TST, Vol. 13, No. 1, PP. 1-21
(Invited paper) Gyro-devices – natural sources of high-power high-order angular momentum millimeter-wave beams
M. Thumm *
Karlsruhe Institute of Technology (KIT), Kaiserstr. 12, 76131 Karlsruhe, Germany
1 Institute for Pulsed Power and Microwave Technology (IHM)
2 Institute of Radiofrequency Engineering and Electronics (IHE)
* Email: manfred.thumm@kit.edu
(Received January 23, 2020)
Abstract: The Orbital Angular Momentum (OAM) carried by light beams with helical phasefront (vortex beams) has been widely employed in many applications such as optical tweezers, optical drives of micro-machines, atom trapping, and optical communication. OAM provides an additional dimension (diversity) to multiplexing techniques, which can be utilized in addition to conventional multiplexing methods to achieve higher data rates in wireless communication. OAM beams have been thoroughly studied and used in the optical regime but in the mm-wave and THz-wave region, they are still under investigation. In these frequency bands, there are difficulties associated with beam-splitting and beam-combining processes as well as with the use of spiral phase plates and other methods for OAM generation, since the wavelength is much larger compared to those at optical frequencies, leading to higher diffraction losses.
The present paper describes the natural generation of high-power OAM modes by gyro-type vacuum electron devices with cylindrical interaction circuit and axial output of the generated rotating higher-order transverse electric mode TEm,n , where m > 1 and n are the azimuthal and radial mode index, respectively. The ratio between the total angular momentum (TAM) JN and total energy WN of N photons is given by m/w, where w is the angular frequency of the operating mode, which in a gyrotron oscillator is close to the TEm,n-mode cutoff frequency in the cavity. Therefore, m/w = Rc/c, where Rc is the caustic radius and c the velocity of light in vacuum. This means that the OAM is proportional to the caustic radius and at a given frequency the same for all modes with the same azimuthal index m. Right-hand rotation (co-rotation with the electrons) corresponds to a positive value of m and left-hand rotation to negative m. The corresponding OAM mode number (topological charge) is l = m – 1. Circularly polarized TE1n modes only possess a Spin Angular Momentum (SAM: s = ±1). TE0n modes have neither SAM nor OAM.
This is the result of the photonic (quasi-optical) approach to derive the TAM of modes generated in gyrotrons. The same result follows from the electromagnetic (EM) wave approach for the TAM within a given waveguide volume per total energy of the EM wave in the same volume.
Such high-power output beams with very pure higher-order OAM, generated by gyrotron oscillators or amplifiers (broadband) could be used for multiplexing in long-range wireless communications. The corresponding mode and helical wavefront sensitive detectors for selective OAM-mode sorting are available and described in the present paper.
Keywords: Orbital angular momentum (OAM), Spin angular momentum (SAM), Millimeter-wave and THz-wave vortex beams, Gyrotron, Gyro-amplifiers, Long-range wireless communication, Multiplexing, Diversity.
doi:
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TST, Vol. 13, No. 1, PP. 22-31
(Invited paper) Terahertz accelerator based electron and x-ray sources
Franz X. Kärtner *
Center for Free-Electron Laser Science, Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany Physics Department and The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Hamburg, Germany
* Email: franz.kaertner@cfel.de
(Received February 23, 2020)
Abstract: The generation and use of THz radiation for electron acceleration and manipulation of electron bunches has progressed over the last decade to a level where practical devices for THz guns, acceleration and a wide range of beam manipulations have become possible. Here, we present our progress on generation of single-cycle THz pulses at the two-hundred micro-Joule level to drive advanced acceleration and beam manipulation devices. Specifically, we use pulses centered at 0.3 THz to power a segmented terahertz electron accelerator and manipulator (STEAM) capable of performing multiple high-field operations on the 6D-phase-space of ultrashort electron bunches. Using this STEAM device, we demonstrate record THz-acceleration of >60 keV, streaking with <10 fs resolution, focusing with >2 kT/m strength, compression to ~100 fs as well as real-time switching between these modes of operation. The STEAM device demonstrates the feasibility of THz-based electron accelerators, manipulators and diagnostic tools enabling science beyond current resolution frontiers with transformative impact.
Keywords: Single-Cycle THz generation, THz guns, THz acceleration, THz beam manipulation
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TST, Vol. 13, No. 1, PP. 32-40
(Invited paper) Repetition frequency locking of a terahertz quantum cascade laser emitting at 4.2 THz
Wen Guan 1, 2, Ziping Li 1, 3, Kang Zhou 1, 3, Wenjian Wan 1, Xiaoyu Liao 1, 3, Yiran Zhao 1, 3, Sijia Yang 1, 3, J. C. Cao 1, 3, and Hua Li 1*, 3
1 Key Laboratory of Terahertz Solid State Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Science, 865 Changning Road, Shanghai 200050, China
2 School of Information Science and Technology, ShanghaiTech University, 393 Middle Huaxia Road, Shanghai 201210, China
3 Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
*1 Email: hua.li@mail.sim.ac.cn
(Received March 10, 2020)
Abstract: The electrically-pumped terahertz quantum cascade laser (QCL) is characterized by high power emission, compact, broad frequency coverage, and so on, which shows abilities for frequency comb operations. Although free-running QCLs can work as frequency combs by designing the laser structure with small group velocity dispersions and/or inserting mirrors to compensate laser intrinsic dispersions, the ideal comb operation can only be obtained by firmly locking the repetition frequency and carrier frequency of a laser. In this work, we have reported a repetition frequency locking of a terahertz QCL emitting around 4.2 THz. When the 6-mm-long laser is operated in continuous wave mode without any locking techniques, the repetition frequency is measured to be ~6.15 GHz with a linewidth of hundred kilohertz. Once a phase lock loop (PLL) is applied to dynamically control the drive current of the QCL, we have demonstrated a successful repetition frequency locking of the laser with a signal to noise ratio of 80 dB. This technique can be employed for the frequency comb and dual-comb operations of terahertz QCLs for high-resolution applications.
Keywords: Terahertz, Quantum cascade laser, Frequency comb, Phase-lock
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