Wire grid polarizer heat sink

FIELD OF THE INVENTION

0001The present application is related generally to wire gird polarizers.

BACKGROUND

0002A wire grid polarizer (WGP) can be used in an application with high temperatures, such as for example computer projectors. As computer projectors decrease in size and increase in brightness, due to customer demand, the need for WGPs that can endure a high temperature environment also increases.

0003Selectively-absorptive WGPs are particularly susceptible to damage in high-light-intensity computer projectors because they absorb a large percent of incident light. Such WGPs typically have wires that include a reflective portion (e.g. aluminum) and an absorptive portion (e.g. silicon). The absorptive portion can absorb about 80% of one polarization of light, and thus about 40% of the total amount of light. Much of the heat from this absorbed light conducts to the reflective portion of the wire, which can melt, thus destroying the WGP. Such a wire grid polarizer is disclosed by <patcit id="pcit0001" dnum="WO2015199948A1"><text>WO 2015/199948 A1</text></patcit>.

SUMMARY

0004It has been recognized that it would be advantageous to provide a wire grid polarizer (WGP) that can endure high temperatures. The present invention is directed to various embodiments of WGPs that satisfy this need and is defined by the appended claims.

0005The WGP can comprise an array of wires over a face of a transparent substrate, with channels between adjacent wires. Each of the wires can include a reflective layer and an absorptive layer. The WGP can further comprise a heat-dissipation layer, which can be located over the array of wires between the array of wires and the transparent substrate. The heat-dissipation layer can be a continuous layer. The heat-dissipation layer can have a high electrical resistivity and a high coefficient of thermal conductivity.

BRIEF DESCRIPTION OF THE DRAWINGS (drawings might not be drawn to scale)

0006<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> is a schematic, cross-sectional side-view of a wire grid polarizer (WGP) 10 comprising an array of wires 12 over a face 11_f of a transparent substrate 11, with channels 13 between adjacent wires 12, each of the wires 12 including a reflective layer 14 and an absorptive layer 15; a heat-dissipation layer 16 located over the array of wires 12; and the absorptive layer 15 sandwiched between the reflective layer 14 and the heat-dissipation layer 16, in accordance with an embodiment of the present invention.</li><li><figref idref="f0001">FIG. 2</figref> is a schematic, cross-sectional side-view of WGP 20, similar to WGP 10, but with the heat-dissipation layer 16 extending into the channels 13 and along sides of the absorptive layer 15 to directly contact the reflective layer 14, in accordance with an embodiment of the present invention.</li><li><figref idref="f0001">FIG. 3</figref> is a schematic, cross-sectional side-view of WGP 30, similar to WGP 10, but with the reflective layer 14 sandwiched between the absorptive layer 15 and the heat-dissipation layer 16, in accordance with an embodiment of the present invention.</li><li><figref idref="f0002">FIG. 4</figref> is a schematic, cross-sectional side-view of WGP 40, similar to WGPs 10, 20, and 30, but with the heat-dissipation layer 16 extending into and filling the channels 13, in accordance with an embodiment of the present invention.</li><li><figref idref="f0002">FIG. 5</figref> is a schematic, cross-sectional side-view of WGP 50, similar to WGP 10, except that each of the wires 12 further comprises a thermal-insulating layer 18 sandwiched between the reflective layer 14 and the absorptive layer 15, in accordance with an embodiment of the present invention.</li><li><figref idref="f0003">FIG. 6</figref> is a schematic, cross-sectional side-view of WGP 60, similar to WGP 10, except that the heat-dissipation layer 16 is sandwiched between the array of wires 12 and the transparent substrate 11, with the absorptive layer 15 sandwiched between the reflective layer 14 and the heat-dissipation layer 16, in accordance with an embodiment of the present invention.</li><li><figref idref="f0003">FIG. 7</figref> is a schematic, cross-sectional side-view of WGP 70, similar to WGP 60, but with the reflective layer 14 sandwiched between the absorptive layer 15 and the heat-dissipation layer 16, in accordance with an embodiment of the present invention.</li><li><figref idref="f0003">FIG. 8</figref> is a schematic, cross-sectional side-view of WGP 80, similar to WGP 60, except that each of the wires 12 further comprises a thermal-insulating layer 18 sandwiched between the reflective layer 14 and the absorptive layer 15, in accordance with an embodiment of the present invention.</li><li><figref idref="f0004">FIG. 9</figref> is a schematic, cross-sectional side-view of WGP 90, similar to WGP 10, but further comprising a second absorptive layer 15_b and a second heat-dissipation layer 16_b, in accordance with an embodiment of the present invention.</li><li><figref idref="f0004">FIG. 10</figref> is a schematic, cross-sectional side-view of WGP 100, similar to WGP 90, but with the heat-dissipation layer 16 extending into the channels 13 and along sides 15_s of the absorptive layer 15 to directly contact the reflective layer 14, in accordance with an embodiment of the present invention.</li><li><figref idref="f0005">FIG. 11</figref> is a schematic, cross-sectional side-view of WGP 110, similar to WGP 90, except that each of the wires 12 further comprises thermal-insulating layers 18 sandwiched between the reflective layer 14 and each adjacent absorptive layer 15, in accordance with an embodiment of the present invention.</li><li><figref idref="f0005">FIG. 12</figref> is a schematic, cross-sectional side-view of WGP 120, similar to other WGPs described herein, but further comprising heat sinks 121, in accordance with an embodiment of the present invention.</li><li><figref idref="f0006">FIG. 13</figref> is a schematic, cross-sectional side-view of WGP 130, similar to other WGPs described herein, but further comprising an antireflection layer 131 including multiple thin-film layers 132 located on the heat-dissipation layer 16, in accordance with an embodiment of the present invention.</li><li><figref idref="f0006">FIG. 14</figref> is a schematic, cross-sectional side-view of WGP 140, similar to other WGPs described herein, but further comprising an antireflection layer 131 including multiple protrusions 142 located on the heat-dissipation layer 16, in accordance with an embodiment of the present invention.</li><li><figref idref="f0007">FIG. 15</figref> is a schematic, perspective view of WGP 150, similar to other WGPs described herein, but without the heat-dissipation layer 16 in order to clearly show the structure of the array of wires 12, in accordance with an embodiment of the present invention.</li><li><figref idref="f0007">FIG. 16</figref> is a schematic, cross-sectional side-view of WGP 160, with a continuous thin film layer 161 sandwiched between the reflective layer 14 and the absorptive layer 15, forming two separate and distinct arrays of wires 12a and 12b.</li></ul>

