Zero-index metamaterials show the unique feature of uniform spatial phase distributions, enabling the interaction of single electromagnetic mode with matter over an infinite area in an arbitrary shape. This feature brings various novel optical physics and devices, such as supercoupler, large-area single-mode laser, and extended superradiance. However, the state-of-the-art zero-index waveguide shows a propagation loss as high as 1000 dB/mm, hampering most potential applications of zero-index metamaterials. Although zero-index metamaterials based on bound state in the continuum can show a lower propagation loss of 45 dB/mm, the photonic crystal slab configuration which are boundless in the in-plane direction limits the devices’ footprint and flexibility drastically. Here we demonstrated a one-dimensional metawaveguide with zero refractive index along the propagation direction, featuring a high flexibility, a compact footprint, and a low propagation loss of 5.45 dB/mm near the zero-index wavelength. This metawaveguide could enable many zero index-based linear, nonlinear, and quantum photonic devices such as entangled photon pair sources based on spontaneous four-wave mixing.
Optical bound states in the continuum (BICs) have recently stimulated a research boom, accompanied by demonstrations of abundant exotic phenomena and applications. With ultrahigh quality (Q) factors, optical BICs have powerful abilities to trap light in optical structures from the continuum of propagation waves in free space. Besides the high Q factors enabled by the confined properties, many hidden topological characteristics were discovered in optical BICs. Especially in periodic structures with well-defined wave vectors, optical BICs were discovered to carry topological charges in momentum space, underlying many unique physical properties. Both high Q factors and topological vortex configurations in momentum space enabled by BICs bring new degrees of freedom to modulate light. BICs have enabled many novel discoveries in light–matter interactions and spin–orbit interactions of light, and BIC applications in lasing and sensing have also been well explored with many advantages. In this paper, we review recent developments of optical BICs in periodic structures, including the physical mechanisms of BICs, explored effects enabled by BICs, and applications of BICs. In the outlook part, we provide a perspective on future developments for BICs.
The bound state in the continuum (BIC) is widely used in optical field confinement because of its potential in radiation suppression, providing an energy confinement method different from traditional microcavities. However, the size compression of BIC cavities still remains challenging. In this paper, we report an ultra-small size, high-Q BIC microcavity on silicon on insulator (SOI) platfom. We used the integer topological charges carried by the BICs to suppress the vertical radiation loss, while photonic crystal hetero-structures were utilized to reduce the transverse energy leakage. We designed and fabricated photonic crystal microcavity with optical confinement region as tiny as 5×5 period. The measured Q is greater than 170 thousand with the peak wavelength at 1550.39 nm. This work shows the potential of BIC microcavities in size compression. The study has positive effects on integrated optics, information optics, biooptics, topological optics and nonlinear optics.
The high-speed, high-efficient, compact phase modulator array is indispensable in the Optical-phased array (OPA) which has been considered as a promising technology for realizing flexible and efficient beam steering. In our research, two methods are presented to utilize high-contrast grating (HCG) as high-efficient phase modulator. One is that HCG possesses high-Q resonances that origins from the cancellation of leaky waves. As a result, sharp resonance peaks appear on the reflection spectrum thus HCGs can be utilized as efficient phase shifters. Another is that low-Q mode HCG is utilized as ultra-lightweight mirror. With MEMS technology, small HCG displacement (~50 nm) leads to large phase change (~1.7π). Effective beam steering is achieved in Connie Chang-Hasnian’s group. On the other hand, we theoretically and experimentally investigate the system design for silicon-based optical phased array, including the star coupler, phased array, emission elements and far-field patterns. Further, the non-uniform optical phased array is presented.
A novel hollow-core (HW) Y-branch waveguide splitter based on high-contrast grating (HCG) is presented. We calculated and designed the HCG-HW splitter using Rigorous Coupled Wave Analysis (RCWA). Finite-different timedomain (FDTD) simulation shows that the splitter has a broad bandwidth and the branching loss is as low as 0.23 dB. Fabrication is accomplished with standard Silicon-On-Insulator (SOI) process. The experimental measurement results indicate its good performance on beam splitting near the central wavelength λ = 1550 nm with a total insertion loss of 7.0 dB.
We discuss the relationship between Sagnac effect and "slow light" phenomenon, and point out that although the medium and waveguide dispersion can in no way affect the magnitude of Sagnac effect, the highly dispersive structure is still beneficial to the enhancement of Sagnac effect and can be utilized to detect absolute rotation for navigation purpose.
Based on the EIT-like property of coupled resonator structure, a miniature highly sensitive gyroscope is possible. This EIT-like phenomenon occurs through a classical mean in a coupled resonator structure due to all-optical classical interference, called coupled resonator induced transparency (CRIT). With the analogy between optical and atomic parameters, we treat Sagnac effect as a phase perturbation to resonators' optical parameters, and then analyze Sagnac effect in a CRIT structure with a transfer function approach and derive the explicit expression of relative Sagnac phase shift. We find that Sagnac effect is enhanced as a factor as light slows, and can be tailored by adjusting the optical parameters of structure.
Furthermore, as a potential highly sensitive, compact size rotation sensor, some issues for the implementation of CRIT structure based gyroscope are discussed and considered, such as the fabrication possibility, line-width, shot noise limit sensitivity and integration issues. With the improvement of micro-fabrication technique, this gyroscope should have all-solid configuration, compact size and also be expected to achieve comparable sensitivity to common optic-fiber gyroscope. It would be easily integrated to all-optical application and construct a high performance rotation sensor.
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