Within this project, great progress has been made in laying the groundwork for a new class of bio-photonic devices and technologies that are based on bio-derived and biocompatible lasers. During the final phase of the project, much of this ground work was deployed in various areas, specifically for the study of biological systems.
Within work package 1, fundamentally new insights into the optical properties of fluorescent proteins were gained and the unique features of these proteins were exploited to develop novel types of lasers. Using a combination of several innovative characterization methods, the research has shed further light on questions about the photophysical properties of fluorescent proteins and their origin. A further activity in work package 1 has been the development of unconventional lasers based on thin films of undiluted proteins. Although synthetic organic materials have been used to produce solid state lasers for a number of years, concentration quenching, i.e. a loss of emission when the material is present in high concentration, has been a problem. Within this project, it was found that this holds true in particular for the emerging class of polariton lasers that operate by stimulated scattering of exciton polaritons into a common ground state rather than by stimulated emission. For these lasers, strong intracavity absorption and hence high amounts of organic material are required within a thin optical microcavity (thickness of a few wavelengths). Fluorescent proteins have turned out to be ideally suited for this application and have indeed allowed realization of polariton lasers with considerably improved performance compared to previously reported polariton lasers that used synthetically produced materials. The work on polariton lasing in particular has led to a multitude of publications, many of which have been "world firsts" that continue to generate attention and inspire further work. Within the PI's team, insights gained from the project now begin to be exploited for the realization of synthetic polariton devices.
Work package 2 was focused on developing lasers that either comprise of single cells or that are sufficiently small and biocompatible to be inserted into single living cells. The goal was to pioneer applications of these microlasers, e.g. for cell tagging via ultra-dense wavelength multiplexing, or for intracellular sensing by measuring small spectral shifts of laser wavelength. The team has made very substantial improvements in this area. In particular, a considerably more efficient way of introducing microlasers into cells has been established. This technique exploits a method from molecular biology that is normally used to transfer foreign DNA into cells. There has also been considerable progress in developing better understanding of lasers that are comprised of single living cells. In particular, a method was developed and optimized that uses externally administered fluorescent markers rather than having to rely on the cellular machinery to produce fluorescent proteins. The cellular lasers obtained in this way were then characterized by a range of innovative spectroscopy methods, including angled resolved Fourier mapping, to study the photonic confinement effect induced by the cell. In the process of developing more and more compact lasers, the project also yielded a configuration that can be considered the world's most light-weight and possibly thinnest laser -- a membrane shaped device that can be attached as an optical barcode to essentially any object. Work is underway to exploit this development further.
