
- Photonics Research
- Vol. 10, Issue 3, 810 (2022)
Abstract
1. INTRODUCTION
As a young branch of the electromagnetic spectrum, the terahertz band has broad prospects in many important applications, including fingerprint spectroscopy, security imaging, and wireless communication [1]. Terahertz devices with high quality factors (
Bound state in the continuum (BIC), first introduced by von Neumann in quantum mechanics [13], refers to a localized confined mode without radiation located in the continuum spectrum coexisting with waves that have access to the free space, which has been proved to be an efficient way to achieve strong light confinement and find high-
In this work, we demonstrated a route to realize high-
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2. DEMONSTRATION OF BIC FAMILY
Figure 1(a) schematically illustrates the design of the proposed dual-period dielectric metagrating. Two different silicon grating ridges [Ridge I and Ridge II as shown in Fig. 1(a)] constitute a supercell with a period of
Figure 1.Schematic of dual-period dielectric gratings and eigenmode analysis. (a) Schematic illustration of dual-period grating and structural parameters of the grating; (b) microscopy image of the sample; (c) band structure of confined eigenmodes simulated with dual- (green) and mono- (red) period boundary and calculated by the GMR theory (dark blue); (d) electric field distribution of the fourth group of modes at 0.479 and 0.492 THz along the gray axis in the right panel, respectively; the light and dark blue regions indicate the grating layer and substrate, respectively; (e) electric field profile of
To interpret the mechanism of the proposed BICs, the intrinsic band structures of TE modes from the dual-period metagrating were investigated by COMSOL Multiphysics. Here,
Since no coupling to free space is allowed for BICs, their experimental demonstration of existence can only be demonstrated by investigating the quasi-BICs where the symmetry of the gratings is broken [21,30,32–35]. In this work, the widths of the ridges
Figure 2.
Although
More importantly, the BICs supported in the dual-period metagrating possess great robustness over broad incident angles, which has rarely been reported in previous research. Here we explain this advantage by investigating the
Figure 3.Investigation of
3. EIT ANALOG AND EXTREMELY SLOW LIGHT
The demonstrated
To investigate the quasi-BIC-caused EIT analogs,
Figure 4.Manipulation of two quasi-BICs and EIT-like effect. (a) Transmission evolution with varying
Although the EIT analog caused by two quasi-BICs was elaborated above, there is an interesting and important situation that needs to be discussed carefully—that is, whether an EIT analog formed by a BIC and a quasi-BIC can exist. We still use the example shown in Figs. 4(b) and 4(d) for the following analysis, and the situation corresponds to
What is more, due to the ultrahigh
Figure 5.Simulated and measured slow-light effect. (a) Simulated and (d) measured transmission with
4. CONCLUSION
In summary, we theoretically and experimentally investigated terahertz BICs induced by dual-period metagratings. Two types of BICs distinguished by the field distributions were demonstrated based on electric field profiles. Derived from the Bloch theory, we demonstrated that BIC can exist at tilted incidence and form a BIC family that endows incidence robustness to our metagrating, which was verified in the experiments under a series of incident angles. By tuning asymmetry factors, a quasi-BIC induced EIT analog with a narrow transparent transmission window was realized and systematically investigated. A measured group delay up to 117 ps was achieved by strong slow-light effect. The robustness against incidence demonstrated here extends the application scenarios of BIC-based devices. It also paves the way for the generation of terahertz waves through lasing and nonlinearity for its ultrahigh quality factor.
APPENDIX A: SAMPLE FABRICATION AND THz-TDS
To meet the design concept the sample was fabricated on a high-resistance silicon wafer with a thickness of 500 μm and conductivity above 10,000 Ω · cm. The sample was fabricated using the following procedure. First, the grating pattern was transferred onto the silicon wafer by photolithographic exposure and development. Second, the grating structure was formed by deep reactive ion etching on the wafer; the photoresist pattern serves as the protecting layer. The size of the fabricated grating sample has a total area of
The transmission spectra of the sample were measured using a THz-TDS system. The distance between the transmitting antenna and the sample was around 25 cm, and the aperture of the sample holder was around 1.4 cm, leading to an incident angle of around 1.6°, which can basically be regarded as a collimated beam. The holder was placed on a rotator with the accuracy of 1/800° to sweep the incident angle of the terahertz wave. A lens was placed in front of the receiver to concentrate the transmitted light and improve the signal-to-noise ratio. In order to manifest resonances with high-
APPENDIX B: THE Q FACTOR AT ARBITRARY WAVE VECTOR
With a structure perturbation
From CMT, we know that the
Combining Eqs. (
Because
On the other hand, according to Eq. (
To investigate the relationship between coupling coefficient
Substituting Eq. (
APPENDIX C: TRANSMISSION SPECTRUM CALCULATED BY CMT
The CMT equation in a lossless system is represented as [
The background coefficients
APPENDIX D: EXTREMELY SLOW-LIGHT EFFECT
According to Eq. (
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