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When light propagates in the atmosphere, it will be absorbed and scattered by the atmospheric molecules and aerosols. Using a detector to collect the scattered light, a non­line­of­sight (NLOS) optical information transmission system can be constructed. Its basic structure consists of an optical transmitting system and a receiving system. The light emitter emits modulated laser pulses into the atmosphere, and the light receiver collects the scattered light signal and converts it into electrical signal. In this transmission model, the optical impulse response of the receiver is a key parameter that affects the communication rate. Ultraviolet (UV) light has a short transmission distance in the atmosphere, so in the existing research, ultraviolet light source is usually used to realize short­range NLOS communication system. Due to the short transmission distance, the impulse response performance of the receiver is usually analyzed under the assumption of uniform atmosphere distribution. But when the transmitting and receiving pitch angle are large, the signal scattering domain may be in a relatively large region, as a result the influence of nonuniform atmospheric distribution on signal transmission needs to be considered. In this paper, a Monte­Carlo (MC) simulation program based on nonuniform transmission model is developed to simulate this NLOS transmission model, and the relationship between impulse response and pitch angle is studied. The results show that when the pitch angle is large the atmospheric molecular density and aerosol density will decrease with the increase of altitude, resulting in the decrease of absorption coefficient and scattering coefficient. By comparison, it is found that the uniform model overestimates the pulse response width and the pulse energy. Therefore, in the design of NLOS communication system, the nonuniform transmission model can provide more accurate analysis.

The new way of generating the attosecond pulses via Thomson scattering of linear polarization Gaussian beam propagating on solid surface is investigated. It is found that a single attosecond laser pulse is generated which renders the appropriate strength electrostatic field to the environment. After studying the influence of the strength of the electrostatic field on it, the results show that a single attosecond pulse is generated when intensity of Gaussian beam is 5 and intensity of the electrostatic field is Es = 5 ×10−4, which is more intense and direction simplification.

We propose and theoretically study a silica double-toroid microcavity to confine the whispering-gallery mode (WGM) in an ultra-small space, which consists of two toroid-to-toroid coupled cavities separated by a nanoscale gap region with low refractive index, such as the air gap. Benefitting from the strong field localization of the “slot” effect, power enhancement of symmetric WGMs in the gap results in the ultra-small mode volume of 4. 8 μ m which is quite smaller than the conventional toroidal microcavity with mode volume of about hundreds of cubic micrometers. The confined modes hold the potential advantages over conventional photonic devices, especially in the applications of the sensing and optical trapping. In the application of refractometer, a high refractive index sensitivity of up to 468 nm per refractive index unit (nm/RIU) can be obtained in the 1000 nm wavelength band. In the optical trapping application, the double-toroid microcavity enables a significant field enhancement in the confined WGM, and the gradient force could reach as high as 22 pN/W for a single nanoparticle with the radius of 5 nm. As the potential advantages, this study of silica double-toroid microcavity provides a good reference for realizing the high-efficient photonic applications such as optical sensing and trapping.

The combination of graphene and high quality (Q) factor microcavity provides a promising way for realizing high-efficient modulation. In this paper, an electro-optic tuning of a graphene–silica microdisk is investigated. Two graphene flakes, separated by Al2O3 isolation layer, are embedded in the silica microdisk to significantly enhance the light-matter interaction for the achievement of high-efficient modulation. Maximal resonant wavelength shift of 9.5 nm is presented as the bias voltage of about 189.5 V is applied on the graphene flakes, which is beneficial for realizing the optical modulation with the extinction ratio of about 16 dB. In addition, two partially reflecting elements are embedded in the waveguide which is side-coupled with the microcavity for realizing the electromagnetically induced transparency (EIT), subsequently achieving the high-efficient modulation with the extinction ratio of about 33.3 dB. Meanwhile, it is found that the 3 dB modulation bandwidth, maximum of as high as 58.1 GHz, gradually decreases as the thickness of Al2O3 isolation layer is reduced. As the potential applications, this improved study of graphene–silica microdisk opens up great potential for realizing high-efficient electro-optic devices such as modulator, optical switch and ultra-short pulsed laser.

