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A novel scheme of staged laser-plasma accelerators produces extremely brilliant γ-rays with photon energies up to GeV level., Recent developments in laser-wakefield accelerators have led to compact ultrashort X/γ-ray sources that can deliver peak brilliance comparable with conventional synchrotron sources. Such sources normally have low efficiencies and are limited to 107–8 photons/shot in the keV to MeV range. We present a novel scheme to efficiently produce collimated ultrabright γ-ray beams with photon energies tunable up to GeV by focusing a multi-petawatt laser pulse into a two-stage wakefield accelerator. This high-intensity laser enables efficient generation of a multi-GeV electron beam with a high density and tens-nC charge in the first stage. Subsequently, both the laser and electron beams enter into a higher-density plasma region in the second stage. Numerical simulations demonstrate that more than 1012 γ-ray photons/shot are produced with energy conversion efficiency above 10% for photons above 1 MeV, and the peak brilliance is above 1026 photons s−1 mm−2 mrad−2 per 0.1% bandwidth at 1 MeV. This offers new opportunities for both fundamental and applied research.

The bremsstrahlung x-ray emission profile from high intensity laser-solid interactions provides valuable insight to the internal fast electron transport. Using penumbral imaging, we characterise the spatial profile of this bremsstrahlung source as a function of laser intensity by incrementally increasing the laser focal spot size on target. The experimental data shows a dual-source structure; one from the central channel of electrons, the second a larger substrate source from the recirculating electron current. The results demonstrate than an order of magnitude improvement in the intensity contrast between the two x-ray sources is achieved with a large focal spot, indicating preferable conditions for applications in radiography. An analytical model is derived to describe the transport of suprathermal electron populations that contribute to substrate and central channel sources through a target. The model is in good agreement with the experimental results presented here and furthermore is applied to predict laser intensities for achieving optimum spatial contrast for a variety of target materials and thicknesses.

Ultra-fast synchrotron radiation emission can arise from the transverse betatron motion of an electron in a laser plasma wakefield, and the radiation spectral peak is limited to tens of keV. Here, we present a new method for achieving high-energy radiation via accelerated electrons wiggling in an additional laser field whose intensity is one order of magnitude higher than that for the self-generated transverse field of the bubble, resulting in an equivalent wiggler strength parameter K increase of approximately twenty times. By calculating synchrotron radiation, we acquired a peak brightness for the case of the laser wiggler field of 1.2 × 1023 ph/s/mrad2/mm2/0.1%BW at 1 MeV. Such a high brilliance and ultra-fast gamma-ray source could be applied to time-resolved probing of dense materials and the production of medical radioisotopes.

When an ultra-intense relativistic laser is irradiated on a solid target, terahertz (THz) pulses can be generated by coherent transition radiation when the laser-driven electron beams cross the rear surface of the target. The radiation energy depends on the number and energy of the electrons. By introducing a milli-joule picosecond ablation laser pulse, an underdense preplasma with a scale length of micrometers is generated at the front surface of the target. Electron beams with more charge and higher energy can be produced during the interaction between the following main laser pulse and the preplasma, which enhance the THz radiation and affect the radiation angle. Two dimensional particle-in-cell simulations demonstrate the improvement of electron beams and a nearly tenfold enhancement of THz radiation energy is observed.

One order of magnitude energy enhancement of the target surface electron beams with central energy at 11.5 MeV is achieved by using a 200 TW, 500 fs laser at an incident angle of 72° with a prepulse intensity ratio of 5×10−6. The experimental results demonstrate the scalability of the acceleration process to high electron energy with a longer (sub-picosecond) laser pulse duration and a higher laser energy (120 J). The total charge of the beam is 400±20 pC(E>2.7 MeV). Such a high orientation and mono-energetic electron jet would be a good method to solve the problem of the large beam divergence in fast ignition schemes and to increase the laser energy deposition on the target core.

