Abstract—We constructed a laser scanning terahertz (THz) imaging system for high-speed imaging by using a galvano scanner and an organic nonlinear optical crystal, DASC, as a two-dimensional THz emitter. Using this system, we succeeded in obtaining high-resolution THz images of a test sample. I. INTRODUCTION AND BACKGROUND ERAHERTZ (THz) imaging technique has attracted much attention for many practical applications in the various fields from bio to security [1] [2]. For practical use of this technique, there are several problems to be overcome, such as a spatial resolution, imaging speed, etc. As a first step toward the practical use of THz imaging system, we have constructed a prototype of laser scanning THz imaging system using a galvano scanner and an organic nonlinear optical crystal, DAST, as a two dimensional THz emitter as shown in Fig.1 [3]. In this system, 1.56 μm femtosecond laser pulses are modulated at 2 kHz by an optical chopper and scanned over the THz emitter by using a galvano scanner. THz wave pulses which are locally generated at the laser irradiation spots transmit through a sample that is set on the emitter, and are detected by a photoconductive antenna (PCA). Therefore, we can observe a THz image of the sample by monitoring the amplitude of the THz wave pulses. In addition, the reflected laser beam from the emitter is detected as a laser reflection image by photodiode, so we can compare the THz image and laser reflection image and confirm the THz emission points. In addition, we can use this system for THz time-domain spectroscopy (THz-TDS) when the galvano scanner is fixed. In this study, we used a DAST and a DASC crystal as a THz emitter and evaluated the system performance for THz imaging. Fig.1: A schematic illustration of laser scanning THz imaging system II. SYSTEM DESIGN AND EXPERIMENT Prior to this experiment, we studied the most appropriate system model. Because we use a crystal, we must consider the fact that imaging results might be affected by the thickness and quality of the crystal. Most especially, the difference of the reflective indices at each part of the crystal may cause a decrease in the spatial resolution. Thus, we constructed a lens system which makes the laser pulses perpendicular to the emitter. We also studied the dependence of the structure of detectors. Fig.2 (a)-(c) shows the THz emission images of DAST and their frequency spectrums using a dipole, bowtie and spiral shaped detector, respectively. The ranges of detection have broadened in order of dipole, bowtie, and spiral shaped detectors. It is also understood that we can obtain the most broadband frequency range using dipole shaped detector. DASC is one of the groups of organic nonlinear optical crystals and its optical characteristics are almost similar to those of DAST [4] [5]. Fig.3 (a) and (b) show the THz emission images of DASC when the bowtie shaped detector was focused and unfocused, respectively. It is clearly seen that the range of detection is wider when the detector is unfocused. We also observed a uniform area of THz intensity about 1.5 mm long in the emission images of DASC crystal when the detector was unfocused. As a result of these studies, we need to choose an appropriate detector which is suitable for our experiments such as THz imaging or THz-TDS. K. Serita a , S. Mizuno a , H. Murakami a , I. Kawayama a and M. Tonouchi a Y. Takahashi b , M. Yoshimura b , Y. Kitaoka b , Y. Mori b a Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita, Osaka 565-0871, Japan b Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0876, Japan Development of Laser Scanning Terahertz Imaging System Using Organic Nonlinear Optical Crystal T Fig.2: THz emission images of DAST crystal, using (a) dipole (b) bowtie and (c) dipole shaped detectors, and (d) comparison of frequency spectrum data obtained by these 3 detectors, respectively. (a) dipole (b) bowtie (c) spiral (d) Frequency spectrums 1 mm 1 mm 1 mm