Hirokazu Fukidome

The terahertz (THz) frequency range (0.1–10 THz) bridges the electronic and photonic spectral domains and offers key opportunities for high-speed communication, sensing, and spectroscopy. However, the realization of compact, coherent, and room-temperature THz sources and detectors remains still a long-standing challenge. Recent advances in two-dimensional (2D) materials, hosting graphene-like massless Dirac fermions, have opened some new paths toward overcoming this limitation. This paper reviews recent advances in the physics of Dirac plasmons in graphene and related 2D heterostructure materials and their THz device applications. It first outlines the fundamentals of 2D plasmon hydrodynamics, nonlinearities, and current-driven instabilities, including Dyakonov–Shur Ryzhii–Satou–Shur, and Cherenkov-type mechanisms. A new mechanism, Coulomb drag instability, recently discovered by the authors, is theoretically shown to provide the largest plasmonic gain among these mechanisms, offering a new route to efficient THz amplification and lasing. Its experimental verification is currently in progress. The review also discusses graphene-based plasmonic lasers, amplifiers, and detectors, and recent developments in graphene/black-arsenic–phosphorus heterostructures that enable band-structure and plasmonic engineering. Finally, topological-insulator-based heterostructures are introduced as promising material systems. These advances demonstrate that Dirac plasmon physics provides a robust foundation for next-generation THz device technology.

Graphene is well suited to biological applications, including biosensing and drug delivery, through uses such as field-effect transistors, surface-enhanced Raman scattering (SERS), fluorescence imaging, and photothermal therapy. For SERS-based biochemical sensing, the chemical stability of graphene against oxidation and other reactions is particularly advantageous. However, its low SERS enhancement factor hinders practical application. To address this limitation, we demonstrate that H2/Ar annealing at temperatures above 350 °C significantly improves sensitivity to a level comparable to that of metal nanostructured SERS substrates. We developed a sandwich enzyme-linked immunosorbent assay on graphene substrates as a biosensor, enabling the detection of interleukin-6, a cytokine associated with mastitis, at concentrations as low as 0.5 pg/ml. We also investigated the mechanism underlying the SERS enhancement observed in H2/Ar-annealed graphene. This enhancement can be attributed to the removal of polymethyl methacrylate residues and oxygen-containing groups from the graphene surface, as well as the adsorption of hydrogen and hydrocarbons, which facilitates the binding of negatively charged antibodies. Additionally, increased surface hydrophobicity enhances hydrophobic interactions, promoting the adsorption of antibodies.

We designed and fabricated an epitaxial-graphene-channel field-effect transistor (EG-FET) featuring an asymmetric dual-grating-gate (ADGG) structure working as a current-driven terahertz detector and experimentally demonstrated a 10 ps-order fast response time and a high responsivity of 0.3 mA/W to 0.95 Terahertz (THz) radiation incidence at room temperature. The ADGG and drain–source bias dependencies of the measured photoresponse showed a clear transition between plasmonic detection under periodic electron density modulation conditions with depleted regions and photothermoelectric (PTE) detection under entirely highly doped conditions without depleted regions. We identified the PTE detection that we observed as a new type of unipolar mechanism in which only electrons or holes contribute to rectifying THz radiation under current-driven conditions. These two detection mechanisms coexisted over a certain wide transcendent range of the applied bias voltages. The temporal photoresponses of the plasmonic and PTE detections were clearly shown to be comparably fast on the order of 10 ps, whereas the maximal photoresponsivity of the PTE detection was almost twice as high as that of the plasmonic detection under applied bias conditions. These results suggest that the ADGG-EG-FET THz detector will be promising for use in 6G- and 7G-class high-speed wireless communication systems.

This paper reviews recent advances in the research and development of graphene‐based plasmonic metamaterials for terahertz (THz) laser transistors. The authors’ theoretical discovery on THz laser transistors in 2007 was realized as a distributed‐feedback dual‐gate graphene‐channel field‐effect transistor (DFB‐DG‐GFET) in 2018, demonstrating ∼0.1 µW single‐mode emission at 5.2 THz and ∼80 µW amplified spontaneous 1–7.6 THz emission at 100 K. To realize room‐temperature, dry‐cell‐battery operating intense THz lasing with fast direct modulation, various approaches based on graphene plasmonic metamaterials are investigated and introduced as real device implementations, including (i) replacement of the laser photonic cavity with plasmonic cavity enormously improving the THz photon field confinement with larger gain overlapping, (ii) introduction of THz amplification of stimulated emission via current‐driven graphene Dirac plasmons (GDPs), and (iii) controlling the parity and time‐reversal symmetry of GDPs enabling ultrafast direct gain‐switch modulation. Possible real device structures and design constraints are discussed and addressed toward coherent light sources applicable to future 6G‐ and 7G‐class THz wireless communication systems.

Graphene is promising for next-generation devices. However, one of the primary challenges in realizing these devices is the scalable growth of high-quality few-layer graphene (FLG) on device-type wafers; it is difficult to do so while balancing both quality and affordability. High-quality graphene is grown on expensive SiC bulk crystals, while graphene on SiC thin films grown on Si substrates (GOS) exhibits low quality but affordable cost. We propose a new method for the growth of high-quality FLG on a new template named “hybrid SiC”. The hybrid SiC is produced by bonding a SiC bulk crystal with an affordable device-type wafer and subsequently peeling off the SiC bulk crystal to obtain a single-crystalline SiC thin film on the wafer. The quality of FLG on this hybrid SiC is comparable to that of FLG on SiC bulk crystals and much higher than of GOS. FLG on the hybrid SiC exhibited high carrier mobilities, comparable to those on SiC bulk crystals, as anticipated from the linear band dispersions. Transistors using FLG on the hybrid SiC showed the potential to operate in terahertz frequencies. The proposed method is suited for growing high-quality FLG on desired substrates with the aim of realizing graphene-based high-speed devices.

In this chapter, the authors experimentally observe fast relaxation and relatively slow recombination dynamics of photogenerated electrons/holes in a heteroepitaxial graphene-on-silicon material under pumping with a 1550 nm, 80 fs pulsed fiber laser and probing with the corresponding terahertz (THz) beam generated by and synchronized with the pumping laser. The time-resolved electric-nearfield intensity originating from the coherent THz photon emission is electrooptically sampled in total-reflection geometry. The authors observe amplified stimulated THz emission in optically pumped and THz-probed epitaxial graphene heterostructures. The incident photon spectra are expected to be reflected in the THz photoemission spectra as evidence that such a process occurs. The time-resolved field emission properties are measured by an electrooptic sampling method in total-reflection geometry. The authors also observes coherent amplified stimulated THz emissions arising from the fast relaxation and relatively slow recombination dynamics of photogenerated electrons/holes in an epitaxial graphene heterostructures.

Gallium nitride (GaN)-based transistors are promising for realizing ultralow latency of wireless information communications that are indispensable for smart societies. However, surface states have slow time constants of (de)trapping electrons and delay electrical signal responses of the transistors, which prevent the ultralow latency. To clarify the mechanism of surface electron trapping (SET) of GaN-based transistors, which varies spatiotemporally, we studied the dynamics of the SET by using spatiotemporally resolved x-ray spectroscopy. We directly observed that neutralization of donor-like surface states occurs only near the gate edge in the drain side just after switching off the bias. We found that the large local electric field formed by the bias application induces site- and time-specificity in the SET, which leads to the delayed responses. Our proposed mechanism will be useful for optimizing the transistor structure to achieve the ultralow latency.