We report a significant increase in the reflectivity of a soft x-ray Co/C multilayer at grazing incidence after low-temperature annealing. By monitoring the enhancement of the first-order modulation peak on annealing, an effective interdiffusion coefficient as low as has been measured, which is nearly equal to the true macroscopic interdiffusion coefficient as the modulation wavelength of the Co/C multilayer studied is well above the critical wavelength. The negative true macroscopic interdiffusion coefficient indicates that there is a tendency to phase separation in the Co - C system, which can be interpreted as the positive enthalpy of mixing calculated on the basis of Miedema's macroscopic atom model. The decrease in reflectivity above some annealing temperature is also discussed and explained as the competition between interfacial sharpness and roughness. This is the first report of observations of enhanced reflectivity in the Co/C multilayered system.

Low cost terahertz filters (copper gratings) have been fabricated by using plastic substrates and printed circuit board processes. The optical properties of these gratings have been investigated. These sub-wavelength copper film gratings with various periods display remarkably enhanced transmission over the terahertz wavelength range. The maximum amplitude transmission efficiency reaches 86% for the grating with a period of 500 µm. The resonance peaks shift to lower frequencies on increasing the period of the gratings. By analyzing three kinds of models, we demonstrate that the excitation of surface plasmons on the upper and lower interfaces of the grating is the prime mechanism responsible for the extraordinary transmission. Various phase shifts between samples and the bare substrate have been detected, leading to the transmission dip frequencies of our experimental results deviating from the theoretical predictions of a surface plasmon polariton model. In addition, apparent angle-dependent behaviors have been observed in the transmission spectra of the grating with the period of 500 µm and the slit width of 180 µm, which confirms that the copper film grating is a promising geometry for producing terahertz polarizers.