and a flash of light at the focal spot is observed. The emitted light intensity increases monotonically with the incident fluence. We postulate that the efficient generation of electrons in the material leads to a plasma similar to that observed in carbon black suspension (CBS) limiters. There are some disadvantages to the plasma mechanism for limiting, most notably the slow recovery of the material. However, in cases where it could be useful, the intrinsic liquid PCs have some advantages. The plasma mechanism is an addition to RSA limiting, which provides substantially lower limiting thresholds. Further, in the PCs, systematic changes in the molecular structure can be used to enhance the electrical properties.

11:45 AM S2.5 
ULTRAFAST AND NONLINEAR OPTICAL CHARACTERIZATION OF OPTICAL LIMITING PROCESSES IN FULLERENES, Duncan McBranch, Victor Klimov, R. Kohlman, Los Alamos National Laboratory, Dept of Chemical Sci & Tech, Los Alamos, NM; Aaron C. Koskelo, Los Alamos National Laboratory, Chem Sci & Tech Div, Los Alamos, NM; Jeanne M. Robinson, Benjamin R. Mattes, Hsing-Lin Wang, Los Alamos National Laboratory, Dept of Chemical Sci & Tech, Los Alamos, NM; Fred Wudl, Univ of California-S Barbara, Inst for Polymers & Organic Solids, Santa Barbara, CA.

We review the results of spectral studies of the ultrafast excited-state absorption in fullerenes, which allow determination of both the spectral response of reverse saturable absorption (RSA) nonlinearities in these compounds, as well as the dynamical response for different morphologies (solution, thin film, and sol-gel glass composites). Derivitized fullerenes which have been optimized for solubility in organic solvents enable enhanced control of blended materials properties. Nanosecond studies of optical limiting performance are presented for various geometries (thin concentrated cell, thick dilute cell, variable concentration ' 'bottleneck'' cell, and series of discrete plates). The experimental data are compared with modeling using a five-level model, and conclusions drawn about the tradeoff between dynamic range and nonlinear performance for various geometries.

Chair: Keith L. Lewis
Monday Afternoon, March 31, 1997
Salon 11

1:30 PM *S3.1 
OPTICAL SWITCHING IN THIN FILM NdNiO, Jeffrey F. DeNatale, Paul Kobrin, Rockwell Intl Science Center, Thousand Oaks, CA.

The rare-earth nickelates, ReNiO (Re = Nd, Pr, Sm, Eu), represent a class of materials for which electrical and optical switching has been observed in bulk polycrystalline ceramics. In the work to be presented, we have successfully fabricated crystalline thin films of NdNiO which exhibit the characteristic thermally-induced switching response. Fabrication of these films has been demonstrated on a number of substrate materials. For lattice-matched substrates, epitaxial film orientation has been achieved. Infrared transmission measurements on these films have demonstrated the corresponding reversible optical switching response, transforming between IR-transparent and IR-opaque states. Their low-temperature transmission spectra exhibit a sharp increase in transmittance at an onset wavelength of approximately 3 m (0.4 eV), with an appearance suggesting formation of a distinct bandgap structure. The transmission and reflection characteristics of these films differ significantly from those predicted by the optical constants previously reported for bulk polycrystalline material.

2:00 PM S3.2 
SPATIAL AND TEMPORAL CHARACTERISTICS OF THE SEMICONDUCTOR TO METALLIC PHASE TRANSITION IN VO THIN FILMS, Mark A. Corbett, Defence Research Agency, Malvern Worcs, UNITED KINGDOM; Keith L. Lewis, DRA Malvern, Dept of Lasers & Photonics, Malvern, UNITED KINGDOM; Andrew M. Pitt, DRA Malvern, Lasers & Photonics, Worcester, UNITED KINGDOM.

Vanadium dioxide (VO) exhibits a semiconductor to metallic phase transition at a temperature around 68C. This can be exploited in the form of optical thin films which will exhibit nonlinear behavior when exposed to focused infrared radiation. Since the phase transition is structural in nature, it is of interest to explore the temporal and spatial properties when irradiated using pulsed laser sources. Measurements with pulsed DF (3.8 m) and CO2 (9-11 m) lasers have been made of the transmission characteristics for increasing energy densities. Fast CMT detectors have been used to resolve subnanosecond time detail and the spatial characteristics of the phase transition have been examined. A technique has been developed to image the focal plane at the film by refocusing transmitted radiation to a final focal plane, then magnifying this spot onto a thermal imager array.

2:15 PM S3.3 
HIGH-TEMPERATURE OPTICAL PROPERTIES OF THIN-FILM VANADIUM OXIDES-VO AND VO, Patrick J. Hood, Air Force Wright Laboratory, Materials Directorate, Wright Patterson AFB, OH; Michael S. Thomas, Wright Patterson AFB, Wright Patterson AFB, OH; Jeffrey F. DeNatale, Rockwell Intl Science Center, Thousand Oaks, CA.

Thin-film vanadium dioxide (VO) and vanadium sesquioxide (VO) undergo shear phase transformations at 340 K and 120 K, respectively. The electrical and infrared optical properties of these materials are dramatically impacted by this phase transformation. Below the phase transition temperature, both VO and VO are "poor" electrical insulators and have minimal absorption in the infrared spectral region. Above the phase transition temperature, both VO2 and V2O3 are poor conductors and are opaque in the infrared. The infrared thermochromic properties of these materials makes them interesting candidates for several optical applications, including optical limiting. Utilization of eider VO or VO necessitates an accurate knowledge of the optical properties both above and below the phase transition temperature. This paper reports on some recent infrared ellipsometric measurements on the optical properties of these materials above the transition temperature. These measurements found that both are strongly dispersive through the 3-12 micron spectral region. In addition, the temperature dependence of the optical properties of VO above the transition temperature can be related to the film morphology and stoichiometry.

2:30 PM S3.4 
DIELECTRIC STUDY ON GRAIN BOUNDRY PROPERTIES OF BaTiO CERAMICS, X. B. Chen, W. S. Gao, X. Y. Mao, Yangzhou Univ, Dept of Physics, Yangzhou, CHINA.

The dielectric characteristics of BaTiO ceramics doped with YO has been experimentally studied. By using Heywang model, the grain resistance, the width of grain boundary and the potential barrier are calculated for investigating the PTC effect of the material. The results show that the complex impedance spectrum existing two summits are in good agreement with Debye model, the relative dielectric constant changes from 10 to 10 and the potential barrier from 0.04 eV to 0.85 eV below and above the phase transition temperature, the effective electron density is about 1.5 10 cm and the width of the grain boundary is about 1.8.

Chair: Patrick J. Hood
Monday Afternoon, March 31, 1997
Salon 11

3:15 PM *S4.1 
SEMICONDUCTOR INFRARED OPTICAL LIMITERS, Shekhar Guha, Air Force Wright Laboratory, WL/MLPJ, Wright Patterson AFB, OH.

