Search

Strain engineering is an attractive method to induce and control anisotropy for polarized optoelectronic applications with two-dimensional (2D) materials. Herein, we have investigated the nonlinear optical coefficient dispersion relationship and the second-harmonic generation (SHG) pattern evolution under the uniaxial strains for graphene, WS

Palladium diselenide (PdSe2) exhibits air stability, low symmetry, and high carrier mobility, resulting in unique in-plane anisotropy for polarized optoelectronic devices. However, the relationship of the symmetry and the terahertz (THz) radiation remains elusive yet significant for both the THz source in technology and nonlinear optical physics in science. Herein, we observed large in-plane anisotropic THz radiation from multilayer PdSe2 under femtosecond laser excitation. The THz emission demonstrates 2α dependence on the optical polarization angle from the resonant optical rectification combined with a background from the photocarrier acceleration under the surface depletion field. Interestingly, the in-plane THz emission along and perpendicular to the puckered direction demonstrates large anisotropy. Furthermore, the THz time-domain signals exhibit reversed polarities along the positive and negative puckered directions. This asymmetric polarization could relate to the bonding of Pd-Se, resulting in the unidirectional photon-induced current. Our results bridge the gap between the low-symmetry two-dimensional materials and the THz technology, which could promote the development of THz-polarized devices based on low-symmetry layered materials.

Coherent polarization control of terahertz (THz) wave radiation in both the time-domain and the frequency-domain is significant in information technology, material science, and spectroscopic analysis. Elliptically polarized THz radiation is generally limited to chiral materials induced by circularly polarized light excitation. Herein, we demonstrate the coherent elliptically polarized THz radiation from few-layer tungsten diselenide (WSe2) in both the time-domain and the frequency-domain under linearly polarized femtosecond laser excitation. This coherent elliptical THz radiation is mainly dominated by in-plane anisotropic shift current and out-of-plane drift current, which is verified by the THz radiation dependence on the pump laser polarization angles, incident angles, and sample azimuthal angles systematically. The ellipticity and major axis direction of the elliptical THz wave can be efficiently controlled by either pump light polarization or sample azimuthal angle due to the controllable amplitudes and phases of two coherent orthogonal THz wave components. Our finding provides a method to distinguish drift and shift photocurrents in different directions and offers a unique design concept for elliptical THz generation with two-dimensional (2D) material physics.

Surface nonlinear optical response of layered tin disulfide (SnS2) is investigated by terahertz (THz) emission spectroscopy using a femtosecond photoexcitation at 400 nm. Under above-bandgap excita...

The interfacial effect in mixed-dimensional van der Waals heterostructure can improve the performance of terahertz (THz) components, such as THz emitter and THz modulator, which highlight the development of THz technology. However, this improvement has only been demonstrated in heterostructure based on graphene or small bandgap two-dimensional materials. We present the enhancement for THz generation and modulation from Si by insulating hexagonal boron nitride (h-BN) layer coating. The surface adsorption charges and the interface trapped charges lower the Fermi energy on Si surface, leading to an enlarged built-in electric field. Consequently, THz generation from h-BN/Si demonstrated significant enhancement compared with that by bare Si surface. Furthermore, the enlarged built-in electric field separates the interface photocarriers and reduces the surface recombination rate. The resulting dense charge accumulation raises up the photoconductivity after optical illumination and the enhanced THz modulation at h-BN/Si interface is approximately ten times higher than that by Si surface. These results not only present an efficient THz generation and modulation from the h-BN/Si interface but also pave the way for the deep understanding of interfacial effect in mixed-dimensional van der Waals heterostructures.

Palladium diselenide (PdSe

Coherent polarization control of terahertz (THz) wave radiation in both the time-domain and the frequency-domain is significant in information technology, material science, and spectroscopic analysis. Elliptically polarized THz radiation is generally limited to chiral materials induced by circularly polarized light excitation. Herein, we demonstrate the coherent elliptically polarized THz radiation from few-layer tungsten diselenide (WSe

