Photonic Crystals: Shaping the Flow of Thermal Radiation Ivan Čelanović Massachusetts Institute of Technology Cambridge, MA 02139
Overview: Thermophotovoltaic (TPV) power generation Photonic crystals, design through periodicity Tailoring electronic- and photonic bandgap properties: a path towards record efficiencies Photovoltaic module: design and characterization TPV system design challenges Quasi-coherent thermal radiation via photonic crystals
Thermophotovoltaic power generation: basic ideas and concepts
Thermo-photo-voltaic conversion TPV power conversion describes the direct conversion of thermal radiation into electricity. Photons Brief History 1956 - Dr. H. Kolm / Dr. P. Aigrain independently propose TPV power conversion concept 1970 s - Loss of interest in TPV due to low efficiencies 1990 s - Advancements in microfabrication technology allow for production of low-bandgap diodes, opening the door for more efficient TPV 1994 - First NREL Conference on TPV Generation of Electricity 2000 s -Photonic crystals for thermal radiation control
Basic TPV energy conversion diagram P N Heat Emitter Blackbody Radiation Cell Surface Reflection GaSb + Waste Heat Pout
PV vs. TPV PV (Solar Cells) TPV Properties: Sensitivity Range Visible and NIR NIR and IR Source Sun Thermal emitter Source Temperature Distance from Source Energy reflected from cell surface Over 5000K (sun s surface) 1000-1500K Over 90 million miles Lost to atmosphere µm to cm Courtesy of DOE/NREL, Credit - Beck Energy. Recycled to the emitter
TPV Technologies and applications AA radioisotope TPV battery: ~10 mwe 30 years life time 24% efficiency micro-tpv power generator (propane/butane operated) 1 W e 15% efficiency Plutonium pellet Courtesy of Klavs Jensen. Used with permission. Photo courtesy of LLNL. 18 W/kg, (PuO 2 fuel) for 30 years 24% efficient sun radiation concentrator absorber/photonic crystal Solar cell Solar TPV T PhC Radioisotope TPV power system for deep space and Mars missions Images courtesy of NASA. Photo courtesy of Sandia National Labs.
Thermophotovoltaics: converting thermal radiation into electricity, with no moving parts good photons thermal emitter filter PV diode Heat sink bad photons Heat Input Waste Heat Si (1.23 ev) InGaAsSb (0.53 ev) InGaAs (0.6 ev) GaSb (0.72 ev)
Photonic Crystals: shaping thermal radiation 1 0.9 0.8 Normalized radiated power 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1 2 3 4 5 6 7 8 wavelength [µm]
TPV Technology roadmap: the time is now Si and Ge PV diode III-V s (GaSb, InGaAs, GaInAsSb) rare earth oxides Spectral control Dielectric stack filters Photonic Crystals System design JX Thermo Power 1950 s 1960 s 1970 s 1980 s 1990 s 2000 s
Photonic crystals, design through periodicity
Photonic crystals are periodical structures with 1D, 2D or 3D periodicity ),, ( ),, ( z y x z y x x λ ε ε + = 1-D Periodicity 2-D Periodicity 3-D Periodicity ),, ( ),, ( z y x z y x y λ x λ ε ε + + = ),, ( ),, ( z y x z y x z y x λ λ λ ε ε + + + =
Metamaterial: optical properties determined from its nano- structure (rather than its composition) High h index e x of o f refraction c n Low w index e x of o f refraction c n 3D photonic crystal: a semiconductor for photons Frequency Allowed Forbidden Allowed Refs: E.Yablonovitch, Phys. Rev. Lett. 58, 2059, (1987). S.John, Phys. Rev. Lett. 58, 2486, (1987). frequency wavevector
