Discriminating electromagnetic radiation based on angle of incidence

Abstract

The present invention provides systems, articles, and methods for discriminating electromagnetic radiation based upon the angle of incidence of the electromagnetic radiation. In some cases, the materials and systems described herein can be capable of inhibiting reflection of electromagnetic radiation (e.g., the materials and systems can be capable of transmitting and/or absorbing electromagnetic radiation) within a given range of angles of incidence at a first incident surface, while substantially reflecting electromagnetic radiation outside the range of angles of incidence at a second incident surface (which can be the same as or different from the first incident surface). A photonic material comprising a plurality of periodically occurring separate domains can be used, in some cases, to selectively transmit and/or selectively absorb one portion of incoming electromagnetic radiation while reflecting another portion of incoming electromagnetic radiation, based upon the angle of incidence. In some embodiments, one domain of the photonic material can include an isotropic dielectric function, while another domain of the photonic material can include an anisotropic dielectric function. In some instances, one domain of the photonic material can include an isotropic magnetic permeability, while another domain of the photonic material can include an anisotropic magnetic permeability. In some embodiments, non-photonic materials (e.g., materials with relatively large scale features) can be used to selectively absorb incoming electromagnetic radiation based on angle of incidence.

Description

RELATED APPLICATIONS

n_A^TM sin θ_A^TM=n_air sin θ_inc   (4)

From the expressions for n_A^TE and n_A^TM, one can see that TE-polarized electromagnetic radiation is affected only by ∈_yy^A, μ_xx^A, and μ_zz^A, while TM-polarized electromagnetic radiation is affected only by μ_yy^A, ∈_xx^A and ∈_zz^A. In particular, TM-polarized electromagnetic radiation is affected by ∈_zz^A, while TE-polarized electromagnetic radiation is not. One can also observe that for angles θ_A close to the normal, sin θ_A≈0. Accordingly, in view of Equations 1 and 2, n_A increases only slightly with increasing anisotropy. However, for incidence angles not close to the normal, n_A increases more rapidly with increasing anisotropy, and therefore higher anisotropy in ∈ or μ results in higher index contrast and wider frequency gaps at those angles.

The multilayer structure illustrated in FIG. 6A discriminates angles only for TM-polarized electromagnetic radiation. One can generalize this structure and design a different multilayer structure which is capable of discriminating angles of incidence irrespective of the polarization of the electromagnetic radiation. Such a multilayer structure can include anisotropy in both the dielectric function and in the magnetic permeability. More specifically, the proposed structure can have ∈_B=μ_A=γ₁ while ∈_A=μ_B=(γ₁,γ₁,γ₂). Note that a structure having ∈_A=μ_A=(γ₁,γ₁,γ₂) and ∈_B=μ_B=γ₁ would work equally well and would have a larger fractional frequency gap (at the quarter-wave condition which is satisfied differently at different incidence angles). The structure illustrated and analyzed in FIGS. 7A-7C can offer somewhat more flexibility in material choice, in the sense that anisotropic ∈ and μ would not have to occur in the same material.

For simplicity, we consider the case ∈_inc=μ_inc=γ₁ in this example. FIGS. 7A-7B includes schematic diagrams showing (A) TM-polarized and (B) TE-polarized electromagnetic radiation incident at nonzero angles from ∈_inc=μ_inc=γ₁ on an anisotropic multilayer structure with ∈_A=μ_B=(γ₁,γ₁,γ₂), ∈_B=μ_A=γ₁. Here, γ₂≠γ₁. In both of these cases, electromagnetic radiation experiences index contrasts and hence photonic bandgaps. For example, as shown in the schematic of FIG. 7A, TM-polarized electromagnetic radiation incident from air at a nonzero angle on this structure experiences a photonic bandgap because according to Eq. (2), n_B^TM=√{square root over (γ₁)} while n_A^TM≠√{square root over (γ₁)}. On the other hand, TE-polarized electromagnetic radiation incident (from air) at the same nonzero angle also experiences an index contrast because according to Eq. (1), n_A^TE=√{square root over (γ₁)} while n_B^TE≠√{square root over (γ₁)}. FIG. 7C includes a contour plot showing how the relative size of the TE (and also TM) photonic bandgap changes with θ_inc and with the degree of anisotropy γ₂/γ₁ for electromagnetic radiation incident from ∈_inc=μ_inc=γ₁ on the anisotropic multilayer structure of FIG. 7A. This result was obtained using TMM, and the thickness of layers A and B were chosen to be equal (h_A=h_B=0.5a) so that the structure discriminates different angles over the same frequency interval and for both polarizations simultaneously. Note that this choice of h_A=h_B does not result in the largest possible fractional gap for either polarization, which actually occurs at the quarter-wave condition, satisfied non-simultaneously for the two different polarizations when ∈_A=μ_B=(γ₁,γ₁,γ₂≠γ₁).

