DARKENET: Engineering dark modes for energy trapping

Growing energy requirements of the world have made scientists work towards tapping more from sustainable resources like solar energy. A major challenge in this direction so far has been about improving efficiency of solar cell devices. Surprisingly, for millions of years, nature has been performing the most efficient energy capture and transfer process known to us: photosynthesis. It is through this process that plants, algae, and bacteria collect solar energy and store it as chemical energy to sustain life on this planet. In the first steps of photosynthesis, sunlight is captured by ring-shaped chlorophyll molecules (called LH2s), then transported from one molecule to another and eventually to reaction centres where it is stored for subsequent chemical reactions. This energy transport and trapping process has efficiencies close to 100%, something that invited a great deal of curiosity among physicists and chemists alike in the last few decades. The LH2, from a physics perspective, is simply a ring of electric dipoles whose interactions depend on their separations and relative orientations. For separations smaller than the optical wavelengths, these interactions result in an energy landscape containing areas with suppressed radiative properties or longer lifetimes, compared to those of individual dipoles. These areas are called dark states. It is then intuitive to assume that in LH2s, nature has optimized such dark states to trap the solar energy efficiently for longer times before transferring it to the reaction centre. With the help of ultrafast lasers, energy transport studies on single LH2s with femtosecond time-resolution have been made possible. However, those were extremely challenging, due to their limited photostability and low quantum efficiency. In this project, awarded by the FWF as part of the 1000 Ideas Program, a group of physicists from Innsbruck, led by Vikas Remesh of the Innsbruck Photonics Group, in collaboration with researchers from Canada, wants to create a simulator system using quantum dots (QD) to study the collective radiative effects in LH2. QDs are semiconductor nanocrystals, widely regarded as artificial atoms due to their discrete energy levels that can be controlled precisely by their fabrication parameters. In contrast to single LH2 molecules, QDs are bright and extremely stable emitters that can be patterned to arbitrary separations and structures with high accuracy. They provide us with a chance to solve the important task of decoding the physics of natures evolution-optimized light harvesting- trapping system. After realising QD-ring structures, the researchers plan to set up an optical microscope with an ultrafast laser and highly sensitive detectors to investigate the spatial properties of dark-state assisted energy transport. Are dark states the secret behind nature`s highly efficient photosynthetic energy transport mechanism? There is only one way to know.

Growing energy requirements of the world have made scientists work towards tapping more from sustainable resources like solar energy. A major challenge in this direction so far has been about improving efficiency of solar cell devices. For millions of years, nature has been performing the most efficient energy capture and transfer process known to us: photosynthesis. It is through this process that life sustains on this planet. In the first steps of photosynthesis, sunlight is captured by a collective molecular system, before it is stored for subsequent chemical reactions. This energy transport and trapping process has efficiencies close to 100%, something that invited a great deal of curiosity among physicists and chemists alike in the last few decades. It is proposed that when emitters are placed within small separations, it enables strong interaction between them and collective states with suppressed radiative properties may evolve. These states are thought to be the reason behind the efficient energy trapping in the natural light harvesting systems. However, despite a reasonably large number of theoretical works on this area, detailed experimental investigations are still missing, which limits our understanding and perhaps a streamlined effort to engineer artificial light harvesting systems. In this project, we wanted to create a simulator system using quantum dots to study the collective radiative effects in light harvesting complexes. Quantum dots are semiconductor nanocrystals, carrying discrete energy levels which are tunable via growth methods and post-growth tuning knobs. They are bright, and photostable systems and have been well studied in the recent past under various optical coherent control techniques. We started with a model system of nanowire quantum dots arranged in rings to emulate natural light-harvesting systems to look for coherent coupling effects. However, it turns out that the current growth parameters of this system forbid any significant inter-dot coupling. Therefore we have proposed a simpler test system, where multiple quantum dots are grown in the same nanowire, at various heights. This helps to investigate collective effects in a large range of couplings. This sample is currently under investigation. To enable simultaneous study of large range of coherent couplings, we are also developing a robust multiplexed microscopy technique. In parallel, an alternative system of strain-tuned quantum wells are also in development, where we believe direct dipole-dipole interactions can be achieved, in arbitrary geometries, both of which are not probably achievable in nanowire systems. Furthermore, we have also developed advanced pulse-shaping techniques in our laboratories to investigate such collectively evolving states. Proof of principle experiments have been demonstrated in our recent experiments on single quantum dots. Such experiments will soon be extended to collectively interacting quantum dots.