Atmospheric turbulence plays an important role in long-range propagation of light pulses. Mid-infrared pulses can propagate in air upto hundreds of meters by forming long channels of plasma due to lower ionization losses compared to near-IR pulses. Such long filamentation channels are useful in atmospheric sensing, remote laser-induced breakdown spectroscopy (LIBS), steering and triggering of electric discharges and other long-range applications. We study the effects of atmospheric turbulence in long-wavelength infrared (LWIR) femtosecond filamentation in air. We numerically investigate the combined effects of turbulence and nonlinearity in the long-range propagation of LWIR pulses at 6 μm. We model the nonlinear response of the atmosphere by including Kerr effect, multiphoton-ionization and rotational Raman effects in air the dispersive response of several atmospheric gas species such as N2, O2, Ar, CO2 CH4 and H2O. We model the turbulence using a phase-screen model. The inhomogeneous medium is represented by a series of phase screens located at regular intervals along the propagation direction. This provides an understanding of the robustness of long range filamentation and propagation of LWIR pulses over turbulent medium which essential for several long range applications including free-space optical communication.
Pulse propagation through hollow-core fibers (HCFs) filled with noble gas is a stable and efficient technique for pulse self-compression. The scalability of soliton dynamics in gas-filled HCFs, varying over a large range of energies, from sub-μJ to above mJ, allows to tune the energy of the generated few-cycle pulses too a great extent. Scaling relations can be used to produce propagation dynamics and effects that are invariant and essentially identical for multiple sets of input conditions. But, for the same input soliton order, the scaling relations derived under different dispersion conditions, such as different gas pressure, result in somewhat different scaling laws. This leads to an ambiguity in the compression factor and compression length for any particular soliton order N. It is thus necessary to find an accurate soliton order which can describe the self-compression dynamics over different dispersion conditions. We numerically simulate soliton self-compression in an argon gas-filled HCF across a wide range dispersion conditions and present an accurate soliton order for better understanding of the self-compression behavior. We introduce an effective soliton order Neff, for explaining the behavior of soliton dynamics in systems with high third order dispersion (TOD). This provides us with universal scaling laws for generating high-energy few-cycle pulses, which are critical for generating single and trains of attosecond pulses, as well as electron and ion acceleration strategies in intense laser pulses.
We report wide-field, epi-illumination microscopic imaging of biological tissues using single-shot random laser illumination. The random laser provides high-resolution, artefact-free images with better contrast and sharpness of structural details.
We report our results on random lasing from rhodamine 6G based colloidal gain medium consisting of urchin-like TiO2 structures. Multimode behaviour is observed even at low pump laser powers. Emission linewidth narrowing and lasing threshold are investigated. Coherent back scattering is used to obtain the disorder degree of the sample. This urchin based system is demonstrated to possess lower lasing thresholds and enhanced efficiency with multimode behaviour compared to a TiO2 spherical particle system with same disorder degree. This work opens up a new avenue for low threshold, high efficiency lasing.
Dielectric-plasmonic composite media are interesting systems as far as light trapping is considered owing to their enhanced interaction with light. Core-shell particles are of particular interest due to their strong scattering and geometry-dependent plasmon resonances which are tunable from visible to IR region. Design of random media with such highly scattering particles increases the probability of light getting trapped inside the medium due to the enhanced multiple scattering. Here we investigate the light scattering scenario in ZnS-Au core-shell random medium experimentally and also provide theoretical basis using Finite Difference Time Domain (FDTD) simulations. Back scattering and transmission simulations are carried out at standard wavelengths 405, 445, 532 and 632 nm. Effect of geometry on light scattering is also investigated by varying the aspect ratios of the particles. Scattering and absorption efficiency is calculated for particles in this size range using Mie theory. Strength and tunability of plasmon resonances are explained in terms of plasmon hybridization model. Core shell particles with core radius 100 nm and shell thickness of 6 nm are found to be optimal for light trapping over the entire visible region. Dielectric-plasmonic random media consisting of ZnS-Au core shell particles with optimized aspect ratio appear to be efficient candidate materials for light harvesting, sensing and optoelectronics.
Dendrimers have recently been shown to be excellent templates for synthesizing nanoparticles in view of their
unique structural features. The present study, we report on nonlinear absorption and optical limiting properties of
dendrimer-embedded silver nanoclusters and their linear and nonlinear optical (NLO) properties. Optical absorption
spectroscopy reveals prominent plasmon peaks indicating quantum confinement. Z-scan experiments using pulsed
laser excitation within the tail region of Plasmon absorption in the nanosecond regimes reveal evidence of strong
nonlinear absorption involving two photo absorption followed by saturation absorption. The third ordered
susceptibility (χ(3)) measured using four wave degenarated four wave mixing tecnquie (FWDM).
