fs XUV Laser Spectroscopy
on Nanoparticles
at The University of Texas in Austin

Heating of Silver Nanoparticles with fs XUV Terawatt Laserpulses

I. Introduction

Experiments studying the physics of laser-cluster interactions at extreme ultraviolet (EUV) wavelengths have become feasible only recently, due to developments in laser and accelerator technology. In contrast to cluster interactions in the visible-infrared spectral region, where field driven ponderomotive motion of the electrons dominates the physics and the photon energy is considered very small, EUV-cluster interactions are characterized by a large photon energy and near negligible ponderomotive influence. This leads to the creation of plasmas which are confined to a volume comparable to the initial size of the cluster, allowing more efficient energy transfer from the incident light to the electrons and from electrons to the ions in the cluster. Initial experiments indicate the production of dramatic electrical imbalances in inert gas clusters exposed to 80 nanometer light at an intensity above 1013W/cm2, resulting in the ejection of high energy electrons and ions with charge states far greater than would result from single photon absorption. Because ponderomotive effects are small in this interaction, the cluster model which successfully describes infrared-cluster interactions cannot be applied without significant modification. Experiments studying EUV interactions with clusters therefore will provide empirical tests for emerging theoretical models. With our setup we go a step forward and introduce LAM generated nanoparticles as a new target  for laser plasma experiments.

II. Harmonic Generation Design

 

Figure 1. Schematic of EUV Harmonic Generation and Focusing to Target

Production of harmonic radiation in the ten to thirty electron volt range is accomplished by loosely focusing (f/30) 100-700mJ of the compressed 40fs output of the THOR laser into a jet of argon or xenon gas (visit here the Ditmire group @ /~utlasers). Previous research reveals that the harmonic yield is best when the laser pulse is focused by the longest focal length lens possible. Harmonic separation is accomplished by imaging a mask in the infrared beam before the focusing lens onto an aperture after the focus (Fig. 1), taking advantage of the fact that the EUV harmonics have substantially less divergence than the infrared beam. This allows the infrared radiation to be removed without using thin film aluminum filters, which would absorb a significant fraction of the EUV light. Positioning the separation aperture in the image plane of the beam mask allows separation of the EUV from the infrared limited only by finite optical aperture and infrared refraction by plasma in the gas jet. Vacuum is maintained at the microTorr level between gas jet shots using a turbomolecular pump beneath the gas jet.

III. Target Chamber Systems Design


 

 

Figure 2. Target Chamber with Cluster Source and Diagnostics

 

The main measurement devices for the cluster-EUV interaction are a grazing incidence EUV spectrometer, which will monitor the EUV spectrum by measuring light which is transmitted through the interaction region, and an time-of-flight (TOF) spectrometer which will measure interaction products (Fig. 2). The EUV spectrometer has been adapted to hold an MCP/phosphor screen detector which will allow spectrally and spatially resolved measurements; this will be used to characterize the mirror spectral response, measure harmonic yield when clusters are not present, and check for preferential absorption of harmonic light by metal clusters. The TOF spectrometer will measure silver ions or electrons extracted from the interaction region by charged grids, allowing observation of relative yields versus wavelength and intensity.

IV. Mirror Construction

In the 35 to 60 nanometer wavelength range, conventional metal mirrors have low efficiency at normal incidence, typically less than twenty percent. Multilayer mirrors consisting of alternating layers of disparate compounds offer dramatic improvement to mirror efficiency, up to 45 percent in the case of Sc/Si mirrors used at 47 nanometers. These mirrors must be made under carefully controlled conditions, as minor variations in the layers can significantly reduce efficiency. We have fabricated Sc/Si mirrors using the equipment and expertise of an existing research group at the Lebedev Physical Institute (LPI) in Moscow, Russia. The mirrors will allow us to focus specific EUV harmonics of the THOR laser to intensities where nonlinear effects occur. We have also fabricated W/SiC bilayer mirrors with the help of LPI, which should have near 30% efficiency in the 60-100 nanometer range for nonlinear studies. At UT, we have fabricated unprotected gold (Au) mirrors, which will offer 5-10% reflectivity at normal incidence above 30 nanometers, and will be used to recollimate light into the EUV grazing incidence spectrometer.

V. Metal Nanoparticle Source


 
Figure 3. Silver nanoparticles production in progress. The TEM on the right shows deposited particles with mean diameters of 5nm

Metal clusters of N~105 atoms are made by Laser Ablation of Microparticles (LAM). Because of a wide variety of pressure requirements at various points in the LAM apparatus, differential pumping is used extensively in the source. In order to provide milliTorr pressure in the cluster expansion cell, while maintaining microTorr pressure in the interaction region, a roots blower and mechanical pump combination is used. A typically silver nanoparticle distribution produced with a Nd:YAG ablation laser is shown in Fig. 3.

VI. Future Work

The harmonic generation system is ready for testing now. We plan to first optimize the production of EUV light using the EUV spectrometer as a diagnostic, then measure spectral reflectivity for all of the mirrors discussed above. Initial experiments with noble gas clusters will be performed over a range of wavelengths for comparison with existing data while the metal cluster target chamber is completed. When the metal cluster chamber is ready, we will repeat the above experiments with Ag and Au clusters to compare with the noble gas cluster data, and to test theoretical predictions of EUV resonant absorption by metal clusters.



This page was last updated : April, 2006.
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