有川 敬

<jats:p>Visualizing the electromagnetic waveforms of device outputs is a fundamental aspect in the development of terahertz emitters and the optimization of their performance. However, such optimization processes are often challenging owing to the frequency of terahertz waves exceeding the detection bandwidth of conventional oscilloscopes, which is typically limited to below 100 GHz. In this study, we employed high-sensitivity, single-shot terahertz time-domain spectroscopy using a femtosecond laser and a reflective echelon mirror to visualize the electric field waveforms generated by terahertz devices. This approach enabled real-time observation of terahertz waveforms emitted from both an impact ionization transit-time diode and a resonant tunneling diode. Moreover, this setup allowed us to capture the transient emission of chirped, large-amplitude terahertz waves during device switching, which is likely attributed to an ultrafast transient change in the device capacitance and bias voltage. These findings underscore the importance of real-time terahertz waveform measurements for advancing terahertz technology development</jats:p>

<jats:p>Terahertz transmission and Lamb-dip spectroscopy were performed on the (J, K) = (16, 0) ← (15, 0) rotational transition of acetonitrile molecules. By systematically varying the sample pressure and irradiation intensity, we determined the saturation intensity and homogeneous linewidth, both of which show excellent agreement with predictions from a two-level model. In the zero-pressure limit, the homogeneous linewidth was estimated to be 10 kHz, attributed to transit-time broadening. A frequency accuracy of 10<jats:sup>−9</jats:sup> was achieved, comparable to the best results previously reported using Lamb-dip spectroscopy in the sub-terahertz regime. These results will open the door to compact, high-precision molecular clocks based on sub-terahertz Lamb-dip spectroscopy.</jats:p>

Abstract As a key component for next-generation wireless communications (6 G and beyond), terahertz (THz) electronic oscillators are being actively developed. Precise and dynamic phase control of ultrafast THz waveforms is essential for high-speed beam steering and high-capacity data transmission. However, measurement and control of such ultrafast dynamic process is beyond the scope of electronics due to the limited bandwidth of the electronic equipment. Here we surpass this limit by applying photonic technology. Using a femtosecond laser, we generate offset-free THz pulses to phase-lock the electronic oscillators based on resonant tunneling diode. This enables us to perform phase-resolved measurement of the emitted THz electric field waveform in time-domain with sub-cycle time resolution. Ultrafast dynamic response such as anti-phase locking behaviour is observed, which is distinct from in-phase stimulated emission observed in laser oscillators. We also show that the dynamics follows the universal synchronization theory for limit cycle oscillators. This provides a basic guideline for dynamic phase control of THz electronic oscillators, enabling many key performance indicators to be achieved in the new era of 6 G and beyond.

Abstract The capability for actual measurements—not just simulations—of the dynamical behavior of THz electromagnetic waves, including interactions with prevalent 3D objects, has become increasingly important not only for developments of various THz devices, but also for reliable evaluation of electromagnetic compatibility. We have obtained real-time visualizations of the spatial evolution of THz electromagnetic waves interacting with a single metal micro-helix. After the micro-helix is stimulated by a broadband pico-second pulse of THz electromagnetic waves, two types of anisotropic re-emissions can occur following overall inductive current oscillations in the micro-helix. They propagate in orthogonally crossed directions with different THz frequency spectra. This unique radiative feature can be very useful for the development of a smart antenna with broadband multiplexing/demultiplexing ability and directional adaptivity. In this way, we have demonstrated that our advanced measurement techniques can lead to the development of novel functional THz devices.

<title>Abstract</title>Fundamental understanding of the confinement of water in porous coordination polymers (PCPs) is important not only with respect to their application, such as in gas storage and separation, but also for exploring confinement effects in nanoscale spaces. Here, we report the observation of water in an exotic state in the well-designed hydrophilic nanopores of PCPs. Single-crystal X-ray diffraction finds that nanoconfined water has an ordered structure that is characteristic in ices, but infrared spectroscopy reveals a significant number of broken hydrogen bonds that is characteristic in liquids. We find that their structural properties are quite similar to those of solid-liquid supercritical water predicted in hydrophobic nanospace at extremely high pressure. Our results will open up not only new potential applications of water in an exotic state in PCPs to control chemical reactions, but also experimental systems to clarify the existence of solid-liquid critical points.