Induced radiation of a beam of channeled relativistic particles

In principle, the possibility of producing a tunable laser on a beam of relativistic channeled particles is shown. A formula for the gain coefficient is obtained. A discussion is presented of the possibility of cooling the relativistic beam using channeling.     

The influence of anharmonic potentials in calculations of planar channeling radiation from GeV positrons

The frequency of radiation from planar channeled positrons in the GeV region is discussed for realistic planar potentials when the dipolarity condition is not satisfied. It is found that there is a critical beam momentum at which the maximum radiated frequency is practically the same for all channeled positrons. For this momentum a sharp peak in the calculated composite spectrum is to be expected and this could explain the sharp peaking observed in a recent CERn experiment for 5 GeV/c positrons.     

Simulation of the acceleration mechanism by light-propulsion for the powder particles at laser direct material deposition

The results of the numerical analysis of heat- and mass-transfer processes at powder particles' motion in a gas flow and laser beam by light-propulsion force during the laser cladding and direct material deposition are presented. Under consideration were the stainless steel particles, the radiation power range of the CO₂ laser were 1000, 3000 and 5000 W. Finally, the particles of 45 μm in diameter reach the maximum velocity of about 80, 220, 280 m/s. It is shown that as particles are heated by the laser up to the temperature approaching the boiling point, the particles' velocity in the light field by the vapor recoil pressure may increase significantly. The radius of the particles slightly varies due to the evaporation; the losses in the clad material mass are negligibly small. Comparisons of numerical results with known experimental data on light-propulsion acceleration of single particles (aluminum, aluminum oxide and graphite) under the influence of pulse laser radiation are also presented. Particle acceleration resulting from the laser evaporation depends on the particle diameter, powder material properties, focusing degree and attenuation laser beam intensity by the direction of its propagation.     

Gemessene und berechnete Lichtausbeute bei planaren Channeling- und Blocking-Experimenten mitα-Teilchen in Anthrazen

The energy loss, scintillation response and multiple scattering of channeled and blockedα-particles along the (100)- and (110)-plane of anthracene are investigated. The energy loss of channeled and blocked particles is calculated including plasma excitation and the shell structure of the crystal atoms. A model is presented which describes the scintillation response of completely stopped, initially channeled and blockedα-particles as a function of angle of incidence. At the first time the half angles and depths of the scintillation response of those particles can be explained well.     

Nonlinear Raman scattering of photons by a channeled particle

Channeled particles are characterized by the discrete spectrum of bound transverse motion. The interaction of photons with channeled particles in single crystals can be accompanied by energy transitions between the levels of transverse motion of the channeled particle. The Raman scattering of photons at a quasibound channeled particle leads to the appearance of a combination of frequencies: the incident radiation frequency ω₀ and the frequency Δω_{m, n}, i.e., ω = ω₀ ± Δω_m,n where Δω_m,n = 2Δε_m,nγ²; Δε_{m, n} is the energy of the transition between quantum states (m and n) of the transverse motion of the channeled particle; and γ = E/mc² is the Lorentz factor of the channeled particle. The appearance of a violet satellite (the anti-Stokes component) in the Raman scattering spectrum is analyzed. The three-photon Raman-type transition, which is the process of the simultaneous absorption of two photons with the frequency ω₀ with the emission of a photon with the frequency ωs = 2ω₀ ± 2Δε_m,nγ², is considered. The conditions for resonance observation during the formation of the second harmonic (ω = 2ω₀) are discussed.     

Effect of diffusion on absorption and saturation of infrared radiation in molecular gases

Effect of diffusion on absorption of infrared radiation and saturation of absorption in molecular gases is considered. Incorporating the diffusion term, a theory is developed from multi-level rate equations, both for homogeneous and inhomogeneous broadening cases. It is shown that the effect of diffusion is characterized by a diffusion functionK'(k ₂ a), which varies inversely with the radius of the laser beam at low pressures and small beam sizes. Experimental results with seven molecular gases pumped by the CO₂ laser beam around 10.6 μm are reported.     

