Laser-cluster interactions - How do intense laser-heated clusters explode?

Atoms or molecules exhibit short-range attractions for one another owing to Van der Waals forces. Under an appropriate rapid cooling process, such as nozzle ejection of a gas puff into vacuum, hundreds to few tens of millions of atoms can aggregate together to make nano-scale clusters, typically of diameter less than a few hundred angstroms.

Recently, there has been great interest in the interaction of intense laser pulse with clusters. If an intense, ultra short laser pulse is focused in a gas of clusters, the clusters are almost instantaneously heated to temperatures up to ~107 K – many times hotter than the sun – and they explode violently. Such high temperatures means laser-heated clusters are a copious source of X-rays. The speeds of the particles thrown off by the explosion are high enough that fusion has been demonstrated from the collisions of deuterium nuclei from nearby explosions.

However, exactly how these nano-fireballs explode is still a controversial question, and one of more than just academic interest. For it turns out that the clusters explode so quickly that the heating laser pulse—even one as short as 100 fs—is still on while they’re doing so. And the time-dependent details of the ultra-rapidly evolving cluster ion and electron spatial distributions fully determine the manner in which the laser couples energy to the cluster and with what efficiency.

To date, most experiments have examined the issue of cluster explosion dynamics by measuring X-ray spectra or fast particles that emerge from the interaction long after the explosion has occurred. The explosion scenario is then inferred from this data.

In our paper, we describe measurements of the cluster explosion as it happens. To do this, we first developed a new ultra fast optical diagnostic, which we call Single Shot Supercontinuum Spectral Interferometry (SSSI). The diagnostic measures the ultra-rapid changes in the complex index of refraction -- the most important and fundamental optical property—induced by an intense laser pulse. We applied SSSI to measuring the transient refractive index of a gas of exploding clusters. As the refractive index in this case is simply a sum of the polarizabilities of the individual exploding clusters, this measurement tells us how the average cluster polarizability evolves in time.

Our diagnostic reveals that the polarizability is positive at early times in the cluster explosion (while it is still at high density), and then goes negative at later times when the expanding fireballs finally dissolve into normal uniform plasmas. The full time evolution of the polarizability shows the exploding cluster throwing off hot material layer-by-layer, with resonant laser coupling occurring near the outer layers. This resonant coupling is essential to all of the applications of laser-heated clusters such as x-ray generation and fusion.

One surprising phenomenon we have recently discovered is that the microscopic cluster evolution described above has a macroscopic effect on beams. The higher intensities on the laser beam axis induce bigger positive polarizabilities compared to the those at the beam periphery. This results in self-generated lensing action. That is, pulses self-focus in clustered gases. There are several applications of this result. One is the more efficient generation of plasma waveguides. Optical guiding of intense laser beams over a long distance is greatly challenging but it is essential for the realization of soft x-ray lasers and laser-plasma-based charged particle accelerators. Exploding clustered gases may be one of the key enabling technologies.
 

Laser-droplet interactions

In the quest for a clean high-power extreme ultraviolet (EUV) light source for next-generation lithography, many research groups are achieving significant success with a source based on a laser-produced plasma (LPP). One of the most efficient LPP sources is produced at Sandia National Laboratory by the irradiation of a spray of micron-size xenon droplets with intense pulses from a 1500 W, Q-switched (~10 ns), 300-mJ, kHz-repetition rate laser system. This interaction produces EUV radiation with conversion efficiencies comparable to those of solid targets (approximately 0.2% conversion of laser energy into EUV radiation into 2π steradians and 2% bandwidth). The mass-limited noble gas droplet source allows for improved control of the plasma emission by careful selection of the material, phase, and size of the droplets, and produces negligible debris, which can help extend the life of the sensitive multilayer optics. An important question is: If the driving laser pulse is of fixed energy, what pulse duration results in the most efficient emission of EUV radiation?
           

We have found in previous experiments using fixed energy, variable-width laser pulses (100 fs-10 ns) that EUV generation from micron-sized krypton droplets is most efficient for pulse durations of ~100-300 ps. Recently, we concentrated exclusively on the ~100ps timescale regime, which is most appropriate for droplets. These recent experimental studies have shown that the pulse duration is an important parameter for EUV source efficiency. In general, the EUV emission of interest is associated with a particular ion state that is present in high relative concentration for the full duration of the laser pulse. We have found that if this spectral feature is associated with an excitation process, the optimal laser pulse duration is set mainly by droplet size. In that case laser heating should occur when the droplet is at or above the critical electron density. For our droplet sizes of <10 microns, this necessitates driver pulses of less than ~300 ps. If the spectral feature is associated with recombination, however, this condition is relaxed, and the most EUV efficient laser pulse duration is generally longer and is at least several recombination cycles, which could amount to a few nanoseconds.

Optical guiding of extremely intense laser pulses

Extreme focused laser intensities can now be achieved with a new generation of ultrashort pulse, high-energy solid state lasers. Many applications of these intense laser pulses would benefit greatly from a large intensity-interaction length product. Usually, the effective interaction length of such pulses has been limited to the effective extent of the focal region, typically less than 1 mm, given by twice the Rayleigh length. This is the minimum natural length scale over which a beam will diffract and spread. Our development of the plasma fiber waveguide has achieved a longstanding goal in laser–matter interaction physics: the propagation of intense pulses over distances greatly in excess of this natural spreading length scale.

Plasma is the unavoidable medium for intense pulse propagation: the intensities of interest for most applications are in excess of minimum ionization thresholds of typical atoms. In a manner analogous to guiding in an optical fiber, light can be guided in plasmas if the refractive index at beam center can be increased sufficiently with respect to the beam edge to balance the effect of diffraction. Guiding in plasma fibres can be linear up to the point at which additional ionization occurs or relativistic effects begin. The plasma fibre guides not only high-intensity optical pulses, but it also guides x-rays, and can do so with mode structure identical to the optical pulse driver. The degree of control possible in the plasma fibre exceeds that of almost any other known guiding structure, with the parameters of channel composition, density, depth, and curvature adjusted by choosing the gas type, density, guided pulse injection delay, and channel breakdown laser parameters.