Inducing Photonic Transitions between Discrete Modes in a Silicon Optical Microcavity

We show the existence of direct photonic transitions between modes of a silicon optical microcavity spaced apart in wavelength by over 8 nm. This is achieved by using ultrafast tuning of the refractive index of the cavity over a time interval that is comparable to the inverse of the frequency separation of modes. The demonstrated frequency mixing effect, i.e., the transitions between the modes, would enable on-chip silicon comb sources which can find wide applications in optical sensing, precise spectroscopy, and wavelength-division multiplexing for optical communications and interconnects.

Figure 1

(a) Spectrum of the drop port measured for a ring resonator with a radius $100\mu \mathrm{m}$. A band-pass fiter is used to eliminate amplified spontaneous emission of the input probe laser. The green line shows the reference spectrum when pump is off, and the probe beam is on resonance. The red line shows the spectrum following the pump pulse where 15 new wavelengths are generated simultaneously. Inset, the top-view microscope image of the fabricated device. (b),(c) The diagram of the discrete cavity modes of an optical microcavity before (b) and after (c) pump is incident on the sample.

Figure 2

(a) Spectrum of the drop port. The input wavelength of the cw laser is on resonance with the ring cavity (large peak). Other resonance peaks appear, resulting from amplified spontaneous emission of the laser. (b) Spectrum of the drop port when the pump is present. The input probe wavelength is detuned from the ring resonance. (c) Same as in (b) when the input wavelength is on resonance with the ring cavity. Seven new wavelengths appear in the spectrum. The black arrows indicate the original modes and the green arrows indicate the newly generated wavelengths. (d) The amplitudes of the generated wavelengths versus the probe power. The dashed lines are linear fittings of the data. (e) The wavelength shift for the 0-order mode versus the pump energy. (f) Total conversion efficiency (the sum of all efficiency for new generated waves) versus the pump energy. In (e)–(f) the dashed lines serve as a guide to the eye.

Figure 3

(a) Spectra with different pump pulse durations. It is evident that the probability of nonadiabatic transitions increase as the pulse duration becomes shorter.