Two-dimensional/ three-dimensional (2D/3D) van der Waals (vdW) heterostructures are one of the most potential candidates for table-top pulse terahertz (THz) emitters. Hence, manipulating carrier information at the interface is important to optimize THz emission performance. Photoinduced doping is an effective way to embellish the carrier characteristic at the interface. Here, we applied the photoinduced doping effect to dynamically manipulate the THz generation process from graphene-silicon (Gr-Si) heterostructure. When photoinduced doping is applied by using 532 nm continuous wave (CW) laser, THz radiation decreases with the increase of CW pump power at the reverse bias. The photoinduced doping attenuates the interfacial built-in electric field, resulting in the decrease of transient photocurrent and further reduction of THz radiation. This photogenerated carrier screening effect has achieved a 95.4% intrinsic THz modulation depth (MD) at the external excitation of 200 mW CW laser under reverse bias voltage -30 V. The intrinsic THz MD is much higher than previous report value due to the optimized heterostructure fabrication and the optimized light spot overlap of CW laser and femtosecond laser. This work proposes a non-destructive and reversible method to actively manipulate the THz emission at the vdW interface and provides an optimized route to realize high intrinsic THz MD in THz region.

We report the optical response of layered bulk and monolayer SnS2 at the surface. The physical mechanism for terahertz (THz) emission in bulk and monolayer SnS2 has been proposed. At 40° incident angle, the drift current accompanied by the surface nonlinear polarization concurrently contribute to THz surface emission in bulk SnS2. The THz radiation in monolayer SnS2 is mainly attributed to the drift photocurrent, which is insensitive to the crystalline symmetry and the pump polarization direction. The corresponding investigation could not only help to clarify the relationship of layer-dependent optical properties, but also make a significant contribution in understanding the nonlinear physical process in other transition metal dichalcogenides (TMDs) materials.

Tunable optical properties play an important role in the high performance of optoelectronic applications based on two-dimensional (2D) transition metal carbide and nitride (MXene) materials. Herein, the optical properties of functionalized MXene monolayers Sc2CT2 (T = O and OH) are investigated by strain engineering. The strain-dependent linear optical properties of Sc2CT2 possess broadband optical response due to the geometry and orbital overlap effect. The peaks from the second-order nonlinear coefficient elements d (d15, d16, and d31) at around half the band-gap exhibit a redshift for Sc2CO2 (blueshift for Sc2C(OH)2) with the increase of strain. The strain-dependent d reveals that Sc2CO2 with -1268 pm V-1 %-1 has a larger photoelastic coefficient than that of Sc2C(OH)2 with -574 pm V-1 %-1 at 1% strain. Meanwhile, the photoelastic tensors can not only be increased but also reduced with the increase of strain due to the dispersion relation. Moreover, the azimuthal angle-dependent second harmonic generation (SHG) from strained Sc2CT2 monolayers depends highly on the strained states and the pumping photon energy. The results pave the way for the tunable, broadband, and anisotropic applications of nonlinear optoelectronic devices based on MXenes based on strain engineering.

BackgroundThe efficacy of repetitive transcranial magnetic stimulation (rTMS) in depression is nonuniform across patients. This study aims to determine whether baseline neuroimaging characters can provide a pretreatment predictive effect for rTMS.MethodsTwenty-seven treatment-naive patients with major depressive disorder (MDD) were enrolled and scanned with resting-state functional magnetic resonance imaging (fMRI) and diffusion tensor imaging. Clinical symptoms were assessed pre- and post-rTMS. Functional and structural connectivity between the left dorsolateral prefrontal cortex (DLPFC) and bilateral insula were measured, and the connectivity strength in each modality was then correlated to the clinical efficacy of rTMS.ResultsWhen the coordinates of left DLPFC were located as a node in the central executive network, the clinical efficacy of rTMS was significantly correlated with the functional connectivity strength between left DLPFC and bilateral insula (left insula: r = 0.66; right insula: r = 0.65). The structural connectivity strength between the left DLPFC and left insular cortex also had a significantly positive correlation with symptom improvement (rs = 0.458).ConclusionThis study provides implications that rTMS might act more effectively when the pretreatment functional and structural connectivity between the insula and left DLPFC is stronger.

Tunable optical properties play an important role in the high performance of optoelectronic applications based on two-dimensional (2D) transition metal carbide and nitride (MXene) materials. Herein, the optical properties of functionalized MXene monolayers Sc

Outcome-based education is not only a talent-training mode but also a theoretical base of curriculum design. Although OBE has been widely studied and practiced around the world, few researches could be found to discuss the roles and effects of blended learning method in OBE framework. This paper develops an outcome-based ESP course with blended learning method for Chinese undergraduates who major in software engineering to acquire adequate working skills in English for the preparation of their future professions in the IT industry. It provides a comprehensive description of the course design, class practices and evaluation method. The paper also verifies the effectiveness of the course from the students’ feedback, teachers’ opinions and experts’ evaluation.

