Abstract

An ultrashort pulse can impulsively excite molecular vibrational/rotational modes in a Raman-active medium provided its duration is less than the Raman period. Previous experiments have excited specific transitions by controlling the pulse duration, chirp or the excitation scheme [1]. Here we study the response of ro-vibrational transitions in hydrogen to a rapid sequence of two identical linearly polarized pulses of duration 30 fs and central wavelength 800 nm. In the experiment the pulses propagate along a 20 cm length of kagomé-style photonic crystal fibre (kagomé-PCF) filled with 8 bar of hydrogen, resulting in anomalous dispersion at 800 nm. A variable delay line (step-size ~1 fs) was used to control the time-delay τ_D between the pulses, up to a maximum value of 1 ps, Fig. 1(a). Upon entering the gas-filled hollow core, the pulses undergo soliton-effect compression down to durations of a few optical cycles – less than the vibrational period in hydrogen (~8 fs). This results in impulsive excitation of ro-vibrational modes and a periodically oscillating refractive index "wave" that survives long after the pulse has left the fibre [2]. If the second pulse is then launched within the coherence time of these modes, it will be modulated by the oscillating refractive index as well as undergoing self-compression, creating quite complex dynamics. In particular, if the time delay is such that the second pulse arrives exactly (8n + 4) fs after the first (where n is an integer), the vibrational motion will be coherently suppressed. As a result the spectral width of the transmitted light, originating from the second pulse, will oscillate with a period of 8 fs. Similar behavior is expected for the ortho-rotational transition, except that the period will be 57 fs.

© 2015 IEEE