Erwin Lau 1 Integrated Photonics Laboratory, UC Berkeley
Nano-LEDs for Optical
Interconnects
Erwin K. Lau
E3S Seminar
November 18, 2010
Erwin Lau 2 Integrated Photonics Laboratory, UC Berkeley
Outline
• Motivation
• Classical lasers: modulation limit
– Gain compression effects
• nanoLED
– Why nanoLEDs over lasers?
– Bandwidth enhancement
– nanoLED design
• Summary
Erwin Lau 3 Integrated Photonics Laboratory, UC Berkeley
Optical Communication Over the Ages
year
0 A.D. 1800’s 1900’s 2000’s
Erwin Lau 4 Integrated Photonics Laboratory, UC Berkeley
Internet Traffic Forecast
source: www.cisco.com
Approaching the Zettabyte Era: Global Internet Traffic
• There is an immediate and global need for more bandwidth capability.
Erwin Lau 5 Integrated Photonics Laboratory, UC Berkeley
Comparison of Electrical and Optical
Interconnects
• Electrial interconnects (@ 10 GHz) are > 80 dB worse than optical interconnects.
• 2009 Nobel Prize: Charles Kao for development of moder optical fiber
• But optical interconnects are harder to implement and E/O/E process is inefficient.
Loss/length of coaxial cable
Lo
ss (
dB
/100 f
t.)
Frequency (GHz)
Lo
ss (
dB
/km
)
> 10 THz
Wavelength (μm)
Loss/length of optical fiber
Erwin Lau 6 Integrated Photonics Laboratory, UC Berkeley
Evolution of Optical Interconnects
-Lee, et al., CLEO, May 2010.
Distance 10’s-100’s km 100 m - 2 km
Erwin Lau 7 Integrated Photonics Laboratory, UC Berkeley
Evolution of Optical Interconnects
-Lee, et al., CLEO, May 2010.
Distance 10’s-100’s km 100 m - 2 km
Erwin Lau 8 Integrated Photonics Laboratory, UC Berkeley
Computercom Copper Displacement
IBM Supercomputers
Erwin Lau 9 Integrated Photonics Laboratory, UC Berkeley
Evolution of Optical Interconnects
-Lee, et al., CLEO, May 2010.
Distance 10’s-100’s km 100 m - 2 km
Erwin Lau 10 Integrated Photonics Laboratory, UC Berkeley
Microprocessor Bandwidth
and Power Projections
- D. Miller, “Device Requirements for Optical Interconnects to Silicon Chips,”
Proc. IEEE, 97, 2009, pp. 1166-1185.
The End to Moore’s Law?
ITRS Specs
Erwin Lau 11 Integrated Photonics Laboratory, UC Berkeley
Global Carbon Footprint of Information
and Communications Technology (ICT)
1.25%
2.7%
Carbon footprint
Erwin Lau 12 Integrated Photonics Laboratory, UC Berkeley
Inter- and Intra-chip Optical
Interconnects
Size Energy/bit
• There are no electrical solutions to extend Moore’s Law.
• How do we solve this problem?
- D. Miller, “Device Requirements for Optical Interconnects to Silicon Chips,” Proc. IEEE, 97, 2009, pp. 1166-1185.
Erwin Lau 13 Integrated Photonics Laboratory, UC Berkeley
ITRS Future Interconnect Technologies
– Source: 2009 International Technology
Roadmap for Semiconductors (ITRS)
Erwin Lau 14 Integrated Photonics Laboratory, UC Berkeley
Interconnects: Optical vs. Electrical
Advantages
• Higher bandwidth
• Reduced latency
• Low loss
• Minimum crosstalk
• Wavelength division multiplexing (WDM)
Disadvantages
• Cost (wires are cheap)
• Complexity (on-chip integration or III-V carrier chip)
– Further reference: D.A.B. Miller, “Device Requirements for Optical Interconnects to
Silicon Chips,” Proc. IEEE, 97, 2009, pp. 1166-1185.
Erwin Lau 15 Integrated Photonics Laboratory, UC Berkeley
Optical
Interconnects:
Lasers vs.
