Guiding Lights

UCSD postdoctoral researcher Mercedeh Khajavikhan has spent countless hours at work inside the Nano3 facility and the optics lab on the third floor of the Engineering building. But all her work was not in vain; her study on lasers, a collaboration with her colleagues at the UCSD Jacobs School of Engineering, has resulted in the development of the smallest-ever, room-temperature nanolaser along with a highly efficient threshold-less laser — or a laser that starts lasing immediately with very little power. The new, innovative design could serve as an important tool in medical research.

The unique aspects of the laser have big implications for scientific studies. The miniscule size of the opening, around 100 nanometers, allows for the detection of HIV viruses and single molecules — an advancement in spectroscopy (the study of the interaction between matter and radiated energy). The laser may also be used in optical devices and future computer chips — opening possibilities for incredibly cutting-edge technology.

The secret lies in the coaxial structure of the device: Instead of the previously used box that wasted much of the generated light and its energy, a cylinder with a metal rod at its center is used, which can support a laser beam (however small it may be).

“We can play with the size of the structure and the metal rod,” Khajavikhan said about the cutting-edge design. “When we squeeze light into this very small structure, we start to play with its environment.”

For Khajavikhan, the lead author of the study, the idea of creating a threshold-less laser first came to her while experimenting in the lab in October 2010.

“First, we made these lasers with only a rough idea as to their potential,” Khajavikhan said. “We were trying different sizes and then I postulated that if the structure can isolate one mode, then the lasing should occur in a threshold-less fashion.”

She immediately emailed her colleagues after her initial discovery, and the project was soon underway. But recognizing the right structure didn’t make things simpler; there was still the tedious process of finding the correct size.

“It took me some time to tune the sizes to find a threshold-less one,” Khajavikhan said. “In the coaxial structure, you get all sorts of new modes inside the cavity. To assort the modes and find their properties, I asked my colleague, Dr. Amit Mizrahi, for help. By changing the size even slightly, new modes appear and old modes move out of the material’s natural window of emission. It was hard to find the coaxial laser cavity that was actually the right size. And it took us a bit of time to find the one with the threshold-less behavior, but we found it.”

On March 31, 2011, the project was done and Khajavikhan and her team had officially built a threshold-less laser.

“We understood the potential of these coaxial structures and finally found the right ways to exploit them,” Khajavikhan said.

At this stage of the work, Khajavikhan turned to her colleague, Michael Katz, to compare the experimental data to the rate equation model (a set of equations that governs the laser behavior), mainly to get a fresh and unbiased opinion about the work. The results conclusively confirmed the threshold-less predication.

“Because of the unprecedented capability of squeezing light into these very small structures, it allows us to experimentally study some quantum electro-dynamical effects,” Khajavikhan said. “We can show and measure many effects that physicists have theoretically predicted in the last 70 years.”

By shrinking the structure down to practically the size of the light waves themselves, or “nurturing” the light’s environment, researchers can begin to really observe what has been held to be supposedly true by “nature.”

Khajavikhan, who moved from Iran in 2004 and received her Ph.D. in Electrical Engineering in 2009 at the University of Minnesota, explained how in some cases, nurture trumps nature. 

“For a very long time, people used to think that [the generation of glow-in-the-dark light] was the property of the material, but now, from 1946 onwards, it’s not really all about nature; it’s equally, and sometimes more importantly, about nurture,” Khajavikhan said. “It’s about what you put around the light, and this effect only shows itself when you squeeze light into very small areas, like what we have done now.”

The properties of a laser are dependent on the size of the structure, which supports modes (standing waves within the metal-based structure). Once you reach the size at which the cavity supports a single mode, it’s considered a threshold-less laser that generates a coherent beam of light as soon as it starts emitting (this light source is only one-fifteenth the size of the light waves it produces). This is unlike conventional lasers that first emit incoherent light and take a lot of energy to begin lasing.

Khajavikhan emphasized that no research discovery is possible without numerous sleepless nights, going back to the fundamentals one learns in school and perhaps most importantly, a lot of discussions and exchange of ideas with colleagues and mentors.

“I think I owe a lot to the University of California, San Diego — to many of the faculty, including Professors Shaya Fainman,Vitaliy Lomakin and Tara Javidi,” Khajavikhan said. “UCSD has the environment to allow this kind of research, and most people that I have worked with are just wonderful people. The fact that they were there listening to my thoughts, sharing their ideas with me and giving me the chance to go and think about those ideas too, eventually helped the project. A lot of good ideas have been generated and developed during the discussions with my colleagues. I think the discussions are the main part of research, and to me, that was collaboration and team work.”

Khajavikhan, now a staff research volunteer at UCSD, is hopeful about her research and wants to continue her study on lasers in the future.

“I still have high hopes in this line of research; I think there is still a lot more to be done,” Khajavikhan said. “On a few occasions, after presenting our work, many of our colleagues at other schools, like Stanford, USC and Berkeley, have come up and told us that this looks very promising. There seems to be a consensus that our discovery will have a major impact, so at least for the next few years I sure hope to be busy.”

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