Semiconductors are used very effectively to limit short duration laser pulses in the infrared. Optical limiting in different spectral regions is obtained by choosing materials having the appropriate bandgap. The dominant processes that contribute to short pulse limiting in semiconductors are nonlinear absorption of light as well as nonlinear refraction. Nonlinear absorption arises mainly from two-photon absorption as well as excited-state absorption from the charge carriers created by the two-photon absorption process. Nonlinear refraction predominantly arises from the created charge carriers. To characterize the optical limiting behavior of a semiconductor, it is important to accurately measure the two-photon absorption coefficient, the excited state absorption coefficient and the decay rate of the charge carriers. To compare the performance of different materials, it is necessary to test their limiting capability in identical experimental situations. Recent optical limiting work in the near, mid-wave and the long-wave infrared regions will be discussed in this talk. Experimental determination of the material parameters relevant to optical limiting will also be described in detail. Measurements performed on a variety of materials including compounds such as InSb, InAs, CdGeAs, Te, GaSe and alloys such as HgCdTe and TeSe will be presented. Several of these materials have either been newly developed or they have been recently improved to provide optical characteristics desired for specific applications.

3:45 PM S4.2 
BROAD BAND OPTICAL POWER LIMITING IN VANADIUM DOPED CdMnTe CRYSTALS, S. B. Trivedi, R. J. Chen, K. Grasza, S. Kutcher, Brimrose Corporation of America, Baltimore, MD; W. H. Steier, David Bacher, Univ of Southern California, Dept of Electrical Engr, Los Angeles, CA; Shekhar Guha, Air Force Wright Laboratory, WL/MLPJ, Wright Patterson AFB, OH; Patrick J. Hood, Air Force Wright Laboratory, Materials Directorate, Wright Patterson AFB, OH; J. V. Jagannathan, Brimrose Corporation of America, Baltimore, MD.

We present, for the first time, optical power limiting in vanadium doped Cd Te crystals in a broad wavelength band from visible to near infrared. These crystals are capable of limiting the high intensity optical beams from 0.630 m to 1.6 m. Transmission spectra and photoconductivity were measured in these crystals. Also, for the first time, the measurements of the electrooptic coefficients in CdTe:V at visible and near infrared wavelengths will be presented. 6 mm x 6 mm x 6 mm samples of CdTe:V crystals with dark resistivity greater than 10'' ohm-cm have been used for the limiting measurements. The transmission drops from 27 to 1 in going from 300 W/cm to 10 W/cm with V of 4.8 kV. The material parameters affecting the performance of optical limiters are discussed. Effects of thermal annealing and strain induced birefringence in these crystals are also discussed. Optical limiting performance of CdTe:V is compared with other II-VI materials CdTe:V and ZnTe:V, grown by us.

4:00 PM S4.3 

Electro-optical properties of thin films of hydrate transition metal oxides which can reversibly change their light absorption in the near IR region due to double charge injection upon applied electric field in electrolyte media have been studied in-situ using photoelectrochemical methods. Thin films of hydrate oxides WO2(HO) and WO1/3(HO) were synthesized on glass substrates coated with conductive ITO layer. It has been found that the ratio of the induced optical density to the injected charge density is in a range of 60 to 85 cm/C at the absorption band maximum (0.9-1.0 eV). For the induced optical density D = 0.6, the switching time is 0.8 1.0 s. The obtained films have a very high stability in acid-electrolyte media and no changes in the structure, morphology and electro-optical properties were observed with SEM, LEED methods after 10 switching cycles and storage for 6 months. Taking into account the high stability and fast switching time, these hydrated tungsten oxide films can be utilized for efficient light modulation in the near infrared region of the spectrum.

4:15 PM S4.4 
SYNTHESIS OF SOL-GEL OPTICS DOPED WITH ORGANIC OPTICAL LIMITERS, Benjamin R. Mattes, Marina N. Grigorova, Duncan McBranch, Hsing-Lin Wang, Los Alamos National Laboratory, Dept of Chemical Sci & Tech, Los Alamos, NM; Fred Wudl, Univ of California-S Barbara, Inst for Polymers & Organic Solids, Santa Barbara, CA.

Sol-gel glass hosts, doped with reverse saturable absorbing molecular guests, have been demonstrated to form high damage threshold composite materials. These materials hold promise for broadband optical limiting devices. In this work we describe ongoing efforts to develop custom sol gel optical components doped with various nonlinear active organic molecules. This presentation will focus on the preparative strategies we have employed to achieve molecular dispersions of optically active hydrophobic guests in the sol phase separation, aggregation, or precipitation. Thermodynamically stable solutions are formed above the upper critical solution temperature. Once formed, the kinetics of gelation for these solutions further determines the properties of the dried gel composite, i.e., hardness, scattering loss, and transparency. We show that transparent windows, as well as other optical components, may be easily fabricated using these solutions. The final gel composites have controlled thickness and concentration of the optically active molecules, and are formed into the desired geometry by molding techniques. Once formed, the composite glass components do not require high processing temperatures or subsequent polishing to achieve good optical quality. Versatile net-shape molding of custom optics for optimized optical limiting geometries (graded-concentration stacks of thin plates, and microlens arrays) is hence possible using our approach.

4:30 PM S4.5 
MODEL OF THE LIGHT YIELD FALLING AT LARGE ION BEAM IRRADIATION DOSES FOR THE PBWO CRYSTAL, Vladimir Egorov, Inst of Microelectronics Technology, Nuclear Research Lab, Chernogolovka, RUSSIA; A. P. Zuev, Inst of Microelectronics Technology, Lab Nuclear Res, Chernogolovka, RUSSIA; E. V. Egorov, ISSP, Chernogolovka, RUSSIA.

The assumed model is based on the experimental data of the light yield (L.Y.) falling excitated by ion beams H and He in PbWO crystals at the growth of an absorbed irradiation dose. The data was obtained by used of this ions with energy E=0.98 MeV, current I=1-10 nA and beam spot size D=1 mm. The dose kinetic dependences of the ion beam excitated L.Y. were registrated by fast single channel analyzer (shaping time =20 nsec) in narrow optical range 425-435 nm. The model proposes that electron subsystem of the PbWO crystal has reached a state of the power saturation when the main part of the excitation energy dissipates in the structure by non-radiation path. There are two energy excitation intervals of it subsystem, low energy interval being characterized by growing relationship between L.Y. and ion beam initiated dose and high energy one being correlated with the decreasing of that function. Moreover, the every interval is described by own concentration of the virtual defects and radiation damages in volume. The discusses model places at our disposal a size estimate procedure of the excitation zone near fast ion trace in the structure PbWO crystal. The execution estimates shown that the radius near H ion trace is close to 0.5 nm and near He one - 4nm.

