Researchers have shown that they can switch light flow through a cadmium sulfide nanowire on and off by using another light pulse. Light switching is currently done in telecommunications and photonic chips using electrical signals, but using optical signals instead would be more efficient. The nanowire switches are “among the smallest all-optical switches ever produced,” said Ritesh Agarwal, a professor of materials science and engineering at the University of Pennsylvania, who led the new work. Agarwal, his graduate student Brian Piccione, and their colleagues combined the optical switches to create a logic gate, a fundamental building block of computing. The idea is used in electronic circuits, where the two output levels of an electronic switch represent the ‘1’ and ‘0’ of digital logic, and multiple switches are combined to create logic circuits. The advance, published in Nature Nanotechnology, is a step towards fully optical nanowire logic circuits, Agarwal says. Such photonic circuits could form the backbone of gadgets that work on light instead of electricity, and hence would be faster and less power-hungry than today’s silicon-based electronic devices. More immediately, all-optical switches could impact telecommunications. In today’s telecom systems, information is encoded onto light pulses that zip through optical fibers. But when the data needs to be processed, it first has to be converted to an electrical signal that is manipulated and then converted back into light. “[This] introduces a delay and requires significant amounts of power,” Agarwal says. Another approach is to channel light from the fiber into light-guiding structures on a semiconductor chip. Here, the light signals can be modulated by slightly changing the refractive index of the semiconductor material. However, this method has its own drawbacks. Tweaking the refractive index either requires an electrical signal or a high-power optical signal. Plus, the light-guiding structures take up space. “Some fabricated silicon all-optical switches have been demonstrated to function using very little power,” says Agarwal, “but this comes at a cost of footprints an order of magnitude larger than what we have demonstrated.”The researcher team knew from previous work that cadmium sulfide nanowires exhibit strong light-matter coupling, which makes them especially good at manipulating light. The challenge was to guide light into these structures that are less than 200-nm wide in order to manipulate it. To do that, they used the nanowire itself as a light source. First, they used vapor-liquid-solid synthesis to grow the nanowires out of cadmium sulfide powder. Next, they used a focused gallium ion beam to precisely slice a gap into the nanowire, splitting it into two portions. Then they continuously pumped a pulsed Ti:sapphire laser beam into the first portion so that it emitted laser light from its end, becoming a light source. Because the two segments are from the same nanowire, they are perfectly aligned and the light crossed the gap and flowed through the second segment. Now, when the researchers shine another beam of light on the second segment, the laser light traveling through it turns off. The researchers believe the switching happens because of a mechanism called stimulated polariton scattering, which is enhanced at the nanoscale.By combining two nanowire switches into a Y configuration, the researchers constructed a NAND gate. A NAND (which stands for Not AND) gate gives a 0 output when both its inputs are 1. To make practical switches, Agarwal says one would need to make new designs to create the pump and probe lasers on the chip itself for ultimate miniaturization.Read the abstract at http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2012.144.html.