From the contour plot in FIG. 7C, one can observe that the size of the fractional gap increases only slightly with increasing anisotropy γ₂/γ₁ beyond γ₂/γ₁≈2, which can also be seen by inspection of Equation 1 and Equation 2, and noticing that the achievable index contrast “saturates” for large γ₂/γ₁ anisotropy values. Therefore, materials with very large anisotropy do not necessarily lead to much larger bandgap in these structures. This is somewhat contrary to conventional photonic crystals where substantially large index contrast typically lead to substantially larger bandgaps. Note also that the size of the fractional frequency gap also increases with θ_inc.

The structures described above allow one to open an angular gap for both TE and TM polarizations over a certain frequency range. One might also consider how to enlarge the frequency range over which this angular discrimination is exhibited. To achieve a relatively large fractional frequency gap that occurs simultaneously for both polarizations, one can operate at the quarter-wave condition (the structure obeying the quarter-wave condition at 45° is used in the calculations of FIG. 8A, While the structure obeying the quarter-wave condition at 22.5° is used in FIGS. 8B and 8C). In this case, anisotropic ∈_A=μ_A is used (as opposed to the previously considered case when anisotropic ∈_A=μ_B). One can start with a single stack consisting of 30 homogeneous bilayers with ∈_A=μ_A=(1.23, 1.23, 2.43) and ∈_B=μ_B=1.23. The size of each bilayer in this stack is a. Electromagnetic radiation incident at 45° (from air) on this stack experiences a simultaneous TE and TM photonic bandgap having a fractional frequency width of 6.94% (at quarter-wave condition). To widen this fractional frequency range, one can consider a multilayer consisting of 17 such stacks, each stack being made out of 30 bilayers, however the period of each stack is chosen so that frequency gaps of different stacks are contiguous and merge together, resulting in a much larger frequency gap (≈17 times the size of the gap in the single gap case). In the system analyzed for this example, the period a_i of the i^th stack (i=1, 2, . . . , 17) was chosen to be a_i=1.0694⁽ⁱ⁻¹⁾a, where a is the period of the first stack facing the incident electromagnetic radiation. The thickness h_Aⁱ of layer A in the i^th stack was chosen to be 0.473a_i so that the quarter-wave condition (which maximizes the relative size of the frequency gap) is satisfied. FIG. 8A includes a transmission spectrum, obtained using TMM, for electromagnetic radiation incident from air at an angle of incidence of 45° (i.e., 45° from normal) on the 17-stack multilayer structure described above. As shown in FIG. 8A, a reflection window with a relative frequency size of about 104% was observed for both TE and TM polarizations simultaneously.

FIG. 8B includes additional transmission spectra obtained using TMM. The TMM results in FIG. 8B show a 107% wide frequency gap for both TE-polarized and TM-polarized electromagnetic radiation incident at 22.5° from the normal, irrespective of its polarization. This structure analyzed in FIG. 8B includes 71 stacks each consisting of 130 bilayers each having ∈_A=μ_A=(1.23, 1.23, 2.43), ∈_B=μ_B=1.23, and a_i=1.0164⁽ⁱ⁻¹⁾a. The thickness hⁱ_A of layer A was 0.494a_i in this case (which satisfies the quarter-wave condition).

