This contribution is concerned with the channeling of a relativistic laser pulse propagating in an underdense plasma, and with the subsequent generation of fast electrons in the cavitated ion channels. Specifically, we study the interaction of laser pulses of duration of several 102 femtoseconds, having their intensity Iλ2 in the range [1019; 1020]Wcm−2μm2 and focused in underdense plasmas, with electron densities n0 such that the ratio n0=nc lies in the interval [10−3, 10−1], nc denoting the critical density. The laser power PL exceeds the critical power for laser channeling Pch = 1:09Pc, Pc denoting the critical power for relativistic self-focusing. The laser-plasma interaction under such conditions is investigated by means of three dimensional (3D) Particle-In-Cell (PIC) simulations. It is observed that the steep laser front gives rise to the excitation of a surface wave which propagates along the sharp radial boundaries of the electron free channel created by the laser pulse. The mechanism responsible for the generation of relativistic electrons observed in the PIC simulations is then analyzed by means of a 3D test particles code. The fast electrons are thus found to be generated by the combination of the electron acceleration caused by the surface wave and of the betatron mechanism. The maximum electron energy observed in the simulations is scaled as a function of PL/Pc; it reaches 350 - 400 MeV for PL/Pc = 70 - 140.
A microfluidic flow cytometric technique capable of obtaining information on nanometer-sized organelles in single cells in a label-free, noninvasive optical manner was developed. Experimental two-dimensional (2D) light scattering patterns from malignant lymphoid cells (Jurkat cell line) and normal hematopoietic stem cells (cord blood CD34+ cells) were compared with those obtained from finite-difference time-domain simulations. In the simulations, we assumed that the mitochondria were randomly distributed throughout a Jurkat cell, and aggregated in a CD34+ cell. Comparison of the experimental and simulated light scattering patterns led us to conclude that distinction from these two types of cells may be due to different mitochondrial distributions. This observation was confirmed by conventional confocal fluorescence microscopy. A method for potential cell discrimination was developed based on analysis of the 2D light scattering patterns. Potential clinical applications using mitochondria as intrinsic biological markers in single cells were discussed in terms of normal cells (CD34+ cell and lymphocytes) versus malignant cells (THP-1 and Jurkat cell lines).
A fiber-coupled microfluidic cytometer has been recently developed in our group for the obtaining of 2D light scatter
patterns from single biological cells. The obtained scatter patterns may be used for label-free characterization of
biological cells. Understanding of these 2D scatter patterns can be better achieved by comparing the experimental results
with those obtained by theoretical simulations, such as the finite-difference time-domain (FDTD) simulation of light
scattering from biological cells. In this paper, we provide detailed study for applying the FDTD method in our fibercoupled
microfluidic cytometer. The FDTD simulation results agree reasonably well with the experimentally obtained
THP-1 cell 2D scatter patterns. Methods for scatter pattern analysis are under development in our group for new light
scattering parameters that may potentially be used in clinics.
In this paper, we report the preliminary development of a fiber coupled microfluidic flow cytometer with its potential
application of sorting the very small embryonic like (VSEL) stem cells out of a mixture of platelets and VSEL stem
cells. The identification of a VSEL stem cell from a platelet is based on the large difference of their abilities to scatter
light. A simple cytometer prototype was built by cutting the fluidic and other channels into a polymer sheet and bonding
it with epoxy between two standard glass slides. Standard photolithography was used to expose an observation window
over the upper coated glass to reduce background scattered light. Liquid sample containing micro-particles (such as
cells) is injected into the microfluidic channel. Light from a 532-nm CW diode laser is coupled into the optical fiber that
delivers the light to the detection region in the channel to interrogate the flowing-by micro-particles. The scattering light
from the interrogated micro-particle is collected by a photodiode placed over the observation window. The device sorts
the micro-particle into the sort or waste outlet depending on the level of the photodiode signal. We used fluorescent latex
beads to test the detection and sorting functionalities of the device. It was found that the system could only detect about
half of the beads but could sort almost all the beads it detected.
The characterization of single biological cells in a microfluidic flow by using a 2D light scattering microfluidic
cytometric technique is described. Laser light is coupled into a microfluidic cytometer via an optical fiber to illuminate a
single scatterer in a fluidic flow. The 2D light scattering patterns are obtained by using a charge-coupled device (CCD)
detector. The system is tested by using standard polystyrene beads of 4 μm and 9.6 μm in diameter, and the bead
experimental results agree well with 1D Mie theory simulation results. Experiments on yeast cells are performed using
the microfluidic cytometer. Cell results are studied by finite-difference time-domain (FDTD) method, which can
simulate light scattering from non-homogeneous cells. For example, a complex biological cell model with inner
mitochondrial distribution is studied by FDTD in this paper. Considering the yeast cell size variations, the yeast cell 2D
scatter patterns agree well with the FDTD 2D simulation patterns. The system is capable of obtaining 2D side scatter
patterns from a single biological cell which may contain rich information on the biological cell inner structures. The
integration of light scattering, microfluidics and fiber optics described here may ultimately allow the development of a
lab-on-chip cytometer for label-free detection of diseases at a single cell level.
