Dimas, Wang continue exciting research in semiconductor photonics

	Electrical engineering professor Boon S. Ooi (center) with award winning Ph.D. students Clara Dimas (left) and Yang Wang (right).

Clara Dimas had already made a name for herself before she joined Lehigh’s Center for Optical Technologies (COT) last year.
Several years after graduating from the University of Pennsylvania, Dimas joined Boston Micromachines Corp., where she tested, developed and packaged micro-mirror devices. She published six papers in conference proceedings and filed for a U.S. patent in adaptive optics.
Yang Wang also traveled a circuitous route before joining the COT in 2003. He earned a B.S. in mechanical engineering from the University of Science and Technology in Hefei, China, then won a scholarship from the Singapore-MIT Alliance to attend the National University of Singapore, where he studied advanced materials for microelectronics and earned an M.S. in electrical engineering.
Today, the careers of Dimas and Wang are intersecting.
Both are Ph.D. students with Boon S. Ooi, associate professor of electrical engineering, who did his Ph.D. in Scotland and led optoelectronics research in Singapore and at a company in California before joining Lehigh’s faculty in 2003.
Both are tackling challenges in semiconductor photonics. Dimas works on broadband light emitters and integrated photonic subsystems for bio-imaging. Wang also seeks to integrate bandgap sizes as they vary from one region to another on a compound semiconductor chip.
And both students recently won major awards from the International Society for Optical Engineering (SPIE). Dimas received the SPIE 2005 Scholarship, which is given to students around the world for “potential long-range contributions to optics and photonics.” Wang was one of 13 student researchers to receive SPIE’s Newport Spectra-Physics Research Excellence Award at Photonics West 2006, a major optics conference that drew more than 10,000 delegates to San Jose, Calif.
Wang and Dimas are now launching Lehigh’s first SPIE chapter. Their goals for the new student group are to promote networking, technical discussions and lectures on optics topics, and to help other students with conference travel and scholarship applications.
Playing catch-up
One of the first things Wang discovered when he arrived at Lehigh was how rapidly semiconductors have evolved.
Wang had acquired a thorough grounding in the properties of the silicon that is used as an elementary semiconductor for electronic and microelectronic devices.
But he knew very little about compound semiconductors, which utilize two or more elements from Groups III and V of the Periodic Table and which are valued for their application to optoelectronic and optical devices.
“In Singapore, I became familiar with silicon processing and devices,” says Wang. “But when I came to Lehigh, I knew nothing about compound semiconductors. The processing and the devices of compound semiconductors are much different from those of silicon. I had to start from the beginning, and it took me one year to learn.
“But Professor Ooi said I could qualify for the program as long as I had the background and the desire to learn.”
In the two years since he completed his transition to compound semiconductors, Wang’s career has taken off. He has published four technical journal articles, including one each in the prestigious Applied Physics Letters and Journal of Applied Physics. Recently, in IEEE Electronics Letter, he reported the monolithic extended cavity laser with lowest propagation loss. Two more of his journal papers are under review, and he has made six presentations at international conferences.
Wang’s research relates to photonic integrated circuits (PICs), which combine several optical components, including lasers, photodetectors and modulars in a single chip. PICs have applications in telecommunications, especially fiber-optic communications. But PICs are an emerging technology, says Wang, and their components are still discrete.
Wang’s goal is to integrate PIC components on a single chip by regulating and changing their bandgaps, which vary from near to far infrared. The achievement of multiple bandgaps on a single chip would be beneficial for telecommunications, he says, enabling the transmission of greater amounts of information, as well as for biophotonics and biosensing, which also receive a boost from a diversity of wavelengths.
To achieve the integration of multiple bandgaps, Wang uses processes called semiconductor quantum-dot and quantum-well intermixing. A quantum dot is a region in a semiconductor crystal that confines electrons or electron-hole pairs in a three-dimensional region measuring nanometers in size.
Wang is also seeking to develop high power semiconductor lasers using interdiffused quantum structures and long wavelength quantum-dot vertical-cavity surface-emitting lasers (VCSELs) for imaging applications.
“I like to handle a lot of projects at one time,” says Wang. “You learn a lot that way. If you give 150 percent, you might not accomplish 150 percent, but you will probably get 100 percent. If you shoot for 100 percent, you might only end up with 80 percent.”
Eyes on a medical application
Dimas, who also makes photonic integrated devices, says improvements in broadband emitters could lead to breakthroughs in biomedical imaging, medical diagnostics and treatment.
Broadband, high-power, light-emitting diodes, she says, have applications to Optical Coherence Tomography, a non-invasive tissue-imaging technique that shows promise in diagnosing diseases of the eye, oral tissues and heart.
“OCT is like ultrasound,” says Dimas, “but instead of using acoustic waves, it uses light waves. It uses constructive and destructive interference of light to generate the location of tissue constituents. This interferometric technique allows the light to penetrate into body tissue with 10 to 100 times better depth resolution than ultrasound.
“Because the eye is so translucent, one of the biggest applications of OCT is ophthalmology. OCT enables doctors to penetrate and image the retina and to detect glaucoma and macular degeneration early and non-invasively.”
In the Center for Optical Technologies, Dimas designs and fabricates super-luminescent diodes, a type of optoelectronic semiconductor device. SLDs can be made in large quantities at a time, or batches, and they offer the additional advantage of being able to be integrated with other optoelectronic devices.
“We would like to integrate more OCT components on one chip,” says Dimas. “That would give us an alternative to big bulky lasers that sometimes must be powered by another laser. And it would be great for portability and expense.”
Dimas’s project ties in with Ooi’s collaboration with Carl Zeiss Meditec Inc. , a German-based international maker of ophthalmology systems. Dimas is continuing the team’s work on developing a semiconductor light source with advanced sensitivity and resolution. Ooi and his group have filed a patent application for the development of broadband emitters for sensor and bio-imaging applications.
“Professor Ooi has so many collaborators and original ideas,” says Dimas. “He is definitely a visionary in the field. He filters out the noise and gets to the essence of a problem. He’s so talented, and very inspiring.”
--Kurt Pfitzer