Journal of Infrared, Millimeter, and Terahertz Waves最新文献

The pulsed magnet can generate a high magnetic field (up to 100T), promoting the development of high-frequency high-power gyrotrons. In this article, a fast cooling and long lifetime 40 T pulsed magnet is developed for a 1 THz gyrotron. To address the various challenges in thermal and mechanical aspects, copper conductors and internal fibers have been combined to enhance mechanical strength and minimize temperature rise, prolonging the lifetime and the admissible duration of the magnet operation. Internal cooling channels are introduced to increase the cooling speed without compromising mechanical strength. Experimental results have demonstrated the magnet's reliability and repeatability in achieving a 40 T pulsed field for tens of milliseconds with a cooldown time of 6 min. It is estimated that the lifetime is about 20,000 shots at 40 T. Importantly, the magnet is capable of realizing a 40 T flat-top pulsed field (flat-top time 10 ms) with a cooldown period of 16 min. This can facilitate the long-pulsed quasi-steady operation of gyrotron operation, which is attractive for numerous high-power THz applications.

This study introduces a novel, highly compact broadband millimeter-wave (mm-wave) antenna design and its Multiple-Input-Multiple-Output (MIMO) configuration proposed for 28 GHz applications targeting 5G networks. The antenna is designed over a 0.25 mm thick Rogers RT5880LZ substrate having a relative dielectric permittivity of 2 and an overall size of 16 mm × 16 mm. Its MIMO configuration utilizes polarization diversity and includes four elliptical-slot radiators integrated with microstrip-line structures, specifically optimized for 28 GHz operation. The performance of the proposed mm-wave MIMO configuration is validated through simulation of its S-parameters using CST software and measurements obtained with a vector network analyzer (VNA). The proposed antenna demonstrates excellent S-parameter performance, achieving a gain of up to 6 dBi and a radiation efficiency of 94% across the operational frequency band. Each antenna element exhibits an impressive wide operating bandwidth of 9 GHz, spanning from 22.2 to 31.4 GHz at a − 10 dB threshold. Evaluations of the MIMO system’s performance indicate promising results, including an exceptionally low envelope correlation of about ({10}^{-5}) and a diversity gain of around 10 dB throughout the operating bandwidth. The design also ensures significant isolation between MIMO elements without requiring decoupling structures. A physical prototype of the proposed antenna is fabricated and subjected to measurements, depicting a strong corelation between the measured and simulated data, with some minor variations attributed to fabrication tolerances and cable losses. Comparative analysis further emphasizes the potential viability of the proposed MIMO antenna positioning it as a viable candidate for future compact-sized mm-wave MIMO systems.

We present novel PIN photodiode (PD) continuous wave (cw) terahertz (THz) emitters with an increased responsivity and reduced substrate thickness compared to the state-of-the-art. Our improved devices feature up to 4 dB higher output power below 500 GHz with maximum power of -0.53 dBm at 115 GHz and strongly reduced THz absorption of the substrate for frequencies above 3 THz. The latter enables us to measure coherent cw THz spectra with a record bandwidth of 5.5 THz, for the first time, which is 1 THz (22%) more than the state-of-the-art.

THz radiation can be generated routinely from water plasmas by focusing femtosecond laser pulses on a water film or water line. However, there are no relevant reports on the THz generation from rotated water molecules driven directly by strong electric field of femtosecond laser pulses under non-plasma condition. Here, we develop a theoretical model of water molecules to study the interaction between laser electric field and water molecules based on quantum mechanics theory. We find that broadband THz radiation (bandwidth ~ 10 THz) can be generated through the transitions of rotational energy levels of the rotated water molecules driven by a linear polarization mid-infrared laser (laser intensity ≤ 10¹² W∙cm⁻²). We demonstrate that the generated THz spectrum can be controlled by changing the laser intensity and its pulse duration. Moreover, for a Gaussian pump laser beam, the high-frequency components of THz wave increase gradually, while the low-frequency components decrease gradually when the laser pulse duration increases from 150 to 400 fs. Our results provide a new idea and possibility for THz generation from water.

We have constructed a millimeter-wave free-space absorption spectrometer for high-resolution studies of jet-cooled molecules and weakly bound van der Waals complexes. The spectrometer employs a microwave frequency generator supplemented by active frequency multipliers and covers the spectral region from 50 to 170 GHz. A nozzle source is fixed on a rooftop mirror placed in the vacuum chamber in such a way that the millimeter-wave radiation propagates parallel to the molecular jet and makes two passes along it. Such arrangement of radiation and nozzle provides better sensitivity and higher spectral resolution compared to more common perpendicular configuration. Schottky diode detectors are used to measure absorption in the molecular gas flow. The observed linewidths are 30–40 kHz, and the accuracy of the line center determination is about 3–4 kHz. The presented spectra of the astrophysically relevant rare CO isotopologues, and the weakly bound NH₃–H₂ complexes demonstrate the potential of the newly built spectrometer. The line splittings arising from the hyperfine interactions of the nuclear spins of ¹³C for CO and ¹⁴N for NH₃–H₂ were resolved and analyzed.

This paper outlines the design and analysis of a compact antenna and its multiple-input multiple-output (MIMO) arrangement for use in applications at millimetre wave (mmW). The antenna was constructed with the Rogers substrate RT Durioid 5880, which possesses a total dimension of 44 × 44 mm² with a height of 1.575 mm. The coplanar waveguide fed (CPW) antenna was originally intended to function within a 30–50-GHz frequency spectrum. Subsequently, the MIMO configuration of the same antenna is designed and developed over which a metamaterial superstrate was positioned in the design. The proposed MIMO configuration offers a good impedance match over a wide frequency range, considerable gain, minimal coupling among the antennas, and a low ECC, in addition to attaining a high diversity gain. To further highlight the importance of the suggested work, a comparison with related studies has been established.

Evaluating the output of emitters and calibrating detectors using absolute power sensors is critical for public use. In free-space thermal power sensors, the measurement accuracy and sensitivity depend on the absorption characteristics of the absorber, and the response time relates to its intrinsic thermal time constant. A response time of a few seconds is useful in practical scenarios where factors such as power variations caused by changes in the current–voltage characteristics of the emitter are monitored. At the minimum, an intrinsic thermal time constant of a few tens of second is required to achieve a few-second measurement time because the actual time can be enhanced by the control method of the sensors. However, in the sub-terahertz region, conventional broadband absorbers must be adequately thick for high absorption, increasing thermal capacity. Thus, achieving high absorption and a good thermal response remains challenging. In this study, an absorber featuring a resin-hollow pyramidal structure covered with a metal film thinner than the skin depth was devised. To realize the proposed absorber, we used resin 3D printing and Ni–P electroless plating. A comparison with conventional absorbers demonstrated that the thermal time constant was comparable to that of a planar absorber, known for its suitable response but low absorption below 1 THz, while maintaining over 98.6% absorptance in the 0.1- to 0.3-THz range. These results can aid in the construction of a sub-terahertz power sensor with high accuracy, high sensitivity, and a few-second response time.

The optical properties of Microcellular Polyethylene Terephthalate (MCPET) towards light and terahertz waves were compared. Within the measurement range of 0.3–1.3 THz and 30 K to room temperature, the refractive index remained nearly constant at approximately 1.15, close to 1. It was observed that MCPET with a thickness of 0.5 mm transmits over 70% of terahertz waves, while blocking more than 99.9% of light in the 300–2000 nm range. These findings present significant advantages for the design of optical devices using MCPET, highlighting its potential for selective dichroism and high opacity in specific applications.