Recently, a novel method of dynamic modulation of refractive index has been developed by Prof.Yihui Wu’s Group in the State Key Laboratory of Applied Optics (SKLAO), CIOMP, CAS.The researchers theoretically and experimentally demonstrated thermal oscillatory behavior under differing conditions and published their breakthroughs in Optics Letters (DOI: 10.1364/OL.40.001607).

The whispering-gallery-mode resonator is commonly used to study light–matter interactions due to its unique ultra-high quality factor and ultra-small mode volume. Since the electrical field is enhanced in such an optical microcavity, material absorption results in obvious thermal nonlinear effects, such as bistability and oscillatory behavior. During the thermal nonlinear process, the heat generated by photon absorption dissipates from the mode volume to the rest of the microcavity (i.e. the fast thermal relaxation process), and then from the microcavity to the surrounding environment (i.e. the slow thermal relaxation process).

The thermal effects of the microcavity have garnered much attention; in particular, thermal linewidth broadening has already been applied in mode-locking and optical path control. However, these thermal nonlinear effects are usually associated with thermal oscillations that jeopardize the mode-locking and the accuracy of optical path control. Many factors contribute to thermal oscillations; the oscillatory behaviors resulting from mode coupling and Andronov–Hopf bifurcation have been investigated.

Prof.Wu’s team has observed a type of oscillatory phenomenon in a silica microspherical resonator coupled with a fiber, in which the transmission spectrum only generates downward oscillation peaks in the upscan of the pump wavelength and upward peaks related to the downscan process. The researchers also demonstrate that a discontinuous sweeping of the pump wavelength and a fast thermal relaxation process result in the oscillatory behavior of the transmission spectrum.

Using a thermal dynamical model involving two thermal relaxation processes, the team investigatesthe oscillations and obtains results that agree well with experimental findings. With this model, they can also describe dynamic modulation of the refractive index (RI) in the microsphere. They achieved modulation of the RI in the mode volume with frequencies on the order of kilohertz through the fast relaxation process. Although the modulation frequency of the thermal process is lower than that of other nonlinear processes, such as the Kerr effect, a larger modulation range(～10-5) owing to the larger thermo-optical coefficient (dn∕dT=1.2 × 10-5K-1) relative to the Kerr coefficient (2.2 × 10-20m2W-1) can be attained.

This work was supported by the National Natural Science Foundation of China,National High Technology Research and Development Program of China, and others.

For more details, please see Wang et al. (2015) Thermal oscillatory behavior analysis and dynamic modulation of refractive index in microspherical resonator. Optics Letters40(7): 1607-1610. http://dx.doi.org/10.1364/OL.40.001607

Thermo-optic locking during one sweeping period(Image by CIOMP)

Dynamic refractive index modulation in the microspere

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