Fuchuan Lei

Parity–time-symmetric whispering-gallery microcavities

It is now shown that coupled optical microcavities bear all the hallmarks of parity–time symmetry; that is, the system’s dynamics are unchanged by both time-reversal and mirror transformations. The resonant nature of microcavities results in unusual effects not seen in previous photonic analogues of parity–time-symmetric systems: for example, light travelling in one direction is resonantly enhanced but there are no resonance peaks going the other way.

Dynamic Fano-like resonances in erbium-doped whispering-gallery-mode microresonators

We report Fano-like asymmetric resonances modulated by optical gain in a whispering-gallery-mode resonator fabricated from erbium-doped silica. A time-dependent gain profile leads to dynamically varying sharp asymmetric resonances with features similar to Fano resonances. Depending on the scan speed of the frequency of the probe laser and the pump-probe power ratio, transmission spectra of the active microcavity exhibit a resonance dip, a resonance peak, or a Fano-like resonance.

Three-pathway electromagnetically induced transparency in coupled-cavity optomechanical system

Recently Qu and Agarwal [Phys. Rev. A 22, 031802 (2013)] found a three-pathway electromagnetically induced absorption (TEIA) phenomenon within a mechanically coupled two-cavity system, where there exist a sharp EIA dip in the broad electromagnetically induced transparency peak in the transmission spectrum. In this work, we study the response of a probe light in a pair of directly coupled microcavities with one mechanical mode. We find that in addition to the sharp TEIA dip within a broad EIT window as found by Qu and Agarwal, three-pathway electromagnetically induced transparency (TEIT) within the broad EIT window could also exist under certain conditions. We give explicit physical explanations and detailed calculations. Our results provide a method for controlling transition between TEIA and TEIT in coupled optomechanical systems, and reveal the multiple pathways interference is versatile for controlling light.

Fabrication of a microtoroidal resonator with picometer precise resonant wavelength.

Fabricating an optical microresonator with precise resonant wavelength is of significant importance for fundamental research and practical applications. Here, we develop an effective method to fabricate ultra-high Q microtoroid with picometer-precise resonant wavelength. Our method adds a tuning reflow process, using low-power CO2 laser pulses, to the traditional fabrication process. It can tailor resonant wavelength to a red or blue direction by choosing a proper laser power. Also, this shift can be controlled by the exposure time. Meanwhile, quality factor remains nearly unchanged during this tailoring process. Our method can greatly reduce the difficulties of experiments where precise resonances are required.

Gain competition induced mode evolution and resonance control in erbium-doped whispering-gallery microresonators.

Precise control of resonance features in microcavities is of significant importance both for researches and applications. By exploiting gain provided by the doped rare earth ions or Raman gain, this can be achieved through changing the pump. Here we propose and experimentally show that by using gain competition, one can also control the evolution of resonance for the probe signal while the pump is kept unchanged. The transition of Lorentz peak, Fano-like resonance and Lorentz dip can be observed from the transmission spectra of the probe signal through tuning the auxiliary control signal. The theory based on coupled-mode theory and laser rate equations by setting the optical gains as time-dependent was constructed. This method can be used in the precise control of transmission spectra and the coupling regime between the waveguide and microcavities.

Self-sustained oscillation and dynamical multistability of optomechanical systems in the extremely-large-amplitude regime

Optomechanics concerns with the coupling between optical cavities and mechanical resonators. Most early works are concentrated in the physics of optomechanics in the small-displacement regime and consider one single optical cavity mode participating in the optomechanical coupling. In this paper, we focus on optomechanics in the extremely-large-amplitude regime in which a mechanical resonator is coupled with multiple optical cavity modes during the oscillation. We explicitly show that the mechanical resonator can present self-sustained oscillations in a novel way with limit cycles in the shape of sawtooth-edged ellipses and exhibit dynamical multistability. By analyzing the mechanical oscillation process and the accompanied variation of the optical cavity occupation, we develop an energy-balanced condition to ensure the stability of self-sustained oscillation. The effect of the mechanical nonlinearities on the dynamics of the mechanical resonator is also investigated.

Raman gain induced mode evolution and on-demand coupling control in whispering-gallery-mode microcavities.

Waveguide-coupled optical resonators have played an important role in a wide range of applications including optical communication, sensing, nonlinear optics, slow/fast light, and cavity QED. In such a system, the coupling regimes strongly affect the resonance feature in the light transmission spectra, and hence the performance and outcomes of the applications. Therefore it is crucial to control the coupling between the waveguide and the microresonator. In this work, we investigated a fiber-taper coupled whispering-gallery-mode microresonator system, in which the coupling regime is traditionally controlled by adjusting the distance between the resonator and the fiber-taper mechanically. We propose and experimentally demonstrate that by utilizing Raman gain one can achieve on-demand control of the coupling regime without any mechanical movement in the resonator system. Particularly, the application of Raman gain is accompanied by Q enhancement. We also show that with the help of Raman gain control, the transitions between various coupling regimes can affect the light transmission spectra so as to provide better resolvability and signal amplification. This all-optical approach is also suitable for monolithically integrated and packaged waveguide-resonator systems, whose coupling regime is fixed at the time of manufacturing. It provides an effective route to control the light transmission in a waveguide-couple resonator system without mechanically moving individual optical components.

Tunable erbium-doped microbubble laser fabricated by sol-gel coating

In this work, we show that the application of a sol-gel coating renders a microbubble whispering gallery resonator into an active device. During the fabrication of the resonator, a thin layer of erbium-doped sol-gel is applied to a tapered microcapillary, then a microbubble with a wall thickness of 1.3 μm is formed with the rare earth ions diffused into its wall. The doped microbubble is pumped at 980 nm and lases in the emission band of the Er3+ ions at 1535 nm. The laser wavelength can be shifted by aerostatic pressure tuning of the whispering gallery modes of the microbubble. Up to 240 pm tuning is observed with 2 bar of applied pressure. We also show that the doped microbubble could be used as a compact, tunable laser source.

Engineering optomechanical normal modes for single-phonon transfer and entanglement preparation

We propose a scheme that exploits the effect of normal-mode splitting in a three-mode optomechanical system (OMS) to manipulate phonons in a single-excitation level. The three normal modes of the OMS are formed by coupling two mechanical modes to a cavity mode, which is driven by a strong red-detuned pump field via radiation pressure. It is shown that the Landau–Zener dynamics of the three-normal-mode configuration is versatile for quantum information engineering. Explicitly, we show that it is feasible to transfer a single phonon between two mechanical modes with distinct frequencies, and an entangled W state among the three modes can be prepared. The effect of damping is analyzed by numerical simulation, which confirms the validity of our proposal.

All-optical nanopositioning of high-Q silica microspheres

A tunable, all-optical, coupling method is realised for a high-Q silica microsphere and an optical waveguide. By means of a novel optical nanopositioning method, induced thermal expansion of an asymmetric microsphere stem for laser powers up to 211 mW is observed and used to fine tune the microsphere-waveguide coupling. Microcavity displacements ranging from (0.61 ± 0.13) - (3.49 ± 0.13) μm and nanometer scale sensitivities varying from (2.81 ± 0.08) - (17.08 ± 0.76) nm/mW, with an apparent linear dependency of coupling distance on stem laser heating, are obtained. Using this method, the coupling is altered such that the different coupling regimes are achieved.