Thermal Tuning Wanted and unwanted…. Origin of Temperature Dependence Temperature coefficient of the excitonic bandgap –Dominant effect (-4.6 x 10 -4

Thermal TuningThermal Tuning

Wanted and unwanted…

Origin of Temperature DependenceOrigin of Temperature Dependence

• Temperature coefficient of the excitonic bandgap– Dominant effect (-4.6 x 10-4 eV/ºC)– Electron-phonon interactions are main cause of

shrinking energy gap with increasing temperature

• Thermal expansion coefficient– Secondary effect (2.5 x 10-6/ºC)

• Temperature dependence of silicon refractive index depends on wavelength – Near-IR: dn/dT~ 2 x 10-4 K-1

– Visible: dn/dT ~ 4 x 10-4 K-1

Consider silicon first…

Temperature Dependence of Temperature Dependence of Various Materials in Near-IRVarious Materials in Near-IR

• dn/dT silica ~ 1 x 10-5 K-1

• dn/dT InP ~ 1 x 10-4 K-1

• dn/dT silicon ~ 1.83 x 10-4 K-1

• dn/dT GaAs ~ 2.6 x 10-4 K-1

• dn/dT polymer ~ 1 x 10-3 K-1

Thermo-Optic Effect depends on Q-factorThermo-Optic Effect depends on Q-factor

1530 1535 1540 1545 1550 1555

0

20

40

60

80

100

Tra

nsm

issi

on (

%)

Wavelength (nm)

Q = 1250

n = 0.01Si: 50ºCInP: 100ºCGaAs: 40ºCdB = 10.6(in near IR)

1530 1535 1540 1545 1550 1555

0

20

40

60

80

100

Tra

nsm

issi

on (

%)

Wavelength (nm)

Q = 2750

n = 0.01Si: 50ºCInP: 100ºCGaAs: 40ºCdB = 10.6dB = 17.3


The higher the Q-factor, the more sensitive the PBG device is to temperature variations

n = 0.001n = 0.01n = 0.1

100 1000 10000

0

10

20

30

40

50

60

dB a

tten

uatio

n

Q factor

Silicon500ºC50ºC5ºC


Let’s look at some examples

Si Photonic Crystal Waveguide MicrocavitySi Photonic Crystal Waveguide Microcavity

• Fabricated in SOI with 340nm Si core and 3000nm silica cladding

• Design = 1.53m (hole diameter = 250nm)

• Coupling to microcavity improved by size-graded holes along input and output channel WGs

H.M.H. Chong and R.M. De La Rue,

IEEE Photonics Technol. Lett. 16, 1528 (2004).

• Integrated microheater– PECVD silica deposited

on top of microcavity– Nichrome thin film

heater evaporated on top and connected to two probe pads with nichrome bottom layer and gold top layer

– Heater width = 300nm

• TE-pol light end fire coupled into WG using microscope objective



5nm shift, T=160ºC

~7 dB attenuationQ~500

Switching time expected to be

submillisecond

9.2 mW

Heat dissipation in silica core seems to be a problem

• AlGaAs/GaAs heterostructure grown by MBE with three layers of InAs quantum dots in core as internal light source– Hole spacing = 220nm

• InP/InGaAsP heterostructure grown by MOVPE with two GaInAsP quantum wells in core layer as light source– Hole spacing = 440nm

GaAs and InP Photonic Crystal MicrocavityGaAs and InP Photonic Crystal Microcavity

GaAs

InP

B. Wild et al., Appl. Phys. Lett. 84, 846 (2004)

• Samples mounted on Peltier stage using silver paste (range: 20-76°C)

• GaAs photonic crystal PL peak at 1000nm with Q~900 showed 4.5nm red shift for T=56°C

• Measured d/dT=8 x 10-2 nm/°C

• Calculated d/dT=9 x 10-2 nm/°C (based on dneff/dT=3.5 x 10-4 /°C)


• InP photonic crystal Fabry-Perot mode at 1564nm with Q~310 showed 5nm red shift for T=56°C

• Measured d/dT=9 x 10-2 nm/°C

• Calculated d/dT=10 x 10-2 nm/°C (based on dneff/dT=2 x 10-4 /°C)


Due to large thermal conductivity of III-V semiconductors, difficult to exploit temperature tuning

Photonic Crystal LaserPhotonic Crystal Laser

• 19 air holes removed from 2D triangular photonic crystal lattice in InGaAsP

• Undercut (V-shaped groove in SEM) to form membrane

• Optically pumped with 865nm VCSEL

• Multimode fiber collects light and connects to optical spectrum analyzer

P.T. Lee, et al., Appl. Phys. Lett. 81, 3311 (2002).


