Terahertz (THz) science and technology have attracted tremendous attention owing to their potential applications in areas including high-speed communications, nondestructive evaluation, biological and medical sensing, and national security.1,2 Terahertz waves bridge electronics and photonics, as well as classical and quantum physics, making them equally attractive for fundamental studies of novel physical phenomena. To realize a broad range of real terahertz applications, it is essential to develop compact solid- state devices for terahertz sources, detectors, modulators, and compact systems. These devices often employ quantum materials and heterostructures and can incorporate other features including carrier transfer using atomic layer epitaxy, micro- and electromechanical systems, plasmonic resonators, and metamaterials. To optimize these devices, we have to understand and control ultrafast carrier dynamics in these advanced materials. Therefore, we have organized a Special Topic in the Journal of Applied Physics to highlight the state-of-the-art in terahertz solid-state devices while also unveiling the properties of these materials on an ultrafast timescale.
The areas we are interested in are information communication technology (ICT), bio/medical/pharmaceutical applications, nondestructive evaluation, and fundamental science. These areas have been intensively studied for the last two decades. Especially interesting topics include THz wireless communications,3 cancer/bio-sensing/imaging,4 and terahertz time-domain spectroscopy (TDS)5/imaging.6,7 To foster those fields, a variety of THz sources, detectors, and systems have been studied, such as THz quantum cascade lasers (QCLs), resonant-tunneling-diodes (RTDs), injection-seeded THz-wave parametric generators (is-TPG),8 photoconductive switches/detectors for TDS, photomixers, bolometers, Dyakonov-Shur field effect transistors (FETs), THz-CMOS,9 THz-STM,10 and THz single photon detectors.11
The “Advances in Terahertz Solid-State Physics and Devices” Special Topic in the Journal of Applied Physics offers an overview of the most active research areas currently under investigation in the broad field of terahertz solid-state devices while also unveiling the properties of these materials on an ultrafast timescale. In particular, featured topics include THz sources, detectors, and other components,12–29 materials and related physics,30–34 and the challenges involved in achieving real world applications.35–39
Several types of THz sources have been reported in this Special Topic. RTDs are excellent THz oscillators from the electronic device side with moderate output powers, especially below 1 THz. Here, the problem arises when fabricating large-scale arrays, which are essential for the high-power devices discussed here.12 Although FETs have been studied for THz oscillators, there still remains an unclear picture of THz emission. Kasagi et al. examined the emission behavior in AlGaN/GaN high-electron-mobility transistors.13 QCLs are also promising devices above 1 THz from the optical side. The bandwidths of such cavities are presented and discussed to better understand and improve the feedback mechanisms by multiple scattering in terahertz semiconductor random lasers.14 For cryogenic applications, flux-flow Josephson oscillators are promising devices, which provide linewidths of 0.1 MHz and 2 MHz in the phase-locking regime and free-running regime, respectively.15 Photoconductive antennae are also THz emitters for TDS. Ponomarev et al. have studied the strain effect of InGaAs/InAlAs superlattices, which enhance terahertz emission.16 Spintronic THz emitters are also attractive THz sources.17
A variety of THz detectors have been reported in the Terahertz Special Topic. Zhang et al. demonstrate high performance of the THz bolometer with thermomechanical transduction.18 On-chip integrable terahertz microbolometer arrays using nanoscale meander titanium thermistors are also developed.19 Kojima et al. reported a high-electron-mobility transistor (HEMT) with an InGaAs/InAs/InGaAs double heterostructured channel on a glass substrate, which is a promising detector for a THz camera.20 A new type of THz detector with graphene-phosphorene hybrid structures and its bolometric response are reported.21 As one of the cryogenic detectors, the performance of superconductor-insulator-normal metal-insulator-superconductor junctions has been improved with annular antenna array metamaterials.22 Photoconductive switches are important detectors for TDS applications. Gorodetsky et al. studied the carrier dynamics in the InAs/GaAs quantum dot photoconductive THz antenna, and revealed its possible operation at fs laser wavelengths between 1 μm and 1.3 μm.23 Yang et al. discussed wavelength-selective THz absorption in a metallic grating/GaAs-based hybrid photoconductive detector.24 A simple idea to reduce the screening effect in photoconductive antennas with wide dipole antenna structures is introduced, which is effective in enhancing the sensitivity for the sub-THz frequency ranges.25 Schottky diodes are also expanding their operation frequencies.26
THz components are essential for real applications. Plez et al. discussed distributed capacitance compensation in a metal-insulator-metal infrared rectenna incorporating a traveling-wave THz diode.27 THz modulators with vanadium dioxide thin films and terahertz monolithic integrated waveguide transmission lines with wide bandgap semiconductors are also reported.28,29
In addition to those THz devices, several scientific probes of the THz nature of advanced materials are discussed. Anomalous terahertz dielectric properties in charge-ordered La1/3Sr2/3FeO3 thin films, THz response of the K+@C60 endohedral complex in a cavity of carbon nanotube containing polymerized fullerenes, and interband transitions in narrow-gap carbon nanotubes and graphene nanoribbons are discussed.30–32 High-refractive index, low-loss oxyfluorosilicate glasses have been developed for sub-THz applications.33 Recent developments in intense THz sources enable us to explore the nonlinear THz field response of advanced materials. For example, Sivarajah et al. study magnon-phonon-polariton coupling at THz frequencies.34
Finally, we can see several advanced studies for real world applications in this Special Topic. Hübers et al. have written a Perspective on high-resolution terahertz spectroscopy with QCLs.35 Röben et al. also studied QCLs for high-resolution spectroscopy of sharp absorption lines. Both articles highlight a pathway to one of the real world applications of QLCs.36 Mochizuki et al. demonstrated that laser THz emission microscopy (LTEM) is a promising tool for semiconductor device evaluation, which measures the surface potential of the metal-insulator-semiconductor structure in a noncontact manor.37 Bioapplications are among the most important fields; they require many tests and demonstrations. Taranets et al. studied the THz resonance response of biological tissue on a nanostructure.38 For THz wireless communication, it is essential to study THz wave propagation in the atmosphere. Golovachev and co-workers report on propagation properties in foggy conditions.39
In summary, THz science and technology provide a bridge between photonics and electronics over a relatively unexplored region of the electromagnetic spectrum, thereby bringing strong potential for a variety of applications. Terahertz solid-state physics and devices play the most important role in this field, which is the focus of this Special Topic. This collection of papers on THz sources, detectors, components, and THz material physics provides an authoritative assessment of the state-of-the-art of THz science and technology. The few examples of applications give evidence that many routes remain unexplored. We hope the “Advances in Terahertz Solid-State Physics and Devices” Special Topic will inspire many scientists and accelerate the expansion of the THz research field.
We are deeply indebted to the Journal of Applied Physics Lead Editor for the Terahertz Special Topic, Professor Masayoshi Tonouchi, for his supportive and enthusiastic role. We also thank the wonderful staff of AIP Publishing for correspondence with invited authors, preparing the Call for Papers, and promoting the papers published in the collection.