Second harmonic generation (SHG) is one of the most studied nonlinear optical phenomenon thanks to the its extensive practical applications. Efficient SHG can be achieved by either improving light-matter interaction or searching for materials with higher nonlinear susceptibility values. Together with nonlinear process efficiency, low losses and damage thresholds are desirable, as well as large operating bandwidth, scalability, integrability and ease of fabrication. We demonstrate that a hybrid approach that combines dielectric metasurfaces with semiconductor quantum well can satisfy all the aforementioned conditions, paving the way for novel designs of low loss, broadband and efficient ultrathin nonlinear optical devices. Since the advent of laser, SHG has been extensively studied for numerous applications [1]. Because of the typically weak nonlinear response of materials, efficient SHG can be achieved only under phase matching conditions and using high power lasers. However, there is still a need to achieve efficient SHG at moderate pump intensities using ultrathin nonlinear optical devices [2]. Several approaches have been proposed, such as the use of two-dimensional transition metal dichalcogenides [3-6] and multi-quantumwell (multi-QW) semiconductor heterostructures [7-9]. In fact, ISTs allow to engineer extremely large χ(2) values [10] and therefore can be used as nonlinear media for SHG in ultrathin nonlinear devices where phase matching conditions are not required. Nonlinear processes can be also boosted increasing lightmatter interactions, i.e. confining electromagnetic fields to subwavelength volumes using resonators, metasurfaces or photonic crystals [11-18]. So far, the highest nonlinear response for SHG (normalized by length) by a nanostructure has been obtained at mid-infrared wavelengths using a hybrid approach which couples plasmonic metasurfaces to ISTs of semiconductor QWs [10, 19-21]. Although efficient, this approach suffers from the use of plasmonic nanostructures for coupling the pump light to the ISTs and out-coupling the second-harmonic (SH) light, which have high dissipative losses that induce heating effects and lowering of damage thresholds. Another possible strategy involves the use of Mie-like resonances in all-dielectric metasurfaces fabricated from nonlinear crystals such as GaAs [13-16]. This method solves the problems of metal absorption but they have intrinsically low χ(2) and sustain only modest field enhancement. High field enhancement in all-dielectric metasurfaces can be attained by exploiting resonances with higher quality factors (Q) such as Fano resonances and bound states in the continuum. However, a higher Q significantly reduces the spectral bandwidth of the nonlinear device. The inverse relation between Q and spectral bandwidth is a fundamental limitation that dictates the bandwidth of SHG in a nonlinear device. Here we experimentally demonstrate a new hybrid approach for high efficiency SHG that overcomes the limitations described above by combining the advantages of IST nonlinear metasurfaces and alldielectric nonlinear metasurfaces. Our method uses high-Q leaky mode resonances (LMRs) [22-24] in dielectric nanostructures coupled to ISTs of semiconductor QWs. Our all-dielectric device has small dissipative losses and therefore a high damage threshold. Furthermore, since the LMR wavelength can be varied by changing the period of the grating, the high Q resonances can be tuned to fit the wavelength Contributo presentato alla Riunione GTTI-SIEm, Pavia 26-28 giugno 2019

A prominent nonlinear optical phenomenon that is extensively studied using nanostructured materials is second-harmonic generation (SHG) as it has applications in various fields. Achieving efficient SHG from a nanostructure requires a large second-order nonlinear susceptibility of the material system and large electromagnetic fields. For practical applications, the nanostructures should also have low losses, high damage thresholds, large bandwidths, wavelength scalability, dual mode operation in transmission and reflection, monolithic integrability, and ease of fabrication. While various approaches have demonstrated efficient SHG, to the best of our knowledge, none have demonstrated all these desired qualities simultaneously. Here, we present a hybrid approach for realizing efficient SHG in an ultrathin dielectric semiconductor nonlinear device with all the above-mentioned desired properties. Our approach uses high quality factor leaky mode resonances in dielectric metasurfaces that are coupled to intersubband transitions of semiconductor quantum wells. Using our device, we demonstrate SHG at pump wavelengths ranging from 8.5 to 11 mu m, with a maximum second-harmonic nonlinear conversion factor of 1.1 mW/W-2 and maximum second-harmonic conversion efficiency of 2.5 x 10(-5) at modest pump intensities of 10 kW/cm(2). Our results open a new direction for designing low loss, broadband, and efficient ultrathin nonlinear optical devices.

We study semiconductor hyperbolic metamaterials (SHMs) at the quantum limit experimentally using spectroscopic ellipsometry as well as theoretically using a new microscopic theory. The theory is a combination of microscopic density matrix approach for the material response and Green’s function approach for the propagating electric field. Our approach predicts absorptivity of the full multilayer system and for the first time allows the prediction of in-plane and out-of-plane dielectric functions for every individual layer constructing the SHM as well as effective dielectric functions that can be used to describe a homogenized SHM.

