Nanorectenna spectrally-selective plasmonic hot electron response to visible-light lasers.

Active metasurfaces with novel visible and infrared (vis/IR) functionalities represent an exciting, growing area of research. Rectification of vis/IR frequencies would produce needed direct current (DC) with no inherent frequency limitation (e.g. no semiconducting bandgap). However, controlling the materials and functionality of (nano)rectennas for rectifying 100 s of THz to the visible regime is a daunting challenge, because of the small features and simultaneously the need to scale up to large sizes in a scalable platform. An active metasurface of a planar array of nanoscale antennas on top of rectifying vertical diodes is a 'nanorectenna array' or 'microrectenna array' that rectifies very high frequencies in the infrared, or even higher frequencies up to the visible regime. We employ a novel strategy for forming optical nanorectenna arrays using scalable patterning of Au nanowires, demonstrate strong evidence for spectral-selective high-frequency rectification, characteristic of optical antennas. We discover a previously unreported out-of-equilibrium electron energy distribution, i.e. hot electrons arising from plasmonic resonance absorption in an optical antenna characterized by an effective temperature, and how this effect can significantly impact the observed rectification.

Enhanced coupling of broadband light into amorphous silicon via periodic nanoplasmonic arrays.

Achieving enhanced coupling of solar radiation over the full range of the silicon absorption spectrum up to the bandgap is essential for increased efficiency of solar cells, especially thin film versions. While many designs for enhancing trapping of radiation have been explored, detailed measurements of light scattering inside silicon cells is still lacking. Here, we demonstrate experimentally and computationally that plasmonic-assisted localized and traveling modes can efficiently couple red and infrared radiation into ultrathin amorphous silicon (a-Si) layers. Utilizing patterned periodic arrays of aluminum nanostructures on thin a-Si, we perform specular and diffuse reflectivity and transmission measurements over a broad spectrum. Based on these results, we are able to separate parasitic absorption in aluminum plasmonic arrays from enhanced light absorption in the 200 nm thick amorphous silicon layer, as compared to a blank silicon layer. We discover a very efficient near-infrared a-Si absorption mechanism that occurs at the transition from the radiative to evanescent diffractive coupling, analogous to earlier surface-enhanced infrared studies. These results represent a direct demonstration of enhanced radiation coupling into silicon due to large angle scattering and show a path forward to improved ultrathin solar cell efficiency.

Engineering the Structural and Electronic Phases of MoTe2 through W Substitution.

MoTe2 is an exfoliable transition metal dichalcogenide (TMD) that crystallizes in three symmetries: the semiconducting trigonal-prismatic 2H- or α-phase, the semimetallic and monoclinic 1T'- or ß-phase, and the semimetallic orthorhombic γ-structure. The 2H-phase displays a band gap of ∼1 eV making it appealing for flexible and transparent optoelectronics. The γ-phase is predicted to possess unique topological properties that might lead to topologically protected nondissipative transport channels. Recently, it was argued that it is possible to locally induce phase-transformations in TMDs, through chemical doping, local heating, or electric-field to achieve ohmic contacts or to induce useful functionalities such as electronic phase-change memory elements. The combination of semiconducting and topological elements based upon the same compound might produce a new generation of high performance, low dissipation optoelectronic elements. Here, we show that it is possible to engineer the phases of MoTe2 through W substitution by unveiling the phase-diagram of the Mo1-xWxTe2 solid solution, which displays a semiconducting to semimetallic transition as a function of x. We find that a small critical W concentration xc ∼ 8% stabilizes the γ-phase at room temperature. This suggests that crystals with x close to xc might be particularly susceptible to phase transformations induced by an external perturbation, for example, an electric field. Photoemission spectroscopy, indicates that the γ-phase possesses a Fermi surface akin to that of WTe2.

Optical parametric amplification via non-Hermitian phase matching.

