Abstract: A new numerical model of terahertz parametric process is demonstrated, which takes the spontaneous parametric down-conversion into consideration by adding tens of frequency components in the coupled-wave equations. To set the values of some parameters in this model, the residual pump energy and idler spectra during spontaneous parametric down-conversion are measured experimentally for different pump energy and crystal lengths. Compared with previous models which only involve three waves, the new model is more accurate, especially when the seed power is low or the generated terahertz-wave is in the low frequency region. PubDate: 2021-05-11

Abstract: We report on THz pulse generation through optical rectification of femtosecond laser pulses in sugar crystals. The sugar crystals were grown from an aqueous solution of caster cane sugar bought in a local grocery. Sugar exhibits rather good nonlinear coefficients ( \(d_{23}\simeq 56\) pm/V), but the optical-THz conversion efficiency remains quite weak because of the lack of phase-matching, when pumped by a laser beam of 0.8-μm central wavelength. Nevertheless, sugar availability at a low price makes it a relevant material for trial experiments. PubDate: 2021-05-07

Abstract: We demonstrate experimentally the increase of optical-to-terahertz conversion efficiency for GaAs-based photoconductive terahertz emitters. This increase is achieved by preventing device breakdown through series resistors, which act as a current limiter. Pulsed photoexcitation and potential current fluctuations result in heat dissipation leading to local heating, which further increases the current and may lead to device breakdown. We manage to increase the maximum bias field before device breakdown by a factor of 3 under illuminated conditions. For a laser system with 250-kHz repetition rate, the terahertz emission amplitude increases linearly with applied bias field up to 120 kV/cm bias field, which results in 3 times higher signal as compared to the standard device. Furthermore, we have also achieved this expanded breakdown prevention at 78-MHz repetition rate, where an integrated on-chip resistance leads to an enhancement of the terahertz field amplitude by 70%. This simple technique can increase the performance of almost all photoconductive terahertz emitters by using appropriate resistances according to the emitter capacitance and laser repetition rate. PubDate: 2021-05-05

Abstract: The present paper addresses numerical calculations on the eigenvalues of hybrid modes in corrugated circular waveguides with varying diameter and corrugation depth. Such calculations are essential for the numerical optimization of advanced mode converters and diameter tapers for future low-loss high-power microwave applications, like broadband high-power radar sensors for space debris observation in low earth orbit (LEO). Corresponding mode converters and diameter tapers may be synthesized based on coupled mode theory. Of particular importance here is the ability to consider varying mode eigenvalues along the perturbed waveguide. The procedure presented here is able to consider arbitrary variations of the corrugation depth as well as the waveguide diameter and therefore is highly flexible. The required computational effort is low. Limitations of the method are discussed. PubDate: 2021-04-23

Abstract: Modifications of the geometry of a folded dipole antenna operating in the infrared were studied with the objective to increase the antenna input impedance. These modified folded dipoles were designed and optimized using numerical simulations. Candidate antennas were fabricated, and input impedance was measured experimentally using scattering-scanning near-field optical microscopy. Numerical and experimental results confirm that the choice of an appropriate antenna geometry leads to an input impedance of 1 kΩ or above. With this impedance, the modified folded dipole antenna could be implemented as a solution for the impedance mismatch challenge that arises in antenna-coupled infrared detectors. PubDate: 2021-04-20

Abstract: This paper presents a 120-GHz wideband phase-compensated variable gain amplifier (VGA) with a p-type metal–oxide–semiconductor (PMOS) switch using a 40-nm CMOS process. By applying a PMOS switch to the common source (CS) amplifier, the gain of the CS amplifier can be controlled by as much as 6.1 dB with 2° phase variations in 15.1-GHz bandwidth ranging from 100.9 to 115 GHz. The measured gain and 3-dB bandwidth of the VGA are 19.1 dB and 33.8 GHz for high-gain state, and 13 dB and 44.2 GHz for low gain state, respectively. Meanwhile, the DC power consumption in the high-gain state is 45 mW, and the OP1dB is −2.7 dBm. PubDate: 2021-04-17