In the widely-studied two-color laser scheme for terahertz (THz) radiation from a gas, the frequency ratio of the two lasers is usually fixed at $\omega_2/\omega_1=$1:2. We investigate THz generation with uncommon frequency ratios. Our experiments show, for the first time, efficient THz generation with new ratios of $\omega_2/\omega_1=$1:4 and 2:3. We observe that the THz polarization can be adjusted by rotating the longer-wavelength laser polarization and the polarization adjustment becomes inefficient by rotating the other laser polarization; the THz energy shows similar scaling laws with different frequency ratios. These observations are inconsistent with multi-wave mixing theory, but support the gas-ionization model. This study pushes the development of the two-color scheme and provides a new dimension to explore the long-standing problem of the THz generation mechanism., Comment: 6 pages, 3 figures

We present a simulation study of terahertz radiation from a gas driven by two-color laser pulses in a broad range of frequency ratios ω1/ω0. Our particle-in-cell simulation results show that there are three series with ω1/ω0=2n, n+1/2, n±1/3 (n is a positive integer) for high-efficiency and stable radiation generation. The radiation strength basically decreases with the increasing ω1 and scales linearly with the laser wavelength. These rules are broken when ω1/ω0

It is found that there is an upper-limit critical power for self-guided propagation of intense lasers in plasma in addition to the well-known lower-limit critical power set by the relativistic effect. Above this upper-limit critical power, the laser pulse experiences defocusing due to expulsion of local plasma electrons by the transverse ponderomotive force. Associated with the upper-limit power, a lower-limit critical plasma density is also found for a given laser spot size, below which self-focusing does not occur for any laser power. Both the upper-limit power and the lower-limit density are derived theoretically and verified by two-dimensional particle-in-cell simulations. The present study provides new guidance for experimental designs, where self-guided propagation of lasers is essential.

Background: Ion beam has been used in cancer treatment, and has a unique preferable feature to deposit its main energy inside a human body so that cancer cell could be killed by the ion beam. However, conventional ion accelerator tends to be huge in its size and its cost. In this paper a future intense-laser ion accelerator is proposed to make the ion accelerator compact. Subjects and methods: An intense femtosecond pulsed laser was employed to accelerate ions. The issues in the laser ion accelerator include the energy efficiency from the laser to the ions, the ion beam collimation, the ion energy spectrum control, the ion beam bunching and the ion particle energy control. In the study particle computer simulations were performed to solve the issues, and each component was designed to control the ion beam quality. Results: When an intense laser illuminates a target, electrons in the target are accelerated and leave from the target; temporarily a strong electric field is formed between the high-energy electrons and the target ions, and the target ions are accelerated. The energy efficiency from the laser to ions was improved by using a solid target with a fine sub-wavelength structure or by a near-critical density gas plasma. The ion beam collimation was realized by holes behind the solid target. The control of the ion energy spectrum and the ion particle energy, and the ion beam bunching were successfully realized by a multi-stage laser-target interaction. Conclusions: The present study proposed a novel concept for a future compact laser ion accelerator, based on each component study required to control the ion beam quality and parameters.

In this paper, an optical imaging module design for an astigmatic detection system (ADS) is presented. The module is based on a commercial optical pickup unit (OPU) and it contains a coaxial illuminant for illuminating a specimen. Furthermore, the imaging module facilitates viewing the specimen and the detection laser spot of the ADS with a lateral resolution of approximately 1 μm without requiring the removal of an element of the OPU. Two polarizers and one infrared filter are used to eliminate stray laser light in the OPU and stray light produced by the illuminant. Imaging modules designed for digital versatile disks (DVDs) and Blu-ray DVDs were demonstrated. Furthermore, the module can be used for imaging a small cantilever with approximate dimensions of 2 μm (width) × 5 μm (length), and therefore, it has the potential to be used in high-speed atomic force microscopy.

Coherent transition radiation in the terahertz (THz) region with energies of sub-mJ/pulse has been demonstrated by relativistic laser-driven electron beams crossing the solid-vacuum boundary. Targets including mass-limited foils and layered metal-plastic targets are used to verify the radiation mechanism and characterize the radiation properties. Observations of THz emissions as a function of target parameters agree well with the formation-zone and diffraction model of transition radiation. Particle-in-cell simulations also well reproduce the observed characteristics of THz emissions. The present THz transition radiation enables not only a potential tabletop brilliant THz source, but also a novel noninvasive diagnostic for fast electron generation and transport in laser-plasma interactions.

We report a systematic study on backward terahertz (THz) radiation generation from laser-solid interactions by changing a variety of laser/plasma parameters. We demonstrate a high-energy (with an energy flux density reaching 80 μJ/sr), broadband (10 THz) plasma-based radiation source. The radiation energy is mainly distributed either in the10 THz or3 THz regions. A radial surface current formed by the lateral transport of low-energy electrons (LEE) is believed to be responsible for the radiation in the high-THz region (10 THz), while high-energy surface fast electrons (SFE) accelerated along the target surface mainly contribute to lower frequency (3 THz) radiation. The unifying explanation could be applied to backward THz radiation generation from solid targets with presence of relative small preplasmas.