DEFINITIONS

0007As used herein, the terms "on", "located on", "located at", and "located over" mean located directly on or located over with some other material between. The terms "located directly on", "adjoin", "adjoins", and "adjoining" mean direct and immediate contact with no other solid material between.

0008As used herein "continuous" means a layer which may include some discontinuity, such as pinholes, but no major discontinuity, such as a division into a grid or separate wires.

0009As used herein, the term "elongated" means that a length L of the wires 12 (length extending into the page of <figref idref="f0001 f0002 f0003 f0004 f0005 f0006">FIGs. 1-14</figref> &amp; <figref idref="f0007">16</figref> and also shown in the perspective view of <figref idref="f0007">FIG. 15</figref>), is substantially greater than wire width W₁₂ or wire thickness Th₁₂ (e.g. L can be at least 10 times, at least 100 times, at least 1000 times, or at least 10,000 times larger than wire width W₁₂ and/or wire thickness Th₁₂).

0010As used herein, the terms "fill", "fills", and "filling", as in "fills the channels" mean completely fills, fills within normal manufacturing tolerances, or nearly completely fills, such that any deviation from completely fills would have negligible effect for ordinary use of the device.

0011As used herein, the term "parallel" means exactly parallel, parallel within normal manufacturing tolerances, or nearly parallel, such that any deviation from exactly parallel would have negligible effect for ordinary use of the device.

0012As used herein, the term "thermal contact" means that the devices in thermal contact with each other are (a) directly touching; or (b) not directly touching but all material(s) between the devices have a coefficient of thermal conductivity of at least 2.0 W/(m^∗K).

0013As used herein, the term "optical thin film" means a thin layer having a thickness less than 10 µm, less than 1 µm, less than 0.5 µm, or less than 0.3 µm, depending on the light spectrum of interest.

