When combined with an equally tiny semiconductor laser the group has already made, these components can form nanoscale photonic integrated circuits that are a thousand times smaller than those currently in use, and which should be cheaper to fabricate and more efficient to use. The research group is headed by Seng-Tiong Ho, associate professor of electrical and computer engineering at Northwestern's Robert R. McCormick School of Engineering and Applied Science. The findings were presented at the annual Conference on Lasers and Electro Optics in Baltimore May 22, by graduate student Deana Rafizadeh, research associate Dr. Jian-Ping Zhang, and other collaborators. The findings will be published in a forthcoming issue of Optics Letters.
"It's very beautiful work," said Yoshihisa Yamamoto, professor of electrical engineering at Stanford University and one of the world's leading authorities on opto-electronic devices. "It's a great advancement toward very low threshold microcavity laser systems." The first important breakthrough for the Ho group came in 1993 with a theoretical paper showing that the spontaneous emission of photons from atoms is not unchangeable --- that it could be modified by using nanoscale structures to channel the photons into tiny waveguides measuring in the ten thousandths of millimeters. Waveguides in current photonic devices are a few hundredths of millimeters wide.
In May of 1995 the group announced the construction of the first laser to take advantage of this effect, a photonic wire laser that was the smallest operating laser ever constructed. It had a cavity volume of .3 cubic micrometers, or four thousand millionths of a cubic millimeter. It also had an unprecedented level of efficiency, with 70 percent of the photons emitted in its microscopic quantum well entering into the lasing mode at threshold levels. In a typical laser, only one in 100,000 photons enters the lasing mode at the threshold level.
The announcement of the newly developed resonator, called a "nanoscale waveguide-coupled microcavity resonator," means that these tiny lasers can now be used in photonic integrated circuits. The resonator can act as an on and off switch for the stream of photons, or as a modulator to change their power levels. The resonator can be controlled electronically. When the laser and the resonator are combined, a stream of photons moves down a nanoscale waveguide past the resonator. If the resonator is vibrating at the same frequency as the photons, the photons are channeled off into the circular resonator, and channeled into a second waveguide. The nanoscale waveguide is a few hundred nanometers wide (one nanometer is one millionth of a millimeter) and the resonator measures only a few micrometers in diameter.
The paper presented at the CLEO conference showed experimental results demonstrating that the photons had in fact been channeled into the second waveguide. The resonator has a cavity Q of about 10,000, which means that the resonance levels were sharp and clearly distinguished. High cavity Q is difficult to achieve but is very important for frequency or wavelength selectivity.
This means that the Northwestern group now has the main components to manufacture photonic integrated circuits with nanoscale dimensions, and the first nanoscale photonic integrated circuit facility in the world is under construction in the Northwestern University/Evanston Research Park. Photonic integrated circuits are at the heart of fiber optic communications systems. They provide light sources modulated with high-speed digital optical signals and detectors that can recognize those signals. The signals are transmitted down optical fibers and then channeled out to different geographical locations, again by photonic devices.
In order to transmit as many optical signals as possible, many different wavelengths of light can be transmitted at the same time down a single optical fiber. This system is called WDM, for wavelength division multiplexing.
In these WDM systems, as many as 30 different wavelengths are sent down a single optical fiber, and photonic devices are necessary to pick up each one of these wavelengths separately. The resonator announced by the Ho group is capable of picking up each of these wavelengths with high precision because of the high cavity Q.
The first practical application of the nanoscale photonic integrated circuits will probably be for a WDM fiber-optic communications network, which could be very useful in transmitting signals from fiber optic cables to individual homes, allowing for very high data rate fiber optic terminals that are needed to transmit high quality video images.
The research team is made up of Ho, Rafizadeh, Zhang, and Dr. Kathleen Stairs, in collaboration with Susan Hagness and Allen Taflove, professor of electrical and computer engineering, who did the device simulation. Richard Tiberio from the Cornell Nanofabrication Laboratory did the electron-beam lithography for the devices.