Monte Carlo simulation of the interaction between an ultra-short pulse and a strongly scattering medium: The case of large particles

Simulations of the transmission of a short light pulse (50 fs) through a strongly scattering medium constituted by water droplets (50-150 μm diameter) are presented. Temporal emerging signals are computed using a Monte Carlo technique. In case of large particles with respect to the pulse duration, the time delay related to the interaction between light and individual particles has to be taken into account, in addition to the time delay related to the travel of light between particles. After careful comparisons with temporal Lorenz-Mie theory, it has been chosen to pre-calculate scattering characteristics of individual droplets using Debye series. Transmitted signals in forward direction and in a small temporal window (0-400 fs) are presented, showing that temporal information on particle size are still observable facing strongly scattering media.     

Constraints on PSC particle microphysics derived from lidar observations

Based on extensive T-matrix computations of light scattering by polydispersions of randomly oriented, rotationally symmetric nonspherical particles, we analyze existing lidar observations of polar stratospheric clouds (PSCs) and derive several constraints on PSC particle microphysical properties. We show that sharp-edged nonspherical particles (finite circular cylinders) exhibit less variability of lidar backscattering characteristics with particle size and aspect ratio than particles with smooth surfaces (spheroids). For PSC particles significantly smaller than the wavelength, the backscatter color index and the depolarization color index β are essentially shape independent. Observations for type Ia PSCs can be reproduced by spheroids with aspect ratios larger than 1.2, oblate cylinders with diameter-to-length ratios greater than 1.6, and prolate cylinders with length-to-diameter ratios greater than 1.4. The effective equal-volume-sphere radius for type Ia PSCs is about 0.8 μm or larger. Type Ib PSCs are likely to be composed of spheres or nearly spherical particles with effective radii smaller than 0.8 μm. Observations for type II PSCs are consistent with large ice crystals (effective radius greater than 1 μm) modeled as cylinders or prolate spheroids.     

Activation energy of etching for CR-39 as a function of linear energy transfer of the incident particles

In this work, we have studied the effect of the radiation damage caused by the incident particles on the activation energy of etching for CR-39 samples. The damage produced by the incident particle is expressed in terms of the linear energy transfer (LET). CR-39 samples from American Acrylic were irradiated to three different LET particles. These are N (LET₂₀₀ = 20 KeV/μm) as a light particle, Fe (LET₂₀₀ = 110 KeV/μm) as a medium particle and fission fragments (ff) from a ²⁵²Cf source as heavy particles. In general the bulk etch rate was calculated using the weight difference method and the track etch rate was determined using the track geometry at various temperatures (50–90 °C) and concentrations (4–9 N) of the NaOH etchant. The average activation energy F_b related to the bulk etch rate v_b was calculated from ln v_b vs. l/T. The average activation energy E_t related to the track etch rate v_t was estimated from ln v_t vs. l/T. It is shown that activation energy of etching is a constant value for CR-39 detector where E_t was found to be independent on the damage produced by the incident particle.     

Radiative properties of cirrus clouds in the infrared (8–13 μm) spectral region

Atmospheric radiation in the infrared (IR) 8–13 μm spectral region contains a wealth of information that is very useful for the retrieval of ice cloud properties from aircraft or space-borne measurements. To provide the scattering and absorption properties of nonspherical ice crystals that are fundamental to the IR retrieval implementation, we use the finite-difference time-domain (FDTD) method to solve for the extinction efficiency, single-scattering albedo, and the asymmetry parameter of the phase function for ice crystals smaller than 40 μm. For particles larger than this size, the improved geometric optics method (IGOM) can be employed to calculate the asymmetry parameter with an acceptable accuracy, provided that we properly account for the inhomogeneity of the refracted wave due to strong absorption inside the ice particle. A combination of the results computed from the two methods provides the asymmetry parameter for the entire practical range of particle sizes between 1 and 10,000 μm over the wavelengths ranging from 8 to 13 μm. For the extinction and absorption efficiency calculations, several methods including the IGOM, Mie solution for equivalent spheres (MSFES), and the anomalous diffraction theory (ADT) can lead to a substantial discontinuity in comparison with the FDTD solutions for particle sizes on the order of 40 μm. To overcome this difficulty, we have developed a novel approach called the stretched scattering potential method (SSPM). For the IR 8–13 μm spectral region, we show that SSPM is a more accurate approximation than ADT, MSFES, and IGOM. The SSPM solution can be further refined numerically. Through a combination of the FDTD and SSPM, the extinction and absorption efficiencies are computed for hexagonal ice crystals with sizes ranging from 1 to 10,000 μm at 12 wavelengths between 8 and 13 μm.