Abstract: Terahertz radiation has many unique characteristics that make it useful for noninvasive mail inspection. While qualitative analysis of mail for suspicious objects is a relatively instantaneous process, quantitative analysis methods may be time-consuming. Multivariate analysis methods, including principal component analysis (PCA), partial least squares (PLS), and interval partial least squares (iPLS), were used for quantitative model building and to predict the content of substances in mail. The optimal spectral interval was selected by analyzing the influence of different spectral regions on the predicted results. A specific interval partial least squares (SiPLS) model was established to improve prediction accuracy and reduce the root mean square error (RMSE) by an order of magnitude. The content of dangerous substances was calculated using SiPLS, established by referencing spectral data of pure substances. Our methods demonstrated that establishing a multiple regression model based on spectral data of pure substances could predict the content of dangerous substances in mail. PubDate: 2021-04-10

Abstract: Epoxy resin, diglycidyl ether of bisphenol A (DGEBA) and triethylenetetramine (TETA), is known to be hygroscopic in nature, which eventually leads to mechanical performance degradation of the material. The long-standing debate on the nature of water-epoxy interaction has been investigated in this study with the help of terahertz spectroscopy, which is able to probe sub 100 cm− 1 wavenumbers in which hydrogen bonds of water are active as well as infrared spectroscopy, which is able to probe 600–1000 cm− 1 in which the libration motions of the water molecules are dominant. Based on observations, it can be demonstrated that both hydrogen-bonded water (with epoxy) and free interstitial water (hydrogen-bonded with itself) is present when the epoxy resin is exposed to normal or saline water. A schematic diagram is presented to demonstrate epoxy-water interactions. PubDate: 2021-04-07

Abstract: We explore background-free options to detect mid-infrared (MIR) electric transients. The MIR field and a near-infrared probe interact via sum- (SFG) and difference-frequency generation (DFG) in an electro-optic crystal. An intuitive picture based on a phasor representation and rigorous numerical calculations are used for analysis. It turns out that separating photons generated either by SFG or DFG from the local oscillator via spectral filtering leads to a signal purely proportional the MIR intensity envelope. Background-free phase information may be extracted in a spectral window containing both SFG and DFG components and blocking the local oscillator background based on its orthogonal polarization. This variant leads to signal proportional to the square of the MIR field amplitude. It is limited by the finite efficiency of polarization filtering. The Hilbert transform as a conjugate variable to the electric field in the time domain turns out to play a fundamental role for the context discussed in this paper. PubDate: 2021-04-07

Abstract: We recently proposed a multiphase oscillator utilizing a periodic pulse train generated and propagated in a closed one-dimensional lattice of resonant-tunneling diode (RTD) oscillators, each of which establishes the neighbor-to-neighbor coupling through lossy inductors. In this work, we develop the design criteria for the bias dependence of oscillation frequency through the bifurcation analysis based on the quasi-continuous model of the lattice. We then model the lattice in the full-wave scheme using the finite-difference time-domain method and validate the multiphase oscillation in submillimeter-wave frequencies. Finally, several experimental observations conducted in MHz frequencies using tunnel diodes in place of RTDs are presented to validate several synchronization properties of the pulse train in the lattice including the direction-specified transmission of pulse train and its application to the multiphase oscillator. PubDate: 2021-04-01

Abstract: Increasing data rates in wireless communications are accompanied with the need for new unoccupied and unregulated bandwidth in the electromagnetic spectrum. Higher carrier frequencies in the lower THz frequency range might offer the solution for future indoor wireless communication systems with data rates of 100 Gbit/s and beyond that cannot be located elsewhere. In this review, we discuss propagation channel measurements in an extremely broad frequency range from 50 to 325 GHz in selected indoor communication scenarios including kiosk downloading, office room communication, living rooms, and typical industrial environments. PubDate: 2021-04-01

