Off-axis collimation of diode laser beams by means of single-element holographic diffractive optics

Principles of single-element holographic diffractive optics for collimation of diode laser beams with a large divergence, an elliptic cross-section, and astigmatism are presented. Holographic off-axis transformation enables collimation of the beam in two variants: one with a perpendicular input beam and an oblique output beam, and the other with the beams arranged vice-versa. Diffraction due to an elliptic aperture is analyzed. Inspection of experimental samples demonstrates an agreement with theory in the case of diffraction limited focal pattern and shows increase of astigmatism with the departure from diffraction only limitation.     

Sound beams with shockwave pulses

The beam equation for a sound beam in a diffusive medium, called the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation, has a class of solutions, which are power series in the transverse variable with the terms given by a solution of a generalized Burgers’ equation. A free parameter in this generalized Burgers’ equation can be chosen so that the equation describes an N-wave which does not decay. If the beam source has the form of a spherical cap, then a beam with a preserved shock can be prepared. This is done by satisfying an inequality containing the spherical radius, the N-wave pulse duration, the N-wave pulse amplitude, and the sound velocity in the fluid.     

Enhancing high-order above-threshold dissociation of H2+ beams with few-cycle laser pulses

High-order (three-photon or more) above-threshold dissociation (ATD) of H(2)(+) has generally not been observed using 800 nm light. We demonstrate a strong enhancement of its probability using intense 7 fs laser pulses interacting with beams of H(2)(+), HD(+), and D(2)(+) ions. The mechanism invokes a dynamic control of the dissociation pathway. These measurements are supported by theory that additionally reveals, for the first time, an unexpectedly large contribution to ATD from highly excited electronic states.     

Demonstrating various quantum effects with two entangled laser beams

We report on the preparation of entangled two mode squeezed states of yet unseen quality. Based on a measurement of the covariance matrix we found a violation of the Reid and Drummond EPR-criterion at a value of only 0.36 ± 0.03 compared to the threshold of 1. Furthermore, quantum state tomography was used to extract a single photon Fock state solely based on homodyne detection, demonstrating the strong quantum features of this pair of laser-beams. The probability for a single photon in this ensemble measurement exceeded 2/3.     

Instability-free ion acceleration by two laser pulses

We demonstrate the instability-free ion acceleration regime by introducing laser control with two parallel circularly polarized laser pulses at an intensity of I = 6.8 × 10²¹?W/cm², normally incident on a hydrogen foil. The special structure of the equivalent wave front of those two pulses, which contains Gaussian peaks in both sides and a concavity in the centre (2D), can suppress the transverse instabilities and hole boring effects to constrain a high density ion clump in the centre of the foil, leading to an acceleration over a long distance and gain above 1GeV/u for the ion bunches.     

Resistive collimation of electron beams in laser-produced plasmas

Intense relativistic electron beams, produced by high-intensity short-pulse laser irradiation of a solid target, have many potential applications including fusion by fast ignition. Using a unique Fokker-Planck code, supported by analytic calculations, we show that fast electrons can be collimated into a beam even when the fast electron source is not strongly anisotropic, and we derive a condition for collimation to occur.     

Cross-focusing of two laser beams in a plasma

Cross-focusing of two copropagating laser beams in a plasma is investigated using paraxial ray theory. If the lasers have a frequency difference equal to the electron plasma frequency, they can drive a large amplitude plasma wave. The ponderomotive force due to the plasma wave forces the plasma electrons outwards thereby generating a parabolic density profile giving rise to cross-focusing. The results show a decrease in threshold for focusing by two orders of magnitude as compared to focusing due to the ponderomotive force of the laser beams.     

