Effect of a dielectric coating on terahertz surface plasmon polaritons on metal wires

Abstract

The authors present measurements and calculations on the effect of thin dielectric coatings on the propagation of terahertz pulses along the surface of metal wires. Our measurements show that propagation over only a few centimeters of wire having a thin dielectric coating, strongly distorts the terahertz pulse, which results in a several tens of picoseconds long chirped signal. We demonstrate that the terahertz pulses propagate along the wire as surface waves, and show how a thin coating of a nondispersive material makes this propagation strongly dispersive, giving rise to the chirped signal observed in the measurements. Our results show the potential of terahertz surface plasmon polaritons on metal wires for the sensitive detection of thin dielectric layers. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.2011773͔ Recently, there has been an increased interest in the search for a good waveguide for the transportation of terahertz radiation. [1][2][3][4][5][6] The latest development in this field is the propagation of terahertz waves along bare metal wires with very little absorption and dispersion. 7,8 Indeed, measurements were shown in which two metal wires were combined to form what could eventually become a medical probe. Many metal wires, however, are not bare. Thin dielectric layers can often be found on the surface of metal wires, applied intentionally or unintentionally through oxidation or contamination, and the effects of these layers on the propagation of terahertz pulses along the wire are not wellknown.Here we show measurements and calculations on the propagation of terahertz pulses over copper wires with and without a thin polyurethane coating. Our time-domain measurements of a terahertz pulse propagating along a 4 cm long wire show that a coating of tens of micrometers thickness strongly distorts the terahertz pulse resulting in a chirped terahertz signal that lasts tens of picoseconds. A comparison with calculations based on Maxwell's equations shows that the terahertz pulses propagate along the wire as surface plasmon polaritons, and that the distortion of the terahertz pulse originates from the dispersive propagation of these waves along the coated wire. Remarkably, the propagation is dispersive, although we assume that the coating material itself has a frequency-independent refractive index. Our work indicates that thin coatings can seriously distort terahertz pulses propagating along metal wires. At the same time, however, this offers the possibility of using metal wires as sensitive detectors of thin layers. Figure 1 shows a schematic drawing of our setup. The terahertz pulses from our emitter are focused onto one of two types of metal wires. The first wire is a bare copper wire with a diameter of 1 mm. The second wire is a copper wire with a diameter of 1 mm having a polyurethane coating, specified to be about 34 m thick.9 A sharp copper needle is used to couple the terahertz radiation onto the wire. 8 The terahertz surface plasmon polariton propagates over the wire toward the detecti...

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