A frequency comb in the extreme ultraviolet

Abstract

Since 1998, the interaction of precision spectroscopy and ultrafast laser science has led to several notable accomplishments. Femtosecond laser optical frequency 'combs' (evenly spaced spectral lines) have revolutionized the measurement of optical frequencies and enabled optical atomic clocks. The same comb techniques have been used to control the waveform of ultrafast laser pulses, which permitted the generation of single attosecond pulses, and have been used in a recently demonstrated 'oscilloscope' for light waves. Here we demonstrate intra-cavity high harmonic generation in the extreme ultraviolet, which promises to lead to another joint frontier of precision spectroscopy and ultrafast science. We have generated coherent extreme ultraviolet radiation at a repetition frequency of more than 100 MHz, a 1,000-fold improvement over previous experiments. At such a repetition rate, the mode spacing of the frequency comb, which is expected to survive the high harmonic generation process, is large enough for high resolution spectroscopy. Additionally, there may be many other applications of such a quasi-continuous compact and coherent extreme ultraviolet source, including extreme ultraviolet holography, microscopy, nanolithography and X-ray atomic clocks.

Extreme-ultraviolet light generation in plasmonic nanostructures

“…The results illustrate that such tapered plasmonic waveguides are not suitable for efficient nanostructure-enhanced HHG, although sufficient local intensities are achieved. We believe that coherent radiation buildup is limited by the nanoscopic generation length, which is orders of magnitude smaller than in other state-of-the-art HHG concepts [96,19,106,78,49]. Nonetheless, the observed strong EUV fluorescence and the exhibited optical bistability indicate novel aspects of strong-field physics in conjunction with nonlinear plasmonics.…”

“…This is not the first time the signature of ALE has been identified in conventional HHG spectral data [78]. The two distinct phenomena, ALE and HHG, are not mutually exclusive but coexistent when gaseous atoms are illuminated by strong-field laser pulses.…”

“…We believe that for future implementations of nanostructurebased EUV generation, it is very important to carefully and spectroscopically identify relative contributions from incoherent and coherent processes. To become competitive with other HHG concepts at high repetition rates [77,78,79], the number of participating atomic dipoles should be substantially increased using higher pressures and larger interaction volumes. Nonetheless, the observation of stable nonlinear fluorescence signals is encouraging and may find application where spatial and temporal coherence are not essential, for example in an implementation of near-field EUV lithography.…”

Femtosecond Optical Frequency Combs

“…A promising route to solve this issue is to generate HHG's inside an enhancement resonator. Harmonics with up to 110 MHz repetition rate where generated this way [45,46]. In the first generation of these set-ups the extracted extreme ultraviolet (XUV) power was by far not sufficient to be useful for high resolution laser spectroscopy.…”

Oscillator-based High-order Harmonic Generation at 4MHz for Applications in Time-of-Flight Photoemission Spectroscopy

“…However, due to the damages caused by the intense electric field, their results can not be reproduced by other groups and the ideas are currently under critical debate [10]. The desired enhancement of the laser electric field at high repetition rate, nevertheless, can be achieved in a macroscopic resonant cavity, either external to or integrated inside the driving laser [11,12]. With enormous efforts optimizing the output coupler for harmonics, the repetition rate of HHG using an external cavity goes nowadays beyond 150 MHz [13].…”