Transient guided-mode resonance metasurfaces with phase-transition materials.

We investigate transient, photo-thermally induced metasurface effects in a planar thin-film multilayer based on a phase-transition material. Illumination of a properly designed multilayer with two obliquely incident and phase-coherent pulsed pumps induces a transient and reversible temperature pattern in the phase-transition layer. The deep periodic modulation of the refractive index, caused by the interfering pumps, produces a transient Fano-like spectral feature associated with a guided-mode resonance. A coupled opto-thermal model is employed to analyze the temporal dynamics of the transient metasurface and to evaluate its speed and modulation capabilities. Using near-infrared pump pulses with peak intensities of the order of 100 MW/cm2 and duration of a few picoseconds, we find that the characteristic time scale of the transient metasurface is of the order of nanoseconds. Our results indicate that inducing transient metasurface effects in films of phase-transition materials can lead to new opportunities for dynamic control of quality (Q)-factor in photonic resonances, and for light modulation and switching.

An All-Dielectric Polaritonic Metasurface with a Giant Nonlinear Optical Response.

Enhancing the efficiency of second-harmonic generation using all-dielectric metasurfaces to date has mostly focused on electromagnetic engineering of optical modes in the meta-atom. Further advances in nonlinear conversion efficiencies can be gained by engineering the material nonlinearities at the nanoscale, however this cannot be achieved using conventional materials. Semiconductor heterostructures that support resonant nonlinearities using quantum engineered intersubband transitions can provide this new degree of freedom. By simultaneously optimizing the heterostructures and meta-atoms, we experimentally realize an all-dielectric polaritonic metasurface with a maximum second-harmonic generation power conversion factor of 0.5 mW/W2 and power conversion efficiencies of 0.015% at nominal pump intensities of 11 kW/cm2. These conversion efficiencies are higher than the record values reported to date in all-dielectric nonlinear metasurfaces but with 3 orders of magnitude lower pump power. Our results therefore open a new direction for designing efficient nonlinear all-dielectric metasurfaces for new classical and quantum light sources.

Design of vanadium-dioxide-based resonant structures for tunable optical response.

Phase change materials are suitable for tunable photonic devices where the optical response can be altered under external stimuli, such as heat, an electrical or an optical signal. In this scenario, we performed numerical simulations to study the optical properties of a flat unpatterned resonant structure and a grating, both coated with a thin film of vanadium dioxide (VO2). Our results suggest that it is possible to modulate broadband and narrowband reflectance spectra of the resonant structures in the visible to near-infrared range by more than 40 % when the VO2 undergoes an insulator-to-metal phase transition. Resonant devices with a tunable spectral response may find application in sensors, filters, absorbers, and detectors.

Strong Coupling in All-Dielectric Intersubband Polaritonic Metasurfaces.

Mie-resonant dielectric metasurfaces are excellent candidates for both fundamental studies related to light-matter interactions and for numerous applications ranging from holography to sensing to nonlinear optics. To date, however, most applications using Mie metasurfaces utilize only weak light-matter interaction. Here, we go beyond the weak coupling regime and demonstrate for the first time strong polaritonic coupling between Mie photonic modes and intersubband (ISB) transitions in semiconductor heterostructures. Furthermore, along with demonstrating ISB polaritons with Rabi splitting as large as 10%, we also demonstrate the ability to tailor the strength of strong coupling by engineering either the semiconductor heterostructure or the photonic mode of the resonators. Unlike previous plasmonic-based works, our new all-dielectric metasurface approach to generate ISB polaritons is free from ohmic losses and has high optical damage thresholds, thereby making it ideal for creating novel and compact mid-infrared light sources based on nonlinear optics.

Nested plasmonic resonances: extraordinary enhancement of linear and nonlinear interactions.

Plasmonic resonators can provide large local electric fields when the gap between metal components is filled with an ordinary dielectric. We consider a new concept consisting of a hybrid nanoantenna obtained by introducing a resonant, plasmonic nanoparticle strategically placed inside the gap of an aptly sized metallic antenna. The system exhibits two nested, nearly overlapping plasmonic resonances whose signature is a large field enhancement at the surface and within the bulk of the plasmonic nanoparticle that leads to unusually strong, linear and nonlinear light-matter coupling.

