Methods and systems for extracting energy from a heat source using photonic crystals with defect cavities

Methods and systems for extracting energy from a heat source using photonic crystals with defect cavities generally comprise a photonic crystal, a cavity, and a converter. The photonic crystal is responsive to a heat source and generates an electromagnetic beam in response to incidence with the heat source. The photonic crystal exhibits a band gap such that wavelengths within the band gap are substantially confined within the photonic crystal. The cavity is substantially within the crystal and is responsive to the electromagnetic beam such that the cavity transmits the electromagnetic beam to a specified location. The converter is substantially collocated with the specified location and extracts energy in response to incidence with the electromagnetic beam.

BACKGROUND OF INVENTION

Heat emanating from a heat source is often wasted because it is not converted into another useful form of energy. Harnessing this heat and converting it into another form of energy would have many useful applications. Photonic crystals containing defect cavities and waveguides permit extraction of some of this heat energy as narrow waveband, narrowly directed electromagnetic radiation beams.

SUMMARY OF THE INVENTION

In various embodiments, a method and system for extracting energy from a heat source may comprise a photonic crystal, cavities, and waveguides. The photonic crystal is responsive to a heat source and generates an electromagnetic beam in response to incidence with the heat source. The photonic crystal exhibits a band gap such that wavelengths within the band gap are substantially confined within the photonic crystal resonant defect cavities. The cavities and waveguides reside substantially within the photonic crystal and are responsive to the electromagnetic beam such that the cavity/waveguide combination transmits the electromagnetic beam to a particular location. In one embodiment, the method and system for extracting energy from a heat source may also comprise a converter substantially collocated with the particular location and configured to extract energy in response to incidence with the electromagnetic beam.

Elements and steps in the figures are illustrated for simplicity and clarity and have not necessarily been rendered according to any particular sequence. For example, steps that may be performed concurrently or in different order are illustrated in the figures to help to improve understanding of embodiments of the methods and systems for extracting energy from a heat source using photonic crystals with defect cavities.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The method and system for extracting energy from a heat source by a photonic crystal with resonant defect cavities may be described in terms of functional block components and various processing steps. Such functional blocks may be realized by any number of components configured to perform the specified functions and achieve the various results.

Turning now toFIG. 1, a system100for extracting energy150from a heat source110according to various embodiments may comprise a photonic crystal120and a converter130to provide, for example, electrical power. The photonic crystal120may further comprise cavities140, which generate radiation, for example, an electromagnetic beam160. The energy extracting system100may be adapted for operation in conjunction with any appropriate application, for example: cooling electronic components while generating usable narrow band radiation, signal generation, passive infrared tags for identification, passive THz sources, self-powered spectrometers, redirecting surface radiation, medium wavelength infrared illuminators driven by engine waste heat, generating power, charging batteries with waste heat, and/or other applications for removing or utilizing heat.

In an embodiment, the heat source110may comprise any source of heat115, and the heat source110may also comprise any system for transferring heat115or that requires cooling, for example: an engine, an electrical component, a person, a computer, a chemical reaction, the sun, ambient air, etc. The heat source110may thermally stimulate the photonic crystal120to generate the radiation160, which may be either dissipated, converted and used, or any combination of dissipation and converted use. The heat source110may generate heat115by any appropriate manner, for example: by an exothermic reaction, friction, electrical resistance, and the like. Among various embodiments, the heat source110may be in contact with the photonic crystal120, in close proximity to the photonic crystal120, or fully separated from the photonic crystal120. In addition, the heat source110may originate from inside the photonic crystal120(e.g. radioactive materials) or outside the photonic crystal120.

In an embodiment, the heat source110may affect the temperature of the photonic crystal120. For example, the temperature of the photonic crystal120may range from just above ambient temperature to at or below the melting point of the material of the photonic crystal120. Among various embodiments, the temperature of the photonic crystal120may range from 200 K to 2000 K, and more specifically, the temperature of the photonic crystal may range from 300 K to 1000 K. The heat source110may transfer heat to the photonic crystal120by any relevant heat transfer process, for example: conduction, convection, or radiation. Thermal stimulation of the photonic crystal120by the heat source110does not necessarily require atomic motion through a medium, but may also be produced by energy incident on the photonic crystal120in another form, for example electromagnetic radiation. For example, electromagnetic waves from sunlight incident on the photonic crystal120may thermally stimulate the photonic crystal120.

