High-harmonic generation at 250  MHz with photon energies exceeding 100  eV

Generation of isolated attosecond pulses with enhancement cavities—a theoretical study

“…The problems of thermal lensing, mirror damage and resonator stability were addressed in [12,16,27], identifying ways of progressively scaling the intracavity power. Thanks to these results, a state-of-the-art experiment demonstrated a enhancement-cavity-based 250 MHz HHG source reaching photon energies in excess of 100 eV and a photon flux of´-9 10 photons s 1 in a 2% bandwidth around 94 eV [13], which indicates that intracavity HHG has come to a point where it is potentially useful for ultrafast photoelectron spectroscopy and microscopy experiments. For this, 30 fs pulses at 1040 nm with a pulse energy of m 0.7 J were power-enhanced a factor of 60 and focused down to m = · ( ) w w 13.4 m x y 0, 0, 2 , reaching peak intensities around -3 10 W cm 14 2 in a 200 mm long neon gas target with an atomic density of n 5 std placed 0.5 Rayleigh ranges before the focus, where n std is the atomic density of an ideal gas at IUPAC standard temperature and pressure and w x 0, and w y 0, are the beam waists in x and y direction.…”

“…Coherently stacking the pulses of a high-repetition-rate modelocked laser inside of a passive optical resonator, or enhancement cavity (EC), provides a convenient way to combine peak intensities on the order of -10 W cm 14 2 necessary for HHG in a gas target with pulse repetition rates of several (tens of) MHz [10,11]. With the advent of Yb-based lasers, ECs have enabled reaching these intensities at the highest repetition rates so far, providing ultrashort pulses with the highest average powers ever demonstrated [12], and allowing for HHG with photon energies exceeding 100 eV at repetition rates as high as 250 MHz [13]. Just a few years ago, femtosecond ECs have been used for the first frequency comb spectroscopy experiments in the vacuum ultraviolet spectral region [14,15].…”

Phase-stable, multi-µJ femtosecond pulses from a repetition-rate tunable Ti:Sa-oscillator-seeded Yb-fiber amplifier

“…For instance, the zero-offset-frequency pulse train can be used to drive HHG in a suitable femtosecond enhancement cavity [16,17] for the generation of attosecond pulse trains and, ultimately, for the generation of isolated attosecond pulses at multi-MHz repetition rates [18]. This will dramatically decrease the measurement times in photoelectron emission microscopy and spectroscopy, in particular allowing for the study of plasmonic fields with a unique combination of nm-scale spatial resolution with sub-femtosecond temporal resolution [20].…”

“…This will dramatically decrease the measurement times in photoelectron emission microscopy and spectroscopy, in particular allowing for the study of plasmonic fields with a unique combination of nm-scale spatial resolution with sub-femtosecond temporal resolution [20]. Furthermore, the adjustable repetition rate allows for direct studies of cumulative effects such as those observed in HHG in gases at high repetition rates [16]. Another application which will tremendously profit from this MOPA is field-resolved detection of broadband infrared pulses [21], employing the 7-fs pulses generated by the Ti:Sa oscillator for electro-optical sampling with a lock-in detection at half the fundamental oscillator repetition rate [22].…”

“…First, the high phase stability achieved with the Ti:Sa frontend is largely preserved upon amplification of a 10-nm band around 1030 nm by about 54 dB to 80 W. Additional phase fluctuations introduced by the multi-stage, chirped-pulse amplifier (CPA) and by subsequent nonlinear compression to about 30 fs were compensated for by a feed-back loop, resulting in an unprecedentedly small overall phase jitter of the highpower pulse train of less than 100 mrad (out-of-loop phase noise, integrated in the band between 0.4 Hz and 400 kHz). Due to its phase stability, this source is particularly well suited to drive cavity-enhanced highorder harmonic generation (HHG) for the generation of extreme ultraviolet (XUV) frequency combs [15,16] and of XUV attosecond pulses [16][17][18]. Second, the master oscillator power amplifier (MOPA) approach readily enables the use of a high-frequency pulse picker after the low-power oscillator [19], allowing for a tunable repetition frequency (18.5,24.7,37 and 74 MHz).…”