LEDs
Erwin Lau 16 Integrated Photonics Laboratory, UC Berkeley
Laser: Analogy to Mechanical Systems
j
MsH
R
22
MHzkHzm
kR
m
b
k
GHzR of s'10
Frequency Response
Kinetic E. Potential E. photon electron
Mass-Spring-Dashpot Laser
2-pole damped oscillator
a b c
mirrors
Erwin Lau 17 Integrated Photonics Laboratory, UC Berkeley
What Makes a Laser Fast?
• Definition: Efficient conversion of electrical modulation to
optical modulation over a large bandwidth
• Figure-of-merits:
– High 3-dB bandwidth: f3dB
– High resonance frequency: fR
• Increasing the resonance frequency (fR) can
increase the bandwidth (f3dB).
0 20 40 60 80 100-20
-10
0
10
20
Frequency [GHz]
Resp
on
se [
dB
]
f3dB = 15GHz
fR = 10GHz
0 20 40 60 80 100-20
-10
0
10
20
Frequency [GHz]
Resp
on
se [
dB
]
0 20 40 60 80 100-20
-10
0
10
20
Frequency [GHz]
Resp
on
se [
dB
]
0 20 40 60 80 100-20
-10
0
10
20
Frequency [GHz]
Resp
on
se [
dB
]
0 20 40 60 80 100-20
-10
0
10
20
Frequency [GHz]
Resp
on
se [
dB
]
f3dB = 75GHz
fR = 50GHz
Erwin Lau 18 Integrated Photonics Laboratory, UC Berkeley
1980 1990 2000 20100
20
40
60
80
100
120
Year
Reso
nan
ce F
req
uen
cy [
GH
z]
GaAs
Evolution of Semiconductor Laser
Resonance Frequency
1980 1990 2000 20100
20
40
60
80
100
120
Year
Reso
nan
ce F
req
uen
cy [
GH
z]
GaAs
InP
1980 1990 2000 20100
20
40
60
80
100
120
Year
Reso
nan
ce F
req
uen
cy [
GH
z]
GaAs
InP
VCSEL
GaAs InP
VCSEL
Limit of Direct Mod. Lasers1
2
3
1 Weisser, et al. PTL, 1996.
2 Matsui, et al. PTL, 1997.
3 Anan, et al. OFC, 2008.
Erwin Lau 19 Integrated Photonics Laboratory, UC Berkeley
Frequency Response of Free-Running
Lasers
• Relaxation oscillation frequency
increases with laser power. Bad for
interconnects!
• 3-dB bandwidth is fundamentally limited
by damping, thermal effects, and
nonlinear gain compression, which is a
stimulated emission effect 0 10 20 30 40 500
10
20
30
40
50
60
Resonance Frequency [GHz]
Fre
qu
en
cy [
GH
z]
Damping
Relaxation
Oscillation
Frequency
f3dB
Kf dB
1
2
2max,3
108
109
1010
1011
-20
-10
0
10
20
Frequency
Resp
on
se [
dB
]
108
109
1010
1011
-20
-10
0
10
20
Frequency
Resp
on
se [
dB
]
108
109
1010
1011
-20
-10
0
10
20
Frequency
Resp
on
se [
dB
]
108
109
1010
1011
-20
-10
0
10
20
Frequency
Resp
on
se [
dB
]
108
109
1010
1011
-20
-10
0
10
20
Frequency
Resp
on
se [
dB
]
108
109
1010
1011
-20
-10
0
10
20
Frequency
Resp
on
se [
dB
]
108
109
1010
1011
-20
-10
0
10
20
Frequency
Resp
on
se [
dB
]
108
109
1010
1011
-20
-10
0
10
20
Frequency
Resp
on
se [
dB
]
108
109
1010
1011
-20
-10
0
10
20
Frequency
Resp
on
se [
dB
]
Increasing optical power
ncompressioGain :ε ;εΓ
1τ
factor Damping :γωγ
Freq. Osc. Relaxation :τ
ω
ω
γω
ω
ω1
1ω
02
0
2
2
g
GK
K
gS
j
H
p
R
pR
RR
3 dB
Erwin Lau 20 Integrated Photonics Laboratory, UC Berkeley
Examples of Laser Dimensions
Photonic CrystalsMicrodisks
VCSELsEdge-emitters
• There exists a large size mismatch between transistor and photonic devices
Erwin Lau 21 Integrated Photonics Laboratory, UC Berkeley
Conventional LED: Modulation Speed
Frequency-Domain Response
Conventional LED:
Spontaneous Emission
(1-pole system)
a b c
Time-Domain Response
Erwin Lau 22 Integrated Photonics Laboratory, UC Berkeley
Conventional LED: Efficiency
• External Quantum Efficiency (EQE): how many carriers convert to
“useful” light
• EQE limited by emission into a continuum of modes in all
directions & total internal refraction
EQE ~ 2%le
ns
Erwin Lau 23 Integrated Photonics Laboratory, UC Berkeley
Conventional LED: Efficiency
External Quantum Efficiency (EQE): how many carriers convert to “useful” light
– Schnitzer, Yablonovitch, et al., APL, 63(16), 1993.
