Toward 6G Communication Networks:Terahertz Frequency Challenges and Open Research Issues

Future networks communication scenarios by the 2030s will include notable applications are three-dimensional(3D)calls,haptics communications,unmanned mobility,tele-operated driving,bio-internet of things,and the Nanointernet of things.Unlike the current scenario in which megahertz bandwidth are sufficient to drive the audio and video components of user applications,the future networks of the 2030s will require bandwidths in several gigahertzes(GHz)(from tens of gigahertz to 1 terahertz[THz])to perform optimally.Based on the current radio frequency allocation chart,it is not possible to obtain such a wide contiguous radio spectrum below 90 GHz(0.09 THz).Interestingly,these contiguous blocks of radio spectrum are readily available in the higher electromagnetic spectrum,specifically in the Terahertz(THz)frequency band.The major contribution of this study is discussing the substantial issues and key features of THz waves,which include(i)key features and significance of THz frequency;(ii)recent regulatory;(iii)the most promising applications;and(iv)possible open research issues.These research topics were deeply investigated with the aim of providing a specific,synopsis,and encompassing conclusion.Thus,this article will be as a catalyst towards exploring new frontiers for future networks of the 2030s.

Power dependence of terahertz carrier frequency in a plasma-based two-color generation process

We conduct a frequency spectrum experiment to investigate terahertz（THz） emissions from laser-induced air plasma under different laser incident powers. The frequency spectra are measured using both air-biased-coherent detection and a Michelson interferometer. The red-shift of the THz pulse carrier frequency is observed as a response to increased pump power. These phenomena are related to plasma collisions and can be explained by the plasma collision model. Based on these findings, it is apparent that the tuning of the THz carrier frequency can be achieved through regulation of the pump beam.

Energy Scaling of Terahertz Pulses Produced through Difference Frequency Generation

We study the energy scaling of terahertz (THz) emission through difference frequency generation of near-infrared pulses, and demonstrate that Gigawatt few-cycle THz transients at the central frequency of 30 THz are produced from GaSe crystal pumped by two pulses at 1.65 and 1.95 micrometers, with the high quantum yield of 28%. Our analysis indicates that the high yield of DFG originates from the largely reduced group velocity mismatch as the long-wavelength pumping pulses are employed.

Milestones of Wireless Communication Networks and Technology Prospect of Next Generation(6G)

Since around 1980,a new generation of wireless technology has arisen approximately every 10 years.First-generation(1G)and secondgeneration(2G)began with voice and eventually introduced more and more data in third-generation(3G)and became highly popular in the fourthgeneration(4G).To increase the data rate along with low latency and mass connectivity the fifth-generation(5G)networks are being installed from 2020.However,the 5G technology will not be able to fulfill the data demand at the end of this decade.Therefore,it is expected that 6G communication networks will rise,providing better services through the implementation of new enabling technologies and allowing users to connect everywhere.6G technology would not be confined to cellular communications networks,but would also comply with non-terrestrial communication system requirements,such as satellite communication.The ultimate objectives of this work are to address the major challenges of the evolution of cellular communication networks and to discourse the recent growth of the industry based on the key scopes of application and challenges.The main areas of research topics are summarized into(i)major 6G wireless networkmilestones;(ii)key performance indicators;(iii)future new applications;and(iv)potential fields of research,challenges,and open issues.

Ultra-low temperature cofired ceramics based on Li_(2)WO_(4)as perspective substrate materials for terahertz frequencies

With the development of low dielectric permittivity materials having an ultra-low sintering temperature,testing their dielectric properties at terahertz frequencies suitable for 6G communication systems and implementation of the fabricated materials in ultra-low temperature cofired ceramics(ULTCC)were the main goals of the research.Lithium tungstate Li_(2)WO_(4)was synthesized by a solid-state reaction and used for the preparation of green tapes and test structures with cofired internal conductive layers,which are destined for substrates of microwave and submillimeter wave circuits.Sintering behavior,thermal effects,and mass changes of the green tapes during heating were studied using a hot-stage microscope,differential thermal analysis,and thermogravimetry.A single-phase composition was revealed for being undoped and doped with AlF3–CaB_(4)O_(7)ceramics.The impact of frequency,temperature,the addition of AlF3–CaB_(4)O_(7)and CuBi_(2)O_(4)dopants,and sintering temperature was the subject of in-depth characterization of dielectric properties in a terahertz region.A glass-free composition,ultra-low sintering temperature of 590–630℃,low roughness of the green tapes,dense microstructure,compatibility with Ag conductors,low and stable dielectric permittivity of 5.0–5.8 in a broad range of 0.2–2 THz,and low dielectric loss of 0.008–0.01 at 1 THz are the main advantages of the developed ULTCC substrates.

