What do you picture in your mind's eye when you hear the word "laser"? A light saber? A cat toy? The sensor at the supermarket reading barcodes as fast as the eye can blink?
These are all lasers, but there are so many more in so many sizes and colors with capabilities that have yet to be tapped or even imagined. Assistant Professor of Electrical Engineering and Applied Physics Alireza Marandi is in the business of dreaming up these lasers and creating them in the lab.
Marandi's latest investigation involves mode-locked lasers, which emit light in steady pulses rather than in a single continuous beam. These pulses can be extremely short, counted in picoseconds (trillionths of a second) or femtoseconds (quadrillionths of a second), and can carry ultrahigh powers in such short times. Pulses from mode-locked lasers have been used in many applications, for instance, for eye surgery, by providing narrowly targeted cutting power without creating the undue heat that a continuous laser beam would cause.
Mode-locking involves locking the amplitudes and phases of the light waves that traverse a laser's resonant cavity. When mode-locking is achieved, these resonant waves act in concert with one another and typically form a steadily pulsing pattern. Marandi's team is adding topological robustness to a mode-locked laser by introducing specific couplings among the resonant light pulses in the laser cavity.
The resulting topological temporal mode-locking creates laser pulse patterns that can tolerate imperfections and disorders arising from manufacturing or environmental noise sources.
"This fundamental research could potentially have many applications," Marandi says. "By realizing topological behaviors in mode-locked lasers, we are essentially creating a knot that can make the laser's behavior more robust against noise. If the laser is ordinarily mode-locked and you shake it, everything goes crazy. But if the laser pulses are knotted together, you can shake the system, and nothing chaotic will happen, at least for a certain range of shakings."
Topologically protected mode-locked lasers can enable the creation of better frequency combs, which are used in communication, sensing, and computing applications. "The output of a mode-locked laser in the frequency domain is a frequency comb, that is, many equidistant narrow spectral peaks," Marandi explains. "Frequency combs are typically prone to noise sources and environmental instabilities. By utilizing the topological behaviors in a mode-locked laser, the resulting frequency comb can be protected against some of these noise sources."
In the future, Marandi hopes to utilize the rich physics of this new type of laser to access regimes of nonlinear topological physics that are not accessible with other experimental platforms as well as developing advanced types of sensors and computing hardware.
The paper is published in Nature Physics and is titled "Topological Temporally Mode-Locked Laser." Co-authors are Christian R. Leefmans, Midya Parto, James Williams, and Gordon H.Y. Li, all of Caltech; Avik Dutt of Stanford University and the University of Maryland; and Franco Nori of the University of Michigan and the RIKEN Center for Quantum Computing in Japan. Funding sources include the National Science Foundation, the Air Force Office of Scientific Research, the Army Research Office, the Japan Society for the Promotion of Science, the Asian Office of Aerospace Research and Development, the Foundational Questions Institute, and NTT Research.
