Loading the Antenna Gap with Two-Dimensional Electron Gas Transistors: A Versatile Approach for the Rectification of Free-Space Radiation

Light conversion into dc current is of paramount interest for a wide range of upcoming energy applications. Here we integrated dipole antennas with field-effect transistors based on a two-dimensional electron gas, with the specific aim of rectifying free-space radiation exploiting both artificial and natural nonlinearities. In the present work, resonant conditions of antenna-coupled field-effect rectifiers have been identified in a terahertz experiment based on the well-established GaAs transistor technology. Rectification of free-space radiation has been observed in a broad 0.15–0.40 THz range by implementing quasi-optical coupling with a substrate lens to an AlGaAs/GaAs heterostructure transistor into the gap of a cross-dipole antenna. The short- and the open-circuit resonances have been clearly identified through a comparison between experimental photocurrent spectra, electromagnetic simulations, and antenna models. The former depends only on the dipole antenna geometry, while the latter is determined by the impedance matching between the antenna and the integrated device and, as such, can be even tuned to the desired frequency by applying a dc gate bias. In addition, the high-mobility two-dimensional electron gas supports plasma wave cavity resonances featuring natural hydrodynamic nonlinearity. The resonant peaks corresponding to the different rectification mechanisms have been identified and discussed in terms of simple lumped-element models. The demonstrated concepts are extrapolated toward infrared frequencies, where novel application demands and novel two-dimensional electron gas materials for antenna-coupled rectifiers are emerging

Normal Incidence Excitation of Out-of-Plane Lattice Resonances in Bipartite Arrays of Metallic Nanostructures

As a result of their coherent interaction, two-dimensional periodic arrays of metallic nanostructures support collective modes commonly known as lattice resonances. Among them, out-of-plane lattice resonances, for which the nanostructures are polarized in the direction perpendicular to the array, are particularly interesting since their unique configuration minimizes radiative losses. Consequently, these modes present extremely high quality factors and field enhancements that make them ideal for a wide range of applications. However, for the same reasons, their excitation is very challenging and has only been achieved at oblique incidence, which adds a layer of complexity to experiments and poses some limitations on their usage. Here, we present an approach to excite out-of-plane lattice resonances in bipartite arrays under normal incidence. Our method is based on exploiting the electric-magnetic coupling between the nanostructures, which has been traditionally neglected in the characterization of arrays made of metallic nanostructures. Using a rigorous coupled dipole model, we demonstrate that this coupling provides a general mechanism to excite out-of-plane lattice resonances under normal incidence conditions. We complete our study with a comprehensive analysis of a potential implementation of our results using an array of nanodisks with the inclusion of a substrate and a coating. This work provides an efficient approach for the excitation of out-of-plane lattice resonances at normal incidence, thus paving the way for the leverage of the extraordinary properties of these optical modes in a wide range of applications

Hybridization in Three Dimensions: A Novel Route toward Plasmonic Metamolecules

Plasmonic metamolecules have received much interest in the last years because they can produce a wide spectrum of different hybrid optical resonances. Most of the configurations presented so far, however, considered planar resonators lying on a dielectric substrate. This typically yields high damping and radiative losses, which severely limit the performance of the system. Here we show that these limits can be overcome by considering a 3D arrangement made from slanted nanorod dimers extruding from a silver baseplate. This configuration mimics an out-of-plane split ring resonator capable of a strong near-field interaction at the terminations and a strong diffractive coupling with nearby nanostructures. Compared to the corresponding planar counterparts, higher values of electric and magnetic fields are found (about a factor 10 and a factor 3, respectively). High-quality-factor resonances (<i>Q</i> ≈ 390) are produced in the mid-IR as a result of the efficient excitation of collective modes in dimer arrays

Hybridization in Three Dimensions: A Novel Route toward Plasmonic Metamolecules

Plasmonic metamolecules have received much interest in the last years because they can produce a wide spectrum of different hybrid optical resonances. Most of the configurations presented so far, however, considered planar resonators lying on a dielectric substrate. This typically yields high damping and radiative losses, which severely limit the performance of the system. Here we show that these limits can be overcome by considering a 3D arrangement made from slanted nanorod dimers extruding from a silver baseplate. This configuration mimics an out-of-plane split ring resonator capable of a strong near-field interaction at the terminations and a strong diffractive coupling with nearby nanostructures. Compared to the corresponding planar counterparts, higher values of electric and magnetic fields are found (about a factor 10 and a factor 3, respectively). High-quality-factor resonances (<i>Q</i> ≈ 390) are produced in the mid-IR as a result of the efficient excitation of collective modes in dimer arrays

Radially and Azimuthally Pure Vortex Beams from Phase-Amplitude Metasurfaces

To exploit the full potential of the transverse spatial structure of light using the Laguerre–Gaussian basis, it is necessary to control the azimuthal and radial components of the photons. Vortex phase elements are commonly used to generate these modes of light, offering precise control over the azimuthal index but neglecting the radially dependent amplitude term, which defines their associated corresponding transverse profile. Here, we experimentally demonstrate the generation of high-purity Laguerre–Gaussian beams with a single-step on-axis transformation implemented with a dielectric phase-amplitude metasurface. By vectorially structuring the input beam and projecting it onto an orthogonal polarization basis, we can sculpt any vortex beam in phase and amplitude. We characterize the azimuthal and radial purities of the generated vortex beams, reaching a purity of 98% for a vortex beam with l =50 and p = 0. Furthermore, we comparatively show that the purity of the generated vortex beams outperforms those generated with other well-established phase-only metasurface approaches. In addition, we highlight the formation of “ghost” orbital angular momentum orders from azimuthal gratings (analogous to ghost orders in ruled gratings), which have not been widely studied to date. Our work brings higher-order vortex beams and their unlimited potential within reach of wide adoption

