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Advanced Bioderived and Biocompatible Lasers

Periodic Reporting for period 4 - ABLASE (Advanced Bioderived and Biocompatible Lasers)

Okres sprawozdawczy: 2019-12-01 do 2021-11-30

Naturally occurring optical phenomena attract great attention and transform our ability to study biological processes, with “the discovery and development of the green fluorescent protein (GFP)” (Nobel Prize in Chemistry 2008) being a particularly successful example. Although found only in very few species in nature, most organisms can be genetically programmed to produce the brightly fluorescent GFP molecules. Combined with modern fluorescence detection schemes, this has led to entirely new ways of monitoring biological processes.
Just prior to the start of the project, the applicant demonstrated a biological laser – a completely novel, living source of coherent light based on a single biological cell bioengineered to produce GFP. Such a laser is intrinsically biocompatible, thus offering unique properties not shared by any existing laser. However, the physical processes involved in lasing from GFP remained poorly understood and the first biological lasers relied on bulky, impractical external resonators for optical feedback.
Within this project, the applicant and his team developed for the first time an understanding of stimulated emission in GFP and related proteins and created an unprecedented stand-alone single-cell biolaser based on intracellular optical feedback. These lasers were deployed as microscopic and biocompatible imaging probes, thus opening in vivo microscopy to dense wavelength-multiplexing and enabling unmatched sensing of biomolecules and mechanical pressure. A particular breakthrough of the project was the adaption of lasing inside cells for the optical, non-contact sensing of heart muscle contraction. Furthermore, the evolutionarily evolved nano-structure of GFP enabled novel ways of studying strong light-matter coupling and bio-inspired advances of synthetic emitters.
The project was inter-disciplinary by its very nature, bridging photonics, genetic engineering and material science. The impact of the project is far-reaching; over its duration, biological lasers evolved from a scientific curiosity to a vivid and fast-paced field of research.
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.
The high profile scientific results described above have been enabled and underpinned by a number of novel and unconventional methodologies.
Particular examples include the adaption and use of innovative hyperspectral Fourier spectroscopy modalities that allow single shot measurements of the spectral and angular composition of the light emitted by the lasers developed within ABLASE (applies to work package 1 and 2).
Fabrication of the protein polariton lasers in work package 1 has required development of a number of new methodologies. The cavities used for these lasers are produced in a solution-based lamination approach, thus avoiding complex and error prone vacuum deposition of the top mirror but instead allowing to use commercially available high-performance mirrors. Another first in this context is the use of laser writing to intentionally induce local bleaching of the protein thin film and thus modify the microscopic refractive index landscape.
In work package 2, the adaptation and modification of a method known from molecular biology to improve the efficiency of introducing lasers into cells represents a further specific example of a novel and unconventional methodology. More recently, the team has very carefully optimized optical modelling of the spectral position of laser modes to perform quantitative refractive index measurements inside living cells.
Cover of Nature Materials, advertising work originating from the ABLASE action
Cover of Advanced Optical Materials, advertising work originating from the ABLASE action