Controlling density of photonic states controlling thermal emission spectrum u ( ω, T ) = N ( ω) hω e k BT hω 1 energy density density of photonic modes energy in each photonic mode frequency wavevector
Photonic crystals are analogous to semiconductors E Face center cubic lattice E allowed states forbidden states allowed Conduction band electronic bandgap E states valence k k band g
Naturally occurring photonic crystals: Butterfly wings Opal Photo by Megan McCarty at Wikimedia Commons. Images removed due to copyright restrictions. Please see: http://www.tils-ttr.org/photos/mitoura-grymdneo.jpg http://www.tils-ttr.org/photos/mitoura-grymvneo.jpg Fig. 11 in Ghiradella, Helen. "Light and Color on the Wing: Structural Colors in Butterflies and Moths." Applied Optics 30 (1991): 3492-3500. Fig. S1a, S2, and S4a in Vukusic, Pete, and Ian Hooper. "Directionally Controlled Fluorescence Emission in Butterflies." Science 310 (November 18, 2005): 1151. Fig. 3 in Pendry, J. B. "Photonic Gap Materials." Current Science 76 (May 25, 1999): 1311-1316. P. Vukusic, I. Hooper, Directionally controlled fluorescence emission in butterflies, Science, vol. 310, pp. 1151
Tailoring electronic-and photonic bandgap properties: a path towards record efficiencies
Photonic crystal as omnidirectional mirror Selective emitter Front-side filter Heat Waste Heat A B C
1D Si/SiO 2 photonic crystals exhibit omni-directional bandgap thermal emitter filter PV diode Heat sink 0 1 2 n PV T PV Heat Waste Heat θ 1 ε o ε 1 ε 2 ε 3 ε n ε PV y z 0 Angular frequency ω (2πc/a) 0..7 0..6 0..5 0..4 0..3 0..2 Δω gn Projected photonic band diagram vacuum light line SiC light line 0..1 0 TM modes 1 0.8 0.6 0.4 0..2 TE modes 0 0.2 0.4 0.6 0.8 1 Parallel wave vector k y (2π/a) 0 0.2 0.4 0.6 Reflectance 0.8 1
Spectral characterization of 1D photonic crystal 1 0.9 0.8 measured simulated 0.7 Transmittance 0.6 0.5 0.4 0.3 500 nm TEM cross section of LPCVD * grown quarter-wave stack filter with half-layer at the top 0.2 0.1 0 1 1.5 2 2.5 3 Wavelength (µm) Si = lighter layers (170nm) SiO 2 = darker layers (390nm) Image removed due to copyright restrictions. Please see Fig. S2 in Vukusic, Pete, and Ian Hooper. "Directionally Controlled Fluorescence Emission in Butterflies." Science 310 (November 18, 2005): 1151.
Front side PhC designs, 0.72 ev, 0.6 ev, 0.52 ev 1 1 1D 1D & plasma 0.52 0.52 ev ev 0.9 1D 1D w/plasma 0.6 0.6 ev 0.9 ev 1DSi/SiO 2 0.72 0.72 ev ev 2 0.8 0.8 0.7 0.7 Transmittance 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 0 01 1.5 2 2.5 3 3.5 4 4.5 5 1 1.5 2 2.5 3 3.5 4 4.5 5 wavelength [µm] wavelength [µm] BB 0 1 2 n PV BB 0 1 2 n PV θ BB θ BB ε BB ε o ε 1 ε 2 ε 3 ε n ε PV ε BB ε o ε 1 ε 2 ε 3 ε plasma ε n ε PV y L o y L o z z
1D Si/SiO 2 photonic crystals: quarter-wave based stack and genetic algorithm optimized stack as a spectral control tool Quarter-wave photonic crystal 1 2 10 PV T PV 1 (a) (b) Transmittance Transmittance 0.8 0.6 0.4 0.2 0 0 1 2 3 4 5 6 7 8 Wavelength [µm] 1 0.8 0.6 0.4 0.2 ε 1 ε 2 ε 1 ε 2 ε 1 ε 2 ε PV half layer Genetic algorithm optimized stack 1 2 10 PV T PV 0 0 1 2 3 4 5 6 7 8 Wavelength [µm] ε 1 ε 2 ε 1 ε 2 ε 1 ε 2 ε PV
Spectral characterization of fabricated 1D photonic crystal 1 0.8 Reflectance 0.6 0.4 TE 20 TE 30 0.2 TE 40 0 TE 50 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 1 0.8 a) 0.6 0.4 TM 20 TM 30 0.2 TM 40 0 TM 50 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 Wavelength (µm) b)
