A survey on the mechanisms of powerful terahertz (THz) radiation from laser plasmas is presented. Firstly, an analytical model is described, showing that a transverse net current formed in a plasma can be converted into THz radiations at the plasma oscillation frequency. This theory is applied to explain THz generation in a gas driven by two-color laser pulses. It is also applied to THz generation in a tenuous plasma driven by a chirped laser pulse, a few-cycle laser pulse, a DC/AC bias electric field. These are well verified by particle-in-cell simulations, demonstrating that THz radiations produced in these approaches are nearly single-cycles and linear polarized. In the chirped laser scheme and the few-cycle laser scheme, THz radiations with the peak field strength of tens of MV/cm and the peak power of gigawatt can be achieved with the incident laser intensity less than 10^{17} W/cm2.

The development of teranertz (THz) quantum cascade lasers (QCLs) has progressed considerably since their advent almost a decade ago. THz QCLs operating in a frequency range from 1.4 to 4 THz with electron-phonon scattering mediated depopulation schemes are described. Several different types of GaAs/AlGaAs superlattice designs are reviewed. Some of the best temperature performances are obtained by the so-called resonant-phonon designs that are described. Operation above a temperature of 160 K has been obtained across the spectrum for THz QCLs operating at \nu >1.8 THz. The maximum operating temperature of previously reported THz QCLs has empirically been limited to a value of ~\hbar \omega/kB. A new design scheme for THz QCLs with scattering-assisted injection is shown to surpass this empirical temperature barrier, and is promising to improve the maximum operating temperatures of THz QCLs even further.

The progress achieved on power scaling and compact and portable THz sources is reviewed. By reversely stacking the GaP plates, the photon conversion efficiency is improved from 25% to 40% which corresponds to the maximum value. When the number of the plates is increased from four to five, the output power decreases because of back conversion. The THz generation is also investigated by mixing the two frequencies generated by a single Nd:YLF solid-state laser. The average output power reaches 1 \mu W. The introduction of two Nd:YLF crystals significantly improves the output power to 4.5 μW. This configuration facilitates the generation of different output frequencies.

We present a review of the development of a compact and high-power broadband terahertz (THz) source optically excited by a femtosecond photonic crystal fiber (PCF) amplifier. The large mode area of the PCF and the stretcher-free configuration make the pump source compact and very efficient. Broadband THz pulses of 150 μW extending from 0.1 to 3.5 THz are generated from a 3-mm-thick GaP crystal through optical rectification of 12-W pump pulses with duration of 66 fs and a repetition rate of 52 MHz. A strong saturation effect is observed, which is attributed to pump pulse absorption; a Z-scan measurement shows that three-photon absorption dominates the nonlinear absorption when the crystal is pumped by femtosecond pulses at 1 040 nm. A further scale-up of the THz source power is expected to find important applications in THz nonlinear optics and nonlinear THz spectroscopy.

Three time-resolved terahertz (THz) spectroscopy methods (optical-pump/THz-probe spectroscopy, THz-pump/THz-probe spectroscopy, and THz-pump/optical-probe spectroscopy) are reviewed. These are used to characterize ultrafast dynamics in photo- or THz-excited semiconductors, superconductors, nanomaterials, and other materials. In particular, the optical-pump/THz-probe spectroscopy is utilized to investigate carrier dynamics and the related intervalley scattering phenomena in semiconductors. The recent development of intense pulsed THz sources is expected to affect the research in nonlinear THz responses of various materials.

Ultrafast electromagnetic waves radiated from semiconductor material under high electric fields and photoexcited by femtosecond laser pulses have been recorded by using terahertz time domain spectroscopy (THz-TDS). The waveforms of these electromagnetic waves reflect the dynamics of the photoexcited carriers in the semiconductor material, thus, THz-TDS provides a unique opportunity to observe directly the temporal and spatial evolutions of non-equilibrium transport of carriers within sub-picosecond time scale. We report on the observed THz emission waveforms emitted from GaAs by using a novel technology, the time domain THz electro-optic (EO) sampling, which has a bipolar feature, i.e., an initial positive peak and a subsequent negative dip that arises from its velocity overshoot. The initial positive peak has been interpreted as electron acceleration in the bottom of valley in GaAs, where electrons have a light effective mass. The subsequent negative dip has been attributed to intervalley transfer from to X and L valleys. Furthermore, the power dissipation spectra of the bulk GaAs in THz range are also investigated by using the Fourier transformation of the time domain THz traces. From the power dissipation spectra, the cutoff frequency for negative power dissipation (i.e., gain) under step electric field in the bulk GaAs can also be obtained. The cutoff frequency for the gain gradually increases with increasing electric fields up to 50 kV/cm and achieves saturation at approximately 1 THz at 300 K. Furthermore, based on the temperature dependence of the cutoff frequency, we find that this cutoff frequency is governed by the energy relaxation process of electrons from L to valley via successive optical phonon emission.

