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Picosecond laser with high-repetition-rate and high pulse energy is widely favorite in many scientific and industrial applications. Some nonlinear crystals can be used to efficiently convert a near-infrared laser into a green laser or an ultraviolet laser which has a higher photon energy and a smaller focal area. Especially for high-quality and high-speed transparent hard material fabrication, green or ultraviolet picosecond laser has been found to possess unique advantages. In this paper, the experiments on high-efficiency second-harmonic-generation (SHG) and third-harmonic-generation (THG) by using a home-made all-solid-state picosecond laser amplifier and an LBO crystal are reported. The picosecond laser amplifier consists of a seed source, a regenerative amplifier and a two-stage single-pass amplifier. The seed source is a commercial all-solid-state picosecond oscillator with a pulse duration of 8.3 ps and a repetition rate of 68 MHz. The repetition rate is reduced from 68 MHz to 500 kHz by an electro-optic Pockels cell (PC), and the period doubling bifurcation is minimized by reducing the duration of high voltage in PC. Both the regenerative amplifier and the two-stage single-pass amplifier are pumped by three 30-W continuous-wave fiber-coupled laser diodes. After the regenerative amplifier, the seed laser is amplified to 4.86 W with a repetition rate of 500 kHz at 1064 nm. Then the laser power is increased to 23.2 W by a two-stage single-pass amplifier, and the M2 value of the amplified laser in the X direction and in the Y direction are 1.330 and 1.235, respectively. The final pulse duration is 13.4 ps, which is slightly stretched in the amplification chain compared with the seed pulse duration (8.3 ps). For high-efficiency SHG and THG from near-infrared to green and ultraviolet, we carefully study the optical characteristics of some nonlinear crystals, such as LBO, BBO, BIBO, CLBO, etc., and we find that the LBO crystal, which has a high damage threshold, small walk-off and high nonlinear coefficient, is the best choice for both SHG and THG. Then the parameters of the two crystals for SHG and THG are specially designed according to the phase matching condition, the walk-off and the laser parameter. As a result, a 4-mm-long type-I phase matching LBO with cutting angle of θ = 90° and φ = 11.6° is used for SHG, and a 3-mm-long type-II phase matching LBO with cutting angle of θ = 42.2° and φ = 90° is used for THG. Finally, we realize high-efficiency frequency conversion with SHG power of 12.7 W at 532 nm and THG power of 9.25 W at 355 nm. The corresponding optical-optical conversion efficiencies reach 54.7% and 39.6%, respectively.

This paper demonstrates the triggering and guiding of the stationary high voltage (HV) discharges at 5–40 kV by using plasma filaments generated by femtosecond laser pulses in air. A significant reduction of the breakdown voltage threshold due to the pre-ionization of the air gap by laser filamentation is observed. The discharge experiments are performed by using laser pulses with different energy from 15–60 mJ. The electron density of filaments is detected by sonography method. The influence of the electron density of laser filaments on the triggering and guiding HV discharge is experimentally investigated. The results have shown that the behaviour of plasma filaments can strongly affect the efficiency of triggering and guiding HV discharge.

A novel reflected terahertz-emission microscopy is proposed and developed for improving the spatial resolution of THz imaging. When attaching a bow-tie antenna directly onto a thin generation crystal, the reflected THz waves can be collected and detected by a photoconductive antenna, and the spatial resolution is decided by the diameter of focused pump beam. In this way, the detected resolution can be largely improved and tunable. The configuration and characteristics of this microscopy are described in detail.

We investigate the effects of confined laser ablation on laser plasma propulsion. Compared with planar ablation, the cavity ablation provides an effective way to obtain a large target momentum and a high coupling coefficient. When laser pulses are focused into a cavity with 1 mm diameter and 2 mm depth, a high coupling coefficient is obtained. By using a glass layer to cover the cavity, the coupling coefficient is enhanced by 10 times. Meanwhile, it is found that with the increase of the target surface size, the target momentum presents a linear increase.

Long plasma channels induced by femtosecond laser pulses in air are diagnosed using the sonographic method. By detecting the sound signals along the channels, the length and the electron density of the channels are measured. Refocusing is also observed at different laser energies and different focal lengths. We find that the sonographic method has manifest advantages compared to other techniques.

Spatial distribution of hot electrons with energies above 50 keV are investigated by an ethanol micro-droplet spray irradiated by linearly and elliptically polarized 150 fs laser pulses at an intensity of 1016W/cm2. Two symmetric hot electron jets with respect to the laser propagation direction are observed in the polarization plane for a linearly polarized laser field and in the plane of the long electric vector for an elliptically polarized laser field, respectively. Particle-in-cell simulations suggest that the resonance absorption on the spherical surface of the droplets is mainly responsible for the generation of the double-jet emission of hot electrons.