0014Materials used in optical structures can absorb some light, reflect some light, and transmit some light. The following definitions distinguish between materials that are primarily absorptive, primarily reflective, or primarily transparent. Each material can be considered to be absorptive, reflective, or transparent in a specific wavelength range (e.g. ultraviolet, visible, or infrared spectrum) and can have a different property in a different wavelength range. Such materials are divided into absorptive, reflective, and transparent based on reflectance R, the real part of the refractive index n, and the imaginary part of the refractive index / extinction coefficient k. Equation 1 is used to determine the reflectance R of the interface between air and a uniform slab of the material at normal incidence: <maths id="math0001" num="Equation 1"><math display="block"><mi>R</mi><mo>=</mo><mfrac><mrow><msup><mfenced separators=""><mi>n</mi><mo>−</mo><mn>1</mn></mfenced><mn>2</mn></msup><mo>+</mo><msup><mi>k</mi><mn>2</mn></msup></mrow><mrow><msup><mfenced separators=""><mi>n</mi><mo>+</mo><mn>1</mn></mfenced><mn>2</mn></msup><mo>+</mo><msup><mi>k</mi><mn>2</mn></msup></mrow></mfrac></math><img file="EP3545345B1_D0001.tif" /></maths>

0015Unless explicitly specified otherwise herein, materials with k ≤ 0.1 in the specified wavelength range are "transparent" materials, materials with k > 0.1 and R ≤ 0.6 in the specified wavelength range are "absorptive" materials, and materials with k > 0.1 and R > 0.6 in the specified wavelength range are "reflective" materials.

DETAILED DESCRIPTION

0016As illustrated in <figref idref="f0001 f0002 f0003 f0004 f0005 f0006">FIGs. 1-14</figref>, wire grid polarizers (WGPs for plural or WGP for singular) 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, and 140, each configured to polarize light incident thereon, are shown comprising an array of wires 12 over a face 11_f of a transparent substrate 11, with channels 13 between adjacent wires 12. Each of the wires 12 can include a reflective layer 14 and an absorptive layer 15. The array of wires 12 can be elongated and parallel.

0017For each wire 12, the reflective layer 14 and the absorptive layer 15 can be aligned with a plane 17 that is perpendicular to the face 11_f of the transparent substrate 11 and parallel to the length L of the wires 12. Thus, each plane 17 can pass through both the reflective layer 14 and the absorptive layer 15 for the respective wire 12. Sides 12_s of each wire 12 can be parallel to the plane 17.

0018As shown in <figref idref="f0001 f0002 f0003 f0004 f0005 f0006">FIGs. 1-14</figref> &amp; <figref idref="f0007">16</figref>, the WGPs can further comprise heat-dissipation layer(s) 16. The heat-dissipation layer 16 can be an optical thin film and can be a continuous layer. The heat-dissipation layer 16 can draw heat away from the array of wires 12, and can conduct this heat to an external heat sink 121 as described below, or can transfer this heat away from the WGP by convection or radiation. For example, the absorptive layer 15 can generate heat from absorbed light and the heat-dissipation layer 16 can conduct this heat away from the reflective layer 14.

0019As shown in <figref idref="f0001 f0002">FIGs. 1-5</figref> &amp; <figref idref="f0006">13-14</figref>, the heat-dissipation layer 16 can be located over the array of wires 12 and farther from the transparent substrate 11 than the array of wires 12. As shown in <figref idref="f0003">FIGs. 6-8</figref>, the heat-dissipation layer 16 can be located or sandwiched between the array of wires 12 and the transparent substrate 11.

0020As shown on WGPs 90 and 100 in <figref idref="f0004">FIGs. 9-10</figref>, each wire 12 can include a first absorptive layer 15_a and a second absorptive layer 15_b with the reflective layer 14 sandwiched between. WGPs 90 and 100 include two heat-dissipation layers 16, including a first heat-dissipation layer 16_a located over the array of wires 12 and farther from transparent substrate 11 and a second heat-dissipation layer 16_b located under the array of wires 12 and closer to transparent substrate 11. The array of wires 12 can be sandwiched between the first heat-dissipation layer 16_a and the second heat-dissipation layer 16_b with the first absorptive layer 15_a located closer to the first heat-dissipation layer 16_a and the second absorptive layer 15_b located closer to the second heat-dissipation layer 16_b.

