Non-resonant Detection Research Articles

Microwave detection towards marine climate monitoring: fog and humidity

Sea fog severely impacts marine operations and coastal transportation due to the low visibility, metal corrosion, and unpredictable effects. This paper introduces a microwave detection technique for monitoring the sea fog generation in-time, as well as the salt-content within sea fog. Besides, in the fogless time, the microwave sensor can also work for humidity detection. The proposed sensor doesn’t use the sensitive materials which assure the reliability and stability. In humidity detection, the nested split-ring resonator (NSRR) sensor achieves a maximum sensitivity with a 1.19 MHz/% RH frequency shift, 0.034 dB/% RH return-loss change, and 0.24°/% RH phase change. For NaCl content, the 2NSRR sensor exhibits a significantly higher sensitivity of 4.42 MHz/% compared to the 1NSRR sensor. This paper firstly introduces four non-resonant detection parameters (FWHM, HPI, FSI, and IF) to avoid high loss due to the fog. This microwave sensor and non-resonant parameters offer a comprehensive solution for accurate and timely sea fog monitoring, enhancing safety and efficiency in maritime and coastal operations.

All-optical non-resonant photoacoustic spectroscopy for multicomponent gas detection based on aseismic photoacoustic cell

An all-optical non-resonant photoacoustic spectroscopy system for multicomponent gas detection based on a silicon cantilever optical microphone (SCOM) and an aseismic photoacoustic cell is proposed and demonstrated. The SCOM has a high sensitivity of over 96.25 rad/Pa with sensitivity fluctuation less than ± 1.56 dB between 5 Hz and 250 Hz. Besides, the minimal detectable pressure (MDP) of the sensor is 0.55 μPa·Hz−1/2 at 200 Hz, which indicates that the fabricated sensor has high sensitivity and low noise level. Six different gases of CO2, CO, CH4, C2H6, C2H4, C2H2 are detected at the frequency of 10 Hz, whose detection limits (3σ) are 62.66 ppb, 929.11 ppb, 1494.97 ppb, 212.94 ppb, 1153.36 ppb and 417.61 ppb, respectively. The system achieves high sensitivity and low detection limits for trace gas detection. In addition, the system exhibits seismic performance with suppressing vibration noise by 4.5 times, and achieves long-term stable operation. The proposed non-resonant all-optical PAS multi-component gas detection system exhibits the advantages of anti-vibration performance, low gas consumption and long term stability, which provides a solution for working in complex environments with inherently safe.

Numerical Investigation of GaN HEMT Terahertz Detection Model Considering Multiple Scattering Mechanisms.

GaN high-electron-mobility transistor (HEMT) terahertz (THz) detectors have been widely studied and applied in the past few decades. However, there are few reports about the influence of GaN/AlGaN heterostructure material properties on the detection model at present. In this paper, a response voltage model for a GaN HEMT THz detector that considers the carrier scattering in a GaN/AlGaN heterostructure is proposed. The phonon scattering, dislocation scattering, and interface roughness scattering mechanisms are taken into account in the classic THz response voltage model; furthermore, the influence of various material parameters on the response voltage is studied. In a low-temperature region, acoustic scattering plays an important role, and the response voltage drops with an increase in temperature. In a high temperature range, optical phonon scattering is the main scattering mechanism, and the detector operates in a non-resonant detection mode. With an increase in carrier surface density, the response voltage decreases and then increases due to piezoelectric scattering and optical phonon scattering. For dislocation and interface roughness scattering, the response voltage is inversely proportional to the dislocation density and root mean square roughness (RMS) but is positively related to lateral correlation length. Finally, a comparison between our model and the reported models shows that our proposed model is more accurate.

