Suman Bagchi

Filamentation without intensity clamping.

We present measurements of the supercontinuum emission (SCE) from ultrashort Ti:Saph laser pulse filamentation in air in a tightly focused geometry. The spectral broadening of SCE indicates that peak intensities exceed the clamping value of a few 10(13) W/cm(2) obtained for filamentation in a loose focusing geometry by at least one order of magnitude. We provide an interpretation for this regime of filamenation without intensity clamping.

Dynamics of laser induced micro-shock waves and hot core plasma in quiescent air

We present our results on spatio-temporal evolution of laser plasma produced shockwaves (SWs) and hot core plasma (HCP) created by focused second harmonic (532 nm, 7 ns) of Nd-YAG laser in quiescent atmospheric air at f/#10 focusing geometry. Time resolved shadowgraphs imaged with the help of an ICCD camera with 1.5 ns temporal resolution revealed the presence of two co-existing sources simultaneously generating SWs. Each of the two sources independently led to a spherical SW following Sedov-Taylor theory along the laser propagation direction with a maximum velocity of 7.4 km/s and pressure of 57 MPa. While the interaction of SWs from the two sources led to a planar SW in the direction normal to the laser propagation direction. The SW detaches from the HCP and starts expanding into the ambient air at around 3 μs indicating the onset of asymmetric expansion of the HCP along the z-axis. The asymmetric expansion is observed till 10 μs beyond which the SW leaves the field of view followed by a deformation of the irradiated region in the XY-plane due to the penetration of surrounding colder air in to the HCP. The deformation in the XY-plane lasts till 600 μs. The dynamics of rapidly expanding HCP is observed to be analogous to that of cavitation bubble dynamics in fluids.

Bright, low debris, ultrashort hard x-ray table top source using carbon nanotubes

We demonstrate that carbon nanotube coated surfaces produce two orders of magnitude brighter hard x-ray emission, in laser produced plasmas, than planar surfaces. It is accompanied by three orders of magnitude reduction in ion debris which is also low Z and nontoxic. The increased emission is a direct consequence of the enhancement in local fields and is via the simple and well known “lightning rod” effect. We propose that this carbon nanotube hard x-ray source is a simple, inexpensive, and high repetition rate hard x-ray point source for a variety of applications in imaging, lithography, microscopy, and material processing.

Hot ion generation from nanostructured surfaces under intense femtosecond laser irradiation

detector conditions for measurements from the coated and uncoated portions. The typical coating thickness of the NPs is about 0.2 m which is large compared to the optical skin depth of a few nanometers. The base pressure of the experimental chamber is 10 �6 torr. The target is scanned across the laser beam for each shot. The laser is focused by a gold coated off axis parabolic mirror, in an f / 4 focusing geometry with a spot size full width at half maximum of 10 m, giving peak intensities in the 0.5– 6.3 10 16 Wc m �2

Dynamics of tightly focused femtosecond laser pulses in water

The dynamics of tightly focused ultrashort (40 fs) pulses manifested in terms of supercontinuum emission (SCE) and cavitation-induced bubbles (CIB) resulting from propagation in water over a wide range of input powers (6 mW‐1.8 W) are presented. The effect of linear polarization (LP) and circular polarization (CP) on SCE in different external focal geometries (f=6, f=7:5 and f=10) is investigated and the results are discussed. SCE with higher efficiency and a considerable spectral blue shift is observed under tight focusing conditions (f=6) compared to loose focusing conditions (f=10). At higher input powers, CIB along the axis of propagation are observed to be assisting deeper propagation of these short pulses and enhanced SCE. (Some figures may appear in colour only in the online journal)

Surface-plasmon-enhanced MeV ions from femtosecond laser irradiated, periodically modulated surfaces

Enhanced emission of high energy ions is measured from sub-wavelength metallic grating targets under irradiation by intense (1015−1016 W cm−2), p-polarized, 50 fs, 800 nm laser pulses. The maximum ion energy is 55% higher and the ion flux is 60% higher for the modulated surface in comparison with polished surfaces of the same atomic composition. The ion emission, a result of enhanced light coupling and hot electron production in the grating targets, is correlated with enhancement in hard x-ray bremsstrahlung emission. The results are well reproduced by particle-in-cell simulations. The study reveals that the enhanced laser coupling by surface plasmon excitation on metallic gratings is directly responsible for the enhancement of ion energies.