Improving the spectral efficiency via selective thermal emission Selective emitter Front-side filter Heat Waste Heat A B C
But remember thermal emitter is really hot! (up to 1500K) Refractory metals have high melting temperature, especially tungsten, and that is why it has been used for incandescent light bulbs ever since William D. Coolidge, invented the process for producing the ductile tungsten in 1909 that revolutionized light bulbs and X-ray tubes. His first light bulb was named Mazda Images removed due to copyright restrictions.please see: http://www.harvardsquarelibrary.org/unitarians/images/coolidge4.jpg http://www.harvardsquarelibrary.org/unitarians/images/coolidge10.jpg http://www.harvardsquarelibrary.org/unitarians/images/coolidge11.jpg http://www.harvardsquarelibrary.org/unitarians/images/coolidge12.jpg http://www.harvardsquarelibrary.org/unitarians/coolidge.html
Adding an array of resonant cavities in tungsten can help us tailor the emittance Lorentz-Drude model for tungsten ω 2 ω 1
2D W PhC as selective thermal emitter: Emittance 1 0.9 0.8 0.7 0.6 0.5 0.4 meas. 2D W PhC (r=450nm d=560nm) sim. 2D W PhC (r=450nm d=560nm) meas. 2D W PhC (r=440nm d=315nm) sim. 2D W PhC (r=440nm d=315nm) meas. 2D W PhC (r=390nm d=600nm) sim. 2D W PhC (r=390nm d=600nm) meas. flat tungsten sim. flat tungsten 0.3 0.2 0.1 0 1 1.5 2 2.5 3 wavelength [µm]
2D W PhC exhibits tunable cut-off and resonant enhancement
Fabrication process improvements Old New
Fabrication Process Laser Interference Lithography Development Soft-mask etch Hard-mask etch ARC = Anti-Reflective Coating Soft-mask removal Tungsten etch Hard-mask removal
Tailoring electronic-and photonic bandgap properties: a path towards record efficiencies
GaSb and GaInAsSb diode comparison 0.5 V oc (V) V [V ] oc 0.45 0.4 0.35 0.3 0.25 0.2 0.15 GaSb GaInAsSb Quantum efficiency External Quantum Efficiency 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 GaSb GaInAsSb EQE GaSb EQE GaInAsSb 0.1 0-4 -3-2 -1 0 1 1 1.5 2 2.5 3 10 10 10 10 10 10 J [A/cm 2 ] sc I sc (A/cm 2 ) wavelength [µm] Wavelength (µm)
Tuning the PhC and PV diode bandgaps: GaSb (0.72 ev) and GaInAsSb (0.52 ev) GaSb GaInAsSb Quantum efficiency, Transmittance 11 PhC PhC Transmittance 0.9 GaSb 0.9 QE QE 0.8 0.8 EQE, Transmittance 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 η spectral = 41 % T >Eg = 70 % 00 11 1.5 1.5 22 2.5 2.5 33 3.5 3.5 44 wavelength [µm] Quantum efficiency, Transmittance 11 PhC PhC Transmittance 0.9 InGaAsSb 0.9 QE QE 0.8 0.8 EQE, Transmittance 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 00 11 1.5 1.5 22 2.5 2.5 33 3.5 3.5 44 Wavelength (µm) η spectral = 80 % T >Eg = 79 %
Photonic crystals tailoring photonic- and electronic bandgaps
Tuning the PhC and PV diode bandgaps: GaSb (0.72 ev) GaSb EQE, 1D PhC Transmittance, 2D PhC Emittance 1 0.9 0.9 0.8 0.8 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 0 1 1.5 1.5 2 2.5 2.5 3 3.5 3.5 4 Wavelength (µm) 2D 2D PhC PhC Emittance 1D 1D PhC PhC Transmittance GaSb EQE Heat Selective emitter Front-side filter A B C PV diode Electricity out Waste Heat 1D PhC and 2D W PhC Spectral efficiency Above bandgap transmittance 93 % 70 %
Photovoltaic module: design and characterization
Simple TPV diode model
GaInAsSb diode characterization cont d Packaged Cells 0.4 100 99-017-08 00-202-18 90 01-471-15 01-471-16 0.35 80 External Quantum Efficiency 99-017-08 00-202-18 01-471-15 01-471-16 70 V (V) oc 0.3 0.25 EQE (Percent) 60 50 40 30 0.2 20 10 0 0.15 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6-2 10-1 10 0 10 Wavelength (µm) J (A/cm 2 ) sc
