Electrically-controlled dynamic optical component comprising a metasurface

Abstract

An optical component (1) comprising a planar metasurface (2) arranged on a surface of a first substrate (3) and a top layer (4) arranged in a height direction Z above the metasurface (2), wherein the metasurface (2) comprises an array (9) of scattering structures (5, 5a, 5b), wherein the array (9) is a repeating pattern of unit cells (7), wherein a unit cell (7) comprises at least two different scattering structures wherein the optical properties of the metasurface (2) are controllable by a control signal, wherein first scattering structures (5, 5a) are at least partially contacting a layer of a first substance (6a) having a first refractive index and second scattering structures (5, 5b) are at least partially contacting a layer of a second substance (6b), which differs from the first substance (6a) and which provides a variable refractive index depending on the control signal.

Claims

1-16. (canceled)

17. An optical component (1) comprising a planar metasurface (2) arranged on a surface of a first substrate (3) and a top layer (4) arranged in a height direction Z above the metasurface (2), wherein the metasurface (2) comprises an array (9) of scattering structures (5, 5a, 5b) comprising a metal, wherein the array (9) is a repeating pattern of unit cells (7), wherein a unit cell (7) comprises at least two different scattering structures (5, 5a, 5b), characterized in that the optical properties of the metasurface (2) are controllable by a control signal, wherein first scattering structures (5a) are at least partially contacting a layer of a first substance (6a) comprising a dielectric material and having a first refractive index and second scattering structures (5b) are at least partially contacting a layer of a second substance (6b) comprising a polymer, which differs from the first substance (6a) and which provides a variable refractive index depending on the control signal, wherein the real part n and/or the imaginary part K of the variable refractive index of the second substance (6b) is shiftable by the control signal to the respective value of the first sub-stance.

18. The optical component (1) according to claim 17, characterized in that the second substance (6b) comprises a conducting and/or electrochromic polymer, which is switchable between an oxidized and a reduced state, preferably electrochemically switchable between an oxidized and a reduced state, more preferably a polymer comprising conjugated double bonds and/or conjugated p-orbitals, more preferably a plurality of aromatic rings, most preferably a substituted or non-substituted polyaniline.

19. The optical component (1) according to one of the previous claims, characterized in that the metal of the scattering structures (5, 5a, 5b) is selected from a group comprising ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, rhenium, copper and gold, wherein the scattering structures (5, 5a, 5b) preferably mainly consist of one or more of these metals, wherein the scattering structures (5, 5a, 5b) preferably comprise at least 95%, preferably 98%, more preferably 99%, most preferably 99.5% of one of these metals, preferably gold.

21. The optical component (1) according to one of the previous claims, characterized in that the scattering structures (5, 5a, 5b) are deployed as optical antennas, preferably in the form of rods, which are preferably orientated in the plane of the metasurface (2), more preferably in spatially varying orientations.

22. The optical component (1) according to one of the previous claims, characterized in that a geometry and size of the first and/or second scattering structures (5, 5a, 5b) in an array differs by 5%, preferably 3%, more preferably 1% and is most preferably the same, wherein the dimensions of the scattering structures (5, 5a, 5b) are preferably smaller than the wavelength of an incident electromagnetic radiation to be manipulated by the optical component, and/or wherein the spacing between the scattering structures (5, 5a, 5b) is preferably smaller than half of the wavelength of the incident electromagnetic radiation to be manipulated by the optical component (1).

23. The optical component (1) according to one of the previous claims, characterized in that the optical properties of the metasurface (2) are switchable from a first optical proper-ty to a second optical property in less than 500 ms, preferably 250 ms, preferably 100 ms, more preferably 50 ms, most preferably 35 ms.

24. The optical component (1) according to one of the previous claims, characterized in that a height of a volume between the first substrate (3) and the top layer (4) is at least 3, preferably 5, more preferably 10, more preferably 20, most preferably 50 the height of the scattering structures (5, 5a, 5b), wherein a medium, preferably a liquid medium, more preferably a conducting medium, most preferably an ionic solution is arranged in this volume.

25. The optical component (1) according to one the previous claims, characterized in that the first substance (6a) comprises a dielectric polymer, more preferably a acrylate polymer, most preferably polymethyl methacrylate, and is preferably at least partially enclosing the first scattering structures (5, 5a), more preferably contacting the first scattering structures (5, 5a) at all sides which are not contacting the first substrate (3).

