Characteristics of terahertz coherent transition radiation generated from picosecond ultrashort electron bunches

This paper presents a method of generating terahertz (THz) coherent transition radiation (CTR) from picosecond ultrashort electron bunches including single and train bunches,which are produced by a photocathode radio frequency gun.The radiation characteristics of THz CTR including formation factor and energy spectrum are analysed in detail.With the help of a 2-dimensional particle-in-cell simulation,the radiation characteristics including power,energy and magnetic field are analysed.The results show that the radiation frequency can be adjusted by tuning the repetition frequency of the train bunch and the energy can be enhanced with the train bunches.

Grain yield and lodging-related traits of ultrashort-duration varieties for direct-seeded and double-season rice in Central China

Lodging is the most common constraint on grain yield of direct-seeded rice.There is limited information about lodging resistance and its related plant traits in direct-seeded and double-season rice(DDR)in Central China.This study aims to identify the plant traits that achieve high lodging resistance in ultrashort-duration varieties(about 95 days)of DDR.Field experiments were conducted in 2017 and 2018 in Wuxue County,Hubei Province,China,with four ultrashort-duration varieties grown under two nitrogen(N)rates.Lodging-related traits were measured on the 15 th day after heading,and yield and yield attributes were measured at maturity.The grain yield of the four varieties ranged from 4.59 to 7.61 t ha^(-1)across the two N rates,with a total growth duration of 85 to 97 days.Varietal differences in lodging index were mainly explained by the bending moment,which was closely related to plant height.Breaking resistance did not affect the lodging index significantly.Shortening plant height from 95.4 to 80.5 cm decreased the lodging index by 22.4%but did not reduce grain yield.Our results suggested that reducing plant height was effective in improving the lodging resistance of ultrashort-duration varieties of DDR.Lodging resistance should be enhanced by improving breaking resistance rather than reducing plant height to increase DDR grain yield further.

Adiabatic Enhanced Compression of Ultrashort Pulses in Fibres with Slowly Decreasing Dispersion

We investigate the adiabatic enhanced compression of ultrashort pulses from a distributed feedback laser operated at 1550 nm in fibres with slowly decreasing dispersion (FSDD). High-quality pulse compressions from 4.4 ps down to 1.1ps in a l00m FSDD and from 4.4ps down to 830fs in a 500m FSDD are obtained. The Raman self frequency shift of femtosecond pulses in the compression is observed. Theoretical analyses are in good agreement with experimental results.

Quasistatic Magnetic Field Generation by an Intense Ultrashort Laser Pulse in Underdense Plasma

The quasistatic magnetic field created in the interaction of intense ultrashort laser pulses with underdense plasmas has been investigated by two-dimensional particle simulation.The relativistic ponderomotive force and plasma wave excited in self-modulation processes can drive intense electron current mainly in the propagation direction.As a result,an azimuthal,multi-mega Gauss order quasi-static magnetic field can be generated around the laserbeam.

Scattering of ultrashort laser pulses on plasmons in a Maxwellian plasma

On the basis of equations obtained in the framework of second-order quantum-mechanical perturbation theory,the standard approach to the calculation of scattering radiation probability is extended to the case of ultrashort laser pulses.Weinvestigate the mechanism of the appearance of plasmon peaks in the spectrum of the plasma form factor for different parameters of the problem.For the case in which scattering on plasmons dominates over scattering on electron density fluctuations caused by chaotic thermal motion,we derive analytical expressions describing the scattering probability of ultrashort laser pulses on plasmons.Together with this,we obtain a simple expression connecting the frequency of scattered radiation and the energy transmitted from the incident pulse to plasmon,and vice versa.In considering the scattering probability,our emphasis is on the dependence on the pulse duration.Weassess in detail the trends of this dependence for various relations between pulse carrier frequency and plasmon energy.

Visualizing the melting processes in ultrashort intense laser triggered gold mesh with high energy electron radiography

High-energy electron radiography(HEER)is a promising diagnostic tool for high-energy-density physics,as an alternative to tools such as X/γ-ray shadowgraphy and high-energy proton radiography.Impressive progress has been made in the development and application ofHEER in the past fewyears,and its potential for high-resolution imaging of static opaque objects has been proved.In this study,by taking advantage of the short pulse duration and tunable time structure of high-energy electron probes,time-resolved imaging measurements of high-energy-density gold irradiated by ultrashort intense laser pulses are performed.Phenomena at different time scales frompicoseconds to microseconds are observed,thus proving the feasibility of this technique for imaging of static and dynamic objects.