Ultra-short and ultra-intense laser is one of the hottest research spot of laser technology and strong field physics, due to its challenging and the frontier application research. As the key specification of ultrafast ultrahigh intensity laser pulse, the contrast ratio is very influential on the effect of laser-matter interaction. To perform the laser-matter interaction experiments at a high power level, the contrast is required to be as high as 1010 to prevent preplasma dynamics. To solve these problems, one has proposed many methods to improve the contrast of ultrafast laser, such as using the saturable absorbers, double chirped pulse amplification, plasma mirrors and the cross-polarized wave (XPW) generation. The XPW technology can not only enhance the contrast of the pulse by 3-4 orders of magnitude without introducing any space dispersion, but also extend the output spectrum to support shorter pulse duration. The XPW is a nonlinear filter technique in third-order nonlinear crystal with anisotropic susceptibility. Because of its simple and all-solid-state structure, the XPW technique has become one of the most effective methods to enhance the temporal pulse contrast and deliver shorter pulse duration in the field of high peak-power ultrafast lasers. This method has been used in many large laser facilities under construction or upgrades, such as the Apollon and ELI, the contrast ratio as high as 1010 has been achieved. It is known that the conversion efficiency and spectral characteristics of XPW have a strong dependence on the spatial and temporal magnitudes of the input driving pulse. In our experiment, it is found that the various changes of the driven pulse properties have different influences on the characteristics of XPW pulses. The relationship between the linear dispersion of driven pulse and temporal property of XPW is investigated theoretically. In addition, an experiment on verifying the theory is conducted by taking advantage of a programmable acousto-optic dispersion filter. The experimental results fit well to the theoretical results while some new phenomena emerge when the intensity in the BaF2 crystal reaches a saturation threshold. The spectral broadening capability of XPW becomes stronger and exceeds a theoretical upper limit. The pulse width can also be compressed to shorter than the theoretical limit. It is found that there are significant differences in spectral shape and conversion efficiency between the XPW signals by applying the opposite linear chirps to the driving pulse. A further analysis and theoretical explanation of these new phenomena are also presented.

With advent of chirped-pulse amplification, the peak power of femtosecond laser pulse was reached to petawatt (PW) or hundreds of terawatt (TW). Many progresses of high-field physics and ultrafast dynamics in matter are achieved using TW or PW laser. Pre-amplifier is an exponential growth amplifier which is also a bridge between oscillator and power amplifier. The best choice of pre-amplifier is amplification in regenerative cavity, due to its high stability and beam quality. The quality of pre-amplified laser pulse is significant to efficiency and beam quality of the successive power amplifier. High energy pre-amplifier with high beam quality will reduce the requirement of pump laser in final power amplifier. But typical regenerative amplifier only support low output energy of few millijoule. Higher energy from only one regenerative amplifier is crucial to whole laser system. High energy regenerative amplifier can be achieved by increasing the size of TEM00 in cavity. A new femtosecond Ti:sapphire regenerative amplifier with large mode size was demonstrated in this letter. The regenerative cavity is designed as stable linear resonator in which end mirrors are planar, the diameter of beam waist in Ti:sapphire crystal is larger than 2 mm, which can support high energy pulse amplified in cavity. By matching the focal spot of pump laser with the size of mode and optimization of cavity, the output laser energy up to 17.4 mJ was achieved under the pump energy of 60 mJ at repetition rate of 10 Hz, which corresponds to the efficiency of 29%. The amplified laser pulse from regenerative amplifier was compressed in a grating-pair compressor. By carefully alignment of incident angle and distance between the two gratings of compressor, the shortest pulses duration of 40.6 fs and energy of 13.9 mJ are obtained, which is a little bit longer than Fourier-transform limit based on spectrum of laser. The dispersion in the CPA laser system was also analyzed, after optimization of compressor, there are still high order dispersions uncompensated, which results in the duration of compressed pulses longer than Fourier-transform limit. Based on this large mode size regenerative amplifier, peak power of 1.9 TW laser pulses which compressed pulse energy of 81.4 mJ in 43 fs were also further realized by following only one stage of multipass amplifier. The beam quality (M2) was measured to be 1.6 and 1.5 in X and Y directions respectively, and the energy stability is 2.15% (rms). The results show that this large mode size regenerative amplifier is an ideal choice of pre-amplifier in TW laser system.

An optical pulse autocorrelator for rapid and slow scanning is described in this paper. Using an audio loudspeaker on one arm, an interferometric rapid-scanning signal of the output from a high-repetition laser oscillator is obtained. However, by adjusting the positions of the mirrors and using a step-motor on another arm, the intensity autocorrelation function of the output from a low-repetition laser amplifier can be easily measured. Using all-reflecting optics and an adequate nonlinear crystal, the whole instrument is very compact and has been used to measure sub-20 fs light pulses in both configurations with excellent agreement. In the slow-scanning configuration, a pulse train as long as 500ps has been determined. Using this autocorrelator, the home-made JIGUANG-I CPA laser facility was characterized for its pulse duration evolution.

