Broadband wire grid polarizer (WGP) products suitable for MWIR and LWIR (mid- and long-wavelength infrared) spectroscopic
applications requiring high contrast were developed on thin, AR-coated silicon substrates using Moxtek wafer-scale aluminum
NanowireŽ patterning capabilities. The Moxtek WGP performance was characterized from the SWIR to LWIR and shows marked improvement
over competing WGP products, especially in terms of the contrast ratio between passing and blocking state polarizer transmittance.
Polarizers are used in IR spectroscopy for characterization of dielectric and metallic coatings, analysis of birefringent
materials, and for indexing the vibrational modes in crystals and textured films. A real polarizer has a finite contrast ratio,
thus some light with the unwanted polarization state is transmitted, which can impact measurement accuracy. The wire grid
polarizer (WGP) is often preferred for IR applications, and consists of an array of metallic lines with sub-wavelength pitch,
often supported by a transparent substrate. Benefits of the WGP over competing designs include a compact form factor with
improved passing state transmittance, minimal performance variation with angle of incidence or wavelength, and improved stability
in high temperature and high brightness environments. Unfortunately, WGP products designed for MWIR and LWIR applications
have historically suffered from low contrast between transmission of linearly polarized light oriented in the passing and
blocking states, which is mostly due to their relatively large wire grid pitch (typically ≥ 370 nm). By dramatically reducing
the pitch from that found in typical IR WGP products to 144 nm, Moxtek has developed MWIR and LWIR polarizers with greatly
Transmission experiments were performed using both Nexus 870 and CARY 670 FTIR spectrometers. Rotation of the WGP sample with
respect to a fixed pre-analyzer allowed for transmittance measurements in the passing and blocking states. The pre-analyzers
consisted of two aligned Moxtek WGP's (both either MWIR or LWIR) separated by a gap of more than 100 μm. Open and blocked
beam configurations and a silicon reference standard were used to validate instrument accuracy for transmission measurements.
Moxtek's MWIR polarizer typically transmits better than 95% of the passing state between 3.5 and 5.5 μm while maintaining
a contrast ratio of better than 35 dB from 3.0–5.5 μm. The Moxtek LWIR polarizer typically transmits better than 68% of the
passing polarization state between 7 and 15 μm and has a contrast ratio exceeding 38.5 dB. Table I compares the performance
of the 144 nm pitch Moxtek products to a 250 nm pitch WGP from another supplier and clearly demonstrates the dramatic improvement
in contrast ratio.
Table I: Performance comparison between Moxtek infrared wire grid polarizers on AR-coated silicon and a competing product
on thallium bromoiodide (KRS-5) substrate
Wire grid polarizers have many advantages 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. To address these
limitations Moxtek has extended its aluminum NanowireŽ patterning capabilities to AR-coated silicon substrates designed for
the mid- and long-wavelength infrared. These new product offerings offer extraordinary improvements in contrast while maintaining
excellent efficiency in passing state transmittance.
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