Special CM-AMO Seminar | Frequency Combs and Precision Spectroscopy in the Extreme Ultraviolet, Speaker: Arman Cingöz (JILA, National Institute of Standards & Technology, University of Colorado Physics)

Speaker: Arman Cingöz (JILA, National Institute of Standards and Technology, University of Colorado Physics)

Development of the optical frequency comb (a continuous train of stabilized ultra short laser pulses) at beginning of the century has revolutionized optical metrology and precision spectroscopy due to its ability to provide a precise and direct link between microwave and optical frequencies. A novel application that aims to extend the precision and accuracy obtained in the visible and near-infrared part of the electromagnetic spectrum to the extreme ultraviolet (XUV) is the generation of XUV frequency combs via intra-cavity high harmonic generation (HHG). The main idea is to leverage both the ultra short duration of each laser pulse and the exquisite phase coherence of the continuous pulse train by coupling a high power infrared frequency comb into a high finesse optical cavity. At the intra-cavity focus, the peak intensities reach ~1014 W/cm2, required to drive the extremely nonlinear HHG process, which produces radiation at the harmonics of the driving infrared laser frequency.

We will discuss the power scaling of this technique using a new 80-W average power Yb fiber frequency comb. Detailed understanding of plasma dynamics in the high-finesse cavity, coupled with technical improvements, have led to record level HHG yield of >20 µW average power per harmonic reaching down to 50 nm. Recently, we have also demonstrated the comb structure of the high harmonics by resolving atomic argon and neon lines at 82 and 63 nm, respectively, via direct frequency comb spectroscopy. The argon transition line width of 10 MHz, limited by residual Doppler broadening, is unprecedented in this spectral region and places a stringent upper limit on the line width of individual comb teeth.

With these developments, ultrahigh precision spectroscopy in the XUV is within grasp and has a wide range of applications that include experimental tests of bound state and many body quantum electrodynamics in He+ and He, development of next-generation nuclear clocks, and searches for spatial and temporal variation of fundamental constants using the enhanced sensitivity of highly charged ions. We will conclude with a brief discussion of some of these applications that we would like to pursue in the future.