Efficient single-cycle mid-infrared femtosecond laser pulse generation by spectrally temporally cascaded optical parametric amplification with pump energy recycling.

We proposed spectrally temporally cascaded optical parametric amplification (STOPA) using pump energy recycling to simultaneously increase spectral bandwidth and conversion efficiency in optical parametric amplification (OPA). Using BiB3O6 and KTiOAsO4 nonlinear crystals, near-single-cycle mid-infrared (MIR) pulses with maximum energy conversion efficiencies exceeding 25% were obtained in simulations. We successfully demonstrated sub-two-cycle, CEP-stable pulse generation at 1.8 µm using a four-step STOPA system in the experiment. This method provides a solution to solve the limitations of the gain bandwidth of nonlinear crystals and the low conversion efficiency in broadband OPA systems, which is helpful for intense attosecond pulse generation and strong laser field physics studies.

Direct sampling of ultrashort laser pulses using third-harmonic generation with perturbation in ambient air.

We propose a simple and robust all-optical pulse sampling method to characterize the temporal profiles of ultrashort laser pulses. The method is based on a third-harmonic generation (THG) process with perturbation in ambient air, which requires no retrieval algorithm and can be potentially applied to electric field measurement. The method has been successfully used to characterize multi-cycle and few-cycle pulses with a spectral range from 800 nm to 2200 nm. Considering the broad phase-matching bandwidth of THG and extremely low dispersion of air, this method is suitable for ultrashort pulse characterization even for single-cycle pulses in the near- to mid-infrared range. Thus, the method provides a reliable and highly accessible approach for pulse measurement in ultrafast optics research.

All-optical sampling of ultrashort laser pulses based on perturbed transient grating.

We propose and demonstrate an all-optical pulse sampling technique based on the transient grating (TG) procedure with perturbation, which provides a simple and robust manner to characterize an ultrashort laser pulse without employing a retrieval algorithm. In our approach, a two-orders weaker perturbation pulse perturbs the diffracted pulse from the TG, which is generated by another strong fundamental pulse. The modulation of the diffracted pulse energy directly represents the temporal profile of the perturbation pulse. We have successfully characterized few-cycle and multi-cycle pulses, which is consistent with the results verified by the widely employed frequency-resolved optical gating (FROG) method. Our method provides a potential way to characterize ultrashort laser waveform from the deep-UV to far-infrared region.

Facial Emotion Recognition Using a Novel Fusion of Convolutional Neural Network and Local Binary Pattern in Crime Investigation.

The exploration of facial emotion recognition aims to analyze psychological characteristics of juveniles involved in crimes and promote the application of deep learning to psychological feature extraction. First, the relationship between facial emotion recognition and psychological characteristics is discussed. On this basis, a facial emotion recognition model is constructed by increasing the layers of the convolutional neural network (CNN) and integrating CNN with several neural networks such as VGGNet, AlexNet, and LeNet-5. Second, based on the feature fusion, an optimized Central Local Binary Pattern (CLBP) algorithm is introduced into the CNN to construct a CNN-CLBP algorithm for facial emotion recognition. Finally, the validity analysis is conducted on the algorithm after the preprocessing of face images and the optimization of relevant parameters. Compared with other methods, the CNN-CLBP algorithm has higher accuracy in facial expression recognition, with an average recognition rate of 88.16%. Besides, the recognition accuracy of this algorithm is improved by image preprocessing and parameter optimization, and there is no poor-fitting. Moreover, the CNN-CLBP algorithm can recognize 97% of the happy expressions and surprised expressions, but the misidentification rate of sad expressions is 22.54%. The research result provides data reference and direction for analyzing psychological characteristics of juveniles involved in crimes.

Security Analysis of Social Network Topic Mining Using Big Data and Optimized Deep Convolutional Neural Network.

This research aims to conduct topic mining and data analysis of social network security using social network big data. At present, the main problem is that users' behavior on social networks may reveal their private data. The main contribution lies in the establishment of a network security topic detection model combining Convolutional Neural Network (CNN) and social network big data technology. Deep Convolution Neural Network (DCNN) is utilized to complete the analysis and search of social network security issues. The Long Short-Term Memory (LSTM) algorithm is used for the extraction of Weibo topic information in the memory wisdom. Experimental results show that the recognition accuracy of the constructed model can reach 96.17% after 120 iterations, which is at least 5.4% higher than other models. Additionally, the accuracy, recall, and F1 value of the intrusion detection model are 88.57%, 75.22%, and 72.05%, respectively. Compared with other algorithms, the model's accuracy, recall, and F1 value are at least 3.1% higher than other models. In addition, the training time and testing time of the improved DCNN network security detection model are stabilized at 65.86 s and 27.90 s, respectively. The prediction time of the improved DCNN network security detection model is significantly shortened compared with that of the models proposed by other scholars. The experimental conclusion is that the improved DCNN has the characteristics of lower delay under deep learning. The model shows good performance for network data security transmission.

