Optical diffraction grating with non-aligned meta-optical elements

Abstract

One example of a device comprises an optical diffraction grating having a first axis oriented in a first direction and a second axis oriented in a second direction different from the first direction, wherein the dimensions of the optical diffraction grating along the first axis are greater than the dimensions of the optical diffraction grating along the second axis. The optical diffraction grating includes a plurality of cells arranged along the first direction, each of which includes a plurality of metaatoms arranged along the first direction, wherein the plurality of metaatoms in each of a particular cell of the plurality are not aligned with respect to the second direction and are arranged asymmetrically with respect to the first axis of the optical diffraction grating.

Description

Technical Field

[0001] The present disclosure relates to a diffraction grating.

Background Art

[0002] [ A diffraction grating is an optical element that disperses light composed of multiple wavelengths into light components according to wavelength. An optical element based on a metasurface can provide an ultra-small light control element. The ultra-small light control element can be integrated into various optical systems due to its ultra-thin and planar nature, and in some cases, it can be manufactured more easily than traditional sawtooth or blazed gratings. The functionality of such a metasurface has been demonstrated in some cases by a plane wave incident perpendicularly to the grating. However, the diffraction efficiency tends to decrease significantly under irradiation at an incident angle deviating from the vertical. Furthermore, due to various factors, for example, at visible wavelengths, it is difficult to design a metasurface that efficiently directs light to a large deflection angle. Nevertheless, a wide range of acceptance angles and more constant efficiency over an angular range are desirable for optical elements in some applications.

Summary of the Invention

Means for Solving the Problems

[0003] The present disclosure describes a diffraction grating including a meta-optical element (MOE).

[0004] In one aspect, the present disclosure describes an apparatus including a diffraction grating having a first axis facing a first direction and a second axis facing a second direction different from the first direction, and the dimension of the diffraction grating along the first axis is larger than the dimension of the diffraction grating along the second axis. The diffraction grating includes a plurality of cells arranged along the first direction, each of the plurality of cells includes a plurality of meta-atoms arranged along the first direction, and the plurality of meta-atoms in each of a specific cell among the plurality of cells are not aligned with respect to the second direction and are arranged in an asymmetric manner with respect to the first axis of the diffraction grating.

[0005] Some embodiments include one or more of the following features. For example, in some embodiments, each of the plurality of cells has the same arrangement of plurality of metaatoms as the other cells of the optical diffraction grating. In some cases, the arrangement of plurality of metaatoms in at least one of the plurality of cells is rotated 180 degrees with respect to the central axis of the grating with respect to the arrangement of plurality of metaatoms in one of the other cells of the plurality. In some cases, the arrangement of plurality of metaatoms in at least one of the plurality of cells is shifted along a second direction with respect to the arrangement of plurality of metaatoms in one of the other cells of the plurality.

[0006] In some embodiments, the multiple metaatoms in each of a particular cell among a plurality of cells have different sizes, shapes, or orientations from the other metaatoms in the same cell. In some cases, the positions of two metaatoms in at least one of the plurality of cells partially overlap in a first direction.

[0007] In some cases, optical diffraction gratings are one-dimensional gratings, while in other cases, they are two-dimensional gratings (for example, the multiple metaatoms in each particular cell of an optical diffraction grating are arranged two-dimensionally).

[0008] In some embodiments, each of the multiple cells has dimensions shorter than the operating wavelength of the optical diffraction grating. In some embodiments, at least one of the shapes, sizes, or positions of the multiple metaatoms differs among the multiple cells. In some embodiments, the optical diffraction grating is part of a metalens.

[0009] The disclosure also describes an apparatus comprising an optical diffraction grating, a light source operable to emit light, an optical detector operable to detect light, and at least one reflective or transmissive surface for directing light emitted from the light source to the optical diffraction grating or light from the optical diffraction grating to the optical detector. [Effects of the Invention]

[0010] Some embodiments offer one or more of the following advantages. For example, non-alignment can provide greater freedom in lattice design. In some embodiments, the arrangement of metaatoms can result in a reduced phase transition region (PTR), allowing for the design of a phase distribution closer to an ideal sawtooth distribution. In some embodiments, the optical diffraction grating exhibits relatively high diffraction efficiency over a wide range of incident angles. Thus, the performance of the lattice can be improved in some cases.

