Electron acceleration by a tightly focused laser beam

State-of-the-art petawatt laser beams may be focused down to few-micron spot sizes and can produce violent electron acceleration as a result of the extremely intense and asymmetric fields. Classical fifth-order calculations in the diffraction angle show that electrons, injected sideways into the tightly focused laser beam, get captured and gain energy in the GeV regime. We point out the most favorable points of injection away from the focus, along with an efficient means of extracting the energetic electron with a static magnetic field.     

Electron acceleration in a tightly focused laser beam

Electron acceleration in a tightly focused ultra-intensity linear polarized laser beam is investigated numerically. It has been found that the acceleration is strong phase dependent and is periodic to the variety of the initial laser field phase. When optimal initial parameters are chosen, the electron can be accelerated effectively. The accelerated electrons are emitted in pulses of which the full width is less than the half period of the laser field.     

Temperature elevation generated by a focused Gaussian ultrasonic beam at a tissue-bone interface

When a focused Gaussian ultrasonic beam in soft tissue normally impinges on the boundary of a contiguous bone, reflection and transmission occur. Taking account of the reflection, the steady-state temperature elevation along the beam axis for this case was calculated. The effect of perfusion is also included.     

Experimental research on the longitudinal field generated by a tightly focused beam

The longitudinal optical field is a peculiar physical phenomenon that is always involved with the domain of near-field optics. Due to its extraordinary properties, it has recently attracted increasing attention in research and application. In this work, the longitudinal fields generated by the evanescent illumination of tightly focused, different polarized hollow beams are investigated. The focused light fields are numerically simulated according to vector diffraction theory, and their vector analysis is also carried out. The longitudinal fields on the focal plane are demonstrated experimentally using tip-enhanced scanning near-field microscopy. The simulation and experimental results show that the tightly focused radially polarized beam is suited to generating a stronger and purer longitudinal optical field at the focus.     

紧聚焦的飞秒激光脉冲在真空中对电子的加速

研究了紧聚焦的圆偏振飞秒相对论高斯激光脉冲与电子的相互作用，提出了一种激光加速电子的新机制.利用束腰小、强度大的激光脉冲上升沿加速电子，束腰大、强度小的脉冲下降沿减速电子，当光脉冲和电子分离时，电子获得了能量增益.研究发现，初始静止的电子与强度高于10¹⁹Wμm^{2/cm2的光脉冲作用以后，可以获得MeV量级的能量.初始位于焦点附近的电子被加速的效果较好，而远离焦点的电子几乎不能获得能量增益. 关键词： 电子加速 能量增益 高斯脉冲 束腰}     

High-order harmonic generation in a tightly focused laser beam

The force due to the transverse magnetic field of a laser beam drives an electron in the direction of laser propagation, thereby impeding the recollision mechanism for high-order harmonic generation. The longitudinal electric field component of a tightly focused Gaussian beam can sufficiently counteract the magnetic force to enhance the harmonic yield substantially. For tight focusing and a laser intensity of 10(18) W/cm2, it can raise the harmonic yield by several orders of magnitude.     

电子在紧聚焦激光束中加速的最佳入射

通过数值模拟研究了电子在真空紧聚焦激光束中的动力学行为和能量增益。研究发现对应于两种不同的轨迹，电子可以得到纵向和横向两种电场的加速。阐明了电子的能量增益与电子的入射角和入射动量间的关系。对于给定的激光参数，给出了电子的最佳入射参数。     

Light propagation in two-dimensional photonic crystals illuminated by a tightly focused laser beam

By using a tightly focused laser beam, we have studied light propagation in two-dimensional photonic crystals consisting of closed-packed hexagonal arrays of polystyrene particles with diameter of 1∼5 μm. The light propagation is found to depend strongly on the focal point positioning, the incident beam polarization and the interaction between particles and glass substrates.     

Electron-positron pair creation by a strong tightly focused laser field

The analytical calculation of the electron-positron pair creation yield by an extremely strong focused laser pulse in the vacuum is presented. In particular, dependence of the total yield of the pair production on the focusing parameter and the polarization properties of the laser pulse are analyzed. Comparison with the previous consideration of N.B. Narozhny, V.S. Popov, V.D. Mur, and S.S. Bulanov is given and discussed. As a result, we confirm their conclusion that the electron-positron pair production can be observed in principle at the level of intensity of 10²⁷ W/cm².     

Adaptive focusing of a high-intensity beam in an irregular medium

Conclusions These model studies show that the performance in adaptive focusing for high-intensity beams in randomly inhomogeneous media is restricted primarily by nonlinear phenomena. The power density at the object is increased by adaptive control and is only slightly dependent on the fluctuation variance for the modes considered and for the range of nonlinearity parameters.In thermal self-action, large-scale spatial distortions predominate, for which one can use adaptive suppression systems with small numbers of control coordinates. In the case of nonlinear refraction in a moving medium, a considerable improvement is obtained by control on the lower aberrations, i.e., by means of the wavefront tilt and curvatures.A current problem in optimizing adaptive-optics systems is evidently the determination of a sufficient set of control coordinates for focusing a high-intensity beam propagating under real conditions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 30–41, November, 1985.     