Abstract: In this article, an improved fast Fourier transform (FFT)–based millimeter-wave imaging algorithm with range compensation is presented, which can be used to reconstruct 3D images for near-field multiple-input multiple-output synthetic aperture radar (MIMO-SAR). The frequency-domain interpolation is avoided and only one-step spherical-wave decomposition is employed in this algorithm. During the image reconstruction process, the amplitude factor is considered for the compensation of signal propagation loss, and the final target image can be obtained by FFT/IFFT and coherent accumulation steps. As demonstrated with numerical theoretical analysis and experimental results, the proposed method greatly reduces the computational load but ensures the quality of image reconstruction compared to the back-projection (BP) algorithm. Moreover, it is superior to the MIMO-range migration algorithm (MIMO-RMA) in compensating propagation loss and other performance indexes in the image reconstruction results. PubDate: 2021-04-01

Abstract: The effect of delayed reflection on frequency stabilization and pulling in a gyrotron oscillator is studied. Simple theoretical formulas for start-oscillation current, oscillation frequency, and frequency stabilization rate are derived in the framework of the linear (or small-signal) theory and verified by numerical simulation for the gyrotron with fixed axial profile of the RF field. Capabilities of increasing frequency stability are analyzed. A possibility of expanding frequency tunability range owing to excitation of high-order axial modes is demonstrated. PubDate: 2021-04-01

Abstract: In this contribution, we describe the design, technology, and physical parameters of antenna-coupled microbolometer, used for broadband detection of terahertz electromagnetic spectrum. This microbolometer features an application of La0.67Sr0.33MnO3 layer grown on multilayered material stack ensuring lattice matching of the sensing LSMO layer to silicon. By virtue of bulk micromachining of silicon-on-insulator substrates, the sensing structure is built on thin suspended membrane to provide weak thermal link, increasing thermal response, thus sensitivity. Additionally, it helps suppressing excitation of in-plane guided surface modes that will otherwise deteriorate the antenna radiation diagram and affect the spectral responsivity of the sensor. Finally, the operation of sensor is demonstrated using a molecular laser setup at 762 GHz (and also 1.4 THz) emission line. The parameters of the said microbolometers are analyzed in terms of response and time constant. Optimal working temperature of the detectors is about 65 °C. PubDate: 2021-03-10 DOI: 10.1007/s10762-021-00781-y

Abstract: The HE11 hybrid mode, propagating in an overmoded corrugated circular waveguide, is widely used for low loss transmission of high-power microwaves. Due to the inherent broadband frequency behaviour, this will be also essential for future broadband high-power radar applications, like space debris observation in low earth orbit (LEO). A promising amplifier concept for such radar sensors is a helical gyro-TWT. However, since the HE11 hybrid mode is not suitable for electron-beam-wave interaction in this kind of vacuum electron device, an additional mode converter is required. The present paper addresses the design procedure of a broadband high-power mode converter, designed for a helical gyro-TWT intended for future broadband high-power radar applications in the W-band. The interaction mode of the helical gyro-TWT under consideration can be easily transferred to the circular waveguide TE11 mode. Therefore, a TE11 ↦ HE11 mode converter is addressed here. The design procedure is based on a scattering matrix formalism and leads to a high HE11 mode content of ≥98.6% within the considered frequency range from 92 GHz to 100 GHz. Inside this frequency band, the mode content is even better and reaches ≈ 99.7% at ≈95 GHz. This allows broadband frequency operation of a helical gyro-TWT and is suitable for broadband high-power radar applications. PubDate: 2021-03-09 DOI: 10.1007/s10762-021-00773-y

