Publications

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The color of light is a fundamental property of electromagnetic radiation; as such, control of the frequency is a cornerstone of modern optics. Nonlinear materials are typically used to generate new frequencies, however the use of time-variant systems provides an alternative approach. Utilizing a metasurface that supports a high-quality factor resonance, we demonstrate that a rapidly shifting refractive index will induce frequency conversion of light that is confined in the nanoresonator meta-atoms. We experimentally observe this frequency conversion and develop a time-dependent coupled mode theory model that well describes the system. The intersection of high quality-factor resonances, active materials, and ultrafast transient spectroscopy leads to the demonstration of metasurfaces operating in a time-variant regime that enables enhanced control over light-matter interaction.

Quantum well intersubband polaritons are traditionally studied in large scale ensembles, over many wavelengths in size.In this presentation, we demonstrate that it is possible to detect and investigate intersubband polaritons in a single sub-wavelength nanoantenna in the IR frequency range. We observe polariton formation using a scattering-type near-fieldmicroscope and nano-FTIR spectroscopy. In this work, we will discuss near-field spectroscopic signatures of plasmonic antennae withand without coupling to the intersubband transition in quantum wells located underneath the antenna. Evanescent fieldamplitude spectra recorded on the antenna surface show a mode anti-crossing behavior in the strong coupling case. Wealso observe a corresponding strong-coupling signature in the phase of the detected field. We anticipate that this near-fieldapproach will enable explorations of strong and ultrastrong light-matter coupling in the single nanoantenna regime,including investigations of the elusive effect of ISB polariton condensation.

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In this work, we investigate cascaded third harmonic generation in a dielectric metasurface by exploiting high quality factor Fano resonances obtained using broken symmetry unit cells.

Toroidal dielectric metasurface with a Q-factor of 728 in 1500 nm wavelength are reported. The resonance couples strongly to the environment, as demonstrated with a refractometric sensing experiment.

We design a resonant metasurface that uses Mie quadrupole modes to suppress the-1 diffraction order. We show that this suppression can be spectrally tuned using optical pumping on a picosecond timescale.

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The realization of metamaterials or metasurfaces with simultaneous electric and magnetic response and low loss is generally very difficult at optical frequencies. Traditional approaches using nanoresonators made of noble metals, while suitable for the microwave and terahertz regimes, fail at frequencies above the near-infrared, due to prohibitive high dissipative losses and the breakdown of scaling resulting from the electron mass contribution (kinetic inductance) to the effective reactance of these plasmonic meta-atoms. The alternative route based on Mie resonances of high-index dielectric particles normally leads to structure sizes that tend to break the effective-medium approximation. Here, we propose a subwavelength dark-state-based metasurface, which enables configurable simultaneous electric and magnetic responses with low loss. Proof-of-concept metasurface samples, specifically designed around telecommunication wavelengths (i.e., λ ≈ 1.5 μm), were fabricated and investigated experimentally to validate our theoretical concept. Because the electromagnetic field energy is localized and stored predominantly inside a dark resonant dielectric bound state, the proposed metasurfaces can overcome the loss issue associated with plasmonic resonators made of noble metals and enable scaling to very high operation frequency without suffering from saturation of the resonance frequency due to the kinetic inductance of the electrons

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Saturable optical elements lie at the cornerstone of many modern optical systems. Regularly patterned quasi-planar nanostructures - metasurfaces - are known to facilitate nonlinear optical processes. Such subwavelength semiconductor nanostructures can potentially serve as saturable components. Here we report on the intensity-dependent reflectance of femtosecond laser pulses from semiconductor metasurfaces with Mie-type modes, caused by the absorption saturation. Arrays of GaAs nanocylinders with magnetic dipole resonances in the spectral vicinity of the GaAs bandgap demonstrate a reduced saturation intensity and increased self-modulation efficiency, an order of magnitude higher than bulk GaAs or unstructured GaAs films. By contrast, the reflection modulation is shown to be negligible in the CW regime for the same average intensities, indicating that the process is not the result of temperature effects. Our work provides a novel idea for low-power saturable elements based on nonthermal nature of saturation. We conclude by devising a high-quality metasurface that can be used, in theory, to further reduce the saturation fluence below 50 nJ/cm2.

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High-harmonic generation (HHG) is a signature optical phenomenon of strongly driven, nonlinear optical systems. Specifically, the understanding of the HHG process in rare gases has played a key role in the development of attosecond science1. Recently, HHG has also been reported in solids, providing novel opportunities such as controlling strong-field and attosecond processes in dense optical media down to the nanoscale2. Here, we report HHG from a low-loss, indium-doped cadmium oxide thin film by leveraging the epsilon-near-zero (ENZ) effect3–8, whereby the real part of the material’s permittivity in certain spectral ranges vanishes, as well as the associated large resonant enhancement of the driving laser field. We find that ENZ-assisted harmonics exhibit a pronounced spectral redshift as well as linewidth broadening, resulting from the photo induced electron heating and the consequent time-dependent ENZ wavelength of the material. Our results provide a new platform to study strong-field and ultrafast electron dynamics in ENZ materials, reveal new degrees of freedom for spectral and temporal control of HHG, and open up the possibilities of compact solid-state attosecond light sources.

We demonstrate all-optical switching of high quality factor quasibound states in the continuum resonances in broken symmetry GaAs metasurfaces. By slightly breaking the symmetry of the GaAs nanoresonators, we enable leakage of symmetry protected bound states in the continuum (BICs) to free space that results in sharp spectral resonances with high quality factors of ∼500. We tune the resulting quasi-BIC resonances with ultrafast optical pumping at 800 nm and observe a 10 nm spectral blue shift of the resonance with pump fluences of less than 100 μJ cm-2. The spectral shift is achieved in an ultrafast time scale (<2.5 ps) and is caused by a shift in the refractive index mediated by the injection of free carriers into the GaAs resonators. An absolute reflectance change of 0.31 is measured with 150 μJ cm-2. Our results demonstrate a proof-of-concept that these broken symmetry metasurfaces can be modulated or switched at ultrafast switching speeds with higher contrast at low optical fluences (<100 μJ cm-2) than conventional Mie-metasurfaces.

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