Integrated system of traditional THz time-domain spectroscopy and asynchronous optical sampling

Terahertz time-domain spectroscopy(THz-TDS)system,as a new means of spectral analysis and detection,plays an increasingly pivotal role in basic scientific research.However,owing to the long scanning time of the traditional THz-TDS system and the complex control of the asynchronous optical scanning(ASOPS)system,which requires frequent calibration,we combine traditional THz-TDS and ASOPS systems to form a composite system and propose an all-fiber trigger signal generation method based on the time overlapping interference signal generated by the collinear motion of two laser pulses.Finally,the time-domain and frequency-domain spectra are obtained by using two independent systems in the integrated systems.It is found that the full width at half maximum(FWHM)of the time-domain spectra and the spectral width of the frequency-domain spectra are almost the same,but the sampling speed of the ASOPS system is significantly faster than that of the traditional THz-TDS system,which conduces to the study of the transient characteristics of substances.

Quantitative Analysis for Monitoring Formulation of Lubricating Oil Using Terahertz Time-Domain Transmission Spectroscopy

The quantitative analysis of zinc isopropyl-isooctyl-dithiophosphate (T204) mixed with lube base oil from Korea with viscosity index 70 (T204-Korea70) is presented by using terahertz time-domain spectroscopy (THz-TDS).Compared with the middle-infrared spectra of zinc n-butyl-isooctyl-dithiophosphate (T202) and T204,THz spectra of T202 and T204 show the weak broad absorption bands.Then,the absorption coefficients of the T204-Korea70 system follow Beer's law at the concentration from 0.124 to 4.024%.The experimental absorption spectra of T204-Korea70 agree with the calculated ones based on the standard absorption coefficients of T204 and Korea70.The quantitative analysis enables a strategy to monitor the formulation of lubricating oil in real time.

Consistency-dependent optical properties of lubricating grease studied by terahertz spectroscopy

The optical properties of four kinds of lubricating greases （urea, lithium, extreme pressure lithium, molybdenum disulfide lithium greases） with different NLGL （National Lubricant Grease Institute of America） numbers were investigated using terahertz time-domain spectroscopy. Greases with different NLGL grades have unique spectral features in the terahertz range. Comparison of the experimental data with predictions based on Lorentz Lorenz theory exhibited that the refractive indices of each kind of lubricating grease were dependent on the their consistency. In addition, molybdenum disullfide （MoS2） aa a libricant additive shows strong absorption from 0.2 to 1.4 THz, leading to higher absorption of MoS2-1ithium grease than that of lithium grease.

Terahertz generation and detection of LT-GaAs thin film photoconductive antennas excited by lasers of different wavelengths

A new method of generating and detecting terahertz waves is proposed.Low-temperature-grown gallium arsenide(LT-Ga As)thin films are prepared by etching a sacrificial layer(Al As)in a four-layer epitaxial structure constituted with LT-Ga As,Al As,Ga As,and semi-insulating gallium arsenide(SI-Ga As).The thin films are then transferred to clean silicon for fabricating the LT-Ga As thin film antennas.The quality and transmission characteristics of the films are analyzed by an800-nm asynchronous ultrafast time domain spectroscopy system,and the degree of bonding between the film and silicon wafer is determined.Two LT-Ga As thin film antennas for generating and detecting the terahertz waves are tested with a1550-nm femtosecond laser.The terahertz signal is successfully detected,proving the feasibility of this home-made LTGa As photoconductive antennas.This work lays a foundation for studying the mechanism of terahertz wave generation in Ga As photoconductive antennas below the semiconductor band gap.

Transient surface photoconductivity of GaAs emitter studied by terahertz pump-emission spectroscopy

This paper investigates the ultrafast carrier dynamics and surface photoconductivity of unbiased semi-insulating GaAs in detail by using a terahertz pump-emission technique. Based on theoretical modelling, it finds that transient photoconductivity plays a very important role in the temporal waveform of terahertz radiation pulse. Anomalous enhancement in both terahertz radiation and transient photoconductivity is observed after the excitation of pump pulse and we attribute these phenomena to carrier capture in the EL2 centers. Moreover, the pump power- and temperature- dependent measurements are also performed to verify this trapping model.

A Compressed Terahertz Imaging Method

A compressed terahertz imaging method using a terahertz time domain spectroscopy system(THz-TDSS)is suggested and demonstrated.In the method,a parallel THz wave with the beam diameter 4 cm from a usual THz-TDSS is used and a square shaped 2D echelon is placed in front of an imaged object.We confirm both in simulation and in experiment that only one terahertz time domain spectrum is needed to image the object.The image information is obtained from the compressed THz signal by deconvolution signal processing,and therefore the whole imaging time is greatly reduced in comparison with some other pulsed THz imaging methods.The present method will hopefully be used in real-time imaging.

