Efficient coupling of propagating broadband terahertz radial beams to metal wires

Switchable generation of azimuthally- and radially-polarized terahertz beams from a spintronic terahertz emitter

We propose and demonstrate a method of generating two fundamental terahertz cylindrical vector beams (THz-CVBs), namely the azimuthally- and radially-polarized THz pulses, from a spintronic THz emitter. We begin by presenting that the spintronic emitter generates the HE₂₁ mode, a quadrupole like polarization distribution, when placed between two magnets with opposing polarity. By providing an appropriate mode conversion using a triangular Si prism, we show both from experiment and numerical calculation that we obtain azimuthal and radial THz vector beams. The proposed method facilitates the access of CVBs and paves the way toward sophisticated polarization control in the THz regime.

Theoretical Analysis of Terahertz Dielectric-Loaded Graphene Waveguide

The waveguiding of terahertz surface plasmons by a GaAs strip-loaded graphene waveguide is investigated based on the effective-index method and the finite element method. Modal properties of the effective mode index, modal loss, and cut-off characteristics of higher order modes are investigated. By modulating the Fermi level, the modal properties of the fundamental mode could be adjusted. The accuracy of the effective-index method is verified by a comparison between the analytical results and numerical simulations. Besides the modal properties, the crosstalk between the adjacent waveguides, which determines the device integration density, is studied. The findings show that the effective-index method is highly valid for analyzing dielectric-loaded graphene plasmon waveguides in the terahertz region and may have potential applications in subwavelength tunable integrated photonic devices.

Graphene-Coated Elliptical Nanowires for Low Loss Subwavelength Terahertz Transmission

Graphene has been recently proposed as a promising alternative to support surface plasmons with its superior performances in terahertz and mid-infrared range. Here, we propose a graphene-coated elliptical nanowire (GCENW) structure for subwavelength terahertz waveguiding. The mode properties and their dependence on frequency, nanowire size, permittivity and chemical potential of graphene are studied in detail by using a finite element method, they are also compared with the graphene-coated circular nanowires (GCCNWs). Results showed that the ratio of the long and short axes (b/a) of the elliptical nanowire had significant influence on mode properties, they also showed that a propagation length over 200 μm and a normalized mode area of approximately 10−4~10−3 could be obtained. Increasing b/a could simultaneously achieve both long propagation length and very small full width at half maximum (FWHM) of the focal spots. When b/a = 10, a pair of focal spots about 40 nm could be obtained. Results also showed that the GCENW had a better waveguiding performance when compared with the corresponding GCCNWs. The manipulation of Terahertz (THz) waves at a subwavelength scale using graphene plasmon (GP) may lead to applications in tunable THz components, imaging, and nanophotonics.

Induction of subterahertz surface waves on a metal wire by intense laser interaction with a foil

We have demonstrated that a pulsed electromagnetic wave (Sommerfeld wave) of subterahertz frequency and 11-MV/m field strength can be induced on a metal wire by the interaction of an intense femtosecond laser pule with an adjacent metal foil at a laser intensity of 8.5×10^{18}W/cm^{2}. The polarity of the electric field of this surface wave is opposite to that obtained by the direct interaction of the laser with the wire. Numerical simulations suggest that an electromagnetic wave associated with electron emission from the foil induces the surface wave. A tungsten wire is placed normal to an aluminum foil with a gap so that the wire is not irradiated and damaged by the laser pulse, thus making it possible to generate surface waves on the wire repeatedly.

Single-cycle surface plasmon polaritons on a bare metal wire excited by relativistic electrons

Terahertz (THz) pulses are applied in areas as diverse as materials science, communication and biosensing. Techniques for subwavelength concentration of THz pulses give access to a rapidly growing range of spatial scales and field intensities. Here we experimentally demonstrate a method to generate intense THz pulses on a metal wire, thereby introducing the possibility of wave-guiding and focussing of the full THz pulse energy to subwavelength spotsizes. This enables endoscopic sensing, single-shot subwavelength THz imaging and study of strongly nonlinear THz phenomena. We generate THz surface plasmon polaritons (SPPs) by launching electron bunches onto the tip of a bare metal wire. Bunches with 160 pC charge and ≈6 ps duration yield SPPs with 6–10 ps duration and 0.4±0.1 MV m⁻¹ electric field strength on a 1.5 mm diameter aluminium wire. These are the most intense SPPs reported on a wire. The SPPs are shown to propagate around a 90° bend.

Here, the authors demonstrate how ultra-short bunches of relativistic electrons produce coherent transition radiation at the tip of a thin wire. The radiation then propagates as a powerful surface plasmon polariton along the wire, illustrating the potential of this technique for terahertz plasmonics.

Generation of radially-polarized terahertz pulses for coupling into coaxial waveguides

Coaxial waveguides exhibit no dispersion and therefore can serve as an ideal channel for transmission of broadband THz pulses. Implementation of THz coaxial waveguide systems however requires THz beams with radially-polarized distribution. We demonstrate the launching of THz pulses into coaxial waveguides using the effect of THz pulse generation at semiconductor surfaces. We find that the radial transient photo-currents produced upon optical excitation of the surface at normal incidence radiate a THz pulse with the field distribution matching the mode of the coaxial waveguide. In this simple scheme, the optical excitation beam diameter controls the spatial profile of the generated radially-polarized THz pulse and allows us to achieve efficient coupling into the TEM waveguide mode in a hollow coaxial THz waveguide. The TEM quasi-single mode THz waveguide excitation and non-dispersive propagation of a short THz pulse is verified experimentally by time-resolved near-field mapping of the THz field at the waveguide output.

