arXiv subjects

Yigong Luan

Publications and source records attributed to Yigong Luan.

3 recordsLinked to original sources

arXiv2026

Efficient photon-pair emission from a nanostructured resonator and its theoretical description

Spontaneous parametric down-conversion (SPDC) in subwavelength nanostructures is a promising source of quantum light, owing to its multifunctionality and ability to generate complex quantum states. Nevertheless, the mechanisms governing photon-pair generation in such systems remain only partially understood. In particular, experimental investigations of key emission properties in individual resonators, such as directionality and spectral distribution, are still lacking, and predictive theoretical frameworks have not yet been experimentally validated. Here, we report the first measurement of the directional and spectral distributions of photon pairs generated via SPDC in a nanostructured resonator. Both distributions exhibit resonant behaviour, which we describe using an extended quasi-normal-mode theory. This comparison is enabled by photon-pair count rates of up to 0.45 Hz/mW -- to our knowledge, the highest reported for a nanostructured resonator. Our results provide new physical insight into nanoscale SPDC and represent an important step toward designing of efficient miniaturized quantum light sources.

quant-ph↗

arXiv2026

Efficient and tunable narrowband second-harmonic generation by a large-area etchless lithium niobate metasurface

Optical resonances in nanostructures enable strong enhancement of nonlinear processes at the nanoscale, such as second-harmonic generation (SHG), with high-$Q$ modes providing intensified light--matter interactions and sharp spectral selectivity for applications in filtering, sensing, and nonlinear spectroscopy. Thanks to the recent advances in thin-film lithium niobate (TFLN) technology, these key features can be now translated to lithium niobate for realizing novel nanoscale nonlinear optical platforms. Here, we demonstrate a large-area metasurface, realized by scalable nanoimprint lithography, comprising a slanted titanium dioxide (TiO$_2$) nanograting on etchless TFLN for efficient narrowband SHG. This is enabled by the optimal coupling of quasi-bound state in the continuum (q-BIC) modes with a narrowband pulsed laser pump. The demonstrated normalized SHG efficiency is $0.15\%\,\mathrm{cm}^2/\mathrm{GW}$, which is among the largest reported for LN metasurfaces. The low pump peak intensity ($3.64~\mathrm{kW}/\mathrm{cm}^2$) employed, which enables SHG even by continuous-wave pumping, allows envisioning integrated and portable photonic applications. SHG wavelength tuning from $870$ to $920~\mathrm{nm}$ with stable output power as well as polarization control is also achieved by off-normal pump illumination. This versatile platform opens new opportunities for sensing, THz generation and detection, and ultrafast electro-optic modulation of nonlinear optical signals.

physics.optics↗

arXiv2024

All-optical polarization control and routing by nonlinear interferometry at the nanoscale

Optical metasurfaces are rapidly establishing as key-enabling platforms for nanophotonics applications. Along with the ability of taming light in subwavelength thicknesses, they can feature multiple functionalities in one device. The generation and control of light polarization by metasurfaces already provided a route towards ultracompact polarimetry devices in the linear regime. If translated to the nonlinear optical regime it may become a key-enabling tool in nonlinear imaging, optical holography and sensing. Here, we report the experimental ultrafast all-optical polarization modulation of upconverted light by all-dielectric metasurfaces via nonlinear interferometry. By controlling the relative phase between a pump beam at $ω$ and its frequency-double replica at 2$ω$, we can set the phase relation between two frequency-degenerate upconverted processes at 3$ω$ $-$ Sum-Frequency Generation (SFG) and Third-Harmonic Generation (THG) $-$ stemming from an AlGaAs metasurface. By exploiting the opposite parity of the two nonlinear process and tuning their relative powers, we can achieve modulation of the polarization states of the light emitted at 3$ω$ between linear and circular states with a degree of circular polarization (DOCP) up to 83$\%$. In particular, circularly polarized light features opposite handedness symmetrically located in the Fourier space, at coincidence with the first diffraction orders of the metasurface. Moreover, by toggling the phase delay by $π$, the handedness can be fully switched between the diffraction. Our work adds an additional layer of modulation in polarization beyond intensity to all-optical routing with precise phase control. The capability to route circular polarization states in the k-space holds significant potential for chiral sensing and advanced imaging techniques.

physics.optics↗