Experimental and theoretical evidence for the existence of photonic bandgaps and selective transmissions in serial loop structures

摘要

We have investigated the electromagnetic band structure, transmission, and phase time through a one-dimensional structure made of loops pasted together with segments of finite length. In this serial loop structure, the loops and segments are constituted of dielectric monomode materials. Analytic expressions are reported for the band structure for a large number N of loops and for transmission coefficients and phase times for any value of N. Experimental and numerical results show the existence of large gaps in these structures. These gaps originate both from the periodicity of the system and the loop resonant states that create zeroes of transmission. The gap widths depend on the lengths of the finite segment and the loop diameters. Defect modes may occur in these bandgaps by introducing defective segments in the structure. The localized states appear as very narrow peaks both in the transmission spectrum and in the transmission phase time of finite serial loop structures. The localized state behavior is analyzed as a function of the length and of the position of the defect segment. The transmission phase measurements enable us to derive the group velocity as well as the density of states in these structures. The experimental results are obtained using coaxial cables in the frequency range of few hundreds of MHz.