# Time-Resolved Imaging of Femtosecond Laser-Induced Plasma Expansion in a Nitrogen Microjet
## 2 Materials and Methods
### Experimental Setup  
For the study of the gas jet expansion dynamics, we used the experimental setup shown in Figure 2. The ultrashort laser pulses were provided by an ultrafast Ti:Sapphire laser system at 800 nm wavelength (Amplitude, Aurora laser system: 15 mJ, 25 fs, 1 KHz). A fraction of the laser pulse energy (up to 440 uJ) was used to ionize the gas, hence, triggering the plasma dynamics. We tuned the pulse energy between 150 and 400 uJ by rotating a beam splitter. The pulses were focused into a vacuum chamber by an anti-reflection coated plano-convex lens with focal length f = 300 mm. Filaments were formed in air with an elongation of \(\sim\)1 cm. At the entrance, the vacuum chamber was equipped with high-optical quality fused silica windows with a thickness of 500 um. Figure 1: (**a**) Illustration of the De Laval nozzle chip. The top circular housing holds the gas inlet tube (glued to it). In the blow-up image the dimensions of the de Laval nozzle are reported. The three-axis reference system we refer to within the article, is reported above. (**b**) Transmission microscope image of the De Laval nozzle used in the experiments reported in the manuscript. The nozzle was connected to the gas by a 1.5-mm inner diameter metallic tube into the glass device inlet with vacuum-compatible low-vapor-pressure epoxy glue. The metallic gas tube was interfaced with a flexible tube, thus, allowing a free motion for positioning the nozzle into the experimental chamber. The device was mounted on top of a high-precision 3-axis motorized translation system composed of three linear stages (Newport) for accurate control of the nozzle alignment to the laser filament. The nozzle was tested in low-pressure air environment (2 \(\times\) 10\({}^{-1}\) mbar). The vacuum chamber was pumped by a dry screw pump (650 m\({}^{3}\)/hour speed). The nozzle was loaded with a continuous backing pressure, up to 3 bars. The backing pressure can be manually tuned by a needle valve mounted upstream the gas pipeline, and accurately monitored by a capacitive pressure gauge placed after such valve. To collect the plasma signal, we used an optical system consisting of a 30\(\times\) aspheric objective lens with 0.4 N.A. (Newport, 5723-A-H), rigidly screwed on an aluminum tube mounted on a KF40 flange and terminated with a fused silica window. For adjusting the laser beam position to the objective working distance, we put the mirror and the lens on a manual translational stage. The plasma dynamics were imaged orthogonally to the expansion direction by a fast gated camera. The camera is made by two units: a multi-channel plate (MCP) intensifier (C9546-03, Hamamatsu Photonics, Tokyo, Japan) coupled to a low noise CCD camera (Retiga R6, Q-imaging, Surrey, BC, Canada) by a lens system. The CCD camera (2688 \(\times\) 2200 pixels) was operated at 2 \(\times\) 2 binning to match the spatial resolution of the MCP (Modulation Transfer Function \(\approx\) 1024 lines). To acquire time-resolved 2D images, we synchronized the camera acquisition with the laser pulses  
Figure 2: Experimental setup used to image the laser-excited gas dynamics. Ultrashort laser pulses were focused on the gas jet by a 300-mm focal lens. The pulse energy was modulated by a beam-splitter (BS) mounted on a rotational stage and measured by a power meter (P). The fluorescence emitted by the ionized nitrogen was collected through an ICCD interfaced with an aspheric objective (OBJ). A UV bandpass interferometric filter (F) was used to select the emission line at 391 nm. Time-resolved frames are collected by triggering the camera with a delay generator. (M) Movable mirror-lens assembly. The insert shows the fluorescence spectrum emitted by nitrogen excited with femtosecond laser pulses. through a programmable time-delay generator (Stanford Research Systems, DG535). All the images were taken with gate width of 5 ns.