Terahertz shaping technology based on coherent beam combining

The generation of terahertz(THz) waves by focusing a femtosecond pulsed laser beam at a distance is able to overcome the strong absorption properties of air and has rapidly attracted the attention of industry. However, the poor directionality of the THz wave radiation generated by this method is not conducive to THz wave applications. By controlling the morphology of the ultrafast laser-excited plasma filament and its electron density distribution through coherent beam combining technology, we achieve direct THz beam shaping and are able to obtain THz wave radiation of Gaussian or arbitrary transverse distribution. The novel experimental approach proposed in this paper opens up the research field of direct THz wave shaping using plasma. Moreover, it innovates multi-parameter convergence algorithms and, by doing so, has the potential to find beam patterns with higher energy conversion efficiency and break the energy limit of THz waves emitted by lasers at high power.

Progress of Terahertz Devices Based on Graphene

Graphene is a one-atom-thick planar sheet of sp2-hybridized orbital bonded honeycomb carbon crystal. Its gapless and linear energy spectra of electrons and holes lead to the unique carrier transport and optical properties, such as giant carrier mobility and broadband flat optical response. As a novel material, graphene has been regarded to be extremely suitable and competent for the development of terahertz （THz） optical devices. In this paper, the fundamental electronic and optic properties of graphene are described. Based on the energy band structure and light transmittance properties of graphene, many novel graphene based THz devices have been proposed, including modulator, generator, detector, and imaging device. This progress has been reviewed. Future research directions of the graphene devices for THz applications are also proposed.

Terahertz wave polarization rotation in bianisotropic metamaterials

Utilizing a polarization sensitive terahertz detection method where the detector is rotated by either 0° or 90°to measure the electric field Ep,8 （t） of each polarization component, we have characterized the properties of split ring res- onators. The strong polarization dependence of the bianisotropic-circular-current-driven and linear-polarization-induced resonances is in excellent agreement with the simulation when the p-polarized terahertz transmission is measured. How- ever, these electromagnetic responses vanish when the s-polarized terahertz transmission is measured. There is only a transmission minimum at 1.64 THz and the terahertz polarization rotation angle of about 90° is observed. The polarized terahertz transmission amplitudes and spectra detected at orthogonal orientations show that these behaviours are probably attributed to the birefringent effect of the sample.

Angle-sensitive and fast photovoltage of silver nanocluster embeded ZnO thin films induced by 1.064-μm pulsed laser

Silver nanocluster embedded ZnO composite thin film was observed to have an angle-sensitive and fast photovoltaic effect in the angle range from -90° to 90° , its peak value and the polarity varied regularly with the angle of incidence of the 1.064-μm pulsed Nd：YAG laser radiation onto the ZnO surface. Meanwhile, for each photovoltaic signal, its rising time reached -2 ns with an open-circuit photovoltage of -2 ns full width at half-maximum. This angle-sensitive fast photovoltaic effect is expected to put this composite film a candidate for angle-sensitive and fast photodetector.

The design of a photonic crystal filter in the terahertz range

One-dimensional photonic crystal structures for multiple channeled filtering and polarization selective filtering in the terahertz （THz） range are studied theoretically.The design of aperiodic photonic quantum-well （APQW） structures for multiple channeled filtering and different polarization filtering at arbitrary preassigned frequencies are achieved by using the simulated annealing algorithm with a special merit function. The parameters of these filters can be expediently controlled and the transmission characters are polarization dependent. Numerical simulations show that the designed APQWs can meet the desired specification well.

Power dependence of terahertz carrier frequency in a plasma-based two-color generation process

We conduct a frequency spectrum experiment to investigate terahertz（THz） emissions from laser-induced air plasma under different laser incident powers. The frequency spectra are measured using both air-biased-coherent detection and a Michelson interferometer. The red-shift of the THz pulse carrier frequency is observed as a response to increased pump power. These phenomena are related to plasma collisions and can be explained by the plasma collision model. Based on these findings, it is apparent that the tuning of the THz carrier frequency can be achieved through regulation of the pump beam.

Transient optical modulation properties in the terahertz metamaterial of split ring resonators

The ultrafast optical modulation properties of split ring resonators are characterized by utilizing optical pump-terahertz probe spectroscopy.The experimental results show that when the terahertz electric vector is perpendicular to the gap of the split ring resonator,resonant absorption can be quenched significantly under high pump excitation.However,when the terahertz electric vector is parallel to the gap,the resonant absorption is less sensitive to pump excitation due to the structural properties of the metamaterial.Our numerical simulations also demonstrate that the pump pulse significantly influences the split ring resonator current by generating carriers in the substrate.