Experimental verification of epsilon-near-zero plasmon polariton modes in degenerately doped semiconductor nanolayers.

We investigate optical polariton modes supported by subwavelength-thick degenerately doped semiconductor nanolayers (e.g. indium tin oxide) on glass in the epsilon-near-zero (ENZ) regime. The dispersions of the radiative (R, on the left of the light line) and non-radiative (NR, on the right of the light line) ENZ polariton modes are experimentally measured and theoretically analyzed through the transfer matrix method and the complex-frequency/real-wavenumber analysis, which are in remarkable agreement. We observe directional near-perfect absorption using the Kretschmann geometry for incidence conditions close to the NR-ENZ polariton mode dispersion. Along with field enhancement, this provides us with an unexplored pathway to enhance nonlinear optical processes and to open up directions for ultrafast, tunable thermal emission.

Role of antenna modes and field enhancement in second harmonic generation from dipole nanoantennas.

We study optical second harmonic generation from metallic dipole antennas with narrow gaps. Enhancement of the fundamental-frequency field in the gap region plays a marginal role on conversion efficiency. In the symmetric configuration, i.e., with the gap located at the center of the antenna axis, reducing gap size induces a significant red-shift of the maximum conversion efficiency peak. Either enhancement or inhibition of second-harmonic emission may be observed as gap size is decreased, depending on the antenna mode excited at the harmonic frequency. The second-harmonic signal is extremely sensitive to the asymmetry introduced by gap's displacements with respect to the antenna center. In this situation, second-harmonic light can couple to all the available antenna modes. We perform a multipolar analysis that allows engineering the far-field SH emission and find that the interaction with quasi-odd-symmetry modes generates radiation patterns with a strong dipolar component.

Nonlinear spin-orbit coupling in optical thin films.

Tunable generation of vortex beams holds relevance in various fields, including communications and sensing. In this paper, we demonstrate the feasibility of nonlinear spin-orbit interactions in thin films of materials with second-order nonlinear susceptibility. Remarkably, the nonlinear tensor can mix the longitudinal and transverse components of the pump field. We observe experimentally our theoretical predictions in the process of second-harmonic generation from a thin film of aluminum gallium arsenide, a material platform widely spread for its role in the advancement of active, nonlinear, and quantum photonic devices. In particular, we prove that a nonlinear thin film can be used to produce vector vortex beams of second-harmonic light when excited by circularly-polarized Gaussian beams.

Wideband plasmonic beam steering in metal gratings.

We demonstrate controllable light deflection in thick metal gratings with periodic subwavelength slits filled with an active material. Under specific illumination conditions, the grating becomes nearly transparent and acts as a uniform optical phased-array antenna where the phase of the radiating elements is controlled by modifying the index of refraction of the material that fills each slit. The beam-steering operational regime occurs in a wide wavelength band, and it is relatively insensitive to the input angle.

Deep-subwavelength waveguiding via inhomogeneous second-harmonic generation.

We theoretically investigate second-harmonic generation in extremely narrow, subwavelength semiconductor and dielectric waveguides. We discuss a guiding mechanism characterized by the inhibition of diffraction and the suppression of cutoff limits in the context of a light trapping phenomenon that sets in under conditions of general phase and group velocity mismatch between the fundamental and the generated harmonic.

Experimental demonstration of a novel bio-sensing platform via plasmonic band gap formation in gold nano-patch arrays.