[0011] Other aspects, features, and advantages will be readily apparent from the following detailed description, accompanying drawings, and claims. [Brief explanation of the drawing]

[0012] [Figure 1] This figure shows an example of an optical diffraction grating containing multiple cells. [Figure 2] This diagram shows an ideal sawtooth-shaped phase distribution. [Figure 3] This figure shows another example of an optical diffraction grating containing aligned metaatoms. [Figure 4] This figure shows an example of an optical diffraction grating containing non-aligned metaatoms. [Figure 5] This figure shows another example of an optical diffraction grating containing non-aligned metaatoms. [Figure 6] This figure shows yet another example of an optical diffraction grating containing non-aligned metaatoms. [Figure 7] This figure shows another example of an optical diffraction grating containing non-aligned metaatoms. [Figure 8] This figure shows yet another example of an optical diffraction grating containing non-aligned metaatoms. [Figure 9A] This figure shows an example of a one-dimensional lattice containing non-aligned metaatoms. [Figure 9B] This figure shows an example of a two-dimensional lattice containing non-aligned metaatoms. [Figure 10] This figure shows an example of an optical system incorporating a light diffraction grating. [Figure 11] This figure shows an example of a metalens that includes an optical diffraction grating containing cells with non-aligned metaatoms. [Figure 12] This figure shows an example of an optical transmission system. [Figure 13] This figure shows an example of an optical receiving system. [Modes for carrying out the invention]

[0013] This disclosure describes optical diffraction gratings including meta-optical elements (MOEs). MOEs are advanced optical elements including meta-surfaces, where a meta-surface means a surface having distributed small structures (e.g., meta-atoms) arranged to interact with light in a particular manner. A meta-surface may also be called a meta-structure and may be a surface having an array of distributed nanostructures. In some cases, meta-atoms are columnar or cylindrical in shape, but in other cases (e.g., meta-atoms having a square cross-section), other shapes may be used. Meta-atoms may interact with light waves individually or collectively. For example, meta-atoms may alter the local amplitude, local phase, or both of the incident light wave. Meta-atoms may be arranged such that the meta-structure functions, for example, as an optical diffraction grating.

[0014] As will be described in more detail below, the metaatoms forming the optical diffraction grating may be distributed along a first direction such that the metaatoms are not aligned with respect to a second direction different from the first direction. The second direction may be orthogonal (i.e., perpendicular) or non-orthogonal to the first direction. Furthermore, the metaatoms may be arranged such that the non-alignment is asymmetric. Such arrangement of metaatoms can, in some embodiments, provide an optical diffraction grating that exhibits high diffraction efficiency over a relatively wide range of incident angles and / or at relatively large incident angles.

[0015] FIG. 1 shows an example of a photorefractive grating 20 including a plurality of cells 22, each of the plurality of cells including a row of meta-atoms 30, 32, 34 arranged along a first direction (e.g., the direction of the X-axis, which is the longer axis of the cell). In the example of FIG. 1, the meta-atoms are aligned with respect to a second direction (e.g., the direction of the Y-axis), and the second direction is orthogonal to the first direction. That is, the centers of each of the meta-atoms 30, 32, 34 are aligned substantially along the same axis. The plurality of meta-atoms in each particular cell can have different sizes (e.g., diameters) from each other. In some embodiments, at least one of the shape, size, or position of the meta-atoms is different between cells 16. Thus, the meta-atoms do not necessarily need to be repeated in a periodic pattern from cell to cell. Instead, in some examples, one or more of the shape, size, or position of the meta-atoms may vary slightly from one cell to the next. For example, in some embodiments, one or more of the shape, size, or position of the meta-atoms may vary slightly from one cell to the next in order to achieve a desired light deflection angle. In some embodiments, the cell sizes are different between cells.

