Impulsive Raman-induced spectral broadening in hydrogen-filled HC-PCF

Hollow-core photonic crystal fibre (HC-PCF) is unique host for gas-based nonlinear optical experiments. This is because it offers low-loss single-mode guidance in a micron-sized hollow core along with pressure-tunable dispersion and nonlinearity. In previous work, noble gases have been used as Raman-free nonlinear media, permitting efficient soliton-based pulse compression where the interplay between Kerr nonlinearity and anomalous dispersion results in dramatic self-compression of an ultrashort pulse. Novel phenomena such as UV wavelength conversion and even plasma generation from ~50 fs laser pulses of ~1 μJ energy have been reported [1]. In a different context, HC-PCF filled with molecular gases offers excellent performance as an ultra-low threshold modulator and frequency shifter for nano- and picosecond laser pulses [2]. Motivated by this, here we study experimentally and numerically the propagation of a 40 fs laser pulse in a hydrogen-filled HC-PCF [3]. Since the pump pulse duration is well below the period of one rotational cycle of ortho-hydrogen (57 fs), so that the bandwidth of the pulse is broader than the corresponding Raman frequency shift (18 THz), the interaction takes place in the impulsive regime. In other words, the pump pulse already contains Stokes shifted photons and the Raman process is self-seeded. Moreover, since the pulse duration is much shorter than the phase relaxation time T₂ of the molecular coherence, the (Raman) response of the medium is highly non-instantaneous (i.e., nonlocal in time) and affected by the whole pre-history of the interaction [4].