Reconfigurable Optical Devices Utilizing Chalcogenide Phase Change Materials

Abstract

Tunable integrated photonic devices working at the visible and near infrared (NIR) regions are needed in a variety of applications such as in time keeping systems, imagers, displays, and memories, among others. To achieve tunability, a reconfigurable structure or material must be integrated within the device. Electro-optical devices, which are using the change in the material are usually cheaper, faster, more reliable, and are easier to integrate due to their smaller dimensions as compared to opto-mechanical devices. From a large selection of materials which can provide tunability, phase change materials (PCMs) stand out due to their reliable phase transitions and large change in their optical and electrical properties as they go through crystallographic phase transition. Specifically, Germanium Telluride exhibits very different optical and electrical properties when it undergoes phase transitions. Compared to the more commercialized Germanium Antimony Telluride (GST) material, GeTe has a much simpler fabrication process, more reliable phase transitions and lower loss at the visible wavelength. Two stable phases of GeTe exist at room temperature: amorphous (a-GeTe) and crystalline (c-GeTe), which have drastically different optical and electrical properties at NIR and noticeable differences in the visible range. Here, for the first time, a transmissive optical shutter is demonstrated that employs GeTe in a multi-layer sub-wavelength grating structure. The shutter exhibits an excellent contrast of ~30 dB (the ON/OFF transmission ratio) with transmission loss of < 3 dB at the wavelength of 1.55 μm. In our shutter, the sub-wavelength array of gold lines provide both efficient joule heating to change the phase of GeTe (indirect heating scheme) and the heat sink required for fast cooling of GeTe. It is worth mentioning that since both phases of GeTe are stable at room temperature, this shutter consumes zero static power. This is in contrast with Vanadium Oxides (VOx) and liquid crystal-based devices which need constant applied power to maintain the ON or OFF state. A reflection-based shutter is also presented here, which includes only a thin layer of GeTe (~40 nm) on a bottom gold heater and yet exhibits a measured contrast (modulation index) of > 20 dB at the visible and NIR range (780 nm-1.1 μm). This shutter that tunes the intensity of the reflected light at two different wavelengths, is the first integrated shutter (or modulator) working at the visible wavelength using the lossy GeTe. Another contribution of this thesis work is to show, for the first time, the dependency of the phase transition temperature of thin-film GeTe on its thickness and to employ this interesting property to achieve more than two colors using only GeTe as the thermochromic material. It is shown here that with thinner GeTe films of < 20 nm, the glass transition temperature increases as the thickness is reduced. This is further employed to demonstrate the first integrated multi-layer PCM device working at the visible wavelength. This device shows four vivid interchangeable red, yellow, green, and blue colors using a single joule heating element. Finally, a novel grating-based reflective color filter is shown here by integrating GeTe grating on a SiO2 waveguide to enhance the number of colors and the color dynamics. Future work includes studying of the device reliability, exploring the use of other phase change materials, and other methods for achieving phase transition more reliably, efficiently, and/or at a higher speed.