Kanglin Wang

Metal wires for terahertz wave guiding

Sources and systems for far-infrared or terahertz (1 THz = 1012 Hz) radiation have received extensive attention in recent years, with applications in sensing, imaging and spectroscopy. Terahertz radiation bridges the gap between the microwave and optical regimes, and offers significant scientific and technological potential in many fields. However, waveguiding in this intermediate spectral region still remains a challenge. Neither conventional metal waveguides for microwave radiation, nor dielectric fibres for visible and near-infrared radiation can be used to guide terahertz waves over a long distance, owing to the high loss from the finite conductivity of metals or the high absorption coefficient of dielectric materials in this spectral range. Furthermore, the extensive use of broadband pulses in the terahertz regime imposes an additional constraint of low dispersion, which is necessary for compatibility with spectroscopic applications. Here we show how a simple waveguide, namely a bare metal wire, can be used to transport terahertz pulses with virtually no dispersion, low attenuation, and with remarkable structural simplicity. As an example of this new waveguiding structure, we demonstrate an endoscope for terahertz pulses.

Enhanced coupling of terahertz radiation to cylindrical wire waveguides

Wire waveguides have recently been shown to be valuable for transporting pulsed terahertz radiation. This technique relies on the use of a scattering mechanism for input coupling. A radially polarized surface wave is excited when a linearly polarized terahertz pulse is focused on the gap between the wire waveguide and another metal structure. We calculate the input coupling efficiency using a simulation based on the Finite Element Method (FEM). Additional FEM results indicate that enhanced coupling efficiency can be achieved through the use of a radially symmetric photoconductive antenna. Experimental results confirm that such an antenna can generate terahertz radiation which couples to the radial waveguide mode with greatly improved efficiency.

Antenna effects in terahertz apertureless near-field optical microscopy

We have performed measurements on terahertz (THz) apertureless near-field microscopy that show that the temporal shape of the observed near-field signals is approximately proportional to the time-integral of the incident field. Associated with this signal change is a bandwidth reduction by approximately a factor of 3 which is observed using both a near-field detection technique and a far-field detection technique. Using a dipole antenna model, it is shown how the observed effects can be explained by the signal filtering properties of the metal tips used in the experiments.

GUIDED PROPAGATION OF TERAHERTZ PULSES ON METAL WIRES

We demonstrate a new waveguiding structure for terahertz (THz) radiation in which broadband THz pulses are confined and guided along a bare metal wire. The propagation of THz pulses on such a waveguide is characterized with a fiber-coupled terahertz time-domain spectroscopy system. Free-space THz radiation is coupled onto the waveguide at different positions along the wire, and spatially resolved detection of the electric field of the guided wave is performed at the end of the wire. This waveguide exhibits the lowest attenuation of any waveguide for broadband THz pulses reported so far because of the minimal exposed metallic surface area. It also supports propagation of broadband radiation with negligible group-velocity dispersion, making it especially suitable for use in THz sensing and diagnostic systems. In addition, the structural simplicity lends itself naturally to the facile manipulation of the guided pulses, including coupling, directing, and beam splitting. These results can be described in terms of a model developed by Sommerfeld for waves propagating along the surface of a cylindrical conductor.

Propagation effects in apertureless near-field optical antennas

We report a direct observation of the electromagnetic wave propagation on optical antennas. A free-space broadband terahertz pulse is coupled into a propagating mode along the shaft of an optical antenna acting as an apertureless near-field probe. We determine the spatial extent of this guided mode and its velocity. In addition, we consider the possibility of multiply reflected modes propagating along the near-field probe, an effect which will be significant in near-field spectroscopic measurements.

Finite-Element Method Simulations of Guided Wave Phenomena at Terahertz Frequencies

As the science and engineering associated with terahertz time-domain spectroscopy and imaging evolves past the use of conventional free-space optics, the continued development of waveguides for terahertz pulses is increasingly relevant. The ability to model and simulate terahertz wave propagation aids in the development, visualization, and understanding of novel terahertz devices and phenomena. We discuss the use of the finite-element method, a powerful computational tool for the modeling of guided wave phenomena and devices at terahertz frequencies.

Frequency-dependent radiation patterns emitted by THz plasmons on finite length cylindrical metal wires

We report on the emission patterns from THz plasmons propagating towards the end of cylindrical metal waveguides. Such waveguides exhibit low loss and dispersion, but little is known about the dynamics of the terahertz radiation at the end of the waveguide, specifically in the near- and intermediate-field. Our experimental results and numerical simulations show that the near- and intermediate-field terahertz spectra, measured at the end of the waveguide, vary with the position relative to the waveguide. This is explained by the frequency-dependent diffraction occurring at the end of the cylindrical waveguide. Our results show that near-field changes in the frequency content of THz pulses for increasing wire-detector distances must be taken into account when studying surface waves on cylindrical waveguides.

Characterization of apparent superluminal effects in the focus of an axicon lens using terahertz time-domain spectroscopy

We describe time-resolved measurements of the propagating interference pattern at the focus of an axicon lens. The technique of terahertz time-domain spectroscopy permits a direct observation of the electric field within the line focus formed by a converging conical wave front. We extract the apparent superluminal velocity of this pulse from the peak of the time-domain waveform, and also in the frequency domain by Fourier transform. From these measurements, we conclude that pulse reshaping, arising from strong frequency-dependent propagation effects, does not play a substantial role. As a result, tracking the peak of the pulse in the time domain is a valid method for determining the average group velocity of the disturbance. We also elucidate a new mechanism which influences the value of the superluminal velocity, involving the curvature of the incident wave front.

Metal wire waveguides for broadband terahertz pulses

Guided propagation of terahertz pulses on metal wires, characterized with the direct measurement of the electric field in the time domain, shows record low loss and negligible group velocity dispersion.

Dispersionless terahertz waveguides

We report a new type of quasi-optic waveguide for broadband terahertz pulses, which shows low loss, negligible group velocity dispersion, and remarkable structural simplicity. We characterize the spatial and temporal behavior of the guided mode