Emission Of Terahertz Radiation Research Articles

Emission of terahertz radiation from dual grating gate plasmon-resonant emittersfabricated with InGaP/InGaAs/GaAs material systems

This paper reviews recent advances in our original 2D-plasmon-resonant terahertz emitters.The structure is based on a high-electron-mobility transistor and featured with doublyinterdigitated grating gates. The dual grating gates can alternately modulate the 2Delectron densities to periodically distribute the plasmonic cavities along the channel, actingas an antenna. The device can emit broadband terahertz radiation even at roomtemperature from self-oscillating 2D plasmons under the DC-biased conditions. When thedevice is subjected to laser illumination, photo-generated carriers stimulate the plasmaoscillation, resulting in enhancement of the emission. The first sample was fabricated withstandard GaAs-based heterostructure material systems, achieving room temperatureterahertz emission. The second sample was fabricated in a double-decked HEMT structurein which the grating gate metal layer was replaced with the semiconducting upper-deck 2Delectron layer, resulting in enhancement of emission by one order of magnitude.

Effects of post-growth annealing on the structure and electro-optical properties of low-temperature grown GaAs

This paper investigates how postgrowth annealing affects the structure and the electro-optical properties of low-temperature grown GaAs (LT-GaAs). A systematic study of as-grown and annealed LT-GaAs revealed that the carrier lifetime is directly related to the density of the An duster and distance between As clusters. The Ga/As compositional ratio and the crystal structure of As clusters were observed for the first time. The As/Ga ratio of the As clusters is higher than that obtained from the LT-GaAs. The carrier lifetime of the annealed LT-GaAs increases as the annealing temperature increases from 400 (less than 30 fs) to 800 °C (824 fs). Under the annealing temperatures ranging from 600 to 700 °C for 90 s, we observed the emission of terahertz radiation using the annealed LT-GaAs grown at temperatures ranging from 260 to 320 °C.

Room temperature terahertz emission from grating coupled two-dimensional plasmons

Room temperature terahertz (far infrared) radiation emission from double grating coupled GaInAs∕AlGaAs∕GaAs heterojunctions is reported. Theoretical calculations of plasmon absorption spectrum are performed using a first principles electromagnetic approach. They correctly reproduce the frequency range and overall shape of the main (broad-band) part of the experimental spectra. The results clearly indicate that important part of the observed room temperature terahertz emission spectra can be due to the radiative decay of hot two-dimensional plasmons in the grating structure.

Emission of terahertz radiation from GaAsN/GaAs heterostructures in an electric field

Emission of terahertz radiation from strained Be-doped GaAsN layers has been revealed at 4.2 K in postbreakdown electric fields. Heavy and light hole subbands are split in strained layers, and, along with localized acceptor states, resonant states arise. The spectrum of terahertz radiation has been obtained, the current-voltage characteristics of the samples were investigated, and the energy spectra of acceptors were calculated. Peaks in the spontaneous emission spectrum correlate well with the results of the energy spectrum calculations. The transitions between the resonant and localized states of the acceptors make the main contribution to the terahertz radiation intensity.

Generation and Detection of THz Radiation With Scalable Antennas Based on GaAs Substrates With Different Carrier Lifetimes

We report on scalable photoconductive antennas for both emission and detection of terahertz (THz) radiation. The concept yields THz emitters with high efficiencies for the conversion of near-infrared into far-infrared radiation, and provides detectors that do not require tight focusing of both the THz beam and the near-infrared gating beam. GaAs substrates implanted with dual energy implants of N <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> and As <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ions of various doses are compared with semiinsulating (SI) and low-temperature-grown GaAs. We discuss which material properties are desirable for emitters and detectors and identify which material is optimal as either emitter or detector substrate. Best results for detectors are found for implanted samples with doses in the range of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> for GaAs:N and for LT-GaAs. Best emitters for typical excitation conditions with a Ti: sapphire oscillator system are based on SI-GaAs.

Absorption and emission of terahertz radiation in doped GaAs/AlGaAs quantum wells

Absorption and emission of terahertz radiation in doped GaAs/AlGaAs quantum wells

Emission of terahertz radiation from GaAsN/GaAs heterostructures in an electric field

Emission of terahertz radiation from GaAsN/GaAs heterostructures in an electric field

Simultaneous generation of a proton beam and terahertz radiation in high-intensity laser and thin-foil interaction

We have observed simultaneously both the fast proton generation and terahertz (THz) radiation in the laser pulse interaction with a 5-μm thick titanium target. In order to control the proton acceleration and THz radiation, we have changed the duration of the amplified spontaneous emission (ASE) preceding the main pulse generated by the high-intensity Ti:sapphire laser. A fast proton beam with the maximal energy of ∼ 490 keV has been realized by reducing the duration of the ASE. Simultaneously, an intense emission of THz radiation is observed for various ASE durations. We propose the antenna mechanism for the THz radiation, according to which the fast electrons moving along the target surface emit the low-frequency electromagnetic wave.

