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All-optical silicon switching

Cornell University researchers have achieved all-optical switching in silicon using relatively low pulse energies (Nature October 28, 2004).

Cornell University researchers have achieved all-optical switching in silicon using relatively low pulse energies (Nature October 28, 2004). The scientists used highly confining resonant structures to enhance the sensitivity of light to small changes in refractive index. Modulation of up to 94% in less than 500psecs (0.5nsecs) was achieved using light pulses with energies as low as 25pJ. Previous attempts to create all-optical switching on silicon have been hampered by the weak dependence of the material's refractive index and absorption coefficient on free-carrier concentrations. The makes it hard to produce significant modulation of light in small structures.

The Cornell work used a ring resonator coupled to a waveguide built on a Smart Cut silicon-on-insulator (SOI) wafer from Soitec. The ring resonator and waveguide have 450x250nm cross-sections. The ring has a 10micron diameter. The on-resonance transmitted power is 10dB higher than the background off-resonance 3.5dB loss. A pulsed laser pump into the ring creates free carriers through two-photon absorption modulating the transmitted signal. The small resulting shift in refractive index moves the peak of the resonance causing the modulation in signal transmission. The scientists see modulation, switching and routing applications for the structure. Time responses of such devices could be as low as 100psecs.

The team also sees possibilities for a ring coupled to two waveguides - incoming data and a control signal would be coupled to one (input and through ports) and some of the signal could be routed to the second (drop port). Such a device could switch data either to the through or drop port depending or the presence or absence of a control pulse.

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