EQE ~ 30%
EQE ~ 9%
This is EQE for escaping light. What about coupling to waveguide?
Erwin Lau 24 Integrated Photonics Laboratory, UC Berkeley
1980 1990 2000 20100
20
40
60
80
100
Year
3-d
B B
and
wid
th [
GH
z]
Evolution of Semiconductor Laser
3-dB Bandwidth
LED modulation speed
(< 1-2 GHz)
Limit of Direct Mod. Lasers
GaAsInP
VCSEL
Erwin Lau 25 Integrated Photonics Laboratory, UC Berkeley
Nano Light Emitting
Devices
Erwin Lau 26 Integrated Photonics Laboratory, UC Berkeley
Enhanced Spontaneous Emission
• The spontaneous emission rate is enhanced by the Purcell effect (Phys. Rev., 1946).
• As laser cavity volumes become smaller, the enhanced spontaneous emission rate can increase the modulation bandwidth.
nV
QF
2
6
Purcell factor:
Quality factor
Normalized mode volume
(cubic half-wavelengths)
a b ca b cLED Cavity-Enhanced LEDa b ca b c
mirrors
Erwin Lau 27 Integrated Photonics Laboratory, UC Berkeley
Cavity LED Model
ns 10 sp
Typical LED
0
1
spτ
0p
LED inside cavity
pτ
1
0sp
F
Fsp /0Spontaneous
Emission lifetime
0p Photon lifetime
Erwin Lau 28 Integrated Photonics Laboratory, UC Berkeley
Rate Equations Including Purcell Effect
0
0
sp N
sp p
dN N NJ GS F
dt
dS N SGS F
dt
Carrier rate equation:
Photon rate equation:
Pump
Stimulated emission
Spontaneous emission
damping rateresonance frequency
j
M
dJ
dS
R
22current Pump
densityPhoton J pump current
spontaneous
lifetime0sp
spontaneous
coupling factor
S photon density
G gain
N carrier density
E. K. Lau, A. Lakhani, R. S. Tucker, and M. C. Wu, "Enhanced modulation bandwidth of nanocavity
light emitting devices," Opt. Express, vol. 17, pp. 7790-7799, 2009
Non-radiative
carrier lifetime
photon lifetime
F
F
Purcell Factor
Erwin Lau 29 Integrated Photonics Laboratory, UC Berkeley
Simplified Analysis and Physical Picture
for Nanocavity LED
N
S
Current
Output
Carrier
Photon
p
0sp
spF
• This dynamic process does not
involve stimulated emission.
• Therefore, it is not limited by
gain compression.