We propose and numerically investigate the plasmonic whispering-gallery mode (WGM) with high-quality (Q) factor (as high as 285) and ultra-small mode volume (as low as 0 . 04 μ m 3 ) in the hybrid plasmonic microcavity with a sandwiched and electrically controlled graphene. The theoretical results present that the resonant wavelength of hybrid plasmonic microcavity dramatically changes and the corresponding intrinsic loss exhibits a distinct peak by electrically adjusting the permittivity of graphene around the epsilon-near-zero (ENZ) point. The influence of graphene with different layers on the characteristic of hybrid plasmonic microcavity is also analyzed. As a potential application, the plasmonic WGM microcavity with the sandwiched graphene could be applied for a refractometer with the sensitivity of higher than 1000 nm per refraction index unit (nm/RIU), and large figure of merit. The sensitivity can also be tuned by the electrically controlled graphene.

Ultra-strong and ultra-short laser pulses can accelerate free electrons and generate attosecond radiation pulses. In this paper, circularly polarized Laguerre-Gaussian (LG) vortex light beam (VLB) is used to accelerate the electrons. A single electron with an incident energy of 2.1 MeV has been simulated to interact with LG-VLB with a half-waist beam width of 5 λ . Numerical simulations show that the vortex light has a special converging effect on the electrons. The electrons distributed near the center of the spot generate a velocity component with the motion vector toward the center of the spot. Different from conventional activation, the electron radiation field has a non-uniform distribution, and the radiation field close to the concentrated lateral intensity has an even more strong acceleration effect on the electrons. The pulse width of the electron radiation is 2 attoseconds(as) which is much smaller than the pulse generated by using an ordinary intense laser and Gaussian beam. It has an advantage of reducing the intensity of the laser that generates a single attosecond pulse, and its structure is simple and easy to implement, it has more power in probing the electron dynamics process and the phenomenon of atomic transition.

The coupling of Laguerre-Gaussian (LG) vortex modes into the Bessel vortex modes in a multimode fiber was analyzed using the vector form of LG beams. A formula for estimating the transmission coefficients of the excited vortex modes was developed. Analytical calculation results show that the coupling coefficients are affected by the focus size. With proper beam waist radius most the power of incident vortex mode can couple into single vortex mode. The numerical simulation was also carried out, and the simulation results are in good agreement with those from the analytical model.

The generation and transport of fast electrons play an important role in intense laser interaction with dense plasma. Here, we experimentally study fast electron generation using a planar aluminum target obliquely irradiated at a large incident angle by an ultrashort high-temporal-contrast relativistic laser pulse. Simulations show that for near-relativistic laser intensities, s-polarized lasers above a certain energy threshold can generate energetic electrons ( }}\,{\text{500}}\;{\text{keV}}$?> E e > 500 keV ) with a high yield, which is different from the case of low laser contrast. It is also shown that the most energetic electrons are emitted along the target’s surface and accelerated to high energies. The key features of the acceleration regimes of lasers with different polarizations are also given.

InGaN/GaN light-emitting diodes (LEDs) with graded-composition multiple quantum wells (GQWs) were designed and theoretically investigated. In the GQW LED structure, the indium composition in In x Ga1−x N QW was decreased sequentially in increments, $$\Delta x$$ , along the [0001] orientation. Across the composition range, the GQW LEDs have high carrier injection efficiencies and enhanced uniform carrier distributions in the multiple quantum wells because of the band structure. When $$\Delta x = 0.01$$ , the light output power the GQW LED was enhanced by 4.2 % compared with that of the original LED, which corresponds to an increase of IQE of 4.5 % at 180 mA. However, when $$\Delta x$$ increased to 0.02, the light output power of the GQW LEDs was reduced by 3.1 % compared with that of original LED, which corresponds to a decrease of IQE of 3.3 % at 180 mA. Results from the simulations indicated that the device performance was not only related to the carrier injection efficiency and the carrier distribution, but was also related to the balance of electrons and holes in the QWs.

