Linas Minkevičius

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.

Spectroscopic Terahertz Imaging at Room Temperature Employing Microbolometer Terahertz Sensors and Its Application to the Study of Carcinoma Tissues

A terahertz (THz) imaging system based on narrow band microbolometer sensors (NBMS) and a novel diffractive lens was developed for spectroscopic microscopy applications. The frequency response characteristics of the THz antenna-coupled NBMS were determined employing Fourier transform spectroscopy. The NBMS was found to be a very sensitive frequency selective sensor which was used to develop a compact all-electronic system for multispectral THz measurements. This system was successfully applied for principal components analysis of optically opaque packed samples. A thin diffractive lens with a numerical aperture of 0.62 was proposed for the reduction of system dimensions. The THz imaging system enhanced with novel optics was used to image for the first time non-neoplastic and neoplastic human colon tissues with close to wavelength-limited spatial resolution at 584 GHz frequency. The results demonstrated the new potential of compact RT THz imaging systems in the fields of spectroscopic analysis of materials and medical diagnostics.

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.

Solar cell imaging and characterization by terahertz techniques

Enhanced attention to solar cell development stimulates search of innovative solutions to their characterization and identification of possible technological defects in various steps of production in a contactless way. In the given work, investigation of solar cells structures by means of terahertz (THz) imaging is presented. Both continuous wave and pulsed THz imaging set-ups were employed in this study. Investigated objects included typical for various production stages test silicon structures – structured and unstructured surfaces, metalized and unmetalized contact areas, commercial silicon solar cells. We demonstrate that phase sensitive subTHz imaging can be employed for detecting manufacturing defects of the solder tabs, while images at higher frequencies reveal finer details of solar cells, such as cracks or different reflection coefficients for structured and unstructured surfaces with different doping.

InGaAs bow-tie diodes for terahertz imaging: low frequency noise characterisation

Noise characteristics of bow-tie InGaAs diodes in forward and backward directions were measured in frequency range from 10 Hz to 20 kHz at room temperature. It was found that the spectral density of voltage fluctuations changes with frequency approximately as 1/f, indicating that origin of noise is superposition of generation and recombination processes in defects of the structure. It was determined that the dependence of spectral density of current fluctuation on current in backward and forward directions at different frequencies exhibits asymmetry which reflects non-uniform electric field distribution within the structure. As both noise and sensitivity increase with bias current, optimal conditions for device operation are discussed.

Terahertz responsivity enhancement and low-frequency noise study in silicon CMOS detectors using a drain current bias

We report on the study of the enhancement of responsivity and properties of low-frequency noise in silicon 0.25 µm CMOS transistor-based detectors for terahertz radiation under applied dc source-to-drain current. We find that at signal modulation frequencies above 50 kHz the signal-to-noise ratio becomes independent from applied current, whereas the responsivity of detectors can be enhanced up to three times. We present quantitative results of noise measurements in the frequency range from 600 Hz to 1 MHz and currents up to the saturation current.

Influence of Field Effects on the Performance of InGaAs-Based Terahertz Radiation Detectors

A detailed electrical characterization of high-performance bow-tie InGaAs-based terahertz detectors is presented along with simulation results. The local surface potential and tunnelling current were scanned over the surfaces of the detectors by means of Kelvin probe force microscopy (KPFM) and scanning tunnelling microscopy (STM), which also enabled the determination of the Fermi level. Current-voltage curves were measured and modelled using the Synopsys Sentaurus TCAD package to gain deeper insight into the processes involved in detector operation. In addition, we performed finite-difference time-domain (FDTD) simulations to reveal features related to changes in the electric field due to the metal detector contacts. The investigation revealed that field-effect-induced conductivity modulation is a possible mechanism contributing to the high sensitivity of the studied detectors.