Energetic electron beam generation from a thin foil target by the ponderomotive force of an ultra-intense circularly polarized laser pulse is investigated. Two-dimensional particle-in-cell (PIC) simulations show that laser pulses with intensity of 1022–1023 Wcm−2 generate about 1–10 GeV electron beams, in agreement with the prediction of one-dimensional theory. When the laser intensity is at 1024–1025 Wcm−2, the beam energy obtained from PIC simulations is lower than the values predicted by the theory. The radiation damping effect is considered, which is found to become important for the laser intensity higher than 1025 Wcm−2. The effect of laser focus positions is also discussed.

A new method based on a chirped optical pulse interferogram has been proposed to measure terahertz radiation. The frequency domain phase information of the interferogram is used to extract the time-domain terahertz pulse waveform. In principle, the resolution of our method can be as high as the unchirped probe pulse duration, with the advantages of relatively simple measurement setup and signal extracting techniques.

Irradiated by femtosecond laser pulses with different energies, opened cone targets behave very differently in the transmission of incident laser pulses. The targets, each with an opening angle of 71? and an opening of 5 ?m, are fabricated using standard semiconductor technology. When the incident laser energy is low and no pre-plasma is generated on the side walls of the cones, the cone target acts like an optical device to reflect the laser pulse, and 15% of the laser energy can be transmitted through the cones. In contrast, when the incident laser energy is high enough to generate pre-plasmas by the pre-pulse of the main pulse that fills the inner cone, the cone with the plasmas will block the transmission of the laser, which leads to a decrease in laser transmission compared with the low-energy case with no plasma. Simulation results using optical software in the low-energy case, and using the particle-in-cell code in the high-energy case, are primarily in agreement with the experimental results.

Intense femtosecond laser-plasma interactions can produce high power terahertz radiations. In our experiment, the polished copper target was irradiated by a p-polarized laser with intensity of more than 1018 W/cm2 at an incident angle of 67.5° from the target normal. The THz energy from three different detection angles is measured. The maximum emission is found in the direction at an angle of 45° to the laser backward direction, which is more than one order of magnitude higher than in the other two directions. A simple theoretical model has been established to explain the measurements.

A survey on the mechanisms of powerful terahertz (THz) radiation from laser plasmas is presented. Firstly, an analytical model is described, showing that a transverse net current formed in a plasma can be converted into THz radiations at the plasma oscillation frequency. This theory is applied to explain THz generation in a gas driven by two-color laser pulses. It is also applied to THz generation in a tenuous plasma driven by a chirped laser pulse, a few-cycle laser pulse, a DC/AC bias electric field. These are well verified by particle-in-cell simulations, demonstrating that THz radiations produced in these approaches are nearly single-cycles and linear polarized. In the chirped laser scheme and the few-cycle laser scheme, THz radiations with the peak field strength of tens of MV/cm and the peak power of gigawatt can be achieved with the incident laser intensity less than 10^{17} W/cm2.

In this report we address the current situation of laser-driven hard X-ray sources for imaging applications, especially the saturation of X-ray conversion efficiency and the serious impact upon imaging quality. By employing high contrast laser pulses, the conversion efficiency to X-rays from solid foil targets is improved and the structure of the spectrum can be optimized with respect to imaging applications. In addition, bright Ar K-shell X-rays with very little continuum background have been generated by irradiating, with an ultra-high contrast laser, a target of Ar gas clusters created by a gas jet. These improvements show great potential for single-shot ultrafast X-ray imaging.

