Progress of Terahertz Devices Based on Graphene

Graphene is a one-atom-thick planar sheet of sp2-hybridized orbital bonded honeycomb carbon crystal. Its gapless and linear energy spectra of electrons and holes lead to the unique carrier transport and optical properties, such as giant carrier mobility and broadband flat optical response. As a novel material, graphene has been regarded to be extremely suitable and competent for the development of terahertz （THz） optical devices. In this paper, the fundamental electronic and optic properties of graphene are described. Based on the energy band structure and light transmittance properties of graphene, many novel graphene based THz devices have been proposed, including modulator, generator, detector, and imaging device. This progress has been reviewed. Future research directions of the graphene devices for THz applications are also proposed.

Guest Editorial TTA Special Section on Terahertz Devices

Lying between the microwave and infrared frequencies,the electromagnetic spectrum band,normally defined as^0.1 THz to 10.0 THz,was often referred to as a“terahertz（THz）gap”due to the difficulties in efficiently generating,manipulating,and detecting THz radiation.However,driven by the promising applications ranging from non-destructive imaging,spectroscopic sensing,to ultra-high bit rate wireless communications,

Guest Editorial TTA Special Section on Terahertz Devices

Recent advances of artificial structured materials, including photonic crystals and metamaterials, have greatly broadened the functionalities of terahertz （THz） devices and provided more degree of freedom in manipulating THz waves beyond traditional constraints. These materials are usually constituted by periodic or aperiodic sub-wavelength elements, showing significant electromagnetic responses during the wave matter interaction, thus enabling the modulation of amplitude, phase, or propagation direction of incident waves as a result. So far, a variety of applications have been proposed and experimentally validated, such as the THz filters, polarizers, modulators, and biosensors with the advantages of ultrathin profile, easy integration, and simple geometry. By incorporating novel materials like graphene, vanadium dioxide, and liquid crystals in the element design, we are allowed to adjust the characteristics of the THz radiation dynamically, which brings additional flexibilities toward the construction of novel THz functional devices.

Highly efficient vectorial field manipulation using a transmitted tri-layer metasurface in the terahertz band

Polarization is a basic characteristic of electromagnetic waves that conveys much optical information owing to its many states.The polarization state is manipulated and controlled for optical information security,optical encryption,and optical communication.Metasurface devices provide a new way to manipulate wave-fronts of light.A single ultrathin metasurface device can generate and modulate several differently polarized light fields,and thus carries optical information in several different channels.Terahertz(THz)waves have become widely used as carrier waves for wireless communication.Compact and functional metasurface devices are in high demand for THz elements and systems.This paper proposes a tri-layer metallic THz metasurface for multi-channel polarization generation and phase modulation with a high efficiency of approximately 80%.An azimuthally polarized THz vectorial beam generator is realized and characterized for use as a THz polarization analyzer.The incident polarization angle can be observed graphically with high accuracy.Moreover,a vectorial hologram with eight channels for different linear polarization states is demonstrated experimentally.The information in different holograms can be hidden by choosing the polarization channel for detection.This work contributes to achieving multi-functional metasurface in the THz band and can benefit THz communication and optical information security.

Guest Editorial——TTA Special Section on Terahertz Materials and Devices

Lying between radio frequency and infrared radiation, terahertz （THz） wave encounters lots of difficulties to produce, detect, transmit, and modulate. Great efforts have been made to construct THz devices, including sources, detectors, switches, modulators, lenses, and filters. However, only moderate progresses have been made in THz generation and detection. Furthermore, the devices and techniques to control and manipulate THz waves are still in its infancy. Therefore, it is still a challenge to date to develop sophisticated THz application systems such as communication, sensing, safety inspection, imaging and medical diagnose systems. This difficult position of THz wave is essential due to the deficiency of THz materials having a suitable THz electromagnetic response, as compared to its neighboring microwave and infrared regime. Practicable material will largely push the THz technology to real-world applications. The GaAs/AlxGa~_xAs material system, for example, is the heart of THz quantum cascade lasers. High quality NbN films, again, set the basis of THz hot electron bolometer.

Guest Editorial TTA Special Section on Terahertz Sources and Devices

The terahertz wave is considered to have great values and plentiful applications, such as in material science, analysis of molecular spectra, information and communication technology, biology and medical science, nondestructive evaluation, and national security. High-power widely tunable terahertz sources are required in the above practical applications of terahertz technologies. A promising approach for monochromatic terahertz generation is based on second-order nonlinear optical effect, e.g.

Guest Editorial—TTA Special Section on Terahertz Functional Devices

In the past decades, terahertz technology has a great development and steady improvement, which has kept discovering and developing a series of potential applications in terahertz sensing, imaging, spectroscopy, security, and communication. After the recent technical breakthroughs in reliable sources and sensitive detectors, terahertz functional devices, such as waveguides, switches, filters, splitters, isolators, modulators and sensors, are indispensable for the construction of compact application systems and have become a worldwide supreme issue in research.

High Power, Room Temperature Terahertz Emitters Based on Dopant Transitions in 6H-Silicon Carbide

Electrically pumped high power terahertz （THz） emitters that operated above room temperature in a pulse mode were fabricated from nitrogen-doped n-type 6H-SiC. The emission spectra had peaks centered on 5 THz and 12 THz （20 meV and 50 meV） that were attributed to radiative transitions of excitons bound to nitrogen donor impurities. Due to the relatively deep binding energies of the nitrogen donors, above 100 meV, and the high thermal conductivity of the SiC substrates, the THz output power and operating temperature were significantly higher than previous dopant based emitters. With peak applied currents of a few amperes, and a top surface area of 1 mm2, the device emitted up to 0.5 mW at liquid nitrogen temperature （77 K）, and tens of microwatts up to 333 K. This result is the highest temperature of THz emission reported from impurity-based emitters.

Photoconductive devices for terahertz pulsed spectroscopy:a review[Invited]

Photoconductive switches were the key components that allowed the generation and detection of coherent broadband electromagnetic pulses at terahertz frequencies, opening the possibility for performing spectroscopy and,therefore, measuring complex dielectric properties of materials in this band, which was mostly unexplored. In this paper, we present a brief introduction to the operation principles of these devices. Subsequently, we present a review of the current state-of-the-art in this field and discuss the challenges to be faced in future development of these devices.

Terahertz out-of-plane coupler based on compact spot-size converter

Low-loss dielectric terahertz(THz) chips are efficient platforms for diverse THz applications. One of the key elements in the chip is the coupler. Most of the available THz couplers are in-plane and couple the THz wave from the metal waveguide to the dielectric waveguide. However, out-of-plane couplers are more suitable for wafer-scale testing and tolerant of alignment variation. In this work, we propose an out-of-plane THz coupler for coupling the antenna to the dielectric waveguide. The device is constructed using a grating and a compact spot-size converter. As the conventional optical spot-size converters that apply directly to THz chips are too large, we have designed a compact spot-size converter based on a tapered waveguide with a lens. The total device is 2.9 cm long and can couple a 7 mm diameter THz beam to a 500 μm wide waveguide. The device can scan the THz beam, radiate the input rectangular waveguide mode to free space, and drive the rotation angle of the fan beam through the scanning frequency. We fabricated the device using a single lithography step on a silicon wafer.The out-of-plane coupling efficiency was found to be ~5 d B at 194 GHz. The fan-beam steering range was found to be around40° in the frequency range of 170–220 GHz. The proposed out-of-plane coupling technique may provide an effective way for THz wafer-scale testing with a higher degree of freedom for on-chip integration. Also, the proposed technique being nonmechanical, beam steering using it, may therefore find applications in THz radar, communication, and sensing.