Dylan C. Yost

Direct frequency comb spectroscopy in the extreme ultraviolet

The development of the optical frequency comb (a spectrum consisting of a series of evenly spaced lines) has revolutionized metrology and precision spectroscopy owing to its ability to provide a precise and direct link between microwave and optical frequencies. A further advance in frequency comb technology is the generation of frequency combs in the extreme-ultraviolet spectral range by means of high-harmonic generation in a femtosecond enhancement cavity. Until now, combs produced by this method have lacked sufficient power for applications, a drawback that has also hampered efforts to observe phase coherence of the high-repetition-rate pulse train produced by high-harmonic generation, which is an extremely nonlinear process. Here we report the generation of extreme-ultraviolet frequency combs, reaching wavelengths of 40 nanometres, by coupling a high-power near-infrared frequency comb to a robust femtosecond enhancement cavity. These combs are powerful enough for us to observe single-photon spectroscopy signals for both an argon transition at 82 nanometres and a neon transition at 63 nanometres, thus confirming the combs’ coherence in the extreme ultraviolet. The absolute frequency of the argon transition has been determined by direct frequency comb spectroscopy. The resolved ten-megahertz linewidth of the transition, which is limited by the temperature of the argon atoms, is unprecedented in this spectral region and places a stringent upper limit on the linewidth of individual comb teeth. Owing to the lack of continuous-wave lasers, extreme-ultraviolet frequency combs are at present the only promising route to extending ultrahigh-precision spectroscopy to the spectral region below 100 nanometres. At such wavelengths there is a wide range of applications, including the spectroscopy of electronic transitions in molecules, experimental tests of bound-state and many-body quantum electrodynamics in singly ionized helium and neutral helium, the development of next-generation ‘nuclear’ clocks and searches for variation of fundamental constants using the enhanced sensitivity of highly charged ions.

Vacuum-ultraviolet frequency combs from below-threshold harmonics

Frequency combs have revolutionized frequency metrology. High-harmonic generation in atoms has led to fast sources of short-wavelength photons. Combining these two technologies enables the transfer of frequency combs to the vacuum-ultraviolet with potential applications in spectroscopy.

Efficient output coupling of intracavity high-harmonic generation

We demonstrate a novel technique for coupling extreme-ultraviolet (XUV) harmonic radiation out of a femtosecond enhancement cavity. We use a small-period diffraction grating etched directly into the surface of a dielectric mirror. For the fundamental light, this element acts as a high reflector. For harmonic wavelengths, it acts as a diffraction grating, coupling XUV radiation out of the cavity. Using this method, we observed the third through twenty-first odd harmonics with a dramatic increase in usable power over previous results of high-harmonic generation at high repetition rates.

Cavity-enhanced similariton Yb-fiber laser frequency comb : 3 × 1014 W/cm2 peak intensity at 136 MHz

We report on a passive cavity-enhanced Yb-fiber laser frequency comb generating 230 MW of peak power (3 kW of average power) at a 136 MHz pulse repetition rate. The intracativy peak intensity of 3 x 10(14) W/cm2 for the 95 fs pulse is sufficient to ionize noble gases, such as Xe, Kr, or Ar. The laser system is based on a mode-locked Yb-fiber similariton oscillator in conjunction with a cladding-pumped chirped-pulse fiber amplifier. After recompression, 75 fs duration pulses at a 13.1 W average power are obtained. These pulses are then coherently added inside a passive ring cavity by controlling the fiber oscillators pulse repetition rate and carrier-envelope offset frequency. This system is well suited for studying high-field phenomena at very high pulse repetition rates.

Simple piezoelectric-actuated mirror with 180 kHz servo bandwidth

We present a high bandwidth piezoelectric-actuated mirror for length stabilization of an optical cavity. The actuator displays a transfer function with a flat amplitude response and greater than 135 masculine phase margin up to 200 kHz, allowing a 180 kHz unity gain frequency to be achieved in a closed servo loop. To the best of our knowledge, this actuator has achieved the largest servo bandwidth for a piezoelectric transducer (PZT). The actuator should be very useful in a wide variety of applications requiring precision control of optical lengths, including laser frequency stabilization, optical interferometers, and optical communications.

