Based on our previous experiments, the PYP in film form could be a viable option for all-optical switching experiments, as adding glycerol to the protein solution before drying the film helps maintain the integrity of the photocycle even in low humidity environments, allowing for the formation of the intermediate states, which accompanies refractive index changes of the film. PYP, being water-soluble and smaller than the bR membrane patches used previously, potentially enables its combination with special IO passive elements where the application of bR is not possible (e.g., in porous silicon structures). Recently, another light-sensitive biomaterial, the photoactive yellow protein (PYP), has garnered interest for IO applications because of its fast photocycle in solution and its large light-induced refractive index change in dried films. The application of biological materials is appealing because of their easy availability and exceptional NLO properties. Eventually, it was also demonstrated that the primary events of the bR photocycle allow for ultrafast (sub-picosecond) switching as well. First, it was shown that (slow) spectral changes accompanying the photocycle of the chromoprotein bacteriorhodopsin (bR) are sufficient to achieve IO switching, making bR a promising candidate for IO applications. Earlier works suggested the consideration of materials of biological origin for these purposes. It is among the long-term goals in optical telecommunication to find proper NLO materials that make possible all-optical IO switching at the proper efficiencies and speeds. Several materials are being developed and used in hybrid systems-mostly nonlinear crystals with π-conjugated electron systems. Integrated optics (IO) is a new alternative method of information transfer analogous to integrated electronics however, the speed of the system at hand is dependent on the nonlinear optical (NLO) material that is applied as the active element of the IO circuit. Based on our results, we discuss how the special advantages of PYP can be utilized in future IO applications. By exploiting photoreactions in the reaction cycle of PYP, we also show how a combination of exciting light beams can introduce an extra degree of freedom to control the operation of the device. In our current work, we directly demonstrate the all-optical switching capabilities of PYP films combined with an IO Mach–Zehnder interferometer (MZI) for the first time. Using biological materials in IO has recently been proposed, the first material to be investigated for this purpose being the protein bacteriorhodopsin however, since then, other proteins have also been considered, such as the photoactive yellow protein (PYP). Currently, the biggest task in IO is finding or manufacturing materials with the proper nonlinear optical characteristics to implement as active components in IO circuits.
Integrated optics (IO) is a field of photonics which focuses on manufacturing circuits similar to those in integrated electronics, but that work on an optical basis to establish means of faster data transfer and processing.