A newly designed femtosecond KBe2BO3F2 device with pulse duration down to 55 fs for time- and angle-resolved photoemission spectroscopy

Abstract

Developing a widely tunable vacuum ultraviolet (VUV) source with a sub-100 fs pulse duration is critical for ultrafast pump–probe techniques such as time- and angle-resolved photoemission spectroscopy (TrARPES). While a tunable probe source with a photon energy of 5.3–7.0 eV has been recently implemented for TrARPES by using a KBe2BO3F2 (KBBF) device, the time resolution of 280–320 fs is still not ideal, which is mainly limited by the duration of the VUV probe pulse generated by the KBBF device. Here, by designing a new KBBF device, which is specially optimized for fs applications, an optimum pulse duration of 55 fs is obtained after systematic diagnostics and optimization. More importantly, a high time resolution of 81–95 fs is achieved for TrARPES measurements covering the probe photon energy range of 5.3–7.0 eV, making it particularly useful for investigating the ultrafast dynamics of quantum materials. Our work extends the application of the KBBF device to ultrafast pump–probe techniques with the advantages of both a widely tunable VUV source and ultimate time resolution.

Ultrafast Carrier Relaxation Dynamics in a Nodal-Line Semimetal PtSn4

“…TrARPES measurements were performed in our home laboratory at Tsinghua University. , The fundamental femtosecond laser pulses at 740 nm are provided by a Ti:sapphire oscillator together with a pulse picker running at a repetition rate of 4.75 MHz. The fundamental laser pulses are split into two beams, one used as the pump beam and another used for generating the probe beam.…”

“…The fitting function used to fit the relaxation time in Figure g–i is , I false( t false) = A true( 1 + erf ( 2 ⁡ ln ⁡ 2 t − t 0 Δ t − Δ t 4 ln ⁡ 2 τ ) true) e − ( t − t 0 ) / τ + B …”

“…The continuous temporal evolution of photoexcited electrons at different energy and momentum positions is also shown in Figure g–i, further revealing the details of the band-resolved photoexcitation and relaxation processes. The corresponding relaxation time τ is extracted by fitting the time traces using the product of a step function and an exponential decay function convoluted with a Gaussian function , (see details in Methods). The extracted relaxation time for the cone-like band at different energies is plotted in Figure j, which shows that the relaxation time ranges from 153 to 380 fs.…”