In this paper we discuss the possibility of implementing a novel bio-sensing platform based on the observation of the shift of the leaky surface plasmon mode that occurs at the edge of the plasmonic band gap of metal gratings, when an analyte is deposited on top of the metallic structure. We report numerical calculations, fabrication and experimental measurements to prove the sensing capability of a two-dimensional array of gold nano-patches in the detection of a small quantity of Isopropyl Alcohol (IPA) deposited on top of sensor surface. The calculated sensitivity of our device approaches a value of 1000 nm/RIU with a corresponding Figure of Merit (FOM) of 222 RIU(-1). The presence of IPA can also be visually estimated by observing a color variation in the diffracted field. We show that color brightness and intensity variations can be ascribed to a change in the aperture size, keeping the periodicity constant, and to different types of analyte deposited on the sample, respectively. Moreover, we demonstrate that unavoidable fabrication imperfections revealed by the presence of rounded corners and surface roughness do not significantly affect device performance.

Nonlinear response of GaAs gratings in the extraordinary transmission regime.

We theoretically describe a way to enhance harmonic generation from subwavelength slits milled on semiconductor substrates in strongly absorptive regimes. The metal-like response typical of semiconductors, like GaAs and GaP, triggers enhanced transmission and nonlinear optical phenomena in the deep UV range. We numerically study correlations between linear and nonlinear responses and their intricacies in infinite arrays, and highlight differences between nonlinear surface and magnetic sources, and intrinsic χ((2)) and χ((3)) contributions to harmonic generation. The results show promising efficiencies at wavelengths below 120 nm, and reveal coupling of TE and TM polarizations for pump and harmonic signals. A downconversion process that can regenerate pump photons with polarization orthogonal to the incident pump is also discussed.

Plasmonic bandgap formation in two-dimensional periodic arrangements of gold patches with subwavelength gaps.

We report on the formation of plasmonic bandgaps in two-dimensional periodic arrangements of gold patches. Orthogonal arrays of subwavelength slits with different periodicities have been studied by means of a three-dimensional finite-difference time-domain (FDTD) code, changing incident polarization and geometrical parameters. Spectral response of gold patches having different a form factor and surrounded by different media have been also investigated and compared in order to give a full description of bandgap shifts paving the way for the design of polarization-sensitive devices.

Stimuli-Responsive Phase Change Materials: Optical and Optoelectronic Applications.

Stimuli-responsive materials offer a large variety of possibilities in fabrication of solid- state devices. Phase change materials (PCMs) undergo rapid and drastic changes of their optical properties upon switching from one crystallographic phase to another one. This peculiarity makes PCMs ideal candidates for a number of applications including sensors, active displays, photonic volatile and non-volatile memories for information storage and computer science and optoelectronic devices. This review analyzes different examples of PCMs, in particular germanium-antimonium tellurides and vanadium dioxide (VO2) and their applications in the above-mentioned fields, with a detailed discussion on potential, limitations and challenges.

Tunable unidirectional nonlinear emission from transition-metal-dichalcogenide metasurfaces.

Nonlinear light sources are central to a myriad of applications, driving a quest for their miniaturisation down to the nanoscale. In this quest, nonlinear metasurfaces hold a great promise, as they enhance nonlinear effects through their resonant photonic environment and high refractive index, such as in high-index dielectric metasurfaces. However, despite the sub-diffractive operation of dielectric metasurfaces at the fundamental wave, this condition is not fulfilled for the nonlinearly generated harmonic waves, thereby all nonlinear metasurfaces to date emit multiple diffractive beams. Here, we demonstrate the enhanced single-beam second- and third-harmonic generation in a metasurface of crystalline transition-metal-dichalcogenide material, offering the highest refractive index. We show that the interplay between the resonances of the metasurface allows for tuning of the unidirectional second-harmonic radiation in forward or backward direction, not possible in any bulk nonlinear crystal. Our results open new opportunities for metasurface-based nonlinear light-sources, including nonlinear mirrors and entangled-photon generation.

Energy considerations for a superlens based on metal/dielectric multilayers.

We investigate the resolution and absorption losses of a Ag/GaP multilayer superlens. For a fixed source to image distance the resolution is independent of the position of the lens but the losses depend strongly on the lens placement. The absorption losses associated with the evanescent waves can be significantly larger than losses associated with the propagating waves especially when the superlens is close to the source. The interpretation of transmittance values greater than unity for evanescent waves is clarified with respect to the associated absorption losses.