Supercontinuum (SC) generation from laser filamentation in air is found to depend strongly on the pulse duration. Rainbow-like SC generation is observed only for a pulse of appropriate negative chirp that agrees with the predictions put forward by Golubtsov et al. [Quantum Electron. 33 (2003) 525]. The conversion efficiency of an 800-nm laser light to rainbow-like SC is found to be the highest for 257 fs pulses with an initial negative chirp. A larger chirp will lead to filamentation surviving at longer distance.

Strong white light emission is observed from femtosecond laser propagation in air. The divergence angle of the white light emission is measured to be about 5 mrad. Young's double-slits and a Michelson interferometer are used to investigate the coherence. The wavelength components of the white light emission are identified to have a good spatial coherence and a coherence time of about 0.5 ps.

High energy and high repetition rate femtosecond Ti:sapphire lasers are widely used in isolated attosecond pulses and high-order harmonic generation. Enhancing the driving laser energy is a convenient and effective way to improve attosecond pulse energy. A 1 kHz or higher repetition rate millijoule level femtosecond Ti:sapphire amplifier is generally used to generate isolated attosecond. However, due to the limitation of its green pump laser energy, the energy of femtosecond Ti:sapphire laser is limited to several millijoules. Appropriately reducing the requirements for repetition rate, realizing high energy driving laser will significantly improve attosecond pulse energy and extend its application scope. Meanwhile, a 532 nm pump laser from frequency doubled 1064 nm Nd:YAG flash lamp pumped laser at 100 Hz repetition rate can achieve high pump energy with lower cost. Accordingly, we develope a 100 Hz repetition rate high energy amplifier based on Ti:sapphire crystal. The femtosecond amplifier system consists of oscillator, stretcher, ring cavity regenerative amplifier, four-pass amplifier and grating compressor. The ring cavity regenerative amplifier is the first amplifier as pre-amplifier, and the four-pass amplifier is the booster amplified-stage. 80 MHz seed pulse from the oscillator has a full width at half maximum bandwidth of 61 nm with a 20 fs duration. Then the seed pulses are stretched to 200 ps with a Martinez grating stretcher, rotated to vertical polarization and injected into the regenerative amplifier. The amplified uncompressed 1 kHz repetition rate laser pulses with 3 mJ pulse energy are selected to be 100 Hz and input into the four-pass amplifier. With a pulse energy of 75.1 mJ, wavelength at 532 nm flash lamp pumped pump laser at 100 Hz repetition rate, single pulse energy up to 25.4 mJ is obtained from a Ti:sapphire crystal, corresponding to a high energy conversion efficiency of 33.8%. We believe that higher energy should be possible if the pump energy can be further increased. After expanding the beam to 10 mm in diameter, the amplified chirped pulse is compressed using a four-pass, single grating compressor, with an overall efficiency of 72%. The highest pulse energy after compression is 18.3 mJ. For a fluctuation of the 100 Hz pump laser is as high as 3.4% for over 10000 shots, the 3.6% energy stability of the amplifier has a room to be improved. The typical spectrum bandwidth after the compressor is 39 nm, which can support transform-limited pulse duration of 32.8 fs. After fine dispersion compensation by the compressor, A pulse duration of 37.8 fs is measured using a single shot autocorrelator (Minioptic Technology, Inc). In addition, the spatial profile of the output beam from the compressor is measured using a commercial laser beam analyzer (Spiricon, Inc). The beam quality M2 factor are 1.8 and 1.6 in X and Y directions, respectively. In summary, a peak power of 0.48 TW compact 100 Hz femtosecond laser with pulse duration of 37.8 fs, pulse energy of 18.3 mJ is achieved from a two-stage amplifier system based on Ti:sapphire crystal. We believe that with a more stable and better spatial profile pump source, even better performance can be obtained by this scheme. Nevertheless, the current results show that this system should be favorable for high energy attosecond pulse generation and further amplification such as Terawatt system.