[0016] Using a configuration in which the meta-atoms are aligned as shown in FIG. 1 can make it difficult to obtain a smooth sawtooth-shaped phase distribution (see FIG. 2) because the minimum distance between the centers of the meta-atom 34 with the largest diameter and the meta-atom 30 with the smallest diameter is the sum of their radii (see FIG. 3). That distance corresponds to the phase transition region (PTR). As the size of that region increases, the performance of the grating may deteriorate. Even for the smallest PTR (see FIG. 2), the phase distribution does not become a smooth sawtooth as in a conventional pillar-based MOE grating. Furthermore, arranging the pillars so close to each other may cause an increase in crosstalk and may reduce the overall transmission capacity of the grating.

[0017] According to some embodiments of the present disclosure, as shown in the example of FIG. 4, the photorefractive grating 120 includes a plurality of cells 122, and each of the plurality of cells 122 includes a column of meta-atoms 130, 132, 134 arranged along a first direction (e.g., the direction of the X-axis corresponding to the longer dimension of the grating). Generally, the respective diameters of the smallest and largest meta-atoms should be selected to achieve a 2π phase transition (i.e., one full turn). In contrast to the example of FIG. 1, the meta-atoms 130, 132, 134 are not aligned with respect to a second direction (e.g., the direction of the Y-axis corresponding to the shorter dimension of the grating), and the second direction is different from the first direction. That is, at least some of the centers of each of the respective meta-atoms 130, 132, 134 are not aligned along the same axis with respect to the centers of other meta-atoms among the plurality of meta-atoms in the same cell 122. Instead, the plurality of meta-atoms are arranged at different positions in the Y direction. Such an arrangement can facilitate designs having a significantly reduced PTR (see the lower part of FIG. 4), and in some examples, designs in which the phase distribution is close to the ideal distribution of FIG. 2. Therefore, the grating performance can be improved in some examples. In some embodiments, each of the plurality of cells 122 has dimensions that are shorter than the operating wavelength of the diffraction grating 120 (e.g., shorter than the wavelength of light emitted by a VCSEL or other light source with which the grating is used together).

[0018] Generally, depending on the embodiment, the first direction and the second direction may be orthogonal or non-orthogonal to each other. For example, for a rectangular cell 122, the first direction and the second direction may be orthogonal to each other. On the other hand, for a hexagonal cell, the first direction and the second direction may be non-orthogonal to each other.

[0019] As also shown in FIG. 4, the meta-atoms can be arranged such that the misalignment is asymmetric. In some embodiments, the refractive index centers of the meta-atoms 130, 132, 134 in the cell 122 are at or near the central axis of the grating (e.g., a line parallel to the X-axis in FIG. 4).

[0020] Figure 4 shows two cells 122, each having three metaatoms 130, 132, and 134, but other embodiments may include a different number of cells and / or metaatoms per cell. For example, in some embodiments, a cell 122 may have as many as 15 or more metaatoms. Using relatively small cells 122 can facilitate obtaining a lattice that operates at a relatively large deflection angle. A cell 122 can be replicated along a first direction (e.g., the X-axis direction) for a lattice 220 containing cells 122(1), 122(2)...122(n), as shown in Figure 5, for only a few times, or tens of times, hundreds of times, or thousands of times. For example, for a lattice having a length of a few microns, in some cases, a cell 122 may be replicated on the order of thousands of times. Within each particular cell 122(1), 122(2)...122(n), the metaatoms are arranged in an asymmetric manner with respect to the central axis 250 of the lattice. In some examples, cell 122 may be replicated with slight variations, such as the size of the unit elements, the position of the metaatoms, or the size of the metaatoms (e.g., diameter).

