(Fifth Lecture) Techno Forum on Micro-optics and Nano-optics Technologies
Efficient light emission from LEDs, OLEDs, and
nanolasers via surface-plasmon resonance
송 석 호, 한양대학교 물리학과, http://optics.anyang.ac.kr/~shsong
silver grating
Key
notes
1. How does the surface plamon resonance enhance the internal quantum efficiency of light source?
2. Understand the Fermi-Golden rule and Purcell enhancement factor in spontaneous emission
3. What are the p
practical difficulties in realizing
g SP-enhanced LEDs?
4. Summary of the five lectures
Remind!
The next chip-scale technology
Three light-design regimes
λ limit
WAVE DESIGN
(d~λ)
Light extraction
e limit
RAY DESIGN
LED
(d>λ)
Internal QE
PHOTON DESIGN
(d<λ)
Power conversion efficiency of III-Nitride LEDs
Example:
E
l
λ=530nm, I=350mA
PCE ~ 12%
External efficiency of LEDs
⎛ R ⎞
ηexternal = ηextraction ⎜
⎟
R
+
R
nr ⎠
⎝
ηextraction :extraction efficiency
Rnr :nonradiative-recombination rate
R:spontaneous-emission rate
η extraction
θc
⎛1⎞
⎛θ ⎞
= ⎜ ⎟∑s , p ∫ [1 − R (θ )]sin ⎜ ⎟dθ
0
⎝2⎠
⎝2⎠
1
≈
4(n f / ng ) 2
= 4% for
f GaN(2.5)
G N(2 5) - air(1.0)
i (1 0)
Wave Design for efficient extraction of the guided light
-. Geometric optics
-. Random scattering
g
in surface textured structure
APL 63, 2174 (1993)
⎛ R ⎞
ηexternal = ηextraction ⎜
⎟
R
+
R
nr ⎠
⎝
⎛ R ⎞
ηexternal = ηextraction ⎜
⎟
R
+
R
nr ⎠
⎝
Photon Design for increasing the emission rate
What determines spontaneous emission rate of radiating source?
electron
Ei
Energy of EM field
=ω (n + 1/ 2)
Ef
Number
N
b off photon
h t
(Stimulated emission)
Vacuum fl
V
fluctuation
t ti
(Spontaneous emission)
Fermi’s Golden Rule
1
1
SE Rate : R =
=
f p⋅E i
τ (ω ) 2ε 0 =
Dipole moment
of radiation source
eMD Lab.
2
ρ (ω )
Photon DOS
(density of states)
Electric field strength
of half photon (vacuum fluctuation)
Microoptics Lab –Hanyang University
6
Photon Design for increasing the emission rate
R=
1
1
f p⋅E i
=
τ (ω ) 2ε 0 =
2
ρ (ω )
E, ρ increase
ηexternal = ηextraction
⎛ R ⎞
⎜
⎟
+
R
R
nr ⎠
⎝
Ag
g
p-GaN
Quantum Well
n GaN
n-GaN
Atoms in microcavity
• High Q
• Narrow Δν
• Fp ~ 1 – 5
• Low volume filling factor
Photonic crystal cavity
• Moderate Q
• Wider
Wid Δν
Δ
• Fp(Quantum wells) ~ 3
• Fp(Quantum dots) ~ 5 –100
Surface plasmon coupling
• Low Q
• Narrow Δν
• Fp ~ 5 – 100
• lossy and off-resonant
• Off-resonant and
complicated fabrication
www.phys.unt.edu/research/ photonic/website/Surf-Plasmon-OHPs-f.ppt
Department of Physics, University of North Texas, Denton, Texas 76203
Photonic-crystal approach
R=
1
1
=
f p⋅E i
τ (ω ) 2ε 0 =
2
ρ (ω )
E, ρ increase
ηexternal = ηextraction
⎛ R ⎞
⎜
⎟
R
+
R
nr ⎠
⎝
Limited by surface recombination
Baba
LumiLed
G d scheme!
Good
scheme!!!
h
!!!!
100 um device size achievable.
Several layer of PC for extraction.
G d internal
Good
i t
l quantum
t
efficiency
ffi i
Needed (>90%).
Multiple pass limits device size (~10um).
Small volume needed.
Not so good for lighting.
Surface recombination limited
Surface recombination limited.
Noda
Photonic-crystal assisted LEDs
1
1
R=
=
f p⋅E i
τ (ω ) 2ε 0 =
2
ρ (ω )
Very small increase in E, ρ !
Look like a result of wave design rather than photon design!
