New method enables superior light confinement in low-refraction-index microcavity

August 24, 2016, Peking University

The trapping of photons in low-refraction-index materials is thought to be difficult in conventional photonic structures that employ total internal reflection. Specifically, the whispering gallery mode (WGM) microcavity, which is an important optical component, has to rely on a refraction index with high contrast to the surrounding environment to manifest excellent light confinement. A team led by Professor Yun-Feng Xiao at Peking University collaborated with researchers from the Harbin Institute of Technology, and demonstrated that an optical microcavity structure consisting of a low-refraction-index silica microtoroid that was directly bonded on a high-refraction-index silicon substrate supports high-Q fundamental WGMs. This work has been published in a recent issue of Optica.

Among on-chip resonators, high-refraction-index microcavities usually have quality factors no more than a million, which becomes an obstacle to applications in the fields of on-chip nonlinearity and quantum manipulation, etc. In contrast, low-refraction-index materials have advantages in many aspects, especially in the fabrication of high-Q devices. In the publication, the authors demonstrated experimentally excellent light confinement in low-refraction-index material.

The photonic microstructure consists of a donut-shaped microcavity with a circular cross section bonded on a silicon substrate. The refraction indices of silicon and silica are 3.48, 1.44, respectively. "The circular cross section of the microresonator remarkably reduces the electric field distribution area in the vertical direction compared with traditional rectangular cross-section, resulting in a better light confinement in the silica," said Li Wang, one of the first co-authors of this work.

In the transmission spectra, high-Q fundamental modes are observed while most higher-order modes are suppressed due to much stronger energy leakage into the substrate, which produces a very clean mode spectrum. The resonant structure supports high-Q fundamental WGMs in both visible and communication bands. In the communication wavelength band, the quality factors increase exponentially as the minor diameter of the microcavity increases. With a larger minor diameter, the highest quality factor can be even up to 10 million. In the visible wavelength band, the quality factors always maintain large values more than 10 million.

"Low-threshold microcavity Raman lasing was demonstrated to show the potential of the resonant structure in the low-power-consumption integrated photonics," said Prof. Xiao. "This mechanism of light confinement in the photonic microstructure applies to various low-refraction-index materials besides silica, such as polymer. Furthermore, the unique microstructure may open up new possibilities for on-chip applications in fields including nonlinearity and quantum manipulation."

Explore further: To infinity and beyond: Light goes infinitely fast with new on-chip material

More information: Li Wang et al. Light confinement in a low-refraction-index microcavity bonded on a silicon substrate, Optica (2016). DOI: 10.1364/OPTICA.3.000937

Related Stories

3-D laser printing of whispering-gallery-mode microcavities

October 30, 2015

Whispering-Gallery-Mode (WGM) microcavities that confine light in a small volume with high quality (Q) factors and enhance interaction of light with matters inside the cavity have shown promising applications as an element ...

Testing Weyl's law at optical frequencies

April 13, 2016

Stable states (or resonances) are always of importance in understanding reactions and collision processes of all energy scales, but they often prove difficult to detect in experiments, particularly when a system exhibits ...

Recommended for you

Gravitational wave detectors to search for dark matter

August 16, 2018

Gravitational wave detectors might be able to detect much more than gravitational waves. According to a new study, they could also potentially detect dark matter, if dark matter is composed of a particular kind of particle ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.