Bright MM-Wave and THz Luminescence by Down-Conversion of Near-IR Amplified-Spontaneous-Emission

We report the measurement of exceptionally bright, incoherent radiation in the THz region by frequency down-conversion of amplified spontaneous emission around 775-nm wavelength. The down-conversion technique is optical mixing in an interdigital photoconductive capacitor made from ultra fast ErAs:GaAs. The brightness temperature into a single spatial mode is approximately 1.1×10⁵ K, making the new radiation at least 70 times more intense than common incandescent sources in the THz region.     

The electronic and emissive properties of Au-doped porous silicon

The temperature dependences of photovoltage induced by intense pulses of red and white light, along with the time-resolved spectral dependences of photoluminescence, are studied for porous silicon structures por-Si/p-Si). These structures have been obtained by anode etching of p-Si with subsequent Au doping from an aqueous solution with Au ion concentrations of 10^?4 and 10^?3 M. The current-voltage characteristics and electroluminescence of the resulting por-Si/p-Si and por-Si:Au/p-Si structures are also studied after a deposition of semitransparent Au electrodes on por-Si. It is shown that the Au doping changes the sign of the boundary potential of p-Si from positive to negative, alters the magnitude and sign of the photovoltage in the por-Si films, and eliminates photomemory phenomena, which are associated with the capture of nonequilibrium electrons at grain-boundary traps and por-Si traps. The formation of Au nanocrystals in por-Si substantially affects the current-voltage and photoluminescence characteristics. Electroluminescence is observed for the Au/por-Si:(Au, 10^?3 M)/p-Si/Al structures and is attributed to emission from the nanocrystals.     

A Modified 8<Emphasis Type="Italic">f</Emphasis> Geometry with Reduced Optical Aberrations for Improved Time Domain Terahertz Spectroscopy

We present a modified 8f geometry for time domain terahertz (THz) spectroscopy (TDTS) experiments. We show, through simulations and data, that a simple rearranging of the off-axis parabolic mirrors, which are typically used to focus and direct THz radiation in TDTS experiments, results in a nearly 40 % reduction in the THz focal spot diameter. This effect stems from significant reduction of the principle optical aberrations which are enhanced in the conventional 8f geometry but partially compensated in the modified 8f experimental setup. We compare data from our home-built TDTS spectrometer in the modified 8f geometry to that of previous iterations that were designed in the conventional 8f geometry to demonstrate the effect.     

Enhanced Terahertz Imaging of Small Forced Delamination in Woven Glass Fibre-reinforced Composites with Wavelet De-noising

Terahertz (THz) reflection imaging is applied to characterize a woven glass fibre-reinforced composite laminate with a small region of forced delamination. The forced delamination is created by inserting a disk of 25- μ m-thick Upilex film, which is below the THz axial resolution, resulting in one featured echo with small amplitude in the reflected THz pulses. Low-amplitude components of the temporal signal due to ambient water vapor produce features of comparable amplitude with features associated with the THz pulse reflected off the interfaces of the delamination and suppress the contrast of THz C- and B-scans. Wavelet shrinkage de-noising is performed to remove water-vapor features, leading to enhanced THz C- and B-scans to locate the delamination in three dimensions with high contrast.     

Observation of Ultrafast Magnon Dynamics in Antiferromagnetic Nickel Oxide by Optical Pump-Probe and Terahertz Time-Domain Spectroscopies

We have studied the ultrafast magnon dynamics in an antiferromagnetic 3d-transition-metal monoxide, nickel oxide (NiO), using optical pump-probe spectroscopy and terahertz time-domain spectroscopy (THz-TDS). THz damped magnon oscillations were observed in the Faraday rotation signal and in the transmitted THz electric field via optical pump-probe spectroscopy and THz-TDS, respectively. The magnon signals were observed in both the optical pump-probe spectroscopy and THz-TDS experiments, which shows that both Raman- and infrared-active modes are included in the NiO magnon modes. The magnon relaxation rate observed using THz-TDS was found to be almost constant up to the Néel temperature T _N (= 523 K) and to increase abruptly near that temperature. This shows that temperature-independent spin-spin relaxation dominates up to T _N . In our experiment, softening of the magnon frequency near T _N was clearly observed. This result shows that the optical pump-probe spectroscopy and THz-TDS have high frequency resolution and a high signal to noise ratio in the THz region. We discuss the observed temperature dependence of the magnon frequencies using three different molecular field theories. The experimental results suggest that the biquadratic contribution of the exchange interaction plays an important role in the temperature dependence of the sublattice magnetization and the magnon frequency in cubic antiferromagnetic oxides.     

