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(Color online) (a) Photograph of a BT diode with electrical leads and contact pads; (b) photograph of the BT diode active part showing the metallized and unmetallized segments; and (c) attenuation in detection (squares) and heterodyning mode (circles). Horizontal dash line depicts noise floors. Upper inset: IF signal (full circles) for various intermediate frequencies between 43 kHz and 180 kHz (full line: guide to the eye). Lower inset: calculated frequency dependence of the BT diode. Vertical brown line marks the imaging frequency.
(Color online) Set-up for terahertz imaging in transmission and reflection geometries using direct and heterodyne modes. Letters B denote beam splitters, BT—bow-tie diode, K—absorbers, L—lenses, P—parabolic mirrors, A—attenuator, PLL—phase lock loop.
(Color online) 0.591-THz transmission images of a leaf placed in a paper envelope: (a) direct power detection scheme, (b) power image, heterodyne mode, and (c) phase image, heterodyne mode. Inset: photograph of the leaf. Lock-in integration time: 5 ms; pixel size: 200 × 200 μm2; the THz images consist of 380 × 170 pixels.
(Color online) 0.591-THz reflection images of a Lithuanian 1-cent coin placed in the paper envelope: (a) direct mode; (b) power image, heterodyne mode; (c) phase image, heterodyne mode; and (d) photograph of the coin. Lock-in integration time: 10 ms; pixel size: 200 × 200 μm2; the THz images consist of 125 × 125 pixels.
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