GaInAsSb diode characterization 99-017-08 00-202-18 0.35 0.35 Unpackaged Packaged 0.3 0.3 AR Coated Unpackaged Packaged AR Coated V (V) V (V) oc oc 0.2 V (V) oc 0.25 0.25 0.15 0.15 10 10 10 10 10 10 0.35 0.3 0.25-2 -1 0-2 -1 0 J (A/cm 2 ) J (A/cm 2 ) sc sc 01-471-15 01-471-16 0.35 V oc (V) 0.2 0.3 0.25 0.2 0.2 0.15 0.15 10 10 10 10 10 10-2 -1 0-2 -1 0 J sc (A/cm 2 ) J sc (A/cm 2 )
MIT µ-tpv T Generato r P roo jee ctt
Key innovations in: photonic crystals, MEMs reactors, power electronics, PV 1D photonic crystal 2D tungsten photonic crystal Low-bandgap PV cells Power electronics Si micro-fabricated reactor
Photonic crystals tailoring photonic- and electronic bandgaps
Robust, integrated catalytic micro-reactor r dee sii gnn
Integrated power electronics controller single chip integrated MPPT
Quasi-coherent thermal emission via photonic crystals Vertical-cavity resonant thermal emitter 2D PhC slab resonant thermal emission
Broad-band spectral control Reflectance 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 measured simulated 1 1.5 2 2.5 3 Wavelength (µm) Emittance 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 flat W 2D W PhC (r=440 nm,d=315 nm) 2D W PhC (r=390 nm,d=560 nm) 2D W PhC (r=440 nm, d=560 nm) Generation 1 0.1 Generation 2 0 1 1.5 2 2.5 3 3.5 Narrow-band spectral control θ z ε 0 ε H ε H ε L ε H ε L ε H L 0 ε cavity ε M y
Vertical cavity resonant thermal emitter is highlydirectional, quasi-coherent radiation source ε 0 θ z ε H ε H ε L ε H ε L ε H L 0 ε cavity ε M y
Vertical cavity resonant thermal emitter: narrow-band, highly directional and Tungsten cavity mirror
Quasi-coherent thermal emission via photonic crystals Vertical-cavity resonant thermal emitter 2D PhC slab resonant thermal emission
Black/Gray- Body Physics Thermal Thermal Ref: Max Planck, Annalen der Physik, 4, 553, (1901).
Modes of a 2D PhC slab H z odd guided resonance z y even guided resonance mode x H z
Fano resonances of a 2D PhC slab z y x Ref: S. Fan and J. D. Joannopoulos, Phys. Rev. B 65, 235112 (2002).
Thermal emittance of a 2D PhC slab Im(ε) 0.005 z y x Thermal Emittance Ref: D.Chan, I.Celanovic, J.D.Joannopoulos, and M.Soljačić, submitted for publication.
Dependence on angle of observation θ θ increases θ increases Thermal Emittance Thermal Emittance z y x
Analytical understanding of Fano resonances 2 a 2 = Q ABS Q RAD ω 1 1 4 1 + + ω FANO Q RAD Q ABS PhC 2 2 Q ABS = ε R σε I Q ABS = Q RAD a PhC MAX = 50%
Rules for designing thermal emission Thermal Emittance z y x ω EMIT (θ): slab thickness Re(ε) lattice constant Γ EMIT Q RAD : size of holes Peak emission Q ABS : Im(ε) 2 2 Q ABS Q a RAD PhC = 2 ω 1 1 4 1 + + ω FANO QRAD Q ABS 2
An example of thermal design z y Q RAD =370 x Q RAD =2000 Thermal
Quasi-coherent thermal radiation: summary and opportunities PhC s offer unprecedented opportunities for tailoring thermal emission spectra Highly anomalous thermal spectra can be obtained Even dynamical tuning of spectra is possible Research in the combined near-field and quasi-coherent PhC radiation is opening up new frontiers Possible applications include: masking thermal targets, coherent thermal sources, high-efficiency TPV generation, chemical sensing, etc.
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