Second harmonic generation at a time-varying interface.

Time-varying metamaterials rely on large and fast changes of the linear permittivity. Beyond the linear terms, however, the effect of a non-perturbative modulation of the medium on harmonic generation remains largely unexplored. In this work, we study second harmonic generation at an optically pumped time-varying interface between air and a 310 nm Indium Tin Oxide film.

Quantum electrodynamics of time-varying gratings.

Gratings which are in apparent motion reveal some startling properties for classical radiation, especially for luminal gratings traveling at or around the speed of light. We show here that their quantum properties are even more remarkable, their effective refractive index modeling the Schwarzschild singularity which as we show generates spontaneous Hawking radiation in correlated photon pairs. Subjected to external radiation, luminal gratings provoke stimulated emission of photon pairs which we propose as a possible means of observing Hawking radiation in the laboratory.

Overcoming losses in superlenses with synthetic waves of complex frequency.

Superlenses made of plasmonic materials and metamaterials can image features at the subdiffraction scale. However, intrinsic losses impose a serious restriction on imaging resolution, a problem that has hindered widespread applications of superlenses. Optical waves of complex frequency that exhibit a temporally attenuating behavior have been proposed to offset the intrinsic losses in superlenses through the introduction of virtual gain, but experimental realization has been lacking because of the difficulty of imaging measurements with temporal decay.

All-angle reflectionless negative refraction with ideal photonic Weyl metamaterials.

Negative refraction, an unnatural optical phenomenon in which the incident and the refracted waves reside on the same side of the surface normal, has been demonstrated with the invention of negative index media based on artificially engineered photonic structures called metamaterials. It has received wide attention due to its potential applications in imaging, nonlinear optics, and electromagnetic cloaking. However, it is highly challenging to realize negative refraction operating at all angles and with the perfect transmission.

Optical response of hyperbolic metamaterials with adsorbed nanoparticle arrays.

Experimental studies of have been recently performed to determine the optical effect of adsorption of arrays of gold nanoparticles, NPs (16 nm or 40 nm in diameter) on reflective substrates (Ma , , 2018, , 4604-4616; Ma , , 2020, , 328-336) and on transparent interfaces (Montelongo , , 2017, 16, 1127-1135). As predicted by the theory (Sikdar , , 2016, , 20486-20498), a reflection quenching effect was observed on the reflective substrates, in the frequency domain centred around the nanoparticle localised plasmon resonance. Those results showed a broad dip in reflectivity, which was deepening and red-shifting with increasing array densities.

Spatial coherence in 2D holography.

Holography is a long-established technique to encode an object's spatial information into a lower-dimensional representation. We investigate the role of the illumination's spatial coherence properties in the success of such an imaging system through point spread function and Fourier domain analysis. Incoherent illumination is shown to result in more robust imaging performance free of diffraction artifacts at the cost of incurring background noise and sacrificing phase retrieval.

Nanoparticle meta-grid for enhanced light extraction from light-emitting devices.

Based on a developed theory, we show that introducing a meta-grid of sub-wavelength-sized plasmonic nanoparticles (NPs) into existing semiconductor light-emitting-devices (LEDs) can lead to enhanced transmission of light across the LED-chip/encapsulant interface. This results from destructive interference between light reflected from the chip/encapsulant interface and light reflected by the NP meta-grid, which conspicuously increase the efficiency of light extraction from LEDs. The "meta-grid", should be inserted on top of a conventional LED chip within its usual encapsulating packaging.

Metamaterials with index ellipsoids at arbitrary k-points.

Propagation behaviors of electromagnetic waves are governed by the equifrequency surface of the medium. Up to now, ordinary materials, including the medium exist in nature and the man-made metamaterials, always have an equifrequency surface (ellipsoid or hyperboloid) centered at zero k-point. Here we propose a new type of metamaterial possessing multiple index ellipsoids centered at arbitrary nonzero k-points.

Broadband Tunable THz Absorption with Singular Graphene Metasurfaces.

By exploiting singular spatial modulations of the graphene conductivity, we design a broadband, tunable THz absorber whose efficiency approaches the theoretical upper bound for a wide absorption band with a fractional bandwidth of 185%. Strong field enhancement is exhibited by the modes of this extended structure, which is able to excite a wealth of high-order surface plasmons, enabling deeply subwavelength focusing of incident THz radiation. Previous studies have shown that the conductivity can be modulated at GHz frequencies, which might lead to the development of efficient high-speed broadband switching by an atomically thin layer.

Graphene as a Tunable Anisotropic or Isotropic Plasmonic Metasurface.

We demonstrate a tunable plasmonic metasurface by considering a graphene sheet subject to a periodically patterned doping level. The unique optical properties of graphene result in electrically tunable plasmons that allow for extreme confinement of electromagnetic energy in the technologically significant regime of THz frequencies. Here, we add an extra degree of freedom by using graphene as a metasurface, proposing to dope it with an electrical gate patterned in the micron or submicron scale.

Harnessing a Quantum Design Approach for Making Low-Loss Superlenses.

Recently, so-called "superlenses", made from metamaterials that are structured on a length scale much less than an optical wavelength, have shown impressive diffraction-beating image resolution, but they use materials with negative dielectric responses, and they absorb much of the light in a way that seriously degrades both the resolution and brightness of the image. Here we demonstrate an alternative "quantum metamaterials" (QM) approach that uses materials structured at the nanoscale, i.e.

van der Waals interactions at the nanoscale: the effects of nonlocality.