Effect of external electric field on the terahertz transmission characteristics of electrolyte solutions

We fabricated a microfluidic chip with simple structure and good sealing performance,and studied the influence of the electric field on THz absorption intensity of liquid samples treated at different times by using THz time domain spectroscopy system.The tested liquids were deionised water and CuSO_(4),CuC_(l2),NaHCO_(3),Na_(2)CO_(3) and NaCl solutions.The transmission intensity of the THz wave increases as the standing time of the electrolyte solution in the electric field increases.The applied electric field alters the dipole moment of water molecules in the electrolyte solution,which affects the vibration and rotation of the whole water molecules,breaks the hydrogen bonds in the water,increases the number of single water molecules and leads to the enhancement of the THz transmission spectrum.

Anomalous behaviours of terahertz reflected waves transmitted from GaAs induced by optical pumping

Femtosecond pump-terahertz probe studies of carrier dynamics in semi-insulating CaAs have been investigated in detail for various pump powers. It is observed that, at high pump powers, the reflection peaks flip to the opposite polarity and dramatically enhance as the pump arrival time approaches the reflected wave of the terahertz pulse. The abnormal polarity-flip and enhancement can be interpreted by the pump-induced enhancement in the photoconductivity of GaAs and half-wave loss. Moreover, the carrier relaxation processes and surface states filling in GaAs are also studied in these measurements.

Diffraction of terahertz waves after passing through a Fresnel lens

The spatiotemporal and spectral characteristics of ultrawide-band terahertz pulses after passing through a Fresnel lens are studied by using the scalar diffraction theory. The simulation shows that the transmitted terahertz waveforms compress with increasing propagation distance, and the multi-frequency focusing phenomenon at different focal points is observed. Additionally, the distribution of terahertz fields in a plane perpendicular to the axis is also discussed, and it is found that the diffraction not only induces focusing on-axis but also inhibits focusing at off-axis positions. Therefore, the Fresnel lens may be a useful alternative approach to being a terahertz filter. Moreover, the terahertz pulses travelling as a basic mode of a Gaussian beam are discussed in detail.

Optical property and spectroscopy studies on the selected lubricating oil in the terahertz range

The optical property and spectroscopy of selected kinds of lubricating oil are studied based on the terahertz time-domain spectroscopy (THz-TDS) in the spectral range of 0.3-1.6 THz. The samples are classified by their characteristics via the near-infrared spectrum. The experimental results reveal that lubricating oil is more sensitive in the range of terahertz than in the near-infrared,and the specific kinds of lubricating oil can be identified according to their different spectral features in the terahertz range. The THz-TDS technology applied to lubricating oil analysis has potentially significant impact on the petroleum field.

Carrier dynamics and terahertz photoconductivity of doped silicon measured by femtosecond pump-terahertz probe spectroscopy

The carrier dynamics and terahertz photoconductivity in the n-type silicon (n-Si) as well as in the p-type Silicon (p-Si) have been investigated by using femtosecond pump-terahertz probe technique. The measurements show that the relative change of terahertz transmission of p-Si at low pump power is slightly smaller than that of n-Si,due to the lower carrier density induced by the recombination of original holes in the p-type material and the photogenerated electrons. At high pump power,the bigger change of terahertz transmission of p-Si originates from the greater mobility of the carriers compared to n-Si. The transient photoconductivities are calculated and fit well with the Drude-Smith model,showing that the mobility of the photogenerated carriers decreases with the increasing pump power. The obtained results indicate that femtosecond pump-terahertz probe technique is a promising method to investigate the carrier dynamics of semiconductors.

Study on terahertz spectra of SO_2 and H_2S

The spectral characteristics of air pollution gas sulfur dioxide and hydrogen sulfide has been studied experimentally and theo-retically in the range of 0.2-2.6 THz. The gases absorption spectra of sulfur dioxide and hydrogen sulfide, as measured by terahertz Time-Domain Spectroscopy (THz-TDS) technique, show equi-spaced absorption peaks. The peak intervals are varied for different gas and may relate with the molecule structures and rotation modes. We have calculated the intervals of rotational transition frequency according to the gases molecule structure and the rotational modes. The results are consistent with experimental results which confirm the suggestion that the absorption is coming from the molecular rotational transition. The study suggests a technique to detect air pollutants by THz-TDS and the rotational modes of gas molecules.

Diffusion interaction and quantitative analysis of zinc dialkyldithiophosphate content in lube base oils in terahertz regime

We investigate the diffusion interaction and quantitative analysis of zinc dialkyldithiophosphate （ZDDP） mixed with lube base oil （LBO） at different concentrations using terahertz time-domain spectroscopy （THzTDS）.When the concentration exceeds 6.78%,the characteristic absorption peaks exhibit significantly shift,and the absorption coefficient peak value is nonlinear against concentration.Moreover,the absorption coefficients of mixed samples follow the Beer＇s law at a concentration below 6.78%.The quantitative analysis enables a strategy for monitoring the formulation of lubricating oil in real time