Session A23: Novel Plasmonic Effects and Devices

Plasmonic Nanomaterials for Optical-to-Electrical Energy Conversion

High-quality semiconductor solids have been the dominant photovoltaic materials platform for decades. Although several alternative approaches have been proposed, e.g. dye-sensitized cells or polymeric solids, none compete in terms of cost and conversion efficiency, the crucial benchmarks for industrial scale implementation. However, semiconductors suffer from several fundamental limitations relating to the microscopic mechanism of power conversion that preclude them, even theoretically, from achieving conversion efficiency at the Carnot limit of 95{\%}. Indeed, the fundamentally different tasks of semiconductors in photovoltaic devices, both as optical absorbers, and separately, for electron-hole pair separation and collection, often demand opposing trade-offs in materials optimization. Alternatively, recent advances in subwavelength metal optics, e.g. nanophotonics, metamaterials, and plasmonics, provide several new examples where nanostructured metals perform the separate tasks of absorption and charge separation necessary for photovoltaic power conversion. Nanostructured metals are extremely efficient broadband absorbers of radiation, with tailorable optical properties throughout the visible and infrared spectrum. It is traditionally assumed that the lack of a band gap and consequent fast electronic relaxation (fs) and short mean free path ( 100 nm) hinders efficient carrier collection. However, new phenomena resulting from the remarkable energy concentration and nanoscale collection geometry afforded by plasmonic systems suggest new strategies may be possible that use all metal structures. In this talk, I will describe two ongoing studies in our laboratory that exemplify opportunities for metal-based optical energy conversion: (1) Excitation with circularly polarized illumination can induce strong, persistent electrical drift currents in resonant metal nanostructures via the inverse faraday effect. (2) Plasmonic absorption in metal nanostructures provides an entirely new mechanism for generating electrochemical potential from photons. This behavior is termed a `plasmoelectric effect' (\textit{Science, 2014}).   

Hot carrier metamaterial detectors and energy converters

Metamaterials can be used to manipulate the flow of light in ways not typically available with traditional materials. Beyond their optical properties, metamaterials can be used as the basis for optoelectronic devices through the incorporation of a metal-semiconductor interface. The absorbed radiation in the metal can excite surface plasmons, which nonradiatively decay into hot electrons or holes that can be injected into the base semiconductor and contribute to photocurrent generation. In this talk, we will present our latest work on metamaterial photo-detectors and solar energy converters.   

Non-equilibrium hot carrier dynamics in plasmonic nanostructures

Decay of surface plasmons to hot carriers is a new direction that has attracted considerable fundamental and application interest, yet a fundamental understanding of ultrafast plasmon decay processes and the underlying microscopic mechanisms remain incomplete. Ultrafast experiments provide insights into the relaxation of non-equilibrium carriers at the tens and hundreds of femtoseconds time scales, but do not yet directly probe shorter times with nanometer spatial resolution. Here we report the first ab initio calculations of non equilibrium transport of plasmonic hot carriers in metals and experimental observation of the injection of these carriers into molecules tethered to the metal surface. Specifically, metallic nanoantennas functionalized with a molecular monolayer allow for the direct probing of electron injection via surface enhanced Raman spectroscopy of the original and reduced molecular species. We combine first principles calculations of electron-electron and electron-phonon scattering rates with Boltzmann transport simulations to predict the ultrafast dynamics and transport of carriers in real materials. We also predict and compare the evolution of electron distributions in ultrafast experiments on noble metal nanoparticles.   

Plasmon drag in esheric percolation series of metallic arrays

Perforated thin metallic films, which evolve from hole to island arrays, form an Esheric percolation series. The plasmonic response of such a series has been investigated [1], with critical phenomena observed near the percolation threshold. In this work, we investigate the plasmon drag effect in such structures, and propose a microscopic explanation for the recently discovered plasmoelectric effect [2]. [1] E. M. Akinoglu, T. Sun, J. Gao, M. Giersig, Z.F. Ren, and K. Kempa, ``Evidence for critical scaling of plasmonic modes at the percolation threshold in metallic nanostructures'', Appl. Phys. Lett. \textbf{103}, 171106 (2013). doi: 10.1063/1.4826535 [2] M.T. Sheldon, J. v.Groep, A.M. Brown, A. Polman, H.A. Atwater, ``Plasmoelectric potentials in metal nanostructures'', Science \textbf{346}, 828-831 (2014). doi: 10.1126/science.1258405   