High-power nano- and picosecond optoelectronic switches based on high-voltage silicon structures with <Emphasis Type="Italic">p–n</Emphasis> junctions: II. Energy efficiency

The energy efficiency of optoelectronic switches based on high-voltage silicon photodiodes, phototransistors, and photothyristors controlled by picosecond laser pulses during the formation of voltage pulses on resistive load R _L is studied for the first time. It is shown that at the given values of the resistive load R _L , pulse amplitude U _R and duration t _R , there exist optimum device areas, energies, and absorbances of control radiation providing a maximum total switch efficiency of ~0.92. All three switch types feature almost the same efficiency at short t _R ; at longer t _R , photothyristors have a noticeable advantage.     

Spontaneous spin polarization of electrons in quantum wires

The quantum conductance staircase of the one-dimensional (1D) channel is analyzed for a weak filling of the lower 1D subbands, when the exchange electron-electron interaction of carriers dominates over their kinetic energy. Main attention is paid to considering the behavior of the “0.7(2e²/h)” feature split off from the first quantum step, which is identified as a result of the spontaneous spin polarization of the 1D electronic gas through exchange interaction at zero magnetic field. The critical linear concentration of electrons above which fully polarized electron gas starts to depolarize, with the resulting evolution of the split-off substep height from e²/h to 2e²/h, is determined within the framework of phenomenological theory. Moreover, the temperature dependence of the height of this substep at the range 0.5(2e²/h)–0.75(2e²/h), resulting from the partial depolarization of the electron gas near the 1D subband bottom, is predicted. The quantum-mechanical consideration carried out analytically in the context of the Hartree-Fock-Slater approximation with localized exchange potential shows that consideration of the electron-electron interaction in a quantum wire with an arbitrary carrier density leads to spontaneous polarization of the quasi-1D electronic gas in zero magnetic field at small linear concentrations of carriers.     

THz Induced Nonlinear Effects in Materials at Intensities above 26 GW/cm<Superscript>2</Superscript>

Nonlinear refractive index and absorption coefficient are measured for common semiconductor material such as silicon and organic molecule such as lactose in the terahertz (THz) spectral regime extending from 0.1 to 3 THz. Terahertz pulses with field strengths in excess of 4.4 MV/cm have been employed. Transmittance and the transmitted spectrum were measured with Z-scan and single shot noncollinear electro-optic pump-probe techniques. The THz-induced change in the refractive index (Δn) shows frequency-dependence and a maximum change of \(-~0.128\) at 1.37 THz in lactose and up to \(+~0.169\) at 0.15 THz in silicon was measured for a peak incident THz intensity of 26 GW/cm². Furthermore, the refractive index variation shows a quadratic dependence on the incident THz field, implying the dominance of third-order nonlinearity.     

Theoretical Study on the 1.185-THz Third Harmonic Gyrotron

We discuss how the existing University of Fukui (FIR UF) second harmonic double-beam gyrotron with the operating frequency 0.79 THz can be adopted for operation at the third harmonic. The new gyrotron will operate at the frequency 1.185 THz and will significantly increase the frequency of the dynamic nuclear polarization-nuclear magnetic resonance (DNP-NMR) spectrometer. This will allow one to study new bio-molecules.A special attention is payed to the mode competition between the operating \( {TE}_{3,11}^{+} \) mode at the third harmonic and the parasitic modes at the second and fundamental harmonics. The operating parameters of the modified gyrotron are U?=?20?kV,?α?=?1.3,?I?=?0.35?A,?and?B?=?14.60?T and the expected output power about \( 100\mathrm{W} \).     

Insights on the Side Panels of the <Emphasis Type="Italic">Franciscan Triptych</Emphasis> by Fra Angelico Using Terahertz Time-Domain Imaging (THz-TDI)

The side panels of the Franciscan Triptych (St. Jerome, St. John the Baptist, and the Archangel Gabriel and St. Francis, St. Onofrio, and the Virgin Annunciate, by Fra Angelico, before 1429) were scanned by means of terahertz time-domain imaging (THz-TDI). THz analysis supplied information on the stratigraphy of the panel paintings and the associated construction, “gessoing” and gilding techniques. Furthermore, THz-TDI provided information regarding the location of restoration materials within the painting stratigraphy on St. Jerome, St. John the Baptist, and the Archangel Gabriel, as well as on the extension and nature of subsurface cracks in the panel painting of St. Francis, St. Onofrio, and the Virgin Annunciate.     

