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An effective and widely tunable means of producing terahertz (THz) radiation using difference-frequency generation (DFG) in an organic N-benzyl-2-methyl-4-nitroaniline (BNA) crystal was demonstrated by optimizing the pump wavelengths. We calculated the (wideband) refractive index and coherence length mapping of BNA to establish the optimum phase-matching condition of the DFG configuration. To satisfy the phase-matching conditions over the range 0.1-20 THz, both pump wavelengths were varied from 780 to 950 nm, and the optical wavelength dependency of the THz-wave output was clearly observed. We expanded the range of THz-wave generation and increased the power output to ten times that obtained using previous methods.

We propose a surface-emitted cavity configuration for a terahertz-wave parametric oscillator that allows THz wave emission perpendicular to the crystal surface without any output coupler. The oscillating idler and pump waves are reflected at the surface of a nonlinear crystal in a single resonance cavity, satisfying the noncollinear phasematching condition. The radiated THz wave has a Gaussian profile. The measured beam quality factors (M(2)) were 1.15 and 1.25 in the horizontal and vertical directions, respectively. The measured tunable range was 0.8-2.74 THz. A test of transmission imaging using a test pattern was demonstrated.

Terahertz-frequency (THz) waves have shown potential for a wide range of applications. We have developed tunable THz-wave sources using nonlinear optical crystals, which have several advantages, including frequency agility, wide tunability, high output, and high coherency. We found that the organic nonlinear crystal of 4-dimethylamino-N-methyl-4-stilbazolium-tosylate (DAST) had particular potential for ultra-wide THz-wave generation from sub-THz to mid-infrared frequencies. Using DAST, we manufactured a coherent, tunable source (1-40 THz) with frequency agility. Moreover, we demonstrated THz-wave detection through up-conversion using DAST or MgO:LiNbO 3 nonlinear optical crystals, which provided a fast response, high sensitivity, and room-temperature operation.

It is proposed and investigated the 8-cm-long slightly curved parallel plate oversized waveguide as wideband THz interconnecting line. Its use in waveguide TDS is demonstrated by measuring absorbance of tiny amount of water vapor.

We calculated wideband refractive index of BNA crystal and established the optimum phase-matched condition of DFG configuration. Terahertz generation ranges had expanded from 0.1 to 20THz, the maximum output power obtained was ten times.

We demonstrate widely tunable terahertz-wave generation using difference frequency generation (DFG) in an organic N-benzyl-2-methyl-4-nitroaniline (BNA) crystal. The frequency tuning of the BNA-DFG was obtained between 0.1 and 15 THz.

We demonstrate highly sensitive coherent detection of a monochromatic THz-wave pulse at room temperature using the frequency up-conversion in MgO:LiNbO3 combined with balanced homodyne detection. Both the intensity and phase of THz radiation are measured.

Widely tunable terahertz (THz) -wave generation using difference frequency generation (DFG) in an organic N-Benzyl-2-methyl-4-nitroaniline (BNA) crystal was demonstrated. An organic nonlinear optical (NLO) BNA crystal is one of the promising materials for efficient and strong THz-wave generation because of its potential to have a sufficiently large enough second-order optical nonlinearity. Large and high quality single crystals of BNA (Φ8×30mm) were successfully grown by a vertical Bridgman method. The NLO coefficient d33 of BNA crystal is about 230pm/V. It is the largest value reported for any yellow-colored NLO materials. BNA has low refractive index dispersion between the optical and THz-wave region, therefore the colinear phase matching condition of the Type0 configuration is satisfied by using 0.7~1μm band pump wavelength. So, we developed a near-infrared dual-wavelength pump source for BNA-DFG. Two KTiOPO4 (KTP) crystals were mounted on galvano scanners inside a double-pass optical parametric oscillator (OPO). It is pumped using a frequency-doubled Nd:YAG laser (532 nm, 8 ns, 100 Hz). The signal wave of the KTP-OPO output was controlled independently and rapidly using a galvano scanner. We successfully generated THz-wave using organic BNA crystal. The THz-wave generation range is from 0.1 to 15THz, while the pumping dual-wavelength is controlled in the 0.8-0.9μm range.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Widely tunable terahertz (THz)-wave generation using difference frequency generation (DFG) in an organic N-benzyl-2-methyl-4-nitroaniline (BNA) crystal was demonstrated. To our knowledge, this is the first report of THz-wave generation by BNA DFG. Large, high-quality single crystals of BNA (phi 8 mm x 30 mm) were grown using the vertical Bridgman method. The nonlinear optical (NLO) coefficient d(33) of the BNA crystal is approximately 234 pm/V, which is the largest value reported for any yellow NLO material. The collinear phase-matching condition of the type-0 configuration is satisfied using a 0.7-1 microm band pump wavelength. We generated THz waves using an organic BNA crystal; the generation range is 0.1-15 THz.