In an embodiment, the photonic crystal120may comprise various materials and configurations. Among the various embodiments, configurations of the photonic crystal120may exhibit a periodic high-contrast modulation of the local index of refraction (or dielectric constant, for non-magnetic materials) in one, two or three dimensions (see for example J. D. Joannopoulos, R. D. Meade, and J. N. Winn, “Photonic Crystals: Molding the Flow of Light,” Princeton: Princeton University Press (1995), or C. Lopez, “Materials Aspects of Photonic Crystals,” Advanced Materials 15, 1679 (2003)). Any two substances comprising sufficient contrast between their respective indices of refraction may be placed in a stable periodic arrangement comprising a particular geometry, spacing and shape of the constituent substances to create a photonic crystal for a particular range of photon wavelengths. Radiation propagating in such a structure undergoes multiple Bragg scattering from a lattice array and multiple Mie scattering off of individual scattering elements. Under certain conditions, the multiple-scattered waves interfere destructively, resulting in minimal transmission over a broad range of wavelengths, which is termed the “band gap”. The photonic band gap (“PBG”) is complete when transmission is blocked for all angles of incidence and all polarization states within the wavelength band. In one embodiment, the photonic crystal120may comprise materials, for example, silicon and air, but the photonic crystal120may also comprise other materials, for example, SiN, SiO2, plastics, metals, ceramics, composites, and many other materials, whether solid, liquid, or gas. For example, with reference toFIG. 2, a geometric configuration of a 3-D photonic crystal220is shown comprising the repeating periodic structure of two materials.

Among the various embodiments, and returning toFIG. 1, the photonic crystal120generates radiation160in response to heat115by spontaneous emitting radiation160, for example Planck radiation, from thermally excited dipole radiators (not shown) within the photonic crystal120. The radiation160may comprise a wide range of wavelengths that may lie along any portion of the electromagnetic spectrum. For example, the radiation160may comprise: ultraviolet light, visible light, infrared light, light in the terahertz frequency range, etc, and in one embodiment, the photonic crystal120may exhibit a peak in Planck radiation at a selected wavelength. For example,FIG. 3graphically shows the Plank spectrum of blackbody radiation for a material at different temperatures.

Among the various embodiments, the radiation160may travel outward in all directions from the dipole radiator that generates it. The photonic crystal160may exhibit a band gap, i.e., a photonic band gap (“PBG”), such that wavelengths within the band gap are substantially confined within the photonic crystal160in at least one direction. The band gap is an inherent property of the photonic crystal160caused by the destructive interference of certain wavelengths due to scattering events such as diffraction and refraction. For example, A 1-D photonic crystal confines the light within the band gap in only one direction, a 2-D photonic crystal confines the light in a plane, and a 3-D photonic crystal confines the light in all directions. In various embodiments of the photonic crystal160, the band gap may be altered or tuned by applying an external magnetic field or physical pressure. For example, and with reference toFIG. 4, a system400shows one manner by which the photonic crystal420emitting radiation460may be tuned to a particular orientation465by the application of an external magnetic field480.

Among the various embodiments, the photonic crystal120converts thermal energy to radiation160substantially within one or more selected frequency ranges. As the temperature (“T”) of the photonic crystal120increases, the radiation160in one or more selected bands increases almost linearly with T. Heat115supplied to the photonic crystal120may be converted into emissions160in the selected emission bands, for example the THz band. For example, and with reference toFIG. 5, a photonic crystal's Planck blackbody radiation spectrum may be modified with a band gap positioned around a peak595(e.g., 5-10 μm) of the Planck spectrum. Several theoretical and experimental papers in this area have been published, for example, Zhi-Yuan Li, Phys. Rev. B 66, R241103 (2002) and S-Y. Lin, et al, Phys. Rev. B 62, R2243 (2000). Li's paper describes modeling a redistribution of the photon density of states (“DOS”) in the emission region of the Planck spectrum as full and partial photonic band gaps that are manipulated by varying the photonic crystal's geometry and material. A three-dimensional photonic crystal may redistribute the photon DOS among different frequency bands and the redistribution may be used to modify the thermal radiation from the photonic crystal.

