Photoionization-induced broadband dispersive wave generated in an Ar-filled hollow-core photonic crystal fiber

The resonance band in hollow-core photonic crystal fiber (HC-PCF), while leading to high-loss region in the fiber transmission spectrum, has been successfully used for generating phase-matched dispersive wave (DW). Here, we report that the spectral width of the resonance-induced DW can be largely broadened due to plasma-driven blueshifting soliton. In the experiment, we observed that in a short length of Ar-filled single-ring HC-PCF the soliton self-compression and photoionization effects caused a strong spectral blueshift of the pump pulse, changing the phase-matching condition of the DW emission process. Therefore, broadening of DW spectrum to the longer-wavelength side was obtained with several spectral peaks, which correspond to the generation of DW at different positions along the fiber. In the simulation, we used super-Gauss windows with different central wavelengths to filter out these DW spectral peaks, and studied the time-domain characteristics of these peaks respectively using Fourier transform method. The simulation results verified that these multiple-peaks on the DW spectrum have different delays in the time domain, agreeing well with our theoretical prediction. Remarkably, we found that the whole time-domain DW trace can be compressed to ~29 fs using proper chirp compensation. The experimental and numerical results reported here provide some insight into the resonance-induced DW generation process in gas-filled HC-PCFs, they could also pave the way to ultrafast pulse generation using DW-emission mechanism.


Introduction
Phase-matched dispersive wave (DW), arising from a nonlinear energy transfer from a self-compressed soliton, has attracted great research interests in the past few decades [1][2][3][4]. In particular, the low-loss broadband-guiding gas-filled hollow-core photonic crystal fibers (HC-PCFs), as ideal platforms, are widely used for studying the efficient generation of tunable phase-matched DW [5][6][7]. The phase-matched DW can not only be generated in a wide wavelength range such as ultraviolet (UV) [8] and mid-infrared (MIR) spectral regions [9], but its wavelength can also be effectively tuned, which makes this kind of light sources have many applications, especially in ultrafast spectroscopy.
In the gas-filled HC-PCFs, the combined effects of self-compressed soliton and high-order dispersion can result in phase-matched DW generated in the UV region [5,7]. As early as ten years ago, Joly et al. demonstrated the efficient emission of DW in the deep-UV region by using an Ar-filled kagomé-style HC-PCF [8]. The generated bright spatially coherent deep-UV laser source is tunable from 200 to 320 nm through varying the pulse energy and gas pressure. Belli et al. used a short length of hydrogen-filled kagomé-style HC-PCF to develop the generation of vacuum-UV (VUV) to near-infrared (NIR) supercontinuum [10]. The experimental result showed that a strong VUV DW emission generated at 182 nm on the trailing edge of the pulse, and it also proved that kagomé-style HC-PCF works well in the VUV spectral region. Simultaneously, Ermolov et al. reported on the generation of a three-octave-wide supercontinuum from VUV to NIR region in a He-filled kagomé-style HC-PCF [11]. The VUV pulses generated through DW emission is tunable from 120 to 200 nm, with efficiencies >1% and VUV pulse energy >50 nJ. It should be pointed out that in the strong-field regime, the plasma caused by gas ionization modifies the fiber dispersion, allowing phase-matched DW generation in the MIR region. Novoa et al. first predicted the generation of MIR DW, and established a new model to explain it well [12]. Köttig et al. first experimentally demonstrated the existence of MIR DW and realized a 4.7-octave-wide supercontinuum from 180 nm to 4.7 µm, with up to 1.7 W of total average power [9].
The phase-matched DW can not only be generated in the UV and MIR spectral regions, but also in the visible and NIR regions. Sollapur et al. experimentally reported on the generation of a three-octave-wide supercontinuum from 200 nm to 1.7 µm at an output energy of ~23 µJ in a Kr-filled HC-PCF [13]. Simulations showed that the spectra generated in the visible and NIR resonance bands are closely related to the emission of the phase-matched DW. Tani et al. further proved that the narrow-band spectral peaks generated in the resonance bands are based on the phase-matched DW emission due to the anti-crossing dispersion [14]. Recently, Meng et al. demonstrated the generation of NIR DW in the resonance bands of an Ar-filled kagomé-style HC-PCF [15]. In our recent experiments [16,17], we reported the high-efficiency emission of phase-matched DW in the visible spectral region using a He-filled single-ring (SR) HC-PCF. As the input pulse energy increases, the central wavelength of the plasma-driven blueshifting soliton (BS) [18,19] is close to the resonance band of the fiber, high-efficiency energy transfer from the pump light to the DW can be triggered.
In this work, we demonstrated in the experiments that the photoionization-induced broadband DW in a 25-cm-long Ar-filled SR HC-PCF. In a certain pulse energy region, we observed that soliton self-compression of the input pulse results in a spectral expansion that overlaps with resonant DW frequencies, leading to a narrow-band DW spectral peak in the first resonance band of the SR HC-PCF. At high pulse energy levels, we observed that the plasma-driven BS can further excite multiple DW peaks, which leads to the broadening of DW spectrum to the longer-wavelength side. In addition, we theoretically investigated the time-domain characteristics of these spectral peaks filtered by the super-Gauss windows using Fourier transform method, and compressed the DW pulses to ~29 fs through suitable dispersion compensation.

