Sebastian Boppel

THz Active Imaging Systems With Real-Time Capabilities

This paper presents a survey of the status of five active THz imaging modalities which we have developed and investigated during the last few years with the goal to explore their potential for real-time imaging. We start out by introducing a novel waveguide-based all-electronic imaging system which operates at 812 GHz. Its salient feature is a 32-pixel linear detector array heterodyne-operated at the eighth subharmonic. This array in combination with a telescope optics for object distances of 2-6 m reaches a data acquisition speed suited for real-time imaging. The second system described then is again an all-electronic scanner (now for around 300 GHz ), designed for object distances of ≥ 8 m , which combines mechanical scanning in vertical direction, synthetic-aperture image generation in horizontal direction, and frequency-modulated continuous-wave sweeping for the depth information. The third and fourth systems follow an optoelectronic approach by relying on several- to multi-pixel parallel electrooptic detection. One imager is based on a pulsed THz-OPO and homodyne detection with a CCD camera, the other on either continuous-wave electronic or femtosecond optoelectronic THz sources and a photonic-mixing device (PMD) camera. The article concludes with a description of the state of the art of imaging with focal-plane arrays based on CMOS field-effect transistors.

Terahertz heterodyne detection with silicon field-effect transistors

We report on the detection of electromagnetic radiation at 0.65 THz by silicon field-effect transistors operated in heterodyne mode. Aiming at terahertz imaging with numerous pixels in a focal-plane array, we explore the improvement of the dynamic range achieved over power detection when the local-oscillator (LO) power is distributed quasioptically onto all detectors. These consist of resonantly antenna-coupled complementary metal-oxide-semiconductor transistors with a gate length of 0.25 μm, and each has an integrated voltage amplifier. With a LO power of 2 μW per detector, the noise-equivalent power amounts to 8 fW/Hz, leading to an estimated improvement of the dynamic range by 29 dB.

CMOS detector arrays in a virtual 10-kilopixel camera for coherent terahertz real-time imaging

We demonstrate the principle applicability of antenna-coupled complementary metal oxide semiconductor (CMOS) field-effect transistor arrays as cameras for real-time coherent imaging at 591.4 GHz. By scanning a few detectors across the image plane, we synthesize a focal-plane array of 100×100 pixels with an active area of 20×20 mm2, which is applied to imaging in transmission and reflection geometries. Individual detector pixels exhibit a voltage conversion loss of 24 dB and a noise figure of 41 dB for 16 μW of the local oscillator (LO) drive. For object illumination, we use a radio-frequency (RF) source with 432 μW at 590 GHz. Coherent detection is realized by quasioptical superposition of the image and the LO beam with 247 μW. At an effective frame rate of 17 Hz, we achieve a maximum dynamic range of 30 dB in the center of the image and more than 20 dB within a disk of 18 mm diameter. The system has been used for surface reconstruction resolving a height difference in the μm range.

Terahertz heterodyne imaging with InGaAs-based bow-tie diodes

Room-temperature detection and imaging in transmission and reflection geometries at 0.591 THz with planar asymmetrically shaped InGaAs diodes (also called bow-tie diodes) are demonstrated in direct and heterodyne mode. The sensitivity of the diodes is found to be 6 V/W in direct mode, and the noise-equivalent power (NEP) in direct and heterodyne mode is estimated to be about 4 nW/Hz and 230 fW/Hz for a local-oscillator power of 11 μW, respectively. The improvement of the dynamic range by heterodyning over direct power detection amounts to about 20 dB using pixel read-out times relevant to real-time imaging conditions.

Antenna-coupled field-effect transistors for multi-spectral terahertz imaging up to 4.25 THz

We demonstrate for the first time the applicability of antenna-coupled field-effect transistors for the detection of terahertz radiation (TeraFETs) for multi-spectral imaging from 0.76 to 4.25 THz. TeraFETs were fabricated in a commercial 90-nm CMOS process and noise-equivalent powers of 59, 20, 63, 85 and 110 pW/√(Hz) at 0.216, 0.59, 2,52, 3.11 and 4.25 THz, respectively, have been achieved. A set of TeraFETs has been applied in raster-scan transmission and reflection imaging of pellets of sucrose and tartaric acid simulating common plastic explosives. Transmittance values are in good agreement with Fourier-transform infrared spectroscopy data. The spatial distribution of the components in the samples has been determined from the transmission data using principal component analysis.

Terahertz responsivity and low-frequency noise in biased silicon field-effect transistors

We report on a comprehensive study of terahertz responsivity and low-frequency noise of current-biased antenna-coupled silicon field-effect transistors. Our measurements corroborate the findings of earlier studies that the current bias dramatically enhances the responsivity by at least an order of magnitude. However, this impressive improvement is accompanied by an equally drastic increase of the spectral density of the voltage fluctuations with the applied current. Therefore, the resulting signal-to-noise ratio (SNR) increases at most by a small amount and only for sub-threshold conditions. We do not register an absolute improvement beyond the best SNR value of the unbiased device. These findings are compared with theoretical absolute-SNR estimates derived using a hydrodynamic transport description. It predicts the maximal enhancement to be limited by a factor of about 1.35. In practical devices, the onset of excess noise suppresses even this little enhancement and limits the applicability of current bia...

Detectors for terahertz multi-pixel coherent imaging and demonstration of real-time imaging with a 12x12-pixel CMOS array

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.

Terahertz detection and coherent imaging from 0.2 to 4.3 THz with silicon CMOS field-effect transistors

We summarize several lines of investigation of foundry-processed patch-antenna-coupled Si MOSFETs as plasmonic detectors of THz radiation. First, we explore detection at frequencies as high as 4.3 THz, about one hundred times higher than the transit-time-limited cut-off frequency of the devices, searching for the fundamental limits of the detection principle. Then, we address the much-debated issue of enhanced sensitivity by a current bias and conclude that - because of the increased noise - there is no net gain in signal-to-noise ratio. Finally, we simulate operation of a 100×100-pixel heterodyne camera, working with a few detectors of a focal-plane array and quasi-optical coupling of the local-oscillator radiation, and show that real-time operation of a camera should be possible with a dynamic range of 30 dB for a quarter-milliwatt local-oscillator power.

Terahertz heterodyne detection with silicon CMOS transistors

We report on heterodyne detection of 0.65-THz radiation with silicon CMOS transistors. With a fairly low local-oscillator power of −27 dBm delivered to the detector, we measure a noise power of −111 dBm/Hz. The 3-dB sensitivity roll-off of the intermediate frequency (IF) is as high as 750 kHz. The detectors exhibit an almost frequency-independent signal-to-noise ratio of 45 dB over the whole measured IF frequency range (70 kHz–24 MHz).

Heterodyne and subharmonic mixing at 0.6 THz in an AlGaAs/InGaAs/AlGaAs heterostructure field effect transistor

We fabricated a two-dimensional-electron-gas field effect transistor with an asymmetric terahertz antenna connected to the channel terminals and a gate length of 1 μm. We investigated frequency mixing in the transistors channel by measuring, with a quasi-optical setup, the heterodyne, second- and third-order subharmonic mixing signal at 0.592 THz. The dependence on the gate voltage and on the radiation power of both the local-oscillator and the radio-frequency signals was studied for all mixing orders. The conditions for full-plasmonic-mixing are fulfilled in our transistor at room temperature.