Spatiotemporally resolved optical emission spectroscopy and harmonic generation in Cu plasmas

Highlights

• Near-infrared laser ablation plasmas of Cu have been characterized.

• Spatiotemporally resolved optical emission spectroscopy employed to assess plasma.

• Cu plasma studied as nonlinear optical medium through third harmonic generation.

• Ablation laser fluence window identified for maximum third harmonic generation.

• Role of plasma composition and phase matching have been determined.

Abstract

Laser-induced ablation plasmas of Cu generated by nanosecond 1064 nm pulses have been examined as media for third harmonic generation of a nanosecond driving pulse. A narrow window of ablation fluences where third harmonic generation is optimized has been identified in a region around twice the ablation threshold. A detailed analysis of the features of the Cu plasma across this critical fluence window, through spatially and temporally resolved optical emission spectroscopy, has been used to assess critical plasma parameters such as temperature, electron density, and abundance of Cu species in different ionization stages. This description has been correlated with the conditions for optimum low-order harmonic generation in the Cu plasma.

Introduction

The incidence of laser light of sufficient intensity on the surface of a solid initiates a cascade of phenomena that causes the ejection of material from the sample in the form of atoms, clusters, nanoparticles and droplets, [1]^, [2] often with extensive ionization. The created ablation-plasma expands in the three dimensions, with a net velocity in the direction perpendicular to the sample surface, with dynamics dominated by collisions, recombination and radiative processes [3].

A detailed understanding of laser plasmas is important for their applications in fields like laser processing of materials, [4] synthesis of thin films, [5] elemental analysis of complex samples [6] or generation of extreme ultraviolet radiation [7] or X-rays [8]. Laser plasmas are inherently transient media and present sharp gradients of measurable quantities in space and time. The understanding of the mechanisms governing plasma formation and expansion is a challenging task that requires a detailed description of the dynamics in the solid from the laser-matter interaction to the formation of the plasma, and the subsequent plasma evolution and expansion. Some of the experimental techniques employed to describe these media are time-of-flight methods, [9] interferometry and fast photography [10]. The detection of optical emissions by excited particles in the plasma is particularly suitable and informative, since it can provide information not only on the composition of the plasma but also on the electron density and temperature, [11,12,13] with spatial and temporal resolution.

As nonlinear optical media, laser plasmas offer interesting routes for exploration and optimization, due to their often complex composition, the presence of species that do not appear in other environments and the broad range of values that macroscopic variables like temperature or electron density can span across. A significant body of literature exists describing harmonic generation in plasmas generated through laser ablation. Among these, some experiments have the exploited resonance enhancement of a given harmonic due to a resonance with a species present in the plasma, [14]^,[15] others have described harmonic generation in species like fullerenes, [16] clusters [17]^,[18]^,[19] or nanoparticles [18] [20] [21]. Additionally, the macroscopic conditions of the plasma region where harmonics are generated (temperature, gas pressure, electron density) have been shown to play a crucial role on harmonic emission [17] [22].

One of the main drivers of the studies of plasmas as nonlinear optical media has been the search for improved characteristics of harmonic emission, be it in the form of higher efficiency, higher cutoffs in high-order harmonic generation or intense generation of a given harmonic. In that arena, previous work has mainly focused on high-order harmonics generated with ultrashort near-infrared laser pulses [15,17,20]. However, it has been shown that the observation of harmonics from a laser plasma also constitutes a powerful diagnostic technique of the plasma itself, and can be sensitive to properties of the laser plasmas that are elusive through other techniques. This type of study has been dominated by low-order harmonic generation [14,18,19,21,23,24]. Given that laser plasmas are media with local inversion symmetry, so that only odd harmonics can be emitted, the first orders to be observed are the third and fifth. When harmonic generation is employed as a tool to diagnose the laser plasma, it is important to design experiments that allow complete spatio-temporal exploration across broad ranges (at least millimeters in space, tens of microseconds in time). An example of this type of measurement can be found in [19], where a detailed temporal analysis of harmonic generation allowed the detection of middle-sized species in a complex fs plasma.

The crucial roles of both microscopic (i.e. the local composition of the plasma) and macroscopic (i.e. dispersion) characteristics of laser plasmas on harmonic generation have been broadly acknowledged in previous literature [17,25,26,27]. In some previous contributions by us and other authors it had been described how optical emission spectroscopy can be used for a qualitative assessment of species present in the plasma where harmonic emission takes place [19], [21], [22], [28], [29]. Also, OES assessment of the electron density in plasmas employed for harmonic generation has been reported for instance in [25], or even, in a reverse approach, harmonic generation has been employed as a measurement of electron density through OES in a quasi-phase matching configuration [30]. On the other hand, detailed analysis of optical emissions has shown to be an invaluable tool for the description of laser-induced plasmas [13], [31], [32], one that can not only provide information on a given parameter like electron density but that, particularly when performed with high spatial, temporal and spectral resolution, can yield valuable and precise information on a broad range of plasma characteristics like composition, excitation temperature, degree of ionization or plasma expansion behavior. However, a detailed description of the features of the plasma through optical emission spectroscopy, together with the determination of their repercussions on harmonic generation, is, to our knowledge, lacking. Contributing to filling this gap constitutes the main aim of this work, where we report the results of an experiment designed to study the behavior of a laser-generated Cu plasma as a nonlinear optical medium, in a spatiotemporal region diagnosed through optical emission spectroscopy. This allows us to determine the conditions, both micro- and macroscopic, that optimize third-order harmonic generation in a laser-generated Cu plasma.