Characterization of Wire Grid Polarizers for IR Spectroscopy

Moxtek wafer-scale aluminum nanowire patterning capabilities were used to develop broadband wire grid polarizer (WGP) products suitable for MWIR and LWIR (mid- and long-wavelength infrared) spectroscopic applications that need a high contrast. These WGP products were developed on thin AR-coated silicon substrates. The performance of the Moxtek WGP was done from SWIR to LWIR and showed considerable improvement over competing WGP products, particularly with regards to the contrast ratio between blocking and passing state polarizer transmittance.

Introduction

Polarizers find applications in IR spectroscopy for the characterization of metallic and dielectric coatings, analyzing birefringent materials and for vibrational mode indexing in textured and crystal films. A real polarizer has a finite contrast ratio, implying that a certain amount of light with an unwanted polarization state is transmitted, which can affect measurement accuracy. The WGP comprises an array of metallic lines with sub-wavelength pitch that are mostly supported by a transparent substrate. The advantages of WGP over competing designs include a compact form factor with enhanced passing state transmittance, minimal performance variation with angle of incidence or wavelength, and high stability in high temperature and high brightness environments. However, WGP products that have been designed for LWIR and MWIR applications have suffered from low contrast between transmission of linearly polarized light oriented in the passing and blocking states, which is especially due to their considerably large wire grid pitch (typically ≥ 370nm). Moxtek has developed LWIR and MWIR polarizers with a greatly improved contrast by drastically bringing down the pitch from that present in typical IR WGP products to 144nm.

Experimental Conditions

Both the CARY 60 FTIR spectrometer and the Nexus 870 were used for transmission experiments. WGP sample rotation with respect to a fixed pre-analyzer permitted transmittance measurement in the passing and blocking states. Two aligned Moxtek WGP’s both either LWIR or MWIR separated by a gap of over 100µm constitute the pre-analyzers. The validation of instrument accuracy for transmission measurements was done using open and blocked beam configurations.

Results

Moxtek’s MWIR polarizer maintains a contrast ratio better than 35dB from 3 to 5.5µm and transmits better than the passing state between 3.5 and 5.5µm. The Moxtek LWIR polarizer transmits typically better than 68% of the passing polarization state between 7 and 15µm and has a contrast ratio exceeding 38.5dB. The performance of the 144nm pitch Moxtek products to a 250nm pitch WGP from another supplier is compared and clearly shows the dramatic improvement in contrast ratio.

Conclusions

WGPs have several benefits over other designs when used as analyzers in infrared spectroscopic applications, but have historically suffered from poor contrast between transmittance in the passing and blocking states. In order to address these limitations Moxtek has extended its aluminum Nanowire^® patterning capabilities to AR-coated silicon substrates developed for the mid- and long-wavelength infrared. These new product offerings offer extraordinary improvements in contrast while maintaining excellent efficiency in passing state transmittance.

Table 1. Performance comparison between Moxtek Infrared wire grid polarizers on AR-coated Silicon and a competing product on Thallium Bromoiodide (KRS-5) substrate.

This information has been sourced, reviewed and adapted from materials provided by Moxtek, Inc.

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