The Fourier finite-difference propagator and the generalized-screen propagator are two general high-order forms of one-way dual-domain methods. We compare these two propagators mainly on phase accuracy, computational efficiency and 3D extension. A comparison of phase accuracy shows that the high-order generalized-screen propagator is preferable to the Fourier finite-difference propagator for heterogeneous media with a weak velocity contrast and wide dip angle. With increasing velocity contrast, the accuracy improvement gained by the high-order generalized-screen propagator declines rapidly. The Fourier finite-difference propagator is more robust and flexible to lateral velocity variations than the generalized-screen propagator. The 2D Fourier finite-difference propagator is superior to the 2D generalized-screen propagator when the velocity contrast is stronger than 23%. Despite the two-way splitting error, the 3D Fourier finite-difference propagator is more accurate than the second-order generalized-screen propagator when the velocity contrast is stronger than 20% and is more accurate than the fourth-order generalized-screen propagator when the velocity contrast is stronger than 40%. Numerical experiments on the SEG/EAGE salt model demonstrate that the Fourier finite-difference propagator behaves better than the generalized-screen propagator when imaging steep salt boundary and faults beneath the salt body. Under the same hardware and software conditions, the computational cost of the Fourier finite-difference propagator in our implementation is greater than that of the second-order generalized-screen propagator but smaller than that of the third-order generalized-screen propagator. Compared with the Fourier finite-difference propagator, the generalized-screen propagator requires fewer grid points per wavelength and has more potential to improve running speed in the presence of a much faster Fourier transform. These analyses are applicable for both forward modelling and depth migration.

Electron injection into a laser wakefield by the colliding of two circularly polarized laser pulses is analyzed by the Hamiltonian approach and particle-in-cell simulations. If the pump pulse driving the laser wakefield is right-circularly-polarized, electron injection is found only when the counter-propagating injection pulse is left-circularly-polarized and vice versa. This holds when the injection pulse is at low intensity and has a frequency near the pump pulse frequency ω0. For a moderately intense injection pulse, even if the two pulses have the same polarization, electron injection is found but with less efficiency. It is also found that the injection pulse with the frequency within [0.5ω0,3ω0] can still create electron injection efficiently provided it has the opposite polarization with the pump pulse.

Conventional two-way splitting Fourier finite-difference migration for 3D complex media yields azimuthal anisotropy where an additional phase correction is needed with much increase of computational cost. We incorporate the alternating-direction-implicit plus interpolation scheme into the conventional Fourier finite-difference method to reduce azimuthal anisotropy. This scheme retains the high-order remnants ignored by the two-way splitting in the form of a wavefield interpolation in the wavenumber domain. The wavefield interpolation for each step of downward extrapolation is implemented between the wavefields before and after the conventional Fourier finite-difference extrapolation. As the Fourier finite-difference migration is implemented in the space and wavenumber dual space, the Fourier transforms between space and wavenumber domain that were needed for the alternating-direction-implicit plus interpolation in frequency domain (FD) migration are saved in Fourier finite-difference migration. Since the azimuth anisotropy in Fourier finite-difference is much less than that in FD, the application of the alternating-direction-implicit plus interpolation scheme in Fourier finite-difference migration is superior to that in FD migration in handling complex media with large velocity contrasts and steep dips. Impulse responses show that the presented method reduces the azimuthal anisotropy at almost no extra cost.

Recently Terahertz (THz) radiation from laser-produced plasmas has attracted much interest since plasmas can work at an arbitrarily high laser intensity. We will present the generation of strong THz radiation from femtosecond and picosecond relativistic laser-plasma interactions. THz pulses with energies up to >200 μJ/sr have been demonstrated. The temporal waveform, polarization, and angular distribution are measured. We find that the radiation depends on the preplasma density scale length. The THz radiation is probably attributed to the self-organized transient fast electron currents formed along the target surface when the plasma density profile is steep, while, the linear mode conversion when a large preplasma is formed.

It is shown, by simulation and theory, that circularly or elliptically polarized terahertz radiation can be generated when a static magnetic (B) field is imposed on a gas target along the propagation direction of a two-color laser driver. The radiation frequency is determined by $\sqrt{\omega_p^2+{\omega_c^2}/{4}} + {\omega_c}/{2}$, where $\omega_p$ is the plasma frequency and $\omega_c$ is the electron cyclotron frequency. With the increase of the B field, the radiation changes from a single-cycle broadband waveform to a continuous narrow-band emission. In high-B-field cases, the radiation strength is proportional to $\omega_p^2/\omega_c$. The B field provides a tunability in the radiation frequency, spectrum width, and field strength., Comment: 6 pages, 5 figures

A simulation study of the conversion process of laser energy into x rays from a gold foil target irradiated by a 1 ns laser pulse with 1014 W/cm2 intensity at 0.35 μm wavelength is presented. The thickness of the foil is optimized to achieve high conversion efficiency on its rear side. The optimized foil can provide unique plasmas with a low spatial gradient of temperature and density.