We introduce the notion of dissipative optical parametric amplifiers (DOPA) and demonstrate that, even in the absence of the Hermitian phase-matching condition in these structures, the signal beam can be amplified when the idler mode suffers optical attenuation. We discuss the optical implementation of this concept in waveguide platforms, and we propose different methods to control the optical loss of these configurations only at the wavelength of the idler component. Surprisingly, this spectrally selective dissipation process allows the signal beam to draw more energy from the pump and, as a result, attains net amplification. Similar results also apply if the losses are introduced only to the signal component. This intriguing feature can open new avenues for building long wavelength light sources and parametric amplifiers by using semiconductor planar structures, where Hermitian phase-matching requirements can be difficult to satisfy without adding stringent geometric constraints or relatively complex fabrication steps.

Quasiparticle interference, quasiparticle interactions, and the origin of the charge density wave in 2H-NbSe2.

We show that a small number of intentionally introduced defects can be used as a spectroscopic tool to amplify quasiparticle interference in 2H-NbSe2 that we measure by scanning tunneling spectroscopic imaging. We show, from the momentum and energy dependence of the quasiparticle interference, that Fermi surface nesting is inconsequential to charge density wave formation in 2H-NbSe2. We demonstrate that, by combining quasiparticle interference data with additional knowledge of the quasiparticle band structure from angle resolved photoemission measurements, one can extract the wave vector and energy dependence of the important electronic scattering processes thereby obtaining direct information both about the fermiology and the interactions. In 2H-NbSe2, we use this combination to confirm that the important near-Fermi-surface electronic physics is dominated by the coupling of the quasiparticles to soft mode phonons at a wave vector different from the charge density wave ordering wave vector.

Development and application of spherically curved charge-coupled device imagers.

Operation of a CCD imager on a curved focal surface offers advantages to flat focal planes, especially for lightweight, relatively simple optical systems. The first advantage is that the modulation transfer function can approach diffraction-limited performance for a spherical focal surface employed in large field-of-view or large-format imagers. The second advantage is that a curved focal surface maintains more uniform illumination as a function of radius from the field center. Examples of applications of curved imagers, described here, include a small compact imager and the large curved array used in the Space Surveillance Telescope. The operational characteristics and mechanical limits of an imager deformed to a 15 mm radius are also described.

Wavelength conversion and parametric amplification of optical pulses via quasi-phase-matched four-wave mixing in long-period Bragg silicon waveguides.

We present a theoretical analysis supported by comprehensive numerical simulations of quasi-phase-matched four-wave mixing (FWM) of ultrashort optical pulses that propagate in weakly width-modulated silicon photonic nanowire gratings. Our study reveals that, by properly designing the optical waveguide such that the interacting pulses copropagate with the same group velocity, a conversion efficiency enhancement of more than 15 dB, as compared to a uniform waveguide, can readily be achieved. We also analyze the dependence of the conversion efficiency and FWM gain on the pulse width, time delay, walk-off parameter, and grating modulation depth.

Large-area regular nanodomain patterning in He-irradiated lithium niobate crystals.

Large-area ferroelectric nanodomain patterns, which are desirable for nonlinear optical applications, were generated in previously He-implanted lithium niobate crystals by applying voltage pulses to the tip of a scanning force microscope. The individual nanodomains were found to be of uniform size, which depended only on the inter-domain spacing and the pulse amplitude. We explain this behavior by the electrostatic repulsion of poling-induced buried charges between adjacent domains. The domain patterns were imaged by piezoresponse force microscopy and investigated by domain-selective etching in conjunction with focused ion beam etching followed by scanning electron microscopy imaging. In order to optimize the He-irradiation parameters for easy and reliable nanodomain patterning a series of samples subjected to various irradiation fluences and energies was prepared. The different samples were characterized by investigating nanodomains generated with a wide range of pulse parameters (amplitude and duration). In addition, these experiments clarified the physical mechanism behind the facile poling measured in He-irradiated lithium niobate crystals: the damage caused by the energy loss that takes place via electronic excitations appears to act to stabilize the domains, whereas the nuclear-collision damage degrades the crystal quality, and thus impedes reliable nanodomain generation.

30 to 50 ns liquid-crystal optical switches.