双光束对高能电子的捕获特性

为了在实验上增强非线性强场效应的信号强度,基于电子在强激光束上的非弹性散射,提出一种双光束捕获电子的方案,目的是通过延长电子和强场相互作用时间来提高非线性过程发生总概率,实现观测信号的增强。数值模拟结果表明,捕获后的电子和强激光场的相互作用时间可延长10倍以上。     

Power‐ and polarization dependence of two photon luminescence of single CdSe nanowires with tightly focused cylindrical vector beams of ultrashort laser pulses

We investigate two‐photon luminescence (TPL) of single CdSe nanowires (NWs) excited by tightly focused cylindrical vector beams of 150 fs pulses, achieving an optical resolution better than λ/4. Comparing the TPL images recorded by scanning the nanowires through the focal fields created either by a radially or an azimuthally polarized vector beam we observe a distinct anisotropic excitation efficiency which depends on the orientation of the electric field component with respect to the nanowire. The excitation anisotropy of the linear photoluminescence (PL) and TPL can directly be derived from the respective image patterns. Our results show that the highest TPL signal from a single NW is detected when the electric excitation field is parallel to the long axis of the NW, i.e. about one order of magnitude stronger than that excited along the short axis. We attribute the TPL to an emission mechanism based on the recombination of free carriers, which exhibits a fourth order excitation power dependence in low power regime and a second order power dependence in high power regime.

     

Coherent beam combination of two thulium-doped fiber laser beams with the multi-ditheringtechnique

Coherent beam combination of two thulium-doped fiber laser beams using a multi-dithering technique is presented for the first time. In the experiment, two fiber lasers centered at 1948.6 nm are coherently combined, and a phase modulator based on piezoelectric ceramics transducer is connected in one beam path to compensate for the phase errors between the two beams. When the phase control system is closed loop, the fringe contrast of the far-field intensity pattern is improved to be more than 75%, from 15% in open-loop, and the residual phase error is less than λ/20. The experimental results show that the performance of the phase control system is robust and the control bandwidth is more than 1 kHz, which indicates that the above approach can be scaled to facilitate the coherent beam combination of kilo-watt level thulium-doped fiberlaser.     

Laser produced plasma heated by two simultaneous laser pulses

A single laser pulse plasma heating and that of two simultaneous pulses with different frequencies and the same front pulse shaping are compared in a one-dimensional model.     

Investigation of plumes produced by material ablation with two time-delayed femtosecond laser pulses

We experimentally investigated and herewith reported the results of laser ablation of copper and gold with two time-delayed femtosecond laser pulses at 800 nm in vacuum. The ablation plume dynamic was monitored by fast plume imaging and time- and space-resolved optical emission spectroscopy. Optical microscopy was used to follow the ablation depth as a function of the delay between the two laser pulses. Nanoparticles deposition on mica substrates was analysed by atomic force microscopy.We estimate roughly the plume's atomization degree - that is the mass fraction of atomized material over the total ablated mass - from the relative intensities of radiation emitted from atoms and nanoparticles. It is shown that the atomization degree depends critically on the time delay between both laser pulses and on the characteristic time of electron-lattice relaxation. The increase of the atomization degree is accompanied by the decrease of the ablation depth. Atomic force microscopy measurements confirm the partial atomization of nanoparticles, as the analyses of particle deposition on mica substrates show a large decrease of the number of nanoparticles for large delay between the two pulses.     

Microablation with ultrashort laser pulses

This paper reports the micromachining results of different materials (Al, Si, InP and fused silica) using a Ti : sapphire laser at the wavelength of 800 and 267 nm with variable pulse lengths in the range from 100 fs to 10 ps. The hole arrays with a diameter up to a few μm through microdrilling are presented. We discussed how an effective suppression of the thermal diffusion inside the ablated materials and an effective microablation could be realized. If the laser fluence is taken only slightly above the threshold, a hole array can be drilled with diameters even smaller than the wavelength of the laser. Some examples are presented in the paper.     

