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Plasmonic excitations, consisting of collective oscillations of the electron gas in a conductive film or nanostructure coupled to electromagnetic fields, play a prominent role in photonics and optoelectronics. While traditional plasmonic systems are based on noble metals, recent work has established graphene as a uniquely suited materials platform for plasmonic science and applications due to several distinctive properties. Graphene plasmonic oscillations exhibit particularly strong sub-wavelength confinement, can be tuned dynamically through the application of a gate voltage, and span a portion of the infrared spectrum (including mid-infrared and terahertz (THz) wavelengths) that is not directly accessible with noble metals. These properties have been studied in extensive theoretical and experimental work over the past decade, and more recently various device applications are also beginning to be explored. This review article is focused on graphene plasmonic nanostructures designed to address a key outstanding challenge of modern-day optoelectronics – the limited availability of practical, high-performance THz devices. Graphene plasmons can be used as a means to enhance light–matter interactions at THz wavelengths in a highly tunable fashion, particularly through the integration of graphene resonant structures with additional nanophotonic elements. This capability is ideally suited to the development of THz optical modulators (where absorption is switched on and off by tuning the plasmonic resonance) and photodetectors (relying on plasmon-enhanced intraband absorption or rectification of charge-density waves), and promising devices based on these principles have already been reported. Novel radiation mechanisms, including light emission from electrically excited graphene plasmons, are also being explored for the development of compact narrowband THz sources.

Metamaterial photonic integrated circuits with arrays of hybrid graphene–superconductor coupled split-ring resonators (SRR) capable of modulating and slowing down terahertz (THz) light are introduced and proposed. The hybrid device’s optical responses, such as electromagnetic-induced transparency (EIT) and group delay, can be modulated in several ways. First, it is modulated electrically by changing the conductivity and carrier concentrations in graphene. Alternatively, the optical response can be modified by acting on the device temperature sensitivity by switching Nb from a lossy normal phase to a low-loss quantum mechanical phase below the transition temperature (Tc) of Nb. Maximum modulation depths of 57.3% and 97.61% are achieved for EIT and group delay at the THz transmission window, respectively. A comparison is carried out between the Nb-graphene-Nb coupled SRR-based devices with those of Au-graphene-Au SRRs, and significant enhancements of the THz transmission, group delay, and EIT responses are observed when Nb is in the quantum mechanical phase. Such hybrid devices with their reasonably large and tunable slow light bandwidth pave the way for the realization of active optoelectronic modulators, filters, phase shifters, and slow light devices for applications in chip-scale future communication and computation systems.

Ultrashort polariton wave packets, such as terahertz graphene plasmon polaritons, could be used for fast information processing in integrated circuits. However, conventional optical techniques have struggled to integrate the components for controlling polariton signals and have a low conversion efficiency. Here, we show that graphene plasmon wave packets can be generated, manipulated and read out on-chip using terahertz electronics. Electrical pulses injected into a graphene microribbon through an ohmic contact can be efficiently converted into a plasmon wave packet with a pulse duration as short as 1.2 ps and a three-dimensional spatial confinement of 2.1 × 10 −18  m 3 . The conversion efficiency between the electrical pulses and plasmon wave packets can also reach 35% due to the absence of a momentum mismatch. The transport properties of graphene plasmons are studied by changing the dielectric environments, which provides a basis for designing graphene plasmonic circuits. Terahertz electronics that can create and control ultrashort graphene plasmon wave packets with durations as short as 1.2 ps can offer on-chip handling of plasmonic signals.

The terahertz (THz) frequency band has emerged as a promising frontier for next-generation wireless communication systems targeting ultra-high data rates, ultra-low latency, and spectrum expansion beyond conventional millimeter-wave regimes. Realizing practical THz communication links, however, critically depends on stable, tunable, and integrable signal sources capable of delivering sufficient output power while maintaining spectral purity and energy efficiency. Among the various THz generation approaches, optoelectronic techniques offer unique advantages, including large bandwidth, wide frequency tunability and compatibility with fiber-optic infrastructures. This review provides a technology-focused assessment of key optoelectronic THz source technologies, namely photoconductive antennas, quantum cascade lasers, and unitraveling carrier photodiode (UTC-PD)-based photomixers, with particular emphasis on UTC-PD photomixers due to their strong suitability for continuous-wave THz generation and fiber-compatible architectures. The implications of optoelectronic THz sources for system-level architectures, including THz-over-fiber links, coherent detection schemes, and phased-array integration, are further examined. Finally, critical challenges and emerging research directions toward monolithic photonic–terahertz integration and deployable high-capacity wireless front-ends are discussed. This review aims to provide a structured perspective on the state of optoelectronic THz source technologies and their role in enabling practical next-generation communication systems.