[0021] In some examples, one or more cells may be mirror images of other cells in a group of cells. For example, the arrangement of metaatoms in a cell may be rotated 180 degrees about the central axis along the direction in which the cells are distributed. For example, as shown in Figure 6, the metaatoms 130B, 132B, and 134B of the second cell 122B are rotated 180 degrees about the central axis 350 relative to the metaatoms 130A, 132A, and 134A of the first cell 122A. Within each particular cell 122A, 122B, the metaatoms are arranged in an asymmetric manner with respect to the central axis 350. Simulations show that inverting cell 122B with respect to cell 122A along the shorter side of the lattice 320 does not change the optical performance of the lattice.

[0022] In some examples, one or more cells are moved laterally along a second direction (e.g., in the Y-axis direction) relative to other cells in a group of cells. Figure 7 shows an example where a metaatom in cell 122C is moved in a second direction (i.e., along the shorter dimension of the lattice) relative to a metaatom in cell 122A. Simulations show that such a lateral movement of cell 122C relative to cell 122A does not change the optical performance of the lattice.

[0023] As described above, in some embodiments, using an arrangement in which metaatoms are not aligned and are arranged asymmetrically (e.g., Figure 4) can result in a smaller PTR than that achieved using an arrangement in which metaatoms are aligned (Figures 1 and 2). The resulting PTR may be reduced in some cases, but the PTR does not necessarily have to be minimized. Instead, the PTR can be optimized along with other variables to achieve the desired phase distribution and optical performance.

[0024] Furthermore, in some embodiments, the lattice has an arrangement in which the metaatoms are not aligned but arranged in an asymmetric manner, and the PTR may be greater than that which can be achieved using an aligned arrangement of metaatoms. Thus, in some examples, the positions of two adjacent metaatoms may at least partially overlap in the first direction (i.e., in the direction of the longer length of the lattice) (see Figures 4 and 5), while in other examples, the positions of multiple metaatoms may not overlap at all in the first direction (see Figure 8). In each of these states, non-alignment can provide further degrees of freedom in lattice design.

[0025] The metaatoms of the lattice can be composed of a material with a relatively high refractive index (e.g., amorphous silicon) that alters one or more properties of incident light in a particular direction. In some examples, the metaatoms are columnar nanostructures on glass or other substrates. As described above, the metaatoms may have other shapes in some embodiments.

[0026] The above example describes the arrangement of metaatoms in a one-dimensional (1D) (linear) lattice, but the technique can be similarly applied to two-dimensional (2D) lattices. For example, Figure 9A shows an example of a lattice where each cell 122D has three metaatoms in a one-dimensional arrangement, while Figure 9B shows an example of a lattice where each cell 122E has nine metaatoms in a two-dimensional arrangement.

[0027] The optical diffraction gratings described above, shown in Figures 4 through 9B, can be employed in a wide range of applications, including other applications using focusing or diverging lenses, optical collimators, and optical phase distributions. In general, non-aligned techniques can be advantageous for arrangements in which metaatoms have a limited diameter range. Optical diffraction gratings can also be used in devices such as spectrometers that split multi-wavelength light into wavelengths composed of inherent elements that make up the light.

[0028] Figure 10 shows an example of an optical system 700 (e.g., a spectrometer) that can incorporate any of the gratings 706 shown in Figures 4-8 above. The system 700 includes a light source 702 that emits multi-wavelength (e.g., white) light 703, and an optical detector 710 that can detect a wide range of wavelengths of light 709. Mirrors or other reflective surfaces 704, 708 may be provided to direct the light 703, 709 towards the grating 706 or the detector 710, respectively. In some examples, one or more transmissive surfaces may be provided to direct the light towards the grating 706 or the detector.

