Arman Cingöz

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.

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.

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.

Broadband phase noise suppression in a Yb-fiber frequency comb.

We report a simple technique to suppress high-frequency phase noise of a Yb-based fiber optical frequency comb using an active intensity noise servo. Out-of-loop measurements of the phase noise using an optical heterodyne beat with a cw laser show suppression of phase noise by ≥7 dB out to Fourier frequencies of 100 kHz with a unity-gain crossing of ∼700 kHz. These results are enabled by the strong correlation between the intensity and phase noise of the laser. Detailed measurements of intensity and phase noise spectra, as well as transfer functions, reveal that the dominant phase and intensity noise contribution above ∼100 kHz is due to amplified spontaneous emission or other quantum noise sources.

Power Scaling of High-Repetition-Rate HHG

We report on cavity-enhanced HHG with a frequency comb delivering 120-fs pulses and 80-W average power at 154-MHz repetition rates. With 5-kW average intracavity powers, average HHG powers beyond the microwatt level have been achieved.

Frequency comb metrology goes extreme UV

A variety of applications, from precision optical clocks (used for timekeeping) to broadband molecular spectroscopy (used for trace gas detection) have been enabled by the use of optical frequency combs. These devices allow precise frequency determination by providing a ‘comb’ of sharp optical spectral lines referenced to radio-frequency sources.1, 2 The comb is generated by a continuous train of ultrashort laser pulses in the time domain: see Figure 1(a). The invention of the optical frequency comb at the beginning of this century revolutionized visible and near-IR spectroscopy and frequency metrology. Extending this tool into the extreme UV (XUV, light with wavelengths less than 100nm) would enable applications ranging from strong field and molecular physics studies to ultrahigh precision spectroscopy in highly charged ions and nuclear isomer transitions. However, the XUV is a challenging part of the electromagnetic spectrum. XUV spectroscopy has trailed far behind that in the visible because of a lack of precise laser sources. A standard technique for generating coherent radiation at these wavelengths is high harmonic generation (HHG). HHG uses an intense optical pulse focused into a gaseous target to generate multiples (high harmonics) of the fundamental optical frequency.3 However, frequency comb lasers do not have the necessary intensity to drive this extremely nonlinear process, which requires pulse peak intensities exceeding 1013W=cm2. The solution is to passively amplify the laser output in an optical enhancement cavity and generate the XUV light at the intracavity focus, where the light is most intense. In this approach, the output of the laser is coupled into an optical enhancement cavity, which consists of several highly reflective mirrors. The optical path length within the cavity is set precisely, such that when the intracavity pulse returns to the Figure 1. (a) A train of stabilized laser pulses generates a comb of sharp spectral lines defined by two radio frequencies: the comb spacing (frep) and an offset (f0). (b) An IR frequency comb is passively amplified in an optical cavity. Xenon gas is injected at the intracavity focus leading to high harmonic generation (HHG) of extreme ultraviolet (XUV) light. An output coupler acts like a diffraction grating and separates the XUV harmonics.

High Power Fiber Laser Frequency Combs for XUV Spectroscopy

A Yb-fiber frequency comb with 120 fs pulse duration, 154 MHz repetition rate, and 80 W average power is used for cavity-enhanced high harmonic generations. Plateau harmonics beyond the microwatt level have been demonstrated.