2D imaging of absolute methyl concentrations in nanosecond pulsed plasma by photo-fragmentation laser-induced fluorescence

The methyl radical plays a central role in plasma-assisted hydrocarbon chemistry but is challenging to detect due to its high reactivity and strongly pre-dissociative electronically excited states. We report the development of a photo-fragmentation laser-induced fluorescence (PF-LIF) diagnostic for quantitative 2D imaging of methyl profiles in a plasma. This technique provides temporally and spatially resolved measurements of local methyl distributions, including in near-surface regions that are important for plasma-surface interactions such as plasma-assisted catalysis. The technique relies on photo-dissociation of methyl by the fifth harmonic of a Nd:YAG laser at 212.8 nm to produce CH fragments. These photofragments are then detected with LIF imaging by exciting a transition in the B-X(0, 0) band of CH with a second laser at 390 nm. Fluorescence from the overlapping A-X(0, 0), A-X(1, 1), and B-X(0, 1) bands of CH is detected near 430 nm with the A-state populated by collisional B-A electronic energy transfer. This non-resonant detection scheme enables interrogation close to a surface. The PF-LIF diagnostic is calibrated by producing a known amount of methyl through photo-dissociation of acetone vapor in a calibration gas mixture. We demonstrate PF-LIF imaging of methyl production in methane-containing nanosecond pulsed plasmas impinging on dielectric surfaces. Absolute calibration of the diagnostic is demonstrated in a diffuse, plane-to-plane discharge. Measured profiles show a relatively uniform distribution of up to 30 ppm of methyl. Relative methyl measurements in a filamentary plane-to-plane discharge and a plasma jet reveal highly localized intense production of methyl. The utility of the PF-LIF technique is further demonstrated by combining methyl measurements with formaldehyde LIF imaging to capture spatiotemporal correlations between methyl and formaldehyde, which is an important intermediate species in plasma-assisted oxidative coupling of methane.

Online-compatible unsupervised nonresonant anomaly detection

The authors of this paper employ two (or more) autoencoders to provide a complete strategy for unsupervised non-resonant anomaly detection. Both signal extraction and data-driven background estimation can be determined with decorrelated autoencoders. The method shows strong performance on test datasets and has the advantage of being online-compatible.

Generation of Highly Vibrationally Excited CO in Sequential Photodissociation of Iron Carbonyl Complexes

Ultraviolet photochemistry of iron pentacarbonyl, Fe(CO)5, was investigated with resonantly enhanced multiphoton ionization (REMPI) spectroscopy and ion imaging. The REMPI spectrum of CO photofragments, generated by ultraviolet irradiation of Fe(CO)5, showed the generation in the highly vibrationally excited states with v = 11-15. Analysis of the band intensities observed in the 213-235 nm region indicated that the high-v CO generation was maximized at around 220 nm. Generation yields of the coordinatively unsaturated intermediates, Fe(CO)n=1-4, were measured as a function of the photolysis wavelength using a nonresonant detection scheme. The yield spectrum of FeCO was correlated with that of the high-v CO fragments, suggesting high-v CO generation in the photodissociation of FeCO. The density functional theory calculations of the excited states of FeCO showed an intense photoabsorption to the metal-centered state near 220 nm. The theoretical results were consistent with the interpretation of FeCO + hν → Fe + high-v CO, which was experimentally indicated. The momentum distribution obtained from the velocity distributions of Fe, Fe(CO)4, and CO fragments further supported that Fe is the counter-product of the high-v CO fragment. The present results provided selective observation of the photochemistry of the unsaturated iron carbonyl complexes, which has not been well elucidated in laser-based experiments because of the uncontrollable sequential photodissociation producing mixed Fe(CO)n intermediates.

TCAD Simulation for Nonresonant Terahertz Detector Based on Double-Channel GaN/AlGaN High-Electron-Mobility Transistor

We propose a nonresonant terahertz (THz) detector based on double-channel (DC) GaN/AlGaN high-electron-mobility transistor (HEMT) utilizing a technology computer-aided design platform. The hydrodynamic model is simplified as the drift-diffusion model for nonresonant THz detection simulation. Dependence of THz photoresponse on various structure parameters of the detector is analyzed by simulation. The two typical metrics, responsivity and noise equivalent power (NEP), are theoretically calculated for the optimization of the structure parameters. The optimized responsivity and NEP reach 5.8 kV/W and 50 pW/Hz0.5 at the same gate voltage, respectively, and a minimum NEP of 20 pW/Hz0.5 is obtained. The comparison between our simulation results and the experiment data of single-channel HEMT detector proves that the DC HEMT detector shows an excellent THz detection performance.