Generation of plasma channels by low jitter ablative capillary discharges is investigated experimentally in details. A time-resolved evolution and radial distributions of the electron density are measured, and proof-of-principle optical guiding experiment is conducted. A proper time window for optical guiding of a femtosecond laser pulse is found. The generated low density, long plasma channel is believed to be useful in the applications as GeV-class channel-guided laser wakefield accelerators and compact X-ray femetosecond coherent radiation sources.

We have experimentally realized a scheme to enhance betatron radiation by manipulating transverse oscillation of electrons in a laser-driven plasma wakefield with a tilted shock front (TSF). Very brilliant betatron x-rays have been produced with significant enhancement both in photon yield and peak energy but almost maintain the e-beam energy spread and charge. Particle-in-cell simulations indicate that the accelerated electron beam (e beam) can acquire a very large transverse oscillation amplitude with an increase in more than 10-fold, after being steered into the deflected wakefield due to the refraction of the driving laser at the TSF. Spectral broadening of betatron radiation can be suppressed owing to the small variation in the peak energy of the low-energy-spread e beam in a plasma wiggler regime. It is demonstrated that the e-beam generation, refracting, and wiggling can act as a whole to realize the concurrence of monoenergetic e beams and bright x-rays in a compact laser-wakefield accelerator.

It is proposed that single attosecond pulses be generated via electron's Thomson scattering of two counter-propagating laser pulses. In the case of linear polarization, the generation of a single attosecond pulse is highly sensitive to the carrier envelope phase (CEP). However, in the case of circular polarization, it is completely independent on the CEP, which will make circular polarization favorable to generate a stable attosecond X-ray pulse. For either linear or circular polarization, the radiation obtained by using two counter-propagating pulses can be much more intense than that obtained by only using one of these two pulses.

We present the idea of intensified attosecond X-ray generation based on nonlinear Thomson backscattering of an intense laser pulse by electrons trapped in plasma-vacuum boundary. Two frequency up-conversions due to the relativistic Doppler effect and longitudinal γ-spike effect are analyzed, respectively, where γ is the relativistic factor of the plasma surface. Relativistic resonance heating conditions should be used as a criterion for the experimental design to obtain efficient high-order harmonics and energetic electrons' generation at relatively low laser intensities. Shaping the laser field by proposing a detuned second-harmonic can generate a single attosecond pulse without spectral filtering.

We report an efficient continuous-wave self-Raman laser at 1176 nm based on a 20-mm-long composite YVO4/Nd:YVO4/YVO4 crystal and pumped by a wavelength-locked 878.9 nm diode laser. A maximum output power of 5.3 W is achieved at a pump power of 26 W, corresponding to an optical conversion efficiency of 20% and a slope efficiency of 21%. The Raman threshold for the diode pump power was only 0.92 W. The results reveal that in-band pumping by a wavelength-locked diode laser significantly enhances output power and efficiency of self-Raman lasers by virtue of improved pump absorption and relieved thermal loading.

We propose a scheme to minimize the energy spread of an electron beam (e-beam) in a cascaded laser wakefield accelerator to the one-thousandth-level by inserting a stage to compress its longitudinal spatial distribution. In this scheme, three-segment plasma stages are designed for electron injection, e-beam length compression, and e-beam acceleration, respectively. The trapped e-beam in the injection stage is transferred to the zero-phase region at the center of one wakefield period in the compression stage where the length of the e-beam can be greatly shortened owing to the velocity bunching. After being seeded into the third stage for acceleration, the e-beam can be accelerated to a much higher energy before its energy chirp is compensated owing to the shortened e-beam length. A one-dimensional theory and two-dimensional particle-in-cell simulations have demonstrated this scheme and an e-beam with 0.2% rms energy spread and low transverse emittance could be generated without loss of charge.

