V. Kumarappan

Measurement of the average size and density of clusters in a gas jet

We present an all-optical method for measurement of the average size and density of clusters produced by high-pressure gas jet flow. The technique employs Rayleigh scattering imaging combined with interferometry. The cluster size and density in a gas jet under the irradiation of intense laser fields plays a critical role in the laser–cluster coupling dynamics.

Clustered gases as a medium for efficient plasma waveguide generation

Clustered gas jets are shown to be an efficient means for plasma waveguide generation, for both femtosecond and picosecond generation pulses. These waveguides enable significantly lower on-axis plasma density (less than 1018 cm−3) than in conventional hydrodynamic plasma waveguides generated in unclustered gases. Using femtosecond pump pulses, self-guided propagation and strong absorption (more than 70%) are used to produce long centimetre scale channels in an argon cluster jet, and a subsequent intense pulse is coupled into the guide with 50% efficiency and guided at above 1017 W cm−2 intensity over 40 Rayleigh lengths. We also demonstrate efficient generation of waveguides using 100 ps axicon-generated Bessel-beam pump pulses. Despite the expected sub-picosecond cluster disassembly time, we observe long pulse absorption efficiencies up to a maximum of 35%. Simulations show that in the far leading edge of the long laser pulse, the volume of heated clusters evolves to a locally uniform and cool plasma already near ionization saturation, which is then efficiently heated by the remainder of the pulse.

Gases of exploding laser-heated cluster nanoplasmas as a nonlinear optical medium

The manner in which strongly heated nanoclusters explode in the presence of intense laser fields influences all applications of this interaction. By measuring, with femtosecond time resolution, the ensemble average polarizability in a gas of intense laser-heated clusters, we have inferred the cluster explosion dynamics. The time evolution of the polarizability is characteristic of competition in the optical response between supercritical and subcritical density regions of the exploding cluster. These results are consistent with complementary time-resolved Rayleigh scattering measurements and with the predictions of a near-field plasma hydrodynamic model of the laser–cluster interaction. A significant implication of this cluster evolution appears in its macroscopic effect on a laser beam: a gas of exploding cluster plasmas causes nonlinear beam propagation owing to the space and time dependence of the ensemble polarizability. A strong self-focusing effect is observed experimentally that strongly contrasts ...

Diagnostic of Laser‐Plasmas: Single‐shot Supercontinuum Spectral Interferometry

Ultrafast optical diagnostics play a vital role in probing the dynamics in laser‐matter interactions, including those observed in the high intensity ultrashort laser pulse regime. We developed a new femtosecond optical diagnostic, single‐shot supercontinuum spectral interferometry (SSSI), which measures ultra‐rapid transients in the complex index of refraction induced by an intense laser pulse. This measurement provides a direct view on how the laser‐produced perturbations evolve in time and space. To date, SSSI has been successfully used to diagnose femtosecond dynamics in the interaction of intense laser pulses with gases, nanometer‐sized atomic or molecular clusters, and plasmas, including plasma waveguides.

Asymmetric explosion of laser-irradiated hydrogen clusters

Under conditions of small hydrogen cluster size, where we expected angular-dependent time-of-flight (TOF) proton spectra consistent with isotropic coulomb explosions, we instead found strong explosion asymmetry with fast protons emitted preferentially in the direction of the laser polarization.

Efficient Generation of Low-density Plasma Waveguides in Clustered Gases

We demonstrate waveguide generation for intense pulse guiding using end and side-pumped schemes. Up to 3times1017 Wcm-2 pulses are guided in ~8 mm long argon plasma waveguides with little further ionization by the guided pulse

Guiding of intense pulses in fully ionized hydrogen plasma waveguides from a cluster jet