Plasma oscillations in nanotransistors for room temperature detection and emission of terahertz radiation

AbstractIn this work we present experimental results on room temperature terahertz (THz) detection/emission from nanotransistors as well as double grating gates structures. It shows that the emission spectra from GaN/AlGaN HEMT transistor can be successfully interpreted in the frame of the Dyakonov‐Shur model which predicts that by heating/accelerating the electrons up to velocities comparable with plasma wave velocity can lead to instability and generation of plasma waves in the transistor channel. Studies of the current dependence of THz detection by InGaAs based HEMT is also presented. It is shown that in realistic transistor structures the room temperature resonant THz detection can be observed only upon applying a current high enough to produce fast/hot electrons. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Forward THz radiation emission by femtosecond filamentation in gases: theory and experiment

A transition–Cherenkov electromagnetic emission by a femtosecond laser pulse propagating in a self-induced plasma channel in air has been very recently proposed as mechanism for production of terahertz (THz) radiation in the forward direction. In this paper, we study in detail the theory of the transition–Cherenkov process. The theoretical model is developed and compared with recent experimental results for several gases.

Emission of Coherent THz Radiation from Superconductors

Compact solid-state sources of terahertz (THz) radiation are being sought for sensing, imaging, and spectroscopy applications across the physical and biological sciences. We demonstrate that coherent continuous-wave THz radiation of sizable power can be extracted from intrinsic Josephson junctions in the layered high-temperature superconductor Bi2Sr2CaCu2O8. In analogy to a laser cavity, the excitation of an electromagnetic cavity resonance inside the sample generates a macroscopic coherent state in which a large number of junctions are synchronized to oscillate in phase. The emission power is found to increase as the square of the number of junctions reaching values of 0.5 microwatt at frequencies up to 0.85 THz, and persists up to approximately 50 kelvin. These results should stimulate the development of superconducting compact sources of THz radiation.

Terahertz emission from vertically aligned InN nanorod arrays

Terahertz emission from indium nitride (InN) nanorods and InN film grown by molecular-beam epitaxy on Si(111) substrates has been investigated. Terahertz emission from InN nanorods is at least three times more intense than that from InN film and depends strongly on the size distribution of the nanorods. Surface electron accumulation at the InN nanorods effectively screens out the photo-Dember field in the accumulation layer formed under the surface. The nanorods with considerably large diameter than the thickness of accumulation layer are found to be dominant in the emission of terahertz radiation from InN nanorod arrays.

Emission of terahertz radiation from InGaP/InGaAs/GaAs grating-bicoupled plasmon-resonant emitter

Abstract We have designed and fabricated an optically-pumped plasmon-resonant terahertz emitter incorporating doubly interdigitated grating gates and a vertical cavity into an InGaP/InGaAs/GaAs high-electron mobility transistor structure. The two-dimensional (2D) plasmon layer is formed with a quantum well in the InGaAs channel layer. When the dual grating gates are biased at different levels, the sheet carrier density is periodically modulated, making periodic 2D plasmon cavities along with the grating. When the device is photoexcited by laser irradiation, photoelectrons, injected to the cavities, excite the plasmons, leading to the emission of terahertz radiation. The fabricated device is subjected to 1550-nm, 1-mW (a) a single CW-laser, (b) 4-THz photomixed dual CW-laser, and (c) a 70-fs pulsed-laser illumination at room temperature. In case of (a), terahertz emission is detected by a Si bolometer under certain bias conditions, which is inferred to be due to the self oscillation stimulated by the plasmon instability. In case of (b), a resonant peak of injection-locked 4-THz oscillation is clearly observed on the device photoresponse. In case of (c), an impulsive radiation followed by relaxation oscillation is observed by electrooptic sampling, whose Fourier spectrum exhibited resonant peaks of plasmons’ harmonic modes up to 4 THz. Estimated radiation power exceeds 0.1 μW, resulting in excellent conversion efficiency of the order of 10 −4 .

