Coherent light sources with ultrashort wavelengths are crucial for the study of atomic and molecular systems, advanced nanomaterials, plasmas, and biological imaging systems. One technology for generating such wavelengths is the process of high-order harmonic generation, which involves converting ultraviolet-visible-infrared pulses from femtosecond lasers or optical parametric amplifiers to extreme ultraviolet (EUV) or soft X-ray frequencies. This article introduces a multi-thin-plate pulse compression technology for the green wavelength range, which can be extended to the ultraviolet to infrared range. The system diagram is shown in Figure 1, which consists of two parts: BBO frequency doubling and CaF₂ thin-plate group broadening and compression. The system converts infrared light with a repetition rate of 100 kHz and a central wavelength of 1030 nm into green light at 515 nm using a BBO crystal, with a frequency doubling efficiency of 70%. The green light energy after frequency doubling is 56 μJ, and the pulse width is 180 fs. The broadening section uses eight 1 mm thick CaF₂ thin plates as the broadening medium, cut along the [111] direction to minimize temporal and spatial distortions of the pulse.

Compared to commonly used materials such as sapphire and fused silica, CaF₂ offers lower second-order and higher-order dispersion. Tight focusing can cause significant spectral broadening in a single thin plate, but excessive nonlinear accumulation can lead to severe beam distortions and splitting, significantly degrading the compressed pulse quality. Controlling the B-integral of each thin plate within an appropriate range ensures pulse compression quality. In this system, the B-integral of each thin plate is controlled at 1.43 rad. By adjusting the spot size on the first thin plate and the distance between different thin plates (50 mm), the pulse approximately satisfies the spatial soliton condition as it propagates through the thin-plate group, with consistent spot sizes (350±10 μm) and pulse widths (200 ± 10 fs) on each thin plate. Finally, the broadened pulse is compressed and the second-order dispersion is compensated by a prism pair.

Figure 1 Schematic diagram of the multi-thin-plate compression device [1].

Figure 2 shows the spectral broadening results after the pulse passes through the thin-plate group. The transmission rate after passing through eight CaF₂ thin plates is 95%, with an overall compression efficiency of 75%. Figure 2A shows the spectral broadening results after different thin plates, and Figure 2B shows the corresponding transform-limited pulses. It can be seen that the spectrum does not broaden significantly after passing through six thin plates. Figure 2C shows the calculated dispersion values (orders 2-10) after compression by the prism pair. When the second-order dispersion compensation is in the range of 2000-3000 fs², the lower higher-order dispersion values have a minor impact on the temporal shape of the pulse.

Figure 2 A: Spectral broadening results after each thin plate; B: Experimental and simulated results of transform-limited pulses after different thin plates; C: Dispersion values (orders 2-10) after prism pair compression [1].

The compressed pulse measurement results are shown in Figure 3, with the self-diffraction FROG and second-harmonic generation FROG measurements indicating a compressed pulse width of approximately 35 fs.

Figure 3 FROG measurement of compressed VIS pulses below 40 fs [1].

CaF₂ crystals have high transmittance and low dispersion over a broad spectral range. By selecting appropriate parameters, pulses can propagate as spatial solitons through the CaF₂ thin-plate group, making this approach suitable for pulse compression in the ultraviolet to infrared (258 - 1030 nm) range. Table 1 shows the specific parameter selections for different wavelength bands using second, third, and fourth harmonic generation light sources.

This article introduces a pulse compression technology for the green wavelength range based on the CaF₂ thin-plate group, compressing green light pulses with a repetition rate of 100 kHz from 56 μJ, 180 fs to 42 μJ, 35 fs. Driving high-order harmonic generation with this light source is expected to yield 13.5 nm EUV light sources.

Reference:

[1] Wang S, Yan J, Song S, et al. High-performance ultrafast pulse compression in the visible spectral range for extreme nonlinear optics at kHz-MHz repetition rates[J]. arXiv preprint arXiv:2307.01164, 2023.