[0029] In some embodiments, as shown in Figure 11, the above-described optical diffraction gratings (e.g., gratings 120, 220, 230) can all function as a part 401 of a metalens 400. For example, as shown in Figure 11, the optical diffraction grating 120 includes one or more cells 122 having unaligned metaatoms 130, 132, 134, and forms a part 401 of the annular ring of the metalens 400. The structure of the optical diffraction grating 120 can be repeated to form a part of the annular ring of the metalens. Thus, in some cases, the metalens may be symmetrical with respect to the center of the metalens, or asymmetrical in the radial direction. As shown in Figure 12, the metalens 400 can be integrated into an optical transmission system 410, for example, including a light source 402 operable to produce a ray 404 that passes through the metalens. In such embodiments, the metalens 400 can provide, for example, a collimating function for the ray 404. In other cases, as shown in Figure 13, the metalens 400 may be integrated into an optical receiving system 420 (e.g., a camera) that directs the light rays 422 towards, for example, an image sensor 424 or at least one other optical detector.

[0030] While this specification contains many details, these are not intended to limit the scope of disclosure or claims, but should be interpreted as descriptions of the specific features of particular embodiments. Features described in the context of separate embodiments in this specification may also be combined in the same embodiment. Conversely, various features described in the context of a single embodiment may also be implemented separately or in any suitable subcombination in multiple embodiments. Various modifications can be made to the examples described herein. Therefore, other embodiments are also within the scope of the claims.

Claims

1. The optical diffraction grating comprises a first axis pointing in a first direction and a second axis pointing in a second direction different from the first direction, The dimensions of the optical diffraction grating along the first axis are greater than the dimensions of the optical diffraction grating along the second axis. The optical diffraction grating includes a plurality of cells arranged along the first direction, Each of the plurality of cells includes a plurality of metaatoms arranged along the first direction, An apparatus in which the plurality of metaatoms in each of a particular cell among the plurality of cells are not aligned with respect to the second direction and are arranged in an asymmetric manner with respect to the first axis of the optical diffraction grating.

2. The apparatus according to claim 1, wherein each of the plurality of cells has the same arrangement of a plurality of metaatoms as the other cells of the optical diffraction grating.

3. The apparatus according to claim 1, wherein the arrangement of the plurality of metaatoms in at least one of the plurality of cells is rotated 180 degrees with respect to the central axis of the lattice with respect to the arrangement of the plurality of metaatoms in another of the plurality of cells.

4. The apparatus according to claim 1, wherein the arrangement of the plurality of metaatoms in at least one of the plurality of cells is moved along the second direction with respect to the arrangement of the plurality of metaatoms in another of the plurality of cells.

5. The arrangement of the plurality of metaatoms in at least one of the plurality of cells is rotated 180 degrees with respect to the central axis of the lattice with respect to the arrangement of the plurality of metaatoms in another of the plurality of cells. The apparatus according to claim 1, wherein the arrangement of the plurality of metaatoms in at least one of the plurality of cells is moved along the second direction with respect to the arrangement of the plurality of metaatoms in one of the other of the plurality of cells.

6. The apparatus according to any one of claims 1 to 5, wherein each of the plurality of metaatoms in a specific cell among the plurality of cells has a size, shape, or orientation that is different from the size, shape, or orientation of the other metaatoms among the plurality of metaatoms in the same cell.

7. The apparatus according to any one of claims 1 to 6, wherein the positions of two metaatoms in at least one of the plurality of cells partially overlap in the first direction.

8. The apparatus according to any one of claims 1 to 7, wherein the plurality of metaatoms in each specific cell of the optical diffraction grating are arranged in two dimensions.

9. The aforementioned device is A light source capable of emitting light, An optical detector capable of detecting light, The apparatus according to any one of claims 1 to 8, further comprising at least one reflective or transmitting surface for directing light emitted from the light source to the optical diffraction grating, or for directing light from the optical diffraction grating to the optical detector.

10. The apparatus according to any one of claims 1 to 9, wherein each of the plurality of cells has dimensions shorter than the operating wavelength for the optical diffraction grating.

11. The apparatus according to any one of claims 1 to 10, wherein at least one of the shapes, sizes, or positions of the plurality of metaatoms differs among the plurality of cells.

12. The apparatus according to any one of claims 1 to 11, wherein the cell sizes differ among the plurality of cells.

13. The apparatus according to any one of claims 1 to 12, wherein the optical diffraction grating is part of a metalens.

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