Research Interests: Optics beyond the limits

Optical resonators toward single-photon nonlinearity
Optical resonators that store light in the confined geometry has played a unique role in the optical research field. The simplest form of the resonators consisting two pieces of mirrors parallelly aligned with each other was the heart of lasers which led Renaissance of optics from the 1960s, and now they are implemented in various kinds of forms optimized for the specific use ranging from commercial laser devices to the front line of scientific research such as cavity quantum electrodynamics (QED) and the measurement of gravity waves.

Recently, the development of the micro-fabrication technique infuses these essential but traditional elements with totally new vitality, and this research field is entering the new phase where it is possible to precisely control the prime properties of the resonators such as free spectral range, chromatic dispersion, and the most important property, quality factor (Q), which is defined by photon lifetime multiplied by angular frequency. The strong confinement of photons in a micro-volume (V) of such a high-Q resonator with properly-designed prime properties enables to observe and exploit strong light-matter interaction and light-light interaction mediated by matter as demonstrated in emerging research fields such as non-classical light generation, optical quantum information, micro-scale spectral frequency comb, optomechanics, and non-Hermitian physics. We finally aim to challenge the goal, single-photon nonlinearity, where single individual photons can interact with each other and behave like Fermions, under the blessing of the ultimate high-Q/V optical resonator.

Light-matter interactions at the sub-wavelength scale
To maximize the interactions of light and materials, it is necessary to concentrate external light incidence to dimensions comparable with the electron mean free path that is at least an order of magnitude smaller than the diffraction limit. Optical resonances of metallic or dielectric nanostructures interface free-propagating radiations and localized near-fields efficiently. In recent decades, resonant light scattering and absorption have been intensively investigated and employed for various applications, including light emission enhancement, optical sensing, nonlinear optical signal generation, photovoltaics, optomechanics, and metamaterials. Plasmonic resonances of metallic nanostructures manipulate light scattering and absorption extremely beyond the optical diffraction limit. Mie resonances, or termed leaky-mode resonances, allow dielectric optical nanostructures to have compact sizes even less than the wavelength scale. The dielectric nanostructures with low optical losses support both electric and magnetic resonant modes and provide an alternative to their metallic counterparts.

Our research group aims to understand resonant light-matter (photon-electron, photon-phonon, photon-spin, photon-exciton, and so on) interactions based on optical nanostructures, boost the efficiency and quantum yield in various nano-photonic phenomena, and engineer their properties for future applications.

Precision nano-optical measurement
We aim to develop precise and quantitative measurement methods for physical understanding of nano-optical phenomena from the evanescent near-field to the radiating near-field (Fresnel) and the far-field (Fraunhofer) regime. For example, we demonstrated for the first time quantitative measurement of the differential far-field scattering cross-section of a single nanostructure over the full hemisphere.

Currently, we are expanding the scope of our research to time- and space-resolved measurements of light absorption, photocurrent generation, optical nonlinearity, and optical angular momentum. Our research will provide a new way for investigating the physical properties and performance of a variety of nano-optical materials, phenomena, and devices in energy (wavelength)-momentum (wavenumber) space. We also aim to understand and utilize further freedom of lights, including spin/orbital angular momentum of electromagnetic fields, quantum optical entanglements, and nonclassical optical states.