An analytical model for terahertz detection in cylindrical surrounding-gate MOSFETs

An analytical model for detection of terahertz radiation by plasma wave in cylindrical surrounding-gate (SRG) MOSFETs is presented. In comparison with traditional drain-current models, the rectification response of terahertz signal due to current self-mixing in conducting channel is considered by solving coupled plasma fluid equations using perturbation method. The resulted model is for the first time dipicting detector response in above threshold, near threshold and subthreshold regimes by a single expression valid for both resonant and nonresonant detection schemes. As no fitting parameters is adopted, the model is physical and predicative. Model validity has been extensively verified through numerically solving differential equations with a wide range of incident wave frequencies, channel doping concentrations, device working temperatures, SRG MOSFET geometry parameters as well as incident wave amplitudes. Model applicability to large input terahertz signal has also been discussed. The presented model is convenient for finding the optimum detector design from a multiparameter space. Its great universality will make it a candidate compact model for future terahertz integrated circuit simulation.

The cosmic axion spin precession experiment (CASPEr): a dark-matter search with nuclear magnetic resonance

The cosmic axion spin precession experiment (CASPEr) is a nuclear magnetic resonance experiment (NMR) seeking to detect axion and axion-like particles which could make up the dark matter present in the Universe. We review the predicted couplings of axions and axion-like particles with baryonic matter that enable their detection via NMR. We then describe two measurement schemes being implemented in CASPEr. The first method, presented in the original CASPEr proposal, consists of a resonant search via continuous-wave NMR spectroscopy. This method offers the highest sensitivity for frequencies ranging from a few Hz to hundreds of MHz, corresponding to masses – eV. Sub-Hz frequencies are typically difficult to probe with NMR due to the diminishing sensitivity of magnetometers in this region. To circumvent this limitation, we suggest new detection and data processing modalities. We describe a non-resonant frequency-modulation detection scheme, enabling searches from mHz to Hz frequencies (– eV), extending the detection bandwidth by three decades.

Sub-Micron Gate Length Field Effect Transistors as Broad Band Detectors of Terahertz Radiation

We report on room temperature non-resonant detection of terahertz radiation using strained Silicon MODFETs with nanoscale gate lengths. The devices were excited at room temperature by an electronic source at 150 and 300 GHz. A maximum intensity of the photoresponse signal was observed around the threshold voltage. Results from numerical simulations based on synopsys TCAD are in agreement with experimental ones. The NEP and Responsivity were calculated from the photoreponse signal obtained experimentally. Those values are competitive with the commercial ones. A maximum of photoresponse was obtained (for all devices) when the polarization of the incident terahertz radiations was in parallel with the fingers of the gate pads. For applications, the device was used as a sensor within a terahertz imaging system and its ability for inspection of hidden objects was demonstrated.

Terahertz signal detection in a short gate length field-effect transistor with a two-dimensional electron gas

We consider the problem of non-resonant detection of terahertz signals in a short gate length field-effect transistor having a two-dimensional electron channel with zero external bias between the source and the drain. The channel resistance, gate-channel capacitance, and quadratic nonlinearity parameter of the transistor during detection as a function of the gate bias voltage are studied. Characteristics of detection of the transistor connected in an antenna with real impedance are analyzed. The consideration is based on both a simple one-dimensional model of the transistor and allowance for the two-dimensional distribution of the electric field in the transistor structure. The results given by the different models are discussed.

High sensitivity microwave detection using a magnetic tunnel junction in the absence of an external applied magnetic field

In the absence of any external applied magnetic field, we have found that a magnetic tunnel junction (MTJ) can produce a significant output direct voltage under microwave radiation at frequencies, which are far from the ferromagnetic resonance condition, and this voltage signal can be increase by at least an order of magnitude by applying a direct current bias. The enhancement of the microwave detection can be explained by the nonlinear resistance/conductance of the MTJs. Our estimation suggests that optimized MTJs should achieve sensitivities for non-resonant broadband microwave detection of about 5000 mV/mW.