Radiation corrections to the electron inelastic scattering by an intense stationary laser beam

By solving the Dirac-Lorentz equation numerically with a perturbative predictor-corrector method, we find that the electron inelastic scattering by an intense stationary laser beam is heavily modified by the radiation reaction when the electron is injected upon the laser beam with a large crossing angle (θ>10°). This is the first time the Dirac-Lorentz equation is applied to study the interaction of an electron with a real laser beam in the Hermite-Gaussian mode.     

Damage and molecular changes under a laser beam in SEM‐EDX/MRS interface: a case study on iron‐rich particles

The control of damage to individual environmental particles by a laser beam during Raman spectroscopy carried out in ambient air is generally well understood. The nature and control of damage under vacuum conditions (e.g. in the scanning electron microscopy with energy X‐ray detection combined with micro‐Raman spectroscopy—interfaced SEM‐EDX/MRS) are more complex and less well comprehended. The physical and chemical processes that affect the damage caused to small particles by lasers still remain somewhat unclear, but certainly the atmosphere (vacuum/air) and the beam intensity have very significant influences. Furthermore, it has been determined that some particles (e.g. haematite), although stable under an electron beam, are damaged by the laser beam, hampering their analysis. Additionally, when simultaneous analyses by SEM/EDX and MRS are considered, the correct choice of the collection surface plays a crucial role. As a result, the following collection substrates were tested to determine their influence on the laser beam damage process to the particle: silver and aluminium foils and silicon wafers. A test study was performed using artificial examples of haematite (Fe₂O₃) particles. Exposure of Fe₂O₃ particles in vacuum to 514‐ and 785‐nm laser radiation often leads to their melting, transformation and evaporation. The dependence of the damage caused by the laser beam on the particle structure is reported here. Molecular and crystallographic changes have also been revealed. Formation of magnetite (as an effect of re‐crystallisation) and Raman inactive structures was detected. Copyright © 2010 John Wiley & Sons, Ltd.     

The monochromatization of a beam of charged particles by Cherenkov interaction

The possibility of monochromatization of a real beam of charged particles (possessing energy spread and angle divergence) as a result of Cherenkov interaction with laser radiation in a medium is shown. The refraction index of the medium, the Cherenkov angle and the intensity of the wave, necessary for this effect are determined by the initial parameters of the beam. For the real parameters of the beam of charged particles and the laser radiation the degree of monochromatization reaches several orders. A method of raising the monochromaticy of the charged particles of a beam is suggested.     

Angular distributions of channeled pions and protons up to 250 GeV/c

Channeling phenomena are observed for positive particles of momentum up to 250 GeV/c in a germanium crystal. The polar angular distributions of the channeled particles are compared with theoretical predictions based on a diffusion model. The results indicate that at high particle energy there may be additional mechanisms besides those operative at low energy leading to dechanneling of the particles. In spite of this, channeling effects are observed for particles incident at up to several times the critical angle, in contrast with the results from low energy channeling. Statistical equilibrium in the azimuthal angular distribution has also been observed at all measured beam momenta to about twice the calculated channeling critical angle. The breakdown of statistical equilibrium for the 2 cm crystal used occurs at an incident angle 2–3 times smaller than predicted theoretically.     

On the orientation effect of the neutron yield increase in <Emphasis Type="Italic">d</Emphasis>(<Emphasis Type="Italic">d,n</Emphasis>)<Superscript>3</Superscript>Hе reactions at an energy of 7–12 keV in TiD<Subscript>2</Subscript> crystals

The orientation dependence of the d(d, n)³He reaction probability in a TiD₂ crystal at a deuteron energy of 7–12 keV is investigated. The BCM-1.0 code developed for calculating the trajectories of channeled particles within classical mechanics is used to simulate deuteron trajectories at (200)-planar channeling in a 0.15-μm-thick Ti crystal with the angular divergence of the beam taken into account. The enhancement of the reaction probability in the computer experiments is 2.1 in the case of a parallel deuteron beam and near-zero crystal entry angles relative to the (200) planes. In the case of a deuteron beam with its angular divergence equal to 1/5 of the critical channeling angle, the maximum reaction-probability enhancement is 1.5. The results of calculations agree qualitatively with recent experiments performed at Tomsk Polytechnic University.     