26. The optical component (1) according to one of the previous claims, characterized in that the first and/or second scattering structures (5, 5a, 5b) provide a geometry of a cylinder or a parallelepiped, preferably a geometry of a right circular cylinder or a rectangular cuboid, wherein a longitudinal extension is preferably in the range of 100-400 nm, preferably 150-300 nm, more preferably 175-225 nm, most preferably about 200 nm, and a width extension perpendicular to the longitudinal direction in the range of 10-200 nm, preferably 20-150 nm, more preferably 30-100 nm, most preferably 50-80 nm, wherein two width extensions of a cuboid or irregular cylindrical could be independently selected from these preferred ranges.

27. The optical component (1) according to one of the previous claims, characterized in that each unit cell (7) comprises a line (8a) of first scattering structures (5a) and a line (8b) of the second scattering structures (5b), wherein neighboring unit cells (7) are arranged with respect to each other in such a way, that the first and second scattering structures (5, 5a, 5b) are forming in alternating lines (8a, 8b) in the array (9).

28. The optical component (1) according to one of the previous claims, characterized in that the first substrate (3) comprises a carrier, preferably comprising silica, more preferably comprising quartz, most preferably consisting of quartz, which is coated with a conductive coating, preferably a metal containing, most preferably an indium tin oxide coating.

29. The optical component (1) according to one of the previous claims, characterized in that each unit cell (7) is addressable independently by the control signal (11).

30. An optical device comprising the optical component (1) according to one of the previous claims, wherein an application of an electric field causes a modulation of an optical functionality of the optical device due to a modulated phase profile of an incident electromagnetic radiation, which is transmitted through or reflected by the optical component (1).

31. An optical device (100) according to claim 30, characterized in that the optical device is a holographic device or a lens or a beam steering device.

32. A method for producing an optical component (1) comprising a planar metasurface (2), providing optical properties which are controllable by a control signal, comprising the steps of: providing a first substrate (3), providing a top layer (4), producing an array (9) of scattering structures (5, 5a, 5b) comprising a metal on a sur-face of the first substrate (3), optionally by electron-beam lithography, and creating a repeating pattern of unit cells (7) comprising at least two different scattering structures (5, 5a, 5b) in the array, by application of a first substance (6a) comprising a dielectric material and having a first refractive index on at least a part of a surface of a first fraction of the scattering structures (5, 5a) and application of a second substance (6b) comprising a polymer and having a second refractive index on at least a part of a surface of a second fraction of the scattering structures (5, 5b), wherein the second substance (6b) differs from the first substance (6a) and provides a variable refractive index depending on the control signal, wherein the real part n and/or the imaginary part of the variable refractive index of the second substance (6b) is shiftable by the control signal to the respective value of the first substance.

Description

[0049] In the drawings:

[0050] FIGS. 1A and B show an example of an optical component according to the invention from different perspectives;

[0051] FIG. 1C schematic illustration of a holographic image comprising the letters MPI switched on (left) and off (right),

[0052] FIG. 2A shows the chemical structure of polyaniline (PANI) in its emeraldine state (ES) and its leucoemeraldine state (LS) and the electrochemical reaction for the transformation between these two states;

[0053] FIG. 2B an example of an optical component according to the invention from different perspectives;

[0054] FIG. 2C illustrates amendments of the normalized intensity with respect to ;

[0055] FIG. 2D illustrates amendments of the anomalous transmission as a function of the complex refractive indices n.sub.2 and k.sub.2;

[0056] FIG. 3A illustrates amendments of light intensity as a function of the applied voltage;

[0057] FIG. 3B illustrates the switching times for an optical component (off.fwdarw.on (left) and on.fwdarw.off (right));

[0058] FIG. 3C illustrates nearly no degradation of the optical component over at least 100 switching cycles;

[0059] FIG. 4A shows a schematic of an experimental setup;

[0060] FIG. 4B shows a cyclic voltammetry diagram for the electrochemical deposition of PANI on the metasurface sample;

[0061] FIG. 4C shows a SEM image of the metasurface of the optical component;

[0062] FIG. 4D shows an AFM image of the metasurface of an optical component (left) and an overly of two selected height profiles (on the right);

[0063] FIG. 4E shows the in situ recorded normalized intensity of the anomalous transmission during the PANI growth; and

[0064] FIGS. 1A and B show an example of an optical component (1) according to the invention from different perspectives. As illustrated in FIG. 1A, a planar metasurface (2) is arranged between an upper surface (3a) of a first substrate (3) and a top layer (4). While the metasurface arranged directly on the upper surface (3a) of the first substrate (3), the top layer (4, not shown in FIG. 1B) is spaced apart along a height direction (h). The first substrate (3) and the top layer (4) are both conductive and serve as bottom and top electrodes (3, 4), respectively.