Surface-plasmon approach
ηint =
η 'int =
Surface Plasmons
Rp
R p + Rnr
R p + Rsp
R p + Rsp + Rnr
The SP approach was started for organic LEDs
Conventional Structures:
Strongly coupled to SPPs
ITO glass (anode)
Organic molecules
Cathode & Mirror
SPP quenching
(~40%)
Metallic mirror
Main issue:
SPP Î Radiation coupling
Metallic thin film
SPP2
SPP1
SPP band gap Direct coupling
(Λ ~ π / kSPP )
(Λ > π / k SPP )
SPP cross-coupling
(Λ = π /[kSPP1 − kSPP 2 ])
Effect of SPP band gap on PL
11411
Angle resolved PL
of dye molecule (DCM)
1st and 2nd order
diffraction
d
act o of
o SPPs
S s
Tracing 1st order peaks shows SPP band gap.
Modification of Spontaneous Emission Rate of Eu3+
Main emission of Eu3+ (614nm)
SPP quenching
hi
τ (spacer thickness
h k
)
TRPL at 614nm
Self-driven dipole (CPS) modeling
d
p
d2
d
e2
2
p + b0
p + ω0 p = E r
2
dt
m
dt
p = p0 e − i (ω −ib / 2) t , Er = E0 e − i (ω − ib / 2)t
2 unknowns and 2 equations
Metal interface
e2
b / b0 = 1 +
Im{E0 }
mω p0 b0
⎛
⎞
b 2 bb0
e2
Δ
ω
≈
−
−
Re{
E
}
⎜
0 ⎟
8ω 4ω0 2mω0 p0
⎝
⎠
14
Dipole Decay Calculation Test : Metal Mirror Cavity
102
10-4
2
10
dissipated pow
wer
1
10
0
10
-1
10
-2
10
-3
10
perpendicular dipole
parallel dipole
-4
10
0.0
0.5
1.0
k x / k1
1.5
2.0
J. A. E. Wasey and W. L. Barnes, J. Mod. Opt. 47, 725-741, 2000
15
CPS Model Calculation for Spontaneous Emission Rates of an OLED
Emission Spectrum
No guided mode
TM0
TM0+TE0
TM0+TE0+TM
1
70nm
100nm
200nm
390nm
ra
adiation ratte (R0)
30
3.0
2.5
total emission rate
air emission
emission to substrate g
guided modes
emission to active layer guided modes
2.0
1.5
cover (medium c)
dipole
1.0
hs
0.5
active material
((medium a))
(ha = hs + hc )
0.0
0
50
100
150
200
250
300
active layer thickness (nm)
16
hc
350
400
substrate (medium s)
Comparison with an experiment
100
90
90
power ratio (%)
PL Efficiency (%
%)
100
80
70
60
70
60
50
40
30
Pair+Psub+1.0Pguided
Pair+Psub+0.4Pguided
20
Pair+Psub+0.8Pguided
Pair+Psub+0.2Pguided
Pair+Psub+0.6Pguided
Pair+Psub+0.0Pguided
10
100
200
300
400
Film Thickness (nm)
(measured)
17
80
500
0
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
active layer thickness (μm)
(calculated)
SPP Enhanced Spontaneous Emission of Eu3+ Ion
SE rate
90% SPP coupling
li
25 times SE rate
Dipole-SPP
coupling fraction
Maximum internal efficiency
Role of Preferred Orientation of the Dipole Source
Adv. Mater. 14 19 1393
Angle integrated EL
Enhanced PL by Coupled SPP
Cross-Coupled vs Coupled SPP
(1)
(2)
(3)
(4)
SPP Enhanced PL of InGaAs QW
Most cited paper
Un-processed
(a)
Half-processed
(b)
Fully-processed
(c) 480nm period (2nd order coupling)
(d) 250nm period (1st order coupling)
(160nm gap)
1st Result of SPP enhanced PL from InGaN QW
Nature Materials,
Materials VOL 3,
3 p.601-605,
p 601 605 2004
R=
1
1
=
f p⋅E i
τ (ω ) 2ε 0 =
2
ρ (ω )
E, ρ increase
ηexternal = ηextraction
⎛ R ⎞
⎜
⎟
R
+
R
nr ⎠
⎝
Nature Materials, VOL 3, p.601-605, 2004
1st Result of SPP enhanced PL from InGaN QW
Nature Materials, VOL 3, p.601-605, 2004
133nm wide, 400nm period grating
40x100nm2
(no enhancement for 200nm wide, 600nm period grating)
0.42
0.18
x2
0 06
0.06
x28
x14
Average
e age internal
te a qua
quantum
tu efficiency
e c e cy est
estimation
at o
TRPL of SPP enhanced InGaN QW emission
How does the surface-plasmon resonance contribute to emission rate?