The optical switching time of twisted-nematic liquid-crystal cells using the liquid crystals, 5CB (C(5)H(11)-Ph-Ph-CN), 5OCB(C(5)H(11)-O-Ph-Ph-CN) and PCH5 (C(5)H(11)-Cy-Ph-CN) have been characterized as a function of temperature, bias voltage and switching voltage, V. The transition time from 90% to 10% transmission scales as V(-1.9) and is limited to 30 to 50 ns by the liquid-crystal breakdown electric field, ~100 V µm(-1). The time from the initial switching voltage step to 90% transmission, delay time, decreases with increasing bias and switching voltage. For 5CB and 5OCB the delay time approaches a constant value at higher electric fields, >10 V µm(-1). Both the transition and delay times decrease with increasing temperature. The minimum transition time at temperatures a few degrees below the nematic-isotropic temperature are 32, 32, and 44 ns and delay times are 44, 25 and 8 ns for 5CB, 5OCB, and PCH5 respectively.

Nonequilibrium band mapping of unoccupied bulk states below the vacuum level by two-photon photoemission.

We demonstrate angle-resolved, tunable, two-photon photoemission (2PPE) to map a bulk unoccupied band, viz. the Cu sp band 0 to 1 eV below the vacuum level, in the vicinity of the L point. This short-lived bulk band is seen due to the strong optical pump rate, and the observed transition energies and their dispersion with photon energy ℏω, are in excellent agreement with tight-binding band-structure calculations. The variation of the final-state energy with ℏω has a measured slope of ∼1.64 in contrast to values of 1 or 2 observed for 2PPE from two-dimensional states. This unique variation illustrates the significant role of the perpendicular momentum ℏk_{⊥} in 2PPE.

Laser-induced chemistry for microelectronics.

Laser-controlled chemical reactions are being explored for use in all phases of the processing of semiconductor devices. Laser-induced chemical processing can produce submicrometer features without the aid of photolithography. Research is also providing new information on light-excited and light-enhanced interface reactions.

Nonlinear-optical phase modification in dispersion-engineered Si photonic wires.

The strong dispersion and large third-order nonlinearity in Si photonic wires are intimately linked in the optical physics needed for the optical control of phase. By carefully choosing the waveguide dimensions, both linear and nonlinear optical properties of Si wires can be engineered. In this paper we provide a review of the control of phase using nonlinear-optical effects such as self-phase and cross-phase modulation in dispersion-engineered Si wires. The low threshold powers for phase-changing effects in Si-wires make them potential candidates for functional nonlinear optical devices of just a few millimeters in length.

Optical negative-index bulk metamaterials consisting of 2D perforated metal-dielectric stacks.

Numerical simulations of a near-infrared negative-index metamaterial (NIM) slab consisting of multiple layers of perforated metal-dielectric stacks exhibiting a small imaginary part of the index over the wavelength range for negative refraction are presented. A consistent effective index is obtained using both scattering matrix and modal analysis approaches. Backward phase propagation is verified by calculation of fields inside the metamaterial. The NIM figure of merit, [ -Re(n)/Im(n) ], for these structures is improved by ~ 10x compared with previous reports, establishing a new approach to thick, low-loss metamaterials at infrared and optical frequencies.

Second harmonic generation from patterned GaAs inside a subwavelength metallic hole array.

By extending GaAs dielectric posts with a large second-order nonlinear susceptibility through the holes of a subwavelength metallic hole array coupled to the metal surface-plasma wave, strong second harmonic (SH) signal is observed. The SH signal is strengthened as a result of the enhanced electromagnetic fields inside the hole apertures.

Near-infrared double negative metamaterials.

We numerically demonstrate a metamaterial with both negative epsilon and negative mu over an overlapping near-infrared wavelength range resulting in a low loss negative-index material. Parametric studies optimizing this negative index are presented. This structure can be easily fabricated with standard semiconductor processing techniques.

Photoinduced-reaction dynamics of halogenated alkanes on iron oxide surfaces: CH3I on Fe3O4(111)-(2x2).

