Generation of 1 MHz 15 fs pulses in the near-infrared with high energy and their application to time-resolved spectroscopy

Ultrashort pulses with a duration of ∼10 fs are crucial to fully resolve nuclear motions in molecules and materials. Here, we employ nonlinear pulse compression using photonic crystal fiber in the near-infrared region to generate ultrashort pulses at a high repetition rate with significant pulse energy. Femtosecond pulses centered around 1200 nm from a cavity-dumped optical parametric oscillator are compressed to a duration of 15 fs. The pulse energy reaches several tens of nanojoules, sufficient for wavelength conversion via nonlinear processes. Second harmonic generation produces clean 13 fs pulses at around 600 nm with pulse energy of 4 nJ. The effectiveness of nonlinear pulse compression using photonic crystal fiber for ultrafast time-resolved spectroscopy is demonstrated through time-resolved fluorescence (photoluminescence) and transient absorption apparatus, utilizing the second harmonic as pump pulses. Specifically, a time resolution of ∼20 fs is achieved in a time-resolved fluorescence experiment, enabling the recording of nuclear wave packets over 1200 cm^-1 by spontaneous fluorescence. The high repetition rate at the megahertz level significantly improved the signal quality.