Symposium EM7 : Functional Plasmonics

The progresses of metamaterials research has brought it to the stage that increasing attention has been paid to realize large scale metamaterial devices for real applications. In particular, a fascinating application is to implement perfect lens which is enabled by negative refractive index metamaterials (NIM) with both negative effective electric permittivity and magnetic permeability.
However, the realization of such a practical device has been hindered due to the obstacle in achieving isotropic negative index metamaterials at optical frequency which are insensitive to the angle of incidence and polarization of light. The design and fabrication of such an isotropic optical metamaterial has turned out to be an extremely challenging task. In the past years, many attempt to tackle the problem were all based on the canonical designs of split ring resonator (SRR) structures. The idea is to construct SRR in all three dimensions in order for the isotropic light-matter interactions. However, these structures require sophisticated fabrications that are really difficult to achieve, especially for metamaterials at optical frequency and at a large scale. Moreover, the SRR based metamaterials structures is almost impossible to scale to optical frequency due to the saturation effect. Here we show a realistic structural design and experimentally demonstrate large scale fabrication of isotropic negative index metamaterials at optical frequency.

Optical isolation is a key requirement for chip-based information processing networks. As in electronic circuits, isolation of forward and backward propagating signals is of critical importance.¹ Typical commercially available isolators are rather bulky, hard to scale and integrate, and expensive. Design of an efficient and compact optical isolator would foster the development of integrated all-photonic networks.¹
There are three known ways to break the symmetry of light propagation in forward and backward directions¹: with the use of magneto-optical response (commonly employed in optical circulators and isolators), time-varying² and nonlinear structures³. The latter two approaches suffer from a number of fundamental constrains. Hence, isolation in time varying systems would require ultrafast modulation of material parameters² (desirably in a THz range), which is hard to achieve in practice, whereas nonlinear systems (typically third order) require high input power and have a limited operation bandwidth³.
Here we propose theoretically a conceptually different paradigm of the signal isolation based on a hybrid approach that combines the principles of isolation of both time-varying and nonlinear systems. Specifically, we show that optical signal isolation is possible in optical light guiding structures with second order optical nonlinearity. We develop an analytical model and demonstrate that in the presence of a relatively strong pump a dynamic nonreciprocal coupling between signal and idler waves is possible. In particular, we show that in the forward direction of propagation signal wave propagates without any perturbation, whereas in the backward direction it may be fully converted into an idler wave in the presence of a control pump beam. We further elaborate our model and demonstrate that in the undepleted pump approximation it is equivalent to that of ultrafast time-varying systems.
Next we apply our model to a realistic material system. To be specific, we consider optical isolation of a telecom wavelength signal (1550nm) in a 130nm gallium phosphide (GaP) waveguide. Gallium phosphide offers an exciting platforms due to its high (over 100 pm/V) second order nonlinear coefficient, high refractive index, low material losses, and its CMOS compatibility. We show that with a 10 mW pump at ~1350 nm a 1 µW signal may be completely isolated over a 10 mm propagation length. Note that the propagation length in this scheme is several orders of magnitude smaller than that reported for electro-optical time-varying systems and for Brillouin scattering based isolation in fibers. We discuss scenarios to reduce the device footprint with the help of optical ring resonators.

References:
1. Jalas, D.; et al. Nature Photon. 2013, 7, 529.
2. Yu, Z.; Fan, S. Nature Photon. 2008, 3, 91 - 94.
3. Shi, Y.; Yu, Z.; Fan, S. Nature Photon. 2015, 9, 388–392.
4. Poulton, C.G.; et al. B.J. Opt. Express 2012, 20, 21235.

Plasmonic nanoparticles are ideal candidate systems for coupling photons into phonons, allowing for control over opto-mechanical properties in nanoscale dimensions. Upon excitation of a gold nanoparticle with an ultrafast laser, a series of events occur: initial creation of a surface plasmon and dephasing via electron-electron scattering, followed by eventual transfer of energy to the lattice and impulsive excitation of acoustic modes. Although such acoustic modes have been observed on single nanoparticles using optical pump-probe experiments, the inherently subwavelength spatial scale of plasmonic structures and devices make new imaging and spectroscopic techniques critical for understanding the relationship between plasmonic nanostructures and observed acoustic modes.

