Along the spectrum of visible and invisible lightwaves, scientists seeking to develop optical technologies are often guided by a critical factor that also animates real-estate agents – location.
This is especially true with lasers, which can gain or lose the ability to perform optical telecommunications and other functions if the wavelength at which they are emitted shifts by a few tens of nanometers.
Conventional lasers emit light along a single wavelength. Broadband semiconductor lasers achieve greater spectral range by emitting light along multiple wavelengths at the same time.
Boon S. Ooi and his students in Lehigh’s Center for Optical Technologies (COT) have developed a new type of broadband semiconductor laser that emits light over an 85-nm span of the infrared region of the spectrum.
Ooi, an associate professor of electrical and computer engineering, says his group’s broadband laser can be generated at a cost of a few hundred dollars from a device measuring just a few hundred micrometers in size.
By contrast, conventional broadband lasers, which are generated with a short-pulse crystal laser technique, require equipment that costs several hundreds of thousands of dollars and must be housed on a large table.
The small size and low cost of Ooi’s laser, coupled with an exceptional power of more than 500 milliwatts, give the laser potential applications not only in optical telecommunications but also in biosensing and biomedical imaging and diagnosis, Ooi says. One potential application will be to improve optical coherence tomography, a noninvasive imaging technique that obtains high-resolution images of subsurface tissue. The new laser will be able to achieve superior resolution at deeper penetrations, says Ooi, enhancing the accuracy of diagnostic techniques.
Ooi and his group reported the results of their research in 2008 in an invited paper in the IEEE Journal of Selected Topics in Quantum Electronics.
Members of the group include James Hwang, professor of electrical and computer engineering at Lehigh, and several Lehigh alumni, as well as the U.S. Army Research Laboratory (ARL) and IQE Inc., an international supplier of advanced semiconductor wafers based in Bethlehem. The group’s work is supported by ARL, NSF and the state of Pennsylvania.
Beyond dots to dashes
The success of Ooi’s laser requires an understanding of the behavior of lightwaves and of the physical, mechanical and optical properties of semiconducting materials at the nanoscale.
Ooi’s laser device contains an ensemble of light-emitting quantum dashes arrayed on an indium-phosphide substrate at a density of 1 billion per square centimeter. A quantum dash is an elongated version of a quantum dot, a nanosized semiconductor that spatially confines electrons and hole pairs.
Ooi’s team uses quantum dashes made of two semiconducting materials and assembled into a laser structure measuring half a millimeter long and 300 microns wide. The structure’s diode contains four sheets, each with five quantum-dash monolayers, including embedding quantum-well and barrier layers. The dimensions of all these features measure in the tens of nanometers or smaller.
The laser’s inhomogeneous structure, says Chee-Loon Tan, a Ph.D. student, enables it to emit light along a relatively wide range of the spectrum.
“Each of the dashes emits light,” says Tan. “Because the dashes have different sizes, heights, compositions and geometries, they generate different wavelengths.”
After the laser structure has been assembled, Ooi’s team uses an intermixing technique called impurity-free vacancy disordering (IFVD) to enhance bandwidth and achieve the desired wavelength for the laser.
The researchers hope to increase the bandwidth emission of their laser to 160 nm.