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Erwin Lau 1 Integrated Photonics Laboratory, UC Berkeley

Nano-LEDs for Optical

Interconnects

Erwin K. Lau

E3S Seminar

November 18, 2010


Outline

• Motivation

• Classical lasers: modulation limit

– Gain compression effects

• nanoLED

– Why nanoLEDs over lasers?

– Bandwidth enhancement

– nanoLED design

• Summary


Optical Communication Over the Ages

year

0 A.D. 1800’s 1900’s 2000’s


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.


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


Evolution of Optical Interconnects

-Lee, et al., CLEO, May 2010.

Distance 10’s-100’s km 100 m - 2 km


Computercom Copper Displacement

IBM Supercomputers


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


Global Carbon Footprint of Information

and Communications Technology (ICT)

1.25%

2.7%

Carbon footprint


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.


ITRS Future Interconnect Technologies

– Source: 2009 International Technology

Roadmap for Semiconductors (ITRS)


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.


Optical

Interconnects:

Lasers vs.

LEDs


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


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


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.


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


Examples of Laser Dimensions

Photonic CrystalsMicrodisks

VCSELsEdge-emitters

• There exists a large size mismatch between transistor and photonic devices


Conventional LED: Modulation Speed

Frequency-Domain Response

Conventional LED:

Spontaneous Emission

(1-pole system)

a b c

Time-Domain Response


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


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?


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


Nano Light Emitting

Devices


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


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


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


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.


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


108

109

1010

1011

-10

-8

-6

-4

-2

0

2

4


Fre

qu

en

cy R

esp

on

se [

dB

]

108

109

1010

1011

-10

-8

-6

-4

-2

0

2

4


Fre

qu

en

cy R

esp

on

se [

dB

]

108

109

1010

1011

-10

-8

-6

-4

-2

0

2

4


Fre

qu

en

cy R

esp

on

se [

dB

]

108

109

1010

1011

-10

-8

-6

-4

-2

0

2

4


Fre

qu

en

cy R

esp

on

se [

dB

]

108

109

1010

1011

-10

-8

-6

-4

-2

0

2

4


Fre

qu

en

cy R

esp

on

se [

dB

]

108

109

1010

1011

-10

-8

-6

-4

-2

0

2

4


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


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


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


Qualit

y F

acto

r, Q

0.001 0.01 0.1 1 10 100 1000 1

3

10

30

100

300

1000

3000


Qualit

y F

acto

r, Q

f3dB,opt

Normalized Modal Volume, Veff/(λ/2n)3

• We desire small cavities for fast nanoLEDs.


Scaling of Optimum 3-dB Frequency

0.001 0.01 0.1 1 10 100 1000 1

3

10

30

100

300

1000

3000


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.


Structure Design


Dielectric Confinement of Light

|E|2

x

zy


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.


Plasmonic Dispersion Curve

– M. Staffaroni, “A Plasmonic Transducer for Near-Field Recording,” 2008.


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


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


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


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


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


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


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


.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.


Nano-LED: Design

Oxide

30 nm

30 nm

200 nm

40 nm


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



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


Nano-LED: Efficiency

• Unlike lasers, Γrad can be engineered without

significant impact to performance


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


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


Summary of Advantages for nano-LED


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

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