METHOD OF CONCENTRATING LIGHT AND LIGHT CONCENTRATOR

Abstract

The invention relates to a method for concentrating light by coupling light into a thin film waveguide (2, 4) arranged on a substrate (1), in particular via at least one of its parallel surfaces, the method further comprising the step of exciting in the thin-film-waveguide (2, 4) at least one lateral guided mode (5) having at least one node (6), preferably exactly one node (6), by interaction, in particular scattering, diffraction or surface plasmon excitation of the incident light with a nanopattemed discontinuous excitation layer (3) of material, in particular metal, preferably silver, the nanopatterned discontinuous excitation layer (3) being arranged in the thin-film-waveguide (2,4) at the position of the at least one node (6) of the guided lateral mode (5). The invention furthermore relates to alight concentrator comprising a thin film waveguide (2, 4) deposited on a substrate (1), the thin film waveguide (2, 4) having at least two parallel surfaces, light being coupable into the thin film waveguide (2, 4) via at least one of these surfaces, wherein the thin film waveguide (2, 4) is established as a collecting thin film wave-guide (2, 3, 4) for collecting light by arranging a nanopatterned discontinuous excitation layer (3) of material, in particular of metal and preferably of silver at a position corresponding to the node position (6) of a guided mode (5) to be excited in the collecting thin film waveguide (2, 3, 4). The invention also relates to a method of fabricating such a light concentrator.

Claims

1. A method of concentrating light by coupling light into a thin film waveguide arranged on a substrate via at least one of its parallel surfaces, the method comprising the step of exciting in the thin-film-waveguide at least one lateral guided mode having exactly one node by scattering, diffraction or surface plasmon excitation of the incident light with a nanopatterned discontinuous excitation layer of metal, the nanopatterned discontinuous excitation layer being in the thin-film-waveguide at the the at least one node of the guided lateral mode.

2. The method according to claim 1, further comprising the step of: exciting the light of the at least one guided mode in at least one specific lateral direction by providing a nanopatterned discontinuous excitation layer with a scattering or diffracting a grating structure, the diffracting structure being periodic in that lateral direction.

3. A light concentrator comprising: a thin film waveguide deposited on a substrate and having at least two parallel surfaces; means for coupling light into the thin film waveguide via at least one of these surfaces, the thin film waveguide being a collecting thin film waveguide for collecting light having a nanopatterned discontinuous excitation layer of metal at a position corresponding to the node position of a guided mode to be excited in the collecting thin film waveguide.

4. The light concentrator according to claim 3, wherein the nanopatterned discontinuous excitation layer is arranged in a plane parallel to the surfaces, in particular the nanopatterned discontinuous excitation layer having a thickness of 20 nm or less in the direction of the normal vector of this plane.

5. The light concentrator according to claim 3, wherein the nanopatterned discontinuous excitation layer is formed of a silver island film having a mass thickness of 5 to 15 nm or particle size of 10 to 100 nm or nanoparticles being deposited from liquid suspension, by printing, dip- or spin-coating.

6. The light concentrator according to claim 3, wherein the nanopatterned discontinuous excitation layer is between two layers of transparent dielectric material.

7. The light concentrator according to claim 3, wherein the nanopatterned discontinuous excitation layer comprises or is formed of at least one scattering or diffracting structure extending parallel to the surface and comprises or is formed of at least one grating structure having at least one periodicity in a direction parallel to the surface.

8. The light concentrator according to claim 3, wherein the substrate is formed of a flexible foil to which the collecting thin film waveguide is attached by material deposition.

9. The light concentrator according to claim 3, further comprising: at least two stacked collecting thin-film-waveguides attached to a common substrate.

10. The light concentrator according to claim 3, further comprising: several collecting thin film waveguides spaced in the same plane in a propagation direction, waveguides for distribution or extraction of light between the collecting thin film waveguides and comprising tapered or bent or stripe waveguide structures.

11. The light concentrator according to claim 10, further comprising: several stripe shaped collecting thin film waveguides parallel to each other on the substrate and each having a stripe shaped discontinuous excitation layer extending and discontinuous in the propagation direction and arranged in the node of the excited mode with respect to both transverse directions.