Calculations of the cirrus bulk scattering and absorption properties are performed for 30 size distributions obtained from various field campaigns for midlatitude and tropical cirrus cloud systems. Ice crystals are assumed to be hexagonal columns randomly oriented in space. The bulk scattering properties are parameterized through the use of second-order polynomial functions for the extinction efficiency and the single-scattering albedo and a power-law expression for the asymmetry parameter. We note that the volume-normalized extinction coefficient can be separated into two parts: one is inversely proportional to effective size and is independent of wavelength, and the other is the wavelength-dependent effective extinction efficiency. Unlike conventional parameterization efforts, the present parameterization scheme is more accurate because only the latter part of the volume-normalized extinction coefficient is approximated in terms of an analytical expression. After averaging over size distribution, the single-scattering albedo is shown to decrease with an increase in effective size for wavelengths shorter than 10.0 μm whereas the opposite behavior is observed for longer wavelengths. The variation of the asymmetry parameter as a function of effective size is substantial when the effective size is smaller than 50 μm. For effective sizes larger than 100 μm, the asymmetry parameter approaches its asymptotic value. The results derived in this study can be useful to remote sensing studies of ice clouds involving IR window bands.     

Secondary particle contribution to LET spectra on LDEF

Four experiments utilizing passive detectors (P0006, P0004, A0015, M0004) were flown on LDEF to study the radiation environment. These experiments have been summarized in a companion paper (Benton et al., 1996). One of the experimental goals was to measure LET spectra at different locations and shielding depths with plastic nuclear track detectors (PNTD). It was found that the LET spectra extended well above the LET cutoff imposed by the geomagnetic field on GCR particle penetration into LEO. The high LET particles detected were mostly short-range (range < 2000 μm), indicating that they were secondaries produced locally within the PNTD. The presence of these high LET particle fluences is important for the determination of dose equivalent because of the high Quality Factors (Q) involved. A relatively small fraction of particle fluence can contribute a large fraction of dose equivalent.

Short-range, inelastic secondary particles produced by trapped protons in the South Atlantic Anomaly (SAA) were found to be a major contributor to the LET spectra above 100 keV/μm. The LET spectra were found to extend beyond the 137 keV/μm relativistic GCR Fe peak to over 1000 keV/μm. The high LET tail of the LET spectra was measured in CR-39 and polycarbonate PNTDs using different techniques. GCR made a relatively modest contribution to the LET spectra as compared to the contributions from short-range secondary particles and stopping protons.

LET spectra intercomparisons were made between LDEF measurements and exposures to 154 MeV accelerated proton beams. The similarities support the role of nuclear interactions by trapped protons as the major source of secondary particles in the PNTDs. Also techniques were employed to reduce the range cutoff for detection of the short-range secondaries to 1 μm, so that essentially all secondary particles were included in the LET spectra. This has allowed a more realistic assessment of secondary contribution to dose equivalent.

Comparisons of measured and calculated LET spectra have been made that demonstrate the need for more accurate modeling of secondary particles in radiation transport codes. Comparisons include preliminary calculations in which attempts have been made to include secondary particles.     

Optical Multiple Scattering by Particles

When an optical beam is incident on particles that are randomly distributed, and if the fractional volume is small, single scattering theory is adequate to explain the scattering characteristics of the medium. However, when the fractional volume is increased, multiple scattering effects cannot be ignored. This paper reviews the fundamental theories of multiple scattering including radiative transfer and diffusion theories. Also included are recent studies on polarization effects, localization, enhanced backscattering, resonant localization, pulse scattering and scattering in dense media.     