Chair: Kenneth J. McEwan
Tuesday Morning, April 1, 1997
Salon 11

8:30 AM *S5.1 

Photorefractive materials have been extensively investigated by many groups for their nonlinear optical properties. Most techniques have used either two-beam coupling, or the closely related processes of beam fanning, in materials such as barium titanate or strontium barium niobate. Recently, other materials such as Fe:lithium niobate have been demonstrated to have a strong photorefractive response, the nature of which depends markedly on the direction of beam propagation through the crystal. Beam fanning perpendicular to the c-axis results in symmetrical fanning, while unidirectional gain effects are found along the c-axis. This can result in either strong attenuation of amplification, depending on the poling direction of the crystal axis. Other materials of interest include the photorefractive polymers such as those based on guest-host approaches using polyvinyl carbazole. Here, the processes of photorefraction depend on the effectiveness of photoreceptor molecules and the ability of the host to accommodate localized reorientational effects. A review will be presented of recent progress highlighting possible ways forward in improving speed of response and increase of sensitivity of such materials.

9:00 AM S5.2 
PHOTOREFRACTIVE OPTICAL LIMITERS, Mary J. Miller, Gary L. Wood, U.S. Army Research Laboratory, Dept of Lasers & Nonlinear Effects, Fort Belvoir, VA; Gregory J. Salamo, Univ of Arkansas, Dept of Physics, Fayetteville, AR.

We have investigated the potential of photorefractive phenomena for the development of an optical limiting device. The photorefractive concept is attractive as an optical limiter since it provides the possibility of allowing the outside world to be monitored, while it simultaneously rejects a laser threat. As a device, it also has the potential to be sensitive over a broad spectral range, operate with a large field-of-view, provide protection against continuous and pulsed laser sources, protect against a laser threat from any direction, simultaneously block multiwavelength laser radiation, and transmit a scene without significant degradation or attenuation . The major obstacle preventing the photorefractive limiter from reaching this potential has been speed of response. Because the photorefractive phenomenon involves charge transport, it is inherently slow. Its relatively slow speed and the corresponding reputation it has acquired suggests that photorefractive limiters may not switch fast enough to stop a laser threat before sensor damage occurs. In the present paper, we report on several techniques that are successful in enhancing the photorefractive beam fanning response time. In particular, we demonstrate and characterize the use of applied electric fields, gratings to seed the beam fan, cylindrical focusing and controlling the acceptor and donor concentrations. Each has led to an improved response time. We also report on a demonstration of a prototype beam fanning optical limiter. Using a combination of spherical and cylindrical lenses and an applied dc electric field, we demonstrated the beam fanning limiter for a laser threat of 1 m w/cm over a two-inch diameter aperture. The limiter switched in 6 msec with an 0.D. of 3 to 4. This limiter, therefore, allows only 6 joules to be transmitted before the O.D. of 3 to 4 was in effect. For both higher and lower intensities, the limiter switched faster and slower, as expected, still limiting at 6 joules with an 0.D. of 3 to 4 at a wavelength of 514.5 nm. While the limiter operated, the outside world could still be monitored, a large field-of-view of about 30 was in effect, and about 50 absorption existed for outside world scenes.

9:15 AM *S5.3 
OBSERVATION OF BEAM FANNING IN A PHOTOREFRACTIVE POLYMER, W. E. Moerner, A. Grunnet-Jepsen, C. L. Thompson, Univ of California-San Diego, La Jolla, CA.

We report the first observation of beam fanning (amplified scattering) in a photorefractive polymer. This effect arises from the extremely large two-beam coupling gain (200 cm) available from a new photorefractive polymer composite [1] consisting of poly(n-vinyl carbazole), the nonlinear optical chromophore PDCST, the liquid plasticizer butyl benzyl phthalate, and the sensitizer, C. Since the total gain depends upon maximizing L, where L is the interaction length, the fanning direction for a 100 micron thick sample is in the plane of the sample. With only one beam present, we have directly observed the fan appearing out of the side of the sample only when the applied electric field is in a direction so as to produce large gain in the plane, and not for the opposite field direction. For a single sample, the fan-induced loss for the incident laser beam is in the range of 40 with an applied field of 80 V/micron. The size of the effect is relatively independent of intensity. The angle dependence and field dependence of the effect are consistent with the underlying photorefractive mechanism. This constitutes an example of the photorefractive optical limiting device originally proposed by Yariv and Cronin-Golomb which works only for coherent laser beams and not for incoherent light. Here instead of requiring an inorganic crystal such as barium titanate, the effect works with a low-cost, low-density photorefractive polymer. In addition, by cascading several samples, the fan-induced loss can be increased. The speed of growth of the fan is on the order of 1 s, which may be too slow for some applications; however, seeding the fan would be expected to reduce the response time. Additional potential applications of this effect and further mechanistic studies will be discussed.

9:45 AM *S5.4 
PHOTOREFRACTIVE EFFECT IN CdSSe:V CRYSTALS, Zhengda Pan, Michelle Davis, Steve H. Morgan, Kuo-Tong Chen, Henry Chen, Fisk Univ, Dept of Physics, Nashville, TN; Swanson L. Davis, Fisk Univ, Dept of Chemistry, Nashville, TN; Arnold Burger, Fisk Univ, Dept of Physics, Nashville, TN.

We present two-wave mixing results obtained with CdSSe:V crystals. A large photorefractive gain of 0.24 cm was observed qt 633 nm with an optical intensity of 60 mW/cm and a grating period of 1.6 m. At this wavelength, we measured the photorefractive gain as a function of the grating period and incident optical intensity. To our knowledge, this is the first observation of the photorefractive effect in doped CdSSe crystals. The CdSSe crystals were grown by physical vapor transport (PVT) and doped with 150 ppm (nominal) vanadium for creating trap centers. The grown crystal has a large crystal size, good optical quality, and a medium resistivity of 10 - 10 -cm. Low temperature photoluminescence spectroscopy measurements and the effects of post-growth annealing treatments will also be discussed. With a significant photorefractive effect, the CdSSE:V crystals are promising for many device applications based on photorefractive effect, including optical limiting devices.

Chair: Richard L. Sutherland
Tuesday Morning, April 1, 1997
Salon 11

10:30 AM *S6.1 
INFRARED LIQUID CRYSTAL TUNABLE FILTERS, Shin-Tson Wu, Chiung-Sheng Wu, Hughes Research Laboratories, Malibu, CA.

Design and performance of tunable liquid crystal filters in the 3-5 m range will be described. New fluoro liquid crystal compounds that possess high birefringence, low absorption, low operation voltage and low viscosity arc particularly attractive for such applications.

11:00 AM S6.2 

The results of a series of quantitative experimental studies of the infrared nonlinear optical responses of aligned nematic liquid crystals, using lasers in the near IR [1.55 m], mid-IR [5 m] and IR [10.6 m] region are reported. Several mechanisms that result in unusually large optical nonlinearities, such as molecular reorientation, density/thermal effects are observed. These processes are characterized by response times spanning milliseconds to submicrosecond, and are therefore well suited for infrared application. We will describe optical wave mixing, switching, and limiting studies conducted with 10.6 m [CO], 5 m [CO] and 1.5 m [communication channel] lasers. Owing to the large birefringence of LCs, the required laser power/intensity thresholds are very low, while the process efficiencies and times are highly competitive in comparison with other infrared materials.

