Abstract-Dual-Wide-Band Dual Polarization Terahertz Linear to Circular Polarization Converters based on Bi-Layered Transmissive Metasurfaces

Ayesha Kosar Fahad,  Cunjun Ruan,  Kanglong Chen,

https://www.mdpi.com/2079-9292/8/8/869/htm

Transmissive metasurface-based dual-wide-band dual circular polarized operation is needed to facilitate volume and size reduction along with polarization diversity for future THz wireless communication. In this paper, a novel dual-wide-band THz linear polarization to circular polarization (LP-to-CP) converter is proposed using transmissive metasurfaces. It converts incident X polarized waves into transmitted left-hand circular polarized (LHCP) and right-hand circular polarized (RHCP) waves at two frequency bands. The structure consists of bi-layered metasurfaces having an outer conductor square ring and three inner conductor squares diagonally intersecting each other. The proposed converter works equally well with incident Y polarizations. Operational bandwidths for the dual-band LP-to-CP are 1.16 THz to 1.634 THz (34% fractional bandwidth) and 3.935 THz to 5.29 THz (29% fractional bandwidth). The electromagnetic simulation was carried out in two industry-standard software packages, High Frequency Structure Simulator (HFSS) and Computer Simulation Technology (CST), using frequency and time domain solvers respectively. Close agreement between results depicts the validity and reliability of the proposed design. The idea is supported by equivalent circuits and physical mechanisms involved in the dual-wide-band dual polarization operation. The impact of different geometrical parameters of the unit cell on the performance of LP-to-CP operation is also investigated.

Abstract-Broadband and high-power terahertz radiation source based on extended interaction klystron

Renjie Li, Cunjun Ruan, Ayesha Kosar Fahad, Chenyu Zhang,  Shasha Li

https://www.nature.com/articles/s41598-019-41087-3

Terahertz applications require high performance and high reliability terahertz radiation sources, especially the urgent demands of high output power and broad bandwidth. The extended interaction klystron (EIK) has the great potential to generate hundreds of watt output power in terahertz band. The terahertz EIK adopts multiple gap cavities and unequal-width slots structure is proposed with methodological improvement of bandwidth and output power. The unequal-width slots are the key design of the multiple gap cavity, and the influences of unequal-width slots on the electromagnetic field distribution and beam-wave interaction are analyzed in detail. With multiple gap cavities and unequal-width slots structure, EIK has advantages of wider frequency separation and larger effective characteristic impedance. Particle in cell (PIC) simulation indicates that the bandwidth of unequal-width slots structure can reach to 550 MHz in our initial G-band EIK design. Then, we utilize two kinds of resonance cavities with different width ratios to build a six-cavity beam-wave interaction system and make it operate at the state of stagger-tuning, the bandwidth can be extended to 1–1.5 GHz. Our research shows that the unequal-width slots structure has wider tuning frequency range. Furthermore, the bandwidth can be further broadened to over 2 GHz when dynamic-tuning is adopted, while maintains a high output power of 560 W with efficiency of 11.3% and gain of 47.5 dB. Thus, the methods of multiple gap cavities with unequal-width slots structure, stagger-tuning and dynamic-tuning are much important for the bandwidth improvement of EIK in terahertz band.

Abstract-Enhancement of Spintronic Terahertz Emission via Annealing in Ferromagnetic Heterostructures

Xiaojun Wu, Yang Gao, Yanbin He, Tianxiao Nie, Chun Wang, Deyin Kong, Chandan Pandey, Bo Wang, Lianggong Wen, Weisheng Zhao, Cunjun Ruan, Jungang Miao, Li Wang, Yutong Li, Kang L. Wang

https://arxiv.org/abs/1812.07113

We systematically investigate the influence of annealing effect on terahertz (THz) generation from CoFeB based magnetic nanofilms driven by femtosecond laser pulses. Three times enhancement of THz yields are achieved in W/CoFeB through annealing effect, and double boosting is obtained in Pt/CoFeB. The mechanism of annealing effect originates from the increase of hot electron mean free path induced by crystallization, which is experimentally corroborated by THz transmission measurement on time-domain spectroscopy. Comparison studies of the thickness dependent THz efficiency after annealing are also implemented, and we eventually conclude that annealing and thickness optimization are of importance for scaling up THz intensity. Our observations not only deepen understanding of the spintronic THz radiation mechanism but also provide normal platform for high speed spintronic opto-electronic devices.

Abstract-Broadband Magnetic-Manipulated Spintronic Terahertz Emitter with Arbitrarily Tunable Polarizations

Xiaojun Wu, Deyin Kong, Tianxiao Nie, Bo Wang, Meng Xiao, Chandan Pandey, Yang Gao, Lianggong Wen, Weisheng Zhao, Cunjun Ruan, Jungang Miao, Li Wang, Yutong Li

https://arxiv.org/abs/1809.10474

Flexible manipulation of terahertz-wave polarization during the generation process is very important for terahertz applications, especially for the next-generation on-chip functional terahertz sources. However, current terahertz emitters could not satisfy such demand, hence calling for new mechanism and conceptually new terahertz source. Here we demonstrate a magnetic-field-controlled, highly-efficient, cost-effective, and broadband terahertz source with flexible switch of terahertz polarization states in ferromagnetic heterostructures driven by femtosecond laser pulses. We verify that the chirality, azimuthal angle, and ellipticity of the generated elliptical terahertz waves can be independently manipulated by delicately engineering of the external applied magnetic fields via effectively manipulating the photo-induced spin currents. Such an ultrafast photomagnetic interaction-based, magnetic-field-controlled, and broadband tunable solid-state terahertz source integrated with terahertz polarization tunability function not only has the capability to reveal physical mechanisms of femtosecond spin dynamics, but also demonstrates the feasibility to realize novel on-chip terahertz functional devices, boosting the potential applications for controlling elementary molecular rotations, phonon vibrations, spin precessions, high-speed terahertz communication, and accelerating the development of ultrafast terahertz opto-spintronics.