Laser-optical path to nuclear energy without radioactivity: Fusion of hydrogen–boron by nonlinear force driven plasma blocks

Anomalous interaction of terawatt-picosecond laser pulses allows side-on ignition of solid state density fusion fuel with the unexpected possibility of igniting uncompressed hydrogen–boron p-¹¹B. Suppression of relativistic self-focusing by using very clean laser pulses with an extremely high contrast ratio is essential to achieve ignition thresholds only ten times more difficult than fusion of deuterium–tritium (DT). This opens the possibility for laser driven fusion energy without neutrons and less radioactivity than from burning coal. The complex nonlinear optical properties involved are elaborated.     

Theory of laser ion acceleration from a foil target of nanometer thickness

A theory for ion acceleration by ultrashort laser pulses is presented to evaluate the maximum ion energy in the interaction of ultrahigh contrast (UHC) intense laser pulses with a nanometer-scale foil. In this regime, the ion energy may be directly related to the laser intensity and subsequent electron dynamics. This leads to a simple analytical expression for the ion energy gain under the laser irradiation of thin targets. Significantly higher energies for thin targets than for thicker targets are predicted. The theory is concretized with a view to compare with the results and their details of recent experiments.     

Experimental study and computer simulations of the implantation of laser plasma ions in pulsed electric fields

Results are presented from experimental studies of pulsed plasma flows generated by nanosecond laser pulses with an intensity of 7 × 10⁸ W/cm² from a solid-state target in a strong electric field. The current pulses through the laser target and the depth distributions of the iron ions implanted in a silicon substrate to which a negative high-voltage pulse was applied are measured. The physical processes occurring in laser plasma with an initial iron ion density of 6 × 10¹⁰ cm⁻³ are simulated numerically by the particle-in-cell method for different delay times and different shapes of the accelerating high-voltage pulse. The model developed allows one to calculate the ion flows onto the processed substrate, the electron flows onto the target, and the energy spectra of the implanted ions. The results from computer simulations are found to be in good agreement the experimental data.     

Quasimonoenergetic deuteron bursts produced by ultraintense laser pulses

We report on the generation and laser acceleration of bunches of energetic deuterons with a small energy spread at about 2 MeV. This quasimonoenergetic peak within the ion energy spectrum was observed when heavy-water microdroplets were irradiated with ultrashort laser pulses of about 40 fs duration and high (10(-8)) temporal contrast, at an intensity of 10(19) W/cm(2). The results can be explained by a simple physical model related to spatial separation of two ion species within a finite-volume target. The production of quasimonoenergetic ions is a long-standing goal in laser-particle acceleration; it could have diverse applications such as in medicine or in the development of future compact ion accelerators.     

Modeling a distributed feedback dye laser pumped by a Nd-glass laser

A mathematical model has been developed to describe the dynamic emission of a distributed feedback dye laser (DFDL) pumped by a Nd-glass laser. The model is based on the coupled-wave theory. It allows the investigation of the temporal behavior of the Nd-glass pumping laser source and the DFDL pulses. The model allows studying the effect of the variation of the laser input parameters of the Nd-glass laser, such as maximum amplification coefficient, loss coefficient and pumping rate on the characteristics of DFDL pulses regarding the pulse width, delay time and separation time. The feedback process of the DFDL is provided either by changes of the refractive index or by optical gain or by both together. The model estimates the following: temporal behavior of the density of emitted radiation, energy densities of the first excited singlet and triplet states, DFDL output power, cavity decay time and the temperature of the produced grating. The numerical solution of the nonlinear coupled rate equation system predicts the generation of DFDL picosecond pulses. The calculated results are in good agreement with the available experimental data. The calculations were done using rhodamine 6G dissolved in ethanol as the investigated matrix.     

激光脉冲作用下囚禁离子在Lamb-Dicke区域精确的量子运动

研究驻波型激光脉冲作用下,囚禁于Paul阱中的单离子在Lamb-Dicke区域的久期运动.通过试探解方法,得到系统的量子力学精确解.基于精确解描述的概率波包串,发现：1）波包串中心以及波包串的高度和宽度受激光脉冲强度的控制,通过调节激光强度可以控制波包串的形变和传播;2）在激光脉冲作用瞬间,离子的能量期待值发生跳变,而在激光关闭时段,有窄的能带形成;3）存在一个激光脉冲强度的临界值,在临界点附近,系统的稳定性发生变化. 关键词： 囚禁离子 激光脉冲 Lamb-Dicke近似 精确解     

激光脉冲作用下囚禁离子在Lamb-Dicke区域精确的量子运动

研究驻波型激光脉冲作用下,囚禁于Paul阱中的单离子在Lamb-Dicke区域的久期运动.通过试探解方法,得到系统的量子力学精确解.基于精确解描述的概率波包串,发现：1）波包串中心以及波包串的高度和宽度受激光脉冲强度的控制,通过调节激光强度可以控制波包串的形变和传播;2）在激光脉冲作用瞬间,离子的能量期待值发生跳变,而在激光关闭时段,有窄的能带形成;3）存在一个激光脉冲强度的临界值,在临界点附近,系统的稳定性发生变化.     