12. A method of fabricating a light concentrator, the method comprising the steps of: depositing a first layer of a transparent, particularly transparent dielectric material on a substrate; depositing a nanopatterned discontinuous excitation layer of material, in particular metal, preferably silver on the first layer; depositing a second layer of a transparent dielectric material on the nanopatterned discontinuous excitation layer, the first and second transparent layer being deposited in a thickness of 200 nm to 400 nm respectively.

13. The method according to claim 12, further comprising the step of: depositing the nanopatterned discontinuous excitation layer as a discontinuous islandized film/layer of silver or as printed, dip- or spin-coated nanoparticles.

14. The method according to claim 12, further comprising the step of: transferring the nanopatterned discontinuous excitation layer is transferred to the first layer by transfer-printing, in particular using a stamp or roll made of elastomeric material, the stamp or roll containing a nano relief.

15. Use of a concentrator according to claim 3 for collecting sun light, and concentrating the light to heat engines with electrical power generator or solar cells or solar-to-gas or solar-to-fuel generators.

Description

[0066] Preferred embodiments are shown in the figures.

[0067] FIG. 1 shows in the upper part a typical construction in a cross sectional view of a light concentrator according to the invention, the cross section being perpendicular to one lateral direction (z). The light concentrator comprises a lower substrate 1, particularly a foil, a layer of dielectric material 2 deposited on the substrate, a discontinuous excitation layer 3 deposited on the first dielectric layer 2 and a second dielectric layer 4 deposited on the layer 3. On the side opposite to the substrate 1 the dielectric layer 4 is contacted by air. Both dielectric layers 2 and 4 are formed with the same thickness thus leading to an asymmetric mode profile 5 since only the lower layer 2 is contacted by the additional substrate.

[0068] Light incident to the upper surface 4a of the second dielectric layer 4 or from the bottom 1a through the substrate 1 is coupled into the collecting thin film waveguide construction constituted by the three layers 2, 3, 4 and interacts with the nanopatterned discontinuous excitation layer 3 thus exciting a mode having an intensity profile parallel to the normal vector of the waveguide surface x and traveling in lateral direction, i.e. in y-direction. The excited mode, depicted by its intensity profile 5, has its minimum of intensity 6 exact in the position of the nanopatterned discontinuous excitation layer 3. This nanopatterned discontinuous excitation layer 3 is formed in the y-z-plane of the collecting thin film waveguide constituted by the deposited layer 2. In particular since a part of the mode is extending into the substrate 1 this substrate 1 may also be understood as a part of the collecting thin film waveguide.

[0069] Hence even if the lower and upper dielectric layer 2,4 are identical the guided mode would still be slightly asymmetric as the substrate refractive index is larger compared to the refractive index of air.

[0070] In the example given in FIG. 1 a TEi-mode is excited and guided in the collecting thin film waveguide 2,3,4 of the light concentrator. Due to the position of the nanopatterned discontinuous excitation layer 3 in the node 6 of the excited mode the nanopattern of the excitation layer does scarcely interact with the TEi-mode. Accordingly this mode may travel in lateral y-direction (or any lateral direction in the plane yz) with minimum losses. Consequently light being trapped in the collecting thin film waveguide through the air/layer interface 4a of the upper dielectric layer 4 or through the substrate 1 may be guided in lateral direction and concentrated to a smaller extraction area, this extraction area being somewhere in the light concentrator in a lateral position not shown in the figure.

[0071] The typical TEi-mode intensity profile 5 shown in the FIG. 1 gives the intensity distribution of the mode as a function of the dimension x.

[0072] The nanopatterned discontinuous excitation layer 3 in the collecting thin film waveguide may comprise statistically distributed nanoscaled particles of material, preferably silver. For example it may be a silver island film (SIF).

[0073] The lower part of FIG. 1 shows an embodiment according to which the nanopatterned discontinuous excitation layer 3 is formed of a diffracting grating structure having periodically arranged structure elements 7. Each of the structure elements 7 may be formed by a silver island film or other discontinuous material or even by a continuous layer area since in such a case the nanoscaled spacing 9 between each two structure elements 7 breaks continuity as well.

[0074] In this embodiment furthermore the thickness of layer 2 and 4 encapsulating the nanopatterned discontinuous excitation Iayer3 is the same and a capping 8 is chosen to have the same or at least similar refractive index as the index of the substrate thus forming a symmetric propagating mode 5. For best symmetry results the thickness of the capping 8 must also be large compared to the penetration depth of the evanescent field in that field that is shown by the exponential decay of the intensity profile 5.