• Mounted on copper and fixed onto Peltier thermal electric cooler with heat sink

• Thermistor monitors temperature

• Emission at 1.55m, FWHM =200nm Q ~ 10!!!

Emission wavelength shifts ~ 0.5Å/K

1.5 nm


• Interesting to note that threshold pump power increases significantly as temperature increases

• Issue for practical applications

Thermal tuning is not always a desirable effect

Laser for WDM

Laser

Detector

Transmission medium

0

0.2

0.4

0.6

0.8

1

1.5 1.55 1.6

Wavelength (m)

Tra

nsm

issi

on

Laser for WDM

Laser

Detector

Transmission medium

0

0.2

0.4

0.6

0.8

1

1.5 1.55 1.6

Wavelength (m)

Tra

nsm

issi

on

Laser for WDM

Laser

Detector

Transmission medium

0

0.2

0.4

0.6

0.8

1

1.5 1.55 1.6

Wavelength (m)

Tra

nsm

issi

on

The higher the Q-factor, the more sensitive the PBG device is to temperature variations

Q factor

Q = 6000PC laser sensor

Channel drop filter

Q = 45,000Nanocavity in 2-D

PC slab

Applications:

n = 0.001n = 0.01n = 0.1

100 1000 10000

0

10

20

30

40

50

60

dB a

tten

uatio

nHow Significant is the Thermal Drift?

Porous silicon 1-D PBG microcavities

Achieving Temperature Insensitivity

Exploit mismatch of coefficient of thermal expansion between Si and oxide Coat silicon walls with oxide (thermal evaporation) Silicon ~ 2.5 x 10-6 K-1

Oxide ~ 0.5 x 10-6 K-1

PP

nT

T

nn

Reflectance shift due to refractive index change

+ +/-

How can a controlled pressure change be introduced?

Porous Silicon PBG Microcavity

Investigate temperature dependence of two different size scale porous silicon microcavities

Pore size ~ 20 nmSilicon walls ~ 5 nm


Mesopores Macropores




1

1.5

2

2.5

3

3.5

0 20 40 60 80 100

Effective Medium Approximation

Estimates refractive index of porous silicon

M f(porosity, nSi, npore)

For a given wavelength, an increase in porosity results in a decrease in refractive index

Bruggeman approximation

Porosity (%)

Effe

ctiv

e in

de

x

Air in pores, = 1500 nm




HP = 75%, n ~ 1.44

LP = 50%, n ~ 2.16

HP = 80%, n ~ 1.34

LP = 70%, n ~ 1.60


Investigated temperature dependence of porous silicon microcavity (1-D PBG with defect)

1000 1400 18000

20

40

60

80

100

Re

flect

an

ce (

%)

Wavelength (nm)650 700 750 800 8500

10

20

30

40

Re

flect

an

ce (

%)

Wavelength (nm)


visiblenear IR

How Serious is the Problem?

Mesoporous silicon microcavity temperature dependence

dndT

silicon

cannot be neglected

~ 3 nm redshift for 100°C

30 40 50 60 70 80 90 100

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Res

onan

ce r

edsh

ift (

nm)

Temperature (ºC)

experiment

simulation

1300 1320 1340 1360

70

75

80

85

90

95

100

Ref

lect

ance

(%

)

Wavelength (nm)

25ºC

Q ~ 1700

> 10 dB> 10 dB80ºC

~ 2.8 nm

How Serious is the Problem?Mesoporous silicon microcavity

How Serious is the Problem?