Successive particle acceleration by use of two tightly focused ultra-short intense laser pulses

We suggest a scheme of electron acceleration by use of two tightly focused ultra-short intense laser pulses at a 100TW level. Electrons obtain a preliminary acceleration with a small angular spread by the longitudinal ponderomotive force of the first pulse. They are then injected and further accelerated to hundreds of MeV by the second laser pulse.     

Scattering of relativistic electrons by high-power laser light tightly focused

We obtain an approximate solution for the drift and oscillatory components of the motion of relativistic electrons in the field of temporally extended high-power laser light under strong focusing of the light (the size of the focal region is of the order of the light wavelength). This makes it possible to start numerically integrating the equations of electron motion near the focal region. We estimate the impact parameters of the electrons when they are still efficiently accelerated in the focal region. Zh. éksp. Teor. Fiz. 111, 1554–1562 (May 1997)     

Third-harmonic generation in a vacuum at the focus of a high-intensity laser beam

A new physical effect lying in the harmonic generation by a focused high-intensity (I ～ 10²⁷ W/cm²) laser beam in a vacuum is predicted. The probability for the third-harmonic generation is calculated for a specific model of a monochromatic laser beam (symmetric beam) with the optical frequency.     

Properties of a laser beam generated from an axicon resonator

A high power cw CO₂ laser beam is generated from a resonator configuration having an axicon mirror as its end reflector. The properties of this beam are evaluated in terms of its polarization, focusing capability and temporal stability and the resulting effects on processing materials are presented. A comparison is made between a folded and a straight resonator having a spherical focusing mirror as its end reflector.     

Degree of polarization of a tightly focused partially coherent dark hollow beam

Propagation of a linearly polarized partially coherent dark hollow beam (DHB) through a high numerical aperture (NA) thin lens is formulated. The tight focusing properties, such as the degree of polarization, the intensity distribution, and the degree of coherence, of a partially coherent DHB are investigated in detail. It is found that the focusing properties of a partially coherent DHB are closely determined by the initial coherence, beam order, and the NA of the thin lens.     

Continuum generation of the third-harmonic pulse generated by an intense femtosecond IR laser pulse in air

The continuum generation by intense femtosecond IR laser pulses focused in air including the effect of third-harmonic generation is investigated. We have used a theoretical model that includes the full spatio-temporal dynamics of both the fundamental and the third-harmonic pulses. Results of our numerical calculations show that a two-color filamentation effect occurs, in which the third-harmonic conversion efficiency remains almost constant over the whole filament length. It is found that this effect is rather independent of the wavelength of the input beam and the focal geometry. During the filamentation process the third-harmonic pulse itself generates a broad continuum, which can even overlap with the continuum of the fundamental pulse for the longer pump wavelengths. In consequence, the continuum generation generated by intense IR laser pulses is further extended into the UV. PACS 42.65.Jx; 42.65.Ky; 52.35.Mw     

Propagation of a filamentary femtosecond laser beam with high intensities at an air-solid interface

The propagation of a filamentary laser beam at an air-glass surface is studied by setting the incident angle satisfying the total reflection condition. The images of the trajectory of the filamentary laser beam inside the sample and the output far-field spatial profiles are measured with varying incident laser pulse energies. Different from the general total reflection, a transmitted laser beam is detected along the propagation direction of the incident laser beam. The energy ratio of the transmitted laser beam depends on the pulse energies of the incident laser beam. The background energy reservoir surrounding the filament core can break the law of total reflection at the air-glass surface, resulting in the regeneration of the transmitted laser beam.     

Cathode spot energy transfer simulated by a focused laser beam

Minimum conditions for the formation of surface craters by laser irradiation were studied experimentally and theoretically for various metals. The critical power density for crater formation within 20 ns was about 10¹¹ W/m². It is therefore concluded that crater formation by ion bombardment requires an ion current density on the order of 10¹⁰ A/m²     

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.     

Structure of a tightly focused vortex beam in the presence of primary coma

Structure of a tightly focused circularly polarized vortex beam in the presence of primary coma has been studied by using Debye-Wolf diffraction integral. The role of topological charge and handedness of the polarization on the focused structure has been discussed. Results have been presented for the total intensity and squares of the polarization components for left-, and right circularly polarized vortex beams. Impact of coma, on the dark core of an azimuthally polarized non-vortex beam is also investigated and compared with the dark core of a circularly polarized vortex beam. The presence of comatic aberration in the focusing system results in a positional shift of the high intensity lobes, and reduction of the intensity on one side of the center. Effect of coma on the focused structure has also been discussed in the context of STED microscopy.