600-GHz Fourier Imaging Based on Heterodyne Detection at the 2nd Sub-harmonic

Fourier imaging is an indirect imaging method which records the diffraction pattern of the object scene coherently in the focal plane of the imaging system and reconstructs the image using computational resources. The spatial resolution, which can be reached, depends on one hand on the wavelength of the radiation, but also on the capability to measure - in the focal plane - Fourier components with high spatial wave-vectors. This leads to a conflicting situation at THz frequencies, because choosing a shorter wavelength for better resolution usually comes at the cost of less radiation power, concomitant with a loss of dynamic range, which limits the detection of higher Fourier components. Here, aiming at maintaining a high dynamic range and limiting the system costs, we adopt heterodyne detection at the 2nd sub-harmonic, working with continuous-wave (CW) radiation for object illumination at 600 GHz and local-oscillator (LO) radiation at 300 GHz. The detector is a single-pixel broad-band Si CMOS TeraFET equipped with substrate lenses on both the front- and backside for separate in-coupling of the waves. The entire scene is illuminated by the object wave, and the Fourier spectrum is recorded by raster scanning of the single detector unit through the focal plane. With only 56 uW of power of the 600-GHz radiation, a dynamic range of 60 dB is reached, sufficient to detect the entire accessible Fourier space spectrum in the test measurements. A lateral spatial resolution of better than 0.5 mm, at the diffraction limit, is reached

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

20 μm gate width CVD graphene FETs for 0.6 THz detection

We have fabricated 20 μm gate width graphene field effect transistors (GFETs) based on graphene grown by chemical vapor deposition (CVD). These GFETs are integrated with split bow-tie antennae for room temperature, direct detection of a 0.6 THz signal. Our detectors reach a maximum optical responsivity of 3.0 V/W and a minimum noise-equivalent power (NEP) of 700 pW/Hz^0.5. The successful demonstration of THz detection using CVD graphene introduces the possibility for scalable detector production

Electrical Noise in Colossal Magnetoresistors and Ferroelectrics

NR 2014080

Electronic model of FitzHugh-Nagumo neuron

For investigation into neurodynamical systems FitzHugh-Nagumo model is often suggested. One neuron can be easily modeled using numerical methods, but numerical modeling of the entire network of FitzHugh-Nagumo neurons requires a lot of calculation resources and time. To overcome that problem the electronic model of FitzHugh-Nagumo neuron is proposed. The article discusses and compares some electronic models of FitzHugh-Nagumo neurons. The advantages and shortcomings of each model are discussed. Microcontroller based model of the FitzHugh-Nagumo neuron is proposed. The microcontroller calculates the nonlinear part of the FitzHugh-Nagumo equations mathematically correctly, i.e. without any approximations. Simulation results of the suggested model are presented and discussed. Ill. 4, bibl. 8 (in English; abstracts in English and Lithuanian)

Enhanced sensitivity AlGaN/GaN HEMT terahertz detector without ungated regions

AlGaN/GaN HEMTs [1,2] and nMOS [3] transistors are reported as the most sensitive field effect transistor-based terahertz detectors. The advantage of HEMT is high electron mobility (up to 1500 cm2 /Vs) and the disadvantage is the presence of passive ungated regions which introduce additional series impedance contributing to the loss of high-frequency signal. The advantage of nMOS is the absence of ungated regions and the disadvantage is low electron mobility (about 250 cm2/Vs) due to high acceptor density (about 2e18 cm-3) in the channel. Here, we propose the HEMT-based THz detector with 5 nm HfO2 dielectric between the gate electrode and the AlGaN layer, which allows to separate the gate from the source and drain terminals without involving ungated regions. For numerical calculations of detector characteristics, we have employed two-dimensional hydrodynamic modeling performed with Synopsys TCAD Sentaurus program package comprising Poisson's equation, continuity equation, current density equation and energy balance equation for electrons and holes. It accounts for the formation of spontaneous and piezoelectric polarization charges in GaN and AlGaN layers, as well as the dependence of carrier mobility on doping density and carrier temperature. The comparison of current responsivity of the HEMT with and without ungated regions and the gate length LG = 100 nm is shown in Fig. 1. The results clearly indicate that the presence of ungated regions with the length LUG = 100 nm reduces the maximum of the current responsivity at 1 THz by about 2 times. The minimum NEP at 1 THz is about 3 times lower in the HEMT without ungated regions

Investigation into dynamic non-symmetrical breaking modes of linear induction motor

The paper presents investigation into non-symmetrical dynamic breaking modes. The non-symmetrical model of linear induction drive wilh different connection ofinductor vvindings at dynamic braldng is presented. Model comprises threc models oflinear induction drive made for direct, inverse and zero components ofsupply voltage. Developed model gives possibility to invcstigate starting process and dynamic characterisdcs by direct current breaking, compare dynamic characteristics and evaluate influence ofmotor parameteis to that. Presented results of simulation show the force, developed by motor and braking time dependence on the resistance of secondary element The greatest braldng force and the smallest braking time are obtained at the supplying by braldng voltage one phase windmg. 111. 9, bibl. 10 (in Lithuanian, summaries m English, Russian and Lithuanian)

Homodyne Spectroscopy with Broadband Terahertz Power Detector Based on 90-nm Silicon CMOS Transistor

Over the last two decades, photomixer-based continuous wave systems developed into versatile and practical tools for terahertz (THz) spectroscopy. The high responsivity to the THz field amplitude of photomixer-based systems is predetermined by the homodyne detection principle that allows the system to have high sensitivity. Here, we show that the advantages of homodyne detection can be exploited with broadband power detectors combined with two photomixer sources. For this, we employ a THz detector based on a complementary metal-oxide-semiconductor field-effect transistor and a broadband bow-tie antenna (TeraFET). At 500 GHz and an effective noise bandwidth of 1 Hz, the response from one photomixer-based THz source resulted in an about 43 dB signal-to-noise ratio (SNR). We demonstrate that by employing a homodyne detection system by overlaying the radiation from two photomixers, the SNR can reach up to 70 dB at the same frequency with an integration time 100 ms. The improvement in SNR and the spectroscopic evidence for water vapor lines demonstrated up to 2.2 THz allow us to conclude that these detectors can be successfully used in practical continuous wave THz spectrometry systems

Passive detection and imaging of human body radiation using an uncooled field-effect transistor-based THz detector

This work presents, to our knowledge, the first completely passive imaging with human-body-emitted radiation in the lower THz frequency range using a broadband uncooled detector. The sensor consists of a Si CMOS field-effect transistor with an integrated log-spiral THz antenna. This THz sensor was measured to exhibit a rather flat responsivity over the 0.1–1.5-THz frequency range, with values of the optical responsivity and noise-equivalent power of around 40 mA/W and 42 pW/√Hz, respectively. These values are in good agreement with simulations which suggest an even broader flat responsivity range exceeding 2.0 THz. The successful imaging demonstratestheimpressivethermalsensitivitywhichcanbeachievedwithsuchasensor. Recording of a 2.3×7.5-cm2-sized image of the fingers of a hand with a pixel size of 1 mm2 at a scanning speed of 1 mm/s leads to a signal-to-noise ratio of 2 and a noise-equivalent temperature difference of 4.4 K. This approach shows a new sensing approach with field-effect transistors as THz detectors which are usually used for active THz detection