Third-harmonic and sum-frequency generation in a quadratically nonlinear polymer by time-ordered ultrashort laser pulses

Third-harmonic and sum-frequency generation in quadratically nonlinear azopolymer films is experimentally studied using femtosecond chromium forsterite laser pulses. A noncollinear geometry of sum-frequency and third-harmonic generation developed and implemented in this work allows the influence of the time ordering of ultrashort laser pump pulses on nonlinear-optical phenomena to be experimentally observed. Femtosecond laser pulses induce transitions of azopolymer molecules to an electronically excited state and produce vibrational wave packets, leading to an asymmetry in the dependence of the efficiency of second-and third-order nonlinear-optical processes on the delay time between the pump pulses.     

Atomistic simulation of laser ablation of gold: Effect of pressure relaxation

The process of ablation of a gold target by femto- and picosecond laser radiation pulses has been studied by numerical simulations using an atomistic model with allowance for the electron subsystem and the dependence of the ion-ion interaction potential on the electron temperature. Using this potential, it is possible to take into account the change in the physical properties of the ion subsystem as a result of heating of the electron subsystem. The results of simulations reveal a significant difference between the characteristics of metal ablation by laser pulses of various durations. For ablation with subpicosecond pulses, two mechanisms of metal fracture related to the evolution of electronic pressure in the system are established.     

Experimental study of relativistic self-focusing andself-channeling of an intense laser pulse in an underdense plasma

This paper reports on experimental investigations on relativistic self-focusing and self-channeling of a terawatt laser pulse (0.7 TW⩽P⩽15 TW) in an underdense plasma. We present results obtained with picosecond (τ=1 ps) and subpicosecond (τ=0.4 ps) pulses. In the “long pulse” regime, modifications in the laser propagation are observed for Pc, the critical power for self-focusing. By contrast, self-guiding of subpicosecond pulses is observed for P≈P_c. Using a paraxial envelope model describing the laser propagation and taking into account the plasma response to the ponderomotive force, it is shown that a maximum laser intensity of 5-15 times that reached in vacuum may be achieved when P is in the (1.25-4)×P_c range. It is also demonstrated that ion motion may significantly reduce the effective P_c     

Femtosecond time-resolved laser-induced desorption of positive ions from MgO

We have used the pump-probe technique to measure the photostimulated positive ion yield as a function of time delay between two sub-threshold femtosecond laser pulses. We find that the ion yield from UV femtosecond irradiated MgO depends critically on the laser pulse delay, (t, in two-pulse experiments. In single-pulse experiments, excitation of MgO produces a variety of ions including Mg⁺, MgO⁺, and a significant yield of H⁺. In contrast, if the femtosecond laser pulse is split into two sub-threshold beams and then recombined with a variable time delay, the ion yield may be drastically altered depending on the delay between pulses. The Mg⁺ desorption yield displays three distinct lifetimes and persists for laser delays of over 100 ps. A pulse delay of only (t=500 fs nearly eliminates ion desorption except for Mg⁺. The use of a pair of delayed femtosecond laser pulses can thus control the species of the desorbed ion. The mechanism for femtosecond laser desorption is clearly different from nanosecond laser desorption. We hypothesize that the creation of electron-hole pairs by nonresonant two-photon excitation contributes to the ultrafast desorption mechanism.     

Absorption and ionization of molecular nitrogen by UV femtosecond laser pulses

Mechanisms of non-linear absorption and ionization of molecular nitrogen gas by UV femtosecond laser pulses were studied using photogalvanic and photoacoustic techniques. The effect of the intermediate Rydberg resonance, its dynamic Stark perturbation and ponderomotive upshift of the first ionization potential of nitrogen molecules by the intense laser pulses has been revealed by observing an increase of a power slope of ion yield from three to four at increasing laser intensity.     

On the possibility of obtaining incoherent femtosecond X-ray pulses from a laser plasma

It is proposed to use a high rate of collisional ionization in a superdense laser plasma to generate incoherent femtosecond X-ray pulses. The calculations indicate that the use of picosecond laser pulses with a contrast of about 10¹⁰ will allow the generation of an X-ray pulse with a duration of about 10 fs. The adequacy of the proposed model of the excitation of linear X-ray radiation from the plasma has been tested in the experiments with a picosecond laser of a moderately high contrast.     

