The performance of intense Bessel femtosecond pulse propagation in dispersive medium is investigated. Water is chosen as the Kerr medium with large dispersion. We find that within the dispersion length, the temporal profile integrated over the beam radius is controlled mainly by dispersion, whereas the spectral profile is nearly undistorted. We also show the energy flow between different transverse parts of the beam, which provides a reservoir for the intense part of the Bessel beam and extends the nonlinear effect length between the pulse and the Kerr medium as we have previously found in argon.

Nonlinear processes associated with terahertz radiation generated via optical rectification from undoped GaP crystal, including second harmonic generation (SHG) and multiphoton absorption processes, are examined. Experimental results of polarization-resolved SHG are obtained.

Terrorism has become an international problem in recent years as evidenced by toxins mailed through the post, liquid explosives planted in airplanes, and so on. Clearly, the security screening of packing materials is highly required. We conduct nondestructive and contactless detection of some packing materials used in daily life by terahertz time-domain spectroscopy. The THz time-domain spectra of five typical kinds of packing materials are measured in the frequency range of 0.3–2.5 THz. THz absorption spectra and transmittance are also analyzed.

The characteristics of terahertz radiation from an n-type InN excited by femtosecond laser pulses tunable from 750 to 840 nm are experimentally studied. Terahertz emission from InN is closely bound up with the Dember effect. Terahertz emission can be interpreted as being emitted from accelerated photo-carriers excited by a femtosecond pulse in Dember field. Terahertz radiation from InN shows a strong dependence on excitation wavelength. Results show that under laser pulse excitation with a different center wavelength, the terahertz radiation shows different characteristics, such as radiation intensity, radiation efficiency, and spectrum width. This work will be propitious to the development of terahertz time-domain spectrum technology and the optimization of experiment system, as well as being a reference for conducting research on terahertz emission with higher radiation intensity and efficiency.

Terahertz (THz) wave transmission through a thin metal film with periodic arrays of subwavelength rectangular holes is investigated systematically. The roles of the waveguide resonances and surface plasmon polaritons (SPPs) in extraordinary transmission are also clarified. The transverse waveguide resonances (hole-shape dependence) dominates the position and width of maximum transmission. In addition, the periodicity of the hole arrays dominates the position of minimum transmission due to the excitation of SPP, which acts as a band-stop filter in the THz transmission spectra. Our results clearly demonstrate that the extraordinary transmission in THz frequency is originated from the transverse waveguide resonances that behave with a SPP-like character. Simulation based on finite-difference time-domain (FDTD) agrees well with the experimental results.

A new scheme to realize terahertz (THz) radiation generation, as well as high-speed THz modulation, for future high-speed THz communication systems, is proposed and discussed. The proposed experimental scheme includes two elemental parts: dual-wavelength mode-locking oscillating cavity and photomixing elements. The conditions for double-wavelength operation using the proposed experimental setup have been theoretically discussed and obtained. The mode-locked output optical short pulses with two highstability wavelengths are obtained and demonstrated. These can be used to produce THz radiation, as well as high-speed signal modulation, through the photomixing effects in an ultrafast oxygen-ion-implanted and epitaxial InGaAs:O for the future high-speed THz communication systems.

The electro-optic (EO) technique used in the terahertz spectral technology is described. Response functions—which are used to describe the EO detecting efficiency—of five types of cubic EO crystals are calculated. Based on these results, the choice of different crystal thicknesses is discussed. In the EO detecting procedure, certain factors, such as reflection, dispersion, absorption and velocity mismatch, may distort the temporal profile of the electric field being detected; therefore, these factors are calculated. The relationship between the field distortion and the bunch duration as well as the crystal thickness is considered.

We investigate the optical response of silicon-based VxOy film for terahertz (THz) transmission. We find that absorption of the THz wave by the film can be controlled by laser excitation. Using THz time-domain spectroscopy (THz-TDS), we observe that the amplitude of the THz pulse is modulated by the external optical beam. The linearity of the optical modulation is also analyzed. Weak modulation nonlinearity is found to be within tolerable range.

The transit characteristics of gain electrons in the dynamic electric field in one micro-channel, the relation of the temporal resolution, and the gating electric pulse are discussed in detail. The simulation analyses provide guidance on how to select parameters of the gating electric pulse for designing the X-ray picosecond framing camera, based on micro-channel plates, especially with regard to the aspect of the temporal resolution of the camera. Certain experimental results are presented.