Summary form only given. The first demonstration of guiding of intense laser pulses in a preformed plasma channel involved the use of a Bessel beam pulse (produced by an axicon) to generate an extended line focus, producing a long plasma channel in either in backfill gases or in a gas jet, using non-clustered gases. As the plasma evolves and expands in the radial direction, it develops the profile required for wave guiding a few nanoseconds later. This method was found to have some limitations. Channel generation through efficient breakdown and inverse bremsstrahlung heating occurs only with resultant electron densities in excess of /spl sim/10/sup 19/ cm/sup -3/, which is too high for some applications. Also, both in backfill and gas jet realizations, these channels had a significant taper (over /spl sim/0.5-1 mm) at the entrance which impeded the injection coupling of the high-intensity pulse. Since the mid-nineties, clusters have been known to absorb laser energy very efficiently. This is primarily due to the solid-like density of an individual cluster. A hydrocode has been used to calculate the complex polarizability of the cluster as it evolves. At early times, when the cluster is mostly supercritical, the real part of the ensemble polarizability is positive, and its radial profile is concave. In this regime the laser pulse can undergo self-focusing. The evolution of the polarizability was measured using spectral interferometry, and these experiments confirmed the general predictions of the model. Self-focusing was also demonstrated in a different set of experiments. Since a clustered gas is expected to guide a laser pulse under appropriate conditions, a plasma channel can be generated which is much longer than the Rayleigh range of the focused beam. The channel can therefore be produced by end-pumping, greatly simplifying the experimental arrangement compared to the axicon case, We have constructed a pump probe experiment using a 1 cm long cluster jet in which a pump pulse self-guides and creates a /spl sim/1 cm plasma channel into which an intense co-propagating probe pulse is injected. This has been done for both argon and hydrogen cluster jets.

Self-guiding and red shifts of intense pulses propagating in clustered gases

Summary form only given. Intense laser interaction with clusters, Van der Waals-bonded aggregates of up to /spl sim/10/sup 7/ atoms, is of great interest owing to applications which include the generation of X-rays, energetic ions and electrons, and nuclear particles. In recent work, the time-resolved explosion of intense-laser-heated clusters has been observed, and this explosion scenario leads to intense laser self-focusing in a clustered gas. Here, we report measurements of the frequency shifts of intense laser pulses propagating in clustered gases. This measurement is consistent with our previous observations and model. We use a pump-probe setup for the frequency shift measurement. For a series of time-delayed 2/spl omega/ probe (sub-80 fs, 399 nm) spectra modulated by an intense pump (1.5 mJ, 80 fs, 798 nm) pulse co-propagating in a gas of argon clusters, we find that the probe increasingly red-shifts during the phase where the pump self-focuses. At later times, the shift goes blue and then relaxes back to zero. The red-shifts at early delays indicate the positive transients of the refractive index of pump-heated clusters, in agreement with our recent observation of self-focusing. This red-shift strongly contrasts with the blue-shift commonly observed in the ionization of non-clustered gases. At later times, however, the probe frequency shift becomes blue and then relaxes to zero owing to the dominant response of the underdense portion of the cluster plasmas at later times, with that response becoming progressively weaker.

Guiding of Intense Laser Pulses in Efficient End‐pumped Plasma Channels Generated by Self‐guiding in Ar and H2 Clusters

We demonstrate that self‐guiding of intense short pulses in clustered gases can be utilized to generate long plasma channels, which upon expansion form waveguides suitable for propagation of laser pulses at high intensity. This scheme has several advantages over waveguide‐generation in non‐clustered gases. The absorption of energy by the target depends on the size of the clusters and not on the average density of the gas, which allows greater control of the density encountered by the guided pulse. In particular, electron densities less than 1018 cm−3 are feasible. Moreover, since clusters absorb sub‐picosecond pulses very efficiently, channel generation by an auxiliary long‐pulse laser is no longer necessary and a considerably simpler setup suffices. The problem of taper at the channel entrance, an old bugbear of side‐pumped waveguides in gases, is shown to be significantly reduced. Evidence will be presented of waveguide generation in gases of argon and hydrogen clusters, using different cryogenic source...