Optical vortex crystals with dynamic topologies

Vortex crystals are geometric arrays of vortices found in various physics fields, owing their regular internal structure to mutual interactions within a spatially confined system. In optics, vortex crystals may form spontaneously within a nonlinear resonator but their usefulness is limited by the lack of control over their topology. On the other hand, programmable devices used in free space, like spatial light modulators, allow the design of nearly arbitrary vortex distributions but without any intrinsic dynamics. By combining non-Hermitian optics with on-demand topological transformations enabled by metasurfaces, we report a solid-state laser that generates vortex crystals with mutual interactions and actively-tunable topologies. We demonstrate 10x10 coherent vortex arrays with nonlocal coupling networks that are not limited to nearest-neighbor coupling but rather dictated by the crystal's topology. The vortex crystals exhibit sharp Bragg diffraction peaks, witnessing their coherence and high topological charge purity, which we resolve spatially over the whole lattice by introducing a parallelized analysis technique. By structuring light at the source, we enable complex transformations that allow to arbitrarily partition the orbital angular momentum inside the cavity and to heal topological charge defects, making these resonators a robust and versatile tool for advanced applications in topological optics

All-optical Reconfiguration of Ultrafast Dichroism in Gold Metasurfaces

Optical metasurfaces have come into the spotlight as a promising platform for light manipulation at the nanoscale, including ultrafast all-optical control via excitation with femtosecond laser pulses. Recently, dichroic metasurfaces have been exploited to modulate the polarization state of light with unprecedented speed. Here, we theoretically predict and experimentally demonstrate by pump-probe spectroscopy the capability to reconfigure the ultrafast dichroic signal of a gold metasurface by simply acting on the polarization of the pump pulse, which is shown to reshape the spatio-temporal distribution of the optical perturbation. The photoinduced anisotropic response, driven by out-of-equilibrium carriers and extinguished in a sub-picosecond temporal window, is readily controlled in intensity by tuning the polarization direction of the excitation up to a full sign reversal. This work proves that nonlinear metasurfaces offer the flexibility to tailor their ultrafast optical response in a fully all-optically reconfigurable platform

Fractal-Like Plasmonic Metamaterial with a Tailorable Plasma Frequency in the near-Infrared

In this work, we show that modulating the fractal dimension of nanoporous gold allows its effective dielectric response to be tailored over a wide spectral range of infrared wavelengths. In particular, the plasma edge and effective plasma frequency depend linearly on the fractal dimension, which can be controlled by varying the pore and ligament sizes. Importantly, the fractal porous metal exhibits superior plasmonic properties compared to its bulk counterpart. These properties, combined with a longer skin depth on the order of 100–200 nm, enables the penetration of optical energy deep into the nanopores where molecules can be loaded, thus, achieving more effective light–matter coupling. These findings may open new pathways to engineering the optical response of fractal-like or self-similar metamaterials without the need for sophisticated lithographic patterning

Nanocrystal Film Patterning by Inhibiting Cation Exchange via Electron-Beam or X‑ray Lithography

In this Letter we report patterning of colloidal nanocrystal films that combines direct e-beam (electron beam) writing with cation exchange. The e-beam irradiation causes cross-linking of the ligand molecules present at the nanocrystal surface, and the cross-linked molecules act as a mask for further processing. Consequently, in the following step of cation exchange, which is performed by directly dipping the substrate in a solution containing the new cations, the regions that have not been exposed to the electron beam are chemically transformed, while the exposed ones remain unchanged. This selective protection allows the design of patterns that are formed by chemically different nanocrystals, yet in a homogeneous nanocrystal film. Spatially resolved compositional analysis by energy-dispersive X-ray spectroscopy (EDS) corroborates that the selective exchange occurs only in the nonirradiated regions. We demonstrate the utility of this lithography approach by fabricating conductive wires and luminescent patterns in CdSe/CdS nanocrystal films by converting nonirradiated regions to Cu_2–<i>x</i>Se/Cu_2–<i>x</i>S. Furthermore, we show that X-ray irradiation too can lead to protection from cation exchange

Surface-Enhanced Infrared Spectroscopy Using Nanometer-Sized Gaps

We report on the near-field coupling of individual gold nanoantennas arranged in tip-to-tip dimer configuration, leading to strong electromagnetic field enhancements in the infrared, which is of great interest for sensing applications such as surface-enhanced infrared spectroscopy. We quantitatively evaluated the enhancement of vibrational excitations of a 5 nm thick test layer of 4,4′-bis(N-carbazolyl)-1,1′-biphenyl as a function of different gap sizes. The dimers with the smallest gaps under investigation (∼3 nm) lead to more than 1 order of magnitude higher signal enhancement with respect to gaps of 50 nm width. The comparison of experimental data and finite-difference time-domain simulations reveals a nonperfect filling of the gaps with sizes below 10 nm, which means that morphological information on the nanoscale is obtained additionally to chemical information