Visualization of photoassisted polarization switching and its consequences in BiFeO3 thin films probed by terahertz radiation

The principle of terahertz radiation emission from multiferroic BiFeO3 thin films was employed to acquire high-resolution images of their photoassisted ferroelectric polarization switching. These images are evidence that poling in “electric-field assisted by illumination of band gap light” enhances the terahertz-emission amplitude by creating additional switchable ferroelectric dipoles.

Possibility of Terahertz Injection-Locked Oscillation in an InGaP/InGaAs/GaAs Two-Dimensional Plasmon-Resonant Photomixer

We experimentally investigated terahertz photomixing operation at room temperature in an InGaP/InGaAs/ GaAs two-dimensional plasmon-resonant photomixer incorporating grating-bicoupled dual-gate structure. Photoelectrons drifting into a high-density plasmon cavity grating from an adjacent low-density one extensively excite the plasmon resonance, resulting in emission of terahertz radiation. A vertical cavity formed between the two-dimensional plasmon grating plane and an indium-tin-oxide mirror at the back surface gains the radiation. Self-oscillation initially at around 4.5 THz excited by a dc-photo carrier component was reinforced by the photomixed differential-frequency excitation at 4.0 and 5.0 THz. This indicates a possibility of injection-locked oscillation of the photomixer in the terahertz frequency band.

Model of thermal terahertz light emission from a two-dimensional electron gas

The spontaneous thermal emission of terahertz radiation of a two-dimensional electron gas is analyzed, including its coupling to lattice excitations and the influence of sample-vacuum interface. As a representative example, electronic intraband transitions in semiconductor quantum wells are discussed. We observe broad emission spectra in the terahertz range including enhanced emission at the optical-phonon frequencies of the barrier material.

Generation of Terahertz radiation with two color semiconductor lasers

AbstractWe discuss and analyze concepts for the generation of tuneable continuous wave terahertz (THz) radiation with two color diode lasers. First, different geometries of two color lasers are reviewed. We show that the THz power of two color lasers in combination with external photomixers becomes sufficient for scanning THz imaging applications when optical amplification with a tapered amplifier is implemented. Then, the concept of direct emission of THz radiation out of a two‐color semiconductor laser is reviewed and the potential of this concept with respect to THz bandwidth and achievable THz power is critically analyzed.

Numerical modeling of TeraHertz electronic devices

We investigate by means of Monte Carlo simulations the physical processes associated with the emission of TeraHertz radiation in different electronic devices. We analyze four alternative and complementary strategies which seem to be promising candidates to obtain the TeraHertz emission: (1) a nitride maser based on the optical-phonon transit-time resonance, (2) the high–order harmonic generation in bulk materials and nanometric Schottky-barrier diodes, (3) the excitation of coherent plasma oscillations in micron and submicron diodes, (4) the current instabilities and plasma oscillations in high electron mobility transistors (HEMT). The numerical results show that several physical mechanisms can be exploited to increase significantly the operating frequency of these devices and the best conditions to optimize the radiation emission in the TeraHertz range are studied in detail.

Plasma Instability and Terahertz Generation in HEMTs Due to Electron Transit-Time Effect

We study the coupled spatio-temporal variations of the electron density and the electric field (electron plasma oscillations) in high-electron mobility transistors using the developed device model. The excitation of electron plasma oscillations in the terahertz range of frequencies might lead to the emission of terahertz radiation. In the framework of the model developed, we calculate the resonant plasma frequencies and find the conditions for the plasma oscillations self-excitation (plasma instability) We show that the transit-time effect in the high-electric field region near the drain edge of the channel of high-electron mobility transistors can cause the self-excitation of the plasma oscillations. It is shown that the self-excitation of plasma oscillations is possible when the ratio of the electron velocity in the high field region, u d , and the gate length, Lg, i.e., the inverse transit time are sufficiently large in comparison with the electron collision frequency in the gated channel, v. The transit-time mechanism of plasma instability under consideration can superimpose on the Dyakonov-Shur mechanism predicted previously strongly affecting the conditions of the instability and, hence, terahertz emission. The instability mechanism under consideration might shed light on the origin of terahertz emission from high electron mobility transistors observed in recent experiments.

Terahertz radiation emission from GaMnAs

Terahertz radiation is observed from ferromagnetic GaMnAs samples excited with 400nm wavelength pump pulses and is related to the sample magnetization M. The emission can be explained by the strong influence of M on the photogenerated carrier motion, a phenomenon related to the dc anomalous Hall effect. Results illustrate the potential of ferromagnetic materials to be used as compact terahertz sources emitting in a direction normal to the surface.