0021A decision of where to locate the heat-dissipation layer 16 and the number of heat-dissipation layers 16 can be made based on direction of incident light, amount of heat that must be dissipated, manufacturability, and performance requirements of the WGP.

0022The heat-dissipation layer 16 can have high electrical resistivity in order to electrically isolate each wire 12 from adjacent wires 12. For example, the heat-dissipation layer 16 can have an electrical resistivity of > 10⁴ Ω^∗cm, > 10⁵ Ω^∗cm, > 10⁶ Ω^∗cm, > 10⁷ Ω^∗cm, > 10⁸ Ω^∗cm, > 10⁹ Ω^∗cm, or > 10¹⁰ Ω^∗cm. All electrical resistivity values specified herein are measured at 20 °C.

0023The heat-dissipation layer 16 can have a high coefficient of thermal conductivity k to effectively conduct heat away from the array of wires 12. For example, the heat-dissipation layer 16 can have a coefficient of thermal conductivity k of > 5 W/(m^∗K), > 10 W/(m^∗K), > 15 W/(m^∗K), > 20 W/(m^∗K), or > 25 W/(m^∗K). All coefficient of thermal conductivity k values specified herein are measured at 25 °C.

0024One example of a material that meets the above requirements of a heat-dissipation layer 16 is aluminum oxide. For example, the heat-dissipation layer 16 can comprise ≥ 50%, ≥ 75%, ≥ 90%, ≥ 95%, or ≥ 99% aluminum oxide. Due to imperfections in deposition of material, aluminum oxide can be deposited in nonstoichiometric ratios. Therefore, the term aluminum oxide (Al₂O₃) used herein means approximately two aluminum atoms for every three oxygen atoms, such as for example Al_xO_y, where 1.9≤x≤2.1 and 2.9≤y≤3.1.

0025The array of wires 12 and the heat-dissipation layer 16 can be made of or can include materials for polarization of light, including metals and/or dielectrics, as are typically used in wires of wire grid polarizers. See for example <patcit id="pcit0002" dnum="US7961393B"><text>US 7,961,393</text></patcit> and <patcit id="pcit0003" dnum="US8755113B"><text>US 8,755,113</text></patcit>, which are incorporated herein by reference.

0026As shown in <figref idref="f0001">FIG. 1, 3</figref>, <figref idref="f0002">5</figref>, <figref idref="f0004">9</figref>, and <figref idref="f0005">11-12</figref>, the heat-dissipation layer 16 can span the channels 13 and not extend into the channels 13, or extend only minimally into the channels 13. The channels 13 can be air filled.

0027As shown in <figref idref="f0001">FIGs. 2</figref>, <figref idref="f0004">10</figref>, and <figref idref="f0006">13-14</figref>, the heat-dissipation layer 16 can extend partially into the channels 13 and the channels 13 can be partially air filled. The channels 13 can be mostly filled with the heat-dissipation layer 16 or mostly air filled. Having the channels 13 partially filled with the heat dissipation layer 16 can be particularly beneficial if the absorptive layer 15 is sandwiched between the reflective layer 14 and the heat-dissipation layer 16 and the heat-dissipation layer 16 extends into the channels 13 and along sides of the absorptive layer 15 to directly contact the reflective layer 14. WGPs 130 and 140 in <figref idref="f0006">FIGs. 13-14</figref> with a match head shaped heat-dissipation layer 16 on each wire 12, grown together to form a continuous thin film, is a typical shape. This shape is beneficial for (a) a large area of contact between the absorptive layer 15 and the heat-dissipation layer 16 and (b) for increasing the area for convective and radiation heat transfer away from the heat-dissipation layer 16 (due to the wavy or curved shape on the outer surface). Manufacture of this shaped layer described in patent publication <patcit id="pcit0004" dnum="US20120075699A"><text>US 2012/0075699</text></patcit>, which is incorporated herein by reference. As shown in <figref idref="f0002">FIG. 4</figref>, the heat-dissipation layer can 16 extend into and can fill the channels 13.