High average power femtosecond lasers based on Ti:sapphire are widely used in strong-field physics and ultrafast dynamics.Continued advances include isolated attosecond pulse generation,few-cycle pulse generation,ultrafast spectroscopy,time-resolved photo-chemical reaction dynamics and laser micro-machining benefit greatly from use of such laser systems.The regenerative amplifiers are mostly utilized and have inherent advantages over multipass ones for applications in chirped pulse amplification.In this paper we describe a design,performance,and the characterizations of a novel linear cavity regenerative amplifier which has produced 4.8 W average power with 35 fs pulse durations at 1 kHz repetition rate. The main difficulty in designing and constructing a high average power Ti:sapphire regenerative cavity is thermal lensing effect.In order to generate amplified pulses with an output power exceeding 5 W at 1 kHz,a green pump power higher than 20 W is required.Meanwhile,the focal pump beam diameter on the surface of Ti:sapphire crystal should have sub-millimeter mode size to demonstrate large pump fluence,inducing a focal length of a thermal lens about 100 mm,i.e.,which is much less than the scale of the cavity length.For our experiments,a cavity mode size adjustable geometry is employed to counteract thermal lensing effect and to optimize the conversion efficiency of the amplifier.We first characterize the cavity stability by applying the well-known ABCD matrix formalism.The cavity consisting of R=900 mm concave mirror,an 2=800 mm lens and a plane mirror has two stability ranges with increasing the focal length of the thermal lens.In order to obtain a highest thermal tolerance,the optimal cavity parameters are resolved when two stability zones merge into one.After characterizing the cavity in detail,we calculate the cavity mode and the pump beam size at the position of the Ti:sapphire rod as a function of the thermal focal length.The optimal mode radius occurs at 312 m,corresponding to the intersection point of two curves.Stability curve exhibits a weak thermal sensitivity which is defined as the change of radius of cavity mode size per unit focal power change of thermal lens, keeping well below 10 m/D in a range of 2 D-4 D.The calculated results show that the active compensation for thermal lens focal length from 100 mm to could be achieved by adjusting the lens position,without changing the cavity. 20 fs,3 nJ pulses at a repetition rate of 82 MHz produced by a home-made Kerr-lens mode-locked oscillator are first sent to a Martinez stretcher by using a 1200 lines/mm holographic reflectance grating,which temporally stretches the laser pulses to 200 ps.The seed pulses out of the stretcher is then injected into the regenerative cavity depicted above. The 20 mJ pumping energy at 1 kHz is focused through the R=900 mm concave mirror into a 10 mm Brewster-cut Ti:sapphire rod,which is cooled to 250 K by thermoelectric elements.Condensation was avoided by placing the crystal into a small evacuated chamber.Mode matchings of pump and laser beam are found to be of critical importance for high energy extraction efficiency and high beam quality.In our experiments it is accomplished by fine adjusting the F=800 mm cavity lens and the pump beam size.The amplified power of 6.5 W at 1 kHz is obtained with minimum beam distortion,giving a 33.6% slope efficiency.The trapped pulse is built-up quickly and saturated after 8-round trips. The beam size of the amplified laser is expanded to 15 mm in diameter before compressor.A transmission efficiency of 73.8% is achieved through the grating-pair Treacy-type compressor,leading to a 4.8 mJ pulse energy.The grating has a groove density of 1500 lines/mm,and the compressed output spectrum has a full width at half maximum of 29 nm. The pulse duration measurement is performed by using an interferometric autocorrelation.As a result,a typical autocorrelation trace corresponding to a 35 fs pulse width is displayed,and agrees well with the 32 fs transform limit.The far-field beam profile after the compressor is round and Gaussian in both s and p planes,respectively.This scheme is also sufficiently reliable and robust so that no components of the laser system were damaged over a year of operation. In summary,the theoretical analysis and experimental results show that the regenerative cavity developed in this work exhibits a high conversion efficiency and an extraordinary thermal stability,and it is very suitable for high power and high efficient amplification of femtosecond Ti:sapphire pulses.

We demonstrate a widely tunable near-infrared source from 767 nm to 874 nm generated by the intracavity second harmonic generation (SHG) in an optical parametric oscillator pumped by a Yb:LYSO solid-state laser. The home-made Yb:LYSO oscillator centered at 1035 nm delivers an average power of 2 W and a pulse duration as short as 351 fs. Two MgO doped periodically poled lithium niobates (MgO:PPLN) with grating periods of 28.5–31.5 μm in steps of 0.5 μm and 19.5–21.3 μm in steps of 0.2 μm are used for the OPO and intracavity SHG, respectively. The maximum average output power of 180 mW at 798 nm was obtained and the output pulses have pulse duration of 313 fs at 792 nm if a sech2-pulse shape was assumed. In addition, tunable signal femtosecond pulses from 1428 nm to 1763 nm are also realized with the maximum average power of 355 mW at 1628 nm.

A long-term stable sub-8 fs Ti:sapphire oscillator based on domestic chirped mirrors is reported. It outputs 300 mW mode-locked pulses at 86 MHz under 4 W pump power. The second order and third order of dispersion introduced by the components of the oscillator are analyzed. Two pairs of domestically designed and fabricated chirped mirrors are utilized to compensate the dispersion introduced by the crystal and the air in cavity. By precisely controlling the dispersion of chirp mirrors, the output pulses have an ultra-broad bandwidth exceeding 150 nm (FWHM) without the insertion of wedges. With the assistance of extra-cavity dispersion compensation, a pulse duration of 7.9 fs is achieved. This is the shortest pulse duration ever reported by using domestic chirped mirrors, and the shortest pulse duration achieved without intra-cavity wedges, to the best of our knowledge. Structure design and electronic feed-back loops are employed to improve the stability of the oscillator passively and actively, respectively. With the assistance of piezoelectric ceramic, the power stability within 24 h is measured to be 0.6%, which is significantly better than that without them.

