It could indeed be demonstrated that a grating structure placed in the node allows for the collection of light into the waveguide with a strongly reduced thickness compared to a conventional grating coupler with a surface relief grating of the same geometrical parameters.
This reduced thickness is of critical importance to address the two most important challenges of passive concentrators: the conservation of radiance limiting the static acceptance angle and dispersion limiting the spectral range. Five different topics can be distinguished highlighting how light concentrators benefit from the node geometry:
1. Stacking
Stacking of multiple concentrators benefits from the reduced thickness of concentrators and proved to increase the dynamic acceptance angle. With increasing distance from the equator, however, the required number of concentrators becomes so high that systems with reduced spectral dispersion are necessary.
2. Zero-order-collection
In order to reduce the number of waveguides to be stacked, low dispersion concentrators were investigated. Figure 1 emphasizes the basic idea.
At low dimensions, high-dispersion diffraction of light dominates, and not all wavelengths can be collected. With increasing size, low-dispersion zero order reflection and transmission dominates. However, such elements changing the direction of light by means of geometrical optics have to be positioned in a way that they do not block each other, resulting in bulkiness (Fresnel lenses in CPV, Fig. 2a,b; mirrors in concentrated solar plants (CSPs), Fig. 2c). Our approach considers the placement of a low-dispersion mirror structure in the node plane of a symmetric node waveguide (Fig. 2d). This way, the mirrors can be placed behind without blocking each other and enable much thinner concentrators. Numerical simulations predict a large optical efficiency and a concentration factor above 1000 at a thickness of 10 µm for this design. Strikingly, the concentrators can be designed to collect a well-controlled spectral range only. A few such foils designed for different colours can be stacked in a way that the focus areas are slightly shifted, and solar cells of perfectly matching bandgap can be placed at the focus areas of suitable colour. This way, the Shockley–Queisser limit for CPV increases from 41% (ideal concentration, single bandgap) to 86.8% (ideal concentration and bandgap matching).
This promising approach, that was also awarded an ERC Proof of Concept Grant, was filed as patent in January 2020.
3. Self-adaption
To change the acceptance angle of a concentrator, the geometric parameters of the structure would have to be changed actively. This is usually not possible. We found experimental evidence that silver nanoparticles grown on top of an excited waveguide by light-induced processes form structures with optimized coupling to waveguide modes. This finding provides a first step towards reversibly adaptive beam steering for which the necessary large propagation lengths are enabled by the node concept.
4. Bound States in Continuum
The node concept has been developed further towards node-induced bound state in the continuum (BIC) which form from the interaction between a symmetric node mode and a second symmetric resonator. This has been experimentally confirmed for dielectric and plasmon second resonators. In the latter case hybrid plasmon dielectric BICs are formed that show large optical sensitivity, making them interesting for light concentrators with implemented beam steering.
5. Nonlinear Time Variance
Node modes and node-induced BICs, that are extremely sensitive to slight changes, were further found to exhibit time-variant (oscillating) optical transmission and output power, a phenomenon I term blinking. Combined with ultrafast nonlinear materials, such phenomena might result in passive light concentrators that increase radiance during concentration. Neither sun-tracking, nor mechanical sun-tracking, steering, or adaption would thus be necessary anymore.