0028WGP 160 of <figref idref="f0007">FIG. 16</figref> includes a continuous layer 161, in a plane 162 parallel to the face 11_f of the transparent substrate 11, sandwiched between the reflective layer 14 and the absorptive layer 15. Therefore, instead of having a single array of wires 12, as shown in <figref idref="f0001 f0002 f0003 f0004 f0005 f0006">FIGs. 1-14</figref>, this continuous layer 161 creates two, separate arrays of wires 12a and 12b. A sidewall 12a_s of the upper array of wires 12a is interrupted from a sidewall 12b_s of the lower array of wires 12b by this continuous layer 161. This continuous layer 161 can interfere with proper performance of the WGP and can create manufacturing difficulties. Furthermore, channels 13a between the upper array of wires 12a are separate from channels 13b of the lower array of wires 12b, and thus a single channel does not extend across both the reflective layer 14 and the absorptive layer 15.

0029In contrast, sidewalls 12_s of each of the wires 12 of the WGPs in <figref idref="f0001 f0002 f0003 f0004 f0005 f0006">FIGs. 1-14</figref> extend from the reflective layer 14 to the absorptive layer 15 uninterrupted by any continuous layer in a plane 162 parallel to the face of the transparent substrate 17. Thus, each reflective layer 14 is paired with an absorptive layer 15 in each wire 12. Also, each of the channels 13 can extend from a proximal end 12_p of the array of wires 12 closest to transparent substrate 11 to a distal end 12_d of the array of wires 12 farthest from transparent substrate 11.

0030In <figref idref="f0001">FIGs. 1-2</figref>, <figref idref="f0002 f0003">5-6</figref>, and <figref idref="f0003 f0004 f0005 f0006">8-14</figref>, the absorptive layer 15 is sandwiched between the reflective layer 14 and the heat-dissipation layer 16. In <figref idref="f0001 f0002">FIGs. 3-4</figref> and <figref idref="f0003">7</figref>, the reflective layer 14 is sandwiched between the absorptive layer 15 and the heat-dissipation layer 16. In <figref idref="f0001 f0002">FIGs. 3-4</figref>, <figref idref="f0003">6, and 8</figref>, the absorptive layer 15 is located closer to the transparent substrate 11 than the reflective layer 14. A choice between these designs can be made based on manufacturability, performance requirements of the application, and amount of light that needs to be dissipated.

0031In order to allow sufficient removal of heat to the heat-dissipation layer 16, it can be beneficial to have minimal resistance to heat transfer between the absorptive layer 15 and the heat-dissipation layer 16. Therefore, the absorptive layer 15 can adjoin the heat-dissipation layer 16 as shown in <figref idref="f0001">FIGs. 1-2</figref>, <figref idref="f0002 f0003">4-6</figref>, <figref idref="f0003 f0004 f0005">8-11</figref>, and <figref idref="f0006">13-14</figref>. Alternatively, the absorptive layer 15 can be separated from the heat-dissipation layer 16 by material (e.g. layer 126 in <figref idref="f0005">FIG. 12</figref>) having a small thermal resistance for heat conduction of a unit area (R_A), such as for example &lt; 10^-6 K^∗m²/W, &lt; 10^-7 K^∗m²/W, &lt; 10^-8 K^∗m²/W, &lt; 10^-9 K^∗m²/W, &lt; 10^-10 K^∗m²/W. The term thermal resistance for heat conduction of a unit area (R_A) means a thickness (e.g. Th₁₂₆) of the material 126 divided by a coefficient of thermal conductivity (k). Thus, R_A=Th/<i>k</i>; so for a 100 nm thick layer of Al, R_A=100 nm/[205 W/(m^∗K)]= 5×10^-10 K^∗m²/W; and for a 10 nm thick layer of SiO₂, R_A=10 nm/[0.96 W/(m^∗K)]= 10^-8 K^∗m²/W.

