NQPL

We have successfully implemented nonlinear optical micro-resonators made of LiNbO3, known for its strong x(2) optical nonlinearities, a broadband transparent spectral window, and high-performance electro-optic and piezoelectric properties. Our demonstrations span various quantum and nonlinear optical processes, such as entangled photon pair generation via spontaneous parametric down-conversion, power-efficient second harmonic generation, and coherent electro-optic modulation, all within practical telecommunication wavelength ranges. Our goal is to develop promising quantum photonic devices, such as indistinguishable quantum entangled photon pair sources, non-classical light states (e.g., squeezed light state, cat state), quantum optical parametric oscillators/amplifiers, and soliton frequency combs. We aim to seamlessly integrate these devices on a chip, creating a highly scalable platform for optical quantum information systems.

Optical Resonators for Quantum Photonics and Toward Single-Photon Nonlinearity

Micro-/nano-resonators, which store light in a confined geometry, have played a pivotal role in optical research. The simplest form of resonators, consisting of two pieces of mirrors aligned parallel to each other, was at the heart of the lasers that led the renaissance of optics from the 1960s onward, and today they are implemented in various types of forms optimized for specific applications ranging from commercial laser devices to the frontiers of scientific research such as cavity quantum electrodynamics (QED) and gravitational wave measurement.

Recently, the development of micro-/nano-fabrication techniques has given a new impetus to these essential but traditional elements with a completely new vitality, and this research field is entering a new phase where it is possible to precisely control the primary properties of the resonators, such as free spectral range, chromatic dispersion, and the most important property, the quality factor (Q), which is defined as the photon lifetime multiplied by the angular frequency. The strong confinement of photons in a micro-volume (V) of such a high-Q resonator with properly designed prime properties allows the observation and exploitation of strong light-matter interactions and light-light interactions mediated by nonlinear matter, as demonstrated in emerging research fields such as non-classical light generation, optical quantum information, microscale spectral frequency comb, optomechanics, and non-Hermitian physics. Finally, we aim to challenge the goal of single-photon nonlinearity, where single photons can interact with each other and behave like fermions, and realize "deterministic" quantum optical processes under the blessing of the ultimate high-Q/V optical resonator. 

Advanced Measurement and Analysis in Quantum Optics and Nanophotonics

Our research emphasizes precise, quantitative measurements and a comprehensive understanding of quantum optical phenomena and nanophotonic functionalities. We have developed advanced methods for analyzing the physical properties and performance of various nano-optical materials, phenomena, and devices within energy (wavelength) and momentum (wavenumber) space. Recently, we have established high-precision facilities for measuring quantum correlation functions, single-photon level spectra, and performing quantum tomography. These developments pave the way to explore quantum and nonlinear optics on the platform of photonic integrated circuits. The strong computational power and theoretical expertise of our group support the experimental measurements and demonstrations.

Light-Matter Interactions at the Sub-Wavelength Scale

To maximize light-material interactions, it is essential to focus external light to dimensions comparable to the electron mean free path, significantly smaller than the diffraction limit. Optical resonances in metallic or dielectric nanostructures efficiently bridge free-propagating radiations and localized near-fields. In recent decades, resonant light scattering and absorption have been extensively studied and applied in various applications, including light emission enhancement, optical sensing, nonlinear optical signal generation, photovoltaics, optomechanics, and metamaterials. Plasmonic resonances in metallic nanostructures extend light scattering and absorption well beyond the optical diffraction limit. Mie resonances, also known as leaky-mode resonances, allow dielectric optical nanostructures to achieve compact sizes even smaller than the wavelength scale. These dielectric nanostructures, with their low optical losses and support for both electric and magnetic resonant modes, provide an alternative to metallic counterparts.