A novel design of X-ray spectrometer is implemented, using a high efficiency HOPG (highly oriented pyrolitic graphite) Bragg crystal. The spectral resolution of it can reach above 350 with energy ranging from 6.891 to 9.193 keV, theoretically. And the efficiency are 3 order of magnitude higher than the ordinary X-ray crystal spectrometer. Application of this instrument is demonstrated in high intensity laser-foil interaction experiments. It is shown that HOPG spectrometer has much lower threshold of X-ray intensity than ordinary one. The spectral resolution for 8.048 keV (Cu Kup -line) can reach 40 eV, which gives the spectral resolution 200.

Frequency estimation based on the reconstruction algorithm of the Chinese remainder theorem(CRT) is one of the frontier focuses in the fields of signal processing, electromagnetism, and optics etc. Howerver, the existing studies can only realize a rough frequency estimation of complex exponential signals. Hence this paper generalizes the CRT-based frequency reconstruction from a rough frequency estimation of complex exponential signals to the accurate frequency estimation of sinusoidal signals. The procedure of the proposed estimation scheme is as follows: (1) Detect zero crossing point on the original high-frequency sinusoidal waveform so as to determine the ideal phase information; (2) implement fast Fourier transform(FFT) to each path's undersampled signal, and then use Candan estimator to correct the frequencies at the peak FFT spectral bins so that the frequency biases can be extracted to realize phase correction; (3) use the proposed classification method based on phase features to screen the corrected remainders; (4)substitute the filtered frequency remainders into the closed-form robust Chinese remainder theorem to obtain the high-accuracy frequency estimation of the original signal. Additionally, this paper also deduces the theoretic expressions of the frequency estimation variance, which is also verified through numerical simulation. And the experimental results also reflect that the proposed scheme possesses high precision and high robustness to noise.

We have investigated the vibrational sensitivity of a horizontal-mounted spherical reference cavity, which can be applied to develop an ultra-stable laser. Effects of different magnitudes of height and area as well as acceleration of the cavity support points on the length variation of the cavity are studied. When the cavity support points are totally constrained, the vibration sensitivity can be reduced to below 3.0×10-10/g. After performing extensive numerical simulations, we can find the optimal support position. According to the obtained results we present the mounting scheme of the spherical cavity. Taking into consideration the machining errors, near-horizontal mounting, and unsymmetrical mounting of the cavity, we can describe quantitatively the length variation of the cavity caused by these three factors. We also discuss the contribution of the second order effect to the length variation of the cavity.

In engineering applications, many measurements of physical quantities can be converted into the problems of frequency estimation. The current frequency estimators are mainly divided into two categories: iterative approaches and direct approaches. However, iterative approaches are not suitable for rapid physical measurement occasions due to its complicated process. But most of the direct approaches are the biased estimators, which are incapable of providing quantitative estimates of variance expression. To enhance the accuracy of the direct frequency estimator and derive the closed-form theoretic expression of the estimated error variance, this paper proposes a novel phase difference frequency estimator based on the forward and backward sub-segmenting. This estimator implements forward and backward fast Fourier transform (FFT) on the given samples separately, and then extracts the phase difference from the peak FFT bins of these two sub-segments to estimate the frequency. And it is emphasized that the proposed method is an unbiased frequency estimator, whose closed-form theoretic expression of the frequency estimated error variance is also derived. Moreover, simulation not only verifies the correctness of this closed-form expression but also proves that the proposed frequency estimator's mean square error is closer to the Cramer-Rao lower bound than that of apFFT/FFT phase difference estimator and Candan estimator. In conclusion, the proposed estimator has higher accuracy in measuring frequencies and has a wide application prospect.

Based on chirped-pulse amplification technology, a ring cavity Ti:sapphire regenerative amplifier with high output energy is demonstrated. Under the 532 nm pump energy of 33.1 mJ at a repetition rate of 100 Hz, the chirped laser pulse with energy of 9.84 mJ is obtained, corresponding to a slope efficiency of 33.1%. Instead, using the pump laser with energy of 32.0 mJ at a repetition rate of 10 Hz, we also obtain 9.64 mJ pulse energy with a corresponding slope efficiency of 36.8%. By optimizing the dispersion among all optical materials, stretcher and compressor, the shortest pulse has an energy of 6.36 mJ and a pulse width of 59.7 fs, and the energy fluctuation is 1.85 % (RMS) over 4000 shots after the compressor. The performances show that it may pave the way for ultrafast applications and serves as a front stage toward TW even PW laser system with high contrast ratio.

We demonstrate a 100-TW-class femtosecond Tr:sapphire laser running at a repetition rate of 0.1 Hz based on a 20 TW/10 Hz laser facility (XL-II). Pumping the new stage amplifier with a 25J green Nd:glass laser, we successfully improve the laser energy to 3.4 J with duration of 29 fs, corresponding to a peak power of 117 TW.

