This paper describes the terahertz electromagnetic pulses emitted when femtosecond laser pulses irradiate quantum wells, bulk semiconductors, and photoconductive antennas fabricated on low-temperature-grown GaAs substrates. The continuous-wave (CW) terahertz electromagnetic radiation emitted when these photoconductive antennas are excited by two single-frequency CW semiconductor lasers is also described, as are the applications of the resultant terahertz wave in fields such as imaging and spectroscopy.

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Widely tunable coherent terahertz (THz) wave generation was successfully demonstrated based on the laser light scattering from the lowest A₁-symmetry polariton mode by using a Q-switched Nd: YAG laser pumping. This method exhibits multiple advantages like wide tunability (frequency: 0.9-2.2 THz), coherency and compactness of its system. In this paper, the general performances of this THz generator, and its applications are reported. A spectroscopic application is demonstrated by measuring the water vapor absorption. This THz-wave source is also applied to the differential imaging. In a proof-of-concept experiment, we optically tagged objects embedded in a shade and measured the difference between transmittance at two wavelengths.

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We have generated enhanced Terahertz (THz) -radiation from semiconductors under magnetic field. The average power of THz-radiation, peaked at frequency of -0.3 THz, reaches 650 μW. We have studied the ellipticity of the THz radiation and have found that it depends strongly on magnetic field. In addition, we have demonstrated a new scheme for generating THz-radiation using a saturable Bragg reflector placed inside a mode-locked Ti: sapphire laser cavity. By using this scheme, THz-radiation with 0.8 μW has been observed.

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We describe fabrication of insulator-gap photoconductive switches forming by an atomic force microscope (AFM). The insulator-gap structure prevents discharge in a photoconductive gap, realizing strong electric field in photo-absorbing region. We made 43 nm, 100 nm and multi-gap photoconductive switches which generates strong THz signals. We have measured the transmission modes and the radiation modes. The transmission modes were measured by the electro-optic sampling which can measure the vector components of the electric field for the photoconductive switches with coplanar transmission lines. We have observed clearly the longitudinal components of electric field which can not be ignored in femtosecond region. The radiation modes from the switches with antenna structure were also measured by a Fourier transform polarizing interferometer. The radiation from the switch exceeds more than 1 THz and rather strong compared to the switch made by normal lift-off process.

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Ultrashort elctromagnetic pulses with time width of -0.5 ps are emitted from high temperature superconducting films by exciting with femtosecond laser pulses. The Fourier spectrum of them extends from 0.1 to 3 THz. The characteristics of the radiation are described in detail including various properties of the radiation related to magnetic fields. Various possible applications of this phenomenon are proposed and demonstrated. We propose a new field of “superconducting optoelectronics”, in which ultrafast optoelectronics and superconducting electronics are combined.

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The recent studies on the p-type Ge hot-hole laser oscillating in the terahertz wave region are described in this review. The present stage of development is now very close to the practical use as a tunable solid-state terahertz-wave source. Though only pulse oscillation has been realized due to heating so far, miniaturization of the p-Ge laser is advancing to obtain the continuous wave oscillation by decreasing the input power necessary for lasing. Lasing from a very small p-Ge laser of 1×1×5 mm³ in size has recently achieved with input peak power of only 300W.

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A small aperture or antenna can confine electromagnetic radiation to dimensions far smaller than the wavelength. Scanning such a structure in close proximity to an object can yield microscopic images with correspondingly high resolution. The benefit of using microwaves and millimeter-waves in scanning near-field microscopy lies in the promise of new types of material contrast. A good example is the mapping of electronic transport properties. In this review an overview of scanning near-field microwave, millimeter-wave, and submillimeter-wave microscopy is presented, a millimeter-wave microscope system using a slit-type probe is described in detail, and the application of this microscope for the visualization of photo-excited free carriers in a silicon substrate is demonstrated.

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We investigate the focusability of intense short-pulse THz radiation from InAs using the knife edge method. We find that the spatial distribution of this THz radiation intensity is Gaussian, and we can focus it down to approximately 2-mm in diameter. We also visualize the 3D THz-radiation intensity distribution around the focus point using a knife edge on automatic translation stages.

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A new simple scheme of the amplitude modulated harmonically mode-locked (AM-HML) Er-doped fiber ring laser is developed. The lasing characteristics are experimentally investigated, and a well-stabilized modelocked operation is confirmed. The AM-HML fiber ring laser generates nearly transform-limited sech pulse train.

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Uniform compression is one of the key issues of the laser fusion scheme. A nonsupported pellet has been desired not to degrade spherically symmetric implosion. A Magnetically Suspended Pellet (MSP) is possible to realize the pellet without any physical contact. To clarify characteristics of the electric current in the suspension magnet and the pellet position, we theoretically and experimentally analyzed the MSP system. We determined the essential parameters: the magnetic shape coefficient K_m = 10 and the specific permeability μ_m=15.

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A titanium nitride film for a transparent conductor was formed on a SiO₂ substrate at room temperature by using a pulsed laser deposition method. The ablation is promoted from a Ti target with KrF excimer laser irradiation in an N₂ ambient gas. The structure of the film measured by using XRD was mainly TiN polycrystalline. In the case of the film thickness thinner than the wavelength of visible light, the transmittance and sheet resistance of the films were in the range of 74-47 % and 1000-70Ω/_??_, respectively.

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The molecular nitrogen ion, N₂⁺, and singly and doubly charged atomic nitrogen N⁺, N²⁺ were formed by laser multiphoton ionization and detected in the time-of-flight (TOF) spectra. The excitation lasers were a Ti: sapphire femtosecond laser (up to 1.0×10¹⁶ W/cm² in 100 fs at a wavelength of 790 nm) and a nanosecond laser (up to 1.2×10¹³ W/cm² in 9 ns at 532 nm). The highest signal intensity among these ions resulted from N₂⁺ in the major region of the laser fluence.

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