By irradiating a flat Al target with femtosecond laser pulses at moderate intensities of $\ensuremath{\sim}{10}^{17}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$, we obtained stable collimated quasimonoenergetic electrons in the specular direction but deviated somewhat toward the target normal. An associated local minimum located on the other side of the specular direction seems to indicate that the peak actually results from the deflection of the collimated electrons from their initial ejection direction. We have proposed a two-step model in which some laser-accelerated electrons are able to leave the plasma in a narrow phase-locked window of the moving wave interference pattern, and are then steered toward the target normal by the ponderomotive force of the interference field. The periodic repetition of the electron emission leads to a pulse train of collimated quasimonoenergetic electrons with subcycle duration.

Near-GeV electron beam generation from a two-stage laser wakefield accelerator (LWFA) is reported. Electron injection and acceleration are separated into two distinct LWFA stages and controlled independently from each other by employing two gas cells filled with a He/O2 mixture and pure He gas, respectively. Electrons with a Maxwellian spectrum, generated from the injection stage assisted by ionization-induced injection, are seeded into the acceleration stage with a 3-mm long gas cell and accelerated to produce a 0.8-GeV quasimonoenergetic electron beam for a 45 TW 40 fs laser pulse, corresponding to an acceleration gradient of 187 GV/m. In the injection stage, the produced electron beam properties can be optimized by adjusting the input laser intensity and the plasma density so that quasimonoenergetic electron beams are obtained owing to the self-focusing effects of the laser beam. The ionization-induced injection scheme has been extensively employed for a capillary discharge plasma waveguide to demonstr...

Cascaded laser wakefield acceleration (LWFA) of electrons is promising for producing monoenergetic electron beams well beyond 1 GeV by separating and controlling electron injection and postacceleration in two LWFA stages. We have demonstrated that electrons with Maxwellian spectra generated from the first LWFA assisted by tunnel-ionization-induced injection were seeded into the second LWFA and then accelerated to be a 0.8 GeV quasi-monoenergetic electron beam. Further acceleration toward multi-GeV may be fulfilled with a long plasma channel. Optical guiding of intense femtosecond laser pulses for powers up to 160 TW over a 4-cm long ablative capillary discharge plasma channel and laser wakefield acceleration of electrons well beyond 1 GeV were experimentally demonstrated. By employing an oxygen-containing ablative capillary, electron beams with energies extending up to 1.8 GeV were generated by using 130 TW, 55 fs laser pulses.

We report on near-GeV electron beam generation from an all-optical cascaded laser wakefield accelerator (LWFA). Electron injection and acceleration are successfully separated and controlled in different LWFA stages by employing two gas cells filled with a $\mathrm{He}/{\mathrm{O}}_{2}$ mixture and pure He gas, respectively. Electrons with a Maxwellian spectrum, generated from the first LWFA assisted by ionization-induced injection, were seeded into the second LWFA with a 3-mm-thick gas cell and accelerated to be a 0.8-GeV quasimonoenergetic electron beam, corresponding to an acceleration gradient of $187\text{ }\text{ }\mathrm{GV}/\mathrm{m}$. The demonstrated scheme paves the way towards the multi-GeV laser accelerators.

A scheme for producing nearly single-cycle relativistic laser pulses is proposed. When a laser pulse interacts with an overdense thin foil, because of self-consistent nonlinear modulation, the latter will be more transparent to the more intense part of the laser, so that a transmitted pulse can be much shorter than the incident pulse. Using two-dimensional particle-in-cell simulation and analytical modeling, it is found that a transmitted pulse of duration 4 fs and peak intensity $3\ifmmode\times\else\texttimes\fi{}{10}^{20}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ can be generated from a circularly polarized laser pulse. The intensity of the resulting pulse is only limited by that of the incident pulse, since this scheme involves only laser-plasma interaction.

