Forward-wave enhanced radiation in the terahertz electron cyclotron maser

Based on the principle of electron cyclotron maser(ECM),gyrotrons are among the most promising devices to generate powerful coherent terahertz(THz)radiation and play a vital role in numerous advanced THz applications.Unfortunately,THz ECM systems using a conventional high-Q cavity were theoretically and experimentally demonstrated to suffer from strong ohmic losses,and,accordingly,the wave output efficiency was significantly reduced.A scheme to alleviate such a challenging problem is systematically investigated in this paper.The traveling-wave operation concept is employed in a 1-THz third harmonic gyrotron oscillator,which strengthens electron-wave interaction efficiency and reduces the ohmic dissipation,simultaneously.A lossy belt is added in the interaction circuit to stably constitute the traveling-wave interaction,and a down-tapered magnetic field is employed to further amplify the forward-wave(FW)component.The results demonstrate that the proportion of ohmic losses is nearly halved,and output efficiency is nearly doubled,which is promising for further advancement of high-power continuous-wave operation of the ECM-based devices.

Generation of polarization-multiplexed terahertz orbital angular momentum combs via all-silicon metasurfaces

Electromagnetic waves carrying orbital angular momentum(OAM),namely OAM beams,are important in various fields including optics,communications,and quantum information.However,most current schemes can only generate single or several simple OAM modes.Multi-mode OAM beams are rarely seen.This paper proposes a scheme to design metasurfaces that can generate multiple polarization-multiplexed OAM modes with equal intervals and intensities(i.e.,OAM combs)working in the terahertz(THz)range.As a proof of concept,we first design a metasurface to generate a pair of polarization-multiplexed OAM combs with arbitrary mode numbers.Furthermore,another metasurface is proposed to realize a pair of polarization-multiplexed OAM combs with arbitrary locations and intervals in the OAM spectrum.Experimental results agree well with full-wave simulations,verifying a great performance of OAM combs generation.Our method may provide a new solution to designing high-capacity THz devices used in multi-mode communication systems.

Broadband infinite-Q plasmons enable intense Smith–Purcell radiation

With the rapid development of nanophotonics for enhancing free-electron radiation,bound states in the continuum(BICs)have emerged as a promising approach for emitting intense Smith–Purcell radiation(SPR)with enhanced intensity.However,current BIC-based methods are limited to single-frequency operation,thereby restricting their applications requiring spectral and angular tunability,such as particle detectors and light sources.To overcome this limitation,this work proposes an approach for constructing plasmonic BICs over a broad spectral range in symmetry-broken systems.By leveraging the high-Q resonances near the BICs,we achieve intense SPR with broadband tunability,potentially improving the radiation intensity by six orders compared to traditional methods.Experimentally,we validate the construction of BIC using plasmonic antennas and achieve broadband demonstration.Our proposed concept can be extended to other plasmonic or guided-wave systems,paving the way toward compact and efficient free-electron sources in hard-to-reach frequency regimes.