A real-time ion spectrometer mainly based on a high-resolution Thomson parabola and a plastic scintillator is designed and developed. The spectrometer is calibrated by protons from an electrostatic accelerator. The feasibility and reliability of the diagnostics are demonstrated in laser-driven ion acceleration experiments performed on the XL-II laser facility. The proton spectrum extrapolated from the scintillator data is in excellent agreement with the CR39 spectrum in terms of beam temperature and the cutoff energy. This real-time spectrometer allows an online measurement of the ion spectra in single shot, which enables efficient and statistical studies and applications in high-repetition-rate laser acceleration experiments.

We set up a multi-wavelength ultraviolet terawatt-order chirped pulse amplification(CPA) system according to the CPA technology and the frequency conversion technology. The system offers 1Hz, 5Hz and 10Hz high energy ultraviolet pulses by precise time synchronization among 5 repeat frequencies. The RMS of output energy fluctuation rate of CPA is less than 2% with using a pre-amplified system of energy fluctuation rate less than 0.3% RMS and a time synchronizer with time jitter less than 3 ns. The multi-wavelength high energy ultraviolet pulses with 64 mJ/400 nm,16 mJ/267 nm and 5 mJ/205 nm are obtained, and their corresponding peak powers reach the terawatt level.

According to the theory of chirped pulse amplification (CPA), we analyze numerically the gain characteristic of laser pulses in multi-pass amplifier. A CPA Ti:sapphire laser with 10-pass scheme is designed and built. The experimental result shows that the contrast ratio of the femtosecond pulse is obviously enhanced by two orders from 10-5 to 10-7 under a pump flux of 1.6 J/cm2. This work demonstrates that the contrast ratio of the amplified femtosecond laser pulse could be effectively improved by multi-pass preamplifier.

This paper reports our experimental research on the laser contrast ratio enhancement in the XL-Ⅲ (eXtreme Light Ⅲ) facility with the technology of cross-polarized wave (XPW) generation. Based on the theoretical analysis of the relerant principles, we introduced an XPW filter in the facility, which was upgraded with a double chirped pulse amplification (DCPA) scheme. Under optimized system design and alignment, we measured the conversion efficiency from fundamental wave to cross-polarized wave to be higher than 10%. The pre-pulses in nanosecond scale were deeply suppressed and the contrast ratio in picosecond scale was enhanced from 10-5 to 10-7. Our results show the XPW generation provides an effective technique to improve the contrast ratio of a Chirped-pulse amplification laser system, which is a key issue in ultrahigh intensity laser-matter researches.

A Ni-like Mo soft x-ray laser (SXRLs) operating at 18.9nm has been demonstrated by employing a grazing incidence pumping scheme with 120mJ in the 200ps pre-pulse and 140mJ in the 200fs main pulse. The SXRL gain is estimated to be 1.5–3cm−1 when a grazing incidence angle of 14° is applied. Numerical simulations are also performed to investigate the dynamics of the ion distribution. It is found that a high intensity at 2.4 × 1014 W/cm2 of the 200 fs main pulse could heat the pre-plasma rapidly to an appropriate temperature for population inversion, and could compensate for the shortage of the total pump energy to a certain extent.

The pulse width generated by an ultra-broad-bandwidth Ti: sapphire oscillator is measured using SPIDER technique. The measurement result is 7.86 fs, which is, as presently reported in China, the shortest pulse output directly from an oscillator in which only chirped mirrors was used to compensate for the laser dispersion. Layout of the oscillator and the SPIDER setup are introduced in details, and the spectral phase of the laser pulse obtained through calculation is discussed.

We triggered and guided the stationary electric field discharges in 3—23 cm long air gaps by using plasma channels produced by 40mJ ultrashort laser pulses. We have observed that the plasma channel reduced the discharge breakdown voltage to 40% of its natural value. By analyzing the delay between the laser pulse and the white light emission of discharge, we evaluated the speed of the stepped leader with laser guiding to be about 107 cm/s.

As a novel inner surface processing method, grid enhanced plasma source ion implantation (GEPSII) method can realize uniform inner surface ion implantation of a metal tube and metal binary compound can be produced. In the experiment, three 0.45% carbon steel (45-steel) work pieces were axially put on the inner surface of the tube, and golden titanium nitride (TiN) films were produced on the work pieces by GPSII. Structure analysis shows that the TiN films have the preferred crystal faces (111) and (200), and the depth of the films is about 20 nm with partial embedding in the substrate. Analysis on the electrochemistry corrosion, hardness and friction of the work pieces indicates that the surface property of the 45-steel has been greatly improved by the protective TiN film, and very good axial uniformity has been obtained.