FIG. 8C includes TMM transmission spectra for electromagnetic radiation incident at 45° on the structure described in association with FIG. 8B. In the case illustrated in FIG. 8C, there is reflection at all frequencies that fall inside the reflection window shown in FIG. 8B for 22.5° incidence. That is, the structure designed to reflect 22.5°-incident electromagnetic radiation in the frequency range (0.13c/a→0.43c/a) irrespective of its polarization, is also capable of reflecting electromagnetic radiation incident at all angles greater than 22.5° in this same frequency range and irrespective of the polarization; after all, bandgap of each stack is larger for 45° incidence than for 22.5° incidence. Electromagnetic radiation incident close-to-normal on this same structure is transmitted irrespective of its polarization. In fact, this proposed structure exhibits an angular gap (for θ_inc between 22.5° and 90°) for both polarizations simultaneously over a 107% wide frequency range.

A variety of materials can be used to achieve the above-described concepts. As seen in FIG. 6A, dielectric metamaterial approaches could be used for TM-polarized electromagnetic radiation. For TE polarization (where anisotropic p can be used), one option is to use metallo-dielectric metamaterials as described, for example, in J. B. Pendry, A. J. Holden, D. J. Roddins, and W. J. Stewart, IEEE transactions on microwave theory and techniques, Vol. 47, No. 11, November 1999. In some cases, one might split incoming electromagnetic radiation according to polarization before it enters the structure, rotate TE polarization into TM polarization, and only then allow it to continue to the structure (see e.g., Barwicz, M. R. Watts, M. A. Popovic, P. T. Rakich, L. Socci, F. X. Kaertner, E. P. Ippen, and H. I. Smith, Nature Photonics, 1, pp 57-60, (2007)).

Example 2

This example describes the design and performance of a macroscale filter used to discriminate electromagnetic radiation based on angle of incidence. In this example, a 2-dimensional array of channels is formed in a reflective material to produce the structure illustrated in FIGS. 9A-9F. FIG. 9A is a schematic illustration of the incident surface of the structure (i.e., a top view of the structure). FIGS. 9B-9F are schematic illustrations of the structure in FIG. 9A from various incidence angles. FIG. 9B is a view of the structure from an incidence angle of 15°, FIG. 9C is a view from an incidence angle of 30°, FIG. 9D is a view from an incidence angle of 45°, FIG. 9E is a view from an incidence angle of 60°, and FIG. 9F is a view from an incidence angle of 75°.

The macroscale filter illustrated in FIGS. 9A-9F includes a periodic array of cylinders formed in a reflective material such as a metal. Each of the holes has a cross-sectional diameter and a length (corresponding to the thickness of the reflective material) such that the aspect ratios of the cylinders are 10:1 (i.e., the channel lengths are 10 times greater than the channel cross-sectional diameters). The holes are spaced such that they have a nearest neighbor distance of 1.05 times the cross-sectional diameters of the holes (i.e., the hole cross-sectional diameters are 95% of the nearest neighbor distances).

The amount of incident electromagnetic radiation that is reflected from the device illustrated in FIGS. 9A-9F is dependent upon the angle of incidence. For angles of incidence less than or equal to θ_m (which corresponds to the maximum polar angle at which electromagnetic radiation is exchanged with the environment), the percentage of electromagnetic radiation that will be reflected from the macroscale filter is calculated as:
% Reflected=R_o+a·sin(θ)   [5]
where R_o represents reflection at normal incidence, a represents angular sensitivity (which depends on the geometry), and θ is the angle of incidence. For angles of incidence greater than θ_m, the percentage of electromagnetic radiation that will be reflected from the macroscale filter is calculated as:
% Reflected=R_o+a·sin(θ_m)   [6]

For the macroscale filter illustrated in FIGS. 9A-9F, at an angle of incidence of 0°, about 16% of the electromagnetic radiation incident on the incident surface is reflected by the filter. At an incidence angle of 75°, roughly 90% of the electromagnetic radiation incident on the incident surface is reflected by the filter.

Example 3

This example describes the enhancement in the performance in a solar thermophotovoltaic (TPV) system that can be achieved using a 2-dimensionally periodic photonic crystal to selectively absorb electromagnetic radiation. In this example, the theoretical performance of a standard solar TPV system without an angularly selective absorber is analyzed, both in the ideal case and with a realistic amount of long-wavelength emissivity. Next, the improvement that can be achieved in a structure with long-wavelength emissivity using an angle-sensitive design, as illustrated in FIG. 10, is described.

