Hong-Seok Kim

Ultra-high- Q , single-spatial-mode micro-resonators are essential for high-coherence integrated photonics because they combine long photon lifetimes, clean modal spectra, and predictable phase matching for tailored nonlinear and quantum optical applications. In birefringent thin-film lithium niobate (TFLN), conventional traveling-wave resonators face intrinsic trade-offs between maintaining high Q factors and controlling multimode excitation, polarization rotation, and dispersion. Standing-wave resonators offer a promising alternative, but reaching ultra-high Q remains challenging. Here, a photonic-crystal Fabry–Perot micro-resonator on TFLN achieves a milestone intrinsic quality factor of up to 4.8×10 ⁷ with single-spatial-mode standing-wave resonances. The bending-free, polarization-fixed resonator with mode-selective photonic-crystal mirrors minimizes optical loss and leverages alignment with the nonlinear-optical axis of the material. A wide-core geometry suppresses sidewall scattering to a record-low, to our knowledge, propagation loss of 0.25±0.26 dB/m, approaching the material-absorption limit of TFLN. Length-dependent analysis reveals a photonic-crystal mirror insertion loss of ∼0.03 dB per reflection. The one-dimensional footprint further facilitates denser planar integration than two-dimensional resonator layouts. The scalable architecture of mode-pure, standing-wave TFLN micro-resonators opens opportunities for compact photon-pair sources, power-efficient electro-optic frequency combs, and reconfigurable quantum photonic circuits.

Thin-film lithium niobate (TFLN) has emerged as a powerful platform for integrated nonlinear and quantum photonics, owing to its strong optical nonlinearities, wide transparency window, and electro- and piezo-optic properties. However, conventional traveling-wave resonators—such as micro-rings, disks, and racetracks—suffer from curvature-dependent group dispersion and losses, limited spectral tunability, and parasitic nonlinearities, which constrain their performance, scalability, and operational stability in nonlinear photonic circuits. Here, we present photonic crystal (PhC) Fabry–Pérot (FP) micro-resonators in TFLN that address these limitations. The device features a one-dimensional straight cavity bounded by PhC reflectors and supports well-confined standing-wave resonant modes within an engineered photonic bandgap. We achieve intrinsic quality (Q) factors of up to 1.4 × 106 and demonstrate that both the free spectral range (FSR) and coupling strength can be consistently controlled via cavity length and PhC coupler design, respectively. The photonic bandgap is tunable across the S-, C-, and L-bands without degradation of resonator performance. Spectral confinement of high-Q resonant modes is expected to mitigate parasitic nonlinearities, such as Raman scattering. These advances, together with the one-dimensional geometry, establish PhC FP micro-resonators as compact and scalable building blocks for high-density photonic integrated circuits targeting next-generation nonlinear and quantum applications.

Lithium niobate on insulator (LNOI) is a promising material platform for applications in integrated quantum photonics. A low optical loss is crucial for preserving fragile quantum states. Therefore, in this study, we have fabricated LNOI rib waveguides with a low optical propagation loss of 0.16 dB/cm by optimizing the etching conditions for various parameters. The symmetry and smoothness of the waveguides on ‐cut LNOI are improved by employing a shallow etching process. The proposed method is expected to facilitate the development of on‐chip quantum photonic devices based on LNOI.

As direct formation of p-type two-dimensional transition metal dichalcogenides (TMDC) films on substrates, tungsten disulfide (WS₂) thin films were deposited onto sapphire glass substrate through shadow mask patterns by radio-frequency (RF) sputtering at different sputtering powers ranging from 60 W to 150 W and annealed by rapid thermal processing (RTP) at various high temperatures ranging from 500 °C to 800 °C. Based on scanning electron microscope (SEM) images and Raman spectra, better surface roughness and mode dominant E ¹ _2g and A ¹ _g peaks were found for WS₂ thin films prepared at higher RF sputtering powers. It was also possible to obtain high mobilities and carrier densities for all WS₂ thin films based on results of Hall measurements. Process conditions for these WS₂ thin films on sapphire substrate were optimized to low RF sputtering power and high temperature annealing.

We report on the fabrication and electrical transport properties of superconducting junctions made of β-Ag₂Se topological insulator (TI) nanowires in contact with Al superconducting electrodes. The temperature dependence of the critical current indicates that the superconducting junction belongs to a short and diffusive junction regime. As a characteristic feature of the narrow junction, the critical current decreases monotonously with increasing magnetic field. The stochastic distribution of the switching current exhibits the macroscopic quantum tunneling behavior, which is robust up to T = 0.8 K. Our observations indicate that the TI nanowire-based Josephson junctions can be a promising building block for the development of nanohybrid superconducting quantum bits.

We report the fabrication of strongly coupled nanohybrid superconducting junctions using PbS semiconductor nanowires and Pb_0.5In_0.5 superconducting electrodes. The maximum supercurrent in the junction reaches up to ∼15 μA at 0.3 K, which is the highest value ever observed in semiconductor–nanowire-based superconducting junctions. The observation of microwave-induced constant voltage steps confirms the existence of genuine Josephson coupling through the nanowire. Monotonic suppression of the critical current under an external magnetic field is also in good agreement with the narrow junction model. The temperature-dependent stochastic distribution of the switching current exhibits a crossover from phase diffusion to a thermal activation process as the temperature decreases. These strongly coupled nanohybrid superconducting junctions would be advantageous to the development of gate-tunable superconducting quantum information devices.