We developed an all-in-one THz-wave parametric oscillator driven by an original LD-pumped Q-switched Nd:YAG laser. The pump source and a ring-cavity THz-wave parametric oscillator were embedded into the package with almost A3-paper-size dimensions, with 155-mm-height.

Ultra-broadband as well as frequency-agile THz-wave generation has become feasible via phase-matched DAST-DFG. Covered frequency region is 1 THz to beyond 40 THz. The random-frequency access and the broad tunability provide promising THz-wave applications in various industrial and research fields. The THz-wave source and its applications will be discussed.

A simple instrument has been developed to carry out temperature dependent submillimeter/terahertz-wave spectroscopy using a polyvinylidene fluoride flexible hollow waveguide and an eggplant-shape launching lens.

A spectroscopy and imaging system using a ring cavity terahertz (THz) wave parametric oscillator was developed. We demonstrate the differentiation of tissue types in a watery microtomed biological section using differences of water concentration. The contrast of differentiation for watery tissues was improved over that of the dried tissue.

Using a ring-cavity terahertz (THz)-wave parametric oscillator as a THz-wave source, we demonstrated THz-wave absorption spectroscopy in water vapor. The THz-wave was fed into a Fabry-Perot interferometer to obtain a narrower spectral line width than that supplied by the source.

Total attenuated reflection (ATR) spectroscopy was performed using monochromatic tunable THz-wave parametric sources in the frequency range of 1-7 THz. Complex refractive indices of liquid samples were determined from analysis of the reflectance data using the Fresnel formula.

We have demonstrated a quasi-monolithic THz-wave parametric oscillator (TPO) to confer more stability, a lower threshold, and more compact size on THz-wave generating devices. In this report, we describe narrow linewidth generation in a quasi-monolithic TPO. The cavity configuration was designed so that the noncollinear phase-matching condition was satisfied in the crystal. A 5 mol% MgO:LiNbO 3 crystal within dimensions of 15 mm × 20 mm with three surfaces for total reflection was used as a nonlinear optical crystal. The quasi-monolithic TPO in a ring-cavity configuration consisted of a nonlinear optical crystal and a super-mirror that reflected the idler beam (l > ca. 1067 nm) and transmitted the pump beam (1064 nm). We obtained narrow oscillation linewidth of < 760 MHz at 1.6 THz of THz-wave radiation. The low threshold of the oscillation was around 5.4 mJ/pulse.

we developed a table-top, coherent tunable THz-wave spectrometer based on an injection-seeded THz-wave parametric generator, which was employed to study the far-infrared rotational spectrum of water vapor, N2 collision-broadening coefficient of water vapor is measured.

Real-time terahertz (THz) wave imaging has wide applications in areas such as security, industry, biology, medicine, pharmacy, and the arts. This report describes real-time room-temperature THz imaging by nonlinear optical frequency up-conversion in an organic 4-dimethylamino-N'-methyl-4'-stilbazolium tosylate (DAST) crystal, with high resolution reaching the diffraction limit. THz-wave images were converted to the near infrared region and then captured using an InGaAs camera in a tandem imaging system. The resolution of the imaging system was analyzed. Diffraction and interference of THz wave were observed in the experiments. Videos are supplied to show the interference pattern variation that occurs with sample moving and tilting.

