P. Prem Kiran

Three-photon absorption in ZnSe and ZnSe∕ZnS quantum dots

ZnSe and ZnSe∕ZnS core/shell quantum dots (QDs) of two different sizes (4.5 and 3.5nm) have been synthesized. The nonlinear absorption is measured at 1064nm using a 35ps laser with an open aperture Z-scan setup. Three-photon absorption (3PA) has been observed in ZnSe and ZnSe∕ZnS QDs. 3PA cross section is found to be about four orders of magnitude larger than bulk ZnSe, and three orders of magnitude higher than ZnS QDs. 3PA cross section is found to be increased in ZnSe and in ZnSe∕ZnS QDs with decreasing size from 4.5to3.5nm, due to strong confinement effect.

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.

Two-photon absorption in ZnSe and ZnSe/ZnS core/shell quantum structures

The third order nonlinear optical properties of two different sized ZnSe and ZnSe∕ZnS quantum dots (QDs) are investigated. The nonlinear absorption is measured at 806nm using Ti:sapphire 100fs laser pulses in an open aperture Z-scan setup. Two-photon absorption (2PA) is found to be dominant in core and core shell QDs. 2PA cross section is enhanced by three orders of magnitude compared bulk ZnSe. 2PA cross section is observed to increase with reduction in QD diameter, due to strong confinement effect. ZnSe∕ZnS QDs exhibit higher 2PA cross section compared with corresponding ZnSe QDs, indicating better passivation of the QD surface.

Enhanced optical limiting and nonlinear absorption properties of azoarene-appended phosphorus (V) tetratolylporphyrins

Optical limiting performance, third-order nonlinearity chi(3), and nonlinear absorption properties have been investigated in a new class of azoarene phosphorus (V) porphyrins with charge transfer (CT) states. The introduction of axial azoarene groups into the phosphorus porphyrin structure is found to reduce the limiting threshold by a factor of 2 and lead to a rise in the second hyperpolarizability by 1 order of magnitude in the picosecond time regime and by 2 orders of magnitude in the nanosecond regime. The experimental data show reverse saturation of absorption in the nanosecond time regime and a saturation of the nonlinear absorption above a fluence of 0.5 J/cm2 in the picosecond regime. The presence of the CT state reduces saturation of excited-state absorption (ESA) in the S1 --> Sn transition through the S1 --> CT transition. Faster CT --> T1 transition increases the ESA from T1 --> Tn states in the nanosecond regime. A self-consistent theoretical analysis based on rate equations is used to estimate the high-lying excited-state lifetimes and absorption cross sections from the experimental results.

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

Hotter electrons and ions from nano-structured surfaces

The impact of nano-structured surfaces on particle generation from ultrashort intense laser produced plasmas is presented over an intensity range of 10 15 –10 17 xa0Wcm −2 . The nano-structured surface evidently produces hotter plasma but does not lead to the generation of hotter ions, a counterintuitive result based on present understanding of plasma expansion mechanism. Although the total ion flux and energy is more in the case of structured surfaces, the average energy of the projectiles is found to be lower than that from polished surfaces. The nano-structured surface shows preferential enhancement of lower energy ions and an intensity dependent divergence of the ejected particles.

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.

Focusing of ultrashort sub-TW laser pulses in air: supercontinuum emission

Our recent work [Phys. Rev. A, 82, 013805 (2010)] clearly establishes a new regime of filamentation beyond the conventional intensity clamping by high-resolution spatial imaging of filaments. We present the corresponding measurements of the supercontinuum emission (SCE) from filaments in this regime produced by ultrashort (45 fs) sub- TW (up to 0.2 TW) Ti:Sapphire laser pulses focused air under tight (f/6) focusing geometry. The intimate connection between the spatial and spectral domains in the new regime is established by investigating the spectral broadening of SCE at peak intensities exceeding 1014 W/cm2 pulses at the focus thus giving a more complete picture of this new regime in filamentary propagation of intense fs pulses.

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.