APL Photon. 4, 030806 (2019)

The technique of phase conjugation has long been used to perform time reversal of wavefronts in the frequency domain. More recently, linear approaches to time reversal of pulses in the time domain have been shown effective for microwave signals. However, the application of these approaches is technically challenging at optical frequencies. Now, a team at Kyoto University, Japan, has demonstrated linear time-reversal of infrared light in a planar photonic crystal system using a dynamic approach. The technique relies on fast, non-adiabatic tuning of coupled cavities. Three two-dimensional silicon photonic crystal resonators were fabricated with separations of 41 μm. Each cavity has a Q-factor of about 400,000, is coupled via waveguides and has its temperature controlled for initial tuning purposes. Two cavities were fixed to 1,546.8 nm and the third was detuned by +0.3 nm; rapid (dynamic) tuning was then provided via optical pulses. A pulsed Ti:sapphire laser and an optical parametric oscillator provided synchronized excitation pulses (∼1,547 nm) and control pulses (∼820 nm) and an optical Rabi oscillation was observed. Applying a control pulse to one cavity generates free carriers in the silicon slab, lowering the refractive index on a 3-ps timescale (satisfying the condition for non-adiabaticity) and enabling time reversal of the optical Rabi oscillation.