Photonic Crystals:Shaping the Flow of Thermal Radiation

Ivan ČelanovićMassachusetts 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

Pout + –

Em

itte

r

P N

Heat Waste Heat Blackbody Radiation

Cell Surface

Reflection

GaS

b

PV vs. TPV

Properties:

PV (Solar Cells) TPV

Sensitivity Range Visible and NIR NIR and IR

Source Sun Thermal emitter

Source

Temperature Over 5000K (sun’s surface) 1000-1500K

Distance from

Source Over 90 million miles µm to cm

Energy reflected

from cell surface Lost to atmosphere Recycled to the emitter

Courtesy of DOE/NREL, Credit - Beck Energy.

TPV Technologies and applicationsTPV Technologies and applications

su

n r

adia

tio

n

TPhC

absorber/photonic crystal

Solar cell

concentrator

Solar TPV

Radioisotope TPV power system for

deep space and Mars missions

18 W/kg, (PuO2 fuel)

for 30 years

24% efficient

1 We

15% efficiency

micro-TPV power generator (propane/butane operated) AA radioisotope TPV battery:

~10 mWe

30 years life time

24% efficiency

Plutonium pellet

Photo courtesy of LLNL.

Images courtesy of NASA. Photo courtesy of Sandia National Labs.

Courtesy of Klavs Jensen. Used with permission.

Heat

Input

Waste

Heat

thermal emitter filter PV diode Heat sink

Thermophotovoltaics: converting thermal radiation into electricity, with no moving parts

GaSb (0.72 eV) InGaAs (0.6 eV) InGaAsSb (0.53 eV) Si (1.23 eV)

bad photons

good photons

Photonic Crystals: shaping thermal radiation

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Norm

aliz

ed

radia

ted

pow

er

1 2 3 4 5 6 7 8

wavelength [µm]

System design

TPV Technology roadmap: the time is nowTPV Technology roadmap: the time is now

Si and Ge

PV III-V’s (GaSb, InGaAs, GaInAsSb)diode

rare earth oxides

Spectral Dielectric stack

control filters Photonic

Crystals

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

),,(),,( zyxzyx x λεε +=

1-D Periodicity 2-D Periodicity 3-D Periodicity

),,(),,( zyxzyx yx λλεε ++= ),,(),,( zyx zyxzyx λλλεε +++=

Metamaterial:

optical properties determined from its nano-structure

(rather than its composition)

Hig h ind e x

o f re fra c tio n

Lo w ind e x

o f re fra c tio n

Allowed

Allowed

Forbidden

Freq

uenc

y

3D photonic crystal: a “semiconductor for photons”

High index of refraction

Low index of refraction

freq

uenc

ywavevector Refs: E.Yablonovitch, Phys. Rev. Lett. 58, 2059, (1987).

S.John, Phys. Rev. Lett. 58, 2486, (1987).

Controlling density of photonic states

⇓⇓⇓⇓controlling thermal emission spectrum

hω

( , T ) = N ω ∗u ω ( )

freq

uenc

y

hω

e kBT − 1

energy

density

density of

photonic

modes

energy in each

photonic mode

wavevector

Photonic crystals are analogous to semiconductors

Face center cubic lattice

states electronic

E E

Conduction band

forbidden bandgap states E

allowed g

states valence kk band

allowed

Naturally occurring photonic crystals:

Butterfly wings Opal

P. Vukusic, I. Hooper, “Directionally controlled fluorescence emission in butterflies,” Science, vol. 310, pp. 1151

Photo by Megan McCarty at Wikimedia Commons.Images removed due to copyright restrictions.Please see: http://www.tils-ttr.org/photos/Mitoura-gryMDneo.jpghttp://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.

Tailoring electronic- and photonic bandgap properties:

a path towards record efficiencies

Photonic crystal as omnidirectional mirror

Selective emitter Front-side filter

Waste

Heat

Heat

A B C

1D Si/SiO2 photonic crystals exhibit omni-directional bandgap

εo ε1 ε2 ε3 εn εPV

0 1 2 n PV

y z

θ1

TPV

Heat

thermal emitter

Heat

thermal emitter filter

Waste

Heat

PV diode Heat sink

0

Ang

ula

r fre

quenc

y ω

(2π

c/a

)A

ng

ula

rfre

quenc

yω

(2π

c/a

)

0.0 7.7

0.0 6.6

0.0 5.5

0.0 4.4

0.0 3.3

0.0 2.2

0.0 1.1

00

Projected photonic band diagram

0 0.6 10.80.2 0.4

TE modesTM modes

vacuu

mlig

htlin

e

SiClig

htlin

e

gNωΔ

0.6 10.8

TE modesTM modes

vacuu

mlig

htlin

e

SiClig

htlin

e

0 0.2 0.4

gNωΔ

10.80 0.2 0.4 0.6

Reflectance

1 0.81 0.8 0.6 0.40.6 0.4 0.0 2.2

Parallel wave vector ky (2π/a)Parallel wave vector ky (2π/a)

Spectral characterization of 1D photonic crystal

500 nm

TEM cross section of LPCVD* grown quarter-wave stack filter with half-layer at the top

