A MEMS IR optical chopper based on subwavelength structures

This contribution presents recent results in the design, fabrication and test of an infrared (IR) chopper device with subwavelength gratings. The IR penetrates the device from front to backside. A first subwavelength grating and movable composite membrane (Si3N4 and Al) at the front side builds a frequency selective surface. Surface-plasmon-polariton (SPP) resonances and a Fabry-Pérot (FP) resonance within the gap between the membrane and the silicon substrate of the device allow for high transmittance in the transmission state. When an electric voltage is applied between the membrane and the silicon substrate, the membrane is pulled onto the substrate, which results in breaking of the plasmon resonance and of the FP resonance that leads to low transmittance in the blocking state. Since the movable part of the chopper is a thin film with low mass, it can be used for high speed applications. A further subwavelength structure at the backside of the device reduces the undesired reflection of the IR at the interface from the silicon substrate to the air. The membrane size of the samples is 2.2 mm × 2.2 mm. The transmittance is 50%...70% in the transmission state and less than 20% in the blocking state within the wavelength range 10 mm...13.5 mm. The switch time to change between the both states is less than 40 ms with 30 Volt actuation voltage. The devices showed regular function over 1.9 billion switch cycles during a long term tests with 5 kHz switch frequency.