The adsorption, thermal chemistry, and photoreaction dynamics of methyl iodide on the (2x2) magnetite termination of natural single-crystal hematite have been investigated by time-of-flight quadrupole mass spectrometry (TOF-QMS), temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). The methyl iodide thermal desorption spectra, taken after dosing the (2x2) surface at 100 K with CH(3)I, show a multiple-peak coverage-dependent behavior, consistent with the presence of several distinct adsorbed phases, along with defect-mediated dissociative chemisorption in the first monolayer. At >1 ML, methyl iodide forms a metastable physisorbed second layer, which desorbs at 148 K, but at higher coverage converts to a layer, which desorbs at 170 K. In the presence of low-fluence-pulse irradiation at 248 nm, angle-resolved TOF-QMS measurements show that 1.6 and 0.3 eV CH(3) fragments are ejected from the adsorbate surface; these fragments originate from direct photodissociation and dissociative photoinduced electron transfer, respectively. These energetic photoejected fragments have characteristic angular distributions peaked at approximately 0 degree with respect to the surface normal. These results and the coverage-dependent relative intensities suggest that the predominant orientation in the first monolayer of the adsorbed CH(3)I is normal to the crystal plane.

Parametric light bullets supported by quasi-phase-matched quadratically nonlinear crystals.

We present a comprehensive analysis of the dynamics of three-dimensional spatiotemporal nonspinning and spinning solitons in quasi-phased-matched (QPM) gratings. By employing an averaging approach based on perturbation theory, we show that the soliton's stability is strongly affected by the QPM-induced third-order nonlinearity (which is always of a mixed type, with opposite signs in front of the corresponding self-phase and cross-phase modulation terms). We study the dependence of the stability of the spatiotemporal soliton (STS) on its energy, spin, the wave-vector mismatch between the fundamental and second harmonics, and the relative strength of the intrinsic quadratic and QPM-induced cubic nonlinearities. In particular, all the spinning solitons are unstable against fragmentation, while zero-spin STS's have their stability regions on the system's parameter space.

Influence of the dispersive properties of metals on the transmission characteristics of left-handed materials.

We study numerically the influence of the frequency dispersion of the dielectric function of metals on the physical properties of negative-refractive-index metamaterials. A numerical analysis is performed using the transfer matrix formalism in conjunction with the finite-difference time-domain method. We analyze the dependence of the transmission and absorption properties of a slab of split-ring-type resonators on the parameters characterizing the frequency dispersion of the metallic dielectric function: plasma frequency and damping frequency. Then, using these transmission and reflection coefficients, we show that the refractive index remains negative near the resonant frequency of the rings, despite the presence of frequency dispersion. We also determine the dependence of the position and width of the band gaps of a slab of such a metamaterial on the material dispersion. Finally, we also discuss the influence of the shape of the split-ring resonators on the transmission and reflection coefficients. The calculations are performed for both two- and three-dimensional structures.

Nonlinear optical effects in a two-dimensional photonic crystal containing one-dimensional Kerr defects.

The nonlinear optical effects induced by a one-dimensional (1D) line defect, made of Kerr material, in a 2D photonic crystal are studied. Comprehensive ab initio numerical simulations based on the finite-difference time-domain method show efficient third-harmonic generation in a photonic crystal waveguide consisting of the 1D defect line. The relationship between the third harmonic generation process and the nonlinear modal properties of the waveguide is discussed. We investigate optical limiting in such a device, that is, control of the transmitted power as a function of the Kerr-induced variation of the refractive index. Power dependent spectral changes in such a device and its use as a frequency selector are also examined.

Spatial solitons in type II quasiphase-matched slab waveguides.

The existence and dynamics of one-dimensional spatial solitons formed upon propagation in quasiphase-matched gratings, through three-wave parametric interaction, is analyzed. We study the general case in which the grating exhibits a periodic modulation of both the refractive index and the second-order susceptibility. It is demonstrated that for negative effective wave vector mismatch the induced third-order nonlinearities increase the domain of soliton instability. Finally, the dependence of the efficiency of the second harmonic generation process in the soliton regime, on the parameters of the grating, is discussed.