Sub-50 fs pulses at 2050 nm from a picosecond Ho:YLF laser using a two-stage Kagome-fiber-based compressor

The high-energy few-cycle mid-infrared laser pulse beyond 2μm is of immense importance for attosecond science and strong-field physics.However,the limited gain bandwidth of laser crystals such as Ho:YLF and Ho:YAG allows the generation of picosecond(ps)long pulses and,hence,makes it challenging to generate few-cycle pulse at 2μm without utilizing an optical parametric chirped-pulse amplifier(OPCPA).Moreover,the exclusive use of the near-infrared wavelength has limited the generation of wavelengths beyond 4μm(OPCPA).Furthermore,high harmonic generation(HHG)conversion efficiency reduces dramatically when driven by a long-wavelength laser.Novel schemes such as multi-color HHG have been proposed to enhance the harmonic flux.Therefore,it is highly desirable to generate few-cycle to femtosecond pulses from a 2μm laser for driving these experiments.Here,we utilize two-stage nonlinear spectral broadening and pulse compression based on the Kagome-type hollow-core photonic crystal fiber(HC-PCF)to compress few-ps pulses to sub-50 fs from a Ho:YLF amplifier at 2μm at 1 kHz repetition rate.We demonstrate both experimentally and numerically the compression of 3.3 ps at 140μJ pulses to 48 fs at 11μJ with focal intensity reaching 10^(13)W/cm^(20.Thereby,this system can be used for driving HHG in solids at 2μm.In the first stage,the pulses are spectrally broadened in Kagome fiber and compressed in a silicon-based prism compressor to 285 fs at a pulse energy of 90μJ.In the second stage,the 285 fs pulse is self-compressed in air-filled HC-PCF.With fine-tuning of the group delay dispersion(GDD)externally in a 3 mm window,a compressed pulse of 48 fs is achieved.This leads to a 70-fold compression of the ps pulses at 2050 nm.We further used the sub-50 fs laser pulses to generate white light by focusing the pulse into a thin medium of YAG.