Abstract
The ubiquitous wire grid polarizer remains one of the most useful optical components in the field. Widely used in applications such as displays, imaging, sensors, communications, and scientific instrumentation, the wire grid polarizer (WGP) 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 the ability for very large clear apertures, large acceptance angle with minimal performance variation over broad bandwidths, and improved stability over organic polarizers in high temperature and high brightness environments. Existing WGP products designed for long wavelength thermal IR applications typically suffer from low contrast between transmission of linearly polarized light in the passing and blocking configurations, which is due to their relatively large wire grid pitch (typically ≥ 370 nm). Moxtek has previously demonstrated a dramatic increase in aluminum WGP contrast at visible and ultraviolet wavelengths by reducing the pitch. We postulated that a dramatic reduction in pitch from that found in typical IR WGP products should greatly improve mid- and long-wavelength IR contrast. Moxtek has therefore developed several high contrast IR polarizers on anti-reflection (AR) coated silicon suitable for mid-wavelength IR (MWIR) and long-wavelength IR (LWIR) applications using our aluminum nanowire, large area patterning capabilities. Potential applications for these MWIR and LWIR polarizers include spectroscopic measurement systems, optical isolators for industrial lasers, and polarization sensitive imaging systems for hyperspectral imaging, guided missile technology, and forward-looking infrared thermal imaging.
Accurate WGP metrology was gathered in both transmission and reflection from the SWIR to LWIR using a combination of FTIR and dispersive spectrometers, as well as laser-based light sources. The WGP structures were analyzed using SEM, FIB, and STEM techniques and optical data was derived from IR VASE, transmission, and reflectance measurements. Modeling of device performance was achieved using rigorous coupled wave analysis. Our 144 nm pitch broadband MWIR polarizer transmits better than 95% of the passing state while maintaining a contrast ratio of better than 33dB from 3.3-5.7 μm. Between 7 and 15 μm, our LWIR polarizer transmits better than 70% of the passing polarization state and has a contrast ratio better than 40dB. A narrowband AR-coated WGP for 7.5 μm wavelength transmits better than 94% of the passing state while maintaining a contrast ratio of better than 42dB.
With the widespread use of lasers in defense, academia, and industry, Moxtek has pursued laser damage threshold (LDT) testing in both the passing and blocking polarizer orientations, as well as a narrow band AR coated WGP product for 10.6 μm laser line applications. Preliminary testing and analysis indicate that defects introduced during silicon AR coating are currently limiting the LDT performance in our broadband LWIR product. The MWIR polarizer uses an improved AR coating and does not show the same damage initiation mechanism. The LWIR product showed LDT values of 110 kW/cm2 and 10 kW/cm2 in the blocking and passing states respectively for a continuous wave, 10.6 μm CO2 laser beam with 360 μm beam diameter. The high reflectivity and low absorption of the Aluminum nanowires evidently protects the defects from laser induced damage in the blocking polarizer configuration. Removing these AR coating defects should further improve LDT performance as well as passing state transmission in the thermal IR. The Moxtek MWIR product showed LDT values of 0.65 kW/cm2 and better than 14 kW/cm2 in the blocking and passing states respectively for a 7 ns, 25 kHz pulsed laser beam at wavelength of 4.0 μm. SEM analysis of the damage mechanisms for the passing and blocking configurations will also be presented.
Infrared Wire Grid Polarizers: Metrology, Modeling, and Laser Damage Threshold
The ubiquitous wire grid polarizer remains one of the most useful optical components in the field. Widely used in applications such as displays, imaging, sensors, communications, and scientific instrumentation, the wire grid polarizer (WGP) 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 the ability for very large clear apertures, large acceptance angle with minimal performance variation over broad bandwidths, and improved stability over organic polarizers in high temperature and high brightness environments. Existing WGP products designed for long wavelength thermal IR applications typically suffer from low contrast between transmission of linearly polarized light in the passing and blocking configurations, which is due to their relatively large wire grid pitch (typically ≥ 370 nm). Moxtek has previously demonstrated a dramatic increase in aluminum WGP contrast at visible and ultraviolet wavelengths by reducing the pitch. We postulated that a dramatic reduction in pitch from that found in typical IR WGP products should greatly improve mid- and long-wavelength IR contrast. Moxtek has therefore developed several high contrast IR polarizers on anti-reflection (AR) coated silicon suitable for mid-wavelength IR (MWIR) and long-wavelength IR (LWIR) applications using our aluminum nanowire, large area patterning capabilities. Potential applications for these MWIR and LWIR polarizers include spectroscopic measurement systems, optical isolators for industrial lasers, and polarization sensitive imaging systems for hyperspectral imaging, guided missile technology, and forward-looking infrared thermal imaging.
Accurate WGP metrology was gathered in both transmission and reflection from the SWIR to LWIR using a combination of FTIR and dispersive spectrometers, as well as laser-based light sources. The WGP structures were analyzed using SEM, FIB, and STEM techniques and optical data was derived from IR VASE, transmission, and reflectance measurements. Modeling of device performance was achieved using rigorous coupled wave analysis. Our 144 nm pitch broadband MWIR polarizer transmits better than 95% of the passing state while maintaining a contrast ratio of better than 33dB from 3.3-5.7 μm. Between 7 and 15 μm, our LWIR polarizer transmits better than 70% of the passing polarization state and has a contrast ratio better than 40dB. A narrowband AR-coated WGP for 7.5 μm wavelength transmits better than 94% of the passing state while maintaining a contrast ratio of better than 42dB.
With the widespread use of lasers in defense, academia, and industry, Moxtek has pursued laser damage threshold (LDT) testing in both the passing and blocking polarizer orientations, as well as a narrow band AR coated WGP product for 10.6 μm laser line applications. Preliminary testing and analysis indicate that defects introduced during silicon AR coating are currently limiting the LDT performance in our broadband LWIR product. The MWIR polarizer uses an improved AR coating and does not show the same damage initiation mechanism. The LWIR product showed LDT values of 110 kW/cm2 and 10 kW/cm2 in the blocking and passing states respectively for a continuous wave, 10.6 μm CO2 laser beam with 360 μm beam diameter. The high reflectivity and low absorption of the Aluminum nanowires evidently protects the defects from laser induced damage in the blocking polarizer configuration. Removing these AR coating defects should further improve LDT performance as well as passing state transmission in the thermal IR. The Moxtek MWIR product showed LDT values of 0.65 kW/cm2 and better than 14 kW/cm2 in the blocking and passing states respectively for a 7 ns, 25 kHz pulsed laser beam at wavelength of 4.0 μm. SEM analysis of the damage mechanisms for the passing and blocking configurations will also be presented.