Exploring dynamics in the far-infrared with terahertz spectroscopy.

Terahertz (THz) radiation, which occupies a relatively unexplored portion of the electromagnetic spectrum between the mid-infrared and microwave bands, offers innovative sensing and imaging technologies that can provide information unavailable through conventional methods such as microwave and X-ray techniques. With the advancement of THz technologies, THz sensing and imaging will impact a broad range of interdisciplinary fields, including chemical and biological detections and identifications. In particular, THz radiation offers the opportunity for transformational advances in defense and security. Recent work shows that THz technologies are promising for the standoff detection and identification of explosive targets.

Terahertz Spectroscopy and Imaging for Defense and Security Applications

Terahertz (THz) radiation, which occupies a relatively unexplored portion of the electromagnetic spectrum between the mid-infrared and microwave bands, offers inno- vative sensing and imaging technologies that can provide information unavailable through conventional methods such as microwave and X-ray techniques. With the advancement of THz technologies, THz sensing and imaging will impact a broad range of interdisciplinary fields, including chemical and biological detections and identifications. In particular, THz radiation offers the opportunity for transformational advances in defense and security. Recent work shows that THz technol- ogies are promising for the standoff detection and identifica- tion of explosive targets.

Terahertz Spectroscopy and Imaging for Defense and Security Applications Laboratory experiments indicate that, with more powerful sources and more sensitive detectors, terahertz techniques have great potential for detection and identification of concealed explosives.

Recent development of the terahertz time domain spectroscopy (THz-TDS) and its application to solids have been reviewed. This spectroscopy is unique in that the time-domain wave forms are measured at first and the complex optical constants are deduced directly by the Fourier transformation of them without resort to the Kramers-Kronig analysis. Various types of the THz-TDS systems are briefly described. Applications of the THz-TDS to various solids, i.e., semiconductors, superconductors, polymers, photonic crystals, and so on are also presented to demonstrate how widely this spectroscopy is applicable to characterization of solids.

Terahertz Time-Domain Spectroscopy of Solids: A Review

We demonstrate what is to our knowledge the first mode-locked Yb:KGd(WO4)2 laser. Using a semiconductor saturable-absorber mirror for passive mode locking, we obtain pulses of 176-fs duration with an average power of 1.1 W and a peak power of 64 kW at a center wavelength of 1037 nm. We achieve pulses as short as 112 fs at a lower output power. The laser is based on a standard delta cavity and pumped by two high-brightness laser diodes, making the whole system very simple and compact. Tuning the laser by means of a knife-edge results in mode-locked pulses within a wavelength range from 1032 to 1054 nm. In cw operation, we achieve output powers as high as 1.3 W.

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Diode-pumped femtosecond Yb:KGd(WO(4))(2) laser with 1.1-W average power.

The THz-radiation power from bulk InAs irradiated with femtosecond optical pulses is significantly enhanced and reaches 650 μW in a 1.7-T magnetic field with 1.5-W excitation power. The THz-radiation power is related almost quadratically both to the magnetic field and excitation laser power. We have also found that the power of the THz-radiation from an InAs sample in a magnetic field is over one order of magnitude higher than that from GaAs. Additionally, a dramatic change of ellipticity is observed, and the spectra of the horizontal and vertical polarization components are found to differ.

High average-power THz radiation from femtosecond laser-irradiated InAs in a magnetic field and its elliptical polarization characteristics

The THz radiation spectrum from InAs in a magnetic field irradiated with femtosecond pulses can be controlled by varying the excitation pulse width and chirp direction of the excitation pulse. A longer excitation pulse width produces lower-frequency THz radiation. Also, positively chirped pulse excitation will generate higher power and higher frequency THz radiation.

Spectrum control of THz radiation from InAs in a magnetic field by duration and frequency chirp of the excitation pulses

We report what we believe is the first all-solid-state tunable
ultraviolet laser pumped by the fifth harmonic of a
Q-switched Nd:YAG laser. Our laser based on a
Ce3+:LiLuF4 active medium stably generates a
single, satellite-free, 0.88-ns pulse under 5-ns, 10-Hz repetition rate
pumping conditions. A novel tilted-incident-angle side-pumping
scheme resulted in a simple laser-cavity design.

All-solid-state tunable ultraviolet subnanosecond laser with direct pumping by the fifth harmonic of a Nd:YAG laser

A large Ce3+:LiCaAlF6 (Ce:LiCAF) crystal of 15 mm diameter was grown successfully by the Czochralski method. Owing to its large size, the Ce:LiCAF crystal demonstrated a maximum output pulse energy from the Ce:LiCAF laser of 21 mJ at a wavelength of 290 nm with an absorbed pump energy of 54 mJ from the fourth harmonic of a Q-switched Nd:YAG laser operated at a 10-Hz repetition rate. Furthermore, the lasing threshold was a low 12 mJ, and the slope efficiency was 39%.

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Shinji Izumida

Papers

High average-power THz radiation from femtosecond laser-irradiated InAs in a magnetic field and its elliptical polarization characteristics

Spectrum control of THz radiation from InAs in a magnetic field by duration and frequency chirp of the excitation pulses

All-solid-state tunable ultraviolet subnanosecond laser with direct pumping by the fifth harmonic of a Nd:YAG laser

All-solid-state tunable ultraviolet subnanosecond laser with direct pumping by the fifth harmonic of a Nd:YAG laser

High-Pulse-Energy, All-Solid-State, Ultraviolet Laser Oscillator Using Large Czochralski-Grown Ce:LiCAF Crystal