Tunable terahertz transmission behaviors and coupling mechanism in hybrid MoS_(2)metamaterials

A hybrid metamaterial with the integration of molybdenum disulfide(MoS_(2))overlayer is proposed to manipulate the terahertz(THz)wave.The simulated results indicate that the introduction of MoS_(2) layer could significantly modify the resonant responses with large resonance red-shift and bandwidth broadening due to the depolarization field effect,especially for the structure on the small permitivity substrate.Additionally,the wide-band modulator in off-resonant region and a switch effect at resonance can be achieved by varying the conductivity of MoS_(2) layer.Further theoretical calculations based on the Lorentz coupling model are consistent with the simulated results,explicating the response behaviors originate from the coupling between MoS_(2) overlayer and the metastructure.Our results could provide a possibility for active control THz modulator and switchable device based on the MoS_(2) overlayer and advance the understanding of the coupling mechanism in hybrid structures.

IMPACT OF DYNAMICAL HYDRATION SHELL AROUND HA PROTEIN ON NONLINEAR CONCENTRATION DEPENDENT T-RAYS ABSORPTION

T rays is sensitive to covalently cross linked proteins and can be used to probe unique dynamic properties of water surrounding a protein.In this paper,we demonstrate the unique abeorption properties of the dynamic hydnation shells deternined by hemagglutinin(HA)protein in ter-ahertz frequency.We study the changes arising fom diferent concentrations in detail and show that nonlinear absorption coefficient is induced by the dynamic hydration water.The binary and ternary component model were used to interpret the nonlinearity absorption behaviors and predict the thickness of the hydration shells around the HA protein in aquous phase.

High-performance terahertz modulators induced by substrate field in Te-based all-2D heterojunctions

High-performance active terahertz modulators as the indispensable core components are of great importance for the next generation communication technology.However,they currently suffer from the tradeoff between modulation depth and speed.Here,we introduce two-dimensional(2D)tellurium(Te)nanofilms with the unique structure as a new class of optically controlled terahertz modulators and demonstrate their integrated heterojunctions can successfully improve the device performances to the optimal and applicable levels among the existing all-2D broadband modulators.Further photoresponse measurements confirm the significant impact of the stacking order.We first clarify the direction of the substrate-induced electric field through first-principles calculations and uncover the unusual interaction mechanism in the photoexcited carrier dynamics associated with the charge transfer and interlayer exciton recombination.This advances the fundamental and applicative research of Te nanomaterials in high-performance terahertz optoelectronics.

Ultrafast hydrogen bond dynamics of liquid water revealed by terahertz-induced transient birefringence

The fundamental properties of water molecules,such as their molecular polarizability,have not yet been clarified.The hydrogen bond network is generally considered to play an important role in the thermodynamic properties of water.The terahertz(THz)Kerr effect technique,as a novel tool,is expected to be useful in exploring the low-frequency molecular dynamics of liquid water.Here,we use an intense and ultrabroadband THz pulse(peak electric field strength of 14.9 MV/cm,centre frequency of 3.9 THz,and bandwidth of 1–10 THz)to resonantly excite intermolecular modes of liquid water.Bipolar THz field-induced transient birefringence signals are observed in a free-flowing water film.We propose a hydrogen bond harmonic oscillator model associated with the dielectric susceptibility and combine it with the Lorentz dynamic equation to investigate the intermolecular structure and dynamics of liquid water.We mainly decompose the bipolar signals into a positive signal caused by hydrogen bond stretching vibration and a negative signal caused by hydrogen bond bending vibration,indicating that the polarizability perturbation of water presents competing contributions under bending and stretching conditions.A Kerr coefficient equation related to the intermolecular modes of water is established.The ultrafast intermolecular hydrogen bond dynamics of water revealed by an ultrabroadband THz pump pulse can provide further insights into the transient structure of liquid water corresponding to the pertinent modes.

Terahertz wave emission from water lin es

To make further understanding of terahertz(THz)wave generation from liquid water,we study THz wave emission from water lines of different diameters.The water line with a smaller diameter generates a stronger THz electric field for the diameters from 0.2 mm to 0.5 mm.The THz electric field strength and polarity change with the relative position between the incident laser and water line.Moreover,the THz energy has an optimal radiation angle of about 60°.A two-dimensional dipole array model is introduced to illustrate the phenomenon.Our observations contribute to optimizing the scheme of the liquid THz source.

Broadband THz Sources from Gases to Liquids

Matters are generally classified within four states:solid,liquid,gas,and plasma.Three of the four states of matter(solid,gas,and plasma)have been used for THz wave generation with short laser pulse excitation for decades,including the recent vigorous development of THz photonics in gases(air plasma).However,the demonstration of THz generation from liquids was conspicuously absent.It is well known that water,the most common liquid,is a strong absorber in the far infrared range.Therefore,liquid water has historically been sworn off as a source for THz radiation.Recently,broadband THz wave generation from a flowing liquid target has been experimentally demonstrated through laser-induced microplasma.The liquid target as the THz source presents unique properties.Specifically,liquids have the comparable material density to that of solids,meaning that laser pulses over a certain area will interact with three orders more molecules than an equivalent cross-section of gases.In contrast with solid targets,the fluidity of liquid allows every laser pulse to interact with a fresh area on the target,meaning that material damage or degradation is not an issue with the high-repetition rate intense laser pulses.These make liquids very promising candidates for the investigation of high-energy-density plasma,as well as the possibility of being the next generation of THz sources.