Si = lighter layers (170nm) SiO2 = darker layers (390nm)

1 1.5 2 2.5 3 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wavelength (µm)

Tra

nsm

itta

nce

measured simulated mmeasured simulated m easuredsimulated

easuredsimulated

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

Tra

nsm

itta

nce

T

ran

sm

itta

nce

11

1D & plasma 0.52 eV

1D w/plasma 0.6 eV 1D & plasma 0.52 eV

0.9 1D w/plasma 0.6 eV

1DSi/SiO2 0.72 eV 0.9

1DSi/SiO 0.72 eV 2

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

001 1.5 2 2.5 3 3.5 4 4.5 51 1.5 2 2.5 3 3.5 4 4.5 5

wavelength [µm] wavelength [µm]

0 1 2 n PV BB 0 1 2 n PV BB

θBB θBB

εε2 ε3ε o ε1 n εPV εplasma εε2 ε3εBB ε o ε1 n εPV εBB

y L oy L o z

z

1D Si/SiO2 photonic crystals: quarter-wave based stack and genetic algorithm optimized stack as a spectral control tool

Quarter-wave photonic crystal

1 2 10 PV TPV

1

0.8

0.6 (a)

0.4

0 1 2 3 4 5 6 7 8

ε1 ε2 ε1 ε2 ε1 ε2 εPV

0.2

0 half layer

Wavelength [µm]

Tra

nsm

itta

nce

T

ransm

itta

nce

1

0.8

0.6 (b)

0.4

0.2

0 0 1 2 3 4 5 6 7 8

Genetic algorithm optimized stack

1 2 10 PV TPV

Wavelength [µm]

ε2 ε1ε1 ε2 ε1 ε2 εPV

Spectral characterization of fabricated 1D photonic crystal

0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6

0

0.2

0.4

0.6

0.8

1

0

0.2

0.4

0.6

0.8

1

Refle

cta

nce

a)

b)

TE 20° TE 30° TE 40° TE 50°

TM 20° TM 30° TM 40° TM 50°

0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6

Wavelength (µm)

Improving the spectral efficiency via selective thermal emission

Selective emitter Front-side filter

Waste

Heat

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”

http://www.harvardsquarelibrary.org/unitarians/coolidge.html

Images removed due to copyright restrictions.Please see:http://www.harvardsquarelibrary.org/unitarians/images/coolidge4.jpghttp://www.harvardsquarelibrary.org/unitarians/images/coolidge10.jpghttp://www.harvardsquarelibrary.org/unitarians/images/coolidge11.jpghttp://www.harvardsquarelibrary.org/unitarians/images/coolidge12.jpg

Adding an array of resonant cavities in tungsten can help us tailor the emittance

Lorentz-Drude model for tungsten

ω1ω2

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

2D W PhC as selective thermal emitter:

1 1.5 2 2.5 3 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

wavelength [µm]

Em

itta

nce

1 1.5 2 2.5 3 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

wavelength [µm]

Em

itta

nce

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

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

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

V

]V[

co

GaSb

GaInAsSbV oc

(V

)

1

Qu

an

tum

eff

icie

nc

y

Exte

rnal Q

uantu

m E

ffic

iency

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

GaInAsSb

GaSb EQE

EQE

GaSb

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/cm2]sc

Isc (A/cm2) wavelength [µm]

Wavelength (µµµµm)

Tuning the PhC and PV diode bandgaps: GaSb (0.72 eV) and GaInAsSb (0.52 eV)

GaSb GaInAsSb

1 1.5 2 2.5 3 3.5 4 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

wavelength [µm]

EQ

E, T

ran

sm

itta

nce

PhC Transmittance

GaSb QE

1 1.5 2 2.5 3 3.5 4 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

wavelength [µm]

EQ

E, T

ran

sm

itta

nce

PhC Transmittance

GaSb QE

1 1.5 2 2.5 3 3.5 4 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wavelength (µm)

EQ

E, T

ran

sm

itta

nce

PhC Transmittance

InGaAsSb QE

1 1.5 2 2.5 3 3.5 4 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wavelength (µm)

EQ

E, T

ran

sm

itta

nce

PhC Transmittance

InGaAsSb QE

ηspectral = 80 %

T>Eg = 79 %

ηspectral = 80 %

T>Eg = 79 %ηspectral = 41 %

T>Eg = 70 %

ηspectral = 41 %

T>Eg = 70 %

Qu

an

tum

eff

icie

nc

y,

Tra

nsm

itta

nce

Qu

an

tum

eff

icie

nc

y,

Tra

nsm

itta

nce

Photonic crystals tailoring photonicPhotonic crystals tailoring photonic-- ana dnd

electronic bandgapselectronic bandgaps

Tuning the PhC and PV diode bandgaps: GaSb (0.72 eV)

1 1.5 2 2.5 3 3.5 4 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wavelength (µm)

Ga

Sb E

QE

, 1

D P

hC

Tra

nsm

itta

nce

, 2

D P

hC

Em

itta

nce

2D PhC Emittance

1D PhC Transmittance

GaSb EQE

1 1.5 2 2.5 3 3.5 4 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wavelength (µm)

