08 septembre 2011

Reflectivity as a function of the wave-number for various delays between the probe and pump pulses.

In the same way that the invention of the transistor revolutionized electronics, the development of ultrafast all-optical switches is a major stake for very high rate information processing, for instance for very high rate telecommunications. In collaboration with the University of Twente, the NPSC team has developed a photon switch with a response time below one picosecond, i.e. ten times faster than previous devices.


An optical switch is for instance a resonant system that lets light through only at a well defined wavelength λ0 that can be modified so as to switch from an "off-state" to an "on-state". In our case, we use a semiconductor cavity with a transmission resonance in the infra-red at λ0 = 1.28 µm. Under a “pump” optical beam, λ0 varies as electron-hole pairs are created in the active material (GaAs) and modify its optical index. The “on” switch is ultra-fast, but not the transition back to “off” because it is dependent upon the recombination of the carriers. To improve the device, we have used the Kerr effect: a material under strong optical illumination undergoes an instantaneous modification of its index of refraction. In this case, the pump wavelength (2.4 µm) is such that no carriers are photocreated by a one- or two-photon absorption.

We have shown the relevance of this approach by studying the transmission of the cavity with a probe beam. When the pump beam is shone onto the sample, the microcavity resonance is shifted due to the Kerr effect and the probe beam is blocked. In our experiment (Fig.), we vary the delay between pump and probe pulses; a shift of the cavity resonance l0 during the pulse length is observed. The double commutation time (on-off-on) is below one picosecond. This switch could thus be used to modulate an optical telecom beam at record high rates, above 1 THz. Our study now aims at miniaturizing the device so as to reduce the commutation energy.


Further reading: Ctistis G et al., Applied Physics Letters 98 (2011) 161114


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