Terahertz quantum plasmonics at nanoscales and angstrom scales

Through the manipulation of metallic structures, light-matter interaction can enter into the realm of quantum mechanics. For example, intense terahertz pulses illuminating a metallic nanotip can promote terahertz field-driven electron tunneling to generate enormous electron emission currents in a subpicosecond time scale. By decreasing the dimension of the metallic structures down to the nanoscale and angstrom scale, one can obtain a strong field enhancement of the incoming terahertz field to achieve atomic field strength of the order of V/nm, driving electrons in the metal into tunneling regime by overcoming the potential barrier. Therefore, designing and optimizing the metal structure for high field enhancement are an essential step for studying the quantum phenomena with terahertz light. In this review, we present several types of metallic structures that can enhance the coupling of incoming terahertz pulses with the metals, leading to a strong modification of the potential barriers by the terahertz electric fields. Extreme nonlinear responses are expected, providing opportunities for the terahertz light for the strong light-matter interaction. Starting from a brief review about the terahertz field enhancement on the metallic structures, a few examples including metallic tips, dipole antenna, and metal nanogaps are introduced for boosting the quantum phenomena. The emerging techniques to control the electron tunneling driven by the terahertz pulse have a direct impact on the ultrafast science and on the realization of next-generation quantum devices

Enhanced terahertz conductivity in ultra-thin gold film deposited onto (3-mercaptopropyl) trimethoxysilane (MPTMS)-coated Si substrates

Various material properties change considerably when material is thinned down to nanometer thicknesses. Accordingly, researchers have been trying to obtain homogeneous thin films with nanometer thickness but depositing homogeneous few nanometers thick gold film is challenging as it tends to form islands rather than homogenous film. Recently, studies have revealed that treating the substrate with an organic buffer, (3-mercaptopropyl) trimethoxysilane (MPTMS) enables deposition of ultra-thin gold film having thickness as low as 5 nm. Different aspects of MPTMS treatment for ultrathin gold films like its effect on the structure and optical properties at visible wavelengths have been investigated. However, the effect of the MPTMS treatment on electrical conductivity of ultra-thin gold film at terahertz frequency remains unexplored. Here, we measure the complex conductivity of nanometer-thick gold films deposited onto an MPTMS-coated silicon substrate using terahertz time-domain spectroscopy. Following the MPTMS treatment of the substrate, the conductivity of the films was found to increase compared to those deposited onto uncoated substrate for gold films having the thickness less than 11 nm. We observed 5-fold enhancement in the conductivity for a 7 nm-thick gold film. We also demonstrate the fabrication of nanoslot-antenna arrays in 8.2-nm-thick gold films. The nanoslot-antenna with MPTMS coating has resonance at around 0.5 THz with an electric field enhancement of 44, whereas the nanoslot-antenna without MPTMS coating does not show resonant properties. Our results demonstrate that gold films deposited onto MPTMS-coated silicon substrates are promising advanced materials for fabricating ultra-thin terahertz plasmonic devices

Detection of Single Nanoparticles inside a Single Terahertz Resonator

With the rapid advancement of 5G/6G communications using millimeter wavelengths, the concomitant usage of these long wavelength radiation for remote sensing and monitoring of biological and chemical agents is anticipated. However, the ability to detect and identify these agents with sizes ranging from nanometers to microns is hampered by its millimeter wavelength, which drastically reduces the interaction cross-section. Herein, it is reported that single gold nanoparticles (NPs) drop-casted on the nanoresonator can be observed by monitoring the far-field transmitting spectra of individual terahertz (THz) nanoresonators, which enhance the electric field hundreds of times on the nanoscale. Despite the enormous mismatch in length scales, full-wave 3D numerical modeling of the single THz nanoresonator is also performed to interpret the experimental results, indicating the possibility to turn off the resonance using only one NP embedded in the hotspot of the nanoresonator. Such NP detection becomes the most sensitive when the particle, whose size is comparable to the gap width, is tightly fitted into the nanoresonator. This work unveils the potential associated with refractive index sensing and hyperspectral absorption spectroscopy for detecting and fingerprinting ultra-low density of bio/chemical molecules such as viruses, lipid vesicles, and explosives

Topology-Changing Broadband Metamaterials Enabled by Closable Nanotrenches

One of the most straightforward methods to actively control optical functionalities of metamaterials is to apply mechanical strain deforming the geometries. These deformations, however, leave symmetries and topologies largely intact, limiting the multifunctional horizon. Here, we present topology manipulation of metamaterials fabricated on flexible substrates by mechanically closing/opening embedded nanotrenches of various geometries. When an inner bending is applied on the substrate, the nanotrench closes and the accompanying topological change results in abrupt switching of metamaterial functionalities such as resonance, chirality, and polarization selectivity. Closable nanotrenches can be embedded in metamaterials of broadband spectrum, ranging from visible to microwave. The 99.9% extinction performance is robust, enduring more than a thousand bending cycles. Our work provides a wafer-scale platform for active quantum plasmonics and photonic application of subnanometer phenomena

Rupture of endotracheal tube cuff during robot-assisted endoscopic thyroidectomy -A case report-

