Microwave pulse delay at propagation through the 1D electromagnetic crystals

Abstract

Interest to electromagnetic pulses propagation in matter with the group velocity much smaller than the speed of light in vacuum increases recent decades. Effect of group velocity reduction for a wave in a spatially periodic matter (natural crystal, photonic crystal, etc.) exists for different ranges and variety of waves (X-ray, optical, THz and microwave) due to generality of laws of wave diffraction. Multiple stu-dies report significant reduction of group velocity v gr of an X-ray pulse passing through a crystal, when Bragg diffraction occurs [1]. A neutron delay in crystal in conditions of diffraction was predicted and observed [2]. Slow light, which is a promising solution for time-domain processing of optical signals, is a subject for multiple studies [3]. Experimental observation of this effect for microwave radiation generated by an electron beam in the photonic crystal built of metal threads was reported in [4]. and its frequency response (b) In the present report the experiment to measure microwave pulse delay in a specially designed photonic crystal (spatially periodic structure built of metal rods inside a WR90/R100 waveguide of 1 m length) is presented. Each crystal plane comprises 3 copper rods of 1.25 mm diameter placed normally to the wider waveguide walls (Fig. 1, a). The crystal period d = 23.2 mm corresponds to the half-wavelength in the waveguide for 9.15 GHz frequency. The microwave field structure in the waveguide corresponds to TE 10 mode of a rectangular waveguide. Since the delay time is proportional to the length of photonic crystal, the number of periods N was chosen to be rather high (N = 33) to make the delay time convenient for measuring. The photonic crystal frequency response is presented at Fig. 1, b and shows some amplitude ripples, which number is determined by the number of crystal periods.Microwave pulse propagation in the photonic crystal was simulated by CST Microwave Studio [5] to define the incident pulse width making pulse delay effect evident. The simulation of transmitted and reflected pulses in time-domain was produced at the incident pulse duration from 4 to 30 ns. It was shown that delay effect is well resolved, when the width of the incident pulse is 10 ns and shorter. The time distance between maxima of transmitted and reflected pulses is about 23 ns (Fig. 2), that corresponds to pulse delay time W. Both the transmitted and reflected pulses have chain of equidistant peaks with time interval between adjacent those of about 2W.