We find a renormalized "time-dependent diffusion coefficient," Dt, for pulsed excitation of a nominally diffusive sample by solving the Bethe-Salpeter equation with recurrent scattering. We observe a crossover in dynamics in the transformation from a quasi-1D to a slab geometry implemented by varying the ratio of the radius, R, to the length, L, of the cylindrical sample with reflecting side walls and open ends. Immediately after the peak of the transmitted pulse, Dtfalls linearly with a nonuniversal slope that approaches an asymptotic value for R/L→1. The value of Dtextrapolated to t→0 depends only upon the dimensionless conductance g for R/L→1 and only upon k '0 for R/L 1, where kis the wave vector and '0 is the bare mean free path.

Negative refraction of acoustic waves in two-dimensional phononic crystals has been demonstrated through both analysis and exact numerical simulation. The methods to achieve this behavior have been discussed. A microsuperlens for acoustic waves has also been designed. It is shown that refractive devices based on phononic crystals behave in a manner similar to that of optical systems. Therefore, a negative square root of the effective density or negative refraction index for acoustic waves can be introduced to describe this phenomena very well as the case of electromagnetic waves in the photonic crystals.

We propose a simple, systematic, and efficient method to enlarge the gap of a photonic crystal. In this method, we add a small fraction of a third component into the existing photonic crystal. The dielectric property of the third component as well as its insertion position in a unit cell are chosen according to the field-energy distribution of two Bloch states at the band edges. The method is very general. It can be applied to any microstructure and is independent of the dimensionality of the original photonic crystal. Thus, it opens up a way to engineer photonic band gaps. Here, we demonstrate the validity of this method explicitly in two dimensions for both s and p waves. We show that, for certain microstructures, absolute gaps can be signifi-cantly enlarged. The method is also demonstrated in microwave experiments.

Based on the two-band model, we present a transfer-matrix treatment of the tunnel conductance and magnetoresistance for tunneling through ferromagnet/insulator (semiconductor) single junctions and double junctions subject to a dc bias. Our results are qualitatively in agreement with the experimental measurements for the single junction. For the double junction, we find that there exists, spin-polarized resonant tunneling and giant tunnel magnetoresistance. The highest value of the magnetoresistance in a double junction can reach 90%. We anticipate that our results will stimulate some interest in experimental efforts in designing spinpolarized resonant-tunneling devices.

We propose a simple and efficient method to engineer absolute negative refraction for both polarizations of electromagnetic wave in two-dimensional photonic crystal by adding an additional component to the existing photonic crystal. The dielectric property of the additional component, as well as its insertion position in a unit cell, are chosen according to the field-energy distribution of Bloch states at the band edges. According to our design, the non-near-field focus and images for both polarized waves at the same structure and parameters, explicitly following the well-known wave-beam negative refraction law, can be realized.