August 31, 2009--Earlier this month researchers from Purdue, Cornell and Norfolk State universities reported demonstration of the smallest laser ever --, consisting of a nanoparticle just 44 nm across. Strictly speaking it was a spaser --, or surface plasmon laser. Now it's the turn of researchers at the University of California at Berkeley. This week in an article in Nature the Berkeley researchers claim to have created the smallest semiconductor laser ever. Their new device can generate light in a space just 5 nm in size.
In a typical laser the cavity length must be at least as big as half the wavelength of the light--about 200 nm for visible light. Realizing a device that can break the diffraction limit is expected to pave the way for many new applications, like optical computers that use light instead of electrons to process information, biosensors and nanometer-sized photonic circuits.
One approach to making tiny lasers is to make use of surface plasmons--light coupled to the electrons at the surface of metals. The resulting excitations of light and electrons are known as "surface plasmon polaritons" (SPPs). Until now, however, resistance losses in the metal at optical frequencies have absorbed the SPPs, making such devices difficult to realize.
But by separating a cadmium sulphide semiconductor nanowire from a metallic silver surface with a 5 nm thick insulating layer of magnesium fluoride, the Berkeley researchers have overcome this problem. Because it is non-metallic the insulating layer allows the SPPs to last longer. The so-called "hybrid plasmonic waveguide" can concentrate light into an area as much as 100 times smaller than a diffraction-limited spot.
According to Professor Xiang Zhang, who led the research team, the impact of plasmonic lasers on optoelectronics integration is potentially significant because the optical fields of these devices rival the smallest commercial transistor gate sizes and thereby reconcile the length scales of electronics and optics.
--Posted by Steve Anderson, stevega@pennwell.com; www.laserfocusworld.com.
