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arXiv:1605.07288 · Applied Physics Letters

Controlling slow and fast light and dynamic pulse-splitting with tunable optical gain in a whispering-gallery-mode microcavity

M. Asano, Ş. K. Özdemir, W. Chen, Rikizo Ikuta, Linglu Yang, N. Imoto, T. Yamamoto

Published May 2, 2016• 17 citations

Abstract

We report controllable manipulation of slow and fast light in a whispering-gallery-mode microtoroid resonator fabricated from Erbium (Er3+) doped silica. We observe continuous transition of the coupling between the fiber-taper waveguide and the microresonator from undercoupling to critical coupling and then to overcoupling regimes by increasing the pump power even though the spatial distance between the resonator and the waveguide was kept fixed. This, in turn, enables switching from fast to slow light and vice versa just by increasing the optical gain. An enhancement of delay of two-fold over the passive silica resonator (no optical gain) was observed in the slow light regime. Moreover, we show dynamic pulse splitting and its control in slow/fast light systems using optical gain.

DOI: 10.1063/1.4948922

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We report controllable manipulation of slow and fast light in a whispering-gallery-mode microtoroid resonator fabricated from Erbium (Er3+) doped silica. We observe continuous transition of the coupling between the fiber-taper waveguide and the microresonator from undercoupling to critical coupling and then to overcoupling regimes by increasing the pump power even though the spatial distance between the resonator and the waveguide was kept fixed. This, in turn, enables switching from fast to slow light and vice versa just by increasing the optical gain. An enhancement of delay of two-fold over the passive silica resonator (no optical gain) was observed in the slow light regime. Moreover, we show dynamic pulse splitting and its control in slow/fast light systems using optical gain.

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.

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.

Erbium-doped whispering-gallery-mode (WGM) microcavities have great potential in many important applications, such as the precision detection and the micro/nano laser. However, they are sensitive to the fluctuations from the pump laser and the environment. Here we demonstrate the precise controlling of transmission spectra and optical gains using optothermal scanning methods in erbium-doped WGM microcavities. The transmission spectrum of the probe signal exhibits the transition between asymmetric Fano-like resonance and the Lorentz peak (or dip) through tuning the input frequency and the scanning speed of the pump laser. In particular, the analytical calculations can fit well with our experimental results through adiabatically eliminating the anticlockwise optical mode. This Letter shows that the optothermal control of gains is more robust to external noises, which paves a crucial step toward the application in the ultra-sensitive detection.

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