The energy conversion efficiency of a solar TPV system such as the system illustrated in FIG. 10 is defined to be:

$\begin{array}{cc}\eta =\frac{I_m{V}_m}{I_s{A}_s}& \left[7\right]\end{array}$
where I_m and V_m are the current and voltage of the thermophotovoltaic diode at the maximum power point, C is the concentration in suns relative to the solar constant I_s (usually taken to be 1 kW/m²), and A_s is the surface area of the selective solar absorber. This system can conceptually be decomposed into two halves: the selective solar absorber front end and the selective emitter plus TPV diode back end. Each half can be assigned its own efficiency: η_t and η_p, respectively. The system efficiency can then be rewritten as:
η=η_t(T)η_p(T)   [8]
where T is the equilibrium temperature of the selective absorber and emitter region. The efficiency of each subsystem can be further decomposed into its component parts. In particular, the selective solar absorber efficiency can be represented by:

$\begin{array}{cc}{\eta }_t\left(T\right)=B\stackrel{\_}{\alpha }-\frac{\stackrel{\_}{ϵ}\sigma T^4}{{\mathrm{CI}}_s}& \left[9\right]\end{array}$
where B is the window transmissivity, ∞ is the spectrally averaged absorptivity, ∈ is the spectrally averaged emissivity, and B is the Stefan-Boltzmann constant.

The TPV diode backend efficiency can be represented by

$\begin{array}{cc}{\eta }_p=\frac{I_m{V}_m}{{\stackrel{\_}{ϵ}}_E{A}_E\sigma T^4}& \left[10\right]\end{array}$
where ∈_E E and A_E are the effective emissivity and area of the selective emitter, respectively.

First, one can consider the situation where absorptivity for both the selective absorber and emitter is unity within a certain frequency range, and δ otherwise. δ corresponds to the emissivity which cannot be suppressed due to fabrication and/or material constraints, which one would generally like to be as small as possible. The ranges for the selective absorbers and emitters are optimized separately, and the lower end of the selective emitter frequency range equals the TPV diode bandgap frequency w_g. If one considers the case of unconcentrated sunlight, the limit δ→0 implies a decoupling between the selective absorber and emitter, where the selective absorber is kept relatively cool to maximize η_t, while the selective emitter acts as if it were much hotter with a bandgap frequency ω_g well over the blackbody peak predicted by Wien's law. However, this also leads to declining effective emissivity ∈_E, (which varies proportionally with δ), and thus A_E/A_s varies proportionally with 1/δ. This expectation is supported by the numerical calculations shown in FIGS. 11A-11B, which demonstrate both that efficiency slowly increases with decreasing δ, while the area ratio increases rapidly as 1/δ. The limit where δ→0 and A_E/A_s→∞ is unphysical, both because the time to establish equilibrium in an arbitrarily large system is arbitrarily long, and a perfectly sharp emissivity cutoff requires a step function in the imaginary part of the dielectric constant of the underlying material. However, the latter is inconsistent with the Kramers-Kronig relations for materials, which derive from causality.

Based on previous comprehensive reviews of selective solar absorbers, typical spectrally averaged selective solar absorber emissivities ∈ are about 0.05 at a temperature of approximately 373 K. Assuming δ=0.05 as well, this implies a maximum system efficiency of 10.5% (T=720 K, η_t=0.6937, η_p=0.1510, and A_E=A_s=0.75), as illustrated in FIG. 12A. While a physically relevant result, this efficiency is unfortunately less than a quarter of the asymptotic efficiency calculated above as δ→0.

