We employ tilted-pulse-front techniques to control the propagation direction of the THz pulses emitted from semiconductor photo-switches. In a first step, we successfully demonstrate the manipulation of the THz emission angle from an electrically biased, large-area photoconductive switch on semi-insulating GaAs. With a double-metal waveguide with an optically transparent thin metal window we then show that, at a proper tilt angle of the optical pulse front, the generated THz pulse propagates along the waveguide to radiate off at its end facet. The technique is suitable for optical pumping of THz gain media with short excited-state lifetime, and for THz pulse control and beam steering using any type of coherent THz emitters.
Traditional imaging systems including microscopes depend on the generation of real images to be recorded by the sensor element, which is only sensitive to the intensity and not the phase. The object distance, which is crucial for the spatial resolution of the system, therefore is restricted to be larger than the focal length of the objective lens. This leads to a limitation of the achievable lateral, diffraction-limited spatial resolution. In order to reach a resolution enhancement with the same system components, we explore – in the sub-THz frequency regime – heterodyne detection of the scene’s complex-valued spatial Fourier spectrum in the image-sided focal plane of the optical system. The existence of the Fourier spectrum is independent of the object distance. The measured complex-valued field distribution enables a numerical back-propagation to any place in the object-sided free space from which radiation has reached the detector. Heterodyne Fourier imaging hence enables 3D imaging and – the relevant theme here – it lifts the restriction of the imaging distance. This enables object distances smaller than the focal length of the objective lens and, with it, an enhanced diffraction-limited spatial resolution. In the experiments presented here, the heterodyne data acquisition of the 0.3-THz continuous wave radiation employs Si CMOS TeraFET detectors, i.e., THz sensors based on antenna-coupled field-effect transistors which have been developed in our laboratory. First imaging results show an enhancement of the maximal resolution by a factor of 1.4 for the specific measurement conditions of our experiments, with considerable room for improvement.
Coherent imaging is a well-established technique in the THz range, in this paper we explore the coherent continuous-wave (CW) THz imaging by recording the complex-valued spatial Fourier spectrum in the focal plane of the imaging system, and by utilizing the Fast Fourier Transform (FFT) technique to numerically retrieve both the intensity and the phase information of the object. System is built up based on the heterodyne detection consisting of two electric multiplier chain locked to each other with a slightly frequency off-set and a narrow-band CMOS TeraFET detector, aiming at the recording and reconstruction of 2D and 3D scenes. With a detection area of 80×70mm2 and a sampling rate of 1pixel/mm2 , both 7.6-cm-diameter 2D intensity image and 3D tomography extracted from the phase information of the object are reconstructed, which also reveals the system a capability of covering a large field of view (FOV).
Continuous-wave THz digital holography (DH) is an advanced interference imaging technique, which can be used to reconstruct the amplitude and phase distributions of a sample. In this paper, an in-line holographic system is presented using a 300 GHz source and a highly sensitive broadband CMOS TeraFET (THz Field-Effect Transistor) detector. Numerical reconstruction is achieved using the angular spectrum approach. Experimental results are presented for a sample made of Polyvinyl Chloride (PVC). The results demonstrate that THz digital holography can be readily applied to perform quantitative metrology and may find many applications in 3D digital imaging and microscopy.
THz in-line holography, is a simple and effective way of three-dimensional (3D) imaging. In this work, a THz in-line holography system is presented to investigate the imaging performance. An electrical multiplier-chain emitter working at 300 GHz was utilized as illuminating source. A highly sensitive broad-band CMOS TeraFET detector mounted on a 2D mechanical translation stage for raster-scanning acted as a virtual camera to record the hologram. The data capture area was 60x60 mm2 with a pixel size of 0.25 0.25 mm2. The test objects were polyvinyl chloride (PVC) boards, one taped with 2-mm-wide straight aluminum stripes separated by 2-mm intervals, another one taped with 1.5-mm-wide aluminum stripes forming the word ’GUF’ (size of each letter: 6x8 mm2), and the other two with the same patterns, but now realized as grooves in the material. The imaging results show that a resolution of at least 2 mm is achieved, with a large dynamic range of 60 dB due to the high sensitivity of the TeraFET detector. A property of in-line holography is its self-homodyning capability with the consequence that, in addition to the intensity, also the phase information is encoded in the hologram. The phase information of the transparent part of the object, and of the scattered and diffracted waves are retrieved with the same spatial resolution as the intensity, revealing the 3D imaging capability of the system. The quality and fidelity of the image results can be substantially improved in the future by enlarging the recording area and using an iterative phase recovery method. Furthermore, with a camera substituting the single-pixel scanning technique, the system will have the capability of real-time imaging. Combined with the 3D imaging ability, it will then have a wide application range for various topical areas.
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