11:15 AM S6.3 
EFFECTS OF VARYING SURFACTANTS ON THE ELECTRO-OPTICAL SWITCHING CHARACTERISTICS OF VOLUME HOLOGRAMS RECORDED IN PDLC'S, Vincent P. Tondiglia, Lalgudi V. Natarajan, Science Applications Intl Corp, Dayton, OH; R. M. Neal, Wright Patterson AFB, Materials Dierectorate, Wright Patterson AFB, OH; Richard L. Sutherland, Timothy J. Bunning, Science Applications Intl Corp, Dayton, OH.

We have investigated the effect of various chemical surfactants on the electrical switching and optical properties of Bragg gratings recorded in polymer dispersed liquid crystals (PDLCs). Holographic transmission gratings were recorded in prepolymer recipes substituting hexanoic, heptanoic, propylpentanoic, octanoic and lauric acids as surfactants in the recipe. The addition of surfactants have shown a lowering of the switching electrical field and may also enhance the diffraction efficiency. We have also measured the response times for electrical switching of these gratings.

11:30 AM S6.4 
A CLASS OF EXCEPTIONALLY NONLINEAR OPTICAL FIBER CORE MATERIALS FOR OPTICAL LIMITING APPLICATION, I. C. Khoo, M. V. Wood, B. D. Guenther, Pennsylvania State Univ, Dept of Electrical Engr, University Park, PA.

Liquid crystals possess many unique physical properties and nonlinear optical responses over wide temporal [cw to picosecond] and spectral [visible to infrared] ranges. In our recent work on isotropic liquid crystal (ILC) cored optical fibers and fiber arrays, we have discovered several exceptionally nonlinear liquid phase liquid crystals capable of nanosecond-picosecond responses. These materials possess broadband excited-state and nonlinear sequential absorption characteristics similar to other novel organic molecules such as Fullerenes C and Phthalocyanines systems. For nanosecond and picosecond laser pulses, the observed thresholds and clamped output levels [energy of 0.1 J or less, fluence of <0.01 J/cm] are among the lowest of all the materials currently under investigation. Because of the broadband response of these molecular systems, these limiting characteristics are expected to hold throughout the entire visible spectrum. We have also succeeded in synthesizing several ILCs with similar exceptional broadband nonlinear optical responses. Using these liquids as waveguiding cores will enable us to develop a whole new class of versatile fiber optical elements that are multifunctional, compact, low cost/weight, fast and operational over very wide spectrum and dynamic ranges.

11:45 AM S6.5 
INTELLIGENT MESOSCOPICALLY PERIODIC MATERIALS FOR OPTICAL LIMITING AND SWITCHING, Sanford Asher, Jesse Weissman, Guisheng Pan, Univ of Pittsburgh, Dept of Chemistry, Pittsburgh, PA; Rasu Kesavamoorthy, Indira Gandhi Cntr for Automic Research, Materials Sci Div, Kalpakkam, INDIA.

We are utilizing mesoscopically periodic materials consisting of ca 100 nm diameter polymer spheres organized in a BCC array with a ca 200 nm nearest neighbor spacing. The array diffracts light at wavelengths in the visible and near IR spectral region. These materials are fabricated through the polymerization of a crystalline colloidal array (CCA) in a polyacrylamide hydrogen. We have created a periodic lattice where the real part of the particle refractive index is matched to that of the surrounding medium, but where only the spheres have a significant imaginary refractive index which results from dye attached to the spheres. Low intensity light transmits freely through the material. In contrast, high intensity light heats the spheres and causes the real part of the refractive index to mismatch from that of the medium. The BCC array ''pops up'' in nsec to diffract away the incident light. We will discuss the switching times and efficiencies. We will also discuss an optical limiter and switch based on a poly-N-isopropylacrylamide CCA where the polymer dramatically changes volume with temperature. This temperature alteration switches the diffraction wavelength and efficiency such that the device acts as an optical limiter and switch.

Chair: David J. Hagan
Tuesday Afternoon, April 1, 1997
Salon 11

1:30 PM *S7.1 
THE DESIGN OF OPTICAL LIMITERS BASED ON THE FORMATION OF BIPOLARON-LIKE DICATIONS, Charles W. Spangler, Montana State Univ, Dept of Chemistry & Biochem, Bozeman, MT.

The rapid development of laser weaponry over the past several years has created new problems in the general area of eye and sensor protection. Several approaches to laser hardening have been proposed, and one particularly attractive approach for organic materials is the design of molecules and polymers which can function as optical limiters via reverse saturable absorption (RSA). In this presentation, attention will be focused on new materials which may derive RSA capability from the photogeneration of highly absorptive stable bipolaron-like dications. The absorption characteristics of these new materials can be rationally controlled by organic synthesis, and tailored to match any frequency range in the visible portion of the spectrum and can be extended into the NIR. They can be produced as soluble oligomeric species for solution thin film protectors, as guest-host composites in a variety of host polymers, or as processible polymeric thin films.

2:00 PM S7.2 
OPTICAL LIMITING IN FULLERENE MATERIALS, Ben Zhong Tang, Han Peng, Shuk Mei Leung, Nai-Teng Yu, Hiroyuki Hiraoka, Hong Kong Univ Sci & Tech, Dept of Chemistry, Kowloon, HONG KONG.

We have synthesized and fully characterized a number of new fullerene derivatives, polymers, and sol-gel glasses ("fullerene materials"), including HC(NHCHCHOH), HC[NH(CHCHO)H], HC)60(NH- -CH), H[NH(CH)OH], HC[NH(CH)Si(OEt)], C-polycarbonate (C-PC), C-CR-39, C-poly(vinyl chloride) (C PVC), HC(NH--CH)/SiO, HC[NH(CH)OH]/SiO, and HC[NH(CH)SiO]-SiO. All the fullerene materials effectively limited 8-ns pulses of 532-nm laser light. The fullerene polymers with aromatic main chains (C-PC) and side chains (C-polystyrene) performed better than did a toluene solution of C, irrespective of whether the fullerene polymers were prepared by radical copolymerizations or by photolysis-induced, radical initiated, or acid-catalyzed polymer reactions. Molecular interaction between the buckyballs and the aromatic rings might have played an important role in enhancing the limiting performance of the fullerene polymers. All the fullerene glasses we prepared were morphologically homogeneous with excellent optical quality, even when no drying-control chemical additives (DCCA) were employed during the sol-gel processes. The HC[NH(CH)SiO]-SiO glasses had high optical stability, whose optical limiting properties remained unchanged after subjecting to continuous irradiation of strong laser pulses for more than 1 h.

2:15 PM S7.3 
NON-LINEAR EFFECTS IN THIN FILM FULLERENE STRUCTURES, Keith L. Lewis, DRA Malvern, Dept of Lasers & Photonics, Malvern, UNITED KINGDOM; S. Blacker, Grant Bourhill, DRA Malvern, Malvern, UNITED KINGDOM.

