Abstract
Wire-grid polarizers (WGPs) have been widely used in various fields, such as polarimetry, imaging, display, spectroscopy, and optical isolation. However, conventional WGPs used in diverse mid-wavelength infrared (MWIR) applications show high reflection losses, which intrinsically arise from high refractive indices of their IR-transmitting substrates, such as silicon (Si) and germanium (Ge). This study demonstrated the enhanced transmittance of a transverse magnetic (TM) wave that surpassed ~80% over the entire MWIR range from 3000 to 5000 nm in a narrow air gap of a WGP, where aluminum (Al) was selectively deposited on a nanopatterned Si substrate using an oblique angle deposition method. Moreover, a higher TM wave transmittance was achieved by reducing the air gaps of the WGPs in the nanopatterns, which were distinctly different from the traditional WGPs comprising metal wires patterned directly on a flat substrate. A finite-difference time-domain simulation was performed to investigate optical properties of the proposed WGPs, which showed that the electric field in the air nanogap was remarkably enhanced. The characteristic performances were further investigated using a combination of an effective medium approximation and an admittance diagram, revealing that the broadband transmission enhancement could be attributed to a combined effect of a strong electric field and a better admittance matching. The approach and results described in this paper hold promise for the design and the fabrication of high-quality WGPs, as well as their numerous applications.
Highlights
Polarizers, which control the polarization state of electromagnetic waves, have played a vital role as an essential element in a wide variety of applications such as polarimetry, imaging, switching, spectroscopy, and display[1,2]
A 2D contour plot of the transverse magnetic (TM)-wave transmittance calculated by finite-difference time-domain (FDTD) software for the proposed Wire-grid polarizers (WGPs) structure is provided as a function of the air gap and wavelength in Fig. 2(a), where a 70-nm-thick Al nanowire is deposited only on top of the Si nanopatterns with a 100-nm period
It shows that the measured transmittance of the psWGP with a 6-nm air gap www.nature.com/scientificreports www.nature.com/scientificreports is higher than 80% over the entire midwavelength infrared (MWIR) range of 3000–5000 nm, whereas the calculated transmittance of the fsWGP structure with an air gap of 6-nm drops to 46.0% (Fig. S2 (b) in Supplementary Information (SI))
Summary
Polarizers, which control the polarization state of electromagnetic waves, have played a vital role as an essential element in a wide variety of applications such as polarimetry, imaging, switching, spectroscopy, and display[1,2]. Conventional polarizers made of thin-film polymers or crystal films are vulnerable to high temperature and constant light illumination, which cause a significant performance degradation over time. Both polymer and crystal materials have strong absorption in the infrared (IR) spectral range, which limits their potential for IR applications. Wire-grid polarizers (WGPs), which consist of a periodic array of subwavelength metallic wires on a transparent substrate, have emerged as an attractive alternative to polymer polarizers because of their compactness, durability, broadband transmission, and wide-angle performance[1,3,4]. The WGPs for TE waves function as a typical metal, where the incident light is reflected. (i.e., the ratio of a parallel transmission (TTM) to a perpendicular transmission (TTE), which is one of the most important characteristics of the WGPs, becomes poor as a result of the greater increase in (TTE)[1,3,5]
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