Additional acceleration and collimation of relativistic electron beams by magnetic field resonance at very high intensity laser interaction

For the interpretation of experiments for acceleration of electrons at interaction up to nearly GeV energy in laser produced plasmas, we present a new model using interaction magnetic fields. In addition to the ponderomotive acceleration of highly relativistic electrons at the interaction of very short and very intense laser pulses, a further acceleration is derived from the interaction of these electron beams with the spontaneous magnetic fields of about 100 MG. This additional acceleration is the result of a laser-magnetic resonance acceleration (LMRA) around the peak of the azimuthal magnetic field. This causes the electrons to gain energy within a laser period. Using a Gaussian laser pulse, the LMRA acceleration of the electrons depends on the laser polarization. Since this is in the resonance regime, the strong magnetic fields affect the electron acceleration considerably. The mechanism results in good collimated high energetic electrons propagating along the center axis of the laser beam as has been observed by experiments and is reproduced by our numerical simulations. PACS 41.75.Jv; 52.38.Kd; 52.65.Cc     

Observation of enhanced field-free molecular alignment by two laser pulses

We show experimentally that field-free alignment of iodobenzene molecules, induced by a single, intense, linearly polarized 1.4-ps-long laser pulse, can be strongly enhanced by dividing the pulse into two optimally synchronized pulses of the same duration. For a given total energy of the two-pulse sequence the degree of alignment is maximized with an intensity ratio of 1:3 and by sending the second pulse near the time where the alignment created by the first pulse peaks.     

二极管激光器垂直阵列光束精密准直

为实现二极管激光器垂直阵列输出光束具有小发散角、高指向精度的特点,简述了快轴准直（FAC）微透镜的光束准直原理,分析了调节装置的精度要求及透镜选择等问题。通过光学成像方法实时监测二极管激光bar条的近场像和远场像,对FAC透镜分别进行粗调节和细调节,获得了20个bar条连续输出2 kW,垂直阵列二极管激光快轴准直光束远场发散角4.4 mrad,bar条间准直光束指向精度不大于±1.7 mrad的准直效果,并对监测精度进行了简要分析。对影响光束准直效果的因素进行了分析,指出了工艺优化的重点。     

Active manipulation of the selective alignment by two laser pulses

This paper solves numerically the full time-dependent Schrdinger equation based on the rigid rotor model, and proposes a novel strategy to determine the optimal time delay of the two laser pulses to manipulate the molecular selective alignment. The results illustrate that the molecular alignment generated by the first pulse can be suppressed or enhanced selectively, the relative populations of even and odd rotational states in the final rotational wave packet can be manipulated selectively by precisely inserting the peak of the second laser pulse at the time when the slope for the alignment parameter by the first laser locates a local maximum for the even rotational states and a local minimum for the odds, and vice versa. The selective alignment can be further optimised by selecting the intensity ratio of the two laser pulses on the condition that the total laser intensity and pulse duration are kept constant.     

Processing of metals by double pulses with short laser pulses

Experimental results related to the influence of time delayed pulses for ablation efficiency with short multi pulses (pulse duration of 5 ps) are reported. A significant improvement of the micro structuring quality at relatively high fluence regime in metals is obtained. Less removed or recast matter is observed and the processed surface appears to be smoother with better roughness. Ablation depths and burr heights are compared for single pulses and double pulses in steel, Al and Cu as a function of scans number. Best results are obtained for weak time delays, typically less than 1 ps. PACS 79.20.Ds; 42.62.Cf; 81.65.Cf     

二极管激光器垂直阵列光束精密准直

 为实现二极管激光器垂直阵列输出光束具有小发散角、高指向精度的特点,简述了快轴准直（FAC）微透镜的光束准直原理,分析了调节装置的精度要求及透镜选择等问题。通过光学成像方法实时监测二极管激光bar条的近场像和远场像,对FAC透镜分别进行粗调节和细调节,获得了20个bar条连续输出2 kW,垂直阵列二极管激光快轴准直光束远场发散角4.4 mrad,bar条间准直光束指向精度不大于±1.7 mrad的准直效果,并对监测精度进行了简要分析。对影响光束准直效果的因素进行了分析,指出了工艺优化的重点。