Ultrafast optical control using the Kerr nonlinearity in hydrogenated amorphous silicon microcylindrical resonators

Abstract

Microresonators are ideal systems for probing nonlinear phenomena at low thresholds due to their small mode volumes and high quality (Q) factors. As such, they have found use both for fundamental studies of light-matter interactions as well as for applications in areas ranging from telecommunications to medicine. In particular, semiconductor-based resonators with large Kerr nonlinearities have great potential for high speed, low power all-optical processing. Here we present experiments to characterize the size of the Kerr induced resonance wavelength shifting in a hydrogenated amorphous silicon resonator and demonstrate its potential for ultrafast all-optical modulation and switching. Large wavelength shifts are observed for low pump powers due to the high nonlinearity of the amorphous silicon material and the strong mode confinement in the microcylindrical resonator. The threshold energy for switching is less than a picojoule, representing a significant step towards advantageous low power silicon-based photonic technologies.

Kerr nonlinear switching in a hybrid silica-silicon microspherical resonator

“…To ensure that the observed shifting was due to the pump interacting with the silicon core, these measurements were repeated using the pure silica microsphere, where a maximum red shift of only ~0.05 nm was obtained for both pump sources. Although the power thresholds for the shifts observed in our hybrid resonator are higher than previously reported in the pure silicon resonators [20], this is a consequence of the off-resonance, side pumping geometry. Furthermore, for this scheme it is not possible to ascertain exactly how much of the pump light is coupled into the silicon core, so we can expect that the actual power required for this level of shifting will be lower.…”

“…3(b) is the averaged signal pulse obtained by applying a low pass filter to remove the high frequency oscillations, which cleans up the response but also artificially broadens it in time. However, as for this off-resonance pumping scheme the Kerr index modulation will occur on the timescale of the 720 fs pump duration, we expect the on/off switching time to be sub-picosecond, which is an order of magnitude faster than previously reported in the pure silicon microcylindrical resonators [20].…”

“…It is worth noting that additional measurements conducted on an unshaped section of the fiber in Fig. 1(a) revealed Q l ~4 x 10 4 , which is still more than two times larger than the highest Q, pure silicon fiberbased resonators [20]. As a final comparison we also fabricated a pure silica microsphere with the same outer diameter and a similar Q factor of Q l ~1.55 x 10 5 , from which we obtained a FSR of ~4.5 nm.…”

“…This modulated signal was obtained for an average pump power of 10 mW, for which we recorded an extinction ratio of 5.9 dB. Unfortunately, owing to the bandwidth limitation of our measurement system only the impulse response of the high-speed on/off switching could be detected [20], so that it was not possible to fully resolve the temporal dynamics. Thus the result shown in Fig.…”

Kerr nonlinear switching in a core-shell microspherical resonator fabricated from the silicon fiber platform

“…We note that because the side coupled light is not launched into a circulating mode, the pump thresholds required to observe the nonlinear shifts are larger than what has previously been reported in Refs. [1,5]. However, this scheme does have the advantage of removing many of the restrictions associated with resonance pumping, such as the necessity for a precise pump wavelength as well as bandwidth limited coupling.…”