Mode coupling in a terahertz multi-mode whispering-gallery-mode resonator

In optics, whispering gallery waves/modes (WGM) are a type of wave that can travel around a concave surface. Technically, they are specific resonances (or modes) of a wave field that are confined inside a given resonator (cavity) with smooth edges due to continuous total internal reflection. As a result, they portray unique properties, including: high quality (Q) factors and low mode volume, which make the WGM resonator an excellent platform for both physical and practical researches in the optical domain.

Existing literature show that much efforts were focused on improving Q factors, optimizing coupling and tuning the terahertz WGMs. Nevertheless, recent publications have highlighted that in the optical regime, mode coupling in a multi-mode resonator introduces various shapes of transmission intensity such as dual resonances and mode splitting; desirable attributes which in turn contribute to more compact devices with various functions. In particular, these desirable phenomena in one multi-mode resonator provide a compact solution for designing terahertz devices such as terahertz dual-band filters, absorbers, and delay lines. Unfortunately, as of now, there are no published studies that investigate mode-coupling effects in a single terahertz WGM resonator.

In this context, Huazhong University of Science and Technology researchers Shixing Yuan, Dr. Liao Chen, Ziwei Wang, Ruolan Wang and led by Professor Xinliang Zhang together with Dr. Xiaojun Wu at Beihang University presented a study where their sole goal was to look at the mode-coupling effects of terahertz whispering gallery modes (WGMs) in a multi-mode resonator using an extended transfer matrix method. They aspired to provide a detailed analysis that would not only contribute to better comprehension of physical phenomena, such as mode splitting, but also helps in identifying parameters when designing terahertz devices, such as dual-band filters. Their work is currently published in the research journal, Optics Letters.

Briefly, the research methodology considered involved the adoption of an extended transmission matrix method to analyze the measured results and identify the states of WGMs. In this endeavor, a well-designed terahertz Teflon ring resonator, which supports fewer modes compared to disks, bubbles or spheres, was chosen as the research object. The team then demonstrated the different intensity profiles, such as single resonance and asymmetric dual-band resonances. All in all, they illustrated the mode-splitting effect in theory based on the adopted method in a multi-mode resonator.

The authors reported that the well-designed terahertz multi-mode ring resonator, which supported low-order modes, had an intrinsic Q factor of 1040. In addition, single resonance and dual resonances with different shapes were observed in experiment and reproduced in theory. Furthermore, different specific intensity profiles such as flat-base resonance and the mode-splitting effect were theoretically demonstrated in a single resonator.

In summary, the detailed observation, modeling, and analysis of terahertz mode-coupling effects in a terahertz multi-mode WGM ring resonator were demonstrated theoretically and proven experimentally. Coupling effects between two WGMs were successfully observed on a well-designed high-Q terahertz Teflon ring resonator that supports low-order WGMs. Overall, better understanding that opens up the reported terahertz devices to more applications was established.