Apparatus for generation of nanojoule-class water-window high-order harmonics.

In our recent study [Fu et al., Commun. Phys. 3(1), 92 (2020)], we have developed an approach for energy-scaling of high-order harmonic generation in the water-window region under a neutral-medium condition. More specifically, we obtained a nanojoule-class water-window soft x-ray harmonic beam under a phase-matching condition. It has been achieved by combining a newly developed terawatt-class mid-infrared femtosecond laser and a loose-focusing geometry for high-order harmonic generation. The generated beam is more than 100 times intense compared to previously reported results. The experimental setup included two key parts: a terawatt mid-infrared femtosecond driving laser [Fu et al., Sci. Rep. 8(1), 7692 (2018)] and a specially designed gas cell. Despite the dramatic drop in the optimal gas pressure for phase-matching due to loose-focusing geometry, it still reached the 1 bar level for helium. Thus, we have designed a double-structured pulsed-gas cell with a differential pumping system, which enabled providing sufficiently high gas pressure. Moreover, it allowed reducing gas consumption significantly. A robust energy-scalable apparatus for high-order harmonic generation developed in this study will enable the generation of over ten-nanojoule water-window attosecond pulses in the near future.

Pathways and mechanisms by which biochar application reduces nitrogen and phosphorus runoff losses from a rice agroecosystem.

Biochar application has the potential to reduce nitrogen (N) and phosphorus (P) losses in agricultural runoff, but little is known about how and to what extent biochar is effective in rice agroecosystems. In this study, in a typical double-rice cropping system, N and P runoff losses and soil carbon (C), N, and P contents (soil CNP contents) were observed under three different biochar application rates (0, 24, and 48 t ha-1, which were defined as CK, LB, and HB, respectively) from 2017 to 2019. The results showed that the two-year averages of soil total organic C (TOC), total N (TSN), total P (TSP), available P (Olsen P), microbial biomass N (MBN), and microbial biomass P (MBP) contents were generally higher in the biochar treatments than in CK (P < 0.05). Specifically, the TSP, TOC, and MBN contents increased with the increasing biochar application rate, thus demonstrating the significant effects of biochar application on the paddy soil CNP contents and composition. The HB and LB treatments reduced the seasonal mean runoff flow-weighted total N (TN_wc) and total P (TP_wc) concentrations by 32.4% and 42.1%, respectively, compared to CK. Structural equation modeling (SEM) further revealed that the paths and mechanisms by which biochar reduced the TN_wc and TP_wc were different, depending on the different application rates. HB reduced the TN_wc mainly through the direct absorption of N, followed by the indirect inhibition of N mineralization, whereas LB decreased the TP_wc mainly through the strong P sorption capacity of the biochar. The direct effect of HB on the TN_wc was 1.58 times as strong as the indirect effect (path coefficients: -0.68 vs. 0.43, respectively), and the direct effect of LB on the TP_wc was 1.78 times as strong as the indirect effect (path coefficients: -0.89 vs. 0.50, respectively). Given the distinct pathways and mechanisms by which biochar reduced NP runoff losses, in practice, the biochar application rate should be optimized according to a targeted priority of reducing either N or P runoff losses in rice agroecosystems.

Dispersed pulses created by aperiodic binary spectral phase jump and applications for pulse shaping.

Inspired by pulse-pair generation with periodic phase jump, the generation of dispersed pulses with aperiodic binary spectral phase jump (ABSPJ) is proposed and theoretically investigated. It is presented by the numerical simulations that two dispersed pulses can be generated by ABSPJ of π. The dispersion of one pulse is opposite to the other and can be tuned freely with engineering of the phase jump. The generated dispersed pulse-pair is potentially of great interest for various applications, such as two-dimensional spectroscopy, double pulses laser-wakefield acceleration (LWFA) and chirp management in dual-chirped optical parametric amplification (DC-OPA) system to generate TW single-cycle mid-infrared (MIR) pulses. Furthermore, a pulse shaper configured as a micro-electro-mechanical systems (MEMS) located at the Fourier plane of a 4-f dispersion-free compressor is suggested and the implementation in a high repetition optical parametric chirped pulse amplification (OPCPA) system with picosecond pump has been numerically studied. The simulations showed that MEMS of 900 pixels is enough to pre-compensate TOD of 200000 fs3 for a pulse of 20 fs. Because pixel with only two piston-levels is necessary for such MEMS, the pulse shaper is expected to be compact and reliable.

