Low temperature enhancement of alignment-induced spectral broadening of femtosecond laser pulses

The propagation of femtosecond laser pulses in N₂-filled hollow fibers is studied both theoretically and experimentally. The laser pulse aligns the N₂ molecules and changes the refractive index, which meanwhile modulates the spectrum of the pulse in turn. The dependence of the spectral modulation on the gas temperature is investigated. We find that both spectral broadening and frequency red-shift are enhanced at low temperature. The degree of enhancement is found to be dependent on the pulse duration. Based on our findings, we propose a method for femtosecond pulse spectral broadening and few-cycle pulse generation via the molecular alignment.