Transitions between the 1s and 2p levels of the fundamental heavy-hole exciton in GaAs quantum wells, followed by scattering into the 2s state, are investigated by resonant THz excitations using a free-electron laser. We report on the external control of this intra-excitonic population transfer by an external magnetic field.
Using a double-sided fabrication method, we fabricated Bi2Sr2CaCu2O8 (BSCCO) intrinsic Josephson junctions (IJJs) with a gold-BSCCO-gold structure. Coherent emission is observed at large dc input power, where a hot spot and “cold” part of the stack, coexist. The power, directly detected with a bolometer, is as high as 25 µW, implying the integrated power should be already in the order of sub-mW. We...
We present results from a TDS imaging system for non-destructive evaluation of aeronautics composite materials, like glass fiber laminate and sandwich structures. Time-of-flight information from reflection measurements allows for 3D reconstruction of test sample volumes. Clear identification and spatial positioning of defects like delamination and foreign inclusion is possible.
We discuss photon-phonon interaction in AlN and GaN thin layer structures by spectroscopic analysis in 20 – 30 THz region. Optical absorption by the generation of electric dipole moment based on interface polarization charges is found besides interface phonon polariton propagation. The polariton and absorption properties are analyzed.
We present experimental implementation of high-speed broadband frequency sweep of CW THz radiation applicable to high-speed THz spectroscopy and tomography. A wavelength-swept laser and a distributed-feedback laser diode constitute a high-speed frequency-swept optical beat source used for photomixing.
We report on the continued progress of a portable, battery-operated frequency domain terahertz spectrometer with an integrated, fiber-coupled, lithium-niobate optical phase-modulator. We discuss the progress on shifting the THz phase 90 degrees between consecutive data samples to remove the interference pattern and on developing a method of employing second harmonic nulling to actively adjust the...
A new type of semiconductor local states is revealed in Pb1−xSnxTe(In) narrow-gap semiconductors. The energy position of these states is not linked to any specific location in the semiconductor energy spectrum, but follows the quasiFermi level position, which may be tuned by photoexcitation.
Depolarization fields are evaluated in several model structures with varying percolation degree. On this basis we propose a simple relation between the microscopic photoconductive response of semiconductor nanostructures and their effective response measured in a time-resolved THz experiment.
We demonstrate end-fire excited THz radiation coupled in thin layers of water propagating a distance of 4 cm. As the propagation lengths are critically dependent from the permittivity, we experimentally demonstrate this propagation length dependency by stepwise exchanging the material layer from deionized water to pure ethanol.
We explore the potential of graphene symmetric tunneling field effect transistors (SymFETs) for THz applications. The interplay between the negative differential conductance in these devices and electron plasma waves occurring in the graphene layers might lead to very sensitive THz detection (R > 100 kV/W) or amplifiers with power gains ∼ 7 dB at RT.
The design of focal planes of power detectors for the characterization of radiometric distributed sources in the sub-mm wave regime is a problem of renewed interest for the scientific community. The important question is how to optimize the focal plane sampling and the focal plane feed elements for maximizing the acquisition speed. The key speed limitations for some popular feed solutions have been...
Gallium Selenide crystals having compositions modified with a range of elements are investigated using THz time-domain spectroscopy, Raman, FTIR and ellipsometry. The optical properties are related to modifications in the crystal structure.
Sub-terahertz and terahertz frequencies can be generated by optical heterodyning of two wavelengths. Two laser modes, generated with a Photonic Integrated Circuit which integrates an Arrayed Waveguide Grating, are employed to create terahertz frequencies. The wavelength difference of the two optical modes defines the frequency of the generated wave, which can be discretely tunable by selecting different...
Transistors are possible THz detectors using rectification in the channel that can be applied for detection of high data-rate wireless communications, based on THz-frequency carrier. For the first time, we present the transmission of pseudo-random bit sequence at 0.2 THz using a commercial GaAs transistor and demonstrate open eye patterns up to 0.250 Gbps.
We demonstrate a reduction of loss and group velocity dispersion (GVD) of THz plasmonic waveguides by using a low-loss thin flexible substrate. We present a numerical calculation of the effect of the substrate thickness on the GVD. We show experimentally low absorption and weak distortion of the propagated THz pulses along a THz plasmonic waveguide on a flexible 58µm-thick polyimide substrate.
We present an ultrafast terahertz detector suitable for wavelengths from 30 µm to 220 µm, which is based on a graphene flake. A logarithmic-periodic antenna is used to couple the radiation to the flake. The detector, characterized by a fast rise time combined with room temperature operation, is well suited for determining timing differences of THz laser pulses.
A Continuous Wave (CW) millimeter-wave photonic synthesizer based on Difference Frequency Generation (DFG) is presented. This system encompasses an Optical Frequency Comb Generator (OFCG), an optical mode selection stage and a high-bandwidth photodetector. The employed OFCG is a novel design based on Gain-Switching modulation of a VCSEL. This new OFCG offers equivalent optical span than commonly employed...
The reflection properties of porcine skin are examined in the terahertz (THz) frequency range utilizing a Fourier transform infrared spectrometer with a reflection unit. Their dependence on the frequency and other parameters such as the surface structure is analyzed in order to determine whether porcine skin can be used as surrogate for human skin in the THz frequency range.
Activation energy of phospholipids in aqueous solution was obtained by terahertz (THz) time domain spectroscopy. The THz absorption as a function of temperature, an Arrhenius plot, yields activation energy of water relaxation process. Head group structure and phase dependence of the activation energy is reported.
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