Supercontinuum emission from tightly focused femtosecond pulses in air: beyond intensity clamping

We present the evolution of supercontinuum emission (SCE) from tightly focused fs laser pulses propagating in air. 45 fs laser pulses at 806 nm, 10 Hz repetition rate, from Ti:Sapphire laser (Thales Laser, Alpha 10) with a nanosecond contrast ratio better than 10-6: 1 are focused in air by a lens to an f/12 focusing geometry in one case, and by an off-axis parabolic mirror leading to an f/6 focusing in another. The laser input power is varied in the range of 10 - 90 PCr and 6 - 60 PCr in the f/12 and f/6 focusing geometries, respectively, where the critical power for selffocusing in air is PCr = 3 GW for 806 nm. The effect of the tight focusing condition on the SCE spectrum and the dependence on the input laser polarization are studied. Within the input power range used in the study, the blue edge (the maximum positive frequency shift) of the SCE spectrum is found to decrease continuously when the laser energy is increased. This result is in contrast with previous measurements of SCE in condensed matter and gases with loose focusing geometry, for which a constant blue edge was interpreted as due to intensity clamping. We propose a model, which show that for tight focusing conditions, external focusing prevails over the optical Kerr effect annihilating plasma defocusing and self-focusing, thereby giving access to a new propagation regime featured by an efficient laser energy deposition in fully ionized air and intense 1015 W/cm2 pulses at the focus.

Direction-dependent asymmetric expansion of laser-induced shockwaves in air

Laser induced breakdown of air is used to create a SW by focusing a 7 ns, 532 nm, 10 Hz Nd:YAG laser. Plasma created at the breakdown launches a compression wave into the nearest ambient air molecular layers that propagate at supersonic velocities. The propagation characteristics of SWs are studied double probe-beam deflectometry method, by two noninteracting probe beams that are parallel to each other and perpendicular to the propagation direction of the SW creating laser beam. The evolution of the laser induced SWs in air created at the focus along the direction of propagation (forward probe, FP) and in the direction opposite to the laser propagation (backward probe, BP) of the breakdown creating laser beam, are studied. The nature of the SW is estimated from the arrival time (τ) measured at different distances (Z) from the focal volume. At an input laser energy of 45 mJ, the arrival time information in the backward probe showed τ ∞ Z2.5 behavior indicating spherical SWs and in the forward direction followed τ ∞ Z1.6 showing planar expansion of the SWs revealing direction dependent asymmetric expansion of SWs across the focal plane. The pressure of the SWs estimated using counter pressure corrected point strong explosion theory along with the experimentally measured shock velocities are used to generate the P - U Hugoniot of shock compressed air.

HOT ELECTRON GENERATION AND MANIPULATION ON 'STRUCTURED' SURFACES

We examine ways of generating hotter electrons by coupling more light into structured surfaces (nanoparticle coated surfaces and sub‐lambda gratings). It is known that such surfaces produce enhanced x‐ray yields. We study the effect of laser prepulse conditions on the enhancement of hard x‐ray emission (20 – 200 keV) from plasmas produced on nanoparticle (NP)‐coated optically polished copper surface, under different prepulse conditions and observe that enhancement reduces with increasing prepulse intensity. The dynamics of the process is seen to be in the ps regime. We attribute this to preplasma formation on nanoparticles and subsequent modification/destruction of the nanostructure layer before the arrival of the main pulse. We suggest that high‐contrast ultrashort pulses are essential for nanoparticles to function as yield enhancers. We exploit surface plasmon coupling of light into sub‐lambda grating to switch ‘ON’ a hotter component in hard x‐ray spectra from plasma.

Interferometric and Shadowgraphic Studies Of Shock Wave And Cavitation Bubble Generated With Nd: YAG Nano Second Laser Pulses Induced Breakdown In Water

Laser break down and subsequent plasma formation in liquids, in particular in water is of considerable interest due to its potential application in biomedical applications and also in the generation of micro/nano particle formation. There are two mechanisms which can lead to plasma formation: direct ionization of the medium by multiphoton absorption or avalanche ionization via inverse bremsstrahlung absorption. Spectroscopic investigation was performed in the early stage of breakdown of water at and beyond breakdown threshold and subsequent formation of shock waves and cavitation bubble is studied using shadow graphic and interferometric technique.