We report power limiting metasurfaces based on saturable absorption of the intersubband transition in n-doped multi-quantum wells. Experimentally, we note a 20% decrease in reflectivity with an increase of the pump intensity.

We experimentally demonstrate a novel approach of using coupling between a leaky mode resonance and intersubband transitions in semiconductor quantum wells to realize a hybrid dielectric-semiconductor metasurface with high second-harmonic conversion efficiency and increased bandwidth.

We report difference-frequency generation in nonlinear polaritonic metasurfaces based on coupling of electromagnetic nanocavity modes with intersubband nonlinearities in semiconductor heterostructures. Nonlinear susceptibility of 2.7×105 pm/V and the conversion efficiency of 0.13% were measured.

We report the design and operation of a nonlinear intersubband polaritonic metasurface for mid-infrared second harmonic generation. The metasurface is made of plasmonic nanoresonators filled with a multiple-quantum-well semiconductor heterostructure. Unlike the previously reported nonlinear intersubband polaritonic metasurfaces that employ full-metal backplanes below the etched metal–semiconductor nanoresonators, the metasurface reported here employs an incomplete backplane that is complementary to the pattern of the top metallization of the etched semiconductor heterostructure nanoresonators. The new approach produces high-electric-field localization and enhancement in the nanoresonators, while requiring simplified fabrication and allowing the metasurface to operate in both transmission and reflection regimes.

All-dielectric metasurfaces made from arrays of high index nanoresonators supporting strong magnetic dipole modes have emerged as a low-loss alternative to plasmonic metasurfaces. Here we use oxygen-doped single-walled carbon nanotubes (SWCNTs) as quantum emitters and couple them to silicon metasurfaces to study effects of the magnetic dipole modes of the constituent nanoresonators on the photoluminescence (PL) of individual SWCNTs. We find that when in resonance, the magnetic mode of the silicon nanoresonators can lead to a moderate average PL enhancement of 0.8-4.0 of the SWCNTs, accompanied by an average increase in the radiative decay rate by a factor of 1.5-3.0. More interestingly, single dopant polarization experiments show an anomalous photoluminescence polarization rotation by coupling individual SWCNTs to silicon nanoresonators. Numerical simulations indicate that this is caused by modification of near-field polarization distribution at certain areas in the proximity of the silicon nanoresonators at the excitation wavelength, thus presenting an approach to control emission polarization. These findings indicate silicon nanoresonators as potential building blocks of quantum photonic circuits capable of manipulating PL intensity and polarization of single photon sources.

Dielectric metasurfaces that exploit the different Mie resonances of nanoscale dielectric resonators are a powerful platform for manipulating electromagnetic fields and can provide novel optical behavior. In this work, we experimentally demonstrate independent tuning of the magnetic dipole resonances relative to the electric dipole resonances of split dielectric resonators (SDRs). By increasing the split dimension, we observe a blue shift of the magnetic dipole resonance toward the electric dipole resonance. Therefore, SDRs provide the ability to directly control the interaction between the two dipole resonances within the same resonator. For example, we achieve the first Kerker condition by spectrally overlapping the electric and magnetic dipole resonances and observe significantly suppressed backward scattering. Moreover, we show that a single SDR can be used as an optical nanoantenna that provides strong unidirectional emission from an electric dipole source.

Gradient metasurfaces, or ultrathin optical components with engineered transverse impedance gradients along the surface, are able to locally control the phase and amplitude of the scattered fields over subwavelength scales, enabling a broad range of linear components in a flat, integrable platform(1-4). On the contrary, due to the weakness of their nonlinear optical responses, conventional nonlinear optical components are inherently bulky, with stringent requirements associated with phase matching and poor control over the phase and amplitude of the generated beam. Nonlinear metasurfaces have been recently proposed to enable frequency conversion in thin films without phase-matching constraints and subwavelength control of the local nonlinear phase(5-8). However, the associated optical nonlinearities are far too small to produce significant nonlinear conversion efficiency and compete with conventional nonlinear components for pump intensities below the materials damage threshold. Here, we report multi-quantum-well based gradient nonlinear metasurfaces with second-order nonlinear susceptibility over 10(6) pm/V for second harmonic generation at a fundamental pump wavelength of 10 mu m, 5-6 orders of magnitude larger than traditional crystals. Further, we demonstrate the efficacy of this approach to designing metasurfaces optimized for frequency conversion over a large range of wavelengths, by reporting multi-quantum-well and metasurface structures optimized for a pump wavelength of 6.7 mu m. Finally, we demonstrate how the phase of this nonlinearly generated light can be locally controlled well below the diffraction limit using the Pancharatnam-Berry phase approach(5,7,9), opening a new paradigm for ultrathin, flat nonlinear optical components.