0032Alternatively or in addition, in order to allow sufficient removal of heat to the heat-dissipation layer 16, it can be beneficial to have minimal resistance to heat transfer between the reflective layer 14 and the heat-dissipation layer 16. Therefore, the reflective layer 14 can adjoin the heat-dissipation layer 16 as shown in <figref idref="f0001">FIGs. 2</figref>, <figref idref="f0002">4</figref>, <figref idref="f0003">7</figref>, <figref idref="f0004">10</figref>, and <figref idref="f0006">13-14</figref>. Alternatively, the reflective layer 14 can be separated from the heat-dissipation layer 16 by material (e.g. layer 36 in <figref idref="f0001">FIG. 3</figref>) having a small thermal resistance for heat conduction of a unit area (R_A), such as for example &lt; 10^-6 K^∗m²/W, &lt; 10^-7 K^∗m²/W, &lt; 10^-8 K^∗m²/W, &lt; 10^-9 K^∗m²/W, &lt; 10^-10 K^∗m²/W.

0033Sometimes light is incident on the WGP in pulses and merely having a heat sink to store, then gradually dissipate, absorbed heat can be sufficient. The heat-dissipation layer 16 can be a heat sink for heat absorbed by the absorptive layer 15. Increased volume of the heat-dissipation layer 16 can be beneficial to allow sufficient volume for absorption of this heat. Thus, for example, a volume of the heat-dissipation layer 16 can be at least two times greater, at least three times greater, at least five times greater, at least eight times greater, at least twelve times greater, or at least eighteen times greater than a volume of the absorptive layer 15.

0034Another way to describe the added size of the heat-dissipation layer 16 is by its thickness Th₁₆ compared to a thickness Th₁₅ of the absorptive layer 15. The thickness Th₁₆ of the heat-dissipation layer 16 is a straight-line distance from a distal end 12_d of the array of wires 12 farthest from transparent substrate 11, to an outermost surface of the heat-dissipation layer 16. The thickness Th₁₅ of the absorptive layer 15 and the thickness Th₁₅ of the heat-dissipation layer 16 are both measured perpendicular to the face 11_f of a transparent substrate 11. Following are example relationships of these comparative thicknesses: Th₁₆ ≥ 2^∗Th₁₅, Th₁₅ ≥ 3^∗Th₁₅, Th₁₅ ≥ 5^∗ Th₁₅, or Th₁₆ ≥ 10^∗ Th₁₅. Following are example thicknesses of the absorptive layer 15 and the heat-dissipation layer 16: Th₁₅ ≥ 5 nm or Th₁₅ ≥ 20 nm; Th₁₅ ≤ 200 nm or Th₁₅ ≤ 100 nm; Th₁₆ ≥ 50 nm, Th₁₆ ≥ 100 nm, Th₁₆ ≥ 200 nm, or Th₁₆ ≥ 400 nm; and Th₁₆ ≤ 2000 nm, Th₁₆ ≤ 1000 nm, or Th₁₆ ≤ 500 nm.

0035For some applications, convective and/or radiative heat transfer away from the heat-dissipation layer 16 can keep the WGP at a sufficiently low temperature. For other applications, based on increased heat input and/or materials in the WGP with a lower melting temperature, conductive heat transfer to a heat sink might be needed. As shown in <figref idref="f0005">FIG. 12</figref>, a heat sink 121 can be coupled to the heat-dissipation layer 16 and can conduct heat away from the heat-dissipation layer 16, and thus away from the array of wires 12. The heat sink 121 can be located outside of the array of wires 12.

0036The heat sink 121 can comprise multiple fins 121_f, such as for example at least five fins 121_f as shown in <figref idref="f0005">FIG. 12</figref>. Each of the fins 121_f can have a width W_f and a thickness Th_f much larger than the width W₁₂ and thickness Th₁₂ of the wires 12 and large enough to transfer heat from the WGP 120 to the surrounding air. For example, each of the fins 121_f can have a width W_f of ≥ 0.01 mm, ≥ 0.1 mm, ≥ 1 mm, or ≥ 100^∗W₁₂; and a thickness Th_f of ≥ 0.1 mm, ≥ 2 mm, ≥ 6 mm, or ≥ 100^∗Th₁₂. The heat sink 121, including the fins 121_f, can have a high coefficient of thermal conductivity k, such as for example > 10 W/(m^∗K), > 40 W/(m^∗K), or > 60 W/(m^∗K), where W=watt, m=meter, and K=degrees Kelvin.