Retrieval of cirrus particle sizes using a split-window technique: a sensitivity study

This paper examines the sensitivity of retrieved ice particle sizes using split-window method to the light scattering program for the single scattering calculation. We find that for randomly oriented hexagonal ice particles the retrieval algorithm using the anomalous diffraction theory (ADT) significantly overestimates the mean effective ice particle sizes, D_ge. The retrieved D_ge based on the geometric optics method (GOM) and Mie theory agrees with reference results within 20% when D_ge<30 μm. Based on the speculation that there is no “tunneling” for complex particles, some recent studies suggest that the ADT is an appropriate method to simulate the absorption coefficient for irregularly shaped particles in the infrared. In this study, however, we find that the overestimation of D_ge due to the ADT is largely caused by the neglect of refraction and reflection processes, instead of by the neglect of “tunneling” in the absorption calculations. By considering complex particle shapes such as aggregates with surface roughness, we further find that the retrieved D_ge based on the GOM is not sensitive to the particle shapes. Note that both ADT and GOM do not consider the “tunneling”, but the retrieved D_ge based on the ADT is about two times larger than those based on the GOM. “Tunneling” plays a significant role in the retrieved D_ge only when the D_ge is larger than 30–35 μm. In this study, we also examine the sensitivities of retrieved D_ge to the ice particle size distributions assumed in the retrieval algorithm and to the errors in the emissivities. It is found that when the D_ge is larger than 30–40 μm, the retrieved D_ge becomes very sensitive to the uncertainties related to the ice particle size distributions and to the errors in cirrus emissivities derived from measurements.     

Second harmonic generation by cold-deposited silver films

The second harmonic generation (SHG) of light diffusely scattered by cold-deposited silver films is negligible with respect to the specular intensities. Therefore, a cold-deposited silver film is well approximated by a homogeneous effective medium. The intensities of SHG at 1.06 μm and the optical absorption at 0.53 μm depend on both the square of the internal effective field at 0.53 μm and a change in the same way when annealing the film. This can be understood by the assumption that SHG is also mainly following the electric field within the metal phase and surface contributions to SHG interfere destructively. This is corroborated by the observation that the SHG intensity does not show the ‘chemical first layer effects’ seen by surface-enhanced Raman scattering (SERS).     

Computational study of plasma-assisted photoacoustic response from gold nanoparticles irradiated by off-resonance ultrafast laser

The gold nanoparticles (AuNPs) are capable of enhancing the incident laser field in the form of scattered near field for even an off-resonance irradiation where the incident laser wavelength is far away from the localized surface plasmon resonance (LSPR). If the intensity of the pulse laser is large enough, this capability can be employed to generate a highly localized free electron (plasma) in the vicinity of the particles. The generated plasma can absorb more energy during the pulse, and this energy deposition can be considered as an energy source for structural mechanics calculations in the surrounding media to generate a photoacoustic (PA) signal. To show this, in this paper, we model plasma-mediated PA pressure wave propagation from a 100-nm AuNPs and the surrounding media irradiated by an ultrashort pulse laser. In this model, the AuNP is immersed in water and the laser pulse width is ranging from 70 fs to 2 ps at the wavelength of 800 nm (off-resonance). Our results qualitatively show the substantial impact of the energy deposition in plasma on the PA signal through boosting the pressure amplitudes up to ～1000 times compared to the conventional approach.     

Effect of Lorentz local field correction on propagation of ultrashort laser pulse in one-dimensional para-nitroaniline (PNA) molecules

This paper investigates the effect of Lorentz local field correction (LFC) on the propagation of ultrashort laser pulses in a para-nitroaniline molecular medium under resonant and nonresonant conditions by solving numerically the full-wave Maxwell-Bloch equations beyond slowly-varying envelope approximation and rotating-wave approximation. The effect of the LFC is considerably obvious when pulses with large areas propagate in the dense molecular medium. In the case of resonance,the group velocity of the sub-pulses split from the incident pulse along propagation is severely decreased by the LFC,especially for the latest sub-pulse. However,in the case of nonresonance,the influence of the LFC on the temporal evolution of the pulse is less obvious and lacks homogeneity with an increase in incident pulse area,propagation distance and molecular density.     