Thin-wall tubes for coupling terahertz waves to metal wires

Bare metal wires are among the most promising waveguides for guiding terahertz (THz) surface plasmon polaritons. In this study, a thin-wall tube is proposed for coupling THz waves to a metal wire with ultrahigh efficiency, which results from three high mode matchings for the two waveguides: field distributions, polarization directions, and wave vectors. According to the mode-overlap calculation, the coupling efficiency can be always between 84% and 94% when the frequency of THz waves is in the range of 0.2-3 THz and the metal wire radius is 0.5 mm. The maximum efficiency is as high as 94% at 0.5 THz, which is much higher than that obtained by the previous methods. We further conclude that the optimal coupling efficiency can be obtained when the outer tube radius is equal to the wire radius and simultaneously the real propagation constants of modes in the two waveguides are the same.

Perturbative solution for terahertz two-wire metallic waveguides with different radii

We introduce the technique of perturbation of boundary condition to the problem of terahertz two-wire metallic waveguides with different radii. Based on the quasi-TEM analytical mode fields derived by use of Möbius transformation, a concise expression for the complex effective index is obtained analytically. The expression is in good agreement with the simulation result. Further, the dispersion and attenuation are obtained from the expression. In addition, we find a zero value point of the group velocity dispersion around 1.268 THz. The results show that the technique of perturbation of boundary condition is helpful in the analysis and design of terahertz metal waveguide.

Strong sub-terahertz surface waves generated on a metal wire by high-intensity laser pulses

Terahertz pulses trapped as surface waves on a wire waveguide can be flexibly transmitted and focused to sub-wavelength dimensions by using, for example, a tapered tip. This is particularly useful for applications that require high-field pulses. However, the generation of strong terahertz surface waves on a wire waveguide remains a challenge. Here, ultrafast field propagation along a metal wire driven by a femtosecond laser pulse with an intensity of 10¹⁸ W/cm² is characterized by femtosecond electron deflectometry. From experimental and numerical results, we conclude that the field propagating at the speed of light is a half-cycle transverse-magnetic surface wave excited on the wire and a considerable portion of the kinetic energy of laser-produced fast electrons can be transferred to the sub-surface wave. The peak electric field strength of the surface wave and the pulse duration are estimated to be 200 MV/m and 7 ps, respectively.

Asymmetric transmission of radially polarized THz radiation through a double circular grating

We report on unidirectional and asymmetric transmission of radially polarized THz radiation through a dual circular metallic grating with sub-wavelength slits. Unidirectional transmission is shown theoretically for a super-Gaussian incident beam, and an asymmetric transmission is demonstrated experimentally, when the radially polarized beam of 0.1 THz is obtained by converting a linearly polarized beam with a discontinuous phase retarder and a tapered waveguide. The dual grating does not include nonlinear materials, its operation is reciprocal, and analogous to that of some planar metallic gratings.

Highly precise and accurate terahertz polarization measurements based on electro-optic sampling with polarization modulation of probe pulses

We have developed an electro-optic (EO) sampling method with polarization modulation of probe pulses; this method allows us to measure the direction of a terahertz (THz) electric-field vector with a precision of 0.1 mrad in a data acquisition time of 660 ms using a 14.0-kHz repetition rate pulsed light source. Through combination with a THz time-domain spectroscopy technique, a time-dependent two-dimensional THz electric field was obtained. We used a photoelastic modulator for probe-polarization modulation and a (111)-oriented zincblende crystal as the EO crystal. Using the tilted pulse front excitation method with stable regeneratively amplified pulses, we prepared stable and intense THz pulses and performed pulse-by-pulse analog-to-digital conversion of the signals. These techniques significantly reduced statistical errors and enabled sub-mrad THz polarization measurements. We examined the performance of this method by measuring a wire-grid polarizer as a sample. The present method will open a new frontier of high-precision THz polarization sensitive measurements.

Generation of broadband terahertz vortex beams

We propose and demonstrate a method for generating broadband terahertz (THz) vortex beams. We convert a THz radially polarized beam into a THz vortex beam via achromatic polarization optical elements for THz waves and characterize the topological charge of the generated vortex beam by measuring the spatial distribution of the phase of the THz wave at its focal plane. For example, a uniform topological charge of +1 is achieved over a wide frequency range. We also demonstrate that the sign of the topological charge can be easily controlled. By utilizing the orbital angular momentum of the beam, these results open new THz wave technologies for sensing, manipulation, and telecommunication.

Tapered dual elliptical plasmon waveguides as highly efficient terahertz connectors between approximate plate waveguides and two-wire waveguides

We present a tapered dual elliptical plasmon waveguide for terahertz waves. This element is composed of a pair of tapered elliptical metal structures and is especially suitable for the coupling of terahertz waves from an approximate plate waveguide to a two-wire waveguide. The long axes of the two ellipses gradually reduce to the same sizes as the short axes, and thus the two-ellipse structure is now a two-wire waveguide. The slowly tapered structure eliminates the reflection and scattering during the coupling process according to WKB approximation. The numerical result shows that the coupling efficiency of this connector can reach as high as 94%.