Vincas Tamošiūnas

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.

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.

Electrically controllable photonic molecule laser

We have studied the coherent intercavity coupling of the evanescent fields of two microdisk terahertz quantum-cascade lasers. The electrically controllable optical coupling of the single-mode operating lasers has been observed for cavity spacings up to 30 mum. The strongest coupled photonic molecule with 2 mum intercavity spacing allows to conditionally switch the optical emission by the electrical modulation of only one microdisk. The lasing threshold characteristics demonstrate the linear dependence of the gain of a quantum-cascade laser on the applied electric field.

Modulated reflectance study of InAs quantum dot stacks embedded in GaAs/AlAs superlattice

Optical transitions in vertically stacked InAs quantum dot (QD) superlattice (SL) with and without AlAs barriers were examined by photo- and electroreflectance techniques. The interband transitions corresponding to the QD, wetting layer (WL), and InAs/GaAs/AlAs SL have been identified. Experimental data and numerical calculations show that blueshifts and enhancement in the intensity of WL-related optical transitions in an InAs/GaAs/AlAs SL originate mainly due to off-center position of the QD layers in the quantum wells. The appearance of multiple WL-related features in the modulated reflectance spectra was revealed and discussed.

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.

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.

Compact hybrid solar simulator with the spectral match beyond class A

Abstract. A compact hybrid solar simulator with the spectral match beyond class A is proposed. Six types of high-power light-emitting diodes (LEDs) and tungsten halogen lamps in total were employed to obtain spectral match with <25% deviation from the standardized one in twelve spectral ranges between 400 and 1100 nm. All spectral ranges were twice as narrow than required by IEC 60904-9 Ed.2.0 and ASTM E927-10(2015) standards. Nonuniformity of the irradiance was evaluated and <2% deviation from the average value of the irradiance (corresponding to A class nonuniformity) can be obtained for the area of >3-cm diameter. A theoretical analysis was performed to evaluate possible performance of our simulator in the case of GaInP/GaAs/GaInAsP/GaInAs four-junction tandem solar cells and AM1.5D (ASTM G173-03 standard) spectrum. Lack of ultraviolet radiation in comparison to standard spectrum leads to 6.94% reduction of short-circuit current, which could be remedied with 137% increase of the output from blue LEDs. Excess of infrared radiation from halogen lamps outside ranges specified by standards is expected to lead to ∼0.77% voltage increase.

On-chip integration solutions of compact optics and detectors in room-temperature terahertz imaging systems

On-chip integrated solutions employing properties of Fresnel zone plates with integrated band-pass filters for the room temperature terahertz imaging systems are discussed. Finite-difference time-domain simulations were used to predict properties of conventional zone plates and ones with resonant filter areas as flat optics components. They are produced employing the laser direct writing and characterized by electronic THz sources and an optically pumped terahertz laser. It was shown that more than one order of magnitude detection enhancement can be observed of bow-tie-shaped InGaAs-based terahertz detectors by on-chip incorporation of the secondary diffractive optics.

Discrete spectrum terahertz imaging using bow-tie diodes: optimized antenna designs and arrays

Optimization routes to optimize the operation of InGaAs bow-tie diodes were demonstrated using simulations based on 3D finite-difference time-domain method. Calculations of the electromagnetic field amplitude distribution in a detector plane were performed. Three types of designs – bow-tie diode itself, log-periodic antenna and log-periodic antenna coupled with the bow-tie diode were analyzed; it was determined that frequency properties of such antennacoupled device can be tuned via variation of antenna shape and size. Multi-pixel InGaAs bow-tie diodes arrays of different designs – narrow band and broadband operation – were designed and fabricated. They were demonstrated to be well suited for continuous wave room temperature spectroscopic terahertz imaging at 0.58 THz and at 1.63 THz.