Calculated using classical electromagnetism, the van der Waals force increases without limit as two surfaces approach. In reality, the force saturates because the electrons cannot respond to fields of very short wavelength: polarization charges are always smeared out to some degree and in consequence the response is nonlocal. Nonlocality also plays an important role in the optical spectrum and distribution of the modes but introduces complexity into calculations, hindering an analytical solution for interactions at the nanometer scale.

Electron-energy loss study of nonlocal effects in connected plasmonic nanoprisms.

We investigate the emergence of nonlocal effects in plasmonic nanostructures through electron-energy loss spectroscopy. To theoretically describe the spatial dispersion in the metal permittivity, we develop a full three-dimensional nonlocal hydrodynamic solution of Maxwell's equations in frequency domain that implements the electron beam as a line current source. We use our numerical approach to perform an exhaustive analysis of the impact of nonlocality in the plasmonic response of single triangular prisms and connected bowtie dimers.

Hydrodynamic model for plasmonics: a macroscopic approach to a microscopic problem.

In this concept, we present the basic assumptions and techniques underlying the hydrodynamic model of electron response in metals and demonstrate that the model can be easily incorporated into computational models. We discuss the role of the additional boundary conditions that arise due to nonlocal terms in the modified equation of motion and the ultimate impact on nanoplasmonic systems. The hydrodynamic model captures much of the microscopic dynamics relating to the fundamental quantum mechanical nature of the electrons and reveals intrinsic limitations to the confinement and enhancement of light around nanoscale features.

Transformation-optics description of plasmonic nanostructures containing blunt edges/corners: from symmetric to asymmetric edge rounding.

The sharpness of corners/edges can have a large effect on the optical responses of metallic nanostructures. Here we deploy the theory of transformation optics to analytically investigate a variety of blunt plasmonic structures, including overlapping nanowire dimers and crescent-shaped nanocylinders. These systems are shown to support several discrete optical modes, whose energy and line width can be controlled by tuning the nanoparticle geometry.

Nonlocal effects in the nanofocusing performance of plasmonic tips.

The nanofocusing performance of plasmonic tips is studied analytically and numerically. The effects of electron-electron interactions in the dielectric response of the metal are taken into account through the implementation of a nonlocal, spatially dispersive, hydrodynamic permittivity. We demonstrate that spatial dispersion only slightly modifies the device parameters which maximize its field enhancement capabilities.

Nanoparticle-assisted stimulated-emission-depletion nanoscopy.

We show that metal nanoparticles can be used to improve the performance of super-resolution fluorescence nanoscopes based on stimulated-emission-depletion (STED). Compared with a standard STED nanoscope, we show theoretically a resolution improvement by more than an order of magnitude, or equivalently, depletion intensity reductions by more than 2 orders of magnitude and an even stronger photostabilization. Our scheme may allow improvement of existing STED nanoscopes and assist in the development of low-power, low-cost nanoscopes.

Revealing plasmonic gap modes in particle-on-film systems using dark-field spectroscopy.

Polarization-controlled excitation of plasmonic modes in nanometric Au particle-on-film gaps is investigated experimentally using single-particle dark-field spectroscopy. Two distinct geometries are explored: nanospheres on top of and inserted in a thin gold film. Numerical simulations reveal that the three resonances arising in the scattering spectra measured for particles on top of a film originate from highly confined gap modes at the interface.

Broadband time-reversal of optical pulses using a switchable photonic-crystal mirror.

Recently, Chumak et al. have demonstrated experimentally the time-reversal of microwave spin pulses based on non-adiabatically tuning the wave speed in a spatially-periodic manner [Nat. Comm.

Time reversal in dynamically tuned zero-gap periodic systems.

We show that short pulses propagating in zero-gap periodic systems can be reversed with 100% efficiency by using weak nonadiabatic tuning of the wave velocity at time scales that can be much slower than the period. Unlike previous schemes, we demonstrate reversal of broadband (few cycle) pulses with simple structures. Our scheme may thus open the way to time reversal in a variety of systems for which it was not accessible before.

Plasmonic hybridization between nanowires and a metallic surface: a transformation optics approach.

The interaction between metallic nanowires and a metal substrate is investigated by means of transformation optics. This plasmonic system is of particular interest for single molecule detection or nanolasers. By mapping such a plasmonic device onto a metal-insulator-metal infinite structure, its optical response can be fully derived analytically.

Macroscopic invisibility cloaking of visible light.

Invisibility cloaks, which used to be confined to the realm of fiction, have now been turned into a scientific reality thanks to the enabling theoretical tools of transformation optics and conformal mapping. Inspired by those theoretical works, the experimental realization of electromagnetic invisibility cloaks has been reported at various electromagnetic frequencies. All the invisibility cloaks demonstrated thus far, however, have relied on nano- or micro-fabricated artificial composite materials with spatially varying electromagnetic properties, which limit the size of the cloaked region to a few wavelengths.

Plasmonic interaction between overlapping nanowires.

The plasmonic interaction between overlapping nanowires with and without structure singularities is studied analytically and numerically. A conformal transformation approach is adopted to predict analytically the optical response of overlapping nanowires in the quasi-static limit. Surface plasmon excitations are shown to exhibit a lower bound cutoff frequency, which blue-shifts when the overlap distance increases.