We present a rigorous numerical analysis on tunable characteristics of guided-mode resonances (GMRs) in coupled gratings. Two schemes of strong and negligible evanescent coupling of guided modes are treated. Both show wide range tunability. In the case of strong evanescent coupling, independent control of the center wavelength and the linwidth of the resonance is obtained via variations of a gap size between the gratings and lateral alignment conditions. We believe that this characteristic will provide a useful means to realize a tunable filter in conjunction with micro/nano-electromechanical system technologies. We also present a generalized theoretical analysis on the tunable characteristics of the GMRs in coupled gratings, which is qualitatively in good agreement with the numerical analysis.

Developing quantum key distribution (QKD) has been recently directed toward distance extension and network expansion for real-world secure communications. Considering a recent report on a quantum communication network over 4,600 km, it seems that QKD networks using conventional protocols have been sufficiently studied. However, although the twin-field QKD (TF-QKD) proposed for long-distance QKD has been studied deeply enough to succeed the demonstrations over 428- and 511-km deployed fibers, TF-QKD networks have been verified only for a ring network. In this work, we propose a star topological 2 × N TF-QKD network scheme, where the coherence maintenance issue, being the primary obstacle to implementing TF-QKD, can be minimized by the automatic mode-matching feature of the Sagnac-based plug-and-play architecture. A lower number of active controllers is required for our scheme in comparison with one-way TF-QKD networks. Moreover, our scheme adopts a cost-effective configuration that requires only a single pair of single-photon detectors for the entire network system. We conducted a proof-of-concept experiment over a 50-km fiber successfully, achieving an average secret key rate of 1.31 × 10⁻⁴bit per pulse (1.52 bit per second) with the finite-size effect.

We investigate properties of nonlinear resonant gratings with emphasis on optical bistability. Slab waveguide gratings with various quality factors are designed and their characteristics analyzed with a finite-difference time-domain method. Considerable field enhancements are observed in the gratings and the performance compares favorably with metallic bistable devices. Bistabilitiy based on coupled gratings is also treated. Mechanically controllable switching intensity realized by varying a gap distance between two gratings is demonstrated. Resonant nonlinear elements in this work may find applications in all-optical information processing and optical switching, and our investigation on the dependence of the normalized switching intensity and the response time on quality factor will provide a general guide line for grating-based bistable device design.

We numerically demonstrate optical bistability in a nonlinear multilayer structure by utilizing the unique dispersion of hyperbolic metamaterials. The linear transmission is varied sharply with topological transition of isofrequency contour of the multilayer structure, and this non-resonant scheme enables realization of optical bistability with a short response time and a relatively low switching intensity. We have investigated exhaustively all possible topological transitions in the dispersion characteristics of the multilayer structure for optical bistability, and shown that the hyperbolic metamaterial (HMM) type transition from Type II to Type I, and the transition from Type II HMM to effective dielectric are suitable for realizing high-performances optical bistability. The proposed schemes can overcome the trade-off between a switching intensity and a response time in resonance based optical bistabilities.

We propose the narrowband perfect absorbers with enormously high fabrication tolerance, which consists of a low-contrast grating and a finite distributed Bragg reflector (DBR) layer with an ultrathin absorbing medium (graphene). It is numerically shown that the proposed perfect absorber outperforms the previously proposed schemes in fabrication tolerance. According to the rigorous coupled wave analysis (RCWA) and coupled mode theory (CMT) fitting, over a considerably wide range of grating width and thickness, the proposed absorber provides a proper ratio of leakage rate to loss rate while preserving resonant condition, so that almost perfect absorption (>99.9%) can be obtained. This result is attributed to the strong electric field confinement in the DBR region rather than the grating layer owing to lower index of grating compared to DBR. In addition, without degrading the fabrication tolerance, the bandwidth of the proposed absorber can be controlled by the DBR thickness (the number of pairs) and a narrow absorbing bandwidth of sub-nanometer is achieved with 8.5 Si/SiO₂ pair stacked DBR.

We propose an electrically tunable absorber based on epsilon-near-zero (ENZ) effect of graphene embedded in a nanocavity, which is composed of metal grating and substrate. Due to strong surface-normal electric field confined in ENZ graphene in the proposed structure, greatly enhanced light absorption (~80%) is achieved in an ultrathin graphene monolayer. By electrically controlling the Fermi-level of graphene, a sharp peak absorption wavelength is tuned over a wide range. The proposed device can work as an optical modulator or a tunable absorption filter, which has a unique feature of incident angle insensitiveness owing to the ENZ effect and magnetic dipole resonance. Moreover, existence of a significantly dominant electric field and its uniformity make the device performance independent of the position of the graphene layer in the nanocavity, which provides great fabrication tolerance.