We encountered a case of a rupture of an endotracheal tube cuff during robot-assisted thyroid surgery in a 35-year-old male patient. Two hours after commencing surgery, the bellows of the ventilator were not filled and a rupture of the endotracheal tube cuff was suspected. Once the robot-manipulator is engaged, the position of the operating table cannot be altered without removing it from the patient. Reintubation with direct laryngoscopy was performed with difficulty in the narrow space between the patient's head and robot-manipulator without moving the robot away from the patient. The rupture of the endotracheal tube cuff was confirmed by observing air bubbles exiting from the balloon in water. The patient was discharged 3 days after surgery without complications. In robot-assisted thyroid surgery, a preoperative arrangement of the robot away from the patient's head to obtain easy access to the patient is essential for safe anesthetic care

Dynamic Terahertz Plasmonics Enabled by Phase-Change Materials

Phase-change phenomena have been an attractive research theme for decades due to the dynamic transition of material properties providing extraordinary capabilities for versatile optical device applications. Even at the terahertz (THz) frequency regime, phase-change materials (PCMs) promote the development of dynamic devices, especially when combined with a plasmonic approach delivering strong field enhancement and localization. According to the design of plasmonic metamaterials or hybrid composites, PCMs can actively modulate the electromagnetic properties of THz waves through thermal, electrical, and optical means. In turn, THz waves can affect the PCM properties in the nonlinear regime due to the intense field strength enhancement by plasmonic structures. Here, a few types of PCMs demonstrating promising potential in THz plasmonic applications are introduced. Starting from the best-known transition metal oxide, vanadium dioxide (VO2), which possesses an insulator-to-metal phase transition near room temperature, superconductors, chalcogenides, ferroelectrics, liquid crystals, and liquid metals are covered along with their phase-change properties and the control mechanisms infused with THz plasmonic applications. The corresponding recent progress presenting how PCMs combined with plasmonic structures can demonstrate effective THz modulation is reviewed. This general overview may provide a better understanding of dynamic THz plasmonics and new ideas for future THz technology

Large-Area Metal Gaps and Their Optical Applications

Recent technological advances in fabrication methods have allowed researchers to manipulate light-matter interactions in the subwavelength region and develop a wide variety of innovative optical applications from the visible to the microwave region. Metal patterning at a subwavelength scale plays a crucial role in realizing these optical applications. Various standard lithography techniques including laser beam machining, focused ion beam, photolithography, and electron-beam lithography are used for the subwavelength feature size of the metal patterns. Many recent studies have demonstrated that funneling light into nanometer-wide gaps in metals gives rise to strong field enhancements and nonlocal electromagnetic effects. However, these standard methods encounter difficulties when one tries to fabricate nanometer feature sizes with macroscopic circumferences, crucial for long-wavelength applications, over a large area. Here, new lithography techniques that fabricate an array of metal gaps of nanometer-to-angstrom ngstrom scale are covered. The corresponding photonic applications in the terahertz and microwave regions are also introduced. These next-generation metal gaps will have a great impact on the advancement of field enhancement and confinement toward the next level of applications such as metamaterials, quantum tunneling, active switching devices, and ultrasensitive chemical/biological sensors

Terahertz field confinement and enhancement in various sub-wavelength structures

Terahertz plasmonics is one of the fascinating research fields that includes diverse applications in nonlinear terahertz optics and nano-optics, as well as studies about strong localization and enhancement of millimeter waves. Recent advanced lithography techniques enable one to reach the confinement scale of electromagnetic waves down to subnanometer, which corresponds to wavelength/10 000 000. In particular, the strong electric-field enhancement in metal gaps can produce atomically strong field exceeding ???10 MV/cm in a picosecond time scale. In this article, we review the strong localization and enhancement of electromagnetic waves in terahertz frequency ranges achieved by subwavelength metal structures. Based on understanding the underlying capacitive coupling and antenna effects, we focus on the field confinement and enhancement of various metal structures such as slit, slot antenna, and dipole antenna. We also give a brief introduction about near-field detection methods

Impact of unintended pregnancy on maternal mental health: a causal analysis using follow up data of the Panel Study on Korean Children (PSKC)

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.Background Pregnancy intention is important for maternal and child health outcomes. The purpose of this study was to examine the causal relation between pregnancy intention and maternal depression and parenting stress in Korean women who gave birth during 2008. Methods This study is a retrospective evaluation of prospectively collected data from the Panel Study on Korean Children from 2008 to 2010. Causal analyses were conducted using propensity score matching and inverse probability of treatment weighted methods. In addition, mediation analyses were performed to test mitigating effects of marital conflict, fathers participation in childcare, and mothers knowledge of infant development on the relation between unintended pregnancy and adverse maternal mental health. Results Results showed that the overall effect of an unintended pregnancy on maternal depression and parenting stress was statistically significant. An unintended pregnancy was associated with 20–22% greater odds of maternal depression, 0.28–0.39 greater depression score, and 0.85–1.16 greater parenting stress score. Relations between pregnancy intention and maternal depression, maternal depression score and parenting stress score were moderately explained by marital conflict and fathers participation in childcare. Conclusions Unintended pregnancy contributed to increased risks of maternal depression and parenting stress. Efforts to increase fathers participation in childcare and decrease marital conflict might be helpful to mitigate adverse impacts of unintended pregnancy on perinatal maternal mental health