Fulgide, a kind of thermally irreversible photochromic compound, can be used for polarization holographic recording owing to its photoinduced anisotropy and photochromic property under the irradiation of linear polarization light. In this letter, a new technique of optical image operation based on the polarization multiplexing scheme in the fulgide film is demonstrated, which can implement the readout of two individual orthogonal polarized images separately and the subtraction or summation of the two images by simply rotating a polarizer in front of the charge-coupled device (CCD) detector.

The characteristics of air plasma are studied using laser-induced breakdown spectroscopy at room temperature in air at atmospheric pressure. The electron temperature of 20796 K is determined using the Boltzmann plot method with six ionic nitrogen lines at 444.703, 463.054, 500.515, 566.663, 594.165, and 648.205 nm. The electron number density inferred by measuring the Stark broadened profile of well-isolated Hα line (656.273 nm) is 1.83 ￡ 1017 cm?3. The hypotheses of the local thermodynamic equilibrium and optically thin plasma are verified based on the experimental results. These results are beneficial for better understanding of the terahertz (THz) wave generation in pulsed laser induced air plasma.

The detonation temperature is measured both theoretically and experimentally by means of a complementary metal oxide semiconductors (CMOS) image sensor. An experimental system of multiwavelength temperature measurement based on CMOS image sensing technology is designed and realized. The instant spectral information of each moment is captured by row with the CMOS image sensor, depending on the unique characteristic of its rolling shutter. With the use of multiwavelength temperature measurement theory and by fitting spectrum emissivity through the regressive least square method, the color temperature of the detonation whose temperature is variable can be obtained continuously. The system is calibrated by measuring the standard spectrum of the laser. In our system, four time temperatures are measured within 10-μs detonation. Results show that the system has a higher measuring accuracy and speed, and can be applied in high-speed spectral information collection and storage during the detonation.

We report on a new technique of mutual injection phase-locking fiber-laser array. This technique uses a 45o beam splitter as coupling device. Injection phase-locking process of the fiber-laser array is simulated and analyzed. Results show that constant values of phase differences are related to initial phases. Reflectivity of the beam splitter and transmissivity of cavity reflectors should be properly optimized in order to make injection phase-locking easier.

An all-digital design method consisting of a laser drive circuit for gradually modulating the frequency, pulse width, and amplitude of the output of semiconductor laser is demonstrated. Field programmable gate arrays (FPGAs) and accurate delaying chip are used to generate the pulse. The repetition rate and pulse width of the laser pulse gradually adjust from 1 to 10 kHz and 700 ps to 3 ns, respectively. Furthermore, the time of the rising and falling edges for the laser pulse is less than 800 ps.

To achieve high peak current with narrow pulse width, the circuit model is analyzed based on a fast and high-power metallic oxide semiconductor field effecttransistor (MOSFET) as the high speed switch of the resistor-capacitor (RC) charge and discharge circuits. It is easy to obtain a narrow high-current pulse by adopting the narrow triggering pulse to control the on-off state of the MOSFET switch, and using the driving pulse to modulate the exponential decay pulse in the RC discharge loop. The procedure for the high speed MOSFET switch is then discussed. To make the speed of the MOSFET switch as quick as possible, the push-pull driving circuit for the grid of the MOSFET is brought forward and the circuit for producing the narrow trigger pulse is designed. The experimental result shows that the full width at half maximum (FWHM) of the trigger pulse is about 500 ps when the narrow trigger pulse is connected with the discharge return circuit. Measured results demonstrate that the RC discharge loop produces a narrow high-current pulse, with a peak current of up to 92.5 A and FWHM of 6.2 ns. After adjusting relevant parameters, the peak current could reach up to 115.9 A. However, the corresponding pulse width is broadened. Finally, influencing factors on the narrow pulse width for the discharge loop are briefly analyzed.

High-speed dynamic spectrum data of transient detection has become the major means of access to transient information. However, in terms of the characteristic of spectral data excessiveness and transience in the dynamic spectral detection system, the linear charge-coupled device (CCD) used in the system should have real-time-output and match the high-speed data storage equipment. Based on the transient spectrum characteristic, we introduce a high-speed dynamic spectrum data acquisition system with a high-linear array CCD. Through the field of programming gate array, the system provides an accurate driving clock for CCD and generates the control signals for analog-to-digital (A/D) conversion, storage, and transmission. Finally, the collected data by the peripheral component interconnect bus are summarized and filtered in the host computer. The results show that the CCD can stably work with a 40-MHz clock, and the frame scanning frequency can achieve 73 KHz. This design can remarkably complete the real-time measurement of the denotation transient temperature and achieve high-speed spectral information collection and storage with high accuracy and frame scanning frequency. It can be applied to other transient information acquisition.