We use ultrafast electron microscopy (UEM), a pump probe technique that combines the spatial resolution of TEM with the temporal resolution of ultrafast pump-probe spectroscopic methods, to study the dynamics of gold nanorods. These experiments use an unprecedented combination of spatial and temporal resolution, resolving dynamics at below 3 ps with less than 5 nm spatial resolution.

The experiment uses two optically delayed pulses to trigger dynamics on a materials system in a TEM, and then images the sample with an optically triggered electron pulse. Using pump-probe stroboscopic imaging within the UEM we are able to image the laser induced dynamics within a single gold nanorod. We spatially map through changes in diffraction contrast the cooling rate of the lattice to the environment within a single Au nanorod, showing changes of diffraction strength of up to 5% from the pre-interaction level, and cooling rates to the substrate of approximately 1 ns. In addition to the kinetic response, there are clear oscillations observable in the diffraction contrast of the nanoparticle, which we assign to thermal shock induced acoustic modes of the nanoparticle. These modes are spatially mapped within the single nanorod and are observed at 3.9, 9.4, and 27 GHz, and are assigned to a bend, symmetric stretch, and symmetric overtone stretch, respectively. These assignments are consistent with the predicted frequencies from finite element simulations.

The technique was then extended to observe dynamics on nanoparticle dimers, tetramers, and larger clusters. Notably, we observe acoustic modes that are distinct from the modes observed in single nanoparticles, including very strong localized response where two nanoparticles come into direct contact. This localized response occurs at 25.8 and 27.3 GHz.

These studies indicate the complex relationship between the structure of plasmonic assemblies and the resulting dynamics, and resolve these dynamics with high resolution spatiotemporal mapping. We believe that this technique would allow for observation of dynamics in plasmonic devices, and could be used to answer questions pertaining to thermal transport, nanomechanical behavior, and optical losses.

Optical metamaterials are often fabricated on flat substrates made from materials that are topographically and chemically compatible with conventional nanofabrication techniques, such as electron beam lithography. The ability to fabricate on unconventional substrates will enable the next generation of optical metamaterials.
We fabricate metamaterials using atomic calligraphy, a microelectromechanical systems (MEMS)-based dynamic stencil lithography technique [1]. We present a flip-chip technique that enables us to write on a variety of substrates. While the areal coverage of atomic calligraphy is approximately 100 μm², we use a stage system to extend the range to square centimeters. Because atomic calligraphy is a direct write technique, it is resist- and liftoff-free. Therefore, it can be used to fabricate on substrates with topographical features or that preclude them from use with resists or on substrates that are chemically incompatible with other lithography techniques [2]. We fabricate metamaterials on foreign substrates and characterize their infrared spectra.
This technique will enable us to fabricate metamaterials on novel substrates that lend additional degrees of freedom. For example, fabricating metasurfaces on two dimensional auxetic mechanical metamaterials will enable us to fabricate tunable metasurfaces.

Plasmon excitation enables extreme light confinement at the nanoscale, localizing energy in subwavelength volumes and thus can enable increased absorption in photovoltaic or photoconductive detectors. Nonetheless, plasmon decay also results in energy transfer to the lattice as heat which is detrimental to photovoltaic detector performance. However, heat generation in resonant subwavelength nanostructures also represents a power source for energy conversion, as we demonstrate here via design of resonant thermoelectric (TE) plasmonic absorbers for optical detection. Though TEs have been used to observe resonantly coupled surface plasmon polaritons in noble-metal thin films and microelectrodes, they have not been employed previously as resonant absorbers in functional TE nanophotonic structures.

