Mid-infrared reconfigurable pulse generators

Harvard University, Cambridge MA

Investigators

Abstract

Lasers are indispensable tools that benefit our society on many different avenues. Laser utility comes from it being able to emit light of precisely defined wavelength –– and an extremely stable intensity, making it especially suitable for measurements requiring utmost precision and in connecting the world by using laser light as a carrier of digital information in undersea fiber optic cables. More applications, however, would benefit from a new kind of laser, that emits simultaneously not one, but many wavelengths of light. The goal of the project is to realize precisely this kind of laser. The output intensity of such a laser is no longer constant, but is a train of short light pulses, all spaced by equal time intervals. These pulses will attain extremely high optical intensities –– much higher than a single- wavelength laser could provide. Such intense pulses would be especially useful to optical spectroscopists –– scientists who strive to deepen our understanding of matter by studying how it absorbs and emits laser light. Short intense pulses of light can serve as a precise clockwork mechanism to time the response of the material to laser radiation, which can be used for real time monitoring of chemical reactions and for the studies of the nonlinear optical response of atomic vapors, gas molecules and semiconductor crystals, directly impacting disciplines from solid-state physics to biology, with potential applications in the medical research. The primary goal of the project is the demonstration of a new device architecture ¬¬–– a mid-infrared ring quantum cascade laser with an integrated active directional coupler, driven by a coherent external laser source. By acting on the drive laser intensity and its detuning from the ring laser frequency it will be possible to reconfigure the state of the ring laser on-demand, ranging from bright and dark cavity solitons, representing respectively ultrashort pulses of light that propagate without broadening in the laser medium and “dark pulses” in a background of continuous laser light, respectively, to more complex waveforms known as Turing rolls and phase solitons –– all in one, all-electrically controlled device. Starting with the design and modeling of laser resonators using commercial electromagnetic mode solvers and an in-house custom laser simulator the researchers will proceed with nanoscale semiconductor processing according to the developed designs. Characterization of fabricated devices will be based on basic emission spectroscopy of laser output as well as complex temporal waveform reconstruction using already existing optical interferometers and the setups constructed within the timespan of the project. The project will encompass broader theoretical studies in mathematical physics centered around pattern formation in nonlinear active resonators and will deepen the understanding of the similarities and the differences of the physics of frequency comb formation in passive and active optical resonators. Achieving ultrashort pulse generation in the mid-infrared from compact chip-scale devices may be transformative for fundamental science allowing to probe the temporal dynamics of ultrafast material processes occurring at mid-infrared frequencies — from phonons to polaritons to interactions of complex organic molecules. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.