Broadband terahertz (THz) waves were generated by optical parametric processes based on laser light scattering from the polariton mode of a nonlinear crystal. By using the parametric oscillation of a MgO-doped LiNbO3 crystal pumped by a nanosecond Q-switched Nd:YAG laser, we have realized a broadband, high-energy and compact THz-wave source. We report the development of a THz-wave parametric generator (TPG) using a small pump source with a short pulse width and a top-hat beam profile. These characteristics of the pump beam permit high-intensity pumping especially close to the output surface of the THz wave without thermal damage to the crystal surface. We also calculated the outcoupled THz wave for beams with two different intensity profiles: a top-hat beam (in this experiment) and a Gaussian beam (previously reported). The result shows the mechanism of the output energy and/or power enhancement.

We obtained far-infrared (THz) region absorption spectra of crystallite and single crystal molecules using a THz-wave parametric oscillator (TPO) spectroscopic system. We observed the vibration modes of mono- and disaccharides clearly and differences between the spectra of each molecule. We discuss the mechanism for the absorption peaks of several mono- and disaccharides in the THz-region.

A compact, tabletop, high-resolution THz-wave spectrometer for trace gases was developed based on an injection-seeded THz-wave parametric generator. The chambers were purged with pure nitrogen gas to avoid strong absorption of THz-wave by water vapor. THz-wave spectra were measured over the wide frequency range of 0.8 to 2.4 THz with a resolution better than 100 MHz. Monitoring of sub-ppm H2O concentrations in real time was demonstrated.

We demonstrated a continuously tunable quasi-monolithic THz-wave parametric oscillator. A 5 mol% MgO:LiNbO3 crystal within dimensions of 15 times 20 mm2 was used as a nonlinear optical crystal. Frequency-tunability of THz-wave radiation was obtained by moving the super mirror linearly. The tunable range from 1.65 to 2.71 THz was obtained, which was derived from measured idler wavelength and pump wavelength with an optical spectrum analyzer.

An all-solid-state tunable injection-seeded THz-wave parametric generator (is-TPG) was developed. It can emit the monochromatic THz-wave over a wide tunable frequency range from 0.4 THz to 2.8 THz. The frequency scanning rate was as fast as 3GHz/second for the step resolution of 6 MHz by using the 500 Hz high-repetition-rate Q-switch Nd:YAG laser as a pump source. The linewidth of the THz-wave was shown to be lower than 40 MHz by the measurement of absorption lines of water vapor at low pressure.

We developed two new kinds of terahertz-wave parametric generators (TPG) by using compact pump sources. One TPG generates high energy and broadband THz-waves with high stability, the other has a potential to be a narrow-linewidth injection-seeded TPG.

A narrow linewidth, widely tunable, fast-data-acquisition injection-seeded THz-wave parametric generator (is-TPG) was developed. It is a powerful source for practical applications in the THz-wave regime. Wavelength-modulated THz-wave spectroscopy is developing based on this is-TPG.

A quasi-monolithic ring THz-wave parametric oscillator with the size of 20times35 mm2 was demonstrated. The oscillation threshold at 150 mum radiation was reducible to a ca. 5 mJ/pulse.

We developed a spectroscopic imaging system using a ring-cavity THz-wave parametric oscillator. The system is compact, robust and easy to handle. It is suitable for many practical applications. Powder samples were investigated as a demonstration.

We have developed frequency-agile THz-wave parametric oscillators (TPO). The random-frequency access and the rapid tunability provide promising THz-wave applications in various industrial and research fields. The THz-wave source and its applications will be discussed.

Using THz parametric oscillation of a LiNbO3 crystal pumped by a Q-switched Nd:YAG laser, we have realized widely tunable coherent THz-wave sources. Continuous-wave THz-wave generation using DFM with periodically poled LiNbO3 is also discussed.

We developed twonewkinds ofterahertz-wave parametric generators (TPG)byusing compact pumpsources. OneTPG includes aflash-lamp-pumped multimode Nd:YAGlaser with a top-hat beamprofile, which generates high energy, broadband THzwaves. Theother isanarrow-linewidth injection-seeded TPG,basedon an LD-pumped single-mode microchip

We demonstrated a threshold-reduced compact terahertz-wave parametric oscillator, which uses high performance mirrors. It achieved threshold intensity reduction of 23%. We obtained THz-wave radiation in the range of 0.96 to 3 THz, thereby enhancing the output energy.