Fullerene materials such as C are well known for their nonlinear behavior arising from excited state absorption processes. In solution, optical limiting processes can set in at low irradiance levels, although evidence is building that the magnitude of the effects is dependent on the solvent used in the experiments. In evaporated thin films of C, the solvent if absent and no effective optical limiting is seen until irradiances approach the laser damage threshold of the films. When the chromophore is incorporated into polymer films, nonlinear processes are again only seen close to the damage threshold. However, when toluene is incorporated to form a C/polymer/toluene composite resembling a thixotropic gel, optical limiting ensues. This behavior will be discussed in relation to process of excited state absorption.

2:30 PM S7.4 
ELASTIC POLYMER GUEST-HOST OPTICAL LIMITERS WITH HIGH LASER DAMAGE THRESHOLD, Michael E. De Rosa, Air Force Wright Laboratory, Wright-Patterson AFB, OH; Mark C. Brant, Daniel G. McLean, Science Applications Intl Corp, Dayton, OH.

Conventional guest-host optical limiters utilize either a liquid solvent or solid polymeric material as the matrix for nonlinear absorbing chromophore dopants. New designs of these guest-host materials have demonstrated that specially designed concentration gradients of the chromophore in the matrix can improve optical limiting performance. However, liquid solutions cannot retain a concentration gradient over time because of diffusion of the dye while commonly used solid plastics, such as polymethylmethacrylate (PMMA). have low laser damage thresholds ( 3J/cm). We report the results of using an elastic thermosetting optical grade epoxy as the matrix for guest-host optical limiters. These new epoxy limiters demonstrate high bulk laser damage threshold (>100J/cm) due to their unique viscoelastic properties which can be tailored by controlling the amount of crosslinker added to the resin before curing. A significant increase in bulk laser damage threshold was observed as the crosslink density is decreased, which makes the matrix more soft and flexible. We show the results of the damage threshold as a function of viscoelastic properties such as shear modulus and glass transition temperature. Optical limiting experiments were performed on epoxy networks doped with a silicon naphthalocyanine. Results of recent efforts in processing and characterizing limiters with dye gradients will also be presented.

2:45 PM S7.5 
POLYMER MATERIALS DESIGN FOR OPTICAL LIMITING, B. H. Cumpston, K. Mansour, California Inst of Technology, Jet Propulsion Lab, Pasadena, CA; Seth R. Marder, California Inst of Technology, Beckman Inst, Pasadena, CA; Joseph W. Perry, California Inst of Technology, Jet Propulsion Lab, Pasadena, CA.

Phthalocyanines (PCs) containing heavy metal central atoms have recently been recognized as leading candidates for reverse saturable absorption optical limiting (OL) applications in the visible spectrum. Strong triplet excited state absorption brought about by a large intersystem crossing rate is responsible for the excellent limiting performance of these molecules. Moreover, devices which maximize the excited state population along the light path will demonstrate maximum limiting efficiency. A nonhomogeneous distribution of indium tetra(-butyl) phthalocyanine chloride has been shown to be very effective in attenuating 532 nm nanosecond laser pulses. This was accomplished by approximating a distribution of chromophores using discrete elements of fixed dye concentration. Greater OL should be achieved by fabricating materials containing a continuous concentration gradient of chromophore. This paper focuses on the preparation of solid polymeric materials that contain such a chromophore gradient. This design is achieved by diffusing chromophore containing solutions into partially polymerized poly(methyl methacrylate) (PMMA). The diffusion of PC dyes into swollen, cross-linked PMMA plugs is also discussed. A comparison of the OL performance between the continuous gradient devices and the finite element approximation is made.

Chair: Daniel G. McLean
Tuesday Afternoon, April 1, 1997
Salon 11

3:30 PM S8.1 
OPTICAL LIMITING IN TETRAPHENYLTETRABENZOPORPHYRIN-DOPED XEROGELS BY REVERSE SATURABLE ABSORPTION, Benjamin Campagne, Marc Brunel, CNRS, Institut d'Optique Theorique et Appliquee, Orsay, FRANCE; S. A. Vinogradov, Univ of Pennsylvania, Dept of Biochemistry & Biophysics, Philadelphia, PA; Frederic Chaput, Ecole Polytechnique, Labaoratoire de Physics de la Matiere Condensee,; Michael Canva, CNRS, Institut d'Optique Theorique et Appliquee, Orsay, FRANCE; Jean-Pierre Boilot, Ecole Polytechnique, Laboratoire Physique de la Matiere Condensee, Palaiseau, FRANCE; Alain Brun, CNRS, Institut d'Optique Theorique et Appliquee, Orsay, FRANCE.

Using sol-gel process, organic-inorganic bulk matrices (xerogels) doped with tetraphenyltetrabenzoporphyrins of palladium (PdTPTBP), zinc (ZnTPTBP) and tin (SnTPTBP) have been prepared. The optical properties of these new solid-state materials were studied in optical limiting experimental set-up. Nonlinear reverse saturable absorption was observed for all materials when they were illuminated with nanosecond laser pulses at 532 nm. A numerical modelling, based on a four energy level molecular system, has been developed in order to provide a qualitative and quantitative explanation for experimental results. This simulation is also a guide for the development or choice of new dopants which spectroscopic characteristics will lead to the best optical limiting performance, evidencing the importance of parameters such as, for example, fluorescence lifetime when it is of the comparable or longer than the optical pulse duration. In this study, the TPTBPs were chosen because of their different type of behavior with respect to the decay mechanisms from the first excited singlet state. The 1 mm thick samples had a linear transmission of 50 at 532 nm. The best optical limiting performances were obtained with the PdTPTBP doped samples. The nonlinear threshold value was only about 10 mJ/cm, the induced optical density was close to 1 at high incident fluence: 5 J/cm (for all these samples, the matrix damage threshold was found to be around 5 to 10 J/cm). Concerning ZnTPTBP and SnTPTBP doped xerogels, the nonlinear thresholds were found to be nearly one order of magnitude higher (and the induced optical density at the same maximum incident fluence was about 0.5).

3:45 PM S8.2 
CHARGE-TRANSFER MATERIALS FOR OPTICAL LIMITING, X. L. Wu, California Inst of Technology, Jet Propulsion Lab, Pasadena, CA; A. A. Heikal, I.-Y. S. Lee, California Inst of Technology, Beckman Inst, Pasadena, CA; M. Bohorquez, Drake Univ, Dept of Chemistry, Des Moines, IO; H. Rockel, Seth R. Marder, California Inst of Technology, Beckman Inst, Pasadena, CA; Joseph W. Perry, California Inst of Technology, Jet Propulsion Lab, Pasadena, CA.