Assessment of laser-induced thermal load on silicon nanostructures based on ion desorption yields

Experimental assessment of the thermal load induced by fast laser pulses on micro- and nanostructures through IR imaging is currently too slow and lacks the spatial resolution to be useful. In this paper, we introduce a method based on measuring the laser-induced yields of ions to compare the thermal loads on nanofabricated silicon structures, when exposed to nanosecond laser pulses. The laser fluences at which the ion yields of, for example, sodiated and potassiated peptides ions are equal for two different structures correspond to equivalent thermal loads. Using alkalinated peptides is a convenient choice because the corresponding ion intensities are easily measured up to the melting point of silicon. As an example, we compare the nanosecond laser heating of silicon nanopost arrays with diverse post diameters and periodicities. Assessment of the thermal load through ion yield measurements can also be used to verify model assumptions for heat transport regimes in nanostructures.     

Modulator‐Free Coherent‐One‐Way Quantum Key Distribution

Time‐bin encoding is an attractive method for transmitting photonic qubits over long distances with minimal decoherence. It allows a simple receiver for quantum key distribution (QKD) that extracts a key by measuring time of arrival of photons and detects eavesdropping by measuring interference of pulses in different time bins. In the past, coherent pulses have been generated using a CW laser and an intensity modulator. A greatly simplified transmitter is proposed and demonstrated here that works by directly modulating the laser diode. Coherence between pulses is maintained by a weak seed laser. The modulator‐free source creates time‐bin encoded pulses with a high extinction ratio (29.4 dB) and an interference visibility above 97 %. The resulting QKD transmitter gives estimated secure key rates up to 4.57 Mbit/s, the highest yet reported for coherent‐one‐way QKD, and can be programmed for all protocols using weak coherent pulses.     

双脉冲激光锡等离子体光源的碎屑动力学研究

极紫外光刻是下一代大容量集成电路制造中最有发展前景的技术之一,而碎屑的减缓及阻挡一直是极紫外光刻光源研究中亟需解决的关键问题。研究了双纳秒激光脉冲辐照锡靶产生的等离子体碎屑的动力学演化。结果表明,等离子体碎屑强烈依赖于预脉冲的能量及其与主脉冲的时间延迟,当预脉冲能量为30 mJ, 双脉冲时间间隔150 ns情况下,大部分锡离子的能量从2.47 keV降低到0.40 keV,降低了6.1倍,碎屑得到了有效抑制。通过对碎屑动能角分布的测量,发现此方法可以有效减缓全角度范围的激光锡等离子体碎屑,并且越接近靶材法线方向,碎屑的动能减少得越多。     

Laser acceleration of ion bunches at the front surface of overdense plasmas

The acceleration of ions in the interaction of high intensity laser pulses with overdense plasmas is investigated with particle-in-cell simulations. For circular polarization of the laser pulses, high-density ion bunches moving into the plasma are generated at the laser-plasma interaction surface. A simple analytical model accounts for the numerical observations and provides scaling laws for the ion bunch energy and generation time as a function of pulse intensity and plasma density.     

Anisotropic explosions of hydrogen clusters under intense femtosecond laser irradiation

We report on measurements of ion energy distributions from hydrogen clusters irradiated by intense laser pulses of duration 40 and 250 fs. Contrary to the predictions of a simple Coulomb explosion model, we observe a pronounced spatial anisotropy of the ion energies from these explosions with the highest energy ions ejected along the laser polarization direction. The origin of the anisotropy is distinct from that previously seen in clusters of high Z atoms such as Ar and Xe. Furthermore, a measured increase in H+ ion energy when longer, lower intensity pulses are employed suggests that multiple-pass, vacuum heating of the cluster electrons is important in the deposition of energy by the laser.     

Synchronization of a mode-locked Nd:YAG — Argon ion laser system

A Q-switched active-passive Nd : YAG laser and a c.w. argon ion laser were simultaneously mode-locked using a common R.F. oscillator. Jitter between the output pulses of the two lasers was measured at 18 ps (RMS) and was highly dependent on the modulation depth of the YAG mode-locker. Intensity fluctuations of the argon laser led to significant phase shift between the laser output pulses.     

Spectral dips in ion emission emerging from ultrashort laser-driven plasmas

Deep dips in MeV ion spectra are obtained from water droplet targets irradiated by intense [(0.5-1.2) x 10(19) W/cm(2)] and ultrashort (35 fs) laser pulses. The existence of these dips is ascribed to the generation of a multielectron-temperature plasma, which is confirmed by our experiments. An existing fluid model based on hot-electron components with significantly different temperatures is consistent with the behavior we observe in the ion spectra of the femtosecond laser-driven interaction. The model provides a good simulation of the observed spectral dips and allows us to establish important parameters such as hot- and cold-electron temperatures and the respective electron density ratios. The result may be of interest for spectral tailoring of proton spectra in future applications of laser-generated proton beams.