Spatial Frequency Filter for Real-Time THz Imaging T. Hattori, M. Sakamoto, and R. Rungsawang Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8573, Japan e-mail: hattori@bk.tsukuba.ac.jp Abstract In real-time THz imaging using intense broadband THz pulses, low-frequency THz components degrade the contrast and resolution of the THz images. We improved the contrast by spatially filtering out low-frequency components using the frequency dependence of the beam waist size of a focused THz beam. Introduction Real-time terahertz (THz) imaging has been achieved using intense THz pulses obtained from large-aperture photoconductive antennas [1−4]. This technique has enabled single-shot acquisition of THz images and also 1-kHz high-speed THz movies. Since the intense THz pulses emitted from large-aperture antennas have a broad spectrum extending from dc to a few THz, the spatial resolution of the obtained images is limited by the existence of low-frequency components. Although one can extract high-frequency contributions to the image using Fourier transformation of a time-scanned series of image data [3], this method is not suited for real-time imaging since the data acquisition in this method takes a long time. In this report, we demonstrated that a spatial filtering method can be used for improving the resolution of the real-time THz images by reducing amplitude of low- frequency components of the THz waves. Experiments The large-aperture photoconductive antenna used in the experiments consisted of a semi-insulating GaAs wafer and two electrodes. The distance between the electrodes was 30 mm. The area between the electrodes was excited by amplified optical pulses. The excitation pulses had a center wavelength, a duration, and a repetition rate of 800 nm, 150 fs, and 1 kHz. Intense THz pulses emitted from this antenna were used for real-time imaging using a field-sensitive electro-optic (EO) sampling method and a CCD camera [1−4]. We introduced a spatial filter in the imaging apparatus in order to cut off low-frequency components. Before illuminating the sample, the THz pulses were passed through a spatial filter setup, which was composed of an off-axis parabolic mirror for focusing the THz waves, an aluminum plate having a hole of 1, 2, 3, or 4 mm in diameter, and a collimating off-axis parabolic mirror. The focal length of the parabolic mirrors was 50.8 mm. The aluminum plate was placed on the focal plane of both the parabolic mirrors. Since the beam waist size of focused THz waves depends almost linearly on the wavelength [3], this setup is expected to work as a high- pass filter. The hole size is expected to correspond to the beam waist size of the component of the cut-off frequency. Results and Discussion The properties of the spatial filter were studied first. The THz waves transmitted through the spatial filter were focused by a TPX lens of 98-mm focal length, and their temporal waveforms were measured at the focus using a standard EO sampling method. The Fourier amplitudes of the waveforms obtained with an aluminum plate hav- ing a hole of 4 mm, 2 mm, and 1 mm in diameter and without an aluminum plate are shown in Fig. 1. It is clearly seen that the spatial filters work as high-pass filters and that the smaller the hole size is, the higher is the cut-off frequency. It can be shown that the observed behavior is consistent with the result of the Gaussian beam model [3,5] as follows. This result shows that the cut-off frequency can be tuned by simply changing the hole diameter. It is also noted that the transmittance of high-frequency components is very high. Fig. 1: Fourier amplitude of THz pulses after spatial filtering using metal plates with a hole of 4 mm, 2 mm, and 1 mm in diameter. THz images were obtained using the THz waves passed through the spatial filter. Sample objects were placed at