Fully stabilized multi-TW optical waveform synthesizer: Toward gigawatt isolated attosecond pulses.

A stable 50-mJ three-channel optical waveform synthesizer is demonstrated and used to reproducibly generate a high-order harmonic supercontinuum in the soft x-ray region. This synthesizer is composed of pump pulses from a 10-Hz repetition-rate Ti:sapphire pump laser and signal and idler pulses from an infrared two-stage optical parametric amplifier driven by this pump laser. With full active stabilization of all relative time delays, relative phases, and the carrier-envelope phase, a shot-to-shot stable intense continuum harmonic spectrum is obtained around 60 eV with pulse energy above 0.24 µJ. The peak power of the soft x-ray continuum is evaluated to be beyond 1 GW with a 170-as transform limit duration. We found a characteristic delay dependence of the multicycle waveform synthesizer and established its control scheme. Compared with the one-color case, we experimentally observe an enhancement of the cutoff spectrum intensity by one to two orders of magnitude using three-color waveform synthesis.

Optimization of a multi-TW few-cycle 1.7-µm source based on Type-I BBO dual-chirped optical parametric amplification.

This paper presents the optimization of a dual-chirped optical parametric amplification (DC-OPA) scheme for producing an ultrafast intense infrared (IR) pulse. By employing a total energy of 0.77 J Ti:sapphire pump laser and type-I BBO crystals, an IR pulse energy at the center wavelength of 1.7 µm exceeded 0.1 J using the optimized DC-OPA. By adjusting the injected seed spectrum and prism pair compressor with a gross throughput of over 70%, the 1.7-µm pulse was compressed to 31 fs, which resulted in a peak power of up to 2.3 TW. Based on the demonstration of the BBO type-I DC-OPA, we propose a novel OPA scheme called the "dual pump DC-OPA" for producing a high-energy IR pulse with a two-cycle duration.

Towards a petawatt-class few-cycle infrared laser system via dual-chirped optical parametric amplification.

Expansion of the wavelength range for an ultrafast laser is an important ingredient for extending its range of applications. Conventionally, optical parametric amplification (OPA) has been employed to expand the laser wavelength to the infrared (IR) region. However, the achievable pulse energy and peak power have been limited to the mJ and the GW level, respectively. A major difficulty in the further energy scaling of OPA results from a lack of suitable large nonlinear crystals. Here, we circumvent this difficulty by employing a dual-chirped optical parametric amplification (DC-OPA) scheme. We successfully generate a multi-TW IR femtosecond laser pulse with an energy of 100 mJ order, which is higher than that reported in previous works. We also obtain excellent energy scaling ability, ultrashort pulses, flexiable wavelength tunability, and high-energy stability, which prove that DC-OPA is a superior method for the energy scaling of IR pulses to the 10 J/PW level.

Indirect high-bandwidth stabilization of carrier-envelope phase of a high-energy, low-repetition-rate laser.

We demonstrate a method of stabilizing the carrier-envelope phase (CEP) of low-repetition-rate, high-energy femtosecond laser systems such as TW-PW class lasers. A relatively weak high-repetition-rate (~1 kHz) reference pulse copropagates with a low-repetition-rate (10 Hz) high-energy pulse, which are s- and p-polarized, respectively. Using a Brewster angle window, the reference pulse is separated after the power amplifier and used for feedback to stabilize its CEP. The single-shot CEP of the high-energy pulse is indirectly stabilized to 550 mrad RMS, which is the highest CEP stability ever reported for a low-repetition-rate (10-Hz) high-energy laser system. In this novel method, the feedback frequency of the reference pulse from the front-end preamplifier can be almost preserved. Thus, higher CEP stability can be realized than for lower frequencies. Of course, a reference pulse with an even higher repetition rate (e.g., 10 kHz) can be easily employed to sample and feed back CEP jitter over a broader frequency bandwidth.

Carrier-envelope phase stabilization of a 16 TW, 10 Hz Ti:sapphire laser.

We propose and demonstrate a simple and promising method for stabilizing the carrier-envelope phase (CEP) of a high-energy ultrashort pulse laser operating at a low repetition rate. The method was successfully applied to a Ti:sapphire laser operating at 10 Hz with 400 mJ pulse energy and 25 fs pulse duration (16 TW). The laser system consists of a 1 kHz front-end preamplifier and a 10 Hz back-end power amplifier. By sampling a 500 Hz reference pulse from a 1 kHz seed pulse, the measured single-shot CEP noise of a 10 Hz amplified pulse is stabilized to 670 mrad rms. Our proposed CEP stabilization concept can be applied to single-shot ultrahigh-power lasers, such as a petawatt laser system.