We have been exploring the use of photoinduced charge transfer as a mechanism for optical limiting and report here the design and study of several charge-transfer systems for optical-limiting applications. Nonlinear transmission measurements of systems based on cyanine and porphyrin photosensitizers (photoexcitable electron donors) and viologen derivative electron acceptors have been carried out using nanosecond and picosecond laser pulses. These donor/acceptor systems exhibited optical limiting properties in solution, and in some cases stronger optical limiting than for the donor in the absence of the acceptor was observed. Time-resolved fluorescence measurements on the above systems showed fast quenching dynamic which suggests an efficient charge transfer in all cases. Factors influencing the overall efficiency of charge separation in these systems will be discussed. We have also been examining an approach based on formation of ground state charge-transfer complexes. With the help of spectroelectrochemical studies on donor cation radicals or acceptor anion radicals, novel systems for optical limiting based on ground-state charge-transfer complexes were identified and studied using nanosecond laser pulses.

4:00 PM S8.3 
SHADOWGRAPHIC IMAGING OF REVERSE SATURABLE ABSORBERS IN SOLUTION, Robert Goedert, US Army-AMSTA-TR-R, Survivability Div, Warren, MI; Roger Becker, Univ of Dayton, Dept of E&CE, Dayton, OH; Andrew Clements, US Army-AMSTA-TR-R, Survivability Div, Warren, MI; Thomas Whittaker, US Army-AMSTA-TR-R, Warren, MI.

Time-resovled imagery is presented showing the changes that occur in the focal volume of dilute reverse saturable absorber solutions following the arrival of single Q-switched doubled Nd:YAG laser pulses. Limiting curves are presented with corresponding imagery obtained at 12, 84, 244, 790, and 2900 nanoseconds following the laser pulse in dilute solutions of silicon naphthalocyanine, tetrabenzporphyrin and carbon 60. Analysis of these shadowgraphic images can indicate the amount of absorption which occurs as a function of position along the axial direction within the cell, and the images dramatically show the effects of back propagation and screening. For medium to high input energies, shock waves are imaged, and their affect on limiting performance will be discussed. Limiting performance variations between the materials is readily seen in the images by noting the energy required to cause different features to appear. Once limiting becomes significant, strong absorption sets in near the front window. It is accompanied by bright emission, the creation of strong shock waves, and forward scattering in discreet directions. The shock waves provided information on the energetics of the reponse.

4:15 PM *S8.4 
BIOLOGICALLY INSPIRED DESIGN: COLOR ON WINGS, Mohan Srinivasarao, North Carolina State Univ, College of Textiles, Raleigh, NC.

This talk will focus on issues that are important for generation of color in biological systems. Color generation by means other than absorption or emission of light will form the central theme of the talk. Examples of color generation using physical optics (Bragg diffraction, iridescence, thin film interference) will be given based on butterfly wings, humming bird feathers and scarab beetles. This will entail discussion of colors produced by selective reflection due the existence of a frozen cholesteric phase (in the case of scarab beetles) and how one can produce colors that have close to 100 reflectivities for specific wavelengths. The colors will be characterized in terms of the CIE conventions. The biological relevance of such colors will also be discussed and I will end the talk with a few designs for optical limiting based on an understanding of the structures that gives rise to the colors in biological systems.

Chair: Shekhar Guha
Wednesday Morning, April 2, 1997
Salon 11

8:30 AM *S9.1 
DEVICE DESIGN AND MATERIALS CHARACTERIZATION FOR PASSIVE OPTICAL LIMITERS, Eric W. Van Stryland, David J. Hagan, Salah Khodja, Sidney Yang, D. Kovsh, Jinhong Lim, Univ of Central Florida, CREOL, Orlando, FL.

We use a variety of methods to characterize the nonlinear response of several molecular species in different hosts for optical limiting applications. These methods include the Z-scan, 2-beam pump-probe measurements, femtosecond continuum spectroscopy and shadowgraphic techniques. Such experiments allow us, for example, to determine the absolute value of excited state cross sections, their lifetimes and their dynamic spectral response. These parameters are necessary in designing optical limiting devices and optimizing the utilization of the molecular nonlinearities. The use of a range of characterization techniques removes ambiguities in the determination of nonlinear mechanisms. For example, excited state absorption and multiphoton absorption may be confused unless the temporal dependence is explored in some way. Practical implementations of nonlinear molecules in devices will also be discussed.

9:15 AM S9.3 
SHADOWGRAPHIC IMAGING OF DILUTE CARBON AND MoAgS[PPh] SUSPENSIONS, Robert Goedert, US Army-AMSTA-TR-R, Survivability Div, Warren, MI; Roger Becker, Univ of Dayton, Dept of E&CE, Dayton, OH; Andrew Clements, US Army-AMSTA-TR-R, Survivability Div, Warren, MI; Thomas Whittaker, US Army-AMSTA-TR-R, Warren, MI.

Time-resolved imagery is presented showing the changes that occur in the focal volume of dilute liquid/particle suspensions following the arrival of single Q-switched doubled Nd:YAG laser pulses. Limiting curves and corresponding imagery at 12, 84, 244, 790, and 2900 nanoseconds following the laser pulse are presented for a carbon particle suspension consisting of used (blackened) 10W-15 motor oil in toluene and for a suspension of the inorganic metallic cluster molecule MoAgS[PPh] in acetonitrile. The images show that limiting is due to scattering from bubbles augmented by absorption by a liquid plasma. The extent to which one process dominates depends on particle concentration, cell thickness, and the input energy and fluence. Accurate modeling of the back propagation that occurs, especially for dilute suspensions, is crucial to understanding the reponse of the system. Until back propagation is stopped by the front window, limiting is dominated by scattering from bubbles. The bubbles are much larger in the acetonitrile suspension and the emitted light (white flash) signal is much stronger than the carbon suspension. This is believed to be a result of a larger particle size. The emitted light is shown to be due to incandescence from particles over a very short time interval just before to particles vaporize to nucleate bubbles. The threshold for nucleation is close to that predicted. Forward scattering is very strong, so that the active region is extended well beyond the region directly irradiated by the laser beam once back propagation is blocked by the front window. The response of the two suspensions is similar, with the main differences more closely a function of the particle size than the material composition.

9:30 AM S9.4 

Carbon-black suspensions (CBS) are of great interest for optical limiting in view of their strong nonlinearity, sensitivity, and broadband transmission. The two mechanisms previously observed in these suspensions are microplasmas scattering and absorption, and scattering by vapor bubbles formed around the intensely heated carbon particles. In order to determine the respective influence of plasma and bubbles near the limiting threshold and at higher incident fluence, we performed at both 532 and 1064 nm a colinear pump-probe experiment coupled with a time-resolved measurement of plasma emission. As in z-scan technique, the transmission of the He-Ne probe can be measured through a fully open aperture, the apparatus is thus absorption and diffusion sensitive, or through a finite aperture in the far field. In this case, the setup is also sensitive to refraction. By temporally measuring the pump laser pulse, the plasma emission and the probe beam variation, we have evidenced several nonlinear mechanisms efficient in this limiting geometry such as strong acoustic impulse and plasma formation. The threshold fluences for these nonlinear mechanisms are determined with high accuracy and compared with the limiting threshold. The plasma lifetime is also compared with the probe relaxation time which exhibits two components. The first one is consistent with the plasma lifetime of about one hundred nanoseconds, whereas the second one is much longer and can be attributed to bubbles relaxation. These different effects are compared in several solvents and their nature is discussed in terms of limiting performance.