As further described by Li, orders-of-magnitude enhancement of the DOS may occur in low-DOS bands within the long-wavelength region of a designed photonic crystal. This leads to significantly enhanced thermal radiation emissions in the visible waveband (approximately 0.5 μm) for a modest cavity temperature. Since the underlying physics of the above conclusion should be generic, the physics may be applied to a selected thermally-generated region of the spectrum as well.

The photonic crystal according to various embodiments, shifts thermal energy in the photonic crystal structure towards a selected spectral range, for example the Terahertz spectral region, and collects the internal energy for emission in a single direction. For example, referring toFIGS. 6 and 7, the DOS of a photonic crystal620may be configured to enhance spectral emissions660in the THz region770(0.3 THz to 10 THz) (FIG. 7), particularly when heated by a heat source610. The photonic crystal620may exhibit peaks in its photon DOS, and may be configured to amplify DOS peaks in the desired bands, for example the THz band, and suppressed elsewhere. Therefore, the final emission states are constrained to the desired spectral band by the use of the photonic crystal620having symmetries and lattice constants adjusted to exhibit the appropriate characteristics.

As shown inFIGS. 6 and 7, the photonic crystal620may comprise a single photonic crystal structure620in which the DOS may be designed and optimized to create a modified Planck spectrum775in which the strongest emission peak may be shifted toward a desired spectral range, for example the THz spectrum770, for example by at least a factor of three. The bandwidth780of the emission peak785may be selected according to the application, for example, less than 20% of the center frequency of the peak and, in one example, less than 10%, which may be narrow enough for many imaging and spectroscopy applications.

Among the various embodiments, a photonic crystal may comprise dimensions commensurate with the application, and in one embodiment may comprise dimensions in the sub-millimeter range. In addition, some photonic crystals may exhibit enhanced emission of certain wavelengths of light near the edge of the band gap, and such photonic crystals may be configured to produce that range of wavelengths by customizing the size, geometry, and spacing of the periodic structure, as described above.

Continuing with various embodiments of the methods and systems for extracting energy from a heat source using photonic crystals with defect cavities, an embedded structure approach adds another photonic crystal layer in the overall structure, which provides more degrees of freedom in designing the photon DOS of the radiation core. The higher-frequency emission peak may be at the Planck distribution peak so that it strongly emits the normal Planck radiation, and/or a lower-frequency emission peak may enhance another spectral range, for example THz radiation. Further, the dual-band photonic crystal facilitates physical integration of the radiation core and a network of radiation collecting and radiation guiding structures, which may lead to further improvement in efficiency and reduction of cost, size and weight of the photonic crystal. Moreover, embedding structures within a coarser structure may be implemented with more than two bands. Periodic structures may be embedded on larger and/or smaller scales according to the desired band gap and/or emission peak effects. The number of bands may be selected according to any suitable criteria. For example, and referring toFIG. 8, a triple-embedded structure820may comprise rectangles821embedded within rectangles823, which may be further embedded within rectangle825. Each rectangle821,823,825may comprise a different material, and a matrix827may consist of empty space.

In another embodiment, a photonic crystal may comprise a core that may include two or more distinct photonic crystal structures, i.e., embedded structures, one structure comprising an emission peak near the desired spectral region, for example the THz region, for wave guiding purposes, and the other structure comprising a band gap at an independent frequency higher than the desired spectral region. For example, and with reference toFIG. 9, because the unit-cell sizes of photonic crystal structures giving rise to the two DOS distributions may be considerably different, the fine (higher frequency band) photonic crystal structure920may be embedded within a coarse (lower frequency band, e.g., THz) photonic crystal structure922.