THz (terahertz) properties of MgF2 crystal (sample 1) and MgF2 : Co crystal (sample 2) is measured using transmission THz time domain spectroscopy in the frequency range between 0.5 and 2.5 THz. The absorption coefficients a(v) of sample 1 increases and has a maximum, 24 cm(-1) with increasing frequency v from 0.5 THz to 2.5 THz. The absorption coefficient of sample 2 is much larger than that of the sample I, and its absorption band edge of the crystal lattice shifts to lower frequency side because of the dopant Co. Moreover, there is an absorption peak at 1.9THz and the absorption coefficient is greater than 70 cm(-1), the force constant K of F- -Co2+ ionic chemical bond stretch vibration calculated from this peak is 3.40 x 10(-2) N/cm. The THz spectroscopy can be an important technique for research of chemical bond in crystal. Real dielectric functions between 0.5 and 2.5 THz of the two samples are calculated by relations between optical constants. The epsilon(1) (v) of sample 1 varies from 4.67 to 4.73 and epsilon(1) (v) of sample 2 varies from 4.62 to 5.01.

The characteristics of Kα X-ray sources generated by p-polarized femtosecond laser-solid interactions are experimentally studied in the relativistic regime. By use of knife-edge image technique and a single-photon-counting X-ray CCD camera, we obtaine the source size, the spectrum and the conversion efficiency of the Kα X-ray sources. The experimental results show that the conversion efficiency of Kα photons reaches an optimum value of 7.08×10-6/sr at the laser intensity of 1.6×1018W/cm2, which is different from the Reich's simulation results (Reich et al., 2000 Phys. Rev. Lett. 84 4846). We find that about 10% of laser energy is converted into the forward hot electrons at the laser intensity of 1.6×1018W/cm2.

The angular distribution and energy spectrum of hot electrons produced in the interaction of ultra-short ultra-intense laser pulses with solid foil targets are investigated. Results show that bath the angular distribution and the energy spectrum reveal anisotropic behavior. This anisotropy is believed to be related to the electron generation mechanism. The existence of energy anisotropy explains why different experimental groups obtained quite different elctron temperatures under the same laser intensity.

Nanocomposite films composed of Ag particles embedded in BaTiO3 matrix were grown on MgO(100) substrates by pulsed laser deposition. The structures of the films were determined by X-ray diffraction. The size and shape of the Ag particles were observed by transmission electron microscopy. The X-ray photoelectron spectroscopy results showed that Ag is in the metallic state. The optical absorption properties were measured from 300 to 800 nm, and the absorption peaks due to the surface plasmon resonance of Ag particles were observed. With increasing annealing temperature and Ag concentration, the peak absorption increased and shifted to longer wavelength (red-shift)

Energy spectrum and angular distribution of fast electrons are measured in the interaction of linearly (P and S) and circularly polarized femtosecond laser pulses with an aluminum target. It is interesting to note that the angular distribution of backward going electrons is unique for S and circular polarization. For S polarization, the angular distribution of fast electrons can be explained by taking account of the surface magnetic field qualitatively.

A long plasma channel can be formed when ultra-intense femtosecond laser pulse propagate, in air. We measured the value of the resistivity of the plasma channel and the contact resistance between the plasma channel and the electrodes. We also investigated the main factors affecting the resistivity of the plasma channel.

Strong third-order harmonic (TH) emission is observed from long plasma channels formed when intense femtosecond laser pulses propagate in air. The conversion efficiency between the fundamental wave and the TH emission is roughly constant over the length of the plasma channels. The divergence angle of the TH emission in the conical emission of the plasma channels is also investigated, which is in good agreement with calculations.

Long plasma channels can be formed when intense femtosecond laser pulses propagate in air. In contrast to other conventional diagnostic techniques, the acoustic method to probe the plasma channel has its advantages. By measuring the sound signals along the channel, the channel length and the distribution of electron density along it can be retrieved easily.

The crystal structure of non-metamict Ti- and Fe2+-rich chevkinite-(Ce) has been redetermined with the single-crystal sample collected from Bayan Obo, Inner Mongolia, China. The chemical formula of the sample is Ce4Fe2Ti3Si4O22. The crystals are monoclinic with the unit cell parameters a = 13.4656(15) angstrom, b = 5.7356(6) angstrom, c = 11.0977(12) angstrom, beta= 100.636(2)degrees, V = 842.39 (16) angstrom(3) and Z = 2. The structures of Ti- and Fe2+-rich chevkinite-(Ce) were refined with space groups P2(I)/a and C2/m. Least-squares refinement results show that both structural models of Ti- and Fe2+-rich chevkinite-(Ce) are very good, R[F-2> 2 sigma(F-2)] =0.027 with P2(1)/a and R[F-2> 2 sigma(F-2)] =0.021 with C2/m. In order to illustrate the relationship between the two space groups P2(1)/a and C2/m, the distribution of diffraction intensities was inspected. Pseudo extinction was found, i.e., reflections with h+k=2n are systematically strong, while those with h+k=2n+1 are weak. By neglecting the systematically weak (h+k=2n+1) reflections the space group becomes C2/m. There is a mirror plane in the C2/m perpendicular to the b axis. However, oxygen atoms in the P2(1)/a model are of a symmetrical relationship with the corresponding pseudo mirror plane. It is concluded that the crystal structure of non-metamict Ti- and Fe2+-rich chevkinite-(Ce) is a superstructure with the space group of P2(1)/a, which is of pseudo symmetry corresponding to the space group C2/m.

