Objective Intense few-cycle laser pulses are particularly important for applications in strong-field physics,time-resolved spectroscopy,and nonlinear optics.However,generating ultrastable,high-efficiency few-cycle pulses with high beam quality is challenging.In this study,we demonstrate our achievement of compressing a 50 fs Ti:sapphire laser pulse at a wavelength of 800 nm to～11.8 fs.This corresponds to four to five optical cycles via ultrastable solitary propagation in a periodic layered Kerr medium(PLKM)in ambient air,which results in a broadband supercontinuum spectrum from visible to infrared in the range of 598 nm to 945 nm with a conversion efficiency of～92％.By comprehensively investigating the effects of various experimental parameters on the temporal and spatial modes of the output pulses,we reveal the formation conditions for discrete spatiotemporal solitons.Methods We utilize linearly polarized,800 nm,50 fs Ti:sapphire laser pulses as the driving source,whose laser energy can be controlled using a half-wave plate(HWP)and a thin-film polarizer.A plano-convex lens with a focal length f1=200 cm is used to focus the laser pulses in ambient air.The PLKM comprises 10 pieces of 200-μm-thick fused silica plates,which are placed in parallel with the laser incident angle at the Brewster angle(55.5°).The spacing L between two adjacent plates can be adjusted along the laser-propagation direction using a linear track.The first PLKM plate is placed immediately after the focal spot of the laser beam.The laser pulses resulting from the PLKM are collimated using a concave mirror with a focal length f2=100 cm and then compressed using four pairs of chirped mirrors and a pair of wedges.The laser pulse energy is determined from the laser power measured using a thermopile detector,and the output-beam profiles are characterized by measuring the laser spot on a paper using a commercial camera,where the distance between the final plate and paper is maintained at 300 mm.To perform spectral measurements,an integrating sphere is used to obtain the laser signal,which is then detected using a fiber-coupled spectrometer.The temporal profiles of the laser pulses are measured using a custom-developed second-harmonic-generation frequency-resolved optical gating(SHG-FROG)and then reconstructed using principal component generalized projections(PCGPs).Results and Discussions Based on Eqs.(1),(2),and(3)and the experimental conditions shown in Fig.1,we obtain the initial distance for the PLKM to form a quasi-soliton and then generate the supercontinuum spectra and beam patterns,as shown in Fig.2.By monitoring the variations in the beam modes and the spectra of the output pulses after the PLKM under several different spacing distances L,we discover that under an input laser energy of 350 μJ,the 800 nm,50 fs laser pulses result in effective spectrum broadening with a favorable beam mode only at a specific spacing distance of L=100 mm,in which the self-focusing and diffraction effects are well balanced[Figs.2(a)and(b)],thus resulting in the highest output energy[Fig.2(c)].To further optimize soliton generation by the PLKM,we observe the variations in the laser spectra and beam patterns of the output laser pulses by adjusting the input laser energy from 320 μJ and 370 μJ under L=100 mm.Based on Fig.3,as the input laser energy varies,the broadened spectra remain almost constant but the beam modes change significantly.Specifically,the beam size reaches the minimum when the input pulse energy is 340 μJ[Figs.3(a)and(b)],and the shortest pulse duration(～52 fs)is recorded.This is consistent with the pulse duration(50 fs)of the input pulses[Fig.3(c)].The stable spatial and temporal profiles provide evidence of soliton formation.Consequently,after the PLKM,an excellent discrete spatiotemporal soliton laser pulse with an energy of 312 μJ is obtained,with the broad spectrum encompassing 598 nm to 945 nm at-20 dB.The soliton pulse is then compressed to～11.8 fs using four pairs of chirp mirrors and a pair of wedges,which corresponds to four to five optical cycles(Fig.4).Conclusions We demonstrate the robust generation of intense few-cycle pulses with a broad spectrum ranging from visible to near infrared using a commercial Ti:sapphire femtosecond laser system and a PLKM.By focusing 340 μJ,50 fs,800 nm pulses into a single-stage PLKM,which is composed of 10 pieces of 200-μm-thick fused silica plates with an equal spacing distance L=100 mm,excellent discrete spatiotemporal solitons ranging from 598 nm to 945 nm at-20 dB and an energy of 312 μJ are obtained.Few-cycle(11.8 fs)pulses are achieved unambiguously when the soliton pulse is compressed using a pair of wedges and four pairs of chirped mirrors.Our results provide a new method for generating high-quality,high-efficiency few-cycle pulses using a Ti:sapphire femtosecond laser.
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