An achromatically injection-seeded terahertz-wave parametric generator was constructed with MgO:LiNbO/sub 3/ crystals and a tunable seeder using a stationary dispersion-compensated optical arrangement. We obtained smooth tuning over the 0.6/spl sim/2.6 THz by adjusting the seeder wavelength alone.

We demonstrated the reflection imaging system using a terahertz-wave parametric oscillator (TPO). Reflection images were acquired on a test sample consisting of bands of aluminum film, as well as on pig tissue.

An achromatically injection-seeded terahertz-wave parametric generator (ais-TPG) and a tera-photonics spectra system were closely integrated in an air-free aluminum box with a volume of about 44×66×17 cm3 and successfully used to measure terahertz-wave absorption spectra.

A new type of flexible THz-wave hollow waveguide was developed using ferroelectric polymer PVDF as the cladding material of the waveguide. The transmission efficiency of the PVDF waveguide was better than a metal coated hollow waveguide with the same size and structure as the PVDF waveguide. We also report the tilt-angle dependencies of the transmission efficiency of the hollow PVDF and metal waveguides. The polarization dependency of the PVDF waveguide was better than that of a metal waveguide.

We report on the development of a widely tunable (frequency: 0.6-2.6 THz), injection-seeded THz-wave parametric generator (is-TPG), which operates at room temperature. The spectral resolution (

We demonstrate THz-wave imaging using a widely tunable THz-wave parametric oscillator. Two Si-bolometer detectors were used to obtain stable measurements. The imaging system was controlled by a PC using the program Labview. The targets of the THz-wave imaging were a T-shaped piece of polyethylene and a P-shaped piece of Teflon within an envelope that was opaque for visible light. We obtained clear images at 1.6 and 1.8 THz. Moreover, a unique differential image was successfully derived from the difference between the 1.6 and 1.8 THz images.

Summary form only given. We report a nanosecond optical parametric generator (OPG) using periodically poled lithium niobate (PPLN). The line-width of the OPG radiation was reduced to about 500 MHz by injection seeding with a Rittrow- or Rittman-type LD. Although an injection seeded PPLN-OPG has previously been reported, our system has simpler wavelength tuning using a telecom-type LD. The OPG radiation was easily tuned by changing the wavelength of the injection seeder radiation, and was mode-hop free. The mode-hop free performance of the OPG radiation made gas spectroscopy easy to perform. An example is shown of the measured absorption spectrum of N/sub 2/O gas.

We developed a diode-pumped, Q-switched Nd:YAG laser as a pump source for a THz-wave parametric generator (TPG). A compact TPG system was realized by using this laser, and the broadest THz-wave spectrum (1.2-2.4 THz) was obtained.

A transform-limited, narrow-linewidth (/spl sim/120 MHz), THz-wave parametric generator (TPG) was demonstrated using injection seeding method. This resulted in an wide tunability from 125 to 430 /spl mu/m (frequency: 0.7-2.4 THz, wavenumber: 23-80 cm/sup -1/).

An arrayed Si-prism coupler was introduced into the THz-wave parametric oscillator to increase the efficiency and decrease the diffraction angle. An output of /spl sim/20 mW peak and diffraction angle of 0.5 deg were obtained by this method.

We have developed a Stokes polarimeter system which can be used at an arbitrary, wide terahertz frequency region by using a wave plate and difference frequency generation system with DAST crystal (DAST-DFG). Conventional wave plates made of dielectric materials with intrinsic birefringence at optical wavelengths cannot be applied directly to terahertz frequencies because of the large absorption loss. High resistivity silicon, which remains transparent even at the higher terahertz frequency region, was selected as the wave plate material. Although the silicon has intrinsic birefringence hardly, large form birefringence can be successfully induced by fabricating a one-dimensional grating structure of sub-wavelength scale on its surface and proper operation as wave plate can be confirmed. By using the wave plate with a polarizer on DAST-DFG system, Stokes polarimeter system can be also successfully developed. Our approach can be extended to an arbitrary, wide terahertz frequency region and open many terahertz-wave applications associated with polarization physics at there.