Erwin Lau 30 Integrated Photonics Laboratory, UC Berkeley
108
109
1010
1011
-10
-5
0
5
10
Modulation Frequency [Hz]
Fre
qu
en
cy R
esp
on
se [
dB
]
Classical Laser Frequency Response
Maximum bandwidth when response is critically damped
45×
100×
J = 2×J1
3×
10×
-3 dB
1×
Q = 1000
Veff = 3000(λ/2n)3
Maximum
3-dB Frequency
f3dB,max = 37 GHz
@ J = 45×J1
* J1: net stimulated emission = spontaneous emission
Erwin Lau 31 Integrated Photonics Laboratory, UC Berkeley
108
109
1010
1011
-10
-8
-6
-4
-2
0
2
4
Modulation Frequency [Hz]
Fre
qu
en
cy R
esp
on
se [
dB
]
108
109
1010
1011
-10
-8
-6
-4
-2
0
2
4
Modulation Frequency [Hz]
Fre
qu
en
cy R
esp
on
se [
dB
]
108
109
1010
1011
-10
-8
-6
-4
-2
0
2
4
Modulation Frequency [Hz]
Fre
qu
en
cy R
esp
on
se [
dB
]
108
109
1010
1011
-10
-8
-6
-4
-2
0
2
4
Modulation Frequency [Hz]
Fre
qu
en
cy R
esp
on
se [
dB
]
108
109
1010
1011
-10
-8
-6
-4
-2
0
2
4
Modulation Frequency [Hz]
Fre
qu
en
cy R
esp
on
se [
dB
]
108
109
1010
1011
-10
-8
-6
-4
-2
0
2
4
Modulation Frequency [Hz]
Fre
qu
en
cy R
esp
on
se [
dB
]
1×
10×3×
30×
100×
10-3
10-2
10-1
100
101
102
0
100
200
Pump Current, J/J1
3-d
B F
req
uen
cy [
GH
z]
✖
✖
✖✖ ✖ ✖
J = 0.2×J1
Q = 400
Veff= 0.2(λ/2n)3
F = 1,216
* J1: net stimulated emission
= spontaneous emission
nano-LED Frequency Response
Below threshold
Maximum
3-dB Frequency
= 210 GHz
• Maximum bandwidth is below threshold.
-3 dB
Erwin Lau 32 Integrated Photonics Laboratory, UC Berkeley
0.001 0.01 0.1 1 10 100 1000 1
3
10
30
100
300
1000
3000
Normalized Modal Volume, Vn
Qualit
y F
acto
r, Q
LED
ConventionalLasers
20
40
40
Maximum 3-dB Bandwidth
Classical
regime
(Rst > Rsp)
Purcell-enhancedregime
(Rst < Rsp)
✖
f3dB,opt
Modal Volume, Veff/(λ/2n)3Normalized Modal Volume, Veff/(λ/2n)
3
Erwin Lau 33 Integrated Photonics Laboratory, UC Berkeley
0.001 0.01 0.1 1 10 100 1000
1011
1012
Norm. Modal Volume, Vn
Optim
al 3-d
B F
requency [
Hz]
Optimal 3-dB BandwidthO
ptim
al 3
-dB
Bandw
idth
f 3dB
,opt[H
z]
n
optdBV
f1
~,3
-1/2
0.001 0.01 0.1 1 10 100 1000 1
3
10
30
100
300
1000
3000
Normalized Modal Volume, Vn
Qualit
y F
acto
r, Q
0.001 0.01 0.1 1 10 100 1000 1
3
10
30
100
300
1000
3000
Normalized Modal Volume, Vn
Qualit
y F
acto
r, Q
f3dB,opt
Normalized Modal Volume, Veff/(λ/2n)3
• We desire small cavities for fast nanoLEDs.
Erwin Lau 34 Integrated Photonics Laboratory, UC Berkeley
Scaling of Optimum 3-dB Frequency
0.001 0.01 0.1 1 10 100 1000 1
3
10
30
100
300
1000
3000
Normalized Modal Volume, Vn
Qualit
y F
acto
r, Q
a
0.001 0.01 0.1 1 10 100 1000 10
9
1010
1011
1012
1013
Normalized Modal Volume Vn
Optim
um
3-d
B B
andw
idth
, f
3d
B,o
pt [H
z]
0.001 0.01 0.1 1 10 100 1000 10
1
102
103
104
105
Optim
um
Qualit
y F
acto
r, Q
op
t
b
Normalized Modal Volume, Veff/(λ/2n)3 Normalized Modal Volume, Veff/(λ/2n)
3
• We desire small Q (~10’s) for fast devices.