Experimental Set-up
The experimental set-up is shown in Fig. 1. The 800 nm, ~45 fs, 0.3 mJ pulses from a commercial Ti:Sapphire laser system were coupled into a 1-m-long hollow-core fiber (HCF) by using a concave mirror with a focal length of 1 m. The HCF has a core diameter of 250 µm and was placed in a gas cell filled with 212 mbar Ar. The spectrum of the pulses after propagating the Ar-filled HCF was broadened due to self-phase modulation (SPM), and the energy transmission was measured to be ~70%. Several pairs of chirped mirrors (CMs) were not only used to compensate the output pulses from the gas-filled HCF, but also to pre-compensate the dispersion introduced by some optical elements, including half-wave plate (HWP), wire grid polarizer (WGP), and even plano-convex lens (PCL) and the window at the input port of the second gas cell. Then the compressed pulses were launched into a 25-cm-long SR HC-PCF filled with 1.4 bar Ar using a coated PCL with a focal length of 0.1 m. The insert indicates the scanning electron micrograph (SEM) of the SR HC-PCF that has a core diameter of 24 µm and a wall thickness of ~0.26 µm.  The phase-matched DW is usually generated in the positive direction of the time axis. In Fig.  5(a), we used a super-Gauss window to filter out the leftmost DW spectral band (marked as red dashed line, called DW-SB1), and the green solid line corresponds to the simulated spectrum at the output of the SR HC-PCF with the input pulse energy of 2.6 µJ. Through the Fourier transform of DW-SB1, the corresponding temporal profile in Fig. 5(b) is located on the right side of the time axis and shows a long pulse duration of ~198 fs at FWHM due to a narrow spectral width. However, the plasma-driven BS located in the anomalous dispersion region has a higher group velocity than input pulses, so that the blueshifting pulses accelerate as the propagation distance increases. This causes the DW radiation to approach the zero point of the time axis, and even appear in the negative direction, as shown in Figs. 5(c)-5(f). The pulse durations corresponding to the DW-SB2 and DW-SB3 are ~171 fs and ~36 fs, respectively. In Fig. 5(h), we also plot the temporal intensity of BS [see Fig. 5(g)], and the pulse duration is about 13 fs. Figure 5(j) shows the complete temporal profile of the output pulses. The main peak is the BS, and its peak position is about 44 fs, consistent with Fig. 5(h). In addition, the left side of the main peak is the existing pedestal of the incident pulses [see Fig. 2(e)], while the oscillations on the right are the DW radiation and residual pump light. The self-compressed pulse duration can be as short as ~6 fs. These results can also be observed in Fig. 4(j). Figure 6(a) shows the broadband DW spectrum filtered from the output spectrum of Fig. 5(i) through using a super-Gauss window from 550 nm to 610 nm, and the corresponding temporal profile after the Fourier transform is plotted in Fig. 6(b). Although the pulse duration at FWHM is ~30 fs, the pulses exhibit long-decay pedestals on the trailing edge due to the DW radiation generated at different positions in the fiber. We found that after -4634 fs 2 group delay dispersion (GDD) and -44827 fs 3 third-order dispersion (TOD) compensation, these pedestals can be compressed into the main peak, showing a clean pulse leading edge. The compressed pulse duration is ~29 fs, which is very close to the Fourier transform limit (FTL) of ~24 fs.