Hiroaki Minamide

Effective terahertz-to-near-infrared photon conversion in slant-stripe-type periodically poled LiNbO3

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.

Terahertz wave parametric amplifier

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.

An ultra-broadband frequency-domain terahertz measurement system based on frequency conversion via DAST crystal with an optimized phase-matching condition

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.

Real-time terahertz wave imaging by nonlinear optical frequency up-conversion in a 4-dimethylamino-N′-methyl-4′-stilbazolium tosylate crystal

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 cam...

Bridging a few terahertz to tens of terahertz: Inspection on a cost-effective, room-temperature operated measurement system based on frequency conversion via 4-dimethylamino-N′-methyl-4′-stilbazolium tosylate crystal

Based on experimental studies, we inspected the feasibility of a frequency conversion system, including both terahertz (THz) generation and detection, by using 4-dimethylamino-N′-methy-4′-stilbazolium tosylate crystal for wideband measurement. At 27 THz, more than five orders dynamic range in power have been obtained. Compared with typical pyroelectric detectors, it is four orders better in terms of sensitivity. Power calibration has been implemented and the minimum detectable pulse energy is 3 fJ, with a corresponding noise equivalent power of 22 pW/Hz1/2. For broadband operation, we have achieved good signal level downwards to 2.5 THz. Such a system can be an excellent complement to classical time-domain spectroscopy systems.

Frequency-agile terahertz-wave sources and applications to sensitive diagnosis of semiconductor wafers

We have developed ultra-widely tunable THz-wave source using organic nonlinear optical crystals such as 4- dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) and N-benzyl-2-methyl-4-nitroaniline (BNA). The THz-wave difference frequency generation using these crystals covers the ultra-widely tunable range of 0.1-40 THz with frequency agility. Collaborating with Furukawa Co. Ltd., we used the progressive, frequency-agile THz-wave source for industrial applications and produced a sensitive, non-destructive method for examining carrier-density and electrical properties of semiconductors. This method presents novel possibilities for use in the semiconductor industry.

Nonlinear optical detection of terahertz-wave radiation from resonant tunneling diodes

The sensitive detection of terahertz (THz)-wave radiation from compact sources at room temperature is crucial for real-world THz-wave applications. Here, we demonstrate the nonlinear optical detection of THz-wave radiation from continuous-wave (CW) resonant tunneling diodes (RTDs) at 0.58, 0.78, and 1.14 THz. The up-conversion process in a MgO:LiNbO3 crystal under the noncollinear phase-matching condition offers efficient wavelength conversion from a THz wave to a near-infrared (NIR) wave that is detected using a commercial NIR photodetector. The minimum detection limit of CW THz-wave power is as low as 5 nW at 1.14 THz, corresponding to 2-aJ energy and 2.7 × 103 photons within the time window of a 0.31-ns pump pulse. Our results show that the input frequency and power of RTD devices can be calibrated by measuring the output wavelength and energy of up-converted waves, respectively. This optical detection technique for compact electronic THz-wave sources will open up a new opportunity for the realization of real-world THz-wave applications.

Room temperature terahertz wave imaging at 60 fps by frequency up-conversion in DAST crystal

Terahertz imaging has attracted a lot of interests for more than 10 years. But real time, high sensitive, low cost THz imaging in room temperature, which is widely needed by fields such as biology, biomedicine and homeland security, has not been fully developed yet. A lot of approaches have been reported on electro-optic (E-O) imaging and THz focal plane arrays with photoconductive antenna or micro-bolometer integrated. In this paper, we report high sensitive realtime THz image at 60 frames per second (fps) employing a commercial infrared camera, using nonlinear optical frequency up-conversion technology. In this system, a flash-lamp pumped nanosecond pulse green laser is used to pump two optical parametric oscillator systems with potassium titanyl phosphate crystals (KTP-OPO). One system with dual KTP crystals is used to generate infrared laser for the pumping of THz difference frequency generation (DFG) in a 4- Dimethylamino-N-Methyl-4-Stilbazolium Tosylate (DAST) crystal. The other one is for generation of pumping laser for THz frequency up-conversion in a second DAST crystal. The THz frequency can be tuned continuously from a few THz to less than 30 THz by controlling the angle of KTP crystals. The frequency up-converted image in infrared region is recorded by a commercial infrared camera working at 60 Hz. Images and videos are presented to show the feasibility of this technique and the real-time ability. Comparison with a general micro-bolometer THz camera shows the high sensitivity of this technique.

Sensitive terahertz-wave detector using a quasi-phase-matched LiNbO3 at room temperature

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. LiNbO3 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 LiNbO3 (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.

Pump-beam-induced optical damage depended on repetition frequency and pulse width in 4-dimethylamino-N ′-methyl-4′-stilbazolium tosylate crystal

We investigated the dependence of optical damage to an organic nonlinear optical crystal of 4-dimethylamino-N′-methyl-4′-stilbazolium tosylate (DAST) on the repetition frequency and pulse width of the pump beam used to cause the thermal damage. For a pump beam with a pulse width of 15 ns at a wavelength of 1064 nm, the highest damage threshold of 8.0 J/cm2 was measured for repetition frequencies in the range from 10 to 40 Hz. On the other hand, DAST crystals were easily damaged under the repetition rates from 50 to 100 Hz. For 600-ps pulses, a higher damage threshold that was a factor of 11 to 28 times higher in terms of peak intensity was obtained compared with that of 15-ns pulses. In both the cases of 15-ns pulse duration and 600-ps duration, we demonstrated that the thermal effects in DAST crystals dominated the optical damage, which depended on thermal accumulation and dissipation.