We demonstrate nanostructures composed of TE thermocouple junctions using established TE materials – chromel/alumel and bismuth telluride/antimony telluride – but patterned so as to support guided mode resonances with sharp absorption profiles, and which thus generate large thermal gradients upon optical excitation and localized heat generation in the TE material. Unlike previous TE absorbers, our structures feature tunable narrowband absorption and measured single junction responsivities 10 times higher than the most similar (albeit broadband) graphene structures, with potential for much higher responsivities in thermopile architectures. For bismuth telluride – antimony telluride structures, we measure thermoelectric voltages up to 850 μV with incident optical power densities of 3.4 W/cm². The maximum responsivity of a single thermocouple structure was measured at 119 V/W, referenced to incident illumination power. We also find that the small heat capacity of optically resonant TE nanowires enables a fast, 3 kHz temporal response, 10-100 times faster than conventional TE detectors. We show that TE nanophotonic structures are tunable from the visible to the MIR, with small structure sizes of 50 µm x 100 µm. Our nanophotonic TE structures are suspended on thin membranes to reduce substrate heat losses and improve thermal isolation between TE structures arranged in arrays suitable for imaging or spectroscopy. Whereas photoconductive and photovoltaic detectors are typically insensitive to sub-bandgap radiation, nanophotonic TEs can be designed to be sensitive to any specific wavelength dictated by nanoscale geometry, without bandgap wavelength cutoff limitations. From the point of view of imaging and spectroscopy, they enable integration of filter and photodetector functions into a single structure.

Metal nanoparticles exhibit broad localized surface plasmon resonances that increase in width as the particle size increases. However, when these nanoparticles are organized into arrays with spacings on the order of hundreds of nanometers, narrow lattice plasmon resonances can result. The talk will describe a new way to achieve even narrower resonances via superlattice plasmons, collective excitations that are supported by hierarchical gold nanoparticle arrays, where finite arrays of particles (patches) are organized into arrays with larger periodicities. Superlattice plasmons resonances are often significantly narrower than that of single-patch lattice plasmon resonances and exhibit stronger local peak fields. We will also discuss how ultra-narrow resonances can be achieved and manipulated in emerging plasmon materials.

Planar nanostructures with subwavelength features have enabled miniaturized optical components with superior properties over traditional bulk optics. However, these structures typically suffer from strong chromatic aberration that limits their applications to narrow wavelength ranges. Here we report a platform based on gold nanoparticle (NP) lattices and an evolutionary method that can design flat lenses for a wide range of wavelengths. Our optical components can access the desired wavelengths by tuning the plasmon resonances of the lattice elements with their size and shape. The design approach allows multi-objective optimization, which enable us to design multi-resonance lattices using multiple particle shapes that can focus light to the same focal point at up to three wavelengths. We expect that the design strategy will be applicable to all wavelengths ranges by choosing the suitable materials and structures as the lattice building blocks.

In order to overcome the limitations imposed by the pre-defined dielectric function of metals we implement alloys [1]. First, we built a library of the optical response of thin-film alloys formed by the binary combination of Ag, Au, Cu and Al. For that, we combine ellipsometry and SPP measurements and calculations, and find an excellent agreement between the two methods. Surprisingly, we find that some compositions present a quality factor higher than their pure counterparts. Second, we fabricated alloyed nanoparticles and investigated their optical response by near-field scanning optical microscopy (NSOM), in conjunction with full-field simulations of light-matter interactions in the visible and NIR ranges of the spectrum [2]. Our findings pave the way for the development of optically engineered building blocks for nanophotonics based on alloys, where the chemical composition of the nanostructures can be used as an additional knob to tune their optical properties.

[1] C. Gong et al. ACS Photonics, 3, 507 (2016). Front COVER.
[2] C. Gong, M. Dias et al. Adv. Optical Materilas, DOI:10.1002/adom.201600568 (2016).

Diffraction limit basically limits the resolution of conventional optical microscopy, which means that the object smaller than the diffraction limit is difficult to be distinguished [1]. Many kinds of imaging techniques have been introduced and demonstrated to overcome this physical limitation. Near-field scanning optical method, many fluorescent microscope, like STED, STORM, NSOM