Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - HyMoCo (Hybrid Node Modes for Highly Efficient Light Concentrators)

Teaser

The daily need for energy world-wide is consistently on the rise. One of the most environment-friendly sources for electricity is the sun. For decades, research has been conducted to find ways of converting solar energy into electrical energy. The most prominent example is...

Summary

The daily need for energy world-wide is consistently on the rise. One of the most environment-friendly sources for electricity is the sun. For decades, research has been conducted to find ways of converting solar energy into electrical energy. The most prominent example is photovoltaics, which allows for immediate conversion. However, as the whole light collecting area needs to be covered by solar cells, its production costs play a significant role.

In contrast, it is possible to collect light on a large area and concentrate it to a much smaller one where the conversion takes place. Here, concentrated solar power plants are a good example, as large arrays of mirrors mechanically track the sun in order to reflect light collected at a large area to the much smaller conversion point. Thermal energy can then be converted into mechanical and electrical energy. However, the prime drawback is high maintenance costs for the moving mirrors.

Planar concentrators that cover the area exposed to sunlight and guide it to a certain point without moving parts would represent major progress. State-of-the–art technology consists of active waveguides. Luminescent solar concentrators have been the subject of research for more than three decades. They absorb light and reemit light of a longer wavelength. The emitted light can be collected into the planar waveguide. However, the inevitable absorption and the Stokes losses during the process limit the performance of the devices, so LSCs cannot be used efficiently on large areas.

The idea of HyMoCo is to collect sunlight at a passive planar waveguide surface by directly coupling it into that waveguide. It is thus concentrated and guided to a certain point. Absorption can be kept extraordinarily small and Stokes losses do not exist in that approach. Such types of passive solar concentrators produced cost-effectively would be a major breakthrough.

Perfectly smooth passive waveguides can transport light over long distances. This is possible because the smooth waveguide surfaces enable total reflection and serve as insurmountable barriers, capturing the light inside the waveguide without allowing it to leave. Likewise, the barriers also block the opposite direction, so light from the outside cannot be collected into the waveguide. Roughening the waveguide surfaces reduces the effectiveness of total reflection, allowing light to enter the waveguide, but also to leave it. In fact, in order to create a useful passive concentrator, one would have to collect light and at the same time block the opposite process, which could be mistaken as breaking the Lorentz reciprocity theorem, one of the most important rules in passive optics.

Our approach is to leave the waveguide surfaces smooth, but to include a scattering or diffracting structure at a certain position inside a planar waveguide. The guided mode is still totally reflected between the smooth waveguide surfaces defining a standing wave in between them. For a TE1 mode this standing wave shows a node plane: a position inside the waveguide where no intensity is found. As the guided mode does not exist at this position, it cannot be influenced in the node plane. It is therefore the node concept to place a scattering or diffracting structure in this node plane, so it can excite guided waves into the waveguide without extracting them from the waveguide.

Work performed

First, methods for numerical and analytical simulation of diffraction of light in the node plane were implemented. A technique to determine extraordinarily low waveguide losses was developed. After finding the optimum dielectric the technological basis for the fabrication of node mode grating couplers and collectors was created.

The node mode grating couplers are a useful test device to study node modes. They consist of an optical grating produced by nanoimprint lithography that is coated with silver and embedded into the node of a polymeric planar waveguide. During diffraction, a laser beam incident from the suitable angle is coupled into the node mode. With the incident light and the guided mode being directed beams, the efficiency of collection and extraction as well as the propagation length of the collected mode can be analysed easily. In addition, diffraction is much easier to simulate compared to white scattering. Hence, the node mode grating couplers have enabled theoretical and experimental evaluation of the node concept described above. As the gratings do not (or very weakly) influence guided node modes when placed in the node, one could assume they must also be useless for coupling light into guided modes. The major result of the project HyMoCo so far was to clearly prove that this assumption is wrong. We have theoretically and experimentally demonstrated that a structure placed in the node allows collecting light into the waveguide and at the same time does not (or extremely weakly) extract light from the waveguide. It was also confirmed that this finding is not in conflict with the reciprocity theorem.

Final results

The resulting node grating couplers reach a concentration of 100. This is about an order of magnitude higher than for state-of-the-art grating couplers of the same thickness that carry the grating at the top or bottom of the waveguide. Originally planned as test devices only, the node grating couplers showed a plethora of properties with potential applications in optics. For instance, the strong dependency of the propagation length on the alignment of the structure in the node plane paves the way for the next generation of efficient ultra-fast displays, laser scanners and even all-optical switches. Due to this wide range of benefits we expect the node concept to have a tremendous impact on the field of optics.

For the solar concentrators the mentioned sensitivity to detuning is, of course, not desirable. Here, perfect symmetry is extremely important. With the developed lamination process that goal can be reached. A structure placed in the very centre of a fully symmetric waveguide will always match with the node position, regardless of the wavelength. That means that any colour can be collected and concentrated. First symmetric node grating couplers do in fact collect all visible colours. However, grating diffraction is of course sensitive to the wavelength and incident angle, so any colour is coupled at a different angle. Solar collectors on the other hand must efficiently collect light of any colour and from any angle.

So, while the waveguide is already compatible with white light propagation, the collection must also be compatible with white light incident at any angle. For that purpose, the diffraction gratings will be replaced by white scattering structures. The theoretical description of such collectors is much more complex, but it can be developed using already existing approaches describing diffraction in the node. After the local scattering parameters of such devices have been simulated and experimentally confirmed, the optimum geometry for passive light concentrators can be identified.

We are confident that such optimized devices are going to outperform state-of-the-art luminescent solar concentrators.