Analysis of Pain Scores and Rehabilitation of Patients with Cervical Spondylotic Radiculopathy Receiving Massage Combined with Traction and Ultrashort Wave

OBJECTIVE: To evaluate the pain scores and rehabilitation of patientis with cervical spondylotic radiculopathy receiving massage therapy combined with traction and ultrashort wave therapy. METHODS: A total of 84 patients with cervical spondylotic radiculopathy treated in 105 Hospital of People's Liberation Army from June 2014 to June 2017 were included and divided into study group and control group according to different treatment regimens. A total of 42 patients in the study group was treated with cervical traction, ultrashort wave and massage therapy at the same time, whereas the other 42 patients in the control group were only treated with cervical traction and ultrashort wave. Comparison of the clinical efficacy and adverse reactions of the 2 groups was made and the Visual Analogue Scale (VAS) scores before and after treatment were observed to assess the patient's pain. Comparison of rehabilitation in the 2 groups was made by applying the Clinical Assessment Scale for Cervical Spondylosis (CASCS). RESULTS: After comparing the therapeutic effect between the study group and the control group, it showed that the total effective rate (90.48%) in the study group was significantly higher than that in the control group (73.81%), the difference was statistically significant (P < 0.05). By comparing the incidence of adverse reactions in the study group and the control group, results showed that the incidence of adverse reactions such as nausea, palpitations, vomiting, sweating, dizziness and colorless complexion in the study group (9.52%) was significantly lower than that in the control group (21.43%), the difference was statistically significant (P < 0.05). After the treatment, the VAS pain scores of the study group and the control group were both significantly improved (P < 0.05), but the improvement of the patients in the study group was more significantly (P < 0.05), and the difference was statistically significant. After treatment, the CASCS scores of the patients in the study group and the control group were significantly improved (P < 0.05), but the improvement of the study group was more (P < 0.05), and there was statistically significant difference. CONCLUSION: The combination of massage, traction and ultrashort wave therapy can significantly reduce the pain and reduce the incidence of adverse reactions in patients with cervical spondylotic radiculopathy, which is of great clinical significance to the rehabilitation of patients.

Theoretical investigation on generating terahertz radiation from gas plasma induced by three-color ultrashort lasers

Generation of intense broadband terahertz(THz) waves from gas plasma induced by tri-color ultrashort(fundamental(ω), second harmonic(2ω), and third harmonic(3ω)) laser pulses is theoretically investigated. Simulation results show that the 3ω laser pulse can greatly enhance or suppress the generation of THz wave at different values of relative phase(θ 3) between the 3ω and ω fields. Moreover, the polarities of the generated THz waves can be controlled by changing θ 3,with the relative phase θ 2(between the 2ω and ω fields) fixed to be a certain value. All of our results show that θ 3 plays a key role in the generation process, which promises to control the intensity as well as the polarity of gas plasma-induced THz radiation.

Dynamics of Interaction of H2+ with Intense Ultrashort Laser Pulse

The dynamic process of H2+ in intense ultrashort laser pulses with intensities of 1014 ~ 1015 W/cm2, wavelength of 532nm and duration of 22fs is theoretically investigated by using collinear model and numerically solving the time-dependent Schrodinger equation. The results show that: (1) the dissociation and dissociative ionization compete each other in the case of intensities between 2.5 × 1014 and 4.0 × 1014 W/cm2;(2) both the dissociation and the dissociative ionization are much smaller and appear at about 5 cycles (≈9fs) later than the ionization. In addition, we estimate the spectrum of high harmonic generation and find that the well-known cutoff rule is invalid for the ultrashort laser pulse (e.g. the duration ~ 25fs), because the shorter pulse can produce higher order harmonics than the longer one.

Temporal Behaviour of Harmonics from One-Dimensional H+2 in an Ultrashort Laser Pulse

With the method of wavelet transform,we consider the temporal behaviour of high-order harmonic generation from one-dimensional H+2 exposed to an ultrashort laser pulse with a duration of tens of femtoseconds.The results,which are calculated by numerically solving the corresponding time-dependent Schrödinger equation with the split-operator method in the non-Born-Oppenheimer approach,show that:(1)The high-order harmonics in the cut-off range emitted as a train of pulses have better coherence than those in the plateau.(2)The harmonics are emitted early in time when the intensity of laser pulse increases.

Fabrication of Micro Structures by Ultrashort Voltage Pulse Electrical Discharge Machining

The machining of small holes and array holes has been a difficult problem in machining field. Tiny hole is widely used in mechanical field, for instance, fuel injection nozzles, spinneret holes for synthetic fibers and wire drawing dies. This paper investigated the application of EDM (electrical discharge machining) to the fabrication of micro structures. There are obvious limitations in the process of micro-electrical discharge machining, such as electrode wear, unstable discharge condition and low machining efficiency. The effects of EDM parameters were investigated, such as voltage, pulse frequency, and frequency of ultrasonic vibration applied to electrode. Micro holes were machined with Pt electrode made by focused-ion-beam chemical vapor deposition (FIB-CVD) and Cu electrode made by wire-electrode cutting. The comparison experiments between EDM and ECM (electrochemical machining) indicated that the processing of ECM has serious stray current corrosion and poor machining precision. Moreover, the workpiece vibration was firstly proposed to be utilized in the micro-electrical discharge machining. It can be concluded that maximum machine could be obtained under the amplitude ratio of 76%, which was an appropriate parameter.