We report on the detection of terahertz radiation by an on-chip planar asymmetric plasmonic structure in the frequency region above one terahertz. The detector is based on a field-effect transistor that has a dual grating gate structure with an asymmetric unit cell, which provides a geometrical asymmetry within the structure. Biasing the detector with a dc source-to-drain current in the linear region of the current-voltage characteristic introduces an additional asymmetry (electrical asymmetry) that enhances the detector responsivity by more than one order of magnitude (by a factor of 20) as compared with the unbiased case due to the cooperative effect of the geometrical and electrical asymmetries. In addition to the responsivity enhancement, we report a relatively low noise equivalent power and a peculiar non-monotonic dependence of the responsivity on the frequency, which results from the multi-plasmonic-cavity structure of the device.

By applying the frequency conversion technique to 4-dimethylamino-N-methyl-4-stilbazolium tosylate crystal, a monochromatic terahertz (THz) measurement system, including both generation and detection, has been developed over quite a broad frequency band, from 1.85 to 30 THz. In the case of frequency upconversion detection of THz waves, for the first time, we used gratings instead of filters to tackle the tough phase-matching conditions for broadband operations. By synchronizing the rotation of two gratings to extract the frequency upconverted signal, the infrared (IR) pumping beam can be tuned freely over 300 nm with decent diffraction efficiency and sufficient isolation between the weak frequency upconversion signal and the strong IR pumping beam to be realized. Such a large tuning range has overcome the limit of commercial filters with a fixed passband, while such a high optical density value has been beyond the limit of commercial tunable filters. Consequently, the proposed frequency domain system gives the largest THz frequency band. Unlike THz time-domain spectroscopy systems in which a fs laser is applied and broadband THz pulses are applied, our system works based on a ns laser and it can function at a single THz frequency with random frequency access ability from pulse to pulse.

The importance of terahertz (THz) wave techniques has been demonstrated in various fields, and the range of applications is now expanding rapidly. However, the practical implementation of THz science to solve the real-world problems is restricted due to the lack not only of convenient high power THz wave emitters and sensitive detectors but also of efficient quasi-optical active devices such as amplifiers. In this work, we demonstrate the direct amplification of THz waves in room temperature using magnesium oxide-doped lithium niobate (MgO:LiNbO3) crystals as the nonlinear gain medium. The input THz wave is injected as a seed beam along with the pump beam into the nonlinear crystal and it is amplified by the optical parametric process. We report gain in excess of 30 dB with an input THz pulse energy of less than 1 pJ. We believe that this demonstration will contribute to the convenience and further applicability of THz frequency techniques.

Real-time terahertz (THz) wave imaging has wide applications in areas such as security, industry, biology, medicine, pharmacy, and arts. In this letter, we report on real-time room-temperature THz imaging by nonlinear optical frequency up-conversion in organic 4-dimethylamino-N′-methyl-4′-stilbazolium tosylate crystal. The active projection-imaging system consisted of (1) THz wave generation, (2) THz-near-infrared hybrid optics, (3) THz wave up-conversion, and (4) an InGaAs camera working at 60 frames per second. The pumping laser system consisted of two optical parametric oscillators pumped by a nano-second frequency-doubled Nd:YAG laser. THz-wave images of handmade samples at 19.3 THz were taken, and videos of a sample moving and a ruler stuck with a black polyethylene film moving were supplied online to show real-time ability. Thanks to the high speed and high responsivity of this technology, real-time THz imaging with a higher signal-to-noise ratio than a commercially available THz micro-bolometer camera was proven to be feasible. By changing the phase-matching condition, i.e., by changing the wavelength of the pumping laser, we suggest THz imaging with a narrow THz frequency band of interest in a wide range from approximately 2 to 30 THz is possible.

We propose a slant-stripe-type periodically poled LiNbO3 crystal for the construction of a practical quasi-phase-matched (QPM) device for terahertz (THz) detection. A minimum detectable THz-wave energy of 25 fJ/pulse is demonstrated, and a linear input-output property with a dynamic range of 60 dB is achieved. The working frequency range of 0.15 THz for THz detection is obtained, and the central frequency of the sensitivity can be controlled by the design of the periodically poled structure. THz detection using this QPM device is a promising technique that may allow the detection of a coherent THz photon.