Maris Bauer

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.

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 rectification by plasmons and hot carriers in gated 2D electron gases

Here we discuss on detection using antenna-coupled field effect transistors. We show, that gate-to-channel separation plays strong role for the dispersion of plasmons excited within the channel changing from gated 2D to ungated 2D plasmon. This change also strongly affects the impedance and the efficiency of rectification. We present experimental data which clearly indicates on with plasmonic rectification competing physical phenomenon. There are strong indications, that this additional signal originates from the inhomogeneous heating of charge carriers i.e., diffusion of “warm” electrons.

The potential for sensitivity enhancement by the thermoelectric effect in carbon-nanotube and graphene Tera-FETs

We report on terahertz (THz) measurements with graphene field-effect transistors with integrated antennas (Tera-FETs) lay special emphasis on thermoelectric contributions to the detected THz photoresponse. Graphene Tera-FETs with integrated broad-band bow-tie antennas were fabricated in a CVD-based growth process and were successfully applied for detection at 600 GHz with optical NEPs down to 515 pW/Hz^1/2. While rectification of THz radiation by (distributed) resistive mixing of charge-density waves induced in the gated transistor channel region is well known, significant additional contributions to the detected signal have experimentally been observed and hot-carrier thermoelectric effects have been identified as a possible origin of these signals. We also observe similar signal contributions in carbonnanotube transistors.

Field-effect transistors as electrically controllable nonlinear rectifiers for the characterization of terahertz pulses

We propose to exploit rectification in field-effect transistors as an electrically controllable higher-order nonlinear phenomenon for the convenient monitoring of the temporal characteristics of THz pulses, for example, by autocorrelation measurements. This option arises because of the existence of a gate-bias-controlled super-linear response at sub-threshold operation conditions when the devices are subjected to THz radiation. We present measurements for different antenna-coupled transistor-based THz detectors (TeraFETs) employing (i) AlGaN/GaN high-electron-mobility and (ii) silicon CMOS field-effect transistors and show that the super-linear behavior in the sub-threshold bias regime is a universal phenomenon to be expected if the amplitude of the high-frequency voltage oscillations exceeds the thermal voltage. The effect is also employed as a tool for the direct determination of the speed of the intrinsic TeraFET response which allows us to avoid limitations set by the read-out circuitry. In particular, we show that the build-up time of the intrinsic rectification signal of a patch-antenna-coupled CMOS detector changes from 20 ps in the deep sub-threshold voltage regime to below 12 ps in the vicinity of the threshold voltage.We propose to exploit rectification in field-effect transistors as an electrically controllable higher-order nonlinear phenomenon for the convenient monitoring of the temporal characteristics of THz pulses, for example, by autocorrelation measurements. This option arises because of the existence of a gate-bias-controlled super-linear response at sub-threshold operation conditions when the devices are subjected to THz radiation. We present measurements for different antenna-coupled transistor-based THz detectors (TeraFETs) employing (i) AlGaN/GaN high-electron-mobility and (ii) silicon CMOS field-effect transistors and show that the super-linear behavior in the sub-threshold bias regime is a universal phenomenon to be expected if the amplitude of the high-frequency voltage oscillations exceeds the thermal voltage. The effect is also employed as a tool for the direct determination of the speed of the intrinsic TeraFET response which allows us to avoid limitations set by the read-out circuitry. In particular...

TeraFET detector for measuring power fluctuations of 4.75-THz QCL-generated radiation

We present the field-effect-transistor-based THz (TeraFET) detector as a tool for monitoring power and its low-frequency fluctuations of 4.75-THz quantum cascade laser (QCL). The resonant patch antenna coupled TeraFET was implemented in 90 nm CMOS technology. At 4.75 THz, detector exhibits area-normalized minimal noise-equivalent power (NEP) of 370 pW/√Hz and the maximum responsivity of 82 V/W.

Efficient detection of short-pulse THz radiation with field effect transistors

Here we report on the efficient detection of THz radiation generated from Ti:sapphire femtosecond laser system. The large-area interdigitated antenna is excited by optical pulses with pulse duration from 15 to 47 fs. We have tested two AlGaN/GaN HEMT-based THz detectors with integrated broadband antennas. Detectors are comparable in sensitivity but differ in applied parasitic signals elimination techniques. Despite being sensitive, the pick-up of the first detector without strong parasitic elimination exceeded the theoretically expected noise level about 100 times. For a second detector we have implemented more stringent interference avoiding techniques, such as gate shunting with a low resistance value resistor, and have managed to significantly improve systems detection capabilities. We have lowered a pick-up near to the thermal noise level and have extended signal/noise ratio to 26 dB.

Thermal noise-limited sensitivity of FET-based terahertz detectors

Here we present a detailed study on estimation of noise-dependent parameters, such as signal-to-noise ratio (SNR) or noise equivalent power (NEP), of field-effect-transistor based terahertz detectors (TeraFETs). Commonly, these parameters are estimated from a well-known assumption, that detectors performance is limited by the thermal noise of transistors channel. However, practice shows that the influence of other noise sources or transient effects is considerable. We summarize TeraFET noise measurements performed on different material systems based transistors, such as AlGaN/GaN, AlGaAs/GaAs, silicon CMOS, and monolayer graphene. We have achieved a good agreement between thermal noise and measured data. However, attention has to be paid to gate leakage currents and slow defect charging and discharging effects, which can strongly influence TeraFETs performance estimation.

Hydrodynamic modelling of terahertz rectification in AlGaN/GaN high electron mobility transistors

We report on the numerical modelling of rectification in a gated two-dimensional electron gas. We demonstrate that drift-diffusion-based and energy-relaxation-based models predict different features of rectified terahertz radiation as a function of gate bias. Whereas the widely accepted mechanism for rectification is considered to be plasmonic-based, there are conditions when diffusion currents originating by non-local carrier heating can dominate the response. Moreover, diffusive contributions can substantially enhance the response becoming an important phenomenon, which has to be considered in future designs of efficient transistor-based terahertz rectifiers.

Optimization of the Design of Terahertz Detectors Based on Si CMOS and AlGaN/GaN Field-Effect Transistors

TeraFETs are THz power detectors based on field-effect transistors (FETs) integrated with antennas. The first part of this paper discusses the design of Si CMOS TeraFETs leading to an optimized noise-equivalent power close to the room-temperature limit. The impact of the choice of the gate width and gate length, the role of the parasitic effects associated with the technology node, and the conjugate matching of antenna and FET impedance – which is possible over narrow THz frequency bands because of the frequency dependence of the channel impedance resulting from plasmonic effects – are highlighted. Taking these aspects into account, we implement narrow-band detectors of two different designs. Using a 90-nm and a 65-nm CMOS technology, we reach a room-temperature cross-sectional NEP of 10 pW/√Hz at 0.63 THz. We then explore the optimization of AlGaN/GaN TeraFETs equipped with broadband antennae. A room-temperature optical NEP of 26 pW/√Hz is achieved around 0.5 THz despite the fact that the existence of pr...