Second-order nonlinear optical wavelength-conversion has been attractive for generating terahertz (THz) wave with high peak-power and for THz-wave detection with high sensitivity. Over 50 kilowatt peak-power THz-wave radiation and sensitive THz-wave detection down to several tens of atto-joule using LiNbO<sub>3</sub> or 4-dimethylamino-<i>N’</i>-methyl-4’- stilbazolium tosylate (DAST) crystals have been demonstrated. LiNbO<sub>3</sub> crystal is an eminent nonlinear crystal for converting wavelengths between THz wave and near infrared (NIR) at frequency range from 1 to 3 THz. Mixing THz wave with an intense NIR pump beam in the LiNbO<sub>3</sub> provides generation of a signal light at a different frequency because of efficient figure of merit. Additionally, sensitivity of up-conversion detection working at room temperature is more than that of cryogenically cooled THz detector. Here, we report on a sensitive THz-wave detection based on novel design using a slant-stripe-type periodically poled Mg doped lithium niobate (PPMgLN) for practical use. The efficient scheme that two optical waves, the pump and up-conversion signal beams, propagate collinearly in the PPMgLN to achieve effective parametric amplification for the signal beam was designed. Minimum THz-wave detection was achieved down to energy about 100 aJ at the frequency of 1.6 THz. The result leads to a novel THz detector based on fiber and integrated optics with high sensitivity, robustness, and easy handling. The nonlinear optical up-conversion detection is promising and broadening THz horizons.
Sensitive terahertz (THz)-wave sensor at room temperature is crucial for most applications such as 2-dimensional realtime
imaging and nonlinear phenomena in semiconductors caused by multi-photon absorption, light-induced ionization,
and saturated absorption. LiNbO<sub>3</sub> is a promising material for frequency up- and down-conversion because of its high
nonlinearity and high resistance to optical damage. In this report, we propose a slant-stripe-type periodically poled Mg
doped LiNbO<sub>3</sub> (PPMgLN) crystal for the construction of a practical THz detector. The PPMgLN solves compromised
optical design and low coupling efficiency between THz and infrared (IR) pump beam due to imperfect dichroic coupler.
The effective coupling of both pump beam and THz-wave into identical interaction region of up-conversion device
promotes the THz detector in practical use. The phase-matched-condition in slant-stripe-type PPMgLN was designed to
offer collinear propagation of two optical waves, the pump and up-conversion signal beams, because of efficient
frequency conversion. The phase-mached-condition was calculated and a slant-stripe-type PPMgLN with an angle (α) of
20° and a grating period (Λ) of 29.0 μm was used in this experiment. A minimum detectable energy of 0.3 pJ/pulse at the frequency of 1.6 THz was achieved with the pump energy of 1.8 mJ/pulse in room temperature. The dynamic range of the incident THz-wave energy of 60 dB was demonstrated. Further improving for the sensitivity using longer interaction
length in a PPMgLN crystal was also investigated.
We have suggested a wide range of real-life applications using novel terahertz imaging techniques. A high-resolution terahertz tomography was demonstrated by ultra short terahertz pulses using optical fiber and a nonlinear organic crystal. We also describe a non-destructive inspection system that can monitor the soot distribution in the ceramic filter using millimeter-to-terahertz wave computed tomography. Further we report on the thickness measurement of very thin films using high-sensitivity metal mesh filter. These techniques are directly applicable to the non-destructive testing in industries.
After more than a dozen years of basic research into the submillimeter and far infrared range, terahertz (THz) wave
research has finally come into its own, and is recognized by the world scientific community as a new frontier. While
femtosecond laser pumped THz wave sources have opened up a new vista in applied research, the ideal THz wave source
will likely require high coherence and wide tunability. When this level of quality is finally made available in a userfriendly
device, there is little doubt that applied research efforts into the THz region will enjoy a true renaissance. In this
direction we have developed a widely tunable injection seeded THz-wave parametric generator (is-TPG) that operates at
room temperature. The spectral resolution is the Fourier transform limit of the nanosecond THz wave pulses. In our
laboratory, THz-waves continue to broaden their range of applications as following. We have developed a basic
technology for THz imaging which allows detection and identification of drugs concealed in envelopes by introducing
the component spatial pattern analysis. On the other hand, for inspecting electrical failures in large scale integration
circuit, we developed the laser-THz emission microscope, which records the map of THz emission amplitude in a sample
upon excitation with fs laser pulses.
We introduce several types of terahertz- (THz) wave parametric sources. THz-waves can be generated by optical parametric processes based on laser light scattering from the polariton mode of nonlinear crystals. Using parametric oscillation of MgO-doped LiNbO<sub>3</sub> crystal pumped by a nanosecond Q-switched Nd:YAG laser, we have realized broadband sources as well as coherent (narrow band) and widely tunable THz-wave sources. The THz-wave Parametric Generator (TPG) generates a broadband THz wave using a simple configuration; the THz-wave Parametric Oscillator (TPO) and the injection seeded THz-wave Parametric Generator (is-TPG) are two sources that generate coherent, widely tunable THz radiation by suitably controlling the idler wave. We report the characteristics of the oscillation and the radiation including linewidth and tunability. Further, we show the recent progress about these THz-wave parametric sources. We developed two new kinds of TPG by using compact pump sources. One TPG includes a flash-lamp-pumped multimode Nd:YAG laser with a top-hat beam profile, that allows generating high energy, broadband THz waves. Fitting in a space as small as 12 cm × 22 cm (including the pump source) this TPG outputs more than 100 pJ/pulse, which is about 100 times higher than the best results previously reported for TPG. The other has a potential to be a narrow-linewidth injection-seeded TPG, based on an laser-diode-pumped single-mode microchip Nd:YAG laser. The pump laser linewidth is below 0.009 nm and its size is 105×30×32 mm<sup>3</sup>. This allowed us to achieve a narrow-linewidth compact injection-seeded terahertz-wave parametric generator.