Supporting data for "Combining ultrafast optics and condensed matter physics: experiments on high harmonic generation and its source"

Intense femtosecond pulses generated from Chirped Pulse Amplification (CPA) and Optical Parametric Amplification (OPA)-based laser systems enable us to monitor electron motion in various energy regimes, and with a greater time resolution. By studying the multi-photon processes occurring during laser-matter interactions, we can decipher matter’s electronic and optical properties. To achieve this purpose, we conducted experiments in two directions: the generation of shorter laser pulses, and the high-harmonic spectroscopy of solid-state media.

In this thesis, I demonstrated experiments in nonlinear pulse compression using perturbative optics. This principle was applied to two different pulses with centre wavelength of 515 nm and ∼ 800 nm, respectively. The 515 nm pulse was the second harmonic generation (SHG) of 1030 nm, with a spectral width of 4.7 nm. After two stages of spectral broadening, the spectrum was expanded to 26.2 nm. After proper dispersion compensation, the resulting pulse was measured to be 18 ± 2 fs. Moreover, we developed an optional pulse compression plan that resulted in a shorter pulse, suitable for HHG experiments. For the ∼ 800 nm pulse, by only one spectral broadening stage, the Fourier-transform limit was broadened to ∼ 7.3 fs.

High harmonic generation (HHG), an extremely nonlinear process, resolves electron motion in gaseous and solid-state media through both real space and momentum space pictures. The main focus along this direction was the HHG from bulk-state and 2d-state materials. For the experiment performed with bulk-state materials, a complete setup including a beamline adapted to different input pulses and an extreme ultraviolet (XUV) spectrometer chamber was constructed. By performing the static HHG experiments using SiO2 and MgO materials, we analysed the special interference patterns in high harmonic spectra that are related to the sample thickness, carrier envelope phase (CEP), intensity and pulse duration of the input pulse. In addition, the dynamic measurement from crystalline SiO2 is noteworthy, from which the information of lattice vibration and electron-phonon interaction can extracted from time-resolved high harmonic spectroscopy.

The next experiment focused on the interaction between 2d materials and infrared (IR) pulses. I illustrated the setup construction and proved its effectiveness. This setup is adaptable to different input pulses, allowing for high-precision imaging of 2d materials up to the micrometre level, as well as high-efficiency signal generation and collection. Dynamic measurements were also set to be conducted. Additionally, two options to have high-quality background-free measurements were presented. Via this unique setup, we successfully measured the orientation-dependent high harmonic spectra from 2d Ruddlesden Popper (R-P) perovskite material, which matches well with its crystal structure. In transmission geometry, two orders of wave mixing signal emerged in 800 nm - 2100 nm pump-probe experiments, which can reflect electron dynamics in multi-layer h-BN. Moreover, by using reflection geometry, high harmonics of 2100 nm pulse up to the 7th order were recorded, which is rarely published in any other experimental works.