Contactless measurement of nonlinear conductivity in the radio-frequency range.

We have developed a system for contactless measurement of nonlinear conductivity in the radio-frequency band, and over a wide temperature range. A non-resonant circuit is used to electrically excite the sample, and the induced signal is detected by a resonant circuit whose natural frequency matches higher harmonics of the excitation. A simple modification of the probe allows non-resonant detection suitable for stronger signals. Two measurement procedures are proposed that allow significant excitation power variation, up to 150 W. The apparatus has been validated through the measurement of the nonlinear response at the superconducting transition of a high-Tc superconductor, and the nematic transition of an iron pnictide.

Photoresponse enhancement of plasmonic terahertz wave detector based on asymmetric silicon MOSFETs with antenna integration

We report the experiments of a plasmonic terahertz (THz) wave detector based on silicon (Si) field-effect transistors (FETs) in the nonresonant sub-THz (0.2 THz) regime. To investigate the effects of the overdamped charge asymmetry on responsivity (RV), a FET structure with the asymmetric source and drain area under the gate has been proposed. RV as a function of gate voltage in Si FET-based detectors integrated with an antenna has been successfully enhanced by the asymmetry ratio (ηa = WD/WS) of gate-overlapped drain width (WD) to source width (WS) in agreement with the nonresonant quasi-plasma wave detection theory. The experimentally measured photoresponse has been enhanced by about 36.2 times on average from various samples according to the 10-fold increase in ηa. The effect of the integrated bow-tie antenna on the performance enhancement has also been estimated as 60-fold at the maximum incident angle for the polarized THz wave source.

Citrate-hydrazine hydrogen-bonding driven single-step synthesis of tunable near-IR plasmonic, anisotropic silver nanocrystals: implications for SERS spectroscopy of inorganic oxoanions.

A simplified, single-step aqueous synthesis route to tunable anisotropic silver nanocrystals (NCs) has been developed by tailoring the hydrogen-bonding interactions between a mild stabilizer, sodium citrate, and a mild reductant, hydrazine hydrate. The structure directing ability of the H-bonding interaction was harnessed by keeping a stoichiometric excess of hydrazine under ambient conditions (pH 7, 25 °C). Decreasing the synthesis temperature to 5 °C imparts rigidity to the citrate-hydrazine H-bonding network, and the plasmon peak moves from 500 to 550 nm (using 40 mM hydrazine). On lowering the pH from 7 to 5, the H-bonding is further strengthened due to partial protonation of citrate and the plasmon peak is tuned to 790 nm. Further, we found that, at 5 °C and pH 5, there also exists a sub-stoichiometric regime in which maximum tunability of the plasmon peak (790→1010 nm) is achieved with 1 mM hydrazine. HR-TEM reveals that the near-IR plasmonic NCs are nanopyramids having a pentagonal base with edge length varying from 15 nm to 30 nm. Through second derivative FTIR analysis, a correlation between hydrogen-bonded molecular vibrations and the plasmon tunability has been established. The anisotropic NCs exhibit significant Raman enhancement on the citrate molecules. Further, a solution-phase, non-resonant SERS spectroscopic detection method for an inorganic contaminant of ground water, arsenite, has also been developed.

Modeling of Field Effect Transistor Channel as a Nonlinear Transmission Line for Terahertz Detection

This paper revisits the theory of operation of field effect transistor in the extremely high frequency scale, where the analysis has gone beyond the conventional cutoff frequency of the transistor. In this range, which is typically the terahertz (THz) and sub-terahertz range, the transistor blocks the high frequency signal and generates a rectified signal related to the input high frequency signal. An analytical model is derived for the channel of the FET in the linear mode of operation in non-resonant THz detection conditions. A transmission line distributed circuit model is applied. This is, from the authors’ point of view, the suitable model for high frequency non-quasi static operation and the characteristic parameters of this model are derived from the differential equation governing the electron gas in the channel. A comparison is presented for the calculated photoresponse with previously published experimental one showing good agreement away from the threshold potential. Finally, the effects of coupling between the present model and the external input circuit have been taken into account including the loading effects of the antenna and a discussion is given for the effect on frequency selectivity of the FET.