Tunable terahertz wave difference frequency generation in a graphene/AlGaAs surface plasmon waveguide

Graphene-based surface plasmon waveguides(SPWs) show high confinement well beyond the diffraction limit at terahertz frequencies. By combining a graphene SPW and nonlinear material, we propose a novel graphene/AlGaAs SPW structure for terahertz wave difference frequency generation(DFG) under near-infrared pumps.The composite waveguide, which supports single-mode operation at terahertz frequencies and guides two pumps by a high-index-contrast AlGaAs∕Al Oxstructure, can confine terahertz waves tightly and realize good mode field overlap of three waves. The phase-matching condition is satisfied via artificial birefringence in an AlGaAs∕Al Ox waveguide together with the tunability of graphene, and the phase-matching terahertz wave frequency varies from 4 to 7 THz when the Fermi energy level of graphene changes from 0.848 to 2.456 eV. Based on the coupled-mode theory, we investigate the power-normalized conversion efficiency for the tunable terahertz wave DFG process by using the finite difference method under continuous wave pumps, where the tunable bandwidth can reach 2 THz with considerable conversion efficiency. To exploit the high peak powers of pulses, we also discuss optical pulse evolutions for pulse-pumped terahertz wave DFG processes.

Recent progress on terahertz generation based on difference frequency generation:from power scaling to compact and portable sources

The progress achieved on power scaling and compact and portable THz sources is reviewed.By reversely stacking the GaP plates,the photon conversion efficiency is improved from 25% to 40% which corresponds to the maximum value.When the number of the plates is increased from four to five,the output power decreases because of back conversion.The THz generation is also investigated by mixing the two frequencies generated by a single Nd：YLF solid-state laser.The average output power reaches 1 μW.The introduction of two Nd：YLF crystals significantly improves the output power to 4.5 μW.This configuration facilitates the generation of different output frequencies.

Influence of N-doping on dielectric properties of carbon-coated copper nanocomposites in the microwave and terahertz ranges

Carbon-coated Cu nanocomposites (Cu@C NCs) consisting of core-shell nanoparticles and nanorods weresynthesized by arc discharge plasma under an atmosphere of He and H_(2) gas, and the N-doping of themwas achieved by a post-treatment process using ureal as the precursor. The concentration of N in the Ndoped samples varies in the range of 0.62%-2.31 % (in mole), with a transformation from pyrrolic N tographitic N when increasing the relative content of ureal. Dielectric properties of the NCs without or withN-doping in the microwave and THz bands were investigated. The N-doped samples achieve theenhanced dielectric loss in both microwave and THz bands. In the microwave band, dielectric loss wasdominated by interfacial polarization, dipolar polarization, and conduction loss, while in the THz band,plasma resonance, ionic polarization and conduction loss are responsible for the dielectric loss, with astrong absorption characteristic dominated by conductive effect.

Heterojunction DDR THz IMPATT diodes based on Al_xGa_(1-x)N/Ga N material system

Simulation studies are made on the large-signal RF performance and avalanche noise properties ofhet- erojunction double-drift region （DDR） impact avalanche transit time （IMPATT） diodes based on AlxGal-xN/GaN material system designed to operate at 1.0 THz frequency. Two different heterojunction DDR structures such as n-Al0.4Ga0.6N/p-GaN and n-GaN/p-Al0.4Ga0.6N are proposed in this study. The large-signal output power, con- version efficiency and noise properties of the heterojunction DDR IMPATTs are compared with homojunction DDR IMPATT devices based on GaN and Al0.4Ga0.6N. The results show that the n-Al0.4Ga0.6N/p-GaN heterojunction DDR device not only surpasses the n-GaN/p-Al0.4Ga0.6N DDR device but also homojunction DDR IMPATTs based on GaN and Al0.4Ga0.6N as regards large-signal conversion efficiency, power output and avalanche noise performance at 1.0 THz.

22 nm In0.75Ga0.25As channel-based HEMTs on InP/GaAs substrates for future THz applications

In this work,the performance of Lg = 22 nm In（0.75）Ga（0.25）As channel-based high electron mobility transistor（HEMT） on InP substrate is compared with metamorphic high electron mobility transistor（MHEMT） on GaAs substrate.The devices features heavily doped In（0.6）Ga（0.4）As source/drain（S/D） regions,Si double δ-doping planar sheets on either side of the In（0.75）Ga（0.25） As channel layer to enhance the transconductance,and buried Pt metal gate technology for reducing short channel effects.The TCAD simulation results show that the InP HEMT performance is superior to GaAs MHEMT in terms of fT,f（max） and transconductance（g（mmax））.The 22 nm InP HEMT shows an fT of 733 GHz and an f（max） of 1340 GHz where as in GaAs MHEMT it is 644 GHz and 924 GHz,respectively.InGaAs channel-based HEMTs on InP/GaAs substrates are suitable for future sub-millimeter and millimeter wave applications.