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Publication# Nonlinear Frequency Conversion in III-Nitride Doubly Resonant Photonic Crystal Cavities

Abstract

Nonlinear frequency conversion processes, such as second-harmonic generation (SHG) and spontaneous parametric down-conversion (SPDC), are essential in many applications, including the generation of entangled photons. It's desirable to enhance these processes since the nonlinear effects are generally very small and photonic crystal (PhC) cavities are usually implemented for this purpose thanks to the combination of high quality factors and small footprints. Singly resonant PhC cavities, which support one mode at either the input frequency or the output frequency, are extensively investigated by previous studies, and significant enhancement is observed, while doubly resonant PhC cavities, which support simultaneously one mode at the input frequency and another mode at the output frequency, are not because of the difficulty of designing the two resonant modes, even though the enhancement is supposed to be stronger.This thesis studies the first design of 2D doubly resonant PhC cavities, realized by implementing a bound state in the continuum (BIC), and investigates its applications in SHG and SPDC. The cavities were fabricated on a III-nitride-on-silicon platform with standard semiconductor techniques and characterized on custom optical setups. The doubly resonant condition was fulfilled through passive lithographic tuning and active strain tuning. The SHG was observed with a high conversion efficiency under continuous wave excitation, which is in good agreement with theoretical predictions. Besides, far-field emission patterns with a donut shape and radial polarization at the second harmonic frequency were observed, which also agrees with the BIC confinement theory. The investigation on SPDC was however limited by the sensitivity of classical photodetectors. Nevertheless, the observation of high-efficiency SHG demonstrated the concept of doubly resonant PhC cavities and marked a cornerstone achievement for nonlinear frequency conversion. The fabrication and optical measurement techniques developed in this thesis could also be valuable for future studies.

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Second-harmonic generation

Second-harmonic generation (SHG, also called frequency doubling) is a nonlinear optical process in which two photons with the same frequency interact with a nonlinear material, are "combined", and generate a new photon with twice the energy of the initial photons (equivalently, twice the frequency and half the wavelength), that conserves the coherence of the excitation. It is a special case of sum-frequency generation (2 photons), and more generally of harmonic generation.

Spontaneous parametric down-conversion

Spontaneous parametric down-conversion (also known as SPDC, parametric fluorescence or parametric scattering) is a nonlinear instant optical process that converts one photon of higher energy (namely, a pump photon), into a pair of photons (namely, a signal photon, and an idler photon) of lower energy, in accordance with the law of conservation of energy and law of conservation of momentum. It is an important process in quantum optics, for the generation of entangled photon pairs, and of single photons.

Nonlinear optics

Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typically observed only at very high light intensities (when the electric field of the light is >108 V/m and thus comparable to the atomic electric field of ~1011 V/m) such as those provided by lasers. Above the Schwinger limit, the vacuum itself is expected to become nonlinear.

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