Enhancement cavities for few-cycle pulses

Abstract

We address the challenge of increasing the bandwidth of high-finesse femtosecond enhancement cavities and demonstrate a broad spectrum spanning 1800 cm^-1 (195 nm) at -10 dB around a central wavelength of 1050 nm in an EC with an average finesse exceeding 300. This will benefit a host of spectroscopic applications, including transient absorption spectroscopy, direct frequency comb spectroscopy, and Raman spectroscopy. The pulse circulating in the EC is composed of only 5.4 optical cycles, at a kilowatt-level average power. Together with a suitable gating technique, this paves the way to the efficient generation of multi-megahertz-repetition-rate isolated extreme ultraviolet attosecond pulses via intracavity high-order harmonic generation.

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

“…Scaling laws predict that this photon flux can be achieved at repetition rates compatible with time-of-flight spectrometers when using non-circular gas nozzle orifices and sufficiently large mirror substrates. Given the recent advances of power scaling in resonators [12], broadband cavity mirrors [20], high-power phase-stable Yb-based seed lasers [29], and zero-offset-frequency resonators [30], the implementation of efficient, cavity-enhanced generation of IAPs comes into reach. Compared to state-of-the-art kHz sources of IAPs, the dramatic increase in repetition rate will have a corresponding impact on the signal-to-noise ratio in experiments in attosecond physics, promising to reveal nanoscopic information so far hidden under the measurement noise floor [8].…”

“…This scheme has been shown to produce IAPs from pulses as long as 33 fs at a wavelength of 800 nm, corresponding to 12.4 optical cycles [54]. Efficient intracavity HHG with 30 fs-pulses was already shown [13], approaching the optimum photon flux for time-resolved photoelectron emission experiments, and mirrors supporting even shorter pulses have been demonstrated [20], so CMC-PG is a viable candidate. To avoid damage at high intensities, the silicon plate can also be replaced by a broadband thin-film polarizer.…”

“…This scheme does not impose further conditions on the spectral or the temporal shape of the pulse incident on the target and is therefore compatible with the standard EC geometry. However, efficient amplitude gating has only been shown with sub-two-cycle pulses [49], while the bandwidth of HR mirrors currently limits the intracavity pulse duration to >18 fs at a wavelength of 1050 nm [20], rendering this scheme unviable for intracavity IAP generation.…”

“…The bandwidth of currently available highly reflective (HR) mirrors does not allow to enhance pulses short enough to reach the single-cycle limit. However, recent work in our group [20] shows that a power enhancement of 75 is still possible with mirrors supporting pulses shorter than 20 fs at a central wavelength of 1050 nm, corresponding to 5.4 cycles.…”

“…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]. Owing to recent progress concerning advanced cavity designs [12,16], the quantitative understanding of the intracavity gas target nonlinearity [17][18][19], and thanks to scaling the bandwidth of ECs [20] and of phase-stable, high-power seeding laser systems [21][22][23], it seems feasible as from today's point of view to extend this technology to application in attosecond physics. However, state-of-the-art dielectric multilayer optics cannot cover the bandwidth necessary for single-cycle near-infrared pulses [20], which would enable the direct generation of IAPs in ECs.…”