With the deepening development of educational reform of College English, its transformation has become an inevitable problem. EAP teaching has become a new topic in College English teaching under the objective situation of the great progress where bilingual teaching in key national universities has made. Based on the Sydney School’s scaffolding instruction framework of EAP reading and writing, in combination of the training objectives and course settings for the talents from advanced science and engineering universities, we have conducted an empirical research on web-based scaffolding pedagogy of EAP literacy cultivation. In the follow-up questionnaires, students have general reflex that independent EAP reading and writing ability has met a significant improvement, and the awareness of self-learning and cooperative learning has been fully tapped.

Terahertz (THz) interface physics as a new interdiscipline between the THz technique and condensed matter physics has undergone rapid development in recent years. In particular, the development of advanced materials, such as graphene, transitional metal dichalcogenides, topological insulators, ferromagnetic metals, and metamaterials, has revolutionized the interface field and further promoted the development of THz functional devices based on interface physics. Moreover, playing at the interface of these advanced materials could unveil a wealth of fascinating physical effects such as charge transfer, proximity effect, inverse spin-Hall effect, and Rashba effect with THz technology by engineering the charge, spin, orbit, valley, and lattice degrees of freedom. In this review, we start with a discussion of the basic theory of THz interface physics, including interface formation with advanced materials, THz wave reflection and transmission at the interface, and band alignment and charge dynamics at the interface. Then we move to recent progress in advanced materials from THz wave propagation to THz wave generation at the interface. In THz wave propagation, we focus on THz wave impedance-matching, Goos–Hänchen and Imbert–Fedorov shifts in THz region, interfacial modulation and interfacial sensing based on THz waves. In THz wave generation, we summarize ongoing coherent THz wave generation from van der Waals interfaces, multiferroic interfaces, and magnetic interfaces. The fascinating THz interface physics of advanced materials is promising and promotes novel THz functional devices for manipulating propagation and generation of THz waves at interfaces.

Graphene/semiconductor heterostructures demonstrate an improvement of traditional electronic and optoelectronic devices because of their outstanding charge transport properties inside and at the interfaces. However, very limited information has been accessed from the interfacial properties by traditional measurement. Herein, we present an active THz surface emission spectroscopy for the interface build-in potential and charge detrapping time constant evaluation from the interface of graphene on SiO2/Si (Gr/SiO2/Si). The active THz generation presents an intuitive insight into the depletion case, weak inversion case, and strong inversion case at the interface in the heterostructure. By analyzing the interface electric-field-induced optical rectification (EFIOR) in a strong inversion case, the intrinsic build-in potential is identified as -0.15 V at Gr/SiO2/Si interface. The interface depletion layer presents 44% positive THz intrinsic modulation by the reverse gate voltage and 70% negative THz intrinsic modulation by the forward gate voltage. Moreover, a time-dependent THz generation measurement has been used to deduce the charge detrapping decay time constant. The investigation will not only highlight the THz surface emission spectroscopy for the graphene-based interface analysis but also demonstrate the potential for the efficient THz intrinsic modulation as well as the enhancement of THz emission by the heterostructures.

Objective To study the neuroimaging mechanisms of repetitive transcranial magnetic stimulation (rTMS) in treating major depressive disorder (MDD). Methods Twenty-seven treatment-naive patients with major depressive disorder (MDD) and 27 controls were enrolled. All of them were scanned with resting-state functional magnetic resonance imaging (fMRI) at baseline, and 15 patients were rescanned after two-week rTMS. The amplitude of low frequency fluctuation (ALFF) and functional connection degree (FCD), based on voxels and 3 brain networks (default mode network [DMN], central executive network [CEN], salience network[SN]),were used as imaging indicators to analyze. The correlations of brain imaging changes after rTMS with clinical efficacy were calculated. Results At baseline, patients groups showed increased ALFF in the right orbital frontal cortex (OFC) and decreased ALFF in the left striatal cortex and medial prefrontal cortex (PFC), while increased FCD in the right dorsal anterior cingulate cortex and OFC and decreased FCD in the right inferior parietal lobe and in the CEN. After rTMS, patients showed increased ALFF in the left dorsolateral prefrontal cortex (DLPFC)and superior frontal gyrus, FCD in the right dorsal anterior cingulate cortex, superior temporal gyrus and CEN, as well as decreased FCD in the bilateral lingual gyrus than pre-rTMS . These rTMS induced neuroimaging changes did not significantly correlated with clinical effecacy. Conclusions This study indicated that rTMS resulted in changes of ALFF and FCD in some brain regions and CEN. But we could not conclude this is the neuroimaging mechanism of rTMS according to the correlation analysis.