9:45 AM S9.5 

Nonlinear absorption is currently of great interest for mode-locking and pulse-shaping applications. Of the vast range of materials exhibiting nonlinear absorption, the most widely studied have been those containing macrocyclic ring systems, e.g., porphyins, phthalocyanines and naphthalocyanines. Here we report the wavelength-dependence of the nonlinear absorption of two structurally similar biological macrocyclic molecules: chlorophyll and bacteriocholorophyll. The photophysical parameters, based on a five-level model, responsible for the observed nonlinear absorption, are used to explain the differences observed between these two materials. These parameters were derived from a range of nonlinear spectroscopic techniques including multi-wavelength nanosecond Z-scan measurements (based around an optical parametric oscillator), ultrafast white-light continuum pump-probe experiments, the pump-CW probe technique, and time-correlated single photon counting.

Chair: Eric W. Van Stryland
Wednesday Morning, April 2, 1997
Salon 11

10:30 AM *S10.1 
POWER SATURATION EFFECTS IN OPTICAL LIMITERS UTILIZING THICK NONLINEAR MEDIA, John Arthur Hermann, Philip Bernard Chapple, DSTO Electronics & Surveillance Research Laboratory, Land, Space & Optoelectronics Div, Salisbury, AUSTRALIA.

Optical limiters which utilize nonlinear absorption or nonlinear refraction are most effective towards an incoming laser beam when the active material is somewhat thicker than one Rayleigh length. A variety of thick limiter configurations are described and modeled by appropriately integrating formulas for thin nonlinear media using gaussian-shaped beams, and include the effect of linear optical losses. The results are exact to the first order in power or irradiance. Both irradiance- dependent and fluence-dependent absorbers have been considered and analyzed. We have found that the combined effects of diffraction and nonlinear absorption can produce, in the thick-medium limit, a transmitted beam which retains a gaussian-shaped profile at the far field, but which is now uniformly attenuated according to the magnitudes of the nonlinear and linear absorption parameters. This implies that the normalized transmittance is independent of the size of the receiving aperture. As the optical beam power increases, it is found that the limiting behaviour saturates. We have discovered that a simple saturation formula agrees well with numerically calculated transmittances for thick irradiance-dependent absorbers within the strongly nonlinear regime. Under certain conditions, this formula also describes refractive limiters, providing a simple and accurate technique for modeling limiters over a broad parameter range. Expressions have been obtained for the transmitted optical powers and pulse energies with various types of absorbers and pulse shapes, and should be useful within the strongly nonlinear regime.

11:00 AM S10.2 

We show that reverse saturable absorbers (RSA), such as metallo phthalocyanines and naphthalocyanines, can be used as optical limiters for protecting CCD arrays from destruction due to nanosecond-pulsed laser irradiation in the visible and near infrared. The CCD sensor is placed at the focal image plane of a laboratory-built magnifying sight, with an F/S focusing geometry. Without limiter in place, the damage threshold of the CCD array is around 1.2 J/cm, corresponding to an incident energy E. The limiter is a solution of RSA in an 0.2 mm quartz cell. The concentration of the solution is adjusted in order to give an integrated transmission of 70. Detailed linear and nonlinear optical properties of the RSA will be presented. Comparison of experimental data on nonlinear transmission with theoretical calculations resulting from a modelization of the transient RSA process will also be assessed. We measure the damage threshold of the CCD array after inserting the limiter at, or close to, the intermediate focal plane (IFP) of the magnifying sight. Measurements are performed in single shots of increasing energy, with a solid-state tunable laser source (pulse duration 10 ns). When the limiter is inserted at the IFP, we do not observe damage on the CCD sensor for incident energies up to 60 times E (maximum energy available from our laser source). This value is obtained at a wavelength giving a minimum photoinduced nonlinear absorption, corresponding in general to a maximum linear transmission of the limiter. We will discuss those results in detail and show that broadband protection results from nonlinear absorption and refraction taking place in the limiter.

11:15 AM *S10.3 
NUMERICAL MODELING OF NONLINEAR BEAM PROPAGATION PHENOMENA, Grover A. Swartzlander, Worcester Polytechnic Inst, Worcester, MA; Chiu-Tai Law, Univ of Wisconsin-Milwaukee, Milwaukee, WI.

Recently we have made available via the world-wide web a powerful nonlinear optics program. Presently the numerical code simulates the 4 dimensional propagation of a laser beam through nonlinear refractive, nonlinear absorptive, and thermal media. Soon, atomic systems (3- and 5- level models), and acoustic interactions will also be included. The web site serves as a numerical test-bed for optical limiting, but can also be used to investigate fundamental nonlinear effects such as optical solitons, optical shock, and optical vortices. The site is intended for nonspecialists with access to the world-wide web via any computing platform (Macintosh, workstation, or PC). Public domain visualization software, available from the National Center for Supercomputing Applications (NCSA) is required to view simulated beam, which appears as an animation sequence. Our site can be accessed at 
Examples of both fundamental beam propagation, and limiting action will be presented, and comparisons with experimental results will be discussed. The modeling of solitons provide a particularly convenient means of validating the code, since some analytic solutions exist. Optical vortex solitons are particularly useful for this purpose, since they are the only known optical soliton in physical systems having three spatial degrees of freedom.

11:45 AM S10.4 
NONLINEAR LIGHT SCATTERING IN TWO COMPONENT HETEROGENEOUS MEDIUM: APPLICATION FOR OPTICAL LIMITING, V. Joudrier, ETCA, Arcueil, FRANCE; Jean-Claude Fabre, ETCA, CREA, Arcueil, FRANCE; F. Hache, C. Flytzanis, Ecole Polytechnique, Laboratoire d'Optique Quantique, Palaiscau, FRANCE.

One of the principal effects of the matter radiation interaction that can be used in optical limiting is the nonlinear light scattering. To illustrate this phenomenon, a medium consisting of two components (for instance, a cell containing some nonlinear liquid with small particles as inclusion in it) is considered. At low energy, this medium is rendered homogeneous by a good refractive index matching between the two components, insuring a good optical transmission. At high energy, the intense light inside the medium produces a nonlinear index change nNL, between the two components and makes it heterogeneous. Therefore, the light scattering increases with the intensity and insures the protection of the sensors. Several experiments are performed with different media. Small particles synthesized with the Stober method to calibrate them are embedded in nonlinear liquids, and different configurations have been tested by changing the particle size, their concentration, and the nature of the surrounding liquid (Toluene, CS2...). Preliminary results of these materials are presented and the nonlinear absorption and refraction are studied separately to highlight the nonlinear scattering contribution. Other original materials are also described together with the first test concerning them. A theoretical model is also developed to study the problem of the nonlinear propagation in a medium with spatially random refractive index. It is based on the flux theory of light scattering, which can be justified in the Rayleigh-Gans regime. In this model, the medium is considered as the whole like a scattering medium, the relaxation of the light-induced changes in the refractive indices of the components is introduced and the resulting equations are numerically solved for a Gaussian incident confrontation with the experimental results will be made to see how the model behaves with the experimental parameters.

