Cavity-enhanced field-resolved spectroscopy

Femtosecond enhancement cavities 1 are key to applications including high-sensitivity linear 2 – 4 and nonlinear 5 , 6 gas spectroscopy, as well as efficient nonlinear optical frequency conversion 7 – 10 . Yet, to date, the broadest simultaneously enhanced bandwidths amount to <20% of the central optical frequency 8 , 9 , 11 – 15 . Here, we present an ultrabroadband femtosecond enhancement cavity comprising gold-coated mirrors and a wedged-diamond-plate input coupler, with an average finesse of 55 for optical frequencies below 40 THz and exceeding 40 in the 120–300 THz range. Resonant enhancement of a 50-MHz-repetition-rate offset-free frequency comb spanning 22–40 THz results in a waveform-stable ultrashort circulating pulse with a spectrum supporting a Fourier limit of 1.6 cycles, enabling time-domain electric-field-resolved spectroscopy of molecular samples with temporally separated excitation and molecular response 16 . The contrast between the two is improved by taking advantage of destructive interference at the input coupler. At an effective interaction length with a gas of up to 81 m, this concept promises parts-per-trillion-level ultrabroadband electric-field-resolved linear and nonlinear spectroscopy of impulsively excited molecular vibrations.