Alvydas Lisauskas

A 0.65 THz Focal-Plane Array in a Quarter-Micron CMOS Process Technology

A focal-plane array (FPA) for room-temperature detection of 0.65-THz radiation has been fully integrated in a low-cost 0.25 mum CMOS process technology. The circuit architecture is based on the principle of distributed resistive self-mixing and facilitates broadband direct detection well beyond the cutoff frequency of the technology. The 3 timesZ 5 pixel array consists of differential on-chip patch antennas, NMOS direct detectors, and integrated 43-dB voltage amplifiers. At 0.65 THz the FPA achieves a responsivity (Rv) of 80 kV/W and a noise equivalent power (NEP) of 300 pW/ radic{Hz}. Active multi-pixel imaging of postal envelopes demonstrates the FPAs potential for future cost-effective terahertz imaging solutions.

Rational design of high-responsivity detectors of terahertz radiation based on distributed self-mixing in silicon field-effect transistors

In search of novel detectors of electromagnetic radiation at terahertz frequencies, field-effect transistors (FETs) have recently gained much attention. The current literature studies them with respect to the excitation of plasma waves in the two-dimensional channel. Circuit aspects have been taken into account only to a limited degree. In this paper, we focus on embedding silicon FETs in a proper circuitry to optimize their responsivity to terahertz radiation. This includes impedance-matched antenna coupling and amplification of the rectified signal. Special attention is given to the investigation of high-frequency short-circuiting of the gate and drain contacts by a capacitive shunt, a common approach of high-frequency electronics to induce resistive mixing in transistors. We theoretically study the effect of shunting in the framework of the Dyakonov–Shur plasma-wave theory, with the following key results. In the quasistatic limit, the capacitive shunt induces the longitudinal high-frequency field neede...

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.

Terahertz imaging with Si MOSFET focal-plane arrays

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.

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.

Opportunities for silicon at mmWave and Terahertz frequencies

This paper summarizes opportunities for silicon process technologies at mmwave and terahertz frequencies and demonstrates key building blocks for 94-GHz and 600-GHz imaging arrays. It reviews potential applications and summarizes state-of-the-art terahertz technologies. Terahertz focal-plane arrays (FPAs) for video-rate imaging applications have been fabricated in commercially available CMOS and SiGe process technologies respectively. The 3times5 arrays achieve a responsivity of up to 50 kV/W with a minimum NEP of 400 pW/radicHz at 600 GHz. Images of postal envelopes are presented which demonstrate the potential of silicon integrate 600-GHz terahertz FPAs for future low-cost terahertz camera systems.

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...