In one embodiment, a photonic crystal may comprise a 2-D photonic crystal slab. The cross section of such a structure is representatively illustrated inFIG. 10. The photonic crystal slab1020contains multiple cylindrical cavities1040and a long cuboidal cavity1045, which may be used as a waveguide. SinceFIG. 10comprises a cross section, the cylindrical cavities1040appear circular and the cuboidal cavity1045appears rectangular. The photonic crystal slab1020may also comprise several long cuboidal cavities. In one embodiment, the spherical cavities1040may comprise equal size and spacing so that they all emit the same range of wavelengths, or the spherical cavities1040may comprise different sized spherical cavities (not shown) so that different wavelengths are emitted. Wavelengths may be combined into a single output1060by combining the beams in the long cavity1045that serves as a waveguide, or by combining them once they exit the photonic crystal1020. In addition, one could simply use different slabs for each wavelength, with the slabs being stacked, as illustrated inFIG. 11, to increase heat capacity. In the example shown byFIG. 11, the emitted beams1160from each photonic crystal slab1120may be combined to produce a combined output1169such that the final output may be configured to comprise of a broad or a narrow range of wavelengths. Among the various embodiments, and as further shown inFIGS. 10 and 11, a coupling efficiency between a cavity and a long cavity (i.e., a waveguide) may comprise an adjustable distance. For example,FIG. 10shows distance1048, which is the distance between the cavity1040and the long cavity1045. Distance1048may be customized in other photonic crystal embodiments as desired.

In another embodiment, a photonic crystal may comprise a 2-D photonic crystal fiber. A band gap my be created in a cladding of the fiber by utilizing multilayer dielectric mirrors, as illustrated inFIG. 12, or by utilizing holes extending the length of the fiber, as illustrated inFIG. 13. The mirrors or the holes create the diffraction, refraction, and destructive interference necessary to create a 2-D photonic band gap in the plane that may be substantially perpendicular to the length of the fiber. In either case the center of the fiber contains a cylindrical hole that extends the length of the fiber and serves both as a cavity and a waveguide. For example, inFIG. 13a center hole1340may serve as a cavity simply because it is of a different size than the surrounding holes1341in the cladding1342. In an embodiment, a permanent magnet, such as shown in the embodiment illustrated inFIG. 4, may be placed around either of these fiber types to orient the dipole radiators such that they generate waves in the plane of the band gap which then travel along the cavity to the specified location. By using such a magnet the system becomes more efficient by coercing the dipole radiators to produce radiation that lies in the forbidden plane and thus ends up being transmitted to the cavity/waveguide. In one embodiment, individual fibers may be cemented together in a thermally conducting matrix so as to produce more output, as shown inFIG. 14.

Among various embodiments, and returning toFIG. 1, the photonic crystal120may comprise a cavity140that may lie substantially within the photonic crystal120, and the cavity140may be responsive to the electromagnetic beam160, wherein the cavity140may transmit the electromagnetic beam160to a particular location. In an embodiment, the cavity140may comprise a disruption in the periodic structure of the photonic crystal120. In another embodiment, the cavity140may comprise an absence of material in a location where material would otherwise be present due to the periodic structure of the photonic crystal120. In another embodiment, the cavity140may also comprise a presence of material in a location where material would otherwise not be present due to the periodic structure of the photonic crystal120. In yet another embodiment, the cavity140may also comprise the presence of a material in a location where a different type of material would otherwise be present due to the periodic structure of the photonic crystal120. In still yet another embodiment, the cavity140may also comprise a change in feature sizes. For example, within a structure of the photonic crystal120, the photonic crystal120may comprise a matrix filled with periodic voids comprising similar sizes. Within this example, a void comprising a different size may be considered a cavity. In an example of an embodiment,FIG. 15is an illustrative representation of the cavity140. InFIG. 15, a cavity1540within photonic crystal1520may comprise the presence of material in a location where material would not otherwise be present due to the periodic structure of the photonic crystal1520.

Among various embodiments, the cavity140may comprise any shape. For example, the cavity140may comprise conventional shapes, for example spherical, cylindrical, cubical, elliptical, and the like shapes, however the cavity140may not comprise conventional shapes at all, but rather may comprise any regular or irregular shape. Moreover, the cavity140may comprise any size, and the cavity140may be located anywhere within or substantially within the photonic crystal120.

In one embodiment, a photonic crystal may comprise of more than one cavity, e.g. multiple cavities. One characteristic of multiple cavities is that they may be configured to resonate with one another by adjusting the size and the distance between the cavities; the size and location of the multiple cavities may be configured such that one cavity may couple its energy with the energy from another proximate cavity. In such an embodiment, one cavity effectively “leaks” electromagnetic radiation to the proximate cavity, and the amount of electromagnetic radiation leaked to the proximate cavity may be dependent on the amount of material between the two cavities.