Terahertz imaging using strained-Si MODFETs as sensors

We report on non-resonant (broadband) and resonant detection of terahertz radiation using strained-Si modulation doped field effect transistors. The devices were excited at room temperature by two types of terahertz sources (an electronic source based on frequency multipliers at 0.292THz and a pulsed parametric laser at 1.5THz). In both cases, a non-resonant response with maxima around the threshold voltage was observed. Shubnikov-de Haas and photoresponse measurements were performed simultaneously and showed a phase-shift of π/2 in good agreement with the theory, which demonstrates that the observed response is related to the plasma waves oscillation in the channel. The non-resonant features were used to demonstrate the capabilities of such devices in terahertz imaging. We also cooled our device down to 4.2K to increase the quality factor and resonant detection was observed by using a tunable source of terahertz radiation.

Photodissociation of pyrrole-ammonia clusters below 218 nm: Quenching of statistical decomposition pathways under clustering conditions

The excited state hydrogen transfer (ESHT) reaction in pyrrole-ammonia clusters (PyH·(NH(3))(n), n = 2-5) at excitation wavelengths below 218 nm down to 199 nm, has been studied using a combination of velocity map imaging and non-resonant detection of the NH(4)(NH(3))(n-1) products. Special care has been taken to avoid evaporation of solvent molecules from the excited clusters by controlling the intensity of both the excitation and probing lasers. The high resolution translational energy distributions obtained are analyzed on the base of an impulsive mechanism for the hydrogen transfer, which mimics the direct N-H bond dissociation of the bare pyrrole. In spite of the low dissociation wavelengths attained (~200 nm) no evidence of hydrogen-loss statistical dynamics has been observed. The effects of clustering of pyrrole with ammonia molecules on the possible statistical decomposition channels of the bare pyrrole are discussed.

Enhanced Terahertz Detection in Resonant Tunnel Diode-Gated HEMTs

We report our studies on terahertz detection in high electron mobility transistors (HEMTs) with a resonant-tunneling gate structure, which exhibits negative differential conductance (NDC) from gate to channel; namely, resonant-tunnel-diode (RTD) gated HEMTs. The effect of NDC on detector responsivity is theoretically derived based on Dyakonov-Shur electron-plasma wave theory. The positive gate conductance in traditional HEMTs damps the electron plasma waves, therefore reducing responsivity; conversely, in devices employing NDC gates, detector sensitivity can be greatly enhanced. Our analysis also demonstrates that resonant detection, thus high responsivity, can be obtained even near the threshold voltage in RTD-gated HEMTs, while only non-resonant detection is attainable in conventional HEMTs in this bias regime. Numerical exploration of the design space for GaN HEMTs with double-barrier AlGaN/GaN/AlGaN RTD gates is performed, showing that thin barriers with low Al composition may be the most practical structures to demonstrate this enhanced detection mechanism.

Nonresonant Detection of Terahertz Radiation in High-Electron-Mobility Transistor Structure Using InAlAs/InGaAs/InP Material Systems at Room Temperature

In this paper, we report on nonresonant detection of terahertz radiation using the rectification mechanism of two-dimensional plasmons in high-electron-mobility transistors using InAIAs/InGaAs/InP material systems. The experiments were performed at room temperature using a Gunn diode operating at 0.30 THz as the THz source. The measured response was dependent on the polarization of the incident THz wave; The device exhibited higher response when the electric-field vector of the incident radiation was directed in the source-drain direction. The 2D spatial distribution image of the transistor responsivity extracted from the measured response shows a clear beam focus centered on the transistor position, which ensures the appropriate coupling of the terahertz radiation to the device. The device also demonstrated excellent sensitivity/noise performances of approximately 125 V/W and approximately 10(-11) W/Hz(0.5) under 0.30 THz radiation.