Large-angle channeling radiation from relativistic electrons in optically transparent crystals

In the work within the frame of quantum electrodynamics are obtained new formulae describing the large-angle photon emission from channeled electrons with taking into account of the dispersion of refractive index. Calculations based on these formulae show that the spectral and angular distributions of large-angle optical and ultraviolet radiation from planar channeled sub-GeV electrons in optically transparent crystal reflect the band structure of transverse energy levels of channeled electrons. Comparison with ordinary Cherenkov radiation spectrum reveals that channeling (depending on the beam energy) leads to sufficient change of the large-angle emission spectrum.     

Enhanced near field mediated nanohole fabrication on silicon substrate by femtosecond laser pulse

Investigation of the process of nanohole formation on silicon surface mediated with near electromagnetic field enhancement in vicinity of gold particles is described. Gold nanospheres with diameters of 40, 80 and 200 nm are used. Irradiation of the samples with laser pulse at fluences below the ablation threshold for native Si surface, results in a nanosized surface modification. The nanostructure formation is investigated for the fundamental (λ = 800 nm, 100 fs) and the second harmonic (λ = 400 nm, 250 fs) of the laser radiation generated by ultrashort Ti:sapphire laser system. The near electric field distribution is analyzed by an Finite Difference Time Domain (FDTD) simulation code. The properties of the produced morphological changes on the Si surface are found to depend strongly on the polarization and the wavelength of the laser irradiation. When the laser pulse is linearly polarized the produced nanohole shape is elongated in the E-direction of the polarization. The shape of the hole becomes symmetrical when the laser radiation is circularly polarized. The size of the ablated holes depends on the size of the gold particles, as the smallest holes are produced with the smallest particles. The variation of the laser fluence and the particle size gives possibility of fabricating structures with lateral dimensions ranging from 200 nm to below 40 nm. Explanation of the obtained results is given on the basis simulations of the near field properties using FDTD model and Mie's theory.     

Quantum effects for parametric X-ray radiation during channeling: Theory and first experimental observation

The theory of X-ray radiation from relativistic channeled electrons at the Bragg angles—parametric X-ray radiation (PXR) during channeling (PXRC)—is developed while accounting for two quantum effects: the initial population of bound states of transverse motion and the transverse “form-factor” of channeled electrons. An experiment was conducted using a 255 MeV electron beam from a linac at the SAGA Light Source. We have identified a difference in the angular distributions of PXR and PXRC and obtained a fairly good agreement between the theoretical and experimental results.     

Radiation of channelled particles in crystals: A review

Abstract

The present paper is a review of basic works dedicated to the radiation emitted by channeled particles in crystals in a low (～100 MeV), intermediate (0.1 ? E ? 30 GeV), and high (E ? 100 GeV) energy ranges.     

Polarization effects accompanying penetration of high-energy electrons,positrons and gamma-quanta through crystals

Abstract

Synchrotron-like processes of radiation and pair production in fields of crystal planes are accompanied by large number of polarization effects. These effects can be used for production and analysis of TeV-energy polarized e ⁺, e ^?, photon and hadron beams for their use in high-energy physics. The process of e ⁺ and e ^? self-polarization accomapnies their radiation in bent crystals, where channeled particles are moving in regions having a preferential direction of the electric field. The results of numerical simulation of self-polarization process with both radiation and multiple scattering taken into account are presented. It is shown that the self-polarization process can be observed using present-day CERN secondary positron beam. At TeV-energy region the efficiency of both e ⁺ and e ^? self-polarization can be considerably risen by making the crystal curvature growing along the crystal.     