Erwin Lau 35 Integrated Photonics Laboratory, UC Berkeley
Structure Design
Erwin Lau 36 Integrated Photonics Laboratory, UC Berkeley
Dielectric Confinement of Light
|E|2
x
zy
Erwin Lau 37 Integrated Photonics Laboratory, UC Berkeley
Dispersion in a Surface Plasmon
z
Semiconductor, εs
Metal, εm
y
β • At optical frequencies, the real part
of the dielectric constant for metal
is negative.
• When εm = -εs, the wave number
increases dramatically.
• In other words, the wavelength
decreases.
Erwin Lau 38 Integrated Photonics Laboratory, UC Berkeley
Plasmonic Dispersion Curve
– M. Staffaroni, “A Plasmonic Transducer for Near-Field Recording,” 2008.
Erwin Lau 39 Integrated Photonics Laboratory, UC Berkeley
Surface plasmon mode volume
GOLD
lsp/2
Diffraction limit
Lakhani 39/41
No
rmal
ized
Mo
de
Vo
lum
e (l
/2n
)3
Wavelength (nm)
Qu
ality Facto
r
n=1
n=3.5
Erwin Lau 40 Integrated Photonics Laboratory, UC Berkeley
Overview of Nanolasers
-Oulton, et al. Nature, 2009.
-Noginov, et al. Nature, 460, 2009, pp. 1110-1112.
-Hill, et al. Opt. Exp., 17, 2009, pp. 11107–11112.
-Hill, et al. Nat. Phot., 1, 2007, pp. 589-594.
Veff > 1.64×(λ/2n)3
Veff > 11.4×(λ/2n)3
Veff > 0.38×(λ/2n)3
Veff ~ 0.0075×(λ/2n)3
Erwin Lau 41 Integrated Photonics Laboratory, UC Berkeley
The Nanopatch Laser
micropatch
cavity to
nanocavity
Gold
Active Gain Region
(InGaAsP)
Gold
Theoretical simulation: Manolatou, C. & Rana,
F. ,IEEE J. Quantum Electron 44, 435–447
(2008).
Lakhani 41/41
Erwin Lau 42 Integrated Photonics Laboratory, UC Berkeley
Cavity Modes for the Nanopatch
Resonator
1st mode
Electric Dipole (TM111) mode
2nd mode
Magnetic Dipole (TE011) mode
Simulated
parameters
Electric
dipole mode
Qtotal 65
Qrad ~1600
Γe 0.84
Veff (λ/2n)3 0.54
Vphys (λ/2n)3 6.5
radius @
λ0=1425nm
203
Gold
InGaAsP
Gold
InP
~ λ
/ 2
radius
Simulated
parameters
Electric
dipole mode
Magnetic
dipole mode
Qtotal 65 80
Qrad ~1600 205
Γe 0.84 0.91
Veff (λ/2n)3 0.54 3
Vphys (λ/2n)3 6.5 11
radius @
λ0=1425nm
203 290
Lakhani 42/41
Erwin Lau 43 Integrated Photonics Laboratory, UC Berkeley
Lasing Characteristics
1300 1350 1400 1450 15000.0
2.0x103
4.0x103
6.0x103
8.0x103
1.0x104
Inte
ns
ity
(A
.U.)
Wavelength (nm)
20.4 mW
4.2 mW
2.6 mW
1.0 mW
0.5 mW
1300 1350 1400 1450 15000.0
2.0x103
4.0x103
6.0x103
8.0x103
1.0x104
Wavelength (nm)
In
ten
sit
y (
A.U
.)
22 mW
4.2 mW
2.6 mW
1.0 mW
1300 1350 1400 1450 150010
2
103
104
105
106
Inte
ns
ity
(A
.U.)
Wavelength (nm)
1300 1350 1400 1450 1500
102
103
104
Wavelength (nm)
In
ten
sit
y (
A.U
.)
Magnetic DipoleElectric Dipole
Optical pumping
(78K) with
1060nm, 100ns
pulse @ 5 kHz
Wavelength (nm)Wavelength (nm)
Inte
nsi
ty (
A.U
.)
Inte
nsi
ty (
A.U
.)
