Polarization-maintaining reflection-mode THz time-domain spectroscopy of a polyimide based ultra-thin narrow-band metamaterial absorber

This paper reports the design, the microfabrication and the experimental characterization of an ultra-thin narrow-band metamaterial absorber at terahertz frequencies. The metamaterial device is composed of a highly flexible polyimide spacer included between a top electric ring resonator with a four-fold rotational symmetry and a bottom ground plane that avoids misalignment problems. Its performance has been experimentally demonstrated by a custom polarization-maintaining reflection-mode terahertz time-domain spectroscopy system properly designed in order to reach a collimated configuration of the terahertz beam. The dependence of the spectral characteristics of this metamaterial absorber has been evaluated on the azimuthal angle under oblique incidence. The obtained absorbance levels are comprised between 67% and 74% at 1.092 THz and the polarization insensitivity has been verified in transverse electric polarization. This offers potential prospects in terahertz imaging, in terahertz stealth technology, in substance identification, and in non-planar applications. The proposed compact experimental set-up can be applied to investigate arbitrary polarization-sensitive terahertz devices under oblique incidence, allowing for a wide reproducibility of the measurements

Temperature behavior of optical absorption bands in colored LiF crystals

We measured the optical absorption spectra of thermally treated, gamma irradiated LiF crystals, as a function of temperature in the range 16–300 K. The temperature dependence of intensity, peak position and bandwidth of F and M absorption bands were obtained. Keywords: Lithium fluoride, Optical absorption, Low temperature, Color center

Cancellation of Fabry-Perot interference effects in terahertz time-domain spectroscopy of optically thin samples

Terahertz time-domain spectroscopy is increasingly used in many fields of research. For strongly absorbing materials with refraction index close to 1, optical parameters at terahertz frequencies are most conveniently quantified using transmission measurements through thin samples. Unfortunately, extracting optical parameters from raw data implies the use and/or development of complicated numerical data processing procedures. In this work we present an efficient computational procedure for extracting the optical parameters in very thin samples (≲100μm) from transmission terahertz time-domain spectroscopy. In our procedure, we are able to successfully remove from raw data the Fabry-Perot interference effects, which are commonly recognized to be the leading cause of inaccuracy in the extracted parameters, introducing fictitious oscillations in their frequency dependence. The procedure is based on the Davidenko method to identify the roots of complex functions used to numerically solve the implicit equation obtained by equating the experimental and theoretical transfer functions. The advantage of the method is the possibility of obtaining the roots using the numerical solution of a system of real differential equations using standard mathematical packages. In addition, we show that complete removal of the Fabry-Perot oscillations is achieved by including in the computational procedure, besides the sample thickness, the instrumental error on the starting instant of the terahertz signal sampling. This error could be common to many terahertz time-domain systems, especially those using optical fibers. This correction is necessary in general to preserve the terahertz spectroscopic features in the extracted optical parameters for strongly absorbing materials with refraction index close to 1, such as water, biological matter, and several organic materials

Resistivity tensor in the normal state of BSCCO single crystals as a function of doping

We present measurements of the resistivity tensor of Bi2Sr2CaCu2O8+δ single crystals with different oxygen concentrations. Sample doping varies from underdoped to slightly overdoped. Measurements are performed through multiterminal technique, which allows for a simultaneous determination of both in-plane and out-of-plane components of the resistivity tensor. Data are analyzed in terms of a model that assumes two different mechanisms for the out-of-plane conduction, markedly thermal activation and incoherent tunneling. Within this model we are able to describe data of normal state resistivity for all samples with different doping levels. We also analyze data from the literature. In all cases, the proposed model describes very well the data in the normal state

Excess conductivity of overdoped Bi2Sr2CaCu2O8+x crystals well above Tc.

We have used a multiterminal technique in order to measure the (a,b) plane excess conductivity Δσ in several Bi2Sr2CaCu2O8+x single crystals. We find that the experimental Δσ does not follow a simple power law Δσ∼ε-α, with ε=ln(T/Tc), and that it drops faster than the two-dimensional Aslamazov-Larkin law, α=1, with increasing temperature. In addition, data for samples with different doping do not scale on a universal curve. We discuss our data in terms of microscopic and Ginzburg-Landau theories, where high-momentum fluctuations are either not excited, or phenomenologically cut off. The experimental Δσ drops even faster than the prediction of the extended microscopic theory. However, we can accurately describe all our data up to T≈1.3 Tc with the GL theory, assuming a sample-dependent cutoff value. We relate the cutoff parameter to the doping level of our samples

Study of the vortex dynamics as a function of frequency in the microwave range

The frequency dependence of the transport properties provides noticeable informations on the vortex matter in superconductors. In particular, low (<100 MHz) and high (of order 10 GHz) frequency response give different informations on the vortex dynamics: while at low frequency large distance motion of vortices is predominant, at higher frequencies vortices tend to oscillate around their equilibrium positions. Few reports have been presented in the past related to the study of the intermediate frequency regime, and to the evolution of one regime into the other. We present here an experimental study as a function of frequency, in the range 1-20 GHz, over a continuous frequency spectrum (Corbino disc technique), in presence of an applied magnetic field. Data are presented for both YBa2Cu3O7-delta and MgB2 superconductors. We also discuss the data as a function of frequency and show to what extent existing theories are able to fit the measured data. (C) 2004 Elsevier B.V. All rights reserved

Normal state resistivity of BSCCO single crystals: description with a two barriers model

We present dc resistivity multi-terminal measurements performed in Bi2Sr2CaCu2O8+delta single crystal. Sample doping was varied from underdoped to slightly overdoped. Data are analyzed in term of a model which assumes two different mechanisms for the out-of-plane conduction, markedly thermal activation and incoherent tunneling. Within this model we are able to describe data of normal state resistivity. We also analyze data from the literature. In all cases, the proposed model describes very well the data in the normal state. (C) 2003 Elsevier Science B.V. All rights reserved

Normal-state c-axis transport in Bi2Sr2CaCu2O8+delta: Interlayer tunneling and thermally activated dissipation

We systematically investigate the effect of doping on the temperature dependence of the c-axis resistivity in Bi2Sr2CaCu2O8+delta. We present simultaneous measurements of the resistivity tensor components rho(c) and rho(ab) at different doping from the underdoped to the overdoped regime. The c-axis resistivity behaviors as a function of temperature are interpreted, in the normal state (T>T*), through a single phenomenological model based on the existence of two energy barriers with different heights and widths. Two complementary processes are considered for each barrier: incoherent tunneling and thermal activation. The resistive measurements at different doping are well fitted with a small number of free parameters with well defined physical meanings. The analysis of the behaviors of the fitting parameters as a function of doping gives support and consistency to the two-barrier model