Second order nonlinearity vanishes for centrosymmetric materials in the dipole approximation. For metamaterial this means second-harmonic-generation is negligible in highly symmetric meta-atoms. We show a new type of meta-atom in which the dipolar approximation breaks down.

Optical nonlinearities are intimately related to the spatial symmetry of the nonlinear media. For example, the second order susceptibility vanishes for centrosymmetric materials under the dipole approximation. The latter concept has been naturally extended to the metamaterials’ realm, sometimes leading to the (erroneous) hypothesis that second harmonic (SH) generation is negligible in highly symmetric meta-atoms. In this work we aim to show that such symmetric meta-atoms can radiate SH light efficiently. In particular, we investigate in-plane centrosymmetric meta-atom designs where the approximation for meta-atoms breaks down. In a periodic array this building block allows us to control the directionality of the SH radiation. We conclude by showing that the use of symmetry considerations alone allows for the manipulation of the nonlinear multipolar response of a meta-atom, resulting in e.g. dipolar, quadrupolar, or multipolar emission on demand. This is because the size of the meta-atom is comparable with the free-space wavelength, thus invalidating the dipolar approximation for meta-atoms.

Narrow-bandgap semiconductors such as alloys of InAsAlSb and their heterostructures are considered promising candidates for next generation infrared photodetectors and devices. The prospect of actively tuning the spectral responsivity of these detectors at the pixel level is very appealing. In principle, this could be achieved with a tunable metasurface fabricated monolithically on the detector pixel. Here, we present first steps towards that goal using a complementary metasurface strongly coupled to an epsilon-near-zero (ENZ) mode operating in the long-wave region of the infrared spectrum. We fabricate such a coupled system using the same epitaxial layers used for infrared pixels in a focal plane array and demonstrate the existence of ENZ modes in high mobility layers of InAsSb. We confirm that the coupling strength between the ENZ mode and the metasurface depends on the ENZ layer thickness and demonstrate a transmission modulation on the order of 25%. We further show numerically the expected tunable spectral behavior of such coupled system under reverse and forward bias, which could be used in future electrically tunable detectors.

Strongly coupling metallic nanoresonators with specially designed intersubband-transitions in quantum-wells results in efficient, saturation-limited second-harmonic (SH) generation. This method also grants full control over the polarization and phase-front of the emitted SH radiation.

We use planar metamaterial resonators to enhance by more than two orders of magnitude the near infrared second harmonic generation obtained from intersubband transitions in III-Nitride heterostructures. The improvement arises from two factors: employing an asymmetric double quantum well design and aligning the resonators' cross-polarized resonances with the intersubband transition energies. The resulting nonlinear metamaterial operates at wavelengths where single photon detection is available, and represents a different class of sources for quantum photonics related phenomena.

Coherent superposition of light from subwavelength sources is an attractive prospect for the manipulation of the direction, shape and polarization of optical beams. This phenomenon constitutes the basis of phased arrays, commonly used at microwave and radio frequencies. Here we propose a new concept for phased-array sources at infrared frequencies based on metamaterial nanocavities coupled to a highly nonlinear semiconductor heterostructure. Optical pumping of the nanocavity induces a localized, phase-locked, nonlinear resonant polarization that acts as a source feed for a higher-order resonance of the nanocavity. Varying the nanocavity design enables the production of beams with arbitrary shape and polarization. As an example, we demonstrate two second harmonic phased-array sources that perform two optical functions at the second harmonic wavelength (∼5 μm): a beam splitter and a polarizing beam splitter. Proper design of the nanocavity and nonlinear heterostructure will enable such phased arrays to span most of the infrared spectrum.

We studied the temperature dependence of the photocurrent spectra of a Ge-SiO2 composite thin film. We found that the spectral position of the photocurrent peak is determined by the competition between absorption and non-radiative recombination and that its temperature dependence is associated with the population variation of the energetically deep levels in the system under “thermal quenching” conditions. Combining these results with our previous deep-level transient spectroscopy data enables the association of these levels with the quantum confinement effect. We thus identify here a non-radiative recombination process associated with deep-level sensitization that stems from quantum confinement.

We present an electrodynamic model of strongly coupled metamaterial/intersubband-transition systems that can be used to predict and maximize Rabi splittings. This model can also be used to optimize metamaterial structures that enhance second-order nonlinear processes.