We present an imaging system designed for use in the terahertz range. As the radiation source a backward-wave oscillator was chosen for its special features such as high output power, good wave-front quality, good stability, and wavelength tunability from 520 to 710 GHz. Detection is achieved with a pyroelectric sensor operated at room temperature. The alignment procedure for the optical elements is described, and several methods to reduce the etalon effect that are inherent in monochromatic sources are discussed. The terahertz spot size in the sample plane is 550 microm (nearly the diffraction limit), and the signal-to-noise ratio is 10,000:1; other characteristics were also measured and are presented in detail. A number of preliminary applications are also shown that cover various areas: nondestructive real-time testing for plastic tubes and packaging seals; biological terahertz imaging of fresh, frozen, or freeze-dried samples; paraffin-embedded specimens of cancer tissue; and measurement of the absorption coefficient of water by use of a wedge-shaped cell.

No abstract

Stress development, relaxation, and memory in colloidal dispersions: Transient nonlinear microrheology A colloidal crystal modeled by bead-spring cubesWe present a report about a new approach that can be used to describe the single-particle dynamics of colloidal crystals. This approach regards the colloidal crystal as a classical bead-spring lattice immersed in viscous fluid. In this model, the mean square displacement of a particle ͑MSD͒ and the mean product of displacement of a particle and that of another particle (x-MSD) are obtained exactly using the Langevin treatmentlike method. In other words, MSD and x-MSD are, respectively, an autocorrelation function of a particle and a cross-correlation function of two particles. As the first-order approximation of hydrodynamic interaction, effective Stokes' viscous drag coefficient ␥ eff is introduced to the model that includes all of the hydrodynamic effects due to the presence of all other particles. As a result of the viscous media, traveling phonon modes are transformed into relaxation modes, and the motion of a particle is comprehended as a superposition of these relaxation modes. The predicted MSD for face-centered-cubic lattice type crystals is in good agreement with the MSD observed by Bongers et al. ͓J. Chem. Phys. 104, 1519 ͑1996͔͒. As no experimental study of x-MSD has been published to date, the validity of the predicted x-MSD remains to be evaluated. Moreover, it has been demonstrated that, in the case of dϭ1, dϭ2, and dу3 ͑where d is the dimension of the system͒, MSD and x-MSD diverge, logarithmically diverge and converge, respectively. The presented results show that bead-spring lattices immersed in viscous media are unstable, quasistable, and stable, in the case of dϭ1, dϭ2, and dу3, respectively. These properties of the model are in agreement with the widely believed notions regarding how the dimension of a system affects the stability of a crystal according to solid state physics, as well as statistical mechanics. The presented model may be utilized to account for the elastic properties of colloidal crystals, such as the bulk modulus; the single-particle dynamics of colloidal crystals are also accounted for. The presented model may therefore lead to a better understanding of various macroscopic phenomena in which the corrective motion of particles or the effects of fluctuations play key roles.

We have measured the mean square displacement of a particle (MSD) and mean product of displacement of a particle and that of another particle (x-MSD) in two-dimensional (2d) and three-dimensional (3d) colloidal crystals for the first time using digital video microscopy. These (x-)MSDs have been compared to an overdamped bead-spring lattice model with effective viscous drag γeff (the OBS-γeff model). The observed 3d system contained ordered structures created by sedimentation equilibrium adjacent to the internal cell wall and was regarded as the (1 1 1) surface of a face-centered cubic lattice. The observed 2d system was a single layer of hexagonal-like ordered structures which were generated in a certain region of the space between the flat surface of the internal cell wall and a convex lens. In the observed time region, the MSDs for 2d system and 3d system were in good agreement with the theoretical MSDs for 2d system and 3d system, which were predicted to logarithmically diverge and to converge, respectively. The observed x-MSDs for the 2d system were in disagreement with theoretical ones in short time behavior. For the 3d system, the disagreement between the theory and the experimental results were found in the amplitudes as well as in the short time behavior. It was assumed that these differences were caused by the hydrodynamic coupling which was not fully incorporated in the OBS-γeff, and an improved treatment, the OBS-cutoff model was introduced. The OBS-γeff elongates the relaxation time of each mode with the constant ratio of γeff/γ, while the OBS-cutoff assumes the relaxation time of the modes to be infinity if the wave number of a mode is larger than a certain cutoff wave number. For the 3d system, the MSD and x-MSD for nearest neighbors obtained from the OBS-cutoff were in excellent agreement with the observed ones. In addition, it was found that the hydrodynamic interaction was the dominant factor on the short time behavior of (x-)MSD.

No abstract