1
1
R=
=
f p⋅E i
τ (ω ) 2ε 0 =
2
ρ (ω )
Field enhancement
near the source layer
eMD Lab.
High DOS
due to decrease in
group velocity
Microoptics Lab –Hanyang University
26
1
1
R=
=
f p⋅E i
τ (ω ) 2ε 0 =
2
ρ (ω )
High DOS
due to decrease in
group velocity
Field enhancement
near the source layer
Requirements for enhancing SE rate
-. slow g
group
p velocity
y
-. tight confinement of mode
-. low ohmic loss
-. large field enhancement
B
slow group velocity,
high
g loss
A
fast group velocity,
l
low
loss
l
A
Q.W.
B
Q.W.
Purcell factor defining enhancement of the spontaneous emission
Fp ≡
Roriginal + Radditional
Roriginal
= 1+
Radditional
Roriginal
For a cavity mode:
Rcav
3Q ( λc / n ) 3
=
Fp =
R ffree 4π 2Vmode_volume
For a SP mode :
RSP
1
Fp = 1 +
= 1+
R0
2π
υ SP
d ωSP
=
,L =
dk
∫
∞
−∞
⎛ λ ⎞ kSP / k0
⎜ ⎟
⎝ L ⎠ υ SP / c
∂ (ωε )
2
E( z )
∂ω
2
Eat dipole
dz
We need a slow and confined mode!
Factors influencing Purcell Enhancement Fp(ω)
Ag ~ z
GaN ~ ζ
Single
Si l Q
Quantum
t
W
Well
ll
GaN
Variation with Ag thickness
Variation with GaN thickness
Purcell
factor (F-1)
Purcell enhancement
factor: A numerical
estimation
cover
Cover = 1.0
Cover = 1.5
C
Cover
=2
2.0
0
Î Need a very thin p-GaN layer !!
Improvement
I-L curve
⎛ 2.68 at 10 K
Fp = ⎜
⎝1.75 at 300 K
“… the enhanced Fp … can be attributed to an increase
in the spontaneous emission rate due to SP-QW coupling.”
No improvement
I-V curve
Why
y SP-LED hasn’t been successful y
yet?
Practical Barriers (especially for InGaN/GaN devices)
• Thin p-GaN leads to abrupt occurrence of leakage current
under
d a certain
t i thi
thickness
k
• SP propagation length in blue wavelength along the Ag/GaN interface
is extremely
y short
• Nanopatterning becomes a huge burden at short wavelength
• Damageless p
p-GaN
GaN patterning has been impossible
• SQW devices are prone to leakage current due to carrier overflow
• Silver is a nasty material with poor adhesion to GaN
and tends to agglomerate at an elevated temperature
SP propagation length
ω ⎛ ε′ ε
k ′′ = ⎜⎜ m d
c ⎝ ε m′ + ε d
⎞
⎟⎟
⎠
32
ε m′′
2(ε m′ ) 2
Λ = λsp, 2λsp, 3λsp, …
4000
3500
25
2.5
Surface Plasmon on the Ag/GaN Interface
Freq
quency (2πc/μm)
Propagattion Length of SPs [nm
m]
PLSPs
1
=
2k ′′
Nanopatterning
3000
2500
2000
1500
1000
500
0
450
500
550
600
650
700
750
800
Wavelength of Photon [nm]
Green LEDs might be possible.
2.0
460nm
530nm
15
1.5
λsp~70
λsp
70 nm
λsp~140 nm
1.0
SP-dispersion
S
d spe s o
on Ag/GaN
0.5
0.0
0
2
4
6
8
10 12 14
I
In-plane
l
Wavevector
W
t (2π /μm))
2nd order gratings (Λ~280nm)
might
i ht b
be readily
dil fabricated
f b i t d
by Holo litho at Green.
Schematic structure
Photon
Sapphire
n-GaN
c Exciton generation
InGaN MQW
p-GaN
Metal (Ag-based)
e Radiation
e-h
d Surface plasmon excitation
Silicon submount
Λ
D
h
High
g output
p directionality
y
by grating with non-even fill-factor
1st order grating, fill factor=0.1
1st order grating, fill factor=0.5
2nd order grating, fill factor=0.1
2nd order grating, fill factor=0.7
Extraction efficiency of a metal grating
• Data sampling at λ = 530 nm / w = 5 nm
ηint =
η
FDTD
int
ηext
1 + ηext ⋅ γ sp
1 + γ nr ⋅ γ sp
=
1 + ηext ⋅ γ spp
1 + γ sp
FDTD
ηint
(1 + γ sp ) − 1
i t
=
γ sp
1
1
ηext
0
0
180
10
γ nr
ηint
ηext
γ sp
: nonradiative re-comb. rate
60
100
: internal quantum eff
eff.