Time-resolved diagnosis of the transient process using X-ray frame cameras (XRCs) is an important means in inertial confinement fusion (ICF) experiments. The sensitivity of the photocathode is a key parameter of the entire camera system. This letter aims to raise the quantum efficiency (QE) of the photocathode. With the changes in the deposition parameters, such as deposit angle, thickness of the coating in the channel, and vacuum of evaporation, the QE results are different. After testing and theoretical calculation, we find that there is a best matching value among these parameters. When the coating parameter meet this best value, the gain of the XRC can be improved significantly.

We propose a three-dimensional chiral metamaterial consisting of arrays of multi-layered mutually twisted sandwich metallic spirals. Such structure exhibits a negative refractive index at terahertz frequencies for its chirality. The chirality with varied refractive index can be achieved and tuned by changing the configurations. The presented structure offers flexibility for investigating electromagnetic properties of chiral metamaterials in the terahertz regime, thus leading to a unique route toward terahertz device applications.

A metamaterial (MM) absorber is very attractive in the terahertz (THz) regime for its potential applications as a bolometer and thermal emitter. In this letter, we propose a transmission line model for the MM absorber in order to identify the basic absorption mechanism involved. Some strategies are put forward to widen the absorption bands to over 250 GHz. A new kind of MM absorber is designed, fabricated, and measured. The results show that a strong absorption of over 90% with a bandwidth of over 300 GHz is obtained, facilitating wide-frequency applications.

We present a thermal active control over terahertz (THz) extraordinary transmission induced by a plasmon in a periodic subwavelength holes array and a patch array. Both arrays consist of high transition temperature YBCO superconductors using THz time domain spectroscopy (THz-TDS). In the periodic subwavelength YBCO holes array, we observe a transition between a virtual excitation type surface plasmon polaritons (SPP) mode and a real SPP mode accompanied with transmission amplitude modulation. This can be attributed to the c-axis Josephson plasma frequency. On the other hand, since dipole localized surface plasmon can be excited by direct optical illuminations without phase-matching techniques, we observe a transmission amplitude modulation induced by combining the normal state carriers and the superconducting carriers in the periodic subwavelength YBCO patch array. These THz superconducting plasmonic crystals hold great potential in application for extreme low-loss, large dynamic amplitude modulation, surface plasmon-based function devices.

We present a systematic study of freestanding terahertz (THz) metamaterials fabricated on Mylar film by self-aligned photolithography. THz time-domain spectroscopy (THz-TDS) transmission measurements and numerical simulations reveal the negative index of refraction in the frequency range of 0.66–0.90 THz under normal wave incidence. The observed resonance behaviors can be explained by a theoretical circuit model. The electromagnetic properties of such close-ring metamaterials are also explored in terms of geometrical parameters of the unit cell, thickness of the dielectric film, and conductivity of the close ring. This flexible metamaterial can pave the way for three-dimensional THz metamaterial fabrication and applications.

A new data-acquisition method based on the less movement of the time-delay stage is designed. The method can clearly reduce the time used in the entire imaging process in terahertz pulse imaging. According to the experimental result, the quality of imaging remains the same as the normal method or even better. The applicability and limitation of the method are also discussed.

Terahertz time-domain spectroscopy (THz-TDS) is a new coherent spectral technique to measure transmission spectra and analyze the spectral characteristics of samples. In this letter, the transmission spectra of edible pigments in the THz frequency range are measured using THz-TDS technology at both room temperature and low temperature in nitrogen atmosphere. Their refractive index spectra and absorbance are obtained. The result shows that there is some obviously abnormal dispersion in the refractive index spectra as well as absorption peaks in absorbance at the corresponding frequencies. THz-TDS is expected to have a potential application in the identification of pigments.

The transmission spectra of methylparaben, ethylparaben, propylparaben, and butylparaben in the range of 0.2–2.5 THz are measured using terahertz time-domain spectroscopy (THz-TDS). Their refractive index and absorption spectra are obtained at room temperature in nitrogen atmosphere. The results show that the four kinds of Nipagin esters have obvious absorption peaks at 1.54 and 1.83 THz for methylparaben, 1.98 THz for ethylparaben, 1.37 and 1.61 THz for propylparaben, and 0.76 and 1.68 THz for butylparaben. Their anomalous dispersion of the refractive index appears at the corresponding position. Based on the density functional theory (DFT), the vibration absorption spectra of their single molecule are simulated. The numerical simulation and experimental results generally agree with each other. Then the reasons for their minor differences are analyzed. Overall, the spectral properties of Nipagin esters are found to have great significance for the detection of preservatives and for food safety.