0037In order to allow sufficient transfer of heat from the heat-dissipation layer 16 to the heat sink 121, the heat sink 121 can adjoin the heat-dissipation layer 16 (see heat sink 121_b in <figref idref="f0005">FIG. 12</figref>). Alternatively, as shown on heat sink 121_a in <figref idref="f0005">FIG. 12</figref>, sufficient heat can transfer to the heat sink 121 if the heat sink 121 is separated from the heat-dissipation layer 16 by material 122 with a small thermal resistance for heat conduction (R). For example R&lt;1 K/W, R&lt;10 K/W, R&lt;50 K/W, R&lt;100 K/W, where W=watt, K=degrees Kelvin, R=L₁₂₂/(<i>k</i>^∗A₁₂₂), L₁₂₂ is the length of the material 122 between the heat-dissipation layer 16 and the heat sink 121, A₁₂₂ is the area of the material for heat transfer (extending into the page of <figref idref="f0005">FIG. 12</figref>), and <i>k</i> is the coefficient of thermal conductivity.

0038Addition of the heat-dissipation layer 16 can result in a drop in WGP performance. This drop in WGP performance can be avoided or mitigated by addition of an antireflection layer 131, located over the heat-dissipation layer 16, farther from the substrate 11 than the heat-dissipation layer 16, the heat-dissipation layer 16 being sandwiched between the antireflection layer and the array of wires 12, as shown on WGPs 130 and 140 in <figref idref="f0006">FIGs. 13-14</figref>. The heat-dissipation layer 16 can provide a base or foundation for applying the antireflection layer 131.

0039As shown on WGP 130 in <figref idref="f0006">FIG. 13</figref>, the antireflection layer 131 can include multiple thin-film layers 132 on the heat-dissipation layer 16. These thin-film layers 132 can extend continuously across the heat-dissipation layer 16 and can reduce reflection of incident light on the heat-dissipation layer 16. One example of a thin-film antireflection layer 131 is at least two pairs 133 of thin-film layers 132 with each pair 133 including a thin-film layer 132 of silicon dioxide and a thin-film layer 132 of 95% ZrO₂ plus 5% TiO₂. Examples of a thickness Th₁₃₂ of each layer is between 30 and 300 nanometers.

0040As shown on WGP 140 in <figref idref="f0006">FIG. 14</figref>, the antireflection layer 131 can include multiple protrusions 142, formed in an array, located on the heat-dissipation layer 16. The protrusions 141 can be designed for reducing reflection of incident light on the heat-dissipation layer 16. For example, each protrusion 141 can have a width W₁₄₂ and a height H₁₄₂ that are &lt; 300 nm, &lt; 700 nm, or &lt; 1200 nm.

0041As shown in <figref idref="f0001 f0002">FIGs. 1-4</figref>, <figref idref="f0003">6-7</figref>, <figref idref="f0004">9-10</figref>, and <figref idref="f0005 f0006">12-14</figref>, the reflective layer 14 can adjoin the absorptive layer 15. Alternatively, as shown in <figref idref="f0002">FIGs. 5</figref> and <figref idref="f0003">8</figref>, each of the wires 12 can further comprise a thermal-insulating layer 18 sandwiched between the reflective layer 14 and the absorptive layer 15. As shown in <figref idref="f0005">FIG. 11</figref>, each of the wires 12 can further comprise two thermal-insulating layers 18, with one sandwiched between the reflective layer 14 and the first absorptive layer 15_a and the other sandwiched between the reflective layer 14 and the second absorptive layer 15_b. The thermal-insulating layer 18 can minimize heat transfer from the absorptive layer 15 to the reflective layer 14, thus minimizing the chance of melting the reflective layer 14. The thermal-insulating layer 18 can have a low coefficient of thermal conductivity, such as for example &lt; 8 W/(m^∗K), &lt; 4 W/(m^∗K), &lt; 2 W/(m^∗K), or &lt; 1.5 W/(m^∗K).

0042In order to improve heat transfer from the absorptive layer 15 to the heat-dissipation layer 16, there can be a large contact area between the two materials. For example, the heat-dissipation layer 16 can cover ≥ 50%, ≥ 80%, ≥ 90%, or ≥ 95% of an exposed surface of the absorptive layer 15, the exposed surface of the absorptive layer 15 being any surface of the absorptive layer 15 not in direct contact with the transparent substrate 11 or material of the array of wires 12 (e.g. the reflective layer 14 or the thermal-insulating layer 18).