Photoionization-assisted, high-efficiency emission of dispersive wave in gas-filled hollow-core photonic crystal fibers

We demonstrate that the phase-matched dispersive wave (DW) emission within the resonance band of a 25-cm-long gas-filled hollow-core photonic crystal fiber (HC-PCF) can be strongly enhanced by the photoionization effect of the pump pulse. In the experiments we observe that as the pulse energy increases, the pump pulse gradually shifts to shorter wavelengths due to soliton-plasma interactions. When the central wavelength of the blueshifting soliton is close to the resonance band of the HC-PCF, high-efficiency energy transfer from the pump light to the DW in the visible region can be obtained. During this DW emission process, we also observe that the spectral center of the DW gradually shifts to longer wavelengths leading to a slightly-increased DW bandwidth, which can be well explained as the consequence of phase-matched coupling between the pump pulse and the DW. In particular, at an input pulse energy of 6 uJ, the spectral ratio of the DW at the fiber output is measured to be as high as ~53% together with a conversion efficiency of ~19%. These experimental results, explained by numerical simulations, pave the way to high-brightness light sources based on high-efficiency frequency-upconversion processes in gas-filled HC-PCFs.


INTRODUCTION
In the past decade broadband-guiding anti-resonant hollow-core photonic crystal fibers (HC-PCFs) filled with gases [1][2][3], have become excellent platforms for studying ultrafast nonlinear optics such as ultrashort pulse compression to the single-cycle regime [4], efficient generation of tunable DW at deep and vacuum ultraviolet (UV) wavelengths [5][6][7][8], soliton-plasma interactions [9][10][11][12][13], etc. These successful applications of the HC-PCF in nonlinear optics are generally due to the remarkable feature of the HC-PCF -it can tightly confine light in its μm-sized core while maintaining low-loss guidance over long distances [1], largely enhancing the light-matter interactions in it. In addition, when filled with gases, the HC-PCF can offer waveguideinduced anomalous dispersion across a broad wavelength range through tuning the gas pressure, leading to versatile soliton dynamics [2,3].
While the anti-resonant HC-PCF can provide low-loss optical guidance over a broad wavelength range, its transmission window is discontinuous, disrupted by the presence of several narrow-band resonances [14,15]. Within these resonance bands the guidance of light shows high loss due to the energy coupling between the core and the cladding modes in the fiber. Tani et al. demonstrated the influence of this transmission discontinuousness on some ultrafast nonlinear processes, such as the suppression of both the soliton self-compression and UV DW generation [16]. They also predicted that when the central wavelength of the pump light is close to the resonance band of the antiresonant fiber, strong narrow-band emission of phase-matched DW could be obtained due to the sharp dispersion slope within the resonance band of the fiber [16]. Recently, Sollapur et al. experimentally demonstrated the generation of more-than-threeoctave supercontinuum light from 200 nm to 1.7 μm that spans multiple resonant bands in a Kr-filled anti-resonant HC-PCF [17], in which the emission of multiple phase-matched DWs was involved. More recently, Meng et al. demonstrated in the experiments that nearinfrared DW emission can be obtained in the resonance band of an Arfilled anti-resonant fiber [18,19]. In this experiment, in order to achieve high-efficiency frequency conversion the central wavelength of the pump pulses was tuned to 1300 nm, close to the ~1000 nm resonance of the fiber being used [19]. All of these theoretical and experimental

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ut is sketched in ~45-fs pulses w used to pump a ith 250-μm core fused silica (FS) elf-phase modula broadened durin CMs) were used WP) and a wire gr rgy launched into nto a 25-cm-lon d gas pressure of 0 cm. Both ends S windows. The s a fiber spectrom grating sphere.
tage for DW genera s in panel (a) repre 4-μm core diamet of the fundament CF filled with 10.9 e) and simulated l of the SR-PCF. ing ray (BR) mod f the SR-PCF at 10 ct that assisted b the strong phaseh the wavelength esonances. On th cy-upconversion source generation n Fig. 1(a)

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To th ge kn sim dis ca wi inp ge nalytical model de escribe the dispe gions [23]. The l rst resonant spe urve shows a shar . High-Efficien Fig. 2(a), we me m-long He-filled S 6 μJ, and the nor fferent input puls black dotted lin ulse energy of <2 roadening. While re set at the range a spectral blue pectral peaks nea an be understo eneration.
In particular, for ulses present a cl e plasma-driven oves to the short hown in Fig. 2  lain the resonanc lated temporal an SR-PCF at an inpu n Fig. 3(a)]. In th persion region ar first few centim he position of ~4 m peak power of ~8.2×10 16   3(f). For a lower pulse energy, we can see that the maximum spectral width and spectral redshift in Fig. 3(e) are both smaller than that in Fig.  3(d), while at a higher pulse energy Fig. 3(f) shows the larger spectral width and redshift compared to Fig. 3(d). In addition, in Figs

CONCLUSIONS
In conclusion, we experimentally and theoretically demonstrated that the plasma-driven blueshifting soliton can strongly enhance the emission of resonance-induced phase-matched DW in a 25-cm-long He-filled SR-PCF. At the input pulse energy of 6 μJ, we observed that the generated DW shows a notable spectral ratio of ~53% and a high energy conversion efficiency of ~19%. In addition, the spectral bandwidth of the DW becomes wider as input pulse energy increases, and its spectrum broadens to longer wavelengths, showing a redshift. This is the direct consequence of phase-matched coupling between the plasma-driven blueshifting soliton and the DW. These experimental results, well-supported by numerical simulations, offer some useful insight in understanding the resonance-induced phase-matched DW emission in gas-filled HC-PCFs. Moreover, the set-up demonstrated here provides a simple and efficient light source at visible wavelengths, which could be easily extended in the future to other wavelengths (UV for example) through using properly-designed anti-resonant fibers with resonance bands at different wavelengths.