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Chinese Physics B

Variable optical chirality in atomic assisted microcavity*

Hao Zhang, Wenxiu Li, Peng Han, Xiaoyang Chang, Shuo Jiang, Anping Huang, Zhisong Xiao

Published Sep 9, 2020• 2 citations
Publisher

Abstract

The manipulating of optical waves in a microcavity is essential to developing the integrated optical devices. Generally, the two eigenmodes in a whispering-gallery-mode (WGM) microcavity possess chiral symmetry. Here we show the chiral symmetry breaking is induced by the asymmetric backscattering of counter-propagating optical waves in a whispering-gallery-mode (WGM) microcavity with a cavity-made slot filled with atomic vapor. Through tuning the dispersion relation of the atomic vapor in the cavity-made slot, the chiral modes are continuously steered. The mode frequency splitting in the transmission and reflection spectra stem from the chiral symmetry breaking of the two eigenmodes. The displacement sensitivity of the proposed system in response to the length variation of cavity-made slot exhibits a high sensitivity value of 15.22 THz/nm.

DOI
10.1088/1674-1056/abb660

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The manipulating of optical waves in a microcavity is essential to developing the integrated optical devices. Generally, the two eigenmodes in a whispering-gallery-mode (WGM) microcavity possess chiral symmetry. Here we show the chiral symmetry breaking is induced by the asymmetric backscattering of counter-propagating optical waves in a whispering-gallery-mode (WGM) microcavity with a cavity-made slot filled with atomic vapor. Through tuning the dispersion relation of the atomic vapor in the cavity-made slot, the chiral modes are continuously steered. The mode frequency splitting in the transmission and reflection spectra stem from the chiral symmetry breaking of the two eigenmodes. The displacement sensitivity of the proposed system in response to the length variation of cavity-made slot exhibits a high sensitivity value of 15.22 THz/nm.
We report a novel, to the best of our knowledge, way to achieve phase-controlled dual-wavelength resonance based on whispering-gallery-mode (WGM) microcavities experimentally. With the help of a feedback waveguide, not only two optical pathways but also a unidirectional coupling between counter-propagating waves are formed, which is the requirement of all-optical analogues of electromagnetically induced transparency and Autler-Townes splitting. By adjusting the accumulating phase introduced from the fiber waveguide, we observe the signal lineshape changes from symmetric to asymmetric, i.e., the resonant transmission and extinction ratio of two splitting modes can be controlled, which brings a new degree of freedom to the WGM resonator system. These results may boost the development of quantum state control and pave the way for reconfiguring devices such as narrow-band filters.
The chiral light-matter interaction, which shows great potential in applications ranging from photonic devices to quantum information processing, can be achieved with the development of recent experimental advances of the fabrication technologies on nanoscales. Traditionally, the chiral optics based on spin-momentum locking in micro- and nanophotonics could be observed only for specific optical modes, such as the transverse magnetic mode. Here in this study, we investigate that the chirality of a whispering-gallery-mode (WGM) microresonator can be well controlled with assistance from coupled Rydberg-atom ensembles. The presented asymmetric backscattering also originates from interferences between scattered optical modes in the WGM microresonator, which is proposed by Wiersig [Phys. Rev. A 84, 063828 (2011)]. Therefore the flow of light can be well controlled, and the statistical properties of photons could be changed by regulating this asymmetric coupling. Most importantly, its universal chiral property is not limited by specific optical modes which may also be used as optical diodes and routers.
Backscattering in whispering-gallery-mode resonators is usually a passive mode-splitting mechanism produced by a fixed defect. Here, we show that, when the backscatterer is a mechanical angular degree of freedom, the same process becomes an angular-recoil backaction channel capable of generating chirality under reciprocal driving. A localized movable scatterer coherently converts photons between clockwise and counterclockwise whispering-gallery modes, transferring angular recoil in each circulation-changing event. In a weak-scattering driven-dissipative model, reciprocal bidirectional pumping gives zero net torque at rest, but rotation Doppler-shifts the two opposite scattering rates in opposite directions. For suitable detuning, this feedback produces negative angular friction, destabilizes the nonrotating reciprocal state, and selects one of two symmetry-related steady rotations. The threshold scales inversely with the square of the WGM azimuthal index. The mechanically chiral state produces a direction-dependent weak-probe response, visible as a Doppler splitting of the backscattered spectra, turning passive WGM mode splitting into a minimal mechanism for autonomous chiral optomechanics.
We report measurements of cavity-QED effects for the radiative coupling of atoms in a dilute vapor to the external evanescent field of a whispering-gallery mode (WGM) in a fused silica microsphere. The high QQQ $(5\ifmmode\times\else\texttimes\fi{}{10}^{7})$, small mode volume (10${}^{\ensuremath{-}8}$ cm3){}^{3})3), and unusual symmetry of the microcavity evanescent field enable velocity-selective interactions between fields with photon number of order unity in the WGM and ${N}_{T}\ensuremath{\sim}1$ atoms in the surrounding vapor.
Two coupled resonance modes can lead to exotic transmission spectra due to internal interference processes. Examples include electromagnetically induced transparency (EIT) in atoms and mode splitting in optics. The ability to control individual modes plays a crucial role in controlling such transmission spectra for practical applications. Here we experimentally demonstrate a controllable EIT-like mode splitting in a single microcavity using a double-port excitation. The mode splitting caused by internal coupling between two counter-propagating resonances can be effectively controlled by varying the power of the two inputs, as well as their relative phase. Moreover, the presence of asymmetric scattering in the microcavity leads to chiral behaviors in the mode splitting in the two propagating directions, manifesting itself in terms of a Fano-like resonance mode. These results may offer a compact platform for a tunable device in all-optical information processing.
We theoretically describe and study coupling between two whispering-gallery-mode microcavities through a side coupled fiber taper. The two microcavities can couple to each other bidirectionally with only one fiber taper waveguide due to the modal coupling between a pair of whispering-gallery modes traveling in opposite directions. By choosing appropriate physical parameters, some interesting spectra, such as electromagnetically-induced transparency-like coherent transmission and band-like reflection, can be obtained in such a system. Slow light is discussed in this paper as a direct application of the coupled microresonator structure. This structure also has potential application in coupled cavity quantum electrodynamics.