Ruirui Zhao

Design of intense 1.5-cycle pulses generation at 3.6 µm through a pressure gradient hollow-core fiber.

We theoretically study the nonlinear compression of the 10-mJ, 62-fs, 3.6-µm laser pulses in an argon gas-filled hollow-core fiber with large diameter of 1000 µm. Using a pressure gradient to restrict undesirable nonlinear effect such as ionization, especially at the entrance, we obtain the intense 18.3-fs (~1.5 cycle) pulses at 3.6 µm only through compression with CaF2 crystal, which can be used as an ultrafast source for strong field driven experiments. In addition, we calculate and discuss the relation between optimal fiber length and coupling efficiency for a given bandwidth. These results are useful for the design of using hollow-core fiber to compress the high-energy pulses with long wavelength.

Propagation dynamics of radially polarized pulses in a gas-filled hollow-core fiber

The propagation dynamics of radially polarized (RP) pulses in a gas-filled hollow-core fiber (HCF) is numerically studied. It is found that the stable transverse mode of RP pulse in HCF is not TM01 mode, nor any eigenmodes in terms of Bessel functions. Compared with linearly polarized (LP) pulses, the RP pulses with the same initial pulse duration and energy have higher transmission efficiency, more uniform spectral broadening, and cleaner temporal profile after highly nonlinear propagation in HCF and better focusing properties. These results suggest that energetic few-cycle RP pulses can be generated more efficiently by directly spectral broadening the RP pulses in HCF followed by temporal compression.

Generation of few-cycle radially-polarized infrared pulses in a gas-filled hollow-core fiber

We perform a numerical study for temporally compressing radially-polarized (RP) infrared pulses in a gas-filled hollow-core fiber (HCF). The dynamic transmission and nonlinear compression of RP pulses centered at wavelengths of 0.8 ?m, 1.8 ?m, 3.1 ?m, and 5.0 ?m in HCFs are simulated. By comparing the propagation of pulses with the same optical cycles and intensity, we find that under proper conditions these pulses can be compressed down to 2?3 cycles. In the transverse direction, the spatiotemporal beam profile ameliorates from 0.8-?m to 1.8-?m and 3.1-?m pulses before the appearance of high-order dispersion. These results show an alternative method of scaling generation for delivering RP infrared pulses in gas-filled HCFs, which can obtain energetic few-cycle pulses, and will be beneficial for relevant researches in the infrared scope.

Energetic radially polarized few-cycle pulse compression in gas-filled hollow-core fiber

The compression of high-energy, radially polarized pulses in a gas-filled hollow-core fiber (HCF) is theoretically studied. The simulation results indicate that a 40-fs input pulse can be compressed to a full-width at half-maximum of less than 9 fs when the pulse energy reaches 7.0 mJ with a transmission efficiency of more than 67% after propagating through a 1-m-long, 500-μm diameter HCF filled with neon. Furthermore, the spatio-temporal intensity distributions of the compressed pulses with different initial input energies are studied, and the numerical results indicate that the spatio-temporal intensity distributions are more uniform for lower input pulse energies.

Picosecond pulse compression by modulation of intensity envelope in a gas-filled hollow-core fiber

A method of temporally compressing picosecond pulses is proposed. To increase the spectral broadening, two picosecond pulses at different central wavelengths are overlapped spatiotemporally to induce a modulation on the intensity envelope, thus leading to a high temporal intensity gradient. The combined pulse is then coupled into a gas-filled hollow-core fiber (HCF) to broaden the spectrum through nonlinear propagation. After that the pulse can be compressed by chirp compensation. This method is demonstrated numerically with two 1-ps/5-mJ pulses centered at 1053- and 1064-nm, respectively, which are coupled into a 250-μm-inner-diameter, 1-m-long HCF filled with 5-bar neon. After nonlinear propagation, the spectrum of the combined pulse is broadened significantly compared with the sum of the broadened spectra of a single 1-ps/10-mJ pulse centered at 1053- and 1064-nm. Under proper initial conditions, the pulse can be compressed down to ~16-fs. The influences of the energy ratio, time delay and wavelength gap between two input pulses, as well as the energy scaling are also discussed. These results show an alternative way to obtain ultrashort laser pulses from the picosecond laser technology, which can deliver both high peak power and high average power, and thus will benefit relevant researches in high-field laser physics.

Field-free molecular orientation enhanced by tuning the intensity ratio of a three-color laser field*

We theoretically study the field-free molecular orientation induced by a three-color laser field. The three-color laser field with a large asymmetric degree can effectively enhance the molecular orientation. In particular, when the intensity ratio of the three-color laser field is tuned to a proper value of I-3 : I-2 : I-1 = 0.09 : 0.5 : 1, the molecular orientation can be improved by similar to 20% compared with that of the two-color laser field at intensity ratio I-2 : I-1 = 1 : 1 for the same total laser intensity of 2 x 10(13) W/cm(2). Moreover, we investigate the effect of the carrier-envelope phase (CEP) on the molecular orientation and use the asymmetric degree of the laser field to explain the result. We also show the influences of the laser intensity, rotational temperature, and pulse duration on the molecular orientation. These results are meaningful for the theoretical and experimental studies on the molecular orientation.

Self-compression of 1.8-μm pulses in gas-filled hollow-core fibers*

We numerically study the self-compression of the optical pulses centered at 1.8-mu m in a hollow-core fiber (HCF) filled with argon. It is found that the pulse can be self-compressed to 2 optical cycles when the input pulse energy is 0.2-mJ and the gas pressure is 500-mbar (1 bar = 10(5) Pa). Inducing a proper positive chirp into the input pulse can lead to a shorter temporal duration after self-compression. These results will benefit the generation of energetic few-cycle mid-infrared pulses.

Spatiotemporal propagation dynamics of intense optical pulses in loosely confined gas-filled hollow-core fibers

We numerically study the propagation dynamics of intense optical pulses in gas-filled hollow-core fibers (HCFs). The spatiotemporal dynamics of the pulses show a transition from tightly confined to loosely confined characteristics as the fiber core is increased, which manifests as a deterioration in the spatiotemporal uniformity of the beam. It is found that using the gas pressure gradient does not enhance the beam quality in large-core HCFs, while inducing a positive chirp in the pulse to lower the peak power can improve the beam quality. This indicates that the self-focusing effect in the HCFs is the main driving force for the propagation dynamics. It also suggests that pulses at longer wavelengths are more suitable for HCFs with large cores because of the lower critical power of self-focusing, which is justified by the numerical simulations. These results will benefit the generation of energetic few-cycle pulses in large-core HCFs.

Generation of few-cycle laser pulses: Comparison between atomic and molecular gases in a hollow-core fiber*

We numerically study the pulse compression approaches based on atomic or molecular gases in a hollow-core fiber. From the perspective of self-phase modulation (SPM), we give the extensive study of the SPM influence on a probe pulse with molecular phase modulation (MPM) effect. By comparing the two compression methods, we summarize their advantages and drawbacks to obtain the few-cycle pulses with micro- or millijoule energies. It is also shown that the double pump-probe approach can be used as a tunable dual-color source by adjusting the time delay between pump and probe pulses to proper values.