Abstract
Membrane-bound or cytosolic light-sensitive proteins, playing a crucial role in energy- and signal-transduction processes of various photosynthetic microorganisms, have been optimized for sensing or harvesting light by myriads of years of evolution. Upon absorption of a photon, they undergo a usually cyclic reaction series of conformations, and the accompanying spectro-kinetic events assign robust nonlinear optical (NLO) properties for these chromoproteins. During recent years, they have attracted a considerable interest among researchers of the applied optics community as well, where finding the appropriate NLO material for a particular application is a pivotal task. Potential applications have emerged in various branches of photonics, including optical information storage and processing, higher-harmonic and white-light continuum generation, or biosensorics. In our earlier work, we also raised the possibility of using chromoproteins, such as bacteriorhodopsin (bR), as building blocks for the active elements of integrated optical (IO) circuits, where several organic and inorganic photonic materials have been considered as active components, but so far none of them has been deemed ideal for the purpose. In the current study, we investigate the linear and NLO properties of biofilms made of photoactive yellow protein (PYP) and bR. The kinetics of the photoreactions are monitored by time-resolved absorption experiments, while the refractive index of the films and its light-induced changes are measured using the Optical Waveguide Lightmode Spectroscopy (OWLS) and Z-scan techniques, respectively. The nonlinear refractive index and the refractive index change of both protein films were determined in the green spectral range in a wide range of intensities and at various laser repetition rates. The nonlinear refractive index and refractive index change of PYP were compared to those of bR, with respect to photonics applications. Our results imply that the NLO properties of these proteins make them promising candidates for utilization in applied photonics, and they should be considered as valid alternatives for active components of IO circuits.
Highlights
Ubiquitous applications of photonics and optoelectronics are penetrating into diverse areas from everyday life to the most advanced scientific disciplines, such as optical communication, data processing and storage, quantum computing, holography, just to mention a few
The recorded traces were fitted by Gaussians, in order to obtain the positions of the TE and TM modes
These refractive index data of the biofilms were used for the determination of their n and n2 values from the Z-scan experiments
Summary
Ubiquitous applications of photonics and optoelectronics are penetrating into diverse areas from everyday life to the most advanced scientific disciplines, such as optical communication, data processing and storage, quantum computing, holography, just to mention a few. Examples for the use of NLO materials in photonics ranges from all-optical signal processing (Willner et al, 2014), to alloptical switching (Chai et al, 2017), optical filtering (Dini et al, 2016), with the list of applications continuously growing. To this end, one of the biggest challenges is finding materials with optimal NLO characteristics that could be applied in photonic devices. Chromoproteins have been perfected by evolution for billions of years for utilizing light as a source of energy or information Their protein matrix stabilizes their chromophores, and fine-tunes their optical properties. Building up stable hybrid structures of the protein and the passive substrate (such as a thin film of chromoprotein on a photonic circuit), and a thorough characterization of their NLO properties is inevitable, as well as their optimization for a particular application, such as IO switching, or other optical information processing tasks
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