An overview of the major techniques to generate and detect THz radiation so far, especially the major approaches to generate and detect coherent ultra-short THz pulses using ultra-short pulsed laser, has been presented. And also, this paper, in particularly, focuses on broadband THz spectroscopy and addresses on a number of issues relevant to generation and detection of broadband pulsed THz radiation as well as broadband time-domain THz spectroscopy (THz-TDS) with the help of ultra-short pulsed laser. The time-domain waveforms of coherent ultra-short THz pulses from photoconductive antenna excited by femtosecond laser with different pulse durations and their corresponding Fourier-transformed spectra have been obtained via the numerical simulation of ultrafast dynamics between femtosecond laser pulse and photoconductive material. The origins of fringes modulated on the top of broadband amplitude spectrum, which is measured by electric-optic detector based on thin nonlinear crystal and extracted by fast Fourier transformation, have been analyzed and the major solutions to get rid of these fringes are discussed.

We demonstrate two distinct emerging terahertz (THz) biomedical imaging techniques. One is based on the use of a new single frequency THz quantum cascade laser and the other is based on broadband THz time domain spectrocopy. The first method is employed to derive a metastasis lung tissue imaging at 3.7 THz with clear contrast between cancerous and healthy areas. The second approach is used to study an osseous tissue under several imaging modalities and achieve full THz spectroscopic imaging based on the frequency domain or on a fixed THz propagation time-delay. Sufficient contrast is achieved which facilitated the identification of regions with different cellular types and density compositions.

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 (THz-TDS). 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.

Terahertz (THz) spectra of bentazon are determined within the range of 0.3 2.4 THz at room temperature. Density functional methods are used to compute the THz spectra using three different programs: Gaussian03 for isolated-molecule form, DMol3 and CRYSTAL09 for solid-state forms. Among the three, the computed THz spectrum of CRYSTAL09 shows better bond length and angle agreements with X-ray experimental results, and corresponds with observed THz experiment spectral characteristics. The isolated-molecule vibrational mode values are less by half than those derived from solid-state calculations. The last five peak positions of the two solid-state computations coincide with each other. Moreover, all the experimental THz absorption peaks are assigned by utilizing CRYSTAL09.

An equivalent circuit model for the analysis and design of terahertz (THz) metamaterial filters is presented. The proposed model, derived based on LMC equivalent circuits, takes into account the detailed geometrical parameters and the presence of a dielectric substrate with the existing analytic expressions for self-inductance, mutual inductance, and capacitance. The model is in good agreement with the experimental measurements and full-wave simulations. Exploiting the circuit model has made it possible to predict accurately the resonance frequency of the proposed structures and thus, quick and accurate process of designing THz device from artificial metamaterials is offered.

We numerically investigate the trade-offs between the dispersion properties, coupling efficiency, and geometrical constraints in dual-wire (twin-lead) terahertz (THz) waveguides. In particular, we show that their inherent linearly polarized quasi-transverse electromagnetic (TEM) modes exist for waveguide transverse dimensions comparable with the wavelength, enabling significant end-fire coupling (>10%) for numericalaperture limited Gaussian beams while supporting a relatively low-dispersion propagation of below 0.5 ps2/m, as desired for short-pulse time-domain spectroscopy applications. Starting from the dual-wire structure, we also demonstrate that low-dispersion tapers can be designed to improve coupling efficiency.

We present a theoretical investigation of THz long-range surface plasmon polaritons propagating on thin layers of InSb. The metallic behavior of doped semiconductors at THz frequencies allows the excitation of surface plasmon polaritons with propagation and confinement lengths that can be actively controlled. This control is achieved by acting on the free carrier density, which can be realized by changing the temperature of InSb.

We design and analyze a novel multiband left-handed metamaterial based on a fishnet-like structure at terahertz (THz) frequencies. The metamaterial exhibits simultaneous negative refractions around the frequencies of 0.48, 1.05, and 1.19 THz for the electromagnetic (EM) wave normal incidence, and around the frequencies of 0.20, 0.79, and 1.13 THz for parallel incidence. The simulated results verify the left-handed properties. A particularly important observation is the capability of the proposed metamaterial with a single geometrical structure to display multifrequency operations in a unit cell. The compact metamaterial is a major step toward the miniaturization of THz materials and devices suitable for multifrequencies.