Chair: Thomas M. Cooper
Wednesday Evening, April 2, 1997
8:00 P.M. 
Salon 7


Ab initio calculations have been carried out to determine the structures and properties of phthalocyanine and porphyrin optical limiters, including free-base, copper, and tin phthalocyanines, as well as free-base porphyrin, mesotetraphenyl octobromyl porphyrin, and mesotetra[(trimethylsilyl)ethynal]porphyrin. The effects of solvation were included by using the Effective Fragment Potential (EFP) method, which includes solute-solvent interactions through a sum of multipoles, polarizable points, and repulsive points, designed to mimic explicit solvent molecules in an ab initio calculation. The results are compared to experiment and an outline for the computational design of optical materials is presented.

STRUCTURE-OPTICAL PROPERTY RELATIONSHIPS OF PORPHYRINS, Weijie Su, Thomas M. Cooper, Nansheng Tang, Air Force Wright Laboratory, Materials Directorate, Wright Patterson AFB, OH; Hao Jiang, Anteon Co, Dayton, OH; Donna Brandelik, Science Applications Intl Corp, Dayton, OH; Jiaoshi Si, Univ of Akron, Dept of Polymer Sci, Akron, OH; Paul Fleitz, Mark C. Brant, Daniel G. McLean, Science Applications Intl Corp, Dayton, OH.

Porphyrins are attractive compounds for optical applications. For the last four years we have been investigating the relationship between molecular structure and optical properties of a number of porphyrin compounds. Structural variations explored include insertion of metal ions, extension of conjugation, halogenation and formation of multimers. The characterization of these chromophores embraces measurements of UV Vis, FT-IR, NMR and FL spectra. Furthermore, their nonlinear absorption and refraction, excitation dynamics as well as oxidation/reduction behavior have also been studied. Based on the observation, certain directions in designing optical limiting chromophores are discussed.

SEMI-EMPIRICAL CALCULATIONS AND MEASUREMENTS OF MODIFIED CARBOCYANINES OPTICAL PROPERTIES, Daniel G. McLean, Nansheng Tang, Weijie Su, Air Force Wright Laboratory, Materials Directorate, WPAFB, OH; Donna Brandelik, Science Applications Intl Corp, Dayton, OH; Jiaoshi Si, Univ of Akron, Dept of Polymer Sci, Akron, OH; Ruth Pachter, Air Force Wright Laboratory, Materials Directorate, Wright Patterson AFB, OH.

Semi-empirical calculations of modified diethylthiazolecarbocyanine iodide and diethylthiazoledicarbocyanine iodide are presented, particularly with halogen substitution at the mezzo position in the polymethine bridge. The calculated properties, such as the ground state absorption spectra, ionization potential, and electron affinity, are compared to the measured linear spectra and the redox potentials. In addition, the calculated results are compared to the measured nonlinear absorption and refraction properties.

PREDICTION OF NONLINEAR OPTICAL EFFECTS FOR PHENYL-SUBSTITUTED NICKEL DITHIOLENES, Steven Trohalaki, Ruth Pachter, Robert J. Zellmer, Air Force Wright Laboratory, Materials Directorate, Wright PattersonAFB, OH.

Nickel dithiolene complexes, known for their strong NIR absorptions, high thermal stability, and light fastness, are being considered as materials for nonlinear-optical applications. In particular, phenyl-substituted nickel dithiolenes are investigated since it was previously found that the second-order hyperpolarizability of a phenyl dithiole is dependent on the torsional angle of the phenyl group (Tokada et al., J. Chem. Soc., Chem. Comm. 1990, 1096). We report polarizabilities and hyperpolarizabilities from semi-empirical and ab initio quantum calculations.

DENSITY FUNCTIONAL AND HARTREE-FOCK STUDIES OF BROMINATED TETRAPHENYL AND ALKYNYL PORPHYRINS, Zhiqiang Wang, Paul N. Day, Ruth Pachter, Air Force Wright Laboratory, Materials Directorate, Wright Patterson AFB, OH.

We report Density Functional Theory (DFT) and Hartree-Fock studies of some porphyrin derivatives with interesting optical limiting properties, i.e., meso-acetylene substituents and tetraphenyl porphyrins with bromine substituents on pyrrole rings, as well as their central metal derivatives. The saddle-shaped distorted structure of the octobromotetraphenyl porphyrin is confirmed. The optimized geometries, electronic structures, absorption spectra, and comparisons with available experimental data are presented.

CHITOSAN GEL SYSTEMS AS NOVEL HOST MATERIALS FOR OPTICAL LIMITERS, Hao Jiang, Anteon Co, Dayton, OH; Yeijie Su, Air Force Wright Laboratory, MLPJ, WPAFB, OH; Mark C. Brant, Science Applications Intl Corp, Dayton, OH; Joe Williams, David Tomlin, Technical Management Concepts Inc, Beavercreek, OH; Michael E. DeRosa, Air Force Wright Laboratory, MLPJ, WPAFB, OH; Timothy J. Bunning, Daniel G. McLean, Science Applications Intl Corp, Dayton, OH.

Two chitosan gel systems, chitosan/acetic anhydride (N-acylchitosan) gels and chitosan/glutaraldehyde gels, were studied as host materials for optical limiters. Both gels are transparent and have very high laser damage thresholds. The N-acylchitosan gel has a bulk laser damage threshold greater than 600 J/cm while the chitosan/glutaraldehyde gel, which is slightly yellow in color, has a damage threshold greater than 450 J/cm. Different optical limiting chromophore dopants, including porphyrin and CuPc were tested using these materials as hosts. The guest/host gel systems demonstrated similar optical limiting behavior to their corresponding solution systems. The morphological structure, rheological properties, and the parameter affecting gelation processes are discussed.

SPECTROSCOPY AND OPTICAL LIMITING BEHAVIOUR OF ,-DITHIENYL POLYENES, Lalgudi V. Natarajan, Science Applications Intl Corp, Dayton, OH; A. Sowards, Wright State Univ, Dept of Chem, Dayton, OH; Charles W. Spangler, Montana State Univ, Dept of Chemistry & Biochem, Bozeman, MT; Nansheng Tang, Air Force Wright Laboratory, Materials Directorate, Wright Patterson AFB, OH; Paul Fleitz, Richard L. Sutherland, Science Applications Intl Corp, Dayton, OH; Thomas M. Cooper, Air Force Wright Laboratory, Materials Directorate, Wright Patterson AFB, OH.

The photophysics and optical limiting behavior of a series of -dithienyl polyenes were studied in chloroform and n octane solutions. Steady state fluorescence and fluorescence lifetime measurements were made. Z-scan experiments were conducted to study nonlinear optical response. Preliminary experiments indicate two photon absorption in 2,2-3-methyldithienyl ethylene and in 2,2-dithienyl-1,3-butadiene.