# Time-Resolved Imaging of Femtosecond Laser-Induced Plasma Expansion in a Nitrogen Microjet
###### Abstract  
We report on the study of ultrafast laser-induced plasma expansion dynamics in a gas microjet. To this purpose, we focused femtosecond laser pulses on a nitrogen jet produced through a homemade De Laval micronozzle. The laser excitation led to plasma generation with a characteristic spectral line emission at 391 nm. By following the emitted signal with a detection system based on an intensified charge-coupled device (ICCD) we captured the two-dimensional spatial evolution of the photo-excited nitrogen ions with a temporal resolution on the nanosecond time scale. We fabricated the micronozle on a fused silica substrate by femtosecond laser micromachining. This technique enabled high accuracy and three-dimensional capabilities, thus, providing an ideal platform for developing glass-based microfluidic structures for application to plasma physics and ultrafast spectroscopy.  
De Laval nozzle; femtosecond laser micromachining; ultrafast laser sources +
Footnote †: journal: _Sci.  
approach consists of using gas cells and capillaries. Sophisticated continuous-flow systems based on nozzle-ended glass capillaries and enabling accurate gas velocity and temperature control were recently developed for applications to high-repetition-rate laser sources, above the MHz [4].  
Dielectric-based gas-fluidic devices are particularly suitable for usage in laser-gas interaction experiments due to their high damage threshold and stability. However, to achieve proper tailoring of the gas flux, the geometry of the nozzle must be accurately engineered, down to the micrometer size, thus, requiring a flexible and precise glass manufacturing technology. In this sense, femtosecond laser micromachining [5] provides a powerful technique for the realization of complex microfluidic systems with high accuracy, extreme versatility, and three-dimensional capability.  
Here, we present a [PERSON] conical nozzle realized on a fused-silica slab through femtosecond laser irradiation followed by chemical etching (FLICE). The nozzle has been designed for application to high-order harmonic generation (HHG) [6] and ultrafast spectroscopy. In particular, it has been thought of as a component of a more complex microfluidic FLICE device aiming at achieving gas density manipulation on the micron-scale for efficient high-order harmonic generation on a chip [7]. In this work, we studied the hydrodynamic expansion of a nitrogen micro-jet in the wavefield of intense femtosecond laser pulses.  
For the nozzle characterization, high-intensity femtosecond laser pulses are focused at the nozzle exit, thus, interacting with the gas jet. Driven by the oscillating laser field, the gas intercepted by the laser beam may experience ionization by electron tunneling leading to the generation of electrons correlated to the parent ions [8]. In this regime, the fluorescence emission from the ionized gas at particular wavelengths in the visible/UV spectrum can be detected. Fluorescence from molecular gases provides information on the excited states involved in the strong-field processes, but can also be used for mapping the distribution of the excited molecules on a spatial domain.  
In particular, we detected the radiative emission at 391 nm from N\({}_{2}\)+(\(B^{2}\)\(\Sigma u^{+}\)) to the N\({}_{2}\)+(\(X^{2}\)\(\Sigma g^{+}\)) states through an ICCD-based imaging setup. The micro-nozzle used for nitrogen injection was engineered to allow collecting the fluorescence signal in proximity to the gas jet and orthogonally to the laser propagation direction. The dynamics of the charged particle population was measured with a time step of 5 ns.