Mesoporous and macroporous silicon microcavity temperature dependence

30 40 50 60 70 80 90 100

Res

onan

ce s

hift

(meV

)

Temperature (ºC)

-7-6-5-4-3-2-101

Mesopores (near IR)dn/dT ~ 2 x 10-4 K-1

Macropores (visible)dn/dT ~ 4 x 10-4 K-1

SimulationExperiment

Experiment

Surface Treatment

Outer layers of silicon rods converted to oxide by

annealing (300-1100°C) in O2

Higher temperatures during anneal lead to thicker oxides

“Passive” Oxidation-Induced ShiftResonance permanently shifts to shorter wavelengths

when silicon converted into silicon dioxide

1100 1200 1300 1400 1500

50

60

70

80

90

100

Ref

lect

ance

(%

)

Wavelength (nm)

Native oxide

400ºC

900ºC

Active shift

Experimental Setup for Active Tuning

Resistors = Heat Source

Thermistor = Temperature Measurement Tool

Al PSi

back front

Temperature Effect on ReflectanceO

xide thickness

20 30 40 50 60 70 80 90 100-0.002

-0.001

0.000

0.001

0.002

0.003

0.004 900ºC

400ºC

300ºC native oxide

Re

flect

an

ce s

hift

(e

V)

Temperature (ºC)

Mesoporous Silicon Microcavities

Note: oxidation time ~ 10 min

native oxide

Temperature Effect on ReflectanceO

xide thickness

20 40 60 80 100-0.008

-0.006

-0.004

-0.002

0.000

0.002

0.004

900ºC

Re

flect

an

ce s

hift

(e

V)

Temperature (ºC)

Macroporous Silicon Microcavities

1000ºC

1100ºC

Note: oxidation time ~ 10 min





Understanding Temperature Insensitivity

PP

nT

T

nn

Reflectance shift due to refractive index change

+ +/-

X-ray analysis to determine pressure change

Pressure Increase

Strain Increase

For silicon:dn/dP = -10-5 MPa-1

Unoxidized mesoporous silicon

microcavity

Increasing temperature

Decreasing strain(decreasing pressure)

34.78 34.82 34.86

0.0

0.5

1.0

(S)

85°C

45°C

25°C

Inte

nsity

(a.

u.)

(degrees)

(P)

X-ray Analysis of Strain

Temperature Effect on Strain

20 30 40 50 60 70 80 90-4

-2

0

2

4

6

8

10

a/a

) (x

10-4)

Temperature (°C)

Native oxide

PP

nT

T

nn

Mesoporous silicon microcavity

Inte

nsit

y (a

.u.)

degrees)

25C(P) (S)

34.80

0.0

0.3

0.6

40C

80C

34.75 34.85

X-ray Analysis of Strain

Slightly oxidized mesoporous silicon

microcavity

Increasing temperature

Increasing strain(increasing pressure)

Temperature Effect on Strain

20 30 40 50 60 70 80 90-4

-2

0

2

4

6

8

10

a/a

) (x

10-4)

Temperature (°C)

Native oxide

Temperature insensitive

Slightly oxidized

More heavily oxidized

PP

nT

T

nn


Pressure & Temperature Effect on Refractive Index

Pressure effect compensates temperature effect on refractive index

400°C in O2 leads to temperature insensitive mesoporous silicon PBG

20 30 40 50 60 70 80 90-4

-2

0

2

4

6

8

10

a

/a)

(x10

-4)

Temperature (°C)

as-anodized

Temperature insensitive

Slightly oxidized

More heavily oxidized

20 40 60 80 100-2

-1

0

1

2

3

4

900ºC

400ºC

300ºC native oxide

Ref

lect

ance

shi

ft (

meV

)

Temperature (ºC)


1000°C in O2 leads to temperature insensitive macroporous silicon PBG

Temperature Insensitivity – A General Method

Extension to silicon-based 2-D and 3-D PBG structures Requires longer oxidation times at high

temperatures

Other materials systems Idea of using pressure as compensating

effect still valid Application of method may be slightly more

complicated

Documents

Thermal Tuning Wanted and unwanted…. Origin of Temperature Dependence Temperature coefficient of the excitonic bandgap –Dominant effect (-4.6 x 10 -4