In order to bridge the gap between performance of solar TPV in the cases where δ=0.05 and δ→0, one can employ an absorber with a specific form of angular selectivity. The expression used in this example is as follows:
∈(ω,θ)=[1−(θ/θ_max²][δ+(1−δ)Π_ω1,ω2(ω)   [11]
where Π_ω1,ω2 is the tophat function (equal to 1 if ω₁<ω<ω₂ and 0 otherwise), θ_max is the maximum polar angle at which electromagnetic radiation is exchanged with the environment, and ω₁ and ω₂ are the range of absorbed wavelengths. FIG. 13 includes a plot of emissivity as a function of polar angle θ for frequencies within the window of the top hat. Inserting Equation 11 into Equation 10 and setting δ=0.05 yields the results in FIG. 12B, where the maximum efficiency is 37.0% (T=1600 K, η_t=0.7872, η_p=0.4697, A_E/A_s=0.05). This is 3.5 times higher than the result illustrated in FIG. 12A, and is fairly close to the asymptotic limit where δ→0, without the physically unreasonable requirement of a perfectly sharp emissivity cutoff (which is inconsistent with causality). This result also exceeds the Shockley-Quiesser limit for photovoltaic energy conversion in unconcentrated sunlight of 31% efficiency. Furthermore, as illustrated in FIG. 14, photovoltaic diodes made from group IV compounds such as silicon and germanium have bandgaps that would allow for the system to continue to exceed the Shockley-Quiesser limit. Finally, the much lower area ratio A_E/A_s=0.05 implies that the angle-selective solar absorber illustrated in FIG. 10 would serve as a sort of thermal concentrator, thus allowing for much less thermophotovoltaic area to be used compared to previous designs.

One structure capable of providing the top-hat functionality described above is a 2-dimensionally periodic photonic crystal configured to selectively absorb incident electromagnetic radiation. The 2-dimensionally periodic photonic crystal considered in this example is formed in a 4244 nm-thick layer of single-crystal tungsten. The tungsten layer comprises a plurality of cylindrical holes with a radius of 795 nm (corresponding to cross-sectional diameters of 1590 nm) formed within the layer. The holes have an average nearest neighbor distance of 1591 nm, and are arranged periodically similar to the arrangement illustrated in FIG. 4A. The cylindrical holes have lengths of 4244 nm, corresponding to the thickness of the tungsten layer in which they were formed.

The performance of the 2-dimensionally periodic photonic crystal described above was simulated using S-matrix code as described in Whittaker and Culshaw, “Scattering-matrix treatment of patterned multilayer photonic structures,” Phys Rev B, 60, 2610 (1999). As shown in FIG. 15, the 2-dimensionally periodic photonic crystal exhibits decreasing average emissivity with increasing incident angle. In particular, at a 75° incident angle, the average emissivity is 60% lower than at normal incidence (i.e., an incident angle of 0°).

While several embodiments of the present invention have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the functions and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the present invention. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the teachings of the present invention is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the scope of the present invention.

The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Other elements may optionally be present other than the elements specifically identified by the “and/or” clause, whether related or unrelated to those elements specifically identified unless clearly indicated to the contrary. Thus, as a non-limiting example, a reference to “A and/or B,” when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A without B (optionally including elements other than B); in another embodiment, to B without A (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.

As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and/or” as defined above. For example, when separating items in a list, “or” or “and/or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of” or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e. “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.

Claims

1. An article, comprising:

a photonic material comprising a plurality of periodically occurring separate layers, including at least a first layer and a second layer disposed over the first layer, wherein:

the first or second layer has an isotropic dielectric function while the other of the first and second layers has an anisotropic dielectric function;

the first or second layer has an isotropic magnetic permeability while the other of the first and second layers layer has an anisotropic magnetic permeability; and

the first and second layers are part of a 1-dimensionally periodic photonic crystal comprising a periodic assembly of layers, wherein the 1-dimensionally periodic photonic crystal is arranged in such a way that a line along a first coordinate direction within the 1-dimensionally periodic photonic crystal passes through multiple layers while lines along second and third orthogonal coordinate directions, each of the second and third orthogonal coordinate directions being orthogonal to the first coordinate direction within the 1-dimensionally periodic photonic crystal, do not pass through multiple layers; and

the second layer comprising the anisotropic magnetic permeability comprises a combination of at least two materials arranged to have an anisotropic effective magnetic permeability.

2. The article of claim 1, wherein the first layer has an isotropic dielectric function, and the second layer has an anisotropic dielectric function.

3. The article of claim 1, wherein the first layer has an anisotropic dielectric function, and the second layer has an isotropic dielectric function.