Terahertz-dependent PM2.5 monitoring and grading in the atmosphere

Rapid industrialization and economic development have led to serious pollution in the form of fine particulate matter(PM2.5,particulate matter with a diameter of less than 2.5 μm). In China, PM2.5 has been one of the most debated topics in councils of government and issues of public concern. Terahertz(THz) radiation was employed to measure the PM2.5 in the atmosphere from September 2014 to April 2015 in Beijing. Comparison of the PM2.5 level from the website with THz absorbance revealed a significant phenomenon: THz radiation can be used to monitor PM2.5 in the atmosphere. During Asia-Pacific Economic Cooperation(APEC) 2014, "APEC Blue" was also recorded in a THz system. The relationship between absorbance and PM2.5 demonstrates that THz radiation is an effective selection for air pollution grading. Based on the absorbance spectra, the elemental compositions were studied by two-dimensional correlation spectroscopy(2 DCOS) in conjunction with X-ray fluorescence.Several single absorption peaks were revealed and caused by sulphate from coal combustion, vehicle exhaust emissions and secondary reactions. Furthermore, mathematical algorithms, such as the BPANN and SVM, can process the THz absorbance data and greatly improve the precision of the estimation of PM2.5 mass. Our results suggest that THz spectroscopy can not only reveal the component information for pollution source determination, but quantitatively monitor the PM2.5 content for pollution level evaluation. Therefore, the use of THz radiation is a new method for future air pollution monitoring and grading systems.

Terahertz assessment of the atmospheric pollution during the first-ever red alert period in Beijing

Haze or smog episodes, which are characterized by the presence of particulate matter at diameters less than 2.5 μm （PM2.5）, have attracted increasing attention during the past few decades [ 1 ]. PM2.5 has adverse effects on human respiratory health as well as on air visibility [2,3]. In the Beijing-Tianjin-Hebei （BTH） region of China, haze has become especially serious in recent years because of industrial expansion and traffic-related emissions [4,5] （http：//news.xinhuanet.com/english/2016-01/07/c_134987525.htm; http：// news.cenews.com.cn/html/2016-01/07/content_38614.htm）. To protect public health, the first-ever red alert for smog in China was issued by the Beijing municipal environmental protection bureau （MEPB）,

Revisiting the relationship between composite multiscale entropy and THz optical parameters with exterior product

With the framework of exterior product,we investigate the relationship between composite multiscale entropy[CMSE]and refractive index and absorption coefficient by reanalyzing six concentrations of bovine serum albumin aqueous solutions from the published work.Two bivectors are constructed by CMSE and its square by the refractive index and absorption coefficient under vectorization.The desirable linear behaviors can be captured,not only between the defined two bivectors in normalized magnitudes,but also between the normalized magnitude of bivectors pertinent to CMSE and the magnitude of a single vector on the refractive index or absorption coefficient,with the processing of optimum selection.Besides that,the relationship between the coefficients of two bivectors is also considered.The results reveal that plenty of sound linear behaviors can be found and also suggest the scale of 15,16 and frequency of 0.2,0.21 THz are prominent for those linear behaviors.This work provides a new insight into the correlation between terahertz[THz]time and frequency domain information.

Characterizing PM2.5 in Beijing and Shanxi Province using terahertz radiation

Particles of aerodynamic diameter〈2.5 pm （PM/.5） caused extremely severe and persistent haze pollution is of concern in many cities. In this study, samples of PM2.5 were collected from atmosphere environment of Beijing and Shanxi Province, and analyzed using terahertz （THz） radiation. The transmission spectrum of PM2.5 in Shanxi Province had two distinct absorption peaks at 6.0 and 6.7 THz, and the curve was increasing on the whole. However, the transmission spectrum of PMa.5 in Beijing had obviously different variation tendency and the absorption peak was studied by monitoring PM2.5 masses in conjunction with two-dimensional correlation spectro- scopy （2DCOS）. By comparing the pollutant species and concentrations of Shanxi Province and Beijing over the time of collecting samples, the concentrations of sulfate and ammonium were similar, which contributed to emerge absorption bands in the same position. While the concentrations of organic matter （OM）, nitrate, chloride and elemental carbon （EC） were different. Furthermore, dust and some other inorganic ion are unique to Shanxi province, which lead to different variation tendency of the transmission spectrum of PM2.5. These results will be of importance for environmental monitoring and for control- ling PM emissions. According to this research, optical techniques, and especially spectral methods, should be considered for PM2.5 monitoring.

Terahertz wave generation via pre-ionized air plasma

We report the terahertz （THz） wave generation from a single-color scheme modulated by pre-ionized air plasma via an orthogonal pumping geometry. It is found that the amplitude of the THz signal generated by the pump beam tends to decrease gradually with the increase of the modulation power. We believe that the ponderomotive force plays an important role in the process of the interaction between the pump beam and the pre-ionization beam. The hydrostatic state of the electrostatic separation field caused by the modulation beam will directly affect the generation efficiency of the THz wave. Our results contribute to further understanding of the theoretical mechanism and expanding of the practical applications of THz wave generation and modulation.