Polarization reflection analysis of a binary phase grating filter on an ellipsoidal collector

This Letter systematically discusses the polarization properties of binary phase gratings integrated on an ellipsoidal collector in laser-produced plasma (LPP) light sources. Here, grating structures suppress 10.6 μm IR wavelength while transmitting EUV or X-ray radiation to the following illumination or imaging systems. IR-suppression efficiency is especially critical for the lifespan and spectral purity in high-power LPP systems such as EUV or X-ray sources. Simulations using FEM and Fourier optics with the thin-element approximation (TEA) are employed and compared to study the IR reflectance phenomenon of the grating, revealing differences in diffraction behaviors between s- and p-polarization. With respect to a large incidence angle of IR rays, gratings with adjusted depth are investigated to achieve an optimal IR suppression. Furthermore, we propose a novel, to our knowledge, intuitive "turned wave" model to explain the oscillation characteristics of p-polarized reflectance, which helps to mathematically describe the periodical "in-stabilization" mechanism of the electromagnetic field. The simulation is in validation with a proper experiment, and the IR reflection is suppressed to a ratio of 10^-4 level. In particular, p-polarized IR light tends to exhibit lower 0th-order reflectance than that of s-polarized light. Besides, fabrication tolerances and spectral linewidth are evaluated for practical applications.