4. The article of claim 1, wherein the first layer has an isotropic magnetic permeability, and the second layer has an anisotropic magnetic permeability.

5. The article of claim 1, wherein the first layer has an anisotropic magnetic permeability, and the second layer has an isotropic magnetic permeability.

6. The article of claim 2, wherein the second layer comprising an anisotropic dielectric function comprises a single material having an anisotropic dielectric function.

7. The article of claim 6, wherein the single material having an anisotropic dielectric function comprises TiO₂, calcite, calomel, beryl, lithium niobate, zircon, and/or or mica.

8. The article of claim 2, wherein the second layer comprising an anisotropic dielectric function comprises a combination of at least two materials arranged to have an anisotropic effective dielectric function.

9. The article of claim 8, wherein the combination of at least two materials arranged to have an anisotropic effective dielectric function form a 2-dimensionally periodic photonic crystal.

10. The article of claim 4, wherein the second layer comprising an anisotropic magnetic permeability comprises a combination of at least two materials arranged to have an anisotropic effective magnetic permeability.

11. The article of claim 10 1, wherein the second layer comprising an anisotropic magnetic permeability comprises a combination of a metal and a second material.

12. The article of claim 11, wherein the second material comprises a dielectric material.

13. The article of claim 10 1, wherein the combination of at least two materials arranged to have an anisotropic effective magnetic permeability form a 2-dimensionally periodic photonic crystal.

14. The article of claim 1, wherein, when the photonic material is exposed to electromagnetic radiation, at least about 75% of the electromagnetic radiation within a range of wavelengths that contacts an incident surface of the photonic material within a range of angles of incidence is transmitted through the photonic material, and at least about 75% of the electromagnetic radiation within the range of wavelengths that contacts the incident surface outside the range of angles of incidence is reflected by the photonic material.

15. The article of claim 1, further comprising an energy conversion device configured to produce electricity from electromagnetic radiation received from the photonic material.

16. The article of claim 15, wherein the energy conversion device comprises a photovoltaic cell.

17. The article of claim 16, wherein the photovoltaic cell comprises a solar photovoltaic cell.

18. The article of claim 16, wherein the photovoltaic cell is a thermophotovoltaic cell.

19. The article of claim 16, wherein the energy conversion device comprises a heat engine.

20. The article of claim 1, wherein the 1-dimensionally periodic photonic crystal is configured to selectively transmit electromagnetic radiation based upon the angle of incidence of the electromagnetic radiation.

21. An article, comprising:

a plurality of separate layers, including at least a first layer and a second layer disposed over the first layer, wherein:

the first or second layer has an isotropic dielectric function while the other of the first and second layers has an anisotropic dielectric function;

the first and second layers are part of a structure arranged in such a way that a line along a first coordinate direction within the structure passes through multiple layers while lines along second and third orthogonal coordinate directions, each of the second and third orthogonal coordinate directions being orthogonal to the first coordinate direction within the structure, do not pass through multiple layers; and

the article is configured to selectively transmit electromagnetic radiation based upon the angle of incidence of the electromagnetic radiation such that at least about 75% of the electromagnetic radiation between 400 nm and 760 nm that contacts an incident surface of the article at an angle of incidence within 5° of the 0° angle normal to the incident surface is not reflected by the article, and at least about 75% of the electromagnetic radiation between 400 nm and 760 nm that contacts the incident surface of the article at an angle of incidence outside 45° of the 0° angle normal to the incident surface is reflected by the article.

22. The article of claim 21, wherein the article is configured to selectively transmit electromagnetic radiation based upon the angle of incidence of the electromagnetic radiation such that at least about 75% of the electromagnetic radiation between 400 nm and 760 nm that contacts an incident surface of the article at an angle of incidence within 5° of the 0° angle normal to the incident surface is transmitted by the article, and

at least about 75% of the electromagnetic radiation between 400 nm and 760 nm that contacts the incident surface of the article at an angle of incidence outside 45° of the 0° angle normal to the incident surface is reflected by the article.