Ga

Sb E

QE

, 1

D P

hC

Tra

nsm

itta

nce

, 2

D P

hC

Em

itta

nce

2D PhC Emittance

1D PhC Transmittance

GaSb EQE

Heat Waste

Heat

Selective emitter

Front-side filter

Electricity out

A B C

PV diode

Spectral efficiency Above bandgap

transmittance

1D PhC and 2D W

PhC 93 % 70 %

Photovoltaic module:

design and characterization

Simple TPV diode model

GaInAsSb diode characterization cont’d

Packaged Cells External Quantum Efficiency 0.4 100

99-017-08

00-202-18 90

01-471-15

01-471-16

0.35 80

70

60

50

40

0.3

0.25

30

20 0.2

10

0 0.15 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6

-2 -1 0 10 10 10 Wavelength (µm)

J (A/cm2)sc

99-017-08

00-202-18

01-471-15

01-471-16

EQ

E (

Perc

ent)

V

(V)

oc

GaInAsSb diode characterization

V

(V)

V

(V)

oc

oc

99-017-08 00-202-18 0.35 0.35

0.2

Unpackaged

Packaged

AR Coated

0.2

0.15 0.15

Unpackaged

Packaged

AR Coated

-2 -1 0 -2 -1 0 10 10 10 10 10 10

J (A/cm2) J (A/cm2)sc sc

01-471-15 01-471-16

V

(V)

oc

0.3 0.3

0.25 0.25

0.35

0.3

0.25

0.2

0.15 0.15

0.2

0.25

0.3

0.35

V o

c (V

)

-2 -1 0 -2 -1 0 10 10 10 10 10 10

J sc

(A/cm2) J sc

(A/cm2)

MITMIT µµµµµµµµ--TPV GeneratoT r Pr ror jej ctcPV Generato P o e t

Key innovations in: photonic crystals,Key innovations in: photonic crystals,

MEMs reactors, power electronics, PVMEMs reactors, power electronics, PV

1D photonic crystal 2D tungsten photonic crystal

Power electronicsLow-bandgap PV cells

Si micro-fabricated reactor

Photonic crystals tailoring photonicPhotonic crystals tailoring photonic-- ana dnd

electronic bandgapselectronic bandgaps

Robust, integrated catalyticRobust, integrated catalytic

micromicro--reactorr ded sis gngeactor e i n

Integrated power electronics controllerIntegrated 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

θθθθ

z

y

L0

εM

εcavity

εH

εL

εH

εL

εH

εH

ε0

Narrow-band spectral control

1 1.5 2 2.5 3 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wavelength (µm)

Re

flec

tance

measured

simulated

Broad-band spectral control

1 1.5 2 2.5 3 3.5 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1 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

Generation 2

Emitt

ance

Ref

lect

ance

Vertical cavity resonant thermal emitter is highly-directional, quasi-coherent radiation source

θθθθ

z

y

L0

εM

εcavity

εH

εL

εH

εL

εH

εH

ε0

Vertical cavity resonant thermal emitter:

narrow-band, highly directional and

Tun

gste

n

ca

vity

mirro

r

Quasi-coherent thermal emission via photonic crystals •Vertical-cavity resonant thermal emitter •2D PhC slab resonant thermal emission

Black/Gray-

Body Physics

Th

erm

al

Th

erm

al

Th

erm

al

Th

erm

al

Ref: Max Planck, Annalen der Physik, 4, 553, (1901).

Modes of a 2D PhC slab

odd guided resonance

|Hz|

even guided resonance mode

|Hz|

x

yz

Fano resonances of a 2D PhC slab

z y

x

Ref: S. Fan and J. D. Joannopoulos, Phys. Rev. B 65, 235112 (2002).

x

yz

Thermal emittance of a 2D PhC slab T

herm

al E

mit

tan

ce

Im(ε)≈0.005

Ref: D.Chan, I.Celanovic, J.D.Joannopoulos, and M.Soljačić, submitted for publication.

x

z y

θ

θ increases

θ increases

e

cn

attim

El

amr

eh

Te

c

natti

mE

l a

mre

hT

Dependence on angle of observation

Analytical understanding of Fano resonances

⇒

2

2 QABS QRADa = PhC 2 2 ω 1 1

4 − 1 + + ωFANO QRAD QABS

ε R=QABS σε I

Q = Q ⇒ aPhC MAX = 50%ABS RAD

Rules for designing thermal emission

x

yz

ωωωωEMIT(θθθθ): • slab thickness

• Re(ε) • lattice constant

ΓΓΓΓEMIT ⇔⇔⇔⇔ QRAD: • “size” of holes

Peak emission ⇔⇔⇔⇔ QABS: • Im(ε)

Th

erm

al E

mit

tan

ce

2

2 QABS QRAD a = PhC ω

4 − 2

1 1 1

+ +ωFANO Q QRAD ABS

2

An example of thermal design

QRAD=370

QRAD=2000

Th

erm

al

z y

x

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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