We demonstrate experimentally the efficient fusion neutron generation from Coulomb explosion (CE) of laser irradiated large-size heteronuclear deuterated methane clusters. A conversion efficiency of $2.1\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }\text{neutrons}/\text{J}$ of incident laser energy is obtained with a 120 mJ, 70 fs laser pulse. It is 50 times higher than that of homonuclear deuterium clusters of similar size. This enhancement is attributed to the significant increase in the deuteron kinetic energies by fourfold due to energetic boosting and overrun effects during CE of heteronuclear clusters. The yield of $5.5\ifmmode\times\else\texttimes\fi{}{10}^{6}$ neutrons per pulse is obtained with a 100 TW, 50 fs driving laser pulse at an intensity of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }\text{W}/{\text{cm}}^{2}$. This work may facilitate the development of a high-flux table-top neutron source.

The time-integrated reflectivities are measured with a TW laser on both glass and antireflected coating as plasma mirrors. The ionization process has been observed on the PM at the critical ionization intensity. The improvement of the contrast ratio on nanosecond scale has been detected directly.

In this work, graded barrier InGaN light-emitting diodes with increasing indium composition barriers are proposed and investigated numerically. When the conventional GaN barriers are replaced by this unique graded InGaN barrier design, the forward voltage at 100 mA is reduced from 3.32 V to 3.27 V, and the efficiency droop is improved from 46.6% to 7.5%. The simulation results observed in this work indicate that these improvements can be attributed to increased electron confinement and enhanced hole injection efficiency caused by the modified energy band diagram.

We present the experimental and numerical results of two-dimensional x-ray imaging due to fast electron transport in a solid target. A 40-μm-thick copper film target is irradiated by a 100 mJ, 50 fs normal incident laser pulse. The full width at half maximum of the x-ray photon dose is 25 μm, and the divergence angle of fast electrons is 25°–30°, which is detected by the pin-hole x-ray imaging technique. The target surface plasma layer is compressed by a ponderomotive force into a depth of 0.2λ. The plasma wave accompanied by fast electrons transporting into the target is studied by dividing the plasma into layers in a radial direction. A narrow fast electron channel, which is approximately 8 μm–10 μm in width, mainly contributes to the x-ray dose.

A new scheme that is able to control the electron injection phase in the plasma wakefield based on the ponderomotive acceleration of electrons is proposed. The scheme uses the far-infrared laser as an injection pulse which copropagates behind the pump laser. Electrons located in the rear of the first period of the wakefield are pre-accelerated by the injection pulse and get injected at the phase that is determined by the distance between the two laser pulses. 1D Hamiltonian analysis and 2D particle-in-cell simulations have been performed to investigate the trapping process, as well as the influence of injection phase on electron energy and beam charge. With the proper injection phase, the maximum injected electron charge that is limited by the beam loading can be achieved.

Two segments of plasmas with different densities, which are operated as the electron injector and accelerator, respectively, are designed to realize a cascaded laser wakefield accelerator. Particle-in-cell simulations indicate that the further acceleration of the electrons in the second uniform-density plasma relies on the injection and acceleration in the first stage. It is found that electrons trapped in the second wakefield period in the first stage can be seeded into the next stage with an optimized phase for efficient acceleration and reducing in the relative energy spread. And finally a 700 MeV electron beam with a relative rms energy spread about 0.6% and the normalized transverse emittance of 1.4π mm mrad was obtained after a 5.5-mm-long acceleration in a dark-current free cascaded laser wakefield accelerator. Our results demonstrate that, for a given laser energy, choices in laser and plasma parameters strongly affect the output electron beam energy and quality, and that all of these parameters c...