Viscoelastic optical nonlocality of low-loss epsilon-near-zero nanofilms.

Optical nonlocalities are elusive and hardly observable in traditional plasmonic materials like noble and alkali metals. Here we report experimental observation of viscoelastic nonlocalities in the infrared optical response of epsilon-near-zero nanofilms made of low-loss doped cadmium-oxide. The nonlocality is detectable thanks to the low damping rate of conduction electrons and the virtual absence of interband transitions at infrared wavelengths. We describe the motion of conduction electrons using a hydrodynamic model for a viscoelastic fluid, and find excellent agreement with experimental results. The electrons' elasticity blue-shifts the infrared plasmonic resonance associated with the main epsilon-near-zero mode, and triggers the onset of higher-order resonances due to the excitation of electron-pressure modes above the bulk plasma frequency. We also provide evidence of the existence of nonlocal damping, i.e., viscosity, in the motion of optically-excited conduction electrons using a combination of spectroscopic ellipsometry data and predictions based on the viscoelastic hydrodynamic model.

Negative refraction and sub-wavelength focusing in the visible range using transparent metallo-dielectric stacks.

We numerically demonstrate negative refraction of the Poynting vector and sub-wavelength focusing in the visible part of the spectrum using a transparent multilayer, metallo-dielectric photonic band gap structure. Our results reveal that in the wavelength regime of interest evanescent waves are not transmitted by the structure, and that the main underlying physical mechanisms for sub-wavelength focusing are resonance tunneling, field localization, and propagation effects. These structures offer several advantages: tunability and high transmittance (50% or better) across the visible and near IR ranges; large object-image distances, with image planes located beyond the range where the evanescent waves have decayed. From a practical point of view, our findings point to a simpler way to fabricate a material that exhibits negative refraction and maintains high transparency across a broad wavelength range. Transparent metallo-dielectric stacks also provide an opportunity to expand the exploration of wave propagation phenomena in metals, both in the linear and nonlinear regimes.

Gap solitons in a nonlinear quadratic negative-index cavity.

We predict the existence of gap solitons in a nonlinear, quadratic Fabry-Pérot negative index cavity. A peculiarity of a single negative index layer is that if magnetic and electric plasma frequencies are different it forms a photonic band structure similar to that of a multilayer stack composed of ordinary, positive index materials. This similarity also results in comparable field localization and enhancement properties that under appropriate conditions may be used to either dynamically shift the band edge, or for efficient energy conversion. We thus report that an intense, fundamental pump pulse is able to shift the band edge of a negative index cavity, and make it possible for a weak second harmonic pulse initially tuned inside the gap to be transmitted, giving rise to a gap soliton. The process is due to cascading, a well-known phenomenon that occurs far from phase matching conditions that limits energy conversion rates, it resembles a nonlinear third-order process, and causes pulse compression due to self-phase modulation. The symmetry of the equations of motion under the action of either an electric or a magnetic nonlinearity suggests that both nonlinear polarization and magnetization, or a combination of both, can lead to solitonlike pulses. More specifically, the antisymmetric localization properties of the electric and magnetic fields cause a nonlinear polarization to generate a dark soliton, while a nonlinear magnetization spawns a bright soliton.

Enhanced second-harmonic generation from two-dimensional MoSe2 on a silicon waveguide.

Two-dimensional transition-metal dichalcogenides (TMDCs) with intrinsically broken crystal inversion symmetry and large second-order nonlinear responses have shown great promise for future nonlinear light sources. However, the sub-nanometer monolayer thickness of such materials limits the length of their nonlinear interaction with light. Here, we experimentally demonstrate the enhancement of the second-harmonic generation from monolayer MoSe2 by its integration onto a 220-nm-thick silicon waveguide. Such on-chip integration allows for a marked increase in the interaction length between the MoSe2 and the waveguide mode, further enabling phase matching of the nonlinear process. The demonstrated TMDC-silicon photonic hybrid integration opens the door to second-order nonlinear effects within the silicon photonic platform, including efficient frequency conversion, parametric amplification and the generation of entangled photon pairs.