Reconfigurable Chiral Microlasers

Coherent light sources are one of the most crucial foundations in both scientific disciplines and advanced appli­cations. As a prominent platform, ultrahigh-Q whispering-gallery mode (WGM) micro­cavities have witnessed signi­ficant developments of novel light sources. However, the intrinsic chiral symmetry of WGMs micro­cavity geometry and the resulting equi­valence between the two directions of laser propa­gation in a cavity severely limits further appli­cations of micro­lasers.

Illustration of the principle for a reconfigurable chiral microlaser by spontaneous symmetry breaking. (Source: X. Yun-Feng)

Very recently, a team of researchers led by Xiao Yun-Feng and Gong Qihuang at Peking University, in collaboration with Qiu Cheng-Wei at National University of Singapore and Professor Stefan Rotter at Vienna University of Technology, has demons­trated a spon­taneously symmetry-broken microlaser in an ultrahigh-Q WGM micro­cavity, exhibiting recon­figurable propa­gating directions of the chiral laser.

In previous studies, existing solutions for a chiral microlaser mainly resort to expli­citly breaking the structure symmetry of a WGM micro­cavity. Unfortunately, the scala­bility and recon­figurability of these preceding strategies are strongly limited since the devices, once fabricated, come with a prefixed, non-tailorable laser direc­tionality. In this work, the researchers achieve a recon­figurable chiral microlaser in a symmetric WGM micro­cavity by utilizing the cavity-enhanced optical Kerr non­linearity.

“We employed microcavity Raman lasers in the experiment, which generally involve a pair of balanced clockwise (CW) and counter­clockwise (CCW) waves,” said Cao Qi-Tao, a Ph.D. student at Peking University. The Raman laser waves in the two directions are coupled together through linear surface Rayleigh scattering and nonlinear phase modu­lation by optical Kerr effect. As the power of the microlaser with a particular phase increases and reaches a threshold, the linear coupling is completely compen­sated by the nonlinear coupling. Above this threshold, the chiral symmetry of the laser field breaks spon­taneously, and the Raman wave evolves randomly into a chiral state with a CW or CCW dominated laser propa­gation.

Experi­mentally, an unpre­cedented ratio of counter-propa­gating emission intensities is obtained exceeding 160:1. Furthermore, the directionality of such the chiral microlaser is all-optically and dynamically controlled by the bias in the pump direction, and the symmetry breaking threshold is adjustable using a nanotip scatterer. “Our results break the perception boundary of how to realize a recon­figurable coherent light source, to enable a powerful recon­figurability of a laser’s direc­tionality and chirality, and to extend a long-ranging impact on on-chip nano­photonics and nonlinear processes,” said Xiao. “Such a spon­taneously chiral emitting laser also can be extended to various micro­structures, and is almost free from the material limit due to the ubiquity of the Kerr non­linearity.” (Source: Peking U.)

Reference: Q.-T. Cao et al.: Reconfigurable symmetry-broken laser in a symmetric microcavity, Nat. Commun. 11, 1136 (2020); DOI: 10.1038/s41467-020-14861-5

Link: Frontiers Science Center for Nano-optoelectronics, School of Physics, Peking University, Beijing, China

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