An integrated microfluidic planar optical waveguide system for measuring light scattered from a single scatterer is described. This system is used to obtain 2D side-scatter patterns from single polystyrene microbeads in a fluidic flow. Vertical fringes in the 2D scatter patterns are used to infer the location of the 90-deg scatter (polar angle). The 2D scatter patterns are shown to be symmetrical about the azimuth angle at 90 deg. Wide-angle comparisons between the experimental scatter patterns and Mie theory simulations are shown to be in good agreement. A method based on the Fourier transform analysis of the experimental and Mie simulation scatter patterns is developed for size differentiation.
Recent experiments were carried out on the Prague Asterix Laser System (PALS) towards the
demonstration of a soft x-ray laser Thomson scattering diagnostic for a laser-produced exploding foil. The
Thomson probe utilized the Ne-like zinc x-ray laser which was
double-passed to deliver ~1 mJ of focused
energy at 21.2 nm wavelength and lasting ~100 ps. The plasma under study was heated single-sided using a
Gaussian 300-ps pulse of 438-nm light (3ω of the PALS iodine laser) at laser irradiances of 1013-1014 W
cm-2. Electron densities of
1020-1022 cm-3 and electron temperatures from 200 to 500 eV were probed at
0.5 or 1 ns after the peak of the heating pulse during the foil plasma expansion. A flat-field 1200 line mm-1
variable-spaced grating spectrometer with a cooled charge-coupled device readout viewed the plasma in the
forward direction at 30° with respect to the x-ray laser probe. We show results from plasmas generated
from ~1 μm thick targets of Al and polypropylene (C3H6). Numerical simulations of the Thomson
scattering cross-sections will be presented. These simulations show electron peaks in addition to a narrow
ion feature due to collective (incoherent) Thomson scattering. The electron features are shifted from the
frequency of the scattered radiation approximately by the electron plasma frequency ±ωpe and scale as ne1/2.
We present a review of recent development and applications of soft x-ray lasers, undertaken at the PALS Centre. The applications benefit from up to 10-mJ pulses at the wavelength of 21.2 nm. We describe the pumping regimes used to produce this soft x-ray laser, and outline its emission characteristics. A significant fraction of applications carried out using this device includes probing of dense plasmas produced by IR laser pulses and high-energy-density-in-matter experiments. Results obtained in these experiments are reviewed, including x-ray laser probing of dense plasmas, measurements of transmission of focused soft x-ray radiation at intensities of up to 1012 Wcm-2, measurements of IR laser ablation rates of thin foils, and probing high density plasmas by x-ray laser Thomson scattering
High resolution 2D side scatter patterns from polystyrene beads were obtained by using an integrated microfluidic
waveguide cytometer. A He-Ne laser beam was prism-coupled into a microfluidic chip, and waveguide modes were
excited to illuminate a single scatterer. While immobilizing a single scatterer on chip in the observation window area,
high resolution 2D scatter patterns were obtained by using a CCD array located beneath the microchip. This cytometer is
sensitive to variations in both the refractive index and the size of a single scatterer. Fourier transforms of Mie simulation
results from a single scatterer show that forward scattered light at large angles is optimal for micro-size differentiation.
While side scatter light was reported to contain rich information about organelles in a single cell, we show here that side
scatter light can be used to perform fast micro-size differentiation and cellular analysis. A cross section scan of the
experimental scatter pattern gives an oscillation distribution of the scattered intensity. This oscillation has a frequency
that is typical for a given micro-size scatterer. A Fourier method for quick micro-size differentiation is reported, based on
the comparisons between the Mie simulations and the experimental results. Finite-difference time-domain (FDTD)
simulations of single white blood cells in the waveguide cytometer are studied, which allows extraction of microstructural
and nano-structural information from single cells.
A 3-D code for solving the set of Maxwell equations with the finite-difference time-domain method is developed for simulating the propagation and scattering of light in biological cells under realistic conditions. The numerical techniques employed in this code include the Yee algorithm, absorbing boundary conditions, the total field/scattered field formulation, the discrete Fourier transformation, and the near-to-far field transform using the equivalent electric and magnetic currents. The code is capable of simulating light scattering from any real cells with complex internal structure at all angles, including backward scattering. The features of the scattered light patterns in different situations are studied in detail with the objective of optimizing the performance of cell diagnostics employing cytometry. A strategy for determining the optimal angle for measuring side scattered light is suggested. It is shown that cells with slight differences in their intrastructure can be distinguished with two-parameter cytometry by measuring the side scattered light at optimal angles.