Ultrafast x/γ ray emission from the combination of laser wake-field acceleration and plasma mirror has been investigated as a promising Thomson scattering source. However, the photon energy and yield of radiation are limited to the intensity of reflected laser pulses. We use the 2D particle in cell simulation to demonstrate that a 75TW driven laser pulse can be refocused on the accelerated electron bunches through a hemispherical plasma mirror with a small f number of 0.25. The energetic electrons with the maximum energy about 350 MeV collide with the reflected laser pulse of a0 = 3.82 at the focal spot, producing high order multi-photon Thomson scattering, and resulting in the scattering spectrum which extends up to 21.2 MeV. Such a high energy γ ray source could be applied to photonuclear reaction and materials science.

It is shown by particle-in-cell simulations that powerful terahertz (THz) radiation can be generated by picosecond (ps) laser pulses below 10(14) W/cm(2) via a two-color laser scheme. At such laser intensities, increasing the laser duration can result in significant enhancement in THz intensities. From 0.03 ps to 0.9 ps the enhancement climbs to nearly 40x before saturating until 2 ps. This demonstrates that low intensity, readily available ps laser technology could be utilized for driving powerful THz sources. By contrast, for laser intensities high enough to completely ionize the gas medium, it is found that the THz emission decreases with increasing pulse duration: optimal conversion is found for few-femtosecond drivers.

The interaction of 100-fs laser pulses with solid targets at laser intensities 10(16)-10(18)W/cm(2) has been investigated experimentally by simultaneous measurements of terahertz (THz) and second harmonic signals. THz yield at the front side of the target, which rises from the self-organized transient electron currents along the target surface, is found scaling linearly with the laser intensity basically. Measurements of specularly reflected light spectrum show clear evidence of resonance absorption. The positive effects of resonance absorption on surface current and THz radiation generation have been confirmed by two-dimensional (2D) particle-in-cell (PIC) simulations and angular-dependent experiments, respectively.

We report the generation of a 6 pC, 23 MeV electron bunch with the energy spread ± 3.5% by using 2 TW, 80 fs high contrast laser pulses interacting with helium gas targets. Within the optimized experimental condition, we obtained quasi-monoenergetic electron beam with an ultra-small normalized divergence angle of 92 mrad, which is at least 5 times smaller than the previous LPA-produced bunches. We suggest the significant decrease of the normalized divergence angles is due to smooth transfer from SM-LWFA to LWFA. Since the beam size in LPA is typically small, this observation may explore a simple way to generate ultralow normalized emittance electron bunches by using small-power but high-repetition-rate laser facilities.

We focus on a control of generation of high-quality ion beam. In this study, near-critical density plasmas are employed and are illuminated by high intensity short laser pulses; we have successfully generated high-energy ions by multiple-stages acceleration. We performed particle-in-cell simulations in this paper. Near-critical density plasmas are employed at the proton source and also in the post acceleration. A beam bunching method is also proposed to control the ion beam length. Intense short-pulse lasers are now available. Based on the new laser technology, new acceleration mechanisms have been proposed by using the laser pulse, as an alternative of the conventional accelerator. However, there are issues in the laser particle acceleration method: the issues includes a lack of controllability for particle beam quality and for the particle energy. These issues should be addressed toward a realistic laser particle accelerator. In this study we focus a high-quality laser proton beam generation. In the laser ion acceleration, first target electrons are kicked by the incoming intense laser pulse, and form an electron cloud at the target surfaces. The target substrate becomes positively charged, and a strong electric field is created at the target surfaces. At the target rear surface, if source ions are located there, the ions are accelerated. This ion acceleration mechanism is called as the TNSA (Target Normal Sheath Acceleration). In TNSA, the acceleration electric field is normal to the target surface. The TNSA is widely used for the ion source. When the target is a under-dense-gas target, a laser would penetrate the gas plasma target with a lower speed than the speed of light c in vacuum. When the intense laser illuminates the gas plasma target, electrons are also accelerated and propagate in the plasma. Then the strong electron current is generated, and a strong magnetic field is generated. At the increase phase of the strong magnetic

It is demonstrated by simulations and analysis that a wakefield driven by an ultrashort intense laser pulse in underdense plasma can emit tunable electromagnetic radiation along the laser propagation direction. The profile of such a kind of radiation is closely associated with the structure of the laser wakefield. In general, electromagnetic radiation in the terahertz range with its frequency a few times the electron plasma frequency can be generated in the moderate intensity regime. In the highly nonlinear case, a chain of radiation pulses is formed corresponding to the nonlinear structure of the wake. Study shows that the radiation is associated with the self-modulation process of the laser pulse in the wakefield and resulting transverse electron momenta from modulated asymmetric laser fields.