High-energy infrared femtosecond pulses generated by dual-chirped optical parametric amplification.

We demonstrate high-energy infrared femtosecond pulse generation by a dual-chirped optical parametric amplification (DC-OPA) scheme [Opt. Express19, 7190 (2011)]. By employing a 100 mJ pump laser, a signal pulse energy exceeding 20 mJ at a wavelength of 1.4 µm was achieved before dispersion compensation. A total output energy of 33 mJ was recorded. Under a further energy scaling condition, the signal pulse was compressed to an almost transform-limited duration of 27 fs using a fused silica prism compressor. Since the DC-OPA scheme is efficient and energy scalable, design parameters for obtaining 100 mJ level infrared pulses are presented, which are suitable as driver lasers for the energy scaling of high-order harmonic generation with sub-keV photon energy.

Wavelength scaling of elliptical-polarization dependence of high-order harmonic generation.

We report on an experimental study on the wavelength-scaling law of elliptical-polarization dependence of high-order harmonic generation. In the elliptical-polarization dependence measurements, we keep an identical peak intensity for pump pulses with carrier wavelength ranging from 800 nm to the mid-IR region, and the harmonic emission near the cutoff region is selected for comparison. We find that the experimental measured wavelength scaling of elliptical-polarization dependence could be well fitted with a function (epsilon(1/2) proportional to lambda(-1)) predicted by semiclassical model.

Generation of extended filaments of femtosecond pulses in air by use of a single-step phase plate.

We experimentally demonstrate that by use of a phase plate that is placed between an adjustable aperture and a focusing lens, the length of the filament can be dramatically extended in air with a femtosecond laser pulse. In addition, the far-field beam profile captured after the filament indicates that the supercontinuum is strongly confined on axis, and a single filament appears to be attainable at relatively high input pulse power when the phase plate is used.

Generation of a coherent x ray in the water window region at 1 kHz repetition rate using a mid-infrared pump source.

We demonstrate the generation of a coherent x ray in the water window region in a gas cell filled with neon gas using a wavelength-tunable mid-IR femtosecond laser operating at 1 kHz repetition rate. The cutoff energy and conversion efficiency of the water window x ray can be optimized by tuning gas pressure as well as the focal position.

Theoretical investigation of single attosecond pulse generation in an orthogonally polarized two-color laser field.

We theoretically demonstrate the generation of extreme ultraviolet supercontinua in an orthogonally polarized two-color few-cycle laser field. We show that the ionized electrons can be driven back to their parent ion after traveling along curved trajectories in a plane perpendicular to the beam propagation direction, giving rise to a train of attosecond pulses at different polarization angles. A single isolated attosecond pulse can be obtained by blocking the low-order high harmonics, which contribute to the formation of the satellite pulses.

Asymmetry in visual cortical circuits underlying motion-induced perceptual mislocalization.

Motion signals in the visual field can cause strong biases in the perceived positions of stationary objects. Local motion signal within an object induces a shift in the perceived object position in the direction of motion, whereas adaptation to motion stimuli causes a perceptual shift in the opposite direction. The neural mechanisms underlying these illusions are poorly understood. Here we report two novel receptive field (RF) properties in cat primary visual cortex that may account for these motion-position illusions. First, motion signal in a stationary test stimulus causes a displacement of the RF in the direction opposite to motion. Second, motion adaptation induces a shift of the RF in the direction of adaptation. Comparison with human psychophysical measurements under similar conditions indicates that these RF properties can primarily account for the motion-position illusions. Importantly, both RF properties indicate a spatial asymmetry in the synaptic connections from direction-selective cells, and this circuit feature can be predicted by spike-timing-dependent synaptic plasticity, a widespread phenomenon in the nervous system. Thus, motion-induced perceptual mislocalization may be mediated by asymmetric cortical circuits, as a natural consequence of experience-dependent synaptic modification during circuit development.

Temporal specificity in the cortical plasticity of visual space representation.

The circuitry and function of mammalian visual cortex are shaped by patterns of visual stimuli, a plasticity likely mediated by synaptic modifications. In the adult cat, asynchronous visual stimuli in two adjacent retinal regions controlled the relative spike timing of two groups of cortical neurons with high precision. This asynchronous pairing induced rapid modifications of intracortical connections and shifts in receptive fields. These changes depended on the temporal order and interval between visual stimuli in a manner consistent with spike timing-dependent synaptic plasticity. Parallel to the cortical modifications found in the cat, such asynchronous visual stimuli also induced shifts in human spatial perception.