Time-Resolved Imaging of Femtosecond Laser-Induced Plasma Expansion in a Nitrogen Microjet

Anna Gabriella Ciriolo,Aldo Frezzotti,Caterina Vozzi,Michele Devetta,Salvatore Stagira,Daniela Comelli,Rebeca Martínez Vázquez,Gabriele Crippa,Roberto Osellame,Gianluca Valentini

doi:10.3390/app12041978

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 micronozzle 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.

Highlights

The development of micro-fluidic systems for controlling and manipulating the density, velocity, and composition of fluids on a micrometer scale has been undergoing a great impulse over the last decade due to the advances in micro-fabrication techniques
We studied the hydrodynamic expansion of a nitrogen micro-jet in the wavefield of intense femtosecond laser pulses
We applied time-resolved 2D fluorescence imaging for tracking the spatial evolution of a nitrogen plasma driven by high-energy femtosecond laser pulses

Summary

Introduction

The development of micro-fluidic systems for controlling and manipulating the density, velocity, and composition of fluids on a micrometer scale has been undergoing a great impulse over the last decade due to the advances in micro-fabrication techniques. 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]. To achieve proper tailoring of the gas flux, the geometry of the nozzle must be accurately engineered, down to the micrometer size, 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. 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. The dynamics of the charged particle population was measured with a time step of 5 ns

Glass De Laval Nozzle Fabrication

Experimental Setup

Study of the Plasma Expansion Dynamics

Conclusions