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We present recent developments of two kinds of compact room-temperature detectors for terahertz radiation. These are asymmetrically-shaped bow-tie diodes, and field-effect transistors with sub-micrometer channel lengths. Both kinds of detectors exhibit fast response times which allow operating them as mixers, moreover they are suitable for the fabrication of large multi-pixel arrays. Here, we provide data on the experimental performance of detector arrays and show their applicability in coherent terahertz imaging systems. In addition, we demonstrate real-time operation of a 12×12-pixel CMOS camera in power-detection mode at 590 GHz.

We explore terahertz imaging with CMOS field-effect transistors exploiting their plasmonic detection capability and the advantages of CMOS technology for the fabrication of THz cameras with respect to process stability, array uniformity, ease of integration of additional functionality, scalability and cost-effectiveness. A 100×100-pixel camera with an active area of 20×20 mm² is physically simulated by scanning single detectors and groups of a few detectors in the image plane. Using detectors with a noise-equivalent power of 43 pW/√Hz, a distributed illumination of 432 μW at 591.4 GHz, and an integration time of 20 ms (for a possible frame rate of 17 fps), this virtual camera allows to obtain images with a dynamic range of at least 20 dB and a resolution approaching the diffraction limit. Imaging examples acquired in direct and heterodyne detection mode, and in transmission and reflection geometry, show the potential for real-time operation. It is demonstrated that heterodyning (i) improves the dynamic range substantially even if the radiation from the local oscillator is distributed over the camera area, and (ii) allows sensitive determination of object-induced phase changes, which promises the realization of coherent imaging systems.

We report on imaging at terahertz frequencies using a 3x5 Si MOSFET focal-plane array (FPA) processed by a 0.25-μm CMOS technology. Each pixel of the FPA consists of a 645-GHz patch antenna coupled to a FET detector and a 43-dB voltage amplifier with a 1.6-MHz bandwidth. We achieve a typical single-pixel responsivity of 80 kV/W and a noise-equivalent power (NEP) of 300 pW/√Hz at 30-kHz. The performance data of these all-CMOS devices pave the way for the realization of broad-band THz detectors and FPAs for video-rate active imaging on the basis of established low-cost and integration-friendly CMOS technology.

We report on experiment and theory of the evolution of the electric field in undoped GaAs/AlGaAs semiconductor superlattices subjected to femtosecond optical excitation. We performed time-resolved pump-probe experiments and measured the photocurrent generated by spectrally narrowed and wavelength-tuned probe pulses as a function of delay time, pump power and bias field. The drift of the charge carriers, subsequent to the optical excitation, leads to the buildup of an inhomogeneity of the electric field which was traced via the temporal changes of the Wannier-Stark spectra. Although the photocurrent spectra by themselves only yield information on the absorption integrated spatially over the superlattice, we extract information on the local electric fields and the charge-carrier densities by a comparison of the measured data with the results of Monte-Carlo simulations. We find that at moderate excitation densities (10 16 -cm -3 range) the superlattice within a few picoseconds splits into two moving field regions, one with strong field gradient and low electron density, the other with partially screened field at low gradient and high electron density. The largest field differences are found just when the last electrons are swept out after 10-30 ps, the exact time depending on the superlattice parameters and excitation conditions. The initial homogeneous field is restored on a much longer time scale of hundreds of picoseconds which is defined basically by the drift of the heavy holes. Our calculations show that Bloch gain in optically excited semiconductor superlattice is expected in spite of the inhomogeneous field if the field in the electron-rich region is not heavily screened. The time window during which Bloch gain exists is determined by the sweep-out of the electrons.