Periodic Reporting for period 4 - HyMoCo (Hybrid Node Modes for Highly Efficient Light Concentrators)
Période du rapport: 2019-09-01 au 2020-02-29
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 idea of HyMoCo is to collect sunlight at a passive planar waveguide surface by directly coupling it into a 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 for concentrated photovoltaics (CPV).
Perfectly smooth passive waveguides can transport light over long distances. Our approach is to leave the waveguide’s 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 inside the waveguide where no intensity is found. As the guided mode does not exist at this position with regard to the growth direction, it cannot be influenced in the node plane. At this position guided waves that show an extraordinary large propagation length can be excited in the waveguide.
The idea of HyMoCo is to collect sunlight at a passive planar waveguide surface by directly coupling it into a 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 for concentrated photovoltaics (CPV).
Perfectly smooth passive waveguides can transport light over long distances. Our approach is to leave the waveguide’s 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 inside the waveguide where no intensity is found. As the guided mode does not exist at this position with regard to the growth direction, it cannot be influenced in the node plane. At this position guided waves that show an extraordinary large propagation length can be excited in the waveguide.
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.
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.
Beyond the advances described in the five topics developed from the node concept, further scientific achievements could be reached.
First of all, the realization of the concepts inspired new technological methods utilizing large scale polymer technology, doping of polymers for low-guidance waveguides as well as lamination and delamination of polymers layers for achieving symmetric waveguide geometries on the scale of ISO A4 sheets.
The investigation of silver nanoparticle systems led to highly efficient conductive light absorbers and the technological skill of achieving optical structures by photon-induced nanoparticle growth. Most recently, silver nanoparticles grown by surface plasmon-induced processes could be demonstrated which show sensitivities to environmental changes about three orders of magnitude larger than for conventional surface plasmon resonance sensors.
Inspired by the node mode waveguides a patent for switchable waveguides was filed in 2016. The exploration of node-induced BICs instead of pure node modes enables even better electrical switching between a perfect dark state (the BIC) and a radiative state (a directed laser beam) which theoretically enables infinite contrast ratio. First experiments with node-induced BIC waveguides laminated in between two LiTaO3 wafers have demonstrated a contrast above 300 already. This ability to emit a directed laser beam at an electrically controlled position of the BIC waveguide will have tremendous impact on optical technology as 3D laser scanners and light radar systems, laser projectors and displays will evolve that are orders of magnitudes faster than state-of-the-art MEMS or LCD devices.
Lastly, the phenomenom of blinking on faster time scales will not only be of use for light concentrators, but also inspire novel developments of efficient THz sources, optical switches and modulators.
First of all, the realization of the concepts inspired new technological methods utilizing large scale polymer technology, doping of polymers for low-guidance waveguides as well as lamination and delamination of polymers layers for achieving symmetric waveguide geometries on the scale of ISO A4 sheets.
The investigation of silver nanoparticle systems led to highly efficient conductive light absorbers and the technological skill of achieving optical structures by photon-induced nanoparticle growth. Most recently, silver nanoparticles grown by surface plasmon-induced processes could be demonstrated which show sensitivities to environmental changes about three orders of magnitude larger than for conventional surface plasmon resonance sensors.
Inspired by the node mode waveguides a patent for switchable waveguides was filed in 2016. The exploration of node-induced BICs instead of pure node modes enables even better electrical switching between a perfect dark state (the BIC) and a radiative state (a directed laser beam) which theoretically enables infinite contrast ratio. First experiments with node-induced BIC waveguides laminated in between two LiTaO3 wafers have demonstrated a contrast above 300 already. This ability to emit a directed laser beam at an electrically controlled position of the BIC waveguide will have tremendous impact on optical technology as 3D laser scanners and light radar systems, laser projectors and displays will evolve that are orders of magnitudes faster than state-of-the-art MEMS or LCD devices.
Lastly, the phenomenom of blinking on faster time scales will not only be of use for light concentrators, but also inspire novel developments of efficient THz sources, optical switches and modulators.