Accurately quantifying transient heat transfer in aqueous media under sub-terahertz (110 GHz) continuous-wave irradiation is a critical prerequisite for bio-electromagnetic research, as rapid heat accumulation significantly impedes the assessment of non-thermal biological effects. To address this, we developed a macroscopic thermodynamic framework to establish a reliable physical thermal baseline. Utilizing a cell-free aqueous phantom to isolate macroscopic thermodynamics from biological variables, we integrated a non-linearly modified Newton’s cooling law with an equivalent lumped volumetric heating model. The framework was calibrated via pure cooling kinetics and validated by hold-out prediction under multi-power continuous-wave irradiation (11.77–25.19 mW). Our evaluations indicate that the macroscopic thermal response is governed by an apparent, time-evolving thermal resistance ( R_eff ), which we attribute to the different thermal masses coupled to the liquid: the medium, which responds on a short timescale, and the dish together with the fixture in contact with it, which respond more slowly. Specifically, the Single-Exponential Relaxation Model (SERM) provides a robust and physically reasonable mathematical approximation for this transient process: under model inversion, it reproduces the measured response with a median RMSE of ∼ 0.20 K, and a mean residual below 0.02 K in magnitude when averaged over the eleven measured power levels. The suitability of this single timescale is consistent with the small Biot number of the layer ( Bi ≈ 0.03 ) and with the short conduction time across it, so that the assembly can be treated as a lumped thermal capacitance. This macroscopic lumped-parameter approach provides researchers with an efficient tool for pre-experimental thermal management on a relative power scale: the reported power is the meter reading of an unchanged beam geometry, and the power absorbed by the sample is not determined in absolute terms.
Noncontact gas thermometry through barriers that are opaque at visible and infrared wavelengths remains challenging. Here, we demonstrate gas thermometry based on terahertz (THz) absorption spectroscopy to address this challenge. Terahertz radiation can transmit through a variety of materials and is absorbed by water vapor. We performed THz absorption spectroscopy of water vapor through polystyrene foam near room temperature and measured the integrated absorbance of a rotational transition at several temperatures. We experimentally demonstrated that the relative temperature dependence of the absorption peak area can be quantitatively explained by theoretical predictions based on the Boltzmann distribution. This was achieved by correcting the peak area using independently measured water vapor density. The standard deviations of repeated measurements were reduced by an order of magnitude compared with those reported in previous studies. These results provide a proof of concept that the temperature dependence of the humidity-corrected peak area can be used for THz gas thermometry when the humidity is known or independently measured, thereby enabling temperature measurements in environments inaccessible by conventional optical thermometry.
This work presents a comparative analysis of two full-band WR-3.4 sub-harmonic mixers (SHM) based on Schottky diodes, using alternative anti-parallel and cross-bar diode balancing configurations. Both mixers share equivalent technology and design techniques, offering a very unique opportunity for a direct experimental comparison. Through detailed simulations, we analyze signal coupling, harmonic emission, and intermodulation (IM) products to understand their performance differences. While conversion loss (CL) degradation in anti-parallel mixers is typically attributed to epitaxial layer properties, this study reveals that a significant portion of the degradation is also linked to the architecture itself and is associated with the generation and propagation of unwanted IM products outside the diode cell. Experimental measurements, closely aligned with simulation results, show a 1.5–3 dB CL degradation, which mainly comes from propagation of IM products, with an additional loss attributed to IF and RF coupling in the anti-parallel configuration. In contrast, the cross-bar mixer exhibits minimal susceptibility to these effects, at the expense of reduced LO harmonic suppression and extra required LO power.
Graphene terahertz (THz) photothermoelectric (PTE) detectors are promising for broadband, zero-bias, and ultrafast detection. However, their low-temperature behaviors remain insufficiently understood, which restricts their application in cryogenic environments. In this work, we systematically investigate the resistance and photoresponse characteristics of three-terminal graphene THz PTE detectors with asymmetric antenna coupling from 77 to 300 K. The two-terminal drain-to-source resistance increases overall with temperature, with a positive contribution from the contact-related series resistance extracted by fitting the transfer curves, while the extracted carrier mobility decreases slightly. Meanwhile, the voltage responsivity shows an overall increase with temperature. To interpret this behavior, we develop a coupled photothermoelectric model incorporating both the temperature-dependent Seebeck coefficient and the simulated channel temperature distribution. Thermal asymmetry metrics (TAMs) are further introduced to quantify the lateral temperature imbalance induced by the asymmetric antenna structure. The model reproduces the measured temperature trend. Within the coupled PTE model, the temperature dependence of the PTE response reflects the combined effects of the increasing Seebeck coefficient and the slightly decreasing thermal asymmetry. This work provides physical insights into the cryogenic operation of graphene THz PTE detectors and offers a practical framework for their low-temperature evaluation and design.
Very long baseline interferometry (VLBI) is a powerful technique for detailed observations of the central engine of active galactic nuclei, galactic dynamics, and massive star-forming regions. Converting from circular to linear polarization is necessary for VLBI observation since several receivers use the latter. However, the existing methods for achieving this remain limited. This study addressed this limitation by designing, fabricating, and testing a low-insertion loss, low-reflection coefficient, and low-cross-polarization broadband circular polarizer (CP) for the 70-116 GHz band. The proposed CP is designed to achieve low-insertion loss, a low-reflection coefficient, and low cross-polarization. It consists of a 90 degrees differential phase shifter (PS), which combines three phase-shifting sections to provide the required differential phase over a wide frequency range, and a double-ridged orthomode transducer (OMT), which offers low-loss characteristics and ease of fabrication. By precisely controlling the phase difference and amplitude error of the PS and implementing structural improvements to suppress air gaps at the contact interfaces, low-loss and low cross-polarization performance was achieved over a wide frequency range. A prototype CP was fabricated and tested, showing good agreement with the simulation results. Back-to-back CP measurements demonstrated an insertion loss less than 0.8 dB, a reflection coefficient below -20 dB, and a cross-polarization below -21 dB. These results demonstrate the broadband performance of the proposed CP over the 70-116 GHz band.
