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

Abstract

The generation of extreme-ultraviolet (XUV) isolated attosecond pulses (IAPs) has enabled experimental access to the fastest phenomena in nature observed so far, namely the dynamics of electrons in atoms, molecules and solids. However, nowadays the highest repetition rates at which IAPs can be generated lies in the kHz range. This represents a rather severe restriction for numerous experiments involving the detection of charged particles, where the desired number of generated particles per shot is limited by space charge effects to ideally one. Here, we present a theoretical study on the possibility of efficiently producing IAPs at multi- MHz repetition rates via cavity-enhanced high-harmonic generation (HHG). To this end, we assume parameters of state-of-the-art Yb-based femtosecond laser technology to evaluate several time-gating methods which could generate IAPs in enhancement cavities. We identify polarization gating and a new method, employing non-collinear optical gating in a tailored transverse cavity mode, as suitable candidates and analyze these via extensive numerical modeling. The latter, which we dub transverse mode gating (TMG) promises the highest efficiency and robustness. Assuming 0.7 μ J , 5-cycle pulses from the seeding laser and a state-of-the-art enhancement cavity, we show that TMG bares the potential to generate IAPs with photon energies around 100 eV and a photon flux of at least 10 8 photons s − 1 at repetition rates of 10 MHz and higher. This result reveals a roadmap towards a dramatic decrease in measurement time (and, equivalently, an increase in the signal-to-noise ratio) in photoelectron spectroscopy and microscopy. In particular, it paves the way to combining attosecond streaking with photoelectron emission microscopy, affording, for the first time, the spatially and temporally resolved observation of plasmonic fields in nanostructures. Furthermore, it promises the generation of frequency combs with an unprecedented bandwidth for XUV precision spectroscopy.

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].…”

“…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).…”

Cavity-enhanced noncollinear high-harmonic generation

“…We compare the method to OC using a symmetric fundamental Gaussian mode focused down to the same focal spot area, using an OC mirror with a hole, with an angular diameter chosen for the same round-trip loss (2.30 mrad). Because the divergence of the XUV beamlets depends on the harmonic order, intensity and target gas [16], we consider two cases: OC of the 33 th harmonic produced in argon (39.9 eV, compare [8]), with a peak intensity of 1.5 × 10 14 W/cm 2 in the target plane, and of the 79 th harmonic produced in neon (95.6 eV, compare [26]), with a peak intensity of 3.0 × 10 14 W/cm 2 . The results are plotted in Fig.…”

“…Moreover, a delay equal to an odd number of optical half cycles results in a single, on-axis maximum around the focus [17]. The delay is reversed by a second, identical mirror located such that the first mirror is imaged onto it in good approximation [26,27]. This ascertains that the recycled field can overlap constructively with the original TEM 01 mode, allowing the field to circulate with negligible losses inside a resonator housing this configuration.…”

“…To this end, we employ the TMG (transverse mode gating) method introduced in [26]: A TEM 01 resonator mode is excited, which consists of two lobes of opposite phase, spatially separated by an intensity minimum (see Fig. 1).…”