In typical laser-driven proton acceleration experiments Thomson parabola proton spectrometers are used to measure the proton spectra with very small acceptance angle in specific directions. Stacks composed of CR-39 nuclear track detectors, imaging plates, or radiochromic films are used to measure the angular distributions of the proton beams, respectively. In this paper, a new proton spectrometer, which can measure the spectra and angular distributions simultaneously, has been designed. Proton acceleration experiments performed on the Xtreme light III laser system demonstrates that the spectrometer can give angle-resolved spectra with a large acceptance angle. This will be conductive to revealing the acceleration mechanisms, optimization, and applications of laser-driven proton beams.

Recently Terahertz (THz) radiation from laser-produced plasmas has attracted much interest since plasmas can work at arbitrarily high laser intensity. This paper will discuss the generation mechanisms of plasma-based THz radiation.

Hard X-ray sources from femtosecond (fs) laser-produced plasmas, including the betatron X-rays from laser wakefield-accelerated electrons, have compact sizes, fs pulse duration and fs pump-probe capability, making it promising for wide use in material and biological sciences. Currently the main problem with such betatron X-ray sources is the limited average flux even with ultra-intense laser pulses. Here, we report ultra-bright betatron X-rays can be generated using a clustering gas jet target irradiated with a small size laser, where a ten-fold enhancement of the X-ray yield is achieved compared to the results obtained using a gas target. We suggest the increased X-ray photon is due to the existence of clusters in the gas, which results in increased total electron charge trapped for acceleration and larger wiggling amplitudes during the acceleration. This observation opens a route to produce high betatron average flux using small but high repetition rate laser facilities for applications.

We report the first demonstration of wrinkled membranes as a kind of optical focusing devices, which are low cost, light weight and flexible. Our device consists of concentric wrinkle rings on a gold-PDMS bilayer membrane, which converts collimated illuminations to diffraction-free focused beams. Beam diameters of 300-400 {\mu}m have been observed in the visible range. By comparing the theoretically calculated and experimentally measured focal spot profiles, we predict a focal spot size as small as around 50 {\mu}m if fabrication eccentricity can be eliminated., Comment: 6 pages, 5 figures

Terahertz (THz) radiation via the two-color laser scheme is investigated theoretically and numerically when the second laser is at different harmonic orders of the main laser. It is found that THz radiation can be generated only when the second laser is an even harmonic and the THz field strength does not show a simple scaling with the two laser amplitudes, suggesting that this is different from normal optical rectification or frequency mixing processes. The THz strength generally tends to decrease with increase of the even harmonic order. The strength is also sensitive to the carrier envelope (CE) phases of both the laser pulses even if their durations are quite long. With different CE phases, the strength can be proportional to either the sine or cosine of the relative phase displacement of the two lasers. For a given gas target except hydrogen, it increases with the laser amplitude between several saturation plateaus, each corresponding to a different ionization level. For a given main laser intensity, there is an optimized intensity of the second laser for the strongest THz radiation. DOI: 10.1103/PhysRevE.87.033108

The development of tabletop intense terahertz (THz) radiation sources is extremely important for THz science and applications. This paper presents our measurements of intense THz radiation from relativistic laser–plasma interactions under different experimental conditions. Several THz generation mechanisms have been proposed and investigated, including coherent transition radiation (CTR) emitted by fast electrons from the target rear surface, transient current radiation at the front of the target, and mode conversion from electron plasma waves (EPWs) to THz waves. The results indicate that relativistic laser plasma is a promising driver of intense THz radiation sources.

The terahertz (THz) radiation from the front of solid targets with a large-scale preplasma irradiated by relativistic picosecond laser pulses has been studied. The THz radiation measured at the specular direction nonlinearly increases with laser energy and an optimal plasma density scalelength is observed. Particle-in-cell simulations indicate that the radiation can be attributed to the model of mode conversion. While the THz radiation near the target normal direction is saturated with laser energy and plasma scalelength. Unlike the radiation in the specular direction' the transient current formed at the plasma-vacuum interface could be responsible for the radiation near the target normal.