The field of terahertz (THz) science and technology has achieved significant progress over the last decades. Research interest focuses on THz spectroscopy and imaging. The signal-to-noise ratio (SNR) of the THz system is of great importance in the application of THz spectroscopy and imaging. In this letter, the wavelet de-noising technology is used to improve SNR and increase the speed of the THz time-domain spectroscopy system by reducing the repeating times.

The frequency-dependent absorption coefficients and refractive indexes of three selected pour point depressants (PPDs) are extracted within the spectral range of 0.2–2.5 THz using terahertz time-domain spectroscopy (THz-TDS). The selected PPDs are also characterized by the middle-infrared spectrum. The experimental results reveal that PPD is more sensitive in the THz range than that in the middle-infrared range. Moreover, the different compositions of PPD can be identified according to their different spectral features in the THz range. Due to its properties of better repeatability, shorter testing time, and easier operation, THz-TDS can be used as a complement for identifying the chemical compositions of PPD.

In this letter, we employ terahertz time-domain spectroscopy (THz-TDS) technology to measure the THz absorption spectrum and refractive index of benzoic acid at room temperature. Linearity combination of atomic orbital (LCAO) within the density functional theory (DFT) is also used to compute the vibrational absorption spectra of benzoic acid in monomer and dimer. In addition, based on the results obtained from density functional calculation, the origins of observed intra-molecular and inter-molecular vibrational modes under specific peaks are interpreted.

We report on the far–infrared optical properties of melamine characterized by terahertz time–domain spectroscopy (THz–TDS). Transmission measurement reveals three low–frequency vibration modes at 1.99, 2.25, and 2.61 THz, which may provide fingerprints for direct melamine detection. By combining THz–TDS with Fourier transform infrared (FTIR) spectroscopy, an overall low–frequency optical response of melamine is presented in an extended spectral range of 0.2–6.2 THz. In this range, the low-frequency vibration mode at 3.96 THz is recorded via FTIR. The measured THz spectra are well fit by the multiple–oscillator model, thereby demonstrating that the low–frequency THz response of melamine is a consequence of the lowest four vibration modes.

We observe the instantaneous frequency and amplitude modulations of the C-C stretching mode with a central frequency of 1343 cm?1 in a poly (substituted thiophene) using 6.3 fs laser pulses by the method of real-time vibrational spectroscopy. Spectrogram analysis shows that both amplitude and instantaneous frequency of the mode with a central frequency of about 1343 cm?1 are modulated. Both modulation frequencies are found to be 120?130 cm?1, which provides evidence of dynamical coupling between a stretching mode and another stretching mode mediated by a low-frequency bending mode.

Cylindrical vector beams are described by the representation formalism of finite electromagnetic beams in free space. This is achieved by factorizing the field vector into a mapping matrix and a Jones-like vector. The vectorial property of a finite beam can be described by a degree of freedom of the mapping matrix that can be determined by the azimuthal angle of a fixed unit vector with respect to the wave vector. In addition, it is a simple and flexible experimental device for converting linearly polarized light beams into cylindrical vector beams. The radially or azimuthally polarized beam is easily converted into either one of these by varying the applied voltage.

The pulse broadening and communication capacity in atmospheric turbulent and disperse channels are investigated. Models on pulse broadening are proposed, and the simulated results for a typical atmospheric condition and configuration are provided. The results show that the capacity of the system is affected mainly by atmospheric dispersion rather than atmospheric turbulence. The received pulse width can be minimized, and the system-achievable capacity can be maximized by selecting the optimum input pulse width. The proposed method can be used to predict the achievable capacity of a free space laser communication system in turbulent and disperse channels.

Pulses with a tunable central wavelength from 278-310 nm reaching <27 fs duration and >1 \mu J energy are generated by non-collinear four-wave mixing in a 0.5-mm CaF2 crystal.

We perform ultrafast transient absorption spectroscopy of oxy-hemoglobin by a pump-probe method with ultrashort laser pulses. Negative time-range of the observed traces, where the pump pulse precedes the probe pulse, is analyzed to study the electronic coherence dynamics between the excited state and the ground state, and the vibrational coherence in the electronic excited state. The analysis result shows that the electronic coherence between the electronic ground state and the Q-band state dephases in 45+(-)5 fs. The vibrational dephasing times of modes with frequencies of 210, 393, 554, 731, and 1106 cm?1 are found to be 449, 366, 555, 1085, and >1330 fs, respectively.

