Principal Investigator Juejun Hu
Optical switching is an essential function in photonic integrated circuits. Recently, a new class of devices based on O-PCMs have emerged for on-chip switching. Unlike electro-optic or thermo-optic effects which are minuscule, phase transition in O-PCMs generates huge optical property modulation conducive to ultra-compact device architectures. In addition, such phase changes can be non-volatile, exemplified by the transition between amorphous (a-) and crystalline (c-) states in chalcogenide alloys. Despite these attractive features, the performances of existing PCM-based pho- tonic switches are severely compromised by the high optical absorption in traditional O-PCMs.
Here we report the design and modeling of a new kind of photonic switches combining low-loss phase change alloys and a “nonperturbative” design to boost the switching performance. On the one hand, we use a low-loss O-PCM for this application: Ge2Sb2Se4Te1 (GSS4T1). Fig 1a and 1b show the optical constants of GSS4T1 compared with traditional PCM Ge2Sb2Te5 (GST225), as measured by ellipsometry. At telecommunication wavelength, the material figure- of-merit, which is defined as index change over extinction coefficient, is 6 times higher. Moreover, the loss of amorphous state GSS4T1 is 0.00017 measured by waveguide cutback method, which is two orders of magnitude lower. On the other hand, the switch design is based on the huge index change of O-PCMs. The basic element is a directional coupler comprised of a bare waveguide (WG1) and a waveguide covered with a PCM strip (WG2). At (a-) state, their indices are matched, and light will be coupler from WG1 to WG2. At (c-) state, due to the large index change of O-PCM, the modal profile will be completely different, and effective index of WG2 will increase a lot so that coupling will not happen. This helps to keep the loss at a low level since light will not travel in WG2 when GSS4T1 is in its (c-) state.