Lakhani 43/41
Erwin Lau 44 Integrated Photonics Laboratory, UC Berkeley
Lasing Characteristics
Fβ=10
0.1
1.1
Fβ=10
0.1
1.2
Pthresh~60kW/cm2 Pthresh~90 kW/cm
2
Peak Pump Power (mW)
Inte
nsi
ty (
A.U
.)
Magnetic Dipole
F=11.4
β=0.105
Peak Pump Power (mW)
Electric Dipole
F=49.5
β=0.022
F=Purcell Enhancement
β=Spontaneous emission
coupling factor
Lakhani 44/41
Erwin Lau 45 Integrated Photonics Laboratory, UC Berkeley
Nanopatch: Process Flow
Legend
InP SubstrateEpilayer
GoldSapphireBonding LayerE-beam Resist
InGaAsP epilayer growth Dielectric deposition (5nm)
Flipchip bond to sapphire Substrate removal; dielectric deposition Litho and Metal Evap
RIE Etch/Wet EtchLiftoff
Metal Evaporation
Dielectric
Lakhani 45/41
Erwin Lau 46 Integrated Photonics Laboratory, UC Berkeley
.001 .01 .1 110
9
1010
1011
1012
Vn/(l/2n)
3
3-d
B B
and
wid
th [
Hz]
Cavity Volume vs. Bandwidth, Qcav = 10
• We desire a sub-wavelength cavity for both speed and footprint.
Erwin Lau 47 Integrated Photonics Laboratory, UC Berkeley
Nano-LED: Design
Oxide
30 nm
30 nm
200 nm
40 nm
Erwin Lau 48 Integrated Photonics Laboratory, UC Berkeley
Nano-LED: Electric Energy Density
Confinement Factor: 17% Mode Volume: 0.03 (λ/2n)3
Qloss: 12 , Qrad: 33, Qtotal : ~10
Gold Antenna
InGaAsP (1.55um)
Gold Ground Plane
Erwin Lau 49 Integrated Photonics Laboratory, UC Berkeley
Normalized Modal Volume, Vn
Qualit
y F
acto
r, Q
0.001 0.01 0.1 1 10 100 1000 1
3
10
30
100
300
1000
3000
StE
SpE
20
20030
40
40100
120160
2500
Modulation Bandwidth Versus Cavity-Q and
Modal Volume [in GHz]
Gold Antenna
InGaAsP (1.55um)
Gold Ground Plane
Vn = 0.03 (λ/2n)3
Q = 10
f3dB = 40 GHz
Erwin Lau 50 Integrated Photonics Laboratory, UC Berkeley
Nano-LED: Efficiency
• Unlike lasers, Γrad can be engineered without
significant impact to performance
Erwin Lau 51 Integrated Photonics Laboratory, UC Berkeley
LegendInP SubstrateEpilayerDielectric
GoldCMOS
E-beam Resist
P-side metal contact Flipchip bond to CMOS Remove InP substrate
Lithography and Etch Dielectric Deposition Planarization and Selective RIE
Gold Deposition and Etch
Nano-LED: Fabrication on CMOS
Erwin Lau 52 Integrated Photonics Laboratory, UC Berkeley
Nano-LEDs as Optical Interconnects
Figure-of-Merit Hybrid Si
Evanescent Laser
nanoLED Improvement
Factor
Modulation
Speed
External Mod.,
~ 10 Gbps?
Direct Mod.,
> 100 Gbps
×~10
Size ~ 2000 μm2 ~ 0.008 μm2 ×100,000
Electrical Power
Consumption
50 mW @ threshold 80 μW @ 100 Gbps ×1,000
Current Usage 25 mA @ threshold 100 μA @ 100 Gbps > ×100
Quantum
Efficiency
~ 10% ~ 10-50% Comparable*
* nanoLED QE can be engineered
Erwin Lau 53 Integrated Photonics Laboratory, UC Berkeley
Summary of Advantages for nano-LED
Erwin Lau 54 Integrated Photonics Laboratory, UC Berkeley
Acknowledgement
• UC Berkeley
– Amit Lakhani
– Michael Eggleston
– Kyoungsik Yu
– Prof. Ming C. Wu
– Integrated Photonics Lab
• University of Melbourne
– Prof. Rodney S. Tucker
• Funding:
– DARPA
– Australian Research Council