With 300 fs pump pulses, spectral broadening due to self-phase modulation was observed at the highest peak powers [Menard (2016)]. This caused a slight departure between the measured SH signal and the quadratic dependence predicted by the simple theoretical model. We monitored the SH spectrum as a function of launched pump pulse energy (figure left). At low peak energies, the SH spectrum has a Gaussian-like distribution centred at 515 nm with a full width half maximum (FWHM) of ~3 nm. As the pump power increases, the spectrum gradually broadens, reaching a maximum bandwidth of ~10 nm FWHM. Surprisingly, symmetric spectral broadening of the pump pulse spectrum due to self-phase modulation (SPM) creates an asymmetric SH spectrum with pronounced features on the short-wavelength side. Due to its lower group velocity, the generated SH pulse is delayed with respect to the pump pulse. Therefore, the SH pulse experiences parametric gain on the trailing edge of the pump pulse, where SPM creates higher frequencies. The broad linewidth of the SH signal confirms that the gas-filled HC-PCF is suitable for efficient three-wave mixing of ultrashort pulses. The detected bandwidth corresponds to the FWHM spectral linewidth of a ~100 fs transform-limited pulse. Furthermore, the electrode length, which was 16 cm, could easily be shortened or lengthened so as to adjust the nonlinear phase-matching bandwidth and potentially allow conversion of optical pulses of any duration.

Periodically patterned electrodes were used to generate a SH signal in the LP01 mode by quasi-phase matching (QPM). Upon applying a voltage across the electrodes and scanning the gas pressure, two prominent SH peaks were consistently recorded. The first one was at a Xe pressure of 3.9 bar, corresponding to the LP02 mode as described above. A second peak was seen at different pressures depending on the QPM period L (figure right). For example, at a Xe pressure of 4.1 bar for L = 1.82 mm (dark blue curve) the second SH signal was always in the LP01 mode. Since χ(3) scales linearly with gas pressure, a stronger SH signal is generated for smaller QPM periods because the phase-matching pressure is higher. The dependence of SH power on the gas pressure is quadratic, but saturates above 10 bar as a result of group velocity walk-off between the pump and SH pulses.