Photodoping as a nonvolatile and reversible method can be used to modify the carrier distribution at the heterojunction interface. Herein, we explore the 2D/3D van der Waals (vdW) graphene/silicon (G/Si) heterostructure in real time by terahertz (THz) emission spectroscopy. Photoinduced doping is introduced by a continuous wave laser, which leads to a screening effect to the built-in electric field at the interface. The resulting decrease in transient photocurrent reduces the THz emission amplitude from the G/Si heterostructure. The photoinduced doping effect suggests a 40% THz intrinsic modulation depth at external reverse bias. This work provides an effective way to actively control the THz emission process from the G/Si interface and paves the way for analyzing the interfacial process under photoinduced doping in vdW heterostructures.

We investigated the terahertz (THz) radiation mechanism of the layered tin sulfide (SnS2) crystal via THz emission spectroscopy. Under non-resonant excitation, THz generation from SnS2 is mainly due to the optical rectification effect, which is verified by the azimuthal angle, polarization angle, and pump fluence dependence of the THz signals. Further incident angle dependent THz signals suggest that the optical progress happened at the surface of SnS2 with both in-plane and out-of-plane photon-induced dipoles. This work puts forward THz emission spectroscopy as a noninvasive characterization tool for the in-plane and out-of-plane photon-induced dipoles in layered materials.

Graphene/semiconductor heterostructures demonstrate an improvement of traditional electronic and optoelectronic devices because of their outstanding charge transport properties inside and at the interfaces. However, very limited information has been accessed from the interfacial properties by traditional measurement. Herein, we present an active THz surface emission spectroscopy for the interface build-in potential and charge detrapping time constant evaluation from the interface of graphene on SiO

Schizophrenia patients are at high risk for cigarette smoking, but the neurobiological mechanisms of this comorbid association are relatively unknown. Long-term nicotine intake may impact brain that are independently and additively associated with schizophrenia. We investigated whether altered intrinsic brain activity (iBA) related to schizophrenia pathology is also associated with nicotine addiction. Forty-two schizophrenia patients (21 smokers and 21 nonsmokers) and 21 sex- and age-matched healthy nonsmokers underwent task-free functional MRI. Whole brain iBA was measured by the amplitude of spontaneous low frequency fluctuation. Furthermore, correlation analyses between iBA, symptom severity and nicotine addiction severity were performed. We found that prefrontal cortex, right caudate, and right postcentral gyrus were related to both disease and nicotine addiction effects. More importantly, schizophrenia smokers, compared to schizophrenia nonsmokers showed reversed iBA in the above brain regions. In addition, schizophrenia smokers, relative to nonsmokers, altered iBA in the left striatal and motor cortices. The iBA of the right caudate was negatively correlated with symptom severity. The iBA of the right postcentral gyrus negatively correlated with nicotine addiction severity. The striatal and motor cortices could potentially increase the vulnerability of smoking in schizophrenia. More importantly, smoking reversed iBA in the right striatal and prefrontal cortices, consistent with the self-medication theory in schizophrenia. Smoking altered left striatal and motor cortices activity, suggesting that the nicotine addiction effect was independent of disease. These results provide a local property of intrinsic brain activity mechanism that contributes to cigarette smoking and schizophrenia.

Terahertz (THz) time-domain emission spectroscopy in both transmission and reflection configurations has been employed to understand the THz radiation property and surface properties of tungsten disulfide ($\mathrm{W}{\mathrm{S}}_{2}$). We observed only one polarization of THz radiation under different polarization of pump beam and a saturation effect with the increasing of pump power. The results are different from that of $\mathrm{Mo}{\mathrm{S}}_{2}$ based on optical rectification in spite of similar physical and optoelectronic properties of them. The nonlinear optical coefficient calculation based on first-principle method combined with the azimuthal angle dependence of THz radiation implies that THz radiation is insensitive to the azimuthal angle in $\mathrm{W}{\mathrm{S}}_{2}$. From the pump polarization angle dependence of THz radiation, we find that the contribution due to the nonlinear effect is only 12% approximately. All these suggest the main THz mechanism from $\mathrm{W}{\mathrm{S}}_{2}$ is due to the surface depletion field induced by the surface states. We also analyzed the surface field features of $\mathrm{W}{\mathrm{S}}_{2}$ with the maximum surface depletion field of approximate $1.2\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.16em}{0ex}}\mathrm{V}/\mathrm{cm}$. Fresnel law combined with the dipole radiation model is also used to analyze the angular dependence of THz radiation. The results can not only afford a fundamental THz radiation property of layered materials, but also promote the development of THz devices based on layered materials.