Comparative studies of the reflectance and degree of linear polarization of particulate surfaces and independently scattering particles

We compare measurements of the phase-angle dependencies of the intensity and degree of linear polarization of particles in air and particulate surfaces. The samples were measured at two spectral bands centered near 0.63 and 0.45 μm. The surfaces were measured with the new photometer/polarimeter at the Astronomical Institute of Kharkov National University. The scattering measurements of the particles in air were carried out with the equipment currently located at the University of Amsterdam. We study a suite of samples of natural mineral particles of different sizes all in the micrometer range, i.e. comparable with the wavelengths. The samples are characterized by a variety of particle shapes and albedos. The samples have been studied in several works and in this paper we include new SEM microphotographs of particles and spectra of powders in a wide spectral range, 0.3–50 μm, using the RELAB equipment of Brown University. We made measurements of particulate surfaces in a phase-angle range, 2–60° that is significantly wider than that of our previous studies. We confirm our earlier results that the negative polarization of the surfaces may be a remnant of the negative polarization of the single scattering by the particles that constitute the surfaces. We also find differences in the spectral behavior of the polarization degree of particles in air and particulate surfaces at large phase angles.     

Light scattering by Gaussian particles with internal inclusions and roughened surfaces using ray optics

We study light scattering by Gaussian-random-sphere particles that are large compared to the wavelength of the incident light using ray optics that, in addition to Fresnellian reflection and refraction, accounts for diffuse scattering. We consider two types of diffusely scattering media. One type of media constitutes a uniform medium inside the particle, i.e. a diffuse internal medium. The second type of media constitutes a layer on the surface of the particle that is thin compared to the particle dimensions and acts as a diffuse external medium mimicking the particle surface roughness. We illustrate the effects of the diffuse media on the scattering characteristics for both cases and show that incorporating diffuse scatterers allows us to explain the scattering matrices measured experimentally for Saharan sand particles large compared to the wavelength.     

Scattering phase functions of horizontally oriented hexagonal ice crystals

Finite-difference time domain (FDTD) solutions are first compared with the corresponding T-matrix results for light scattering by circular cylinders with specific orientations. The FDTD method is then utilized to study the scattering properties of horizontally oriented hexagonal ice plates at two wavelengths, 0.55 and 12 μm. The phase functions of horizontally oriented ice plates deviate substantially from their counterparts obtained for randomly oriented particles. Furthermore, we compute the phase functions of horizontally oriented ice crystal columns by using the FDTD method along with two schemes for averaging over the particle orientations. It is shown that the phase functions of hexagonal ice columns with horizontal orientations are not sensitive to the rotation about the principal axes of the particles. Moreover, hexagonal ice crystals and circular cylindrical ice particles have similar optical properties, particularly, at a strongly absorbing wavelength, if the two particle geometries have the same length and aspect ratio defined as the ratio of the radius or semi-width of the cross section of a particle to its length. The phase functions for the two particle geometries are slightly different in the case of weakly absorbing plates with large aspect ratios. However, the solutions for circular cylinders agree well with their counterparts for hexagonal columns.     

Effect of pulse entrapping on diffuse reflection from a resonant random medium: exact solution to the scalar albedo problem

A recently derived radiative transfer equation with three Lorentzian delay kernels is applied to an albedo problem of the scalar wave field produced by the diffuse reflection of a quasi-monochromatic pulse from a semi-infinite random medium consisting of resonant point-like scatterers. The albedo problem is solved exactly in terms of the Chandrasekhar H-function H(μλ), extended analytically into the complex single-scattering albedo λ plane. The resulting analytic solution for the time evolution of a diffusely reflected short pulse is used to study on the whole time axis the effect of the redistribution of the energy of the propagated pulse from the front to the rear of the pulse in cases where the pulse may for most of the propagation time through the medium be 'entrapped' inside resonant scatterers. By considering the power flux through unit area of the boundary of the medium and unit solid angle, it is shown that the relative shift of an 'energy centroid' ('centre of mass') of the pulse diffusely reflected from the resonant random medium (compared with the pulse energy centriod in the non-resonant case) is equal to the parameter describing the energy accumulation inside the scatterers. This result may be used for experimental study of resonant random media with the aid of a short-pulse technique.     

Conical Emission Patterns by Femtosecond Pulses with Different Spectral Bandwidths

Different conical emission （CE） patterns are obtained experimentally at various incident powers and beam sizes of pump laser pulses with pulse durations of 7fs, 44fs and 100fs. The results show that it is the incident power but not the incident power density that determines a certain CE pattern. In addition, the critical powers for similar CE patterns are nearly the same for the laser pulses with the same spectral bandwidth. Furthermore, as far as a certain CE pattern is concerned, the wider the spectral bandwidth of pump laser pulse is, the higher the critical power is. This will hopefully provide new insights for the generation of CE pattern in optical medium.     