[0065] The metasurface (2) comprises a plurality of scattering structures (5, 5a, 5b). The scattering structures (5a, 5b) are arranged in lines (8a, 8b). The (first) scattering structures (5a) of some of these lines (8a) are covered with a first substance (6a) having a first refractive index, whereas the scattering structures (5b) of the other lines (8b) are covered with a second substance (6b), which differs from the first substance and which provides a variable refractive index depending on a control signal, in this example an electric potential (V) applied between the top and bottom electrodes (3, 4). In the illustrated example the first and second scattering structures (5a, 5b) are arranged in alternating lines (8a, 8b). The first substance (6a) covering the first scattering structures (5a) is a dielectric material. The second substance (6b) covering the second scattering structures (5b) is an electrochromic polymer.

[0066] The geometry of all scattering structures (5, 5a, 5b) in this example is the same, a rectangular cuboid. Since these structures are preferably made of gold, also the term gold nanorods is used instead of scattering structures. It should be understood that preferred embodiments only disclosed for gold nanorods (5, 5a, 5b) could also be applied to scattering structures, not comprising gold and/or having a different geometry. However, gold nanorods are preferred since they could be fabricated by electron-beam lithography (EBL) on an ITO-coated quartz substrate. The size of the gold nanorods (5, 5a, 5b) is 200 nm80 nm50 nm. The odd rows (or lines) (8a) are covered by PMMA (h.sub.1=100 nm) through a double-layer EBL process. The gold nanorods (5, 5a, 5b) of the even rows (or lines) (8b) have been covered with PANI by electrochemical polymerization of the metasurface sample in an aqueous electrolyte containing (2 M) HNO.sub.3 and (0.1 M) aniline.

[0067] The longitudinal direction of each cuboid (5) extends in the plane of the upper surface (3a) of a first substrate (3). However, the directions in this plane differs between some of the scattering structures (5, 5a, 5b). The different in-plane orientations of the scattering structures (5, 5a, 5b), which function as optical antennas, allows shaping light wavefronts via a geometric phase, e.g., Pancharatnam-Berry (PB) phase. The scattering structures are of sub-wavelength dimension and also the spacing between adjacent scattering structures (5, 5a, 5b) and/or lines (8a, 8b) of differently covered scattering structures (5, 5a, 5b) is of sub-wavelength dimension.

[0068] FIG. 1B shows a cross section of the metasurface (2) and the bottom electrode (3). The top electrode (4) is not shown. The height of all nanorods (5, 5a, 5b) is identical, however, the form of the covering first and second substance (6a, 6b) differs. While the first substance (6a) covers a line of gold nanorods (5a) and thereby forms a continuous band, the second substance (6b) only covers individual nanorods (5b), thereby forming islands of the second substance (6b). The band of the first substance (6a) has a nearly rectangular cross section. The cross section of the second substance (6a) is similar to a dome surrounding each nanorod (5a).

[0069] The optical properties of such a metasurface (2) could be controlled by applying an electrical potential (V) between the electrodes (3, 4). As a response to such an electrical potential (V), the second substance (6b) in this embodiment polyaniline (PANI) could be switched from its emeraldine state (ES) to its leucoemeraldine state (LS) and vice versa. These two states and the electrochemical reaction for the transformation in each other is illustrated in FIG. 2A.

[0070] As a response to this switching, the optical properties of the metasurface (2) change and a holographic image, in the embodiment illustrated schematically in FIG. 10 comprising the letters MPI, could be switched on (left) or off (right).

[0071] FIG. 2B again shows the optical component (1) from FIGS. 1A and B. However, the top electrode (4) is not shown. In the left illustration of FIG. 2B, a single cell (7) of the array (9) is highlighted. A cross section of such a cell (7) is illustrated on the right of FIG. 2B.

[0072] The complex refractive indices (n.sub.2, k.sub.2) of the second substance (6b), in this embodiment PANI, at different applied voltages are different. Since the anomalous transmission of light through PANI is depending on the complex refractive indices (n.sub.2, k.sub.2) as illustrated in FIG. 2D, the anomalous transmission could be amended by the applied voltage (V). In case the first substance (6a) is PMMA and the first and second substance (6a, 6b) are applied in alternating lines (8a, 8b) on the scattering structure (5a, 5b, 1, 9), the light intensity increases continuously until the applied voltage (V) reaches about 0.6 V (on) (all mentioned electrochemical potentials are measured relative to an Ag/AgCl reference electrode). This could be explained by the gradual change of the refractive index of PANI through electrochemically tuning, while the refractive index of PMMA remains constant. A minimum of the light intensity could be observed at an applied voltage (V) of about 0.2 V (off) as illustrated in FIG. 3A.