Nowadays, graded semiconductors attract developers' interest as prospective material which can improve the interaction of the electric field and the electrons in the devices operating on the intervalley electron transfer effect. This effect increases the efficiency and power output of the generation of current oscillations in Gunn diodes. To obtain the best effect graded semiconductor must be optimal by the dependence of the energy gap between the nonequivalent valleys of the conduction band on the coordinate. This paper deals with the results of the investigation of Gunn diodes operation based on graded InGaP-InPAs by means of the temperature model of intervalley electron transfer in graded semiconductors. The paper presents the results of the numerical experiments on efficient generation of electromagnetic waves in the range from 18 to 80 GHz using graded InxGa1 – xP-InPyAs1 – y Gunn diodes with the active region length of 2.5 mm and concentration of ionized impurities therein of 1016 cm – 3. Our findings are the dependences generation efficiency and output power on frequency for different distributions of GaP and InAs in InxGa1 – xP-InPyAs1 – y. We have compared obtained results with similar AlxGa1 – xAs-GaAs-Ga1 – yInyAs-diodes. The maximal obtained power in InxGa1 – xP-InPyAs1 – y-diode is 11.3 kW×cm – 2 at a frequency of 40 GHz with an efficiency of 10.2 % at x = 0.6 and y = 0.6.

A new possible mechanism of signal detection in the THz range is investigated, based on the excitation of resonances due to the tunneling effect between two graphene nanoribbons. A simple detector is proposed, where two graphene nanoribbons are used to contact two copper electrodes. The terminal voltages are shown to exhibit strong resonances when the frequency of an external impinging field is tuned to the characteristic tunneling frequency of the graphene layer pair. An electrodynamic model for the electron transport along the graphene nanoribbons is extended here to include the tunneling effect, and a coupled transmission line model is finally derived. This model is able to predict not only the tunneling resonance, but also the well-known plasmon resonances, related to the propagation of slow surface waves.

Monolayer (ML) molybdenum disulfide (MoS2) is a typical valleytronic material which has important applications in, for example, polarization optics and information technology. In this study, we examine the effect of continuous wave (CW) laser pumping on the basic optoelectronic properties of ML MoS2 placed on a sapphire substrate, where the pump photon energy is larger than the bandgap of ML MoS2. The pump laser source is provided by a compact semiconductor laser with a 445 nm wavelength. Through the measurement of THz time-domain spectroscopy, we obtain the complex optical conductivity for ML MoS2, which are found to be fitted exceptionally well with the Drude–Smith formula. Therefore, we expect that the reduction in conductivity in ML MoS2 is mainly due to the effect of electronic backscattering or localization in the presence of the substrate. Meanwhile, one can optically determine the key electronic parameters of ML MoS2, such as the electron density ne, the intra-band electronic relaxation time τ, and the photon-induced electronic localization factor c. The dependence of these parameters upon CW laser pump intensity is examined here at room temperature. We find that 445 nm CW laser pumping results in the larger ne, shorter τ, and stronger c in ML MoS2 indicating that laser excitation has a significant impact on the optoelectronic properties of ML MoS2. The origin of the effects obtained is analyzed on the basis of solid-state optics. This study provides a unique and tractable technique for investigating photo-excited carriers in ML MoS2.

The terahertz (THz) regime (0.1–10 THz) is rich with emerging possibilities in sensing, imaging and communications, with unique applications to screening for weapons, explosives and biohazards, imaging of concealed objects, water content and skin. Here we present initial surveys to evaluate the possibility of sensing plastic explosives and bacterial spores using field-deployable electronic THz techniques based on short-pulse generation and coherent detection using nonlinear transmission lines and diode sampling bridges. We also review the barriers and approaches to achieving greater sensing-at-a-distance (stand-off) capabilities for THz sensing systems. We have made several reflection measurements of metallic and non-metallic targets in our laboratory, and have observed high contrast relative to reflection from skin. In particular, we have taken small quantities of energetic materials such as plastic explosives and a variety of Bacillus spores, and measured them in transmission and in reflection using a broadband pulsed electronic THz reflectometer. The pattern of reflection versus frequency gives rise to signatures that are remarkably specific to the composition of the target, even though the target's morphology and position is varied. Although more work needs to be done to reduce the effects of standing waves through time-gating or attenuators, the possibility of mapping out this contrast for imaging and detection is very attractive.

A tunable lens-coupled annular-slot antenna using a Schottky varactor diode has been designed, fabricated and characterised at 140 to 220 GHz. Simulation results show that the resonant frequency of the annular-slot antenna can be effectively tuned by varying the capacitance of the imbedded varactor diode, with an average tunability of ∼2.5 GHz/fF. Prototype demonstration using a varactor with zero-bias capacitance of 3.8 fF has shown a frequency tuning range from 197 to 202.7 GHz by varying the diode DC bias voltage from 1 to −5 V (corresponding to a diode capacitance change from 4.97 to 2.4 fF). The measured tunability is 2.22 GHz/fF, which agrees well with simulation. Projections indicate tuning ranges of ∼50 GHz in the G-band should be possible using varactor diodes with ∼20 fF zero-bias capacitance.