Nav: Home

A new way of generating ultra-short bursts of light

February 05, 2018

Although critical for varied applications, such as cutting and welding, surgery and transmitting bits through optical fiber, lasers have some limitations - namely, they only produce light in limited wavelength ranges. Now, researchers from the Ginzton Lab at Stanford University have modified similar light sources, called optical parametric oscillators, to overcome this obstacle.

Until now, these lesser-known light sources have been mostly confined to the lab because their setup leaves little room for error - even a minor jostle could knock one out of alignment. However, following a counterintuitive decision, the researchers may have found a solution to this weakness that could lead to smaller, lower-cost and more efficient sources of light pulses.

Their work, published Feb. 1 in Physical Review Letters, demonstrates a new way to produce femtosecond pulses - pulses measured by quadrillionths of a second - in desirable wavelength ranges using this light source. The technology could potentially lead to better detection of pollutants and diseases by merely scanning the air or someone's breath.

A counterintuitive innovation

The light source these researchers study consists of an initial step where pulses of light from a traditional laser are passed through a special crystal and converted into a wavelength range that's difficult to access with conventional lasers. Then, a series of mirrors bounce the light pulses around in a feedback loop. When this feedback loop is synchronized to the incoming laser pulses, the newly converted pulses combine to form an increasingly strong output.

Traditionally, people could not convert much of the initial light pulses into the desired output with such a contraption. But to be effective in real-world applications, the group had to bump up that percentage.

"We needed higher conversion efficiency to prove it was a source worth studying," said Alireza Marandi, a staff member in the Ginzton Lab. "So we just said, 'OK, what are the knobs we have in the lab?' We turned one that made the mirrors reflect less light, which was against the standard guidelines, and the conversion efficiency doubled." The researchers published their initial experimental results two years ago in Optica.

Cranking up the power in a conventional design usually results in two undesirable outcomes: The pulses lengthen and the conversion efficiency drops. But in the new design, where the researchers significantly decreased the reflectivity of their mirrors, the opposite occurred.

"We were thinking about this regime based on the standard design guidelines, but the behavior we would see in the lab was different," said Marc Jankowski, lead author of the paper and a graduate student in the Ginzton Lab. "We were seeing an improvement in performance, and we couldn't explain it."

After more simulations and lab experiments, the group found that the key was not just making the mirrors less reflective but also lengthening the feedback loop. This lengthened the time it took for the light pulses to complete their loop and should have slowed them too much. But the lower reflectivity, combined with the time delay, caused the pulses to interact in unexpected ways, which pulled them back into synchronization with their incoming partners.

This unanticipated synchronization more than doubled the bandwidth of the output, which means it can emit a broader span of wavelengths within the range that is difficult to access with conventional lasers. For applications like detecting molecules in the air or in a person's breath, light sources with greater bandwidth can resolve more distinct molecules. In principle, the pulses this system produces could be compressed to as short as 18 femtoseconds, which can be used to study the behavior of molecules.

The decision to reduce the mirror reflectivity had the surprising consequence of making a formerly persnickety device more robust, more efficient and better at producing ultra-short light pulses in wavelength ranges that are difficult to access with traditional lasers.

Getting out of the lab

The next challenge is designing the device to fit in the palm of a hand.

"You talk with people who have worked with this technology for the past 50 years and they are very skeptical about its real-life applications because they think of these resonators as a very high-finesse arrangement that is hard to align and requires a lot of upkeep," said Marandi, who is also co-author of the paper. "But in this regime of operation these requirements are super-relaxed, and the source is super-reliable and doesn't need the extensive care required by standard systems."

This newfound design flexibility makes it easier to miniaturize such systems onto a chip, which could lead to many new applications for detecting molecules and remote sensing.

"Sometimes you completely reshape your understanding of systems you think you know," Jankowski said. "That changes how you interact with them, how you build them, how you design them and how useful they are. We've worked on these sources for years and now we've gotten some clues that will really help bring them out of the lab and into the world."
Additional Stanford co-authors of this paper are Ryan Hamerly, Robert Byer and Martin M. Fejer. Other co-authors are C.R. Phillips of ETH Zurich in Switzerland and Kirk A. Ingold of the U.S. Military Academy at West Point.

This research was funded by the U.S. Department of Defense and the National Science Foundation.

Stanford University

Related Molecules Articles:

The inner lives of molecules
Researchers from Canada, the UK and Germany have developed a new experimental technique to take 3-D images of molecules in action.
Novel technique helps ID elusive molecules
Stuart Lindsay, a researcher at Arizona State University's Biodesign Institute, has devised a clever means of identifying carbohydrate molecules quickly and accurately.
How solvent molecules cooperate in reactions
Molecules from the solvent environment that at first glance seem to be uninvolved can be essential for chemical reactions.
A new way to display the 3-D structure of molecules
Berkeley Lab and UC Berkeley Researchers have developed nanoscale display cases that enables new atomic-scale views of hard-to-study chemical and biological samples.
Bending hot molecules
Hot molecules are found in extreme environments such as the edges of fusion reactors.
At attention, molecules!
University of Iowa chemists have learned about a molecular assembly that may help create quicker, more responsive touch screens, among other applications.
Folding molecules into screw-shaped structures
An international research team describes the methods of winding up molecules into screw-shaped structures.
Artificial molecules
A new method allows scientists at ETH Zurich and IBM to fabricate artificial molecules out of different types of microspheres.
Molecules that may keep you young and alive
A new study may have uncovered the fountain of youth: plant extracts containing the six best groups of anti-aging molecules ever seen.
Fun with Lego (molecules)
A great childhood pleasure is playing with Legos® and marveling at the variety of structures you can create from a small number of basic elements.

Related Molecules Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Bias And Perception
How does bias distort our thinking, our listening, our beliefs... and even our search results? How can we fight it? This hour, TED speakers explore ideas about the unconscious biases that shape us. Guests include writer and broadcaster Yassmin Abdel-Magied, climatologist J. Marshall Shepherd, journalist Andreas Ekström, and experimental psychologist Tony Salvador.
Now Playing: Science for the People

#513 Dinosaur Tails
This week: dinosaurs! We're discussing dinosaur tails, bipedalism, paleontology public outreach, dinosaur MOOCs, and other neat dinosaur related things with Dr. Scott Persons from the University of Alberta, who is also the author of the book "Dinosaurs of the Alberta Badlands".