Influence of Two-Photon Absorption Anisotropy on Terahertz Emission Through Optical Rectification in Zinc-Blende Crystals

We report for the first time on the observation of an angular anisotropy of the THz signal generated by optical rectification in a < 111 > ZnTe crystal. This cubic (zinc-blende) crystal in the <?111 > orientation exhibits both transverse isotropy for optical effects involving the linear χ⁽¹⁾ and nonlinear χ⁽²⁾ susceptibilities. Thus, the observed anisotropy can only be related to χ⁽³⁾ effect, namely two-photon absorption, which leads to the photo-generation of free carriers that absorb the generated THz signal. Two-photon absorption in zinc-blende crystals is known to be due to a spin-orbit interaction between the valence and higher-conduction bands. We perform a couple of measurements that confirm our hypothesis, as well as we fit the recorded data with a simple model. This two-photon absorption effect makes difficult an efficient generation, through optical rectification in <?111 > zinc-blende crystals, of THz beams of any given polarization state by only monitoring the laser pump polarization.     

Integer quantum Hall effect and correlated disorder

The effect of the form of the random potential of impurities and defects on the longitudinal σ _xx and Hall σ _xy components of conductivity in the mode of the integer quantum Hall effect is theoretically investigated. It is shown that the width of the Hall conductivity plateau as well as the peak values of the longitudinal conductivity heavily depend on the ratio λ/a _H between the random potential correlation length and the magnetic length. For the first time, it is established that in the case of the short-wavelength potential λ ? a _H, the peak values of σ _xx ^(N) are directly proportional to the Landau level number N ≥ 1, σ _xx = 0.5Ne ²/h, whereas the peak values of σ _xx ^(N) are independent of the Landau level number in the case of the long-wavelength potential λ ? a _H, and their magnitude is much lower than 0.5e ²/h. The obtained results are in good agreement with the available experimental data.     

Ammonia sensors based on Pd-<Emphasis Type="Italic">n</Emphasis>-Si diodes

The influence of ammonia on the electrical characteristics of Pd-n-Si diode structures is studied. The kinetics of diode characteristics under exposure to ammonia is considered. It is shown that the response speed of ammonia sensors based on Pd-n-Si diodes can be controlled by applying additional potential pulses to the barrier electrode.     

Pulsed Phase-Shift Keyed Signals with Zero Autocorrelation Zone

A coherent additional signal whose zero side-lobe region (its relative width lies in the interval [1 3,1)) is located near the central peak of an aperiodic autocorrelation function is synthesized. It is demonstrated that the given signal is the batch composed of two phase-shift keyed pulses encoded by paired or adjacent sequences of the D-code with the length N = 2^k signal synthesis is proposed to generate from a set of the N/4 ensembles of D-code with length N. The correlation characteristics of the synthesized signal are analyzed.     

Shaping pulse of faster-than-Nyquist signaling with truncated optimal detector

In this paper, we investigate the shaping pulse of faster-than-Nyquist (FTN) signaling by making use of the reduced complexity truncated optimal maximum-likelihood sequence detection. Specifically, the nonorthogonal Gaussian shaping pulse which owns the approximate optimal energy concentration in time-frequency domains is exploited. Moreover, for fair of comparisons, a general benchmark for different shaping pulses is adopted, and based on which, the Euclidean distance (or Mazo limit) and practical information rate performance of FTN signaling with Gaussian pulse and conventional T-orthogonal shaping pulses such as \({\mathrm{sinc}}\) pulse and root raised cosine pulse are evaluated. Theoretical analyses and numerical results demonstrate that when employed with truncated optimal detector and small channel memory at the receiver, the Gaussian pulse could achieve better BER and information rate performance than conventional T-orthogonal pulses.     