[0075] The embodiment of FIG. 2 shows how nanopatterns can be implemented in the discontinuous excitation layer 3. As shown in the upper part of FIG. 2 a stamp 10 may by replicated from a template containing a nano relief pattern 11 that for instance may be fabricated by electron beam lithography and etching. The stamp 10 may be formed of an elastic material which can easily be removed from the pattern template as shown in FIG. 2c. In FIG. 2d silver is deposited on the nano relief surface of the stamp and transfer-printed on top of the bottom dielectric layer 2 (FIG. 2e). Each line of the nano pattern 12 is thus transferred to the transparent layer 2 and forms an island film of silver or a continuous silver film being spaced according to the nano template. In this construction the guided mode is excited in a preferred direction D defined by the direction of nano patterning, particularly the nano patterning being provided with one or more periodicities.

[0076] FIGS. 3a to 3f furthermore show, that a nano structure does not necessarily consist of a line structure only as shown in a) but may also have a patterning in two dimensions. Each nano element 12 may comprise a rotation symmetric pattern or a periodic or nonperiodic repetition in both dimensions and may be formed of islandized areas (FIG. 3a,c,g) as well as of continuous areas (FIG. 3b,d-f).

[0077] FIG. 3g shows a top view of a real grating structure that was transfer printed and comprises grating lines each being formed of islandized material.

[0078] FIG. 4 shows in the top part that different waveguide structures I, II, III, IV may be established in a light concentrator according to the invention in a lateral arrangement. Here the structure I constitutes a waveguide for excitation of film modes, a collecting thin film waveguide 2,3,4 as shown in FIG. 1. According to 1a the nanopatterned discontinuous excitation layer 3 may be an islandized layer having no further structure. According to 1b the nanopatterned discontinuous excitation layer 3 may have a superimposed nanopattern, each pattern element being islandized. According to 1c the nanopatterned discontinuous excitation layer 3 is formed of spaced continuous areas.

[0079] In the waveguide structure II no discontinues excitation layer at all exist. The excited mode is propagating undisturbed.

[0080] In the waveguide structure III extraction of the light takes place. Here this is done by removing the top layer 4 of a colleting thin film waveguide 2,3,4

[0081] The waveguide structure IV is used for lateral confinement.

[0082] The lower part of FIG. 4 shows that a light concentrator of the invention may consist of different waveguide structures being stacked on each other. Here the waveguide structures are stacked in two planes, each plane at least comprising a collecting thin film waveguide and waveguide structures for distribution of light in a side-by-side configuration. It makes use of the legend to correlate the certain areas to the waveguide structures above. The different areas, particularly areas for collection and distribution may be offset in the two planes of such a stacked light concentrator. Distribution areas may guide the collected light to a common spot of extraction 13. As a second way for extraction an external hotspot is shown. It is also symbolized by the dotted pattern for an extraction structure although this is here just symbolizing the extraction as no waveguide is included in this position.

[0083] FIG. 5 shows how the concept of the invention may be applied to a second dimension. FIG. 5a shows a collecting thin film waveguide where the mode is confined in x direction only. A fix position of minimum intensity hence exists in x direction only. The wave freely propagates in y and z.

[0084] When confining the wave in a second dimension (y) according to FIG. 5b a stripe shaped collecting thin film waveguide is build. Higher stripe waveguide modes show minimum intensity position with respect to x and y. For instance in a fully symmetric stripe shaped collecting thin film waveguide (with respect to x and y) the intensity distribution 5 of dielectric HE-n or EHu stripe waveguide modes has a minimum intensity in the very center of the waveguide and hence a minimum in x and y.

[0085] As the concept of node modes adapted to one dimension has successfully reduced waveguide losses it is assumed that adapting the concept to a further dimension leads to further improvement of propagation losses for stripe modes propagating in z direction. A stripe shaped nanopatterned discontinuous excitation layer 3 in the center of such a stripe shaped collecting thin film waveguide 4with respect to x and y that is discontinuous with respect to the propagation direction z may excite such stripe modes of very low propagation losses. A possible implementation of such light concentrators having stripe shaped collecting thin film waveguides on large areas is shown in FIG. 5c.