Controlled rotation and orientation of rubrene particles and Escherichia coli using optical tweezers

Optical trapping and rotating of suspended micro-sized rubrene particles were performed using optical tweezers with circularly polarized light. The experimental results show that the rotation speed of the rubrene particles is proportional to the laser power, and the orientation of the rubrene particles can be controlled by the optical tweezers with linearly polarized light. Interestingly, by combining with the rubrene particle, the Escherichia coli (E. coli) can be rotated and oriented by optical tweezers. However, the rotating and orientating are mainly determined by the characteristics of rubrene particles. Our experiment provides a simple and convenient way to orient biological particles even if they are not sensitive to the polarization of the laser beam. Moreover, the rubrene can emit strong fluorescence when excited by the laser at the wavelength of 532 nm, and which can be potential applied to manipulate other particles with the fluorescence characteristics.     

Modelling laser cleaning of low-absorbing substrates: the effect of near-field focussing

A three-dimensional model for laser cleaning of spherical, transparent particles on low-absorbing substrates has been developed. It takes into account near-field focussing of the laser radiation by the particles. The intensity distribution under a particle was found using Mie theory together with the geometrical optics approximation. This permits the estimation of the beam width at the substrate surface and the focal distance of the radiation coming from the spherical particle. These parameters are used to find the distribution of intensity within the low-absorbing substrate from the formula for a focussed Gaussian beam. This is in contrast with most other models of laser cleaning, which assume that all absorption occurs at the surface of the substrate. The energy criterion was used to calculate the threshold fluence. The model predicts threshold fluences of the order of 10³ J/cm² for silica spheres having a diameter of the order of a micron on silica substrates, assuming adhesion by van der Waals force. As this is well above the damage threshold for silica, it effectively predicts that laser cleaning of silica spheres from silica will be impossible. For glass slides the threshold fluence is predicted to be a factor of 10^-4 times smaller than that for silica slides (about 0.1 J/cm²). This is due to the much higher absorption of glass compared to that of silica at 248 nm. PACS 42.62.Cf; 81.65.Cf     

Anomalous laser energy absorption associated with resonance saturation

Laser energy absorption measurements have been undertaken in an experiment involving the transmission of a pulse of laser radiation through sodium vapour. The wavelength of the laser was tuned to overlap the 589 nm resonance transition of the sodium atoms. Although simple radiation transport appears to account for the attenuation of the laser beam at low values of incident laser irradiance, anomalously large absorption has been observed at high values of incident laser energy. We suggest that this anomalous absorption of laser energy can be regarded as evidence of superlastic electron heating and subsequent ionization.     

Radiation force exerted on nanometer size non-resonant Kerr particle by a tightly focused Gaussian beam

We calculate the radiation force that is exerted by a focused continuous-wave Gaussian beam of wavelength λ on a non-absorbing nonlinear particle of radius a ? 50λ/π. The refractive index of the mechanically-rigid particle is proportional to the incident intensity according to the electro-optic Kerr effect. The force consists of two components representing the contributions of the electromagnetic field gradient and the light scattered by the Kerr particle. The focused intensity distribution is determined using expressions for the six electromagnetic components that are corrected to the fifth order in the numerical aperture (NA) of the focusing objective lens. We found that for particles with a < λ/21.28, the trapping force is dominated by the gradient force and the axial trapping force is symmetric about the geometrical focus. The two contributions are comparable with larger particles and the axial trapping force becomes asymmetric with its zero location displaced away from the focus and towards the beam propagation direction. We study the trapping force behavior versus incident beam power, NA, λ, and relative refractive index between the surrounding liquid and the particle. We also examine the confinement of a Kerr particle that exhibits Brownian motion in a focused beam. Numerical results show that the Kerr effect increases the trapping force strength and significantly improves the confinement of Brownian particles.