Publications

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.

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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.

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.

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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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Strong coupling of an intersubband (ISB) electron transition in quantum wells to a subwavelength plasmonic nanoantenna can give rise to intriguing quantum phenomena, such as ISB polariton condensation, and enable practical devices including low threshold lasers. However, experimental observation of ISB polaritons in an isolated subwavelength system has not yet been reported. Here, we use scanning probe near-field microscopy and Fourier-transform infrared (FTIR) spectroscopy to detect formation of ISB polariton states in a single nanoantenna. We excite the nanoantenna by a broadband IR pulse and spectrally analyze evanescent fields on the nanoantenna surface. We observe the distinctive splitting of the nanoantenna resonance peak into two polariton modes and two ?-phase steps corresponding to each of the modes. We map ISB polariton dispersion using a set of nanoantennae of different sizes. This nano-FTIR spectroscopy approach opens doors for investigations of ISB polariton physics in the single subwavelength nanoantenna regime.