We report on the study of the self-focusing effects on the tunnel-ionization-induced injection in a laser wakefield accelerator. Targets composed of a gas mixture of 94% helium and 6% oxygen were used. The energy, energy spread, and charge of the generated electron beams can be adjusted by changing the input laser intensity and the plasma density, but the different aspects of the properties of the electron beams were not independent. It was inferred that the K-shell electrons of oxygen were ionized and injected into the plasma wake for acceleration when the laser intensity was increased beyond the threshold for generating O7+ by tunnel-ionization, due to the relativistic self-focusing in the propagation. Controlling the self-focusing of the laser beam by adjusting the input laser energy to shorten the distance over which the electrons were injected into the wake, quasi-monoenergetic electron beams were observed.

Laser wakefield acceleration of electrons well beyond 1 GeV and optical guiding of ultraintense laser pulses of peak powers up to 160 TW over a 4-cm long ablative capillary discharge plasma channel were experimentally demonstrated. Electron beams, with energies up to 1.8 GeV, were generated by using the 130 TW, 55 fs driving laser pulses. A comparison of oxygen-containing acrylic resin (C:O:H = 4:2:7) capillary and no oxygen-containing polyethylene (C:O:H = 1:0:2) capillary measurements suggests that the injection of electron into the laser wakefield is assisted by the ionization of oxygen K-shell electrons.

Few-cycle relativistic circularly polarized (CP) laser pulse reflected from overdense plasma is investigated by analysis and particle-in-cell simulations. It is found that through the laser-induced one-time drastic oscillation of the plasma boundary, an ultra-intense single attosecond light pulse can be generated naturally. An analytical model is proposed to describe the interaction and it agrees well with simulation results. They both indicate that peak intensity of the generated attosecond pulse is higher when the plasma density is closer to the relativistic transparency threshold and/or the pulse duration is closer to plasma oscillating period. Two dimensional simulation shows that a two-cycle 1021 W/cm2 CP laser can generate a single 230 attosecond 2 × 1021 W/cm2 pulse of light at a conversion efficiency greater than 10-2.

Acceleration of protons by a circularly polarized laser pulse irradiating on a double-layer target is investigated by a theoretical model and particle-in-cell simulations. The target is made up of a heavy ion layer coated with a proton layer on the rear surface. The results show that when the first layer is transparent induced by the hole-boring effect, the whole proton layer is accelerated by the transmitted laser pulse to high energy with low energy spread. The quality of the proton beam generated from a double-layer target is better than that from a single-layer target. The improvement is attributed to the flat top structure of the electrostatic field caused by the electrons injected into the second layer. It is easier to control the spectrum quality by using a double-layer target rather than using a single-layer one when the radiation pressure acceleration is dominant.

The dynamics of a solid foil irradiated by a circularly polarized laser pulse in the normal incidence is investigated by performing particle-in-cell simulations. After sufficiently compressed by the light pressure, the foil becomes transparent, with a part of the incident pulse transmitted through, and then it turns opaque again, blocking the tail of the pulse. It is found that the transparency dynamically depends on the motion of the compressed foil and relies on the incident pulse. Thus, the foil can be used to shorten the incident pulse as a self-triggered shutter.

In this paper, fusion neutron yields from the Coulomb explosion of (CD4)N clusters under the irradiation of an intense femtosecond laser pulse is optimized by controlling the propagation of the laser pulse in a cluster jet. A correlated study of fusion neutron yields, kinetic energies of deuterons, together with the plasma channels diagnosed by a pump–probe interferometer, is performed. It has been found that by controlling the focal position related to the cluster jet, the plasma defocusing effect can balance with the tight focusing effect of the laser pulse induced by an off-axis parabolic mirror, and results in a cylindrical-shaped and relatively narrow plasma channel crossing the gas jet. The most energetic deuterons and the maximum yields of fusion neutrons are produced at the same time. For better understanding of the experimental results, numerical simulations for the nonlinear propagation of the femtosecond laser pulse in (CD4)N gas-cluster jets are performed. Simulated results show that plasma defocusing and laser attenuation play dominant roles in this case of optimization.