In this paper, nonlinear frequency conversion of second harmonic and third harmonic of femtosecond Ti:sapphire laser is theoretically and experimentally studied. Noncolinear type Ⅰ phase matching process is chosen to generate third harmoni c of femtosecond Ti:sapphire laser, and we have obtained conversion efficiency o f 62% at 260nm output. The pulse duration is about 169fs. This work will s erve as femtosecond ultraviolet source for time_resolved electron microscopy.

Based on the theory of multi_pass amplification, we analyse numerically the character of gain for amplified pulse and design a 6_pass amplifier. With two 13J, 532nm Nd:YAG lasers as the source, amplified pulse energy as high as 1J was obtained, which corresponds to a 38% conversion efficiency.

A frequency_resolved optical gating (PG_FROG) with polarization gating technique is developed to diagnosis the characteristics of laser pulses generated from a 20_TW femtosecond laser amplifier. Based on the data of the correlation signal in time and frequency domains, we obtained the detail information on pulse shape, bandwidth, pulse duration and phase by using the retrieval algorithm. The measured results with different phase compensation are also presented and discussed.

A low-threshold self-starting Kerr-lens mode-locked Ti:sapphire laser was demons trated by use of a tight focusing cavity design and a semiconductor saturable-ab sorber mirror (SESAM). With a 3% and a 12% output couplers, we achieved mode-loc king thresholds as low as 390 and 600mW, respectively. Stable femtosecond laser pulses with average power of 114mW were generated for pump power at 12W, which corresponds to a typical duration of 17fs and bandwidth of 47nm. The pump power range for mode-locking covers from 600 mW to 48W at a 12% output coupler, an d the mode-locking stability is improved.

The dispersive character of ?ffner stretcher was studied, and a concise expression of output pulse duration was presented. An ?ffner stretcher was setup experimentally, which could stretch a 19fs seed pulse to 605 ps after 4 passes in it free of aberration.

The angular distribution and the energy spectrum of hot electrons with energy over 50keV emitted from ethanol droplets irradiated by linearly polarized 150 fs laser pulses at the intensity of 10 16W/cm2 have been measured. The angular distribution of hot electrons is found to be dependent on the laser polarization. Two hot electron jets like double leaves symmetrically with respect to the laser propagation direction are observed within the polarization plane. The maximum energy of the hot electrons is found to be more than 750 keV. A model based on the resonance absorption can be used to interpret the above distribution. The calculation agrees well with the experimental results.

Vertical cavity surface emitting lasers (VCSEL's) lase in a single longitudinal mode, have a circularly symmetric output beam that can be very tightly focused, offer extremely high coupling efficiencies into optical fiber, and can be mass produced in two - dimensional arrays relative to the conventional edge-emitting semiconductor laser. To obtain a high optical output power, besides enlarging of VCSEL's lasing area, the key points are proper number of quantum wells, homogeneous distribution of current density in the active layer and good thermal control. The dependence of VCSEL's performance on the number of quantum wells, the active diameter, the thermal conductivity and the electric resistance of the material; and the distance between electrical contacts are investigated. By optimizing the design parameters for a 980 nm In0.2Ga0.8As/GaAs VCSEL, a maximum continuous wave (cw) output power of 1.95 W at room temperature is obtained, which to our knowledge is the highest cw output power of any single VCSEL so far.

We present a measurement of femtosecond laser pulses using the technique of second harmonic generation frequency-resolved optical gating (FROG) in this paper. Based on the data of the correlation signal in time and frequency domains, we obtained the detailed information on pulse shape, bandwidth, pulse duration and phase by using the retrieval algorithm program. The pulse duration is in reasonable agreement with the measurements using an interferometric autocorrelator.

A cavity-dumped Kerr-lens Mode-Locked (KLM) Ti:S laser is demonstrated with an all solid-state continuous-wave 532nm laser as the pump resource. Stable mode-locked laser pulse with a peak power of 4.5MW was obtained under 5.4W pump power. The pulse duration width is shorter than 18fs and the repetition rates can be tuned as low as 20kHz. To our knowledge, this is the lowest repetition rate and the most compact size from the solid-state cavity-dumped femtosecond laser.

A plasma channel up to 80 Rayleigh lengths is formed when intense femtosecond laser pulses propagate in air. Two methods, fluorescence measurement and acoustic diagnostics, are used to probe the plasma channel simultaneously. Both methods reveal the evolution of the channel in detail. Compared with the fluorescence measurement, the acoustic method is a convenient approach having higher sensitivity and higher spatial resolution in detecting long plasma channels.