: extraction efficiency of metal grating
: re-comb. rate to surface plasmon
Max ~ 80% (at 140 nm / 40 nm)
단일 원기둥 구조 silver-grating
계산
Two-dimensional
(2nd order)
1.2
Normaliz
zed LT / In
nternal QE
Λ = 250nm
Grating depth = 50nm
p to QW = 30 nm
Gap
1.0
2.2
20
2.0
1.8
0.9
1.6
08
0.8
14
1.4
0.7
1.2
0.6
1.0
0.5
0.8
0.6
0.4
0.4
0.3
0.2
50
100 150 200 250 300 350 400 450 500
Diameter (nm)
169 nm
Upward em
mitted pow
wer (a.u.)
Normalized LifeTime
Internal Quantum Efficiencyy
Upward Emitted Power
1.1
Upward
d enhanc
cement
Optimum gap distance between metal and QW
2.5
λ = 530 nm
d = 20 nm
2.0
1.5
1.0
0.5
00
0.0
0
coupling to lossy surface wave
5
10
15
20
Distance [nm]
25
30
coupling to surface plasmons
6nm is a theoretical limit given by self-driven dipole (CPS) modeling
[W. L. Barens and P. T. Worthing, Optics Communications 162, 16 (1999)]
Grating on p-GaN
Substrate
mount
Z
• Little damage to p-GaN
• Enlarged surface area for
low contact resistance
Rotation
otat o
stage
θ
X
Y
Linear
stage
Aperture
Mirro
r
L-Shape
mount
EL Measurement
0.0045
Higher output power
up to
t 70 %
0.004
0.0035
ref
250A _ 3
250B_ 2
250C _ 2
270A _ 4
270B_ 2
270C _ 3
290A _ 3
290B_ 2
Power(a
arb.)
0.003
0.0025
0.002
0.0015
0.001
0.0005
0
0
0.1
0.2
Cu r r e n t (A )
0.3
0.4
Sample images
An Optimistic Estimation for SP-enhanced LEDs
FDTD calculation
l l ti
10 nm
MQW
5 nm
20 nm
grating de
g
epth
At green (530 nm)
with a 1st order grating
2.3 times
more
Photons
generated
60 nm
100 nm
grating
ti period
i d
180 nm
1.0
Ph
hotons escaped
Good directionality
Surface plasmon
140 nm
0.8
82 %
0.6
34.1% within 20o
after escape
0.4
02
0.2
0.0
1/(2n2) = 8 %
400
500
600
700
800
Wavelength (nm)
(Bare-chip LED with 8 % extraction) Î (82 % / 8 %) x 2.3 ~ 24 times Brighter
( Optimized LED with 50 % extraction) Î (82 % / 50 %) x 2.3 ~ 4 times Brighter
Nanocavity lasers
Nanocavity lasers
Final comments
Key
notes
1. How does the surface plamon resonance enhance the internal quantum efficiency of light source?
2. Understand the Fermi-Golden rule and Purcell enhancement factor in spontaneous emission
3. What are the practical difficulties in realizing SP-enhanced
SP enhanced LEDs?
4. Summary of the five lectures
External Efficiencies
Conventional LED η = E p
η'=
SP LED
Rp
Rnr + R p
E p R p + ESP RSP
Rnr + R p + RSP
An Optimistic Estimation for SP-enhanced LEDs
At green (530 nm)
with a 1st order grating
MQW
5 nm
grating d
depth
10 nm
60 nm
100 nm
FDTD calculation
20 nm
2.3
2
3 ti
times
more
Photons
generation
ti
140 nm
grating period
180 nm
Final comments
Summary of the five lectures
(06/23)
(06/30)
(07/07)
(07/14)
(07/21)
Introduction: Micro- and nano-optics based on diffraction effect for next generation technologies
Guided-mode resonance (GMR) effect for filtering devices in LCD display panels
Surface-plasmons: A basic
Surface plasmon waveguides for biosensor applications
Surface-plasmon
Efficient light emission from LED, OLED, and nanolasers by surface-plasmon resonance
R0
T0
GMR grating
Micros
metal strip
Dcor
e
cladding
SPP mode
core
metal slab
cladding
Final comments
Summary of the five lectures
Now, let’s get back to Macros with Nanos and Micros.