A Novel Nanotube-Based Fiber Laser for Ultrashort Pulse Generation and Fast Measurements

We propose a nanotube-based erbium-doped fiber laser that can deliver conventional soliton (CS) and stretched pulse (SP) based on D-shaped fiber saturable absorber (DF-SA) where evanescent-field interaction works. The novel Nanotube-based Fiber Laser can generate SP or CS by tuning pump power and polarization controller (PC) properly. The net cavity dispersion of laser is slightly negative. In our experiment, by optimizing the PC in the cavity, CS and SP can be obtained at the central wavelengths of 1530.6 nm and 1530.3 nm due to on carbon nanotubes and the spectral filtering effect induced by nonlinear polarization rotation. Although the acquired CS and SP nearly have the same central wavelengths, they show distinct optical spectra, 3-dB bandwidths. The proposed fiber laser with switchable CS and SP is attractive for ultrashort pulse generation and fast measurements in practical applications.

Effect of ultrashort wave therapy combined with antioxidant and neurotrophy therapy on nerve conduction function and serum cytokines in patients with diabetic peripheral neuropathy

Objective: To study the effect of ultrashort wave therapy combined with antioxidant and neurotrophy therapy on nerve conduction function and serum cytokines in patients with diabetic peripheral neuropathy. Methods: A total of 128 patients who were diagnosed with DPN in South District of Guang'anmen Hospital of China Academy of Chinese Medical Sciences between May 2014 and February 2017 were selected as the research subjects and randomly divided into electrotherapy + drug group and routine drug group;the nerve conduction velocity as well as serum levels of nerve cytokines and inflammatory cytokines were measured before treatment and 8 weeks after treatment. Results: Median nerve and common peroneal nerve MNCV, median nerve and superficial peroneal nerve SNCV as well as serum CNTF, BDNF, SDF-1α, IGF-1, CAT and HO-1 levels of both groups 8 weeks after treatment were significantly higher than those before treatment while serum ICAM-1, TNF-α, IL-6, ET-1, MDA and 8-OHdG levels were significantly lower than those before treatment;median nerve and common peroneal nerve MNCV, median nerve and superficial peroneal nerve SNCV as well as serum CNTF, BDNF, SDF-1α, IGF-1, CAT and HO-1 levels of electrotherapy + drug group 8 weeks after treatment were significantly higher than those of routine drug group while serum ICAM-1, TNF-α, IL-6, ET-1, MDA and 8-OHdG levels were significantly lower than those of routine drug group. Conclusion: Ultrashort wave therapy combined with antioxidant and neurotrophy therapy for DPN can improve the nerve conduction function and neurotrophic state, and also inhibit the inflammatory response and oxidative stress response.

Terahertz coherent transition radiation based on an ultrashort electron bunching beam

The experimental result of terahertz (THz) coherent transition radiation generated from an ultrashort electron bunching beam is reported.During this experiment,the window for THz transmission from ultrahigh vacuum to free air is tested.The compact measurement system which can simultaneously test the THz wave power and frequency is built and proofed.With the help of improved Martin-Puplett interferometer and Kramers-Krong transform,the longitudinal bunch length is measured.The results show that the peak power of THz radiation wave is more than 80 kW,and its radiation frequency is from 0.1 THz to 1.5 THz.

Optical power limiting of ultrashort hyper-Gaussian pulses in cascade three-level system

Propagation of strong femtosecond hyper-Gaussian pulses in a cascade three-level molecular system is studied by solving numerically the Maxwell–Bloch equations by the iterative predictor-corrector finite-difference time-domain method.Optical power limiting behavior induced by strong nonlinear two-photon absorption is observed for different orders of the femtosecond hyper-Gaussian pulses. Pulses of a higher order temporal profile are found to have a wider power range of optical limiting but a larger output saturation intensity. Both the output saturation value and the damage threshold of optical power limiting decrease with pulse duration increasing. The decrease of the pulse area along the pulse propagation is much slower than that obtained from the two-photon area theorem due to invalidity of the slowly varying amplitude approximation and the monochromatic field hypothesis.

Describing spatiotemporal couplings in ultrashort pulses using coupling coefficients

Three coupling coefficients are defined to describe spatiotemporal coupling in ultrashort pulses.With these coupling coefficients,the first-order spatiotemporal couplings of Gaussian pulse and beam are described analytically.Also, the first-order and the second-order spatiotemporal couplings caused by angular dispersion elements are studied using these coupling coefficients.It can be shown that these coupling coefficients are dimensionless and normalized,and readily indicate the severity of spatiotemporal coupling.

Standing Ultrashort Relativistic Solitons in Overdense Plasmas

By using a one-dimensional self-consistent relativistic fluid model,an investigationis made numerically on relativistic electromagnetic solitons with a high intensity in cold overdenseplasmas with an electrons' initial velocity opposite to the laser propagating direction.Two typesof standing solitons with zero group velocity are found at the given electrons' initial velocities.One is single-humped with a weakly relativistic intensity;the another is multi-humped with astrong relativistic amplitude.The properties of these two types of solitons are presented in detail.