Free-space coupling to symmetric high-Q terahertz whispering-gallery mode resonators

Terahertz disk resonator on a substrateless dielectric waveguide platform

Resonant cavities are fundamental to and versatile for terahertz integrated systems. So far, integrated resonant cavities have been implemented in relatively lossy terahertz platforms. In this Letter, we propose a series of integrated disk resonators built into a low-loss substrateless silicon waveguide platform, where the resonances and associated quality factor (Q-factor) can be controlled via an effective medium. The measurement results demonstrate that the Q-factor can reach up to 9146 at 274.4 GHz due to the low dissipation of the platform. Additionally, these resonators show strong tunability of the resonance under moderate optical power. These terahertz integrated disk resonators can be employed in sensing and communications.

Encaved optical fiber nano-probe exciting whispering gallery mode resonance with focused far off-axis beam

This paper demonstrates whispering gallery mode (WGM) resonance with the help of an encaved optical nano-probe developed inside an optical fiber tip cavity. The nano-probe generates a tightly focused beam with a spot-size of ∼3 µm. A barium titanate microsphere is placed besides the optical axis inside the cavity. The focused beam remains off-axis of the microresonator and excites the WGM. The off-axis excitation shows unique resonating properties depending on the location of the resonator. A resonant peak with quality factor as high as Q ∼7 × 104 is achieved experimentally. Another design with a shorter cavity length for a bigger resonator is also demonstrated by embedding a bigger microsphere on the cleaved fiber tip surface. The optical probe holds great potential for photonic devices and is ideal for studying morphology-based scattering problems.

A Review on Terahertz Technologies Accelerated by Silicon Photonics

In the last couple of decades, terahertz (THz) technologies, which lie in the frequency gap between the infrared and microwaves, have been greatly enhanced and investigated due to possible opportunities in a plethora of THz applications, such as imaging, security, and wireless communications. Photonics has led the way to the generation, modulation, and detection of THz waves such as the photomixing technique. In tandem with these investigations, researchers have been exploring ways to use silicon photonics technologies for THz applications to leverage the cost-effective large-scale fabrication and integration opportunities that it would enable. Although silicon photonics has enabled the implementation of a large number of optical components for practical use, for THz integrated systems, we still face several challenges associated with high-quality hybrid silicon lasers, conversion efficiency, device integration, and fabrication. This paper provides an overview of recent progress in THz technologies based on silicon photonics or hybrid silicon photonics, including THz generation, detection, phase modulation, intensity modulation, and passive components. As silicon-based electronic and photonic circuits are further approaching THz frequencies, one single chip with electronics, photonics, and THz functions seems inevitable, resulting in the ultimate dream of a THz electronic-photonic integrated circuit.

In-fiber zigzag excitation for whispering-gallery modes via evanescent wave and free space coupling

In this paper, we demonstrate a simple capillary-based coupler with optical zigzag transmission to excite whispering-gallery modes (WGMs) in a passive deformed microcavity. The coupler is just composed sections of a single mode fiber, a microsphere and a capillary, while neither corrosion nor tapering process is necessary. Based on the ray optics model, we obtain the criterion between the cone-apex angle and capillary wall thickness for the zigzag beams to excite WGMs effectively. This proper cone-apex angle range can be obtained by only setting the appropriate welding parameters. The smaller and larger cone-apex angles correspond to evanescent wave and free space coupling, respectively. The former has a clear free spectral range (FSR) envelope of WGMs, while the latter has no but compact and fairly intensity-uniform WGM peaks because it first directly excites chaotic sea which then couples into WGMs by dynamical tunneling. Moreover, the modified equations of motion for the photon counts are brought forward to analyze the energy distribution for the free space coupling. The excitation unit can work as a pure and basic building block, which is a simple and flexible coupling scheme for WGMs and has great potential in photonic integrated devices.

Terahertz Gas-Phase Spectroscopy Using a Sub-Wavelength Thick Ultrahigh-Q Microresonator

The terahertz spectrum provides tremendous opportunities for broadband gas-phase spectroscopy, as numerous molecules exhibit strong fundamental resonances in the THz frequency range. However, cutting-edge THz gas-phase spectrometer require cumbersome multi-pass gas cells to reach sufficient sensitivity for trace level gas detection. Here, we report on the first demonstration of a THz gas-phase spectrometer using a sub-wavelength thick ultrahigh-Q THz disc microresonator. Leveraging the microresonator's ultrahigh quality factor in excess of 120,000 as well as the intrinsically large evanescent field, allows for the implementation of a very compact spectrometer without the need for complex multi-pass gas cells. Water vapour concentrations as low as 4 parts per million at atmospheric conditions have been readily detected in proof-of-concept experiments.