Appl. Phys. Lett. 101, 171110 (2012)

Rapidly changing the properties of an optical cavity is a popular technique for converting light from one wavelength to another. Such changes modify the resonant frequency of the resonator and can force the photons to adapt by changing colour to 'fit' into the new cavity. Stefan Preble and colleagues from the Rochester Institute of Technology in the USA have now extended this idea to the low-photon-number regime in a silicon ring resonator. They used a 415 nm pump laser to modify the resonance of the ring cavity through a free-carrier-induced change in its refractive index. The 1,523 nm probe light, produced by an optical parametric oscillator, matched the original resonant frequency of the ring resonator and was attenuated to the single-photon power level. The researchers measured a conversion efficiency of more than 10% for wavelength shifts of around 0.3 nm, although they say this can be increased to 100% in principle. The observed wavelength shift at low photon numbers may have important implications for quantum information applications.