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31 May 2006
A US-developed system could lead to all-optical transistors and single-photon switching for quantum information networks.
Scientists at Duke University, US, are developing an all-optical switching system which they say could become the optical equivalent of the electronic transistor. Research leader, Daniel Gauthier of the university's Department of Physics, believes that the system has some important advantages compared with other attempts at developing all-optical switches.
Gauthier's team has succeeded in switching a high-intensity light beam using a much weaker beam. Most existing attempts at all-optical switching use strong beams to control weaker ones, he told optics.org. The research also suggests that single photon switching might become possible, which is important for quantum information networks.
The Duke University system uses a combination of strong nonlinear interactions and transverse optical patterns. In a nonlinear material, light of sufficiently high intensity changes the optical properties of the medium which in turn affects any light propagating in the material. This means that a beam of light traveling through the material can affect the interaction between the material and another beam.
The team uses rubidium atoms because they exhibit strong nonlinear optical effects and also relies on an approach known as resonant enhancement. Resonant enhancement involves using light with a frequency that matches the energy of a transition between an atom's excited and ground state. This ensures that the light and the atoms interact much more readily than if light with different frequencies was used.
Gauthier and colleagues use two laser beams pointing in opposite directions which overlap inside a rubidium vapour glass cell. Interactions between the laser beams and the rubidium atoms create what is called "counterpropagating beam instability". This instability leads to the generation of new light which is emitted along a cone about the original beams.
When light exits the system, it can be projected onto a screen perpendicular to the direction of propagation. A third laser beam can be used to control the orientation of the pattern emitted from the rubidium vapour. This is possible because the electromagnetic patterns created by the counterpropagating laser beams are very sensitive to additional light entering the system and can be made to rotate.
The researchers observed that 100% of the pattern rotated when they used a strong enough switching beam - corresponding to about 40,000 photons injected into the vapour cell along the switching beam direction. For smaller photon numbers, only a fraction of the power in the pattern rotated to the new orientation - about 50% for about 3000 injected photons.
The team believes that the fraction of the switched pattern depends linearly on the injected photon number. The device might be analogous to the operation of an electronic transistor where the input switching beam corresponds to the signal applied to the transistor base and the counterpropagating beams correspond to the transistor bias.
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