A novel optical fiber evanescent-wave (EW) sensing platform combining a taper and a refractive index (RI) gel residue is proposed. The platform includes two identical large core multimode fibers perpendicularly positioned to each other: one for excitation light delivery (i-fiber) and one for EW fluorescent signal collection (r-fiber). One end of the r-fiber is decladded to expose a segment of a cylindrical fiber core terminated with a taper. A small drop of rhodamine 6G (R6G) solution sample is distributed in such a way that it surrounds the side wall of the cylindrical portion of the core and covers the i-fiber end face. The fluorescent signal is recorded under the following conditions: 1) the entire taper is exposed to air; 2) the entire taper is immersed into a large gel block; 3) the taper is covered with a gel residue. A dramatic rise of the fluorescence signal is observed in the third case, which is over 20 times more than the level achieved from the first two cases. We reveal that the intensive mode coupling in the sandwiched air-gel-taper architecture accounts for this phenomenon, which is discussed in detail.

The features of Moire fringe generated by overlapping a circular and a linear grating are studied. Given that the pitch of circular grating is a and that of linear grating is P, the shapes of the Moire fringe that they form are hyperbola, parabola, and ellipse when a > P, a = P, and a < P, respectively. As the pitches of these two gratings become close to each other, the magnification of the Moire fringe is over 100, which is useful for the measurement of small displacement. This letter also discusses how the fill factor influences Moire fringe visibility.

Core/shell CdSe/ZnS quantum dots (QDs) are synthesized by thermal deposition using cadmium oxide and selenium as precursors for a hot lauric acid and hexadecylamine trioctylphosphine oxide hybrid. The CdSe/ZnS QDs show high photoluminescence efficiency, with a quantum yield greater than 48% and sizetunable emission wavelengths ranging from 500 to 620 nm. A diode that emits white light with color coordinates of (0.311, 0.321) and luminous efficiency of 46.7 lm/W at 20 mA, as standardized by the Commission Internationale de L’Eclairage or International Commission on Illumination, is fabricated by combining a blue indium-gallium nitride (InGaN) chip with multi-CdSe/ZnS QDs.

In order to improve the burn-resistant property of titanium (Ti) alloy, Ti-molybdenum (Mo) burn-resistant alloyed layer is coated onto the Ti6Al4V substrate with the double glow plasma surface alloying technology by permeating the Mo. The experiment is performed at a working temperature of 800 oC to keep the layer warm for 2.5 h under air pressure of 30 Pa. Mo and Ti6Al4V are used as the source electrode and cathode, respectively. The result of X-ray diffraction shows that the alloy phase of Ti and Mo is formed on the surface of Ti6Al4V substrate. The hardness of the alloyed layer on the surface is more than 800 HK. The friction result further indicates that wear rate can be reduced by three orders of magnitude. Laser ablation experiment is used to characterize the burn-resistant properties of the Ti alloy. The result indicates that the laser burn area of the treated Ti alloy is reduced to 1/12 of the untreated sample. Moreover, the burn-resistant properties improved greatly. Current experimental results clearly show that the laser ablation can be used to characterize the burn-resistant property of the material.

Chemical oxygen demand (COD) is a synthetic indicator that represents the degree of organic pollution in water. Compared with the conventional wet chemical method, near-infrared (NIR) transmission and ultraviolet (UV) absorbance method based on photoelectric detection technology and spectroscopy analysis offer advantages such as high precision, speed, non-contact, and absence of secondary pollution, to name a few. NIR transmission spectra and UV absorbance spectra of standard solution configured with phthalate hydrogen potassium were collected using multi purpose analyzer (MPA) Fourier transform infrared (FT-IR) spectrometer and AvaSpec-2048-2 UV spectrometer, respectively. After different pretreatment of the spectra, COD quantitative analysis model was established using partial least squares (PLS) regression and linear regression. The statistical analysis of COD quantitative model was implemented, and the result revealed that UV absorbance method had higher relevance but lower forecast accuracy and precision than the NIR transmission method.

A portable near-infrared (NIR) spectroscopy analyzer is developed for rice wine analysis. The analyzer is composed of a lamphouse, a temperature-controlled sample chamber, a mini-spectrometer, and a control unit. The mini-spectrometer is based on the crossed Czerny-Turner structure grating platform and a linear InGaAs array detector with spectrum range of 1100?2500 nm and spectral resolution of 1.5-12 nm full-width at half-maximum (FWHM). The control software written in VC++ consists of three parts: the spectra acquisition, the model establishment, and the sample analysis. The calibration models of determination of alcohol and nonsugar solids in rice wine are established with 54 samples. After calibration, the portable analyzer is used for real-time analysis of the rice wine sample.