Devices based on optical technology for high speed communication networks require materials with large
nonlinear optical response in the ultrafast regime. Nonlinear optical materials have also attracted wide
attention as potential candidates for the protection of optical sensors and eyes while handling lasers. Optical
limiters have a constant transmittance at low input influence and a decrease in transmittance at higher
fluences and are based on a variety of mechanisms such as nonlinear refraction, nonlinear scattering,
multiphoton absorption and free carrier absorption. As we go from bulk to nanosized materials especially in
the strong quantum confinement regime where radius of the nanoparticle is less than the bulk exciton Bohr
radius, the optical nonlinearity is enhanced due to quantum confinement effect. This paper is on the
ultrafast nonresonant nonlinearity in free standing films of PbS quantum dots stabilized in a synthetic glue
matrix by a simple chemical route which provides flexibility of processing in a variety of physical forms.
Optical absorption spectrum shows significant blue shift from the bulk absorption onset indicating strong
quantum confinement. PbS quantumdots of mean size 3.3nm are characterized by X-ray diffraction and
transmission electron microscopy. The mechanism of nonlinear absorption giving rise to optical limiting is
probed using open z-scan technique with laser pulses of 150 fs pulse duration at 780 nm and the results are
presented in the nonresonant femtosecond regime. Irradiance dependence on nonlinear absorption are
discussed.
Transmission properties of a novel optical waveguide structure based on Nafion polymer are investigated
by the technique of the m-line spectroscopy at a wavelength of 632.8nm. The refractive index profiles for
Nafion film for both TE and TM modes are found to be of quadratic nature with surface refractive indices
values of 1.3408 and 1.3446 respectively. The attenuation loss of this polymeric waveguide is found to be 1.53 dBcm-1
The process of two-beam holographic coupling in optically anisotropic media is analyzed. We consider holographic media possessing birefringence, optically, and spatial modulation of these effects. In this paper energy transfer between two beams interfering in an electro-optic is analyzed as a function of the grating period of the recording hologram. The theory for two-beam coupling in Bi12SiO20 (BSO) with an external electric field and a moving grating is developed. We report high coefficients in a wide range of low spatial frequencies. Exponential gain coefficients of (Gamma) equals 7 cm-1 have been reached for high intensity beams ratio and moving gratings. These results have been compared with GaAs crystals.
A comparative study of the photorefractive response of some commonly used ferroelectric, nonferroelectric and semiconducting crystals is presented. The figure-of-merit parameters such as steady-state change in refractive index, response time and photorefractive sensitivity tare evaluated for crystals of BaTiO3, LiBnO3, BSO, GaAs and InP on the basis of Kukhtarev's theory, using input parameters form reported data. These parameters are calculated as a function of the grating period with and without an application of 10kV/cm electric field. The results indicate that the choice of the ideal material for a given application can be made only after a careful comparison of the figure-of-merit parameters as trade-offs exist among the various parameters for any material.
Photorefractive crystals offer several attractive features such as high resolution and in situ processing. As the images are erasable, these crystals are suitable for read-write applications and hence find potential use in speckle photography, image processing and holography. The BaTiO3 crystal as recording medium has been extensively used as a novelty filter for real-time in-plane displacement measurements employing two beam coupling configuration. This paper presents new optical configurations in speckle shear photography to measure in-plan displacement and the strain in real time using BaTiO3 crystal as recording medium. Speckle photography studies are made using a simple two-beam coupling configuration. In speckle shear photography, a diffused object illuminated with two parallel narrow laser beams is imaged inside the crystal, and a pump beam is added at this plane. The speckle patterns due to each beam and the pump beam produce index gratings. When the object is deformed, the speckle patterns shift consequently. We now have four speckle fields: two generated from the interaction of pump beam with the index gratings and two pertaining to deformed states directly transmitted through the crystal. Thus, the fields from respective points on the object interfere after passage through the crystal and produce the Young's fringe patterns. Due to strain, the fringes in each pattern are of different width and orientation, resulting in the generation of a moire pattern. The strain is obtained from the width and orientation of the fringes in the moire pattern. The experiments are conducted on a specimen with a notch, which is subjected to tensile loading. The in-plane displacement is measured separately in another experiment. Time evolution of the growth and the decay of the signal beam also is studied. The above studies are carried out at different laser wavelengths and the results are compared.
Photorefractive beam coupling in barium titanate crystals is characterized experimentally by measuring the signal beam gain and the exponential gain coefficient at 440 nm, 632.8 nm and 780 nm. The figure of merit parameters such as the change in refractive index, the space charge field and the trap density are estimated using Kukhtarev's theory. A comparative study at multiple wavelengths is presented. The signal beam gain is found to be maximum at 440 nm.
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