A groundbreaking laser mixing technique can manipulate electron-hole collisions to create many frequencies of light simultaneously. This mechanism for ultrafast light modulation has potential applications in high-speed optical communications. Caption: Benjamin Zaks (left) and Mark Sherwin. (Photo: UCSB) Researchers at the University of California, Santa Barbara, use a free-electron laser aimed at a gallium arsenide nanostructure semiconductor to create a quasiparticle called an exciton, a bound electron-hole pair, in the material. Excitons occur when a semiconductor absorbs a photon. The excess energy excites an electron, which causes the electron to jump into another energy level and to leave behind a positively charged hole in the energy level it left. The electron and hole are bound because of their mutual attraction. Normally, the exciton would have a smaller energy than the original electron and hole, but the researchers use a second laser with a lower frequency to smash the electron back into the hole with an energy greater than that with which it left. As a result, the electron-hole recombination emits photons at different frequencies than those it absorbed. "It's fairly routine to mix the lasers and get one or two new frequencies, said Mark Sherwin, the lead researcher, a professor at UCSB and director of the university's Institute for Terahertz Science and Technology. "But to see all these different new frequencies, up to 11 in our experiment, is the exciting phenomenon. I've never seen anything like this before." Artist's rendition of electron-hole recollision. Near infrared (amber rods) and terahertz (yellow cones) radiation interact with a semiconductor quantum well (tiles). The near-IR radiation creates excitons (green tiles) consisting of a negative electron and a positive hole (dark blue tile at center of green tiles) bound in an atom-like state. Intense terahertz fields first pull the electrons (white tiles) away from the hole and then back toward it (electron paths represented by blue ellipses). Electrons periodically recollide with holes, creating periodic flashes of light (white disks between amber rods) that are emitted and detected as sidebands. (Image: Peter Allen, UCSB) Each frequency generated by the electron-hole recollision phenomenon corresponds to a different color, he added. In terms of real-world applications, this technique can be used to transmit more information at a faster rate by sending data through multiple channels — multiplexing — or can be used for high-speed frequency modulation for a faster Internet. "Think of your cable Internet," said Ben Zaks, a doctoral student at UCSB and lead author on a paper about the work. "The cable is a bundle of fiber optics, and you're sending a beam with a wavelength that's approximately 1.5 microns down the line. But within that beam, there are a lot of frequencies separated by small gaps, like a fine-toothed comb. Information going one way moves on one frequency, and information going another way uses another frequency. You want to have a lot of frequencies available, but not too far from one another." Because the laser currently used is the size of a building, the researchers are forced to come up with a more practical way to implement these findings. One solution is to use a transistor that modulates in the near-infrared to produce strong terahertz fields akin to those of the free-electron laser. Sherwin hopes that his discovery opens up more research regarding electron-hole recollisions. Apparatus used for electron-hole recollision experiments. Large flat and curved mirrors guide and focus terahertz radiation, emitted in a different room by one of the UCSB Free-Electron Lasers, through a round cryostat window onto the sample (not visible). Smaller flat mirrors guide near-infrared radiation from the left, through a small hole barely visible at the center of the curved mirror surface, through a round cryostat window, to the sample (not visible). Near-IR laser and sidebands caused by recollisions in the sample exit through a second cryostat window (hidden), are reflected by small round mirror on the right and directed to a spectrometer (not visible). (Photo: Alison McElwee, UCSB) "We have a unique tool ... which gives us a big advantage for exploring the properties of fundamental materials. We just put it in front of our laser beams and measure the colors of light going out. Now that we've seen this phenomenon, we can start doing the hard work of putting the pieces together on a chip," he said. "I want to continue working on it, but I'd like to see a lot of other people join in." Also contributing to the research, which appears in the March 28 online issue of Nature, is R.B. Liu of The Chinese University in Hong Kong. For more information, visit: www.ucsb.edu