Precise control of femtosecond optical frequency combs is a significant research interest of Jun Ye. Combs are being applied to precision spectroscopy, absolute frequency metrology, optical atomic clocks, optical frequency synthesis, pulse-timing synchronization and distribution, coherent pulse manipulation, and direct femtosecond comb spectroscopy. The combined time-and-frequency resolution of frequency combs permits simultaneous investigation of system dynamics and structure. Ye's group can now ensure precise and coherent control at the highest possible spectral resolution.

An illustration of one of Ye's optical cavities

Ye's group studies the interactions between ultrafast pulses and passive optical cavities. This research is expected to impact the development of ultrafast lasers, gainless amplifiers, ultrasensitive wide-bandwidth laser spectroscopy, optical frequency metrology, and nonlinear optics. The researchers can now efficiently couple and coherently store ultrashort light pulses inside high-finesse optical cavities. They have undertaken (1) precision stabilization of ultrafast lasers to high-finesse optical cavities, (2) studies of cavity-enhancement effects on extreme nonlinear optics and optical frequency conversions to other spectral regions, and (3) sensitive detections and efficient excitations of intracavity molecular samples.

Ye expects that his work on cavity-based amplifiers will allow him to optimize cavity designs to improve the nonlinear frequency-conversion efficiencies and pulse-shaping capabilities. These improvements should, in turn, lead to the implementation of cavity-based, wide-bandwidth sensitive spectroscopy for the study of molecular dynamics and the detection of trace gases. An interesting set of intracavity experiments on nonlinear optics and coherent excitations is being pursued.

Ye and his co-workers recently generated the world's first frequency comb in the extreme ultraviolet (EUV) region of the electromagnetic spectrum using a combination of an ultrafast mode-locked laser and a high-finesse optical cavity. It will allow scientists to study the fine structure of atoms and molecules with coherent EUV light. This comb is a short-wavelength version of the optical frequency combs that have revolutionized optical frequency metrology, precision measurement, and optical pulse synthesis and control, including demonstrations of optical atomic clocks.

The invention of optical frequency combs led the awarding of the 2005 Nobel Prize in Physics to long-time JILAn John (Jan) L. Hall and his colleague Ted Hänsch of the Max Planck Institute for Quantum Optics. They shared the prize with Harvard Professor Roy Glauber. Thanks to Jan's work, it is now possible to build lasers with extremely sharp colors. The frequency comb technique makes it possible to precisely measure the frequency of all colors of light. These advances are the foundation for precision optical atomic clocks and improved global positioning system (GPS) technology.

Hall has been working on stabilizing the frequency of lasers since the 1960s. Recently, his group collaborated with Jun Ye's group to devise an improved and less expensive method for laser stabilization. The new method is based on a small, vertically mounted optical cavity. Because the cavity is supported exactly in the middle, the top and bottom halves change in length by equal and opposite amounts in response to vibrations. Its sensitivity to vibration and gravity is a hundredfold less than a comparable horizontally mounted device.