Interest of using metal wire structures as transmission-lines in frequencies much higher than microwave region is recently arised. In this theoretical study guided waves in circular metal wire is investigated in infrared frequency range or even in optical region for transmission-line applications. In this frequency region the
permittivity of the metal is not any more described by the conductivity of the metal but obeys the Drude model. The operational frequency region is between the electron scattering frequency and the plasma frequency. Just below the plasma frequency the permittivity is negative and almost a real number. The axial field components are written in terms of modified Bessel functions. The eigenvalue equation is evaluated for the guided modes by equating the continuity condition of the tangential field components at the interface of the free space and the wire region. The dispersion curves are calculated numerically and the propagation factors inside and outside of the wire region is illustrated as well as the propagation factor in axial direction. Also the field distribution in the cross-section of the structure is analysed. Analysing the properties of the propagating
fields in metal wire, it is found that some modes are more and more tightly bound into the surface of the metal wire when the frequency approaces to the plasma resonant frequency. Using typical values for metals, the proper radius of the wire is in a range of about a hundred nanometers which is much below the wavelength of the guided mode even in optical region. The effect is analogous to that for guided fields in microwave frequency range for the circularly symmetric mode propagating in the metal wire surrounded by a thin dielectric layer. Now the similar effect is occured in subwavelenth region due to plasma phenomena. In this study it is demonstrated that subwavelength metal wire structures may have applications as transmission-line structures in infrared or optical region.