In intense laser plasma interaction for particle acceleration several issues remain to be solved. In this paper we focus on a collimation of ion beam, which is produced by a laser plasma interaction. In this study, the ion beam is collimated by a thin film target. When an intense short pulse laser illuminates a target, target electrons are accelerated, and create an electron cloud that generates a sheath electric field at the target surface. Such the ion acceleration mechanism is called the target normal sheath acceleration (TNSA). The TNSA field would be used for the ion beam collimation by the electric field. We have successfully obtained a collimated beam in our particle-in-cell simulations.

Turbulence in fluids is a ubiquitous, fascinating, and complex natural phenomenon that is not yet fully understood. Unraveling turbulence in high density, high temperature plasmas is an even bigger challenge because of the importance of electromagnetic forces and the typically violent environments. Fascinating and novel behavior of hot dense matter has so far been only indirectly inferred because of the enormous difficulties of making observations on such matter. Here, we present direct evidence of turbulence in giant magnetic fields created in an overdense, hot plasma by relativistic intensity (10 18 W/cm 2 ) femtosecond laser pulses. We have obtained magneto-optic polarigrams at femtosecond time intervals, simultaneously with micrometer spatial resolution. The spatial profiles of the magnetic field show randomness and their k spectra exhibit a power law along with certain well defined peaks at scales shorter than skin depth. Detailed two-dimensional particle-in-cell simulations delineate the underlying interaction between forward currents of relativistic energy “hot” electrons created by the laser pulse and “cold” return currents of thermal electrons induced in the target. Our results are not only fundamentally interesting but should also arouse interest on the role of magnetic turbulence induced resistivity in the context of fast ignition of laser fusion, and the possibility of experimentally simulating such structures with respect to the sun and other stellar environments.

Lifetime of laser plasma channel is significantly prolonged using femtosecond laser pulse sequence, which is generated from a chirped pulse amplification laser system with pure multi-pass amplification chain. Time-resolved fluorescence images and electrical conductivity measurement are used to characterize the lifetime of the plasma channel. Prolongation of plasma channel lifetime up to microsecond level is observed using the pulse sequence.

In this work, an anti-drift and auto-alignment mechanism is applied to an astigmatic detection system (ADS)-based atomic force microscope (AFM) for drift compensation and cantilever alignment. The optical path of the ADS adopts a commercial digital versatile disc (DVD) optical head using the astigmatic focus error signal. The ADS-based astigmatic AFM is lightweight, compact size, low priced, and easy to use. Furthermore, the optical head is capable of measuring sub-atomic displacements of high-frequency AFM probes with a sub-micron laser spot (~570 nm, FWHM) and a high-working bandwidth (80 MHz). Nevertheless, conventional DVD optical heads suffer from signal drift problems. In a previous setup, signal drifts of even thousands of nanometers had been measured. With the anti-drift and auto-alignment mechanism, the signal drift is compensated by actuating a voice coil motor of the DVD optical head. A nearly zero signal drift was achieved. Additional benefits of this mechanism are automatic cantilever alignment and simplified design.

A simple scheme to produce strong magnetic fields due to cold electron flow in an open-ended coil heated by high power laser pulses is proposed. It differs from previous generation of magnetic fields driven by fast electron current in a capacitor-coil target [S. Fujioka et al., Sci. Rep. 3, 1170 (2013)]. The fields in our experiments are measured by B-dot detectors and proton radiography, respectively. A 205 T strong magnetic field at the center of the coil target is generated in the free space at Iλ2 of 6.85 × 1014 W cm−2 μm2, where I is the laser intensity, and λ is the laser wavelength. The magnetic field strength is proportional to Iλ2. Compared with the capacitor-coil target, the generation mechanism of the magnetic field is straightforward and the coil is easy to be fabricated.

Interactions of 100-fs laser pulses with solid targets at intensities of 10(18) W/cm(2) and resultant terahertz (THz) radiation are studied under different laser contrast ratio conditions. THz emission is measured in the specular reflection direction, which appears to decrease as the laser contrast ratio varies from 10(-8) to 10(-6). Correspondingly, the frequency spectra of the reflected light are observed changing from second harmonic dominant, three-halves harmonic dominant, to vanishing of both harmonics. Two-dimensional particle-in-cell simulation also suggests that this observation is correlated with the plasma density scale length change. The results demonstrate that the THz emission is closely related to the laser-plasma interaction processes. The emission is strong when resonance absorption is a key feature of the interaction, and becomes much weaker when parametric instabilities dominate.