A sensing system, with Michelson-type fiber optical interferometer based on single fiber Bragg grating (FBG) as the reflector, is demonstrated. The system used a frequency-matched ring fiber optical laser as the source. The closed Michelson-type fiber optical interferometer system will be helpful in simplifying the developed interferometric sensor by replacing the double reflectors with only one FBG reflecting the double-side light. The basic sensing properties of the system are demonstrated, with a fiber optic piezoelectric ceramic transducer embedded in the arm of the interferometer simulating the sensing signal.

We propose employing multi-beam propagating technology to mitigate the influence of atmospheric scintillation to the wireless optical code division multiple access (WOCDMA) system and then deduce the bit error rate (BER) formulas of systems in weak and strong scintillations, respectively. According to simulation experiment results, multi-beam propagation can improve the system performance very well compared with single-beam propagating technique. Moreover, the more beams we use, the better the performance we get. When the received optical power is –30 dBm, the BER of the system employing four beams is 5 and 1 dB lower than that of using single-beam propagating technique in weak and strong scintillations, respectively.

The influence of GaAs substrate on the transmission performance of a multi-film Fabry-Peerot filter (FPF), fabricated by metalorganic chemical vapor deposition epitaxial growth on GaAs substrate, is investigated using the transfer matrix method. On the basis of the theoretical simulation, we determine that the quality of the resonant transmission peak of this epitaxially grown FPF (EG-FPF) deteriorates through splitting when the substrate is taken into account. Rapid periodic oscillation of peak-transmittivity along with the alteration of substrate thickness is also observed in the simulation results. Finally, a remarkably improved transmission performance of the EG-FPF is obtained by thinning the substrate down to a suitable thickness range through well-controlled grinding and polishing.

Based on the theory of quasi-three-level rate equations modified by amplified spontaneous emission, the stored energy density and the small signal gain of the cryogenic Yb:YAG regenerative amplifier for a given geometry for pulsed pumping in three dimensions are theoretically studied using the Monte Carlo simulation. The present model provides a straightforward procedure to design the Yb:YAG parameters and the optical coupling system for optimization when running at cryogenic temperature. A fiber-coupled laser diode end-pumped cryogenic Yb:YAG regenerative amplifier running at 1 030 nm is demonstrated with a maximum output energy 10.2 mJ at a repetition rate of 10 Hz. A very good agreement between the experiments and the theoretical model is achieved.

The capabilities of a compact and highly efficient NdAl3(BO3)4 (NAB) thin-disk laser are demonstrated. Under a pump power of 8.2 W, the NAB disk laser delivers an average output power of 4.6 W at 1 063 nm, with a slope efficiency of 64%. The difficulty and complexity of the thin-disk laser design are minimized by the high absorbance of the NAB crystal. To reduce the thermal effect, low repetition frequency pulsed 885 nm direct pumping is considered an efficient way to realize a compact and highly efficient NAB thin-disk laser.

A Yb:YAG disk laser with V-shaped stable resonator and active-mirror configuration, end-pumped by a 940-nm InGaAs laser diode array, is demonstrated. Performances and optimization of the disk laser at low temperature over a range of 130–200 K are investigated theoretically and experimentally. Laser output energy of 1.46 J/pulse operating at 10-Hz repetition rate is obtained with the optimum output coupler transmission of 30%, and the corresponding optical-to-optical efficiency is 48.7%.

We demonstrate a high-power single-frequency master oscillator power amplifier (MOPA) fiber laser. The central wavelength of the single-frequency fiber laser seed is 1 063.8 nm, with a linewidth narrower than 20 kHz and output power of 120 mW. By using two-stage amplification, a single-frequency fiber laser with an output power of 122 W is obtained, and the optical-optical conversion efficiency is 72%. No significant amplified spontaneous emission (ASE) or stimulated Brillouin scattering (SBS) is observed. The output power can be further increased by launching more pump power.

We demonstrate that the maximum coherent superposition state can be selectively prepared using a sequence of pulse pairs in lambda-type atomic systems, with the final level as a doublet. In each pair, the Stocks pulse comes before the pump pulse, with their back edges overlapping. Numerical results indicate that by tuning the interval of the adjacent pulse pairs, the selective maximum coherent superposition state preparation between the initial and one of the ˉnal levels can be achieved. The phenomenon is caused by the accumulative property of the pulse sequence.