[0073] The switching times for rise (off.fwdarw.on) and fall (on.fwdarw.off) processes have been measured to be approximately 48 ms and 35 ms, respectively as shown in FIG. 3B. FIG. 3C illustrates that the switching process between the on (0.6 V) and off(0.2 V) states exhibits no significant degradation over at least 100 cycles. As illustrated by FIGS. 3A-C, the tested optical component (1) provides a superior performance with respect to its switching characteristics, including rapid switching speed, high light intensity contrast, and excellent reversibility.

[0074] Responses of the metasurfaces (2) can be in situ monitored and optimized by controlling the conducting polymer thickness during its electrochemical growth. The operation of the metasurfaces (2) is electrochemically powered, showing excellent performance with high intensity contrast as high as 1000:1, fast switching rate around on the millisecond scale, and notable reversibility over 100 cycles without significant degradation.

[0075] FIG. 4C shows a SEM image of the metasurface (2) of the optical component (1). The illustrated rods (5, 5a, 5b) are arranged in alternating lines (8a, 8b). The rods of the first and third lines (8.sub.1 and 8.sub.3) (from the top) are covered with PMMA, the rods of lines 2 (8.sub.2) and 4 (8.sub.4) are covered with PANI. The scale bar (S) at the lower left corner indicates a length of 200 nm. From FIG. 3C it could be derived, that the lines (8.sub.1-8.sub.2n+1) in which the gold rods (5, 5a, 5b) are covered with PMMA (6a) are homogeneously covered. In contrast thereto, PANI (6b) is only present in the direct surrounding of the rods (8.sub.2-8.sub.2n) of the second and forth lines (8.sub.2 and 8.sub.4). Thus, PANI (6b) is not forming a continuous band along the lines with even numbers (8.sub.2-8.sub.2n).

[0076] FIG. 4D shows an AFM image of the metasurface (2) of an optical component (left) (1). On the right, an overlay of two selected height profiles is shown. The lines along which the height profiles are measured are illustrated as lines (L.sub.1, L.sub.2) in the AFM image on the left. Since the height of the nanorods (5, 5a, 5b) is known to be 50 nm, the thickness of the PANI-coating (6a) on the gold nanorods (5a), t.sub.PANI, could be determined to be approximately 50 nm. The height profiles confirm the different coverage of the gold rods (5, 5a, 5b) of the uneven numbered lines (8.sub.1-8.sub.2n+1) and the even numbered lines (8.sub.2-8.sub.2n). The height profile along the line indicated in grey (L1), which passes through the spacings between neighboring nanorods (5, 5a, 5b) of each line (8), shows a very little height along the width of the even lines (8.sub.2-8.sub.2n), whereas the height profile along the width of the uneven lines (8.sub.1-8.sub.2n+1) is about 100 nm. Thus, for both measuring lines (L.sub.1, L.sub.2), the height profile along the width of the uneven lines (8.sub.1-8.sub.2n+1) is similar, independently whether the lines (L.sub.1, L.sub.2) cross a nanorod (5a) or not. In contrast thereto, the height profile of both measuring lines (L.sub.1, L.sub.2) differ significantly when crossing even lines (8.sub.2-8.sub.2n), depending whether the respective line (L.sub.1, L.sub.2) crosses a nanorod (5b) or not.

[0077] FIG. 4B shows a cyclic voltammetry diagram for the electrochemical deposition of PANI (5b) on the metasurface sample (1). A potential range from 0.2 V to 0.8 V and a scan speed of 25 mV/s has been used.

[0078] FIG. 4A shows a schematic of an experimental setup. The metasurface (2) on ITO (working electrode 3) is immersed into an electrolyte in a glass cell along with a Pt wire (counter electrode 4) and an Ag/AgCl reference electrode. Right-handed circularly polarized (RCP) light impinges on the sample at normal incidence and the intensity of the anomalous transmission is recorded.

[0079] FIG. 4E shows the in situ recorded normalized intensity of the anomalous transmission during the PANI (6b) growth. The electrochemical process is halted, when the intensity reaches the minimum (cycle 36, circle). The intensity contrast, defined as the ratio between the maximum and minimum intensities, is as high as 860:1.

[0080] This invention features great potentials to achieve diversified optical functions, such as optical switch for communication systems and dynamic holographic for data storage.

[0081] All the features disclosed in the application documents are claimed as being essential to the invention if, individually or in combination, they are novel over the prior art.