Extreme Nonlinear Optics in a Femtosecond Enhancement Cavity

Intrinsic to the process of high-order harmonic generation is the creation of plasma and the resulting spatiotemporal distortions of the driving laser pulse. Inside a high-finesse cavity where the driver pulse and gas medium are reused, this can lead to optical bistability of the cavity-plasma system, accumulated self-phase modulation of the intracavity pulse, and coupling to higher-order cavity modes. We present an experimental and theoretical study of these effects and discuss their implications for power scaling of intracavity high-order harmonic generation and extreme ultraviolet frequency combs.

The Rydberg constant and proton size from atomic hydrogen

How big is the proton? The discrepancy between the size of the proton extracted from the spectroscopy of muonic hydrogen and the value obtained by averaging previous results for “regular” hydrogen has puzzled physicists for the past 7 years. Now, Beyer et al. shed light on this puzzle (see the Perspective by Vassen). The authors obtained the size of the proton using very accurate spectroscopic measurements of regular hydrogen. Unexpectedly, this value was inconsistent with the average value of previous measurements of the same type. Also unexpectedly, it was consistent with the size extracted from the muonic hydrogen experiments. Resolving the puzzle must now include trying to understand how the old results relate to the new, as well as reexamining the sources of systematic errors in all experiments. Science, this issue p. 79; see also p. 39 The proton radius from hydrogen spectroscopy is consistent with the value from muonic hydrogen spectroscopy. At the core of the “proton radius puzzle” is a four–standard deviation discrepancy between the proton root-mean-square charge radii (rp) determined from the regular hydrogen (H) and the muonic hydrogen (µp) atoms. Using a cryogenic beam of H atoms, we measured the 2S-4P transition frequency in H, yielding the values of the Rydberg constant R∞ = 10973731.568076(96) per meterand rp = 0.8335(95) femtometer. Our rp value is 3.3 combined standard deviations smaller than the previous H world data, but in good agreement with the µp value. We motivate an asymmetric fit function, which eliminates line shifts from quantum interference of neighboring atomic resonances.

Megawatt-scale average-power ultrashort pulses in an enhancement cavity

We investigate power scaling of ultrashort-pulse enhancement cavities. We propose a model for the sensitivity of a cavity design to thermal deformations of the mirrors due to the high circulating powers. Using this model and optimized cavity mirrors, we demonstrate 400 kW of average power with 250 fs pulses and 670 kW with 10 ps pulses at a central wavelength of 1040 nm and a repetition rate of 250 MHz. These results represent an average power improvement of one order of magnitude compared to state-of-the-art systems with similar pulse durations and will thus benefit numerous applications such as the further scaling of tabletop sources of hard x rays (via Thomson scattering of relativistic electrons) and of soft x rays (via high harmonic generation).

Extreme ultraviolet radiation with coherence time greater than 1 s

Many atomic and molecular systems of fundamental interest possess resonance frequencies in the extreme ultraviolet (XUV) where laser technology is limited and radiation sources have traditionally lacked long-term phase coherence. Recent breakthroughs in XUV frequency comb technology have demonstrated spectroscopy with unprecedented resolution at the megahertz level, but even higher resolutions are desired for future applications in precision measurement. By characterizing heterodyne beats between two XUV comb sources, we demonstrate the capability for sub-hertz spectral resolution. This corresponds to coherence times >1 s at photon energies up to 20 eV, more than six orders of magnitude longer than previously reported. This work establishes the ability of creating highly phase-stable radiation in the XUV with performance rivalling that of visible light. Furthermore, by direct sampling of the phase of the XUV light originating from high-harmonic generation, we demonstrate precise measurements of attosecond strong-field physics.

Power optimization of XUV frequency combs for spectroscopy applications [Invited]

We address technical impediments to the generation of high-photon flux XUV frequency combs through cavity-enhanced high harmonic generation. These difficulties arise from mirror damage, cavity nonlinearity, the intracavity plasma generated during the HHG process, and imperfect phase-matching. By eliminating or minimizing each of these effects we have developed a system capable of generating > 200 μW and delivering ~20 μW of average power for each spectrally separated harmonic (wavelengths ranging from 50 nm - 120 nm), to actual comb-based spectroscopy experiments.