It is shown by simulations that terahertz (THz) radiation can be produced more efficiently by a mid-infrared laser pulse from a gas target. The THz amplitude is enhanced by 35 times as the laser wavelength increases from 1 μm to 4 μm; a 4 μm laser at 10(15) W cm(-2) produces 5 MV/cm THz radiation. The THz amplitude changes oscillatingly with increasing laser intensity for a given laser wavelength. In addition, the laser intensity threshold for the THz emission is lower for a longer laser wavelength.

Future ultrashort 10^{22}–10^{25} W cm^{-2} laser pulses offer the possibility of producing ultrashort monoenergetic electron beams in the GeV–TeV level by direct laser ponderomotive force acceleration (LPFA) in distances of millimeters to about 1 m. A scheme is proposed that a thin solid foil and a thick solid foil are placed on the laser axis, where the thin foil supplies the electron source for LPFA and the thick foil reflects the laser away, however allows the accelerated electrons to go through. By optimizing the distance between the foils, one can obtain the maximum electron beam energy. This scheme is demonstrated by particle-in-cell simulations.

Theoretical investigations show a dc bias in plasmas can efficiently produce terahertz radiation with the frequency, amplitude, polarization, and initial phase controllable. The radiation is strengthened considerably for the bias at the plasma frequency.

Single-cycle strong terahertz pulses can be generated by irradiating ultrashort intense laser pulses onto a tenuous plasma slab. At the plasma surface, laser ponderomotive force accelerates electrons and induces net currents, which radiate terahertz pulses. Our theoretical model suggests that if ${\ensuremath{\tau}}_{L}g2\ensuremath{\pi}/{\ensuremath{\omega}}_{p}$, with ${\ensuremath{\tau}}_{L}$ as the laser-pulse duration and ${\ensuremath{\omega}}_{p}$ as the plasma frequency, the emission frequency is around ${\ensuremath{\tau}}_{L}^{\ensuremath{-}1}$. On the other hand, the emission frequency is around ${\ensuremath{\omega}}_{p}/2\ensuremath{\pi}$ if ${\ensuremath{\tau}}_{L}l2\ensuremath{\pi}/{\ensuremath{\omega}}_{p}$. Our numerical simulations support the theoretical model, showing that such a terahertz source is capable of providing megawatt power, field strengths of MV/cm, and broad frequency tunability.

Mechanism of terahertz (THz) pulse generation in gases irradiated by ultrashort laser pulses is investigated theoretically. Quasi-static transverse currents produced by laser field ionization of gases and the longitudinal modulation in formed plasmas are responsible for the THz emission at the electron plasma frequency, as demonstrated by particle-in-cell simulations including field ionization. The THz field amplitude scales linearly with the laser amplitude, which, however, holds only when the latter is at a moderate level. To overcome this limitation, we propose a scheme using chirped laser pulses irradiating on tenuous gas or plasma targets, which can generate THz pulses with amplitude 10-100 times larger than that from the well-known two-color laser scheme, enabling one to obtain THz field up to 10MV/cm with incident laser at approximately 10(16)W/cm(2).

High-speed atomic force microscopy (HS-AFM) enables visualizing dynamic behaviors of biological molecules under physiological conditions at a temporal resolution of 1s or shorter. A small cantilever with a high resonance frequency is crucial in increasing the scan speed. However, detecting mechanical resonances of small cantilevers is technically challenging. In this study, we constructed an atomic force microscope using a digital versatile disc (DVD) pickup head to detect cantilever deflections. In addition, a flexure-guided scanner and a sinusoidal scan method were implemented. In this work, we imaged a grating sample in air by using a regular cantilever and a small cantilever with a resonance frequency of 5.5 MHz. Poor tracking was seen at the scan rate of 50 line/s when a cantilever for regular AFM imaging was used. Using a small cantilever at the scan rate of 100 line/s revealed no significant degradation in the topographic images. The results indicate that a smaller cantilever can achieve a higher scan rate and superior force sensitivity. This work shows the potential for using a DVD pickup head in future HS-AFM technology.