Continuing with an embodiment of multiple cavities, such multiple cavities may comprise varying shapes and sizes. For example, in one embodiment, the cavities may be all cylindrical and identical in size except one cavity, which may be cuboidal in shape. As previously shown inFIG. 10, whereFIG. 10comprises a cross section, the cylindrical cavities1040appear circular, and the cuboidal cavity1045appears rectangular. The cylindrical cavities1040in such a case couple with other proximate cylindrical cavities1040and the cylindrical cavity1040proximate to the cuboidal cavity1045may couple with the cuboidal cavity1045, such that the electromagnetic radiation1060may leak from the cylindrical cavities1040into the cuboidal cavity1045and the electromagnetic radiation1060may travel along the cuboidal cavity1045to a particular location, i.e., location1065.

Among the various embodiments, a cavity may be configured such that a narrow band of wavelengths within a band gap, as described earlier, encounters constructive interference when it enters a cavity, thus resulting in little loss within the cavity, i.e., the cavity may comprise a high Q factor. The cavity may be configured such that the wavelength of the peak Planck radiation of the photonic crystal encounters constructive interference within the cavity and may be leaked to other cavities, as described above. Furthermore, the cavity may be tuned using external pressure or a magnetic field to determine which wavelength within the band gap encounters constructive interference within the cavity, again as shown inFIG. 4.

Among the various embodiments, increasing the number of cavities may be desirable because the increased surface area allows more energy to be extracted from the photonic crystal in the form of electromagnetic radiation. In sum, a photonic crystal may comprise some or all of the above embodiments of a cavity and/or cavities, as well as others now known or further developed.

Among the various embodiments, the methods and systems for extracting energy from a heat source using photonic crystals with defect cavities may comprise a wave guide and/or power combining structure such that radiation energy may be efficiently collected and/or directed, for example to an output antenna For example, a series of cavities comprising variable-Q defect cavities may collect and concentrate the radiation. Among various embodiments, the cavities may be placed in a pattern to allow the electromagnetic radiation to leak to a desired location, and in a waveguide fashion, the pattern of cavities may guide the electromagnetic radiation to a particular location. Similarly, one long cavity may be used as a waveguide to guide the wave of electromagnetic radiation to a particular location. Such patterns of cavities or long cavity need not comprise a linear configuration, but rather may comprise curves, turns, and the like to guide the electromagnetic radiation with little loss.

Among the various embodiments, photonic crystals possessing waveguides and cavities have been developed for fiber optical and millimeter wave applications. Single mode photonic crystal waveguides (A. Scherer et al., IEEE Trans. Nanotech. 1, 4 (2002), optical waveguides with sharp bends (A. Mekis et al., Phys. Rev. Lett. 77, 3787 (1996)), very high-Q cavity resonators (Y. Akahane et al., Nature 425, 944 947 (2003)), and photonic crystals with tunable band gaps (H. Xin et al., IEEE Antennas and Propagation Symp. 2, 435 (2003)) have been demonstrated. Such systems may offer higher efficiency, smaller size and other unique advantages compared to conventional wave guiding components.

In one example, and with reference toFIG. 16, a plurality of cavities1640within a photonic crystal1620may couple radiation1660to a waveguide1645, which subsequently directs the radiation1660to an output antenna1690.

In one example, and with reference toFIG. 17, a dual-band photonic crystal THz structure1720may comprise both the THz radiation generation and wave guiding functions. Cavities1740may be formed in the coarse structure of a dual-band photonic crystal layer1725to collect and concentrate the THz radiation. The structure collects internal Planck radiation from the photonic crystal structure1720, and the cavities1740near the waveguides1745collect and concentrate a narrow spectral band of the THz radiation according to the Q of the cavity1740. In this embodiment, multiple waveguide channels1745may be designed and fabricated in a parallel fashion in the coarse structure of the photonic crystal layer1725to combine into a network of waveguides1745to consolidate and increase power output to a horn antenna1790. In this approach, a network of the cavities1740and waveguide channels1745in the coarse structure may be used to localize and then couple out thermally powered THz radiation1760over a large volume of the entire photonic crystal structure1720to one or more micro-machined THz antennas, such as the horn antenna1790. A thermal source1710may be formed around the exterior of the photonic crystal layer1725. Other various architectures and geometries may be implemented using such a dual-band photonic crystal structure1720.

