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Photonic crystals speed up all-optical switching

21 Feb 2008

A polystyrene photonic crystal that acts as an all-optical switch boasts picosecond response time and low power requirements.

Researchers in China claim to have developed an ultrafast and low-power all-optical switch based on photonic crystals. They claim that their polymer-based device exhibits a high switching efficiency of 80%, a low operational power of 0.1MW/cm2 and an ultrafast response time of the order of a picosecond. (Nature Photonics doi:10.1038/nphoton.2007.299)

"All-optical switching is an essential component of optical communication networks. It requires low pump power, high switching efficiency and fast time response," Qihuang Gong a researcher from the University of Beijing, told optics.org. "We have realized all-optical switching, where operation power is reduced by four orders of magnitude whilst maintaining picosecond response time."

"The picosecond switching time is impressive, despite being due to the polymer used by the team," said Thomas Krauss from the photonic crystal research group at the University of St Andrews, UK, who is also pioneering work in this area.

Until now, the use of photonic crystals has been limited because they can be destroyed by the high pumping intensities required for high switching efficiency. Pump power can be reduced using two methods: either the strong photon confinement effect can be used to enhance the nonlinear interaction of light and matter, or materials with large nonlinear optical coefficients can be used. The problem for scientists, however, is that the larger the nonlinear optical coefficient of the material, the slower the nonlinear response time.

The solution adopted by Gong and his colleagues takes advantage of polystyrene, a material with a large third-order nonlinear optical coefficient. "Operation power and switching efficiency are determined by the third-order nonlinear optical susceptibility of the material," explained Gong. "By using near-resonant enhancement of the optical nonlinearity, we can use ultra-low pump power and achieve large switching efficiency."

The team used polystyrene doped with coumarin as the matrix of the photonic crystal. "We chose coumarin because its linear absorption band overlaps with that of polystyrene," explained Gong. "Polystyrene is widely used as a base material for optical devices thanks to its high environmental and thermal stability, high optical damage threshold, and good film quality."

Femtosecond pump and probe beams were used to study the transmittance properties of the photonic crystal. "The probe beam was centered at 788 nm, we used pulses with a duration of 120 fs and repetition rate of 76 MHz," explained Gong. "The pump beam was centered at 394 nm as this is within the linear absorption band of the photonic crystal."

When the pump pulse is moved far away from the probe pulse in the temporal sequence, the probe pulse propagates through the photonic crystal. The transmittance of this pulse maintains its maximum value and switching is in the "ON" state. When the pump and probe pulses are at a zero time delay, the transmittance of the probe pulse drops to its minimum value and the switching is in the "OFF" state.

"Usually, the high and low transmittance of the probe beam may be considered as 'on' and 'off' states respectively for optical computing," commented Gong. "In this way, the pump beam plays the 'switch' (like the switch in electronics)."

"The fact that both the pump and the probe are resonant in the cavity is elegant, and this leads to low pump power," commented Krauss. "However, the photonic crystal aspects are rather disappointing and far inferior to the work by Notomi et al at NTT basic research laboratories in Japan."

Before the device can find practical applications, several hurdles must be tackled, such as low-light coupling efficiency, the quality of photonic crystal and mass-production. "Our next step is to investigate other composite materials for use with different wavelengths," concluded Gong. "We are also working towards all-optical switching with a response time of the order of femtoseconds, a pump-energy of several femtojoules and a switching efficiency of over 90%."

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