23. The article of claim 21, further comprising:

a third layer disposed over the first layer and the second layer; and

a fourth layer disposed over the first layer, the second layer, and the third layer, wherein:

the first layer has an isotropic dielectric function,

the second layer has an anisotropic dielectric function,

the third layer has an isotropic dielectric function, and

the fourth layer has an anisotropic dielectric function.

24. An article, comprising:

a plurality of separate layers, including at least a first layer and a second layer disposed over the first layer, wherein:

the first layer has an isotropic dielectric function;

the second layer has an anisotropic dielectric function; and

the article is configured to selectively transmit electromagnetic radiation based upon the angle of incidence of the electromagnetic radiation such that at least about 75% of the electromagnetic radiation between 400 nm and 760 nm that contacts an incident surface of the article at an angle of incidence within 5° of the 0° angle normal to the incident surface is not reflected by the article, and at least about 75% of the electromagnetic radiation between 400 nm and 760 nm that contacts the incident surface of the article at an angle of incidence outside 45° of the 0° angle normal to the incident surface is reflected by the article.

25. The article of claim 24, wherein the article is configured to selectively transmit electromagnetic radiation based upon the angle of incidence of the electromagnetic radiation such that at least about 75% of the electromagnetic radiation between 400 nm and 760 nm that contacts an incident surface of the article at an angle of incidence within 5° of the 0° angle normal to the incident surface is transmitted by the article, and

at least about 75% of the electromagnetic radiation between 400 nm and 760 nm that contacts the incident surface of the article at an angle of incidence outside 45° of the 0° angle normal to the incident surface is reflected by the article.

26. The article of claim 21, wherein the article comprises a material having a first index of refraction and is configured to be exposed to electromagnetic radiation from a medium having a second index of refraction that is smaller than the first index of refraction.

27. The article of claim 22, wherein the article comprises a material having a first index of refraction and is configured to be exposed to electromagnetic radiation from a medium having a second index of refraction that is smaller than the first index of refraction.

28. The article of claim 21, wherein the article is configured to selectively transmit electromagnetic radiation based upon the angle of incidence of the electromagnetic radiation such that at least about 75% of said electromagnetic radiation that contacts an incident surface of the article at an angle of incidence within 5° of the 0° angle normal to the incident surface is transmitted through the article, and at least about 75% of said electromagnetic radiation that contacts the incident surface of the article at an angle of incidence outside 5° of the 0° angle normal to the incident surface is reflected by the article.

29. The article of claim 26, wherein the article is configured to selectively transmit electromagnetic radiation based upon the angle of incidence of the electromagnetic radiation such that at least about 75% of said electromagnetic radiation that contacts an incident surface of the article at an angle of incidence within 5° of the 0° angle normal to the incident surface is transmitted through the article, and at least about 75% of said electromagnetic radiation that contacts the incident surface of the article at an angle of incidence outside 5° of the 0° angle normal to the incident surface is reflected by the article.

30. The article of claim 24, wherein the article comprises a material having a first index of refraction and is configured to be exposed to electromagnetic radiation from a medium having a second index of refraction that is smaller than the first index of refraction.

31. The article of claim 25, wherein the article comprises a material having a first index of refraction and is configured to be exposed to electromagnetic radiation from a medium having a second index of refraction that is smaller than the first index of refraction.

32. The article of claim 24, wherein the article is configured to selectively transmit electromagnetic radiation based upon the angle of incidence of the electromagnetic radiation such that at least about 75% of said electromagnetic radiation that contacts an incident surface of the article at an angle of incidence within 5° of the 0° angle normal to the incident surface is transmitted through the article, and at least about 75% of said electromagnetic radiation that contacts the incident surface of the article at an angle of incidence outside 5° of the 0° angle normal to the incident surface is reflected by the article.

33. The article of claim 30, wherein the article is configured to selectively transmit electromagnetic radiation based upon the angle of incidence of the electromagnetic radiation such that at least about 75% of said electromagnetic radiation that contacts an incident surface of the article at an angle of incidence within 5° of the 0° angle normal to the incident surface is transmitted through the article, and at least about 75% of said electromagnetic radiation that contacts the incident surface of the article at an angle of incidence outside 5° of the 0° angle normal to the incident surface is reflected by the article.