An off-axis incident model is presented to analyze the influence of beam pointing fluctuation on the propagation properties of intense laser beams in plasma channels. The equations for the beam spot size and centroid are obtained by applying the variational method. The beam pointing fluctuation contributes additional focusing effect by amplifying relativistic self-focusing, leading to periodically modified oscillations of the spot size. The beam centroid oscillates along the channel axis with the amplitude close to its initial off-axis displacement, while the oscillation frequency is scaled as the square of the dimensionless channel strength parameter.

A technique is developed to reduce the jitter associated with ablative capillary discharges. A laser pulse propagating perpendicularly to the axis of the capillary and focused onto a copper wire creates a plasma that initiates the discharge. This transverse laser ignition method has several advantages over previous techniques employing a laser pulse collinear with the capillary, including increased capillary lifetime and simpler arrangement of the igniting and the driving pulses for laser-wakefield acceleration. Using this technique long, low density plasma channels are produced with low jitter.

Angular and energy distributions of fast electrons generated from the interaction of 60 fs, 795 nm laser pulses with aluminum targets have been experimentally investigated in various conditions. Increasing laser intensities from the nonrelativistic to the relativistic, a transition of the angular distribution of outgoing fast electrons from the specular reflection direction to the target normal has been observed for p-polarized laser irradiation. The fast electrons’ energy spectrum at high laser intensity, e.g., ∼2.6×1018 W/cm2, consists of two peaks, which are found to originate from the target normal with low energy and specular reflection direction with high energy, respectively. By adding a prepulse to generate preplasma, the electron yields at the direction of the reflected laser can be greatly enhanced, and a double-peak angular distribution is observed. Besides, a more collimated electron emission peak in the specular reflection direction can be obtained by employing a larger f-number focusing system.

Using a linearly chirped laser pulse to irradiate antireflection coated targets, the time-varying reflectivity of a plasma mirror (PM) has been measured at various laser intensities from 1012 to 1017 W/cm2. The onset of plasma generation as well as the formation process of a PM with the highest reflectivity has been observed. The rise time of the PM’s reflectivity reaching up to the maximum varies from 300–500 fs at lower laser intensities but goes up to 900 fs at higher intensity of >1016 W/cm2. This long rise time can be attributed to a slowly rising shoulder of the laser pulse, which will trigger the generation of preplasma well in advance of the laser peak. The detailed measurements on both time-integrated and time-resolved reflectivity of a PM, which is induced by p- and s-polarized laser pulses, respectively, indicate that an s-polarized pulse is favorable to obtain the maximal reflectivity and the best contrast improvement as well. This difference can be attributed to the weaker absorption and a sm...

An experimental investigation on the interaction of an ultraintense femtosecond laser pulse at the intensity of 2 × 10 17 W / cm 2 (60 fs, 120 mJ at 800 nm) with clusters in a supersonic jet of deuterated methane gas has shown the generation of energetic deuterons and nuclear fusion events. The deuteron density and the average size of the clusters in the gas jet, as well as the fusionneutron yields under different backing pressures were measured simultaneously as a function of the time delays of the laser pulses with respect to the puffing of the gas jet. The results demonstrate that during the development of the gas jet expanding through a conical nozzle, the clusters grew up with time, and the average size of the clusters reached the maximum when the molecular density in the jet started to drop. The fusionneutron yields were found to increase with the larger average cluster size and the higher deuteron density, in accordance with the theoretical prediction. Experimental data indicate the existence of a ∼ 1 ms steady region in which the fusionneutron yields have reached the maximum of 2.0 × 10 5 per shot at the backing pressure of 74 bars. Consequently, an efficiency of 1.6 × 10 6 neutrons per joule of incident laser energy was realized.

The influence of the carrier-envelope phase (CEP) of the driving laser pulse on the generation of single attosecond (as) pulses from surface harmonics by using the polarization gating technique is investigated in detail. It is found that the modulation depth of the high-order harmonic spectrum depends on the CEP, and a strong single 68 as pulse can be generated when the CEP is stable and has the proper value. The physical origin of the influence of the CEP is explained in terms of the oscillating mirror model. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2997342]