Session H7: Focus Session: Ultrafast Light Sources

XUV frequency combs based on intracavity high harmonic generation

Intracavity high harmonic generation utilizing femtosecond enhancement cavities (fsEC's) has been established as an efficient route for the generation of femtosecond frequency combs in the vacuum-ultraviolet (VUV) to the extreme-ultraviolet (XUV) spectral regions. Such VUV/XUV frequency combs enable precision spectroscopy of atomic and potentially molecular spectra in an otherwise difficult to access spectral region. An improved understanding of the intracavity ionization dynamics that currently limit pulse enhancement has enabled a new generation of XUV frequency comb sources with significantly higher powers, at the $>$10 microwatt level per harmonic order extending below 50nm. We have developed a novel time-resolved pump-probe measurement technique to monitor and characterize the intracavity ionization dynamics by utilizing the sensitive response of the fsEC resonance itself to plasma induced nonlinear phase shifts. In recent work, we have developed a high power dual-frequency comb system based on Yb-fiber laser technology. The two phase-coherent frequency combs can be up-converted to the VUV/XUV using the fsEC. Dual-comb spectroscopy has already been established as a powerful spectroscopic method in the infrared. It's extension to the VUV/XUV spectral region will enable robust and high precision direct frequency comb spectroscopy of complex atomic and molecular structure in this spectral region.   

Extreme Ultraviolet Radiation with Coherence Time Beyond 1 second

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 resolution at the MHz-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-Hz spectral resolution. This corresponds to coherence times \textgreater 1 s, at photon energies up to 20 eV, more than 6 orders of magnitude longer than previously reported. We also identify various noise contributions to the obtainable comb linewidth in the XUV. This work establishes the ability of creating highly phase stable radiation in the XUV with performance rivaling that of visible light. Further, by direct sampling of the phases of the XUV light originating from high harmonic generation, we demonstrate precise measurements of attosecond strong-field physics.   

Frequency divide-and-conquer approach to producing octave-wide frequency combs and few-cycle pulses in the mid-IR

I will present a new technique for extending frequency combs to the highly desirable yet difficult-to-achieve mid-IR spectral range. The technique is based on subharmonic optical parametric oscillation (OPO) that can be considered as a reverse of the second harmonic generation process. The frequency comb of a pump laser is transposed to half of its central frequency and simultaneously spectrally augmented, thanks to an enormous gain bandwidth of the OPO near degeneracy, as well as due to massive cross-coupling between the laser and the OPO frequency comb components. Using ultrafast erbium (1.56 microns) or thulium (2 microns) -based fiber lasers as a pump and using thin, sub-mm-long, quasi phase-matched lithium niobate [1] or gallium arsenide [2] crystals, we produce frequency combs centered correspondingly at 3.1 or 4 micron subharmonic of the pump frequency. With the properly managed OPO cavity group velocity dispersion, octave-wide frequency combs spanning 2.5 - 6 micron range were achieved. Due to the doubly-resonant operation, the threshold of such a system is low (typically 10 mW) and by several experiments including measuring frequency beats between the OPO comb teeth and a narrow-linewidth CW laser and by interfering the outputs of two identical but distinct OPOs pumped by the same laser [3], we established that the frequency comb from a subharmonic OPO is phase-locked to that of the pump laser. Pulse duration measurements show that for the optimal intracavity dispersion conditions, we generate sub 5-cycle pulses at the subharmonic of the pump. I will also talk about applications of our mid-IR frequency combs to trace gas detection, where part-per-billion sensitivity of molecular detection is achieved [4] as well as about Fourier spectroscopy using a dual-comb system consisting of two phase-locked lasers. [1] N. Leindecker, A. Marandi, R.L. Byer, K. L. Vodopyanov, ``Broadband degenerate OPO for mid-infrared frequency comb generation,'' Opt. Express 19, 6296-6302 (2011). [2] N. Leindecker, A. Marandi, R.L. Byer, K. L. Vodopyanov, J. Jiang, I. Hartl, M. Fermann, and P. G. Schunemann ``Octave-spanning ultrafast OPO with 2.6-6.1 $\mu$m instantaneous bandwidth pumped by femtosecond Tm-fiber laser,'' Opt. Express 20, 7047-7053 (2012). [3] A. Marandi, N. Leindecker, V. Pervak, R.L. Byer, K. L. Vodopyanov, ``Coherence properties of a broadband femtosecond mid-IR optical parametric oscillator operating at degeneracy,'' Opt. Express 20, 7255-7262 (2012). [4] M. W. Haakestad, T. P. Lamour, N. Leindecker, A. Marandi, and K. L. Vodopyanov, ``Intracavity trace molecular detection with a broadband mid-IR frequency comb source,'' J. Opt. Soc. Am. B 30, 631-640 (2013).   

Optical Undulators for Free Electron Lasers

Free Electron Lasers (FELs) in the x-ray region are opening new research directions in AMO physics and other fields, but beam time is quite limited at these expensive facilities. There are conceptual designs for much less expensive soft x-ray FELs using sheared pulses from Table Top Terawatt (T$^{3})$ lasers as optical undulators [1]. A nearly co-propagating laser pulse can be angle tuned to yield soft x-rays, and shearing the pulse can optimize use of the laser photons. Undulator K values near unity are available from T$^{3}$ lasers, and angle tuning provides almost arbitrary effective undulator periods. A combination of these optical undulator ideas with pre-``micro-bunching'' at a photocathode followed by electron beam emittance exchange [2] can reduce the energy needed from the T$^{3}$ laser. A combination of a nearly co-propagating optical undulator with a Bragg-reflection diamond mirror cavity [3] may lower the cost of an x-ray frequency comb for metrology. \\[4pt] [1] J. E. Lawler, J. Bisognano, R. A. Bosch, T. C. Chiang, M. A. Green, K. Jacobs, T. Miller, R. Wehlitz, D. Yavuz, {\&} R. C. York, J. of Phys. D: Appl. Phys. \textbf{46}: 325501 (2013). \\[0pt] [2] W. S. Graves, F. X. K\"artner, D. E. Moncton, and P. Piot, Phys. Rev. Letts. 108, 263904 (2012). \\[0pt] [3] K.-J. Kim, Proc. ERL Conf. 2011 (Tsukuba, Japan).   

Combining phase-tagging and CEP-stabilization for increased precision in CEP-dependent measurements*

We have used a stereographic above-threshold-ionization phase meter to provide shot-to-shot carrier-envelope-phase (CEP) tagging on few cycle pulses from our PULSAR ultrafast laser system. PULSAR is based on a commercial Red Dragon$^{\mathrm{TM}}$ Ti:Sapphire system from KM Labs providing 2mJ pulses of nominal 21 fs width, reduced to sub-5 fs with a hollow core fiber and a chirped mirror compressor. By phase-tagging on a shot-to-shot basis, we can make precise measurements on the quality of PULSAR's CEP-locking system, studying long-term lock stability and the effects of shot averaging and variation of locking circuit parameters on the reported lock value. We also demonstrate the utility of a live oscilloscope output from the phasemeter in tuning the laser CEP parameters. * This work was supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy under Grant No. DE-FG02-86ER13491. The PULSAR laser was provided by Grant No. DE-FG02-09ER16115 from the same agency.