Silicon-Nanocrystal-Coated Silica Microsphere Thermooptical Switch

We report on a low-switching-energy, all-optical fiber switch that consists of a silica microsphere resonator coated with a silica layer containing silicon nanocrystals. A signal at 1450 nm and a pump at 488 nm are coupled into the microsphere through a tapered fiber. When a pump pulse is launched into the sphere, it is absorbed by the nanocrystal layer, causing the sphere to heat up and change its refractive index. The index change can be exploited to switch the signal by shifting the microsphere resonance. A resonance wavelength shift of 5 pm, sufficient to fully switch the signal, was observed with a pump pulse energy of only 85 nJ. The rise time of the switch was ~25 ms (limited by the pump peak power) and its fall time was ~30 ms (limited by the sphere´s thermal time constant). The product of the switching peak power (3.4 mu W) and the device´s characteristic dimension (a diameter of 150 mum) is 5.1times10^-10 Wm, one of the lowest values reported for an all-optical fiber switch