Collective plasmonic oscillations in nanostrips arrays and sine wave gratings. Comparative study

Excitation of collective plasmonic modes and their effects on optical behavior were experimentally and theoretically studied in 1 D arrays of gold nanostrips in comparison with those in continuous gold films with a sine wave profile and similar periodicity. Two kinds of collective resonance modes are efficiently excited in gold strips, with participation of gold -air and gold- glass interfaces. These modes correspond to maxima in the angular dependence of reflection, as opposed to minima observed at surface plasmon polariton conditions in a continuous sine wave grating. Spectral and polarization dependences are obtained. A theoretical approach based on the novel combined transfer-matrix coupled-wave analysis and coordinate transformation method is shown to well describe experiments.   

Semiconductor-free hot carrier devices for energy harvesting and photodetection.

The maximum efficiency for a single-junction solar cell is around 30{\%} by the Shockley-Queisser (SQ) limit. The energy loss is typically through a thermalization process between the excited high-energy carriers, e.g. hot carriers, and the lattice. Therefore, the collection of the hot carriers before thermalization would allow for reduced power loss. Recently, photodetectors based on metal-semiconductor Schottky junctions have been exploiting hot electron effects to allow sub-bandgap absorption and hence show promise as near IR wavelength detectors. Here we present a simple, semiconductor-free hot carrier device based on transparent conducting oxides (TCO) electrodes. We experimentally demonstrate the hot carrier generation and extraction under monochromatic and broadband light illumination of normal and oblique incidence. Under optimized conditions, a power conversion efficiency \textgreater 10{\%} is predicted for high-energy photon excitation. The performance of the device shows further improvement by employing nanostructures, which couple the incident light into surface plasmons, leading to absorption enhancement. This semiconductor-free device provides an alternative way of energy harvesting and photodetection.   

High-contrast and fast electrochromic switching enabled by plasmonics

With vibrant colors and simple, room-temperature processing methods, electrochromic polymers have long attracted attention as active materials for flexible, low-power consuming devices such as smart windows and displays. However, despite their many advantages, slow switching speed and complexity of combining several separate polymers to achieve full-color gamut has limited electrochromic materials to niche applications. Here we exploit the enhanced light-matter interaction associated with the deep-subwavelength mode confinement of surface plasmon polaritons propagating in metallic nanoslit arrays coated with ultra-thin electrochromic polymers to build a novel configuration for achieving high-contrast and fast electrochromic switching. The switchable configuration retains the short temporal charge-diffusion characteristics of thin electrochromic films while maintaining the high optical-contrast associated with thicker electrochromic coatings. We further demonstrate that by controlling the pitch of the nanoslit arrays, it is possible to achieve a full-color response with high-contrast and fast switching-speeds while relying on just one electrochromic polymer.   

Alloyed Noble Metal Nanoparticles with Tunable Optical Properties

Noble metal nanoparticles (NPs) have been widely used in sensing, optics, and catalysis applications by taking advantage of surface plasmon resonance (SPR). This response is slightly tuned by varying the size and shape of the NPs; however, a method to obtain truly on-demand plasmonic responses is still lacking due to the intrinsic nature of a metal's dielectric function. Here, we fabricate size and composition controlled metal alloy NP arrays by deposit-and-anneal methods and through-template depositions. We control the composition of the metal NPs by co-sputtering and by alternating electron-beam evaporation of the Ag and Au targets. To characterize the NPs, macroscopic transmission measurements are combined with spectrally dependent near-field scanning optical microscopy to show the local optical properties around the NPs. By varying the atomic fraction of Ag and Au in the alloys, we modulate the optical properties of the NPs for different applications. For example, hot carrier plasmonic devices necessitate high absorption in the visible range, while photovoltaic applications require low absorption by the NPs.   