Abstract: In this paper, a terahertz metasurface supercell consists of two ring resonators with slightly different radii is proposed. Electromagnetic-induced transparency (EIT)–like resonance is excited with an ultrahigh figure of merit. Furthermore, the impact of the coupling between the two rings is investigated on the transmission amplitude response, the quality factor, and the EIT peak amplitude by varying the radius of the top resonator. The achieved figure of merit of the EIT peak reaches 88.65 and almost 20,000 when gold and a perfect electric conductor are used for the metallic layer, respectively. The simplicity and unique properties of the proposed design could render it to be a desirable candidate for filtering, slow light applications, and sensing. PubDate: 2021-02-26 DOI: 10.1007/s10762-021-00775-w

Abstract: A novel type of near-field slit probe with high sensitivity and high spatial resolution is proposed. We tested the implementation of this in passive millimeter-wave microscopy at frequencies around 50 GHz. The slit probe comprises a standard rectangular waveguide incorporating a triple-screw tuner penetrating into the waveguide, followed by a four-section quarter-wave transformer, and a metal-coated silicon chip with a micro-slit aperture fabricated at the probe tip using a bulk micromachining technique. The probe allows the transmission of the thermal radiation collected at the probe aperture to the radiometric receiver used in passive millimeter-wave microscopy to be maximized, resulting in highly sensitive measurements. The system noise temperature of the radiometric receiver including the slit probe used in the passive measurements was found to be 1800 K, meaning that a temperature resolution of 0.18 K with the integration time set to 1 s was achieved. This system noise temperature is four times better than that when a tapered slit probe with no tuning circuit was used. Image acquisition with a spatial resolution of better than 100 μm was demonstrated over the temperature range from 210 to 310 K. PubDate: 2021-02-26 DOI: 10.1007/s10762-021-00777-8

Abstract: We applied microwave light with frequencies 10, 212, and 419 GHz to form octacalcium phosphate (OCP) intercalated with succinate ions (SCI). The composition of starting reagents (i.e., α-type tricalcium phosphate (TCP), calcium hydrogenphosphate dihydrate (DCPD), and SCI in pure water) did not enable us to form OCP intercalated with SCI in a thermostatic water bath. Although microwave irradiation typically acts as a heat source in water, it has brought a valuable contribution to OPC formation. This paper reports the following interesting result; microwave irradiation of power less than 0.1 W enabled us to synthesize OCP intercalated with SCI which was not possible to form with power values exceeding 0.1 W. PubDate: 2021-02-17 DOI: 10.1007/s10762-021-00774-x

Abstract: This work reports in first time a 100-Gb/s, ultra-low noise, variable gain multi-stagger tuned transimpedance amplifier (VGMST-TIA) over the D-band performance. The whole work is binding into two phases. The first phase involves the modeling and characterization of graphene field-effect transistor (GFET) with an optimized transition frequency of operation. While in the second phase, a TIA design employs a T-shaped symmetrical L-R network at the input, which mitigates the effect of photo diode capacitance and achieves a D-band of operation. The proposed work uses a VGMST to establish TIA, which realizes optimum noise performance. The high gain 3-stage VGMST-TIA effectively minimizes the white noise and illustrates a sharp out-of-band roll-off to achieve considerable noise reduction at high frequencies. The active feedback mechanism controls the transimpedance gain by tuning the control voltage which results better group delay. Besides, an L-C circuit is employed at the output to enhance bandwidth. The full TIA is implemented and fabricated using a commercial nano-manufacturing 9-nm graphene film FET on a silicon wafer using 0.065-μm process. The TIA achieves a flat transimpedance gain of 61.2 dBΩ with ± 9 ps group delay variation over the entire bandwidth. The proposed TIA measured an impedance bandwidth of 0.2 THz with ultra-low input-referred noise current density of 2.03 pA/√Hz. The TIA supports a 100-Gb/s data transmission due to large bandwidth; therefore, a bit-error-rate (BER) less than 10−12 is achieved. The chip occupies an area of 0.92 * 1.34 mm2 while consuming power of 21 mW under supply of 1.8 V. PubDate: 2021-01-14 DOI: 10.1007/s10762-021-00771-0