We present a new interference pattern based scheme for the cell diagnostics. In this approach the biological cell is illuminated by a spherical light source. The interference pattern that is created by the unscattered incident light and the light that is scattered by a cell is used for cell diagnostics. A three-dimensional computational code (AETHER) for solving the full set of Maxwell’s equations with the Finite-Difference Time-Domain (FDTD) method has been used to numerically determine the interference light intensity pattern between the unscattered incident light and the scattered light. Features of the interference patterns that are obtained for different cellular parameters and structures are discussed. We have numerically shown that the interference intensity pattern can replace the purely scattered light patterns currently used in cell diagnostics.
Quasi-self-similar solutions to the stationary electron Fokker-Planck equation in diffusive approximation have been found in inhomogeneous plasma. These solutions describe reduction in the number of bulk electrons and formation of the suprathermal tail. The characteristics of the stationary electron distributions have been treated in terms of the collisionality parameter, the ratio of the electron stoping rage to the plasma gradient scale length. The dependencies of the electron distribution functions on density profile has been studied. Fokker-Plank simulations performed demonstrate good agreement with a theory.
In this work we report our numerical modeling results of laser-generated transient inversion and capillary discharge X-ray lasers. In the search for more efficient X-ray lasers we look closely at other approaches in conjunction with experiments at LLNL. In the search for improved X-ray lasers we perform modeling and experimental investigations of low density targets including gas puff targets. We have found the importance of plasma kinetics in transient X-ray lasers by expanding the physical model beyond hydrodynamics approach with Particle In Cell (PIC) and Fokker-Planck codes. The evidence of the Langdon effect was inferred from the recent experimental data obtained with the Ni-like Pd X-ray laser. We continue modeling different kinds of capillary discharge plasma configurations directed toward shorter wavelength X-ray lasers, plasma diagnostics and other applications.
For the first time the three dimensional modeling of laser light scattering in biological tissue has been performed using the spectral technique. The accuracy of the spectral numerical method has been verified by comparison with linear perturbation theory and Mie theory. Comparison with Mie theory has validated that the three-dimensional scalar wave equation is a good approximation to the full Maxwell's set of equations for light scattering at moderate angles. The computational requirements for the spectral method in modeling laser interaction with biological samples are much lower than the requirements for other existing numerical methods: finite-difference time-domain and Monte Carlo. Yet the new logarithm is capable of resolving the variations in the scattered signal with a contrast in intensity of up to six orders of magnitude. The spectral technique can be successfully applied to address to address scattering from individual cells and from biological samples containing many cells. The new method is well suited to recognize the size and composition of biological cells, making it a valuable tool in cell cytometry, for example, in the detection of rare event cells and cancerous cells.
The instability of hot spots leading to angular spreading and frequency broadening of a spatially randomized laser beam has a much lower threshold for collisional plasmas compared to the collisionless regime. The plasma-induced spatial and temporal incoherence in forward-propagating light is consistent with the forward stimulated Brillouin scattering (SBS). It also serves to reduce the reflectivity of backward SBS. The interaction of randomized beams with collisional plasmas produces the angular and frequency spectra of transmitted light that are much broader and smoother than in the collisionless regime.
We discuss the mechanism responsible for the enhancement of Brillouin backscattered light in the context of recent experimental observations. The scattering takes place from the fluctuations produced during the nonlinear evolution of stimulated Raman scattering. The frequency spectra of Brillouin light shows blue and red shifted components consistent with experimental observations.
Measurements of reflectivity of stimulated Brillouin scattering, from an underdense, homogeneous plasma, irradiated by a 10 ps-1.06μm laser pulse, show an increase from 10-4 to 10-2, as the laser intensity is varied between 1013 to 1015 W/cm2. Numerical simulations have been developed to interpret these data. At low laser intensity, (<1014 W/cm2), the low reflectivity is well explained as being due to saturation of the instability in the convective regime, and good agreement with numerical simulations has been obtained. At high laser intensity, the model predicts higher reflectivities than measured in the experiment, although non-linear effects in ion sound waves evolution have been observed.
Intense, subpicosecond lasers are capable of producing plasmas at or near solid density with temperatures of down to a few
eV. Such a plasma is strongly coupled with respect to particle-particle interactions and the use of standard, Spitzer
descriptions of plasma processes is not valid. Here we develop an expression for the electrical resistivity, p, in such a plasma.
Since r governs laser light absorption via inverse bremsstrahlung, it must be calculated with some care to ensure that
modeling of the plasma is correct.
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