Advances in Front-end Enabling Technologies for Thermal Infrared ‘<Emphasis Type="Italic">THz Torch</Emphasis>’ Wireless Communications

The thermal (emitted) infrared frequency bands (typically 20–40 and 60–100 THz) are best known for remote sensing applications that include temperature measurement (e.g. non-contacting thermometers and thermography), night vision and surveillance (e.g. ubiquitous motion sensing and target acquisition). This unregulated part of the electromagnetic spectrum also offers commercial opportunities for the development of short-range secure communications. The ‘THz Torch’ concept, which fundamentally exploits engineered blackbody radiation by partitioning thermally generated spectral radiance into pre-defined frequency channels, was recently demonstrated by the authors. The thermal radiation within each channel can be independently pulse-modulated, transmitted and detected, to create a robust form of short-range secure communications within the thermal infrared. In this paper, recent progress in the front-end enabling technologies associated with the THz Torch concept is reported. Fundamental limitations of this technology are discussed; possible engineering solutions for further improving the performance of such thermal-based wireless links are proposed and verified either experimentally or through numerical simulations. By exploring a raft of enabling technologies, significant enhancements to both data rate and transmission range can be expected. With good engineering solutions, the THz Torch concept can exploit nineteenth century physics with twentieth century multiplexing schemes for low-cost twenty-first century ubiquitous applications in security and defence.     

Formalizing REST APIs for web-based communication and SIP interworking

This paper proposes an instantaneous recovery route design scheme using multiple coding-aware link protection scenarios to achieve higher link cost reduction in the network. In this scheme, two protection scenarios, namely, (i) traffic splitting (TS), and (ii) two sources and a common destination (2SD) are used to integrate multiple sources and a common destination. The proposed scheme consists of two phases. In the first phase, the proposed scheme determines routes for 2SD and TS scenarios of all possible source-destination pairs to minimize the total link cost. In this phase, the network coding is applied to the common path within each scenario, individually. In the second phase, network coding is applied to the common path between two scenarios (or a scenario pair) in order to enhance the resource saving. This phase develops conditions that select the most appropriate combination of scenario pairs, such as TS–TS, 2SD–2SD, and 2SD–TS for network coding, including their proofs. Using these conditions, a heuristic algorithm is introduced in order to select the most appropriate combination of scenario pairs for further enhancing of resource saving. Simulation results show that the proposed scheme outperforms the conventional 1 + 1 protection scheme, the TS scenario, and the 2SD scenario in terms of link cost reduction in the network.     

Computationally Efficient Motion Estimation Algorithm for HEVC

In this paper, we present a novel computationally efficient motion estimation (ME) algorithm for high-efficiency video coding (HEVC). The proposed algorithm searches in the hexagonal pattern with a fixed number of search points at each grid. It utilizes the correlation between contiguous pixels within the frame. In order to reduce the computational complexity, the proposed algorithm utilizes pixel truncation, adaptive search range, sub-sampling and avoids some of the asymmetrical prediction unit techniques. Simulation results are obtained by using the reference software HM (e n c o d e r_l o w d e l a y_P_m a i n and e n c o d e r_r a n d o m a c c e s s_m a i n profile) and shows 55.49% improvement on search points with approximately the same PSNR and around 1% increment in bit rate as compared to the Test Zonal Search (TZS) ME algorithm. By utilizing the proposed algorithm, the BD-PSNR loss for the video sequences like B a s k e t b a l l P a s s_416 × 240@50 and J o h n n y_1280 × 720@60 is 0.0804 dB and 0.0392 dB respectively as compared to the HM reference software with the e n c o d e r_l o w d e l a y_P_m a i n profile.     

Electron-phonon damping factor for the landau quantization of 2D electrons with a fine structure of the energy spectrum

The drift of degenerate 2D electrons in the channel of the heterojunction potential well is considered. In a quantizing magnetic field B, the electrons scan the defects of the heteroboundary, which perturb their momentum and energy equilibrium state (T, T _D ⁰ ). The stationary nonequilibrium state (T, T _D ^* ) is attained by electron-phonon relaxation in energy and momentum. The experimentally observed nonlinear dependence T_D(T) is explained by the admixture of deformation acoustic phonons to the interaction of 2D electrons with piezoelectric phonons.     

Field effect and capacitance of silicon crystals with hopping conductivity over point radiation defects pinning the Fermi level

The static field effect and capacitance of Si crystals with hopping conductivity over defects in the charge states (+1), (0), and (?1), which pin the Fermi level, are calculated. In the Si band gap, the defects in the (0) and (+1) charge states form a v′ band, and in the charge states (?1) and (0), they form a c′ band. The width of the c′ and v′ energy bands is calculated under the assumption of Coulomb interaction of each charged defect with only the nearest ion. The energy gap between the c′ and v′ bands is assumed to be constant. Nonmonotonicity of the dependence of capacitance and surface hopping conductivity on the electric potential on the surface of the highly damaged Si crystals is predicted.