V. Tamosiunas

Imaging with a Terahertz quantum cascade laser.

We demonstrate bio-medical imaging using a Terahertz quantum cascade laser. This new optoelectronic source of coherent Terahertz radiation allows building a compact imaging system with a large dynamic range and high spatial resolution. We obtain images of a rat brain section at 3.4 THz. Distinct regions of brain tissue rich in fat, proteins, and fluid-filled cavities are resolved showing the high contrast of Terahertz radiation for biological tissue. These results suggest that continuous-wave Terahertz imaging with a carefully chosen wavelength can provide valuable data on samples of biological origin; these data appear complementary to those obtained from white-light images.

Subwavelength Microdisk and Microring Terahertz Quantum-Cascade Lasers

We report on the emission characteristics of microcavity quantum-cascade lasers emitting in the terahertz frequency range based on circular-shaped microresonators. Strong mode confinement in the growth and in-plane directions are provided by a double-plasmon waveguide and due to the strong impedance mismatch between the gain material and air. This allows laser emission from devices with overall dimensions much smaller than the free-air emission wavelength (lambda > 100 mum). Hence, for the smallest microdisks we achieved a threshold current as low as 13.5 mA (350 A/cm2) in pulsed-mode operation at 5 K and stable single-mode emission up to 95 K in continuous-wave mode operation. We have observed dynamical frequency pulling of the resonator mode on the gigahertz scale, as a consequence of the gain shift due to the quantum-confined Stark effect. Thus, we were able to estimate the peak gain of the material to 27 cm-1. The smallest microcavities exhibited a strong dependence on the exact placement of the bond wire which resulted in single- as well as double-mode emission. Finite-difference time-domain simulations were performed in order to identify the modes of the recorded spectra. They confirm that most of the observed spectral features can be attributed to the lasing emission of whispering-gallery modes.

Numerical investigation of resonances within the X-band waveguide type

In this letter, we report on the numerical investigation of resonances within waveguide type resistive sensors used for measuring high power microwave pulse radiation in the X-band. We have applied a finite-difference time-domain method to determine the dependencies of the average electric field and its distribution on frequency for several sets of dimensions and specific resistances of the sample

Terahertz quantum cascade lasers in a magnetic field

We have investigated the behavior of a terahertz quantum cascade laser in an external magnetic field. A reduction of the threshold current density and a simultaneous enhancement of the laser emission intensity are observed. Although several mechanisms can induce this effect, the suppression of nonradiative Auger-intersubband transitions through Landau quantization of the in-plane electron motion is the most probable candidate. In addition, the injection rate via resonant inter-Landau-level transfer and the waveguide properties are modulated by the field. We also observed clear shifts of the emission spectra when the external magnetic field is applied, while operating the device at constant voltage or current.

Compact Light-Emitting Diode-Based AAA Class Solar Simulator: Design and Application Peculiarities

In this paper, we report on the efficient design of a light-emitting diode (LED)-based AAA class solar simulator, employing only 19 high-power emitters for a usable illuminated area of at least 5 cm in diameter with at least 1 sun irradiance. Such a low number of emitters was achieved by selectively employing secondary optics for several LED groups and taking advantage of wide emission angle for others. The so-called A class spectrum was also achieved for the larger area of more than 6 cm × 6 cm, covering zones with B and C class irradiance nonuniformity. Five distinct solar cell external quantum efficiency spectra were considered for theoretical evaluation of possible measurement peculiarities related to different solar cell technologies. These computer-generated spectra contained essential features, which is typical for high-efficiency crystalline and amorphous silicon, Cu(In,Ga)Se2, and Cu2ZnSn(S,Se)4 solar cell technologies. Significant photocurrent distribution nonuniformity change is predicted only for amorphous silicon cells due to a much narrower efficient absorption spectrum.

RESISTIVE SENSOR FOR HIGH POWER MICROWAVE PULSE MEASUREMENT OF TE01 MODE IN CIRCULAR WAVEGUIDE

A resistive sensor (RS) devoted for high power microwave pulse measurement in cylindrical waveguide is considered. The modeling results of the interaction of the TE01 (H01) wave with a semiconductor plate with contacts on sidewalls of the plate placed on a wall of the circular waveguide are presented. A finite-difference time-domain (FDTD) method was employed for the calculation of the electromagnetic field components, reflection coefficient from the semiconductor obstacle, and the average electric field in it. The features of the resonances have been used to engineer the frequency response of the RS. It has been found that such electrophysical parameters of the plate can serve as the prototype of the sensing element (SE) for the circular waveguide RS with flat frequency response.

Computation of the Averaged Electric Field in the Semiconductor Obstacle Placed in the Coaxial Line

In this paper we present computed results of the averaged electric field in the semiconductor obstacle placed in the coaxial line. Such layout might be used as a sensor for the measurement of microwave pulse power transmitted through the coaxialline. FDTD (finite different time domain) method was applied for the theoretical investigation. The dependences of the averaged electric field in the obstacle on the microwave frequency, as well as on the dimensions and specific resistance of the obstacle are considered. Both layouts with the obstacle placed on the inner and on the outer conductor of the coaxial line were investigated. Calculations were performed for the 50 Ω impedance coaxial line filled with air and obstacle made from Si.

Microcavity terahertz quantum-cascade laser

We report the realization of microdisk and microring quantum-cascade lasers (QCLs) emitting in the terahertz (THz) region between 3.0 THz and 3.8 THz. The GaAs/Al0.15Ga0.85As heterostructure is based on longitudinal-optical phonon scattering for depopulation of the lower radiative state. A double metal waveguide is used to confine the whispering gallery modes in the gain medium. The threshold current density is 900 A/cm2 at 5 K. Lasing takes place in pulsed-mode operation up to a heat-sink temperature of 140 K. Finite-Difference Time-Domaine (FDTD) simulations were performed in a strong field limit to obtain the field distribution within a microdisk THz QCL resonator.

Magnetic field effects in terahertz quantum-cascade lasers

We have measured the emission intensity and spectra of terahertz quantum-cascade lasers in an external magnetic field applied normal to the epilayers. We have observed a reduction of the threshold current, an enhancement of laser emission intensity and shifts of the emission line. A wider operating range was predicted for the selected waveguide design according to our finite-difference time-domain simulation results. The intensity enhancement and the threshold current reduction are attributed to the suppression of nonradiative Auger-intersubband transitions by Landau quantization of the in-plane electron motion, to the modulation of the injection rate via resonant inter-Landau-level transfer, and to the modulation of waveguide properties.

The response rate of room temperature terahertz InGaAs-based bow-tie detector with broken symmetry

A bow-tie InGaAs with broken symmetry has been designed for terahertz detection at room temperature. An active part of the detector consists of a two-dimensional electron gas which is heated non-uniformly with incident radiation. Main detector performances are operation in a passive scheme, flat frequency response up to 1 THz, the voltage sensitivity of about 5 V/W, the noise equivalent power of roughly 10 nW/Hzfrac12, and the response time less then 7 ns.