Not all depression patients effectively respond to repeated transcranial magnetic stimulation (rTMS). We tested whether the intrinsic functional connectivity (FC) strength between the stimulated left dorsolateral prefrontal cortex (DLPFC) and left nucleus accumbens (NAcc) might predict effects of rTMS. Twenty-two medication-naïve depression patients received rTMS on left DLPFC for 2 weeks and underwent baseline functional magnetic resonance imaging (fMRI). We compared the amplitude of the low-frequency fluctuation (ALFF) and regional homogeneity (ReHo) in the stimulated target (the cortex region directly stimulated by rTMS) located in the left DLPFC, and the left NAcc, as well as the intrinsic FC of the DLPFC–NAcc between early improvers and non-improvers. We evaluated the association between the baseline brain imaging features (ALFF, ReHo, and FC) and improvements in depression and anxiety symptoms. We found that the pretreatment ALFF and ReHo in the stimulated DLPFC and left NAcc did not significantly differ between the subgroups. The early improvers displayed increased negative FC strength between the stimulated DLPFC and left NAcc with respect to non-improvers. The stimulated DLPFC–NAcc FC strength negatively correlated with improved depressive and anxious symptoms. This study is the first to demonstrate that the resting-state FC of the stimulated DLPFC–NAcc, rather than regional brain activity or local synchronization in the stimulated target, might predict the anti-depression and anti-anxiety effects of rTMS for depression.

Broadband terahertz (THz) impedance matching is important for both spectral resolution improvement and THz anti-radar technology. Herein, graphene-silicon hybrid structure has been proposed for active broadband THz wave impedance matching with optical tunability. The main transmission pulse measured in the time domain indicates a modulation depth as high as 92.7% totally from the graphene-silicon interface. The interface reflection from the graphene-silicon junction implies that an impedance matching condition can be actively achieved by optical doping. To reveal the mechanism, we propose a graphene-silicon heterojunction model, which gives a full consideration of both the THz conductivity of graphene and the loss in doped junction layer. The theory fits well with the experimental results. This work proves active THz wave manipulation by junction effect and paves the way for active anti-reflection coating for THz components.

This paper investigates lexical features in two different styles of English newspapers-broadsheets (The New York Times, and The Times), and tabloids (New York Post and The Sun). The comparison and contrast are done in five lexical aspects: the use of compound words, fuzzy words, numbers, proper nouns and vivid words. Statistics show that differences do exist between broadsheets and tabloids. Analysis is done to reveal the possible causes that lead to the difference and provides evidence that lexical features are essential to set the tone of newspaper writing.

This paper tries to provide both theoretical and pedagogical evidence on the organizational pattern of L2 mental lexicon for Chinese ESL learners and teachers. The evidence results from a carefully designed empirical research where the subjects are ESL students with different language proficiencies from software school of DUT. Results show that the organizational pattern of Chinese ESL learners’ mental lexicon is a mix of semantic and non-semantic network, and it is rather dynamic. Efforts of reconstructing the lexicon network are necessary.

This paper traces 9 non-English major EFL students and collects their oral productions in 4 successive oral exams in 2 years. The canonical correlation analysis approach of SPSS is adopted to study the disfluencies developmental traits under the influence of language acquisition development. We find that as language acquisition develops, the total production of difluenices does not decrease correspondingly as we thought, but keeps constant for a period of time. While the proportions of specific disfluencies phenomena change significantly, which features the decrease of pauses and the increase self-repairs. Besides, the grammatical accuracy and language complexity have opposite effects on disfluencies traits. In the first year, disfluencies were displayed mainly as pauses and repetitions since EFL students paid more attention to grammatical accuracy; in the second year, disfluencies featured more self-repairs and less pauses because EFL students transferred their attention to language complexity. We also find language acquisition can only account for partial developmental traits of disfluencies despite of the strong correlations between them, and other factors, such as psychological or social elements, may also take effects.