No abstract

We report an experimental and theoretical study of THz Sommerfeld wave propagation on a single copper wire. THz pulses are optoelectronically generated and launched onto 0.52-mm-diam copper wire, and the guided THz pulses are detected at the end of the wire by a standard photoconductive antenna. Very low attenuation and group velocity dispersion are observed, and the measured radial field amplitude of the Sommerfeld wave is inversely proportional to the radial distance. These results are consistent with theoretical predictions. Experimental results from curved wires show the weakly guiding property of the THz Sommerfeld wave, which will limit its applications.

Via ultrafast optoelectronic THz techniques, we are able to test alternative theories of conduction by precisely measuring the complex conductivity of doped silicon from low frequencies to frequencies higher than the plasma frequency and the carrier damping rate. These results, obtained for both n and p-type samples, spanning a range of more than 2 orders of magnitude in the carrier density, do not fit any standard theory. We only find agreement over the full frequency range with the complex conductivity given by a Cole-Davidson type distribution applied here for the first time to a crystalline semiconductor, and thereby demonstrate that fractal conductivity is not just found in disordered material.[ S0031-9007(96) The frequency dependent complex conductivity is one of the most basic properties describing doped semiconductors. Associated with the conductivity are the key parameters characterizing the dynamics of the free carriers in semiconductors, the plasma frequency v p , and the carrier damping rate G 1͞t, where t is the carrier collision time. Characteristically, v p and G have THz frequencies. Even though the complex conductivity has been a topic of theoretical studies for several decades, complete experimental characterizations from low frequencies to beyond several THz have only started to be performed [1,2].Here we report definitive measurements of the complex conductivity from dc to 2.5 THz on doped silicon. Compared to earlier experimental studies of doped silicon [1,3], these new results have sufficient frequency range and precision to test alternative theories [4][5][6][7][8][9][10][11][12] for the conductivity. However, for both n-and p-type silicon and over a measured range of more than 2 orders of magnitude of the carrier density we do not find agreement with any standard theory, including Drude, lattice-scattering, and impurity-scattering theories. As a result, we were forced to look outside the standard theoretical approaches.Our interest in a Cole-Davidson type distribution was motivated by the close relationship between the Debye theory of dielectric insulators and Drude theory, the simplest theory of electrical conduction. For Debye theory, in response to a step-function electric field, the polarization is established exponentially with a characteristic response time. Similarly, for Drude theory, in response to a stepfunction E field, the current is established exponentially with the carrier collision time. Consequently, in the frequency domain the mathematical representations of these two theories are identical. It has been found experimentally that for relatively low frequencies the complex dielectric constants of disordered materials, such as molecular liquids [13], polymers [14], and more recently ionic glasses [15,16], show better agreement with a modified Debye spectral response, known as the Cole-Davidson (C-D) distribution. In the frequency domain, the C-D distribution corresponds to Debye theory with a fractional exponent b, limited to values between 0 and 1, and reduces to Debye th...

Via THz time-domain spectroscopy, we have measured the absorption and index of refraction of single-crystal 〈110〉 ZnTe from 0.3 to 4.5 THz. We find that the absorption is dominated by two lower-frequency phonon lines at 1.6 and 3.7 THz and not by the transverse-optical (TO) -phonon line at 5.3 THz as previously assumed. However, the index of refraction is determined mainly by the TO-phonon line. Using these data, we discuss a frequency-domain picture of electro-optic detection of THz radiation below the TO-phonon resonance and compare with the photoconductive THz receiver over the same frequency range.

We present an experimental study of the propagation of the THz Zenneck surface wave on an aluminum sheet, now more commonly denoted as the THz surface plasmon ͑TSP͒. Here, the TSP pulse is generated by coupling the THz pulse from a metal parallel-plate waveguide onto the aluminum sheet; the propagated TSP pulse is detected at the output end of the sheet using a standard photoconductive dipole antenna. We separate the associated free-space THz pulse from the TSP pulse using a curved sheet. The observed weakly guided TSP propagation has the expected low group velocity dispersion, but also has anomalously high attenuation and much tighter binding to the metal surface than predicted by Zenneck theory. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2171488͔ Surface electromagnetic ͑EM͒ waves have now been studied for more than a century, starting with Sommerfeld's study of EM propagation on a single metal wire, 1 and including Zenneck's description of EM propagation on a flat metal surface.2 An excellent overview of the early EM surface wave investigations is the work by Barlow and Cullen.3 A more recent work also provides a good description of EM surface wave measurements and experimental techniques.4 A good description of EM surface waves from the equivalent point of view of surface plasmons is given in Ref. 5. Most recently, the study of surface plasmons has been stimulated by the observation of unusually high transmission resonances through thin metal subwavelength hole arrays at optical frequencies, 6 which have now been studied in the optical, infrared and THz regions. Here, we describe an experimental study of THz pulses propagating as surface waves, or equivalently as THz surface plasmons ͑TSP͒, on a metal sheet. We measure a much higher attenuation of the propagating TSP pulses and a much reduced spatial extent of the TSP evanescent field than predicted by theory. In previous work, such pronounced disagreement between theory and experiment has resulted in a long standing and unresolved controversy. [8][9][10][11][12][13][14][15] It has been experimentally difficult to distinguish between freely propagating EM radiation along the surface and the guided surface wave. [8][9][10][11][12][13][14][15] Due to the collinear propagation and equal phase velocities, power transfer between the two waves easily occurs. Extremely flat and optically smooth surfaces appear to be required to obtain the predicted large propagation distances; 9,12 surface roughness has been predicted to bind the wave more tightly to the surface and thereby increase the attenuation.12 Submicron layers of high-index, low-loss dielectrics on the metal surface can reduce the extent of the evanescent field by an order of magnitude, causing much higher propagation loss. 10,11,14 Our experimental study involved measuring TSP propagation on 10-cm-wide by 51-m-thick Al sheets of different lengths with smooth, but not polished surfaces. As shown in Fig. 1͑a͒, the initially freely propagating THz pulses were collimated and focused into the wavegui...