Tao Zhao

Coherent and tunable terahertz radiation from graphene surface plasmon polaritons excited by an electron beam

Although surface plasmon polaritons (SPPs) resonance in graphene can be tuned in the terahertz regime, transforming such SPPs into coherent terahertz radiation has not been achieved. Here, we propose a graphene-based coherent terahertz radiation source with greatly enhanced intensity. The radiation works at room temperature, it is tunable and can cover the whole terahertz regime. The radiation intensity generated with this method is 400 times stronger than that from SPPs at a conventional dielectric or semiconducting surface and is comparable to that from the most advanced photonics source such as a quantum cascade laser. The physical mechanism for this strong radiation is presented. The phase diagrams defining the parameters range for the occurrence of radiation is also shown.

Transformation of surface plasmon polaritons to radiation in graphene in terahertz regime

We demonstrate a concept that allows direct excitation of surface plasmon polaritons (SPPs) by a moving electron bunch above a single layer graphene sheet deposited on a dielectric substrate without any additional coupling requirements. We show that if the two-dimensional current in the graphene is dominated by the third order nonlinear effect when the surface electric field exceeds a moderate strength of ∼5 kV/cm, the SPP mode can cross the light line although the group velocity remains much smaller than the speed of light. This effect gives rise to direct transformation of SPPs into radiation. The underlying mechanism of the crossing of the SPP dispersion into the light line is the energy shift of charged particles in the nonlinear regime and the finite transport scattering time in graphene. Both the energy and lifetime of the SPPs increase with the field intensity. The radiation intensity and frequency can be tuned with an AC bias.

Coherent and tunable terahertz radiation from graphene surface plasmon polarirons excited by cyclotron electron beam

Terahertz (THz) radiation can revolutionize modern science and technology. To this date, it remains big challenges to develop intense, coherent and tunable THz radiation sources that can cover the whole THz frequency region either by means of only electronics (both vacuum electronics and semiconductor electronics) or of only photonics (lasers, for example, quantum cascade laser). Here we present a mechanism which can overcome these difficulties in THz radiation generation. Due to the natural periodicity of 2π of both the circular cylindrical graphene structure and cyclotron electron beam (CEB), the surface plasmon polaritions (SPPs) dispersion can cross the light line of dielectric, making transformation of SPPs into radiation immediately possible. The dual natural periodicity also brings significant excellences to the excitation and the transformation. The fundamental and hybrid SPPs modes can be excited and transformed into radiation. The excited SPPs propagate along the cyclotron trajectory together with the beam and gain energy from the beam continuously. The radiation density is enhanced over 300 times, up to 105 W/cm2. The radiation frequency can be widely tuned by adjusting the beam energy or chemical potential. This mechanism opens a way for developing desired THz radiation sources to cover the whole THz frequency regime.

Electron beam excitation of surface plasmon polaritons.

In this paper, the excitations of surface plasmon polaritons (SPPs) by both perpendicular and parallel electron beam are investigated. The results of analytical theory and numerical calculation show that the mechanisms of these two excitations are essentially different, and the behavior and properties of SPPs in metal structures strongly depend on the methods of excitation. For the perpendicular excitation, SPPs contain plenty of frequency components, propagate with attenuation and are always accompanied with the transition radiation. Whereas for parallel excitation, SPPs waves are coherent, tunable, propagating without attenuation and the transition radiation does not occur. We also show that there are two modes for the parallel excited SPPs on the metal films and they all can be excited efficiently by the parallel moving electron beam. And the operating frequency of SPPs can be tuned in a large frequency range by adjusting the beam energy.

Optical bistability induced by nonlinear surface plasmon polaritons in graphene in terahertz regime

We demonstrate optical bistability in a prism-air-graphene-dielectric structure. Under a moderate electric field in the terahertz frequency regime, the third order nonlinear optical conductivity is comparable to the linear conductivity. The nonlinear conductivity enhances the energy of surface plasmon polaritons. Both the energy and frequency of the surface plasmon polaritons depend on the strength of the nonlinear current in the graphene layer. When considering excitation in the Kretschmann configuration, the reflectance as a function of frequency exhibits bistability. The origin of the bistability is the field dependence of the plasmon mode. We have determined the parameter regime for the occurrence of bistability in this structure.

Cherenkov terahertz radiation from graphene surface plasmon polaritons excited by an electron beam

We demonstrate a mechanism of efficiently transforming surface plasmon polaritons (SPPs) into Cherenkov terahertz (THz) radiation. In a structure where multilayer graphene is deposited on a dielectric substrate with a buffer layer, the energy of the SPPs can be significantly enhanced. The dispersion of SPPs crosses the light line of the substrate if the buffer layer has a low permittivity relative to the substrate. As a result, the SPPs can be readily transformed into radiation without the need of wavevector compensation. Compared to the radiation from structures without graphene, the radiation power density is enhanced by nearly three orders of magnitude due to the field enhancement of SPPs. Our results could provide a promising way for developing room temperature, tunable, coherent, and intense THz radiation sources to cover the entire THz regime.

Mediated coupling of surface plasmon polaritons by a moving electron beam

The mediated coupling of surface plasmon polaritons (SPPs) by a parallel moving electron beam is demonstrated in this paper. The theoretical analysis shows that the electron beam excited spoof surface plasmon polaritons (SSPs) on the grating placed above the metal films play the role as the excitation source in the mediated coupling. The numerical calculations and particle-in-cell simulations demonstrate the significant advantages of the SSPs mediately coupled SPPs in contrast with that coupled by the parallel moving electron beam directly. The photo density of the mediately coupled SPPs reaches up to 106 per cm2 for the electron beam with the charge density 100 nC/cm, which is two orders of magnitude larger than that of the directly coupled SPPs. The tuning band of the mediately coupled SPPs reaches up to 9% for the beam energy ranging from 10 keV to 30 keV, while it almost cannot be tuned for the direct coupling. The lifetime of the mediately coupled SPPs, which reaches up to hundreds of femtoseconds, is also much longer. Accordingly, the mediated coupling may bring great significances for the applications of SPPs.

Plasmon modes of circular cylindrical double-layer graphene.

In this paper, a theoretical investigation on plasmon modes in a circular cylindrical double-layer graphene structure is presented. Due to the interlayer electromagnetic interaction, there exist two branches of plasmon modes, the optical plasmon mode and the acoustic plasmon mode. The characteristics of these two modes, such as mode pattern, effective mode index and propagation loss, are analyzed. The modal behaviors can be effectively tuned by changing the distance between two graphene layers, the chemical potential of graphene and the permittivity of interlayer dielectric. Importantly, the breakup of tradeoff between mode confinement and propagation loss is discovered in the distance-dependent modal behavior, which originates from the unique dispersion properties of a double-layer graphene system. As a consequence, both strong mode confinement and longer propagation length can be achieved. Our results may provide good opportunities for developing applications based on graphene plasmonics in circular cylindrical structure.