Application of phase change material in tunable optical filters and shutters

This paper reports tunable integrated photonic devices in the visible and near-infrared (NIR) regions using a phase change material, Germanium Telluride (GeTe), within sub-wavelength layered optical cavity structures. GeTe exhibits two distinct index of refraction values at its amorphous and crystalline states in this spectral range. Utilizing this property, we demonstrate a multi-color filter working in visible range (400 nm-750 nm), achieving four colors through novel optical and thermal engineering of a thin film stack that includes two GeTe layers with only a single integrated joule heater element. Specifically, ultra-thin GeTe films were sandwiched between a bottom metallic mirror and a top high-index dielectric (titanium dioxide). It is shown that the crystallization temperature (Tx) of GeTe is dependent on the film thickness when less than ~20 nm. The refractive index of GeTe only changes significantly in the NIR region when it undergoes phase transitions when heated. To enhance the color contrast, a 250-nm thick silicon dioxide layer is placed under GeTe to create an optical cavity between GeTe and a bottom metal reflector. Using this optimized design, the tunable color filter shows four distinct colors using the integrated heater. In addition to a color filter, we demonstrate an electrically tunable multi-mode optical shutter in near infrared range of 1.1 μm and 800 nm with more than 20 dB modulation depth. The low static power consumption of these devices achieved through reliable memory-base phase transitioning of GeTe makes them prime candidates for a number of portable consumer electronic applications.