Frequency shifting of sub-100 fs laser pulses by stimulated Raman scattering in a capillary filled with pressurized gas

The technique of Raman conversion of sub-100 fs laser pulses based on excitation of active medium by two orthogonally polarized pulses has been developed for Raman lasers with a glass capillary. 52 fs Stokes pulse at the wavelength of 1200 nm has been generated by stimulated Raman scattering of 48 fs Ti:sapphire laser pulse at the wavelength of 800 nm in hydrogen. 13% energy conversion efficiency has been achieved at pulse repetition rate up to 2 kHz.     

Heating of thin foils with a relativistic-intensity short-pulse laser

K-shell x-ray spectroscopy of sub-100 nm Al foils irradiated by high contrast, spatially uniform, 150 fs, Ilambda (2)=2 x 10(18) W microm(2)/cm(2), laser pulses is obtained with 500 fs time resolution. Two distinct phases occur: At /=500 fs the resonance transitions dominate. Initial satellites arise from a large area, high density, low temperature (approximately 100 eV) plasma created by fast electrons. Thus, contrary to predictions, a short, high intensity laser incident on a thin foil does not create a uniform, hot dense plasma.     

Acceleration of the iterative solver in the discrete dipole approximation: Application to the orientation variation of irregularly shaped particles

We have applied a method of reducing the number of iterations required to solve a system of linear equations in the discrete dipole approximation. This method obtains an initial guess of dipole polarization from those with similar particle characteristics (e.g., the size parameter and refractive index) calculated a priori. If the initial guess is closer to the solution, the number of iterations of the linear equation solution becomes smaller than that calculated with an arbitrary initial value.

This method was applied to various particle orientations using spline interpolation of the initial guess of dipole polarization from orientations calculated a priori.

We studied three types of particle model: an aggregate, a deformed sphere with moderate surface roughness, and a particle with a large number of edges. For the particle with a large number of edges, we propose a new model called the overlapping mixture of multiple tetrahedra (OMMT).

The proposed method is most advantageous for particles with moderate surface roughness (e.g., a deformed sphere), for which the calculation time was reduced to 20–40% of the original calculation time. For OMMT and an aggregate, the computation time was reduced to 30–60% and 40–90%, respectively. The differences in the scattering coefficient, absorption coefficient, intensity and polarization introduced by our method were less than 0.008%, 0.03%, 0.1%, and 0.08%, respectively.

If the light scattering properties vary slowly with the orientation variation, interpolation of the results is more efficient than the proposed method and produces only a small difference in the results. However, the interpolation of the results fails for particles such as BCCA64, for which our proposed method produces more accurate results.     

Proton response of high sensitivity CR-39 copolymer

We studied the track response for the copolymer of CR-39 monomer with N-isopropylacrylamide (NIPAAm) as well as etching properties. It was found that copoly (CR-39/NIPAAm/Naugard 445) composed in wieght ratio of 99/1/0.01 is highly sensitive to low LET particles in the region below 10 keV/μm of LET and able to record normally incident particles of LET down to 1.5 keV/μm, recording protons up to the energy of 27 MeV. These results were compared with the responses for two types of CR-39 detectors containing a small quantity of antioxidant. The threshold energy proton registration is discussed.     

Continuous-wave and passively Q-switched Nd:GdVO4 lasers at 1.06 μm end-pumped by laser-diode-array

We demonstrated a diode-end-pumped continuous-wave and passively Q-switched Nd:GdVO₄ laser operating at 1.06 μm wavelength with a three-mirror folded resonator. The maximum continuous-wave output power of 8.18 W was obtained at the incident pump power of 22.5 W, with the corresponding optical conversion efficiency of 36.4%. For Q-switched operation, the maximum average output power was measured to be 4.64 W with the corresponding optical conversion efficiency of 25.8%, when the initial transmission of Cr⁴⁺:YAG crystal was 90%. The shortest pulse width of 83 ns, the largest pulse energy of 20.7 μJ and the highest peak power of 246.7 W were obtained when the Cr⁴⁺:YAG crystal with an initial transmission of 85% was used.