Plasmonic thickness variation study of gold nanostructures in ultraviolet-visible light regime

Noble metal nanostructures exhibit strong surface plasmon resonances in the ultraviolet-visible light range that are not present in bulk metal. In this study, we have observed the plasmonic properties of different sized gold nanodisks and nanorods with varying thickness. The samples were fabricated by electron beam lithography on silicon dioxide substrates. Depending on the thickness of the nanostructures, strong and well-defined surface plasmon resonances were found (wavelength 400nm - 1000nm). For experimental and theoretical results, we have used Dark field spectroscopy and finite element method, respectively. We found that resonance peak was shifted with nanostructure thickness. By using Dark field spectroscopy, the scattered light from individual structures can be analyzed with less background noise and the incident light was at an angle to the substrate.   

Complex Near-Field Plasmonic Response of Au Nanospirals

Complex metallic nanostructures that support unique near-field surface plasmon modes have shown applications across the fields of photovoltaics, bio-sensing, and even quantum computing. Chiral Au nanospirals not only possess a non-symmetric morphology that results in second-harmonic generation, but possess multiple distinct near-field plasmonic modes that cover a wide range of plasmon frequencies. We use cathodoluminescence (CL) and electron energy loss spectroscopy (EELS) within a scanning transmission electron microscopy (STEM) to study the surface plasmons and map them with nanoscale precision. The two techniques are complementary as EELS measures excitations in the sample, while CL measures the subsequent radiative decays. We STEM-EELS/CL to map and analyze the spatial profile, intensity and polarization response of the intricate near-field plasmon modes in these versatile nanostructures.   

Infrared Resonances in Plasmonic Nanorod and Nanoarc Antennas

Tunability of the frequency and polarization of localized surface plasmon resonances (LSPR) of nanostructures is crucial for their implementation in nanophotonics applications such as photovoltaics, chiroptical spectroscopy, and infrared detection. We report spectroscopic data of plasmonic nanorods and nanoarcs collected by polarized Fourier transform infrared reflectance spectroscopy (FTIR). The effects of the nanostructure material, geometry and substrate material are investigated by patterning gold and aluminum structures with varying length on silicon and glass substrates, as well as on anodic aluminum oxide, a cost effective alternative to standard transparent substrates. By varying such parameters for straight rods and arcs, we find that the measured LSPR frequencies of our nanostructures span the mid-infrared spectral range ($\lambda$=2-12 microns). However, we find that bending the nanostructures (i.e., forming arcs rather than straight rods) results in additional resonances with unique polarizations not observed in straight nanorods. We find that the nanorods exhibit half-wave antenna behavior which can be modeled using antenna theory with a linearly scaled effective wavelength which accounts for structure dimensions and material.   

Localized and Propagating Surface Plasmons in Aluminum Nanostructures: The Effect of Metal Deposition Method on Resonance Quality and Depolarization

The field of plasmonics has provided revolutionary concepts in sensing, nano-optics and energy harvesting. Al plasmonics has recently emerged as an alternative, CMOS-compatible nanofabrication platform for applications in the UV-visible ranges. Previously, we found that high-temperature sputtered Al films showed significantly better plasmonic response than conventional evaporated films. Here, we extend this thin film work to patterned aluminum nanostructures that support both localized and propagating plasmon modes. The nanostructures from sputtered and evaporated aluminum are fabricated side-by-side in a CMOS compatible state-of-the-art facility. The quality of plasmonic resonances is analyzed with Mueller Matrix spectroscopic ellipsometry over a wide range of incidence angles and wavelengths. Full band structure is experimentally obtained and verified with full-field simulations. We find a strong enhancement in the ellipsometric depolarization parameter near the wavelengths of plasmonic resonance. The depolarization parameter is interpreted as a powerful connection between the near and the far field, providing a diagnostic of the quality of plasmonic resonances.   

Optical forces in a cluster of cylinders.

We present a rigorous multiple-scattering method to calculation the optical force exerted on a cylinder placed near a cluster of parallel cylinders. Various kinds of cluster structure with both dielectric and left-hand materials are considered. It is shown that optical forces are sensitive to the position of the test cylinder, the material of the cylinders in the cluster or the structure of the system. The optical force is strong within frequency bands corresponding to surface-plasmon excitations of the cylinders.