In an embodiment, a system for extracting energy from a heat source may further comprise a converter130(FIG. 1) that converts the beam of electromagnetic radiation160into another form of energy150. In an embodiment, the converter130may be substantially collocated with a particular location, wherein the converter130extracts energy in response to incidence from the electromagnetic beam160. Among various embodiments, the converter130may comprise any system for converting electromagnetic radiation160into another form of energy150, for example: electricity, a voltage gradient, capacitance, and the like. The converter130may also comprise: a photovoltaic cell, a quantum well device (for converting infrared radiation), a nanocable or nanotube device, and the like. In another embodiment, the converter130may be configured such that its peak output of energy150corresponds with the peak wavelength of electromagnetic radiation160.

A method1800for extracting energy from a heat source may comprise: providing a photonic crystal with resonant defect cavities (1810); generating an electromagnetic beam by the photonic crystal (1820); transmitting the electromagnetic beam by a waveguide (1830); and converting the electromagnetic beam by a converter (1840). The method1800for extracting energy from a heat source may further comprise, providing a magnet to orient dipole radiators within the photonic crystal (1850), and providing a photonic crystal pressure system to modify the wavelength at which a cavity resonates (1860).FIG. 18illustrates a flow chart of the method1800.

Among the various method embodiments, generating an electromagnetic beam by the photonic crystal may comprise the photonic crystal to generate the electromagnetic beam in response to incidence with the heat source, and the photonic crystal to exhibit a band gap such that wavelengths within the band gap may be substantially confined in at least one direction within the photonic crystal. Where the method comprises transmitting the electromagnetic beam by a waveguide, the waveguide may comprise a waveguide substantially within the photonic crystal, and the waveguide may transmit the electromagnetic beam to a specified location. In one embodiment, the waveguide comprises at least two adjacent cavities

In another embodiment, where the method of converting the electromagnetic beam by a converter, the converter may be substantially collocated with the specified location, and the converter may extract energy in response to incidence with the electromagnetic beam. In one embodiment the converter comprises a photovoltaic cell, or the converter comprises a quantum well device.

In an embodiment of a method for extracting energy from a heat source, the method may further comprise providing the photonic crystal to further comprises cavities within the photonic crystal and the cavities may be configured to resonate at or near a peak of a Planck spectrum for the photonic crystal at a specified temperature.

Among the various method embodiments, the photonic crystal may comprise a fiber comprising a hollow core, wherein the fiber may further comprise dielectric mirrors, and/or the fiber may comprise holes that extend a length of the fiber to create the band gap. The method may also comprise the photonic crystal to comprise a slab that is substantially planar, wherein at least two slabs are stacked to increase a heat capacity, and the at least two slabs are tuned to resonate at a different temperature

In the foregoing specification, the invention has been described with reference to specific exemplary embodiments. Various modifications and changes may be made, however, without departing from the scope of the methods and systems for extracting energy from a heat source using photonic crystals with defect cavities as may be set forth in the claims of any issuing patent. The specification and figures are illustrative, rather than restrictive, and modifications are intended to be included within the scope of the methods and systems for extracting energy from a heat source using photonic crystals with defect cavities. Accordingly, the scope of the invention should be determined by the claims in any issuing patent and their legal equivalents rather than by merely the examples described.

For example, the steps recited in any method or process claims in any issuing patent may be executed in any order and are not limited to the specific order presented in the claims. Additionally, the components and/or elements recited in any system claims in any issuing patent may be assembled or otherwise operationally configured in a variety of permutations and are accordingly not limited to the specific configuration recited in the claims.

The terms “comprise”, “comprises”, “comprising”, “having”, “including”, “includes” and the like refer to a non-exclusive inclusion, such that a process, method, article, composition, system, or apparatus that comprises a list of elements does not include only those elements recited, but may also include other elements not expressly listed or inherent to such process, method, article, composition, system, or apparatus. Other combinations and/or modifications of the structures, arrangements, applications, proportions, elements, materials or components used in the practice of the methods and systems for extracting energy from a heat source using photonic crystals with defect cavities, in addition to those not specifically recited, may be varied or otherwise particularly adapted to specific environments, manufacturing specifications, design parameters or other operating requirements without departing from the general principles of the same.