Thermal Tuning
Wanted and unwanted…
Origin of Temperature Dependence
Consider silicon first…
• 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
Temperature Dependence of
Various 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-factor
Transmission (%)
100
80
60
Q = 1250
Dn = 0.01
dB = 10.6
40
Si: 50ºC
InP: 100ºC
GaAs: 40ºC
(in near IR)
20
0
1530 1535 1540 1545 1550 1555
Wavelength (nm)
Thermo-Optic Effect depends on Q-factor
Transmission (%)
100
80
60
Q = 2750
Dn = 0.01
dB = 10.6
17.3
40
20
0
1530 1535 1540 1545 1550 1555
Wavelength (nm)
Si: 50ºC
InP: 100ºC
GaAs: 40ºC
Thermo-Optic Effect depends on Q-factor
The higher the Q-factor, the more sensitive the
PBG device is to temperature variations
dB attenuation
60
50
40
Dn = 0.1
Dn = 0.01
Dn = 0.001
Silicon
500ºC
50ºC
5ºC
30
20
10
0
100
1000
Q factor
10000
Let’s look at
some examples
Si Photonic Crystal Waveguide Microcavity
H.M.H. Chong and R.M. De La Rue,
IEEE Photonics Technol. Lett. 16, 1528 (2004).
• Fabricated in SOI with
340nm Si core and
3000nm silica cladding
• Design l = 1.53mm (hole
diameter = 250nm)
• Coupling to microcavity
improved by sizegraded holes along
input and output
channel WGs
Si Photonic Crystal Waveguide Microcavity
• 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
Si Photonic Crystal Waveguide Microcavity
5nm shift, DT=160ºC
Q~500
~7 dB attenuation
9.2 mW
Switching time
expected to be
submillisecond
Heat dissipation in silica core seems to be a problem
GaAs and InP Photonic Crystal Microcavity
B. Wild et al., Appl. Phys. Lett. 84, 846 (2004)
• AlGaAs/GaAs
heterostructure grown by
MBE with three layers of
InAs quantum dots in core
as internal light source
– Hole spacing = 220nm
GaAs
• InP/InGaAsP heterostructure
grown by MOVPE with two
GaInAsP quantum wells in
core layer as light source
– Hole spacing = 440nm
InP
GaAs and InP Photonic Crystal Microcavity
• 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 DT=56°C
• Measured dl/dT=8 x 10-2 nm/°C
• Calculated dl/dT=9 x 10-2 nm/°C
(based on dneff/dT=3.5 x 10-4 /°C)
GaAs and InP Photonic Crystal Microcavity
• InP photonic crystal
Fabry-Perot mode at
1564nm with Q~310
showed 5nm red shift
for DT=56°C
• Measured dl/dT=9 x
10-2 nm/°C
• Calculated dl/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 Laser
P.T. Lee, et al., Appl. Phys. Lett. 81, 3311 (2002).
• 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
Photonic Crystal Laser
• Mounted on copper
and fixed onto Peltier
thermal electric
cooler with heat sink
• Thermistor monitors
temperature
• Emission at 1.55mm,
FWHM =200nm
1.5 nm
 Q ~ 10!!!
Emission wavelength shifts ~ 0.5Å/K
Photonic Crystal Laser
• 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
Detector
Transmission
medium
1
Transmission
Laser
0.8
0.6
0.4
0.2
0
1.5
1.55
Wavelength (mm)
1.6
Laser for WDM
Detector
Transmission
medium
1
Transmission
Laser
0.8
0.6
0.4
0.2
0
1.5
1.55
Wavelength (mm)
1.6
Laser for WDM
Detector
Transmission
medium
1
Transmission
Laser
0.8
0.6
0.4
0.2
0
1.5
1.55
Wavelength (mm)
1.6
How Significant is the Thermal Drift?
The higher the Q-factor, the more sensitive the
PBG device is to temperature variations
dB attenuation
60
50
40
Applications:
Dn = 0.1
Dn = 0.01
Dn = 0.001
Q = 6000
PC laser sensor
Channel drop filter
30
Q = 45,000
Nanocavity in 2-D
PC slab
20
10
0
100
1000
Q factor
10000
Porous silicon
1-D PBG microcavities
Achieving Temperature Insensitivity
Reflectance shift due to refractive index change
n
n
Dn 
DT 
DP
T
P
+
+/-
How can a controlled pressure change be introduced?

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
Porous Silicon PBG Microcavity

Investigate temperature dependence of two
different size scale porous silicon microcavities
Mesopores
Macropores
Pore size ~ 20 nm
Silicon walls ~ 5 nm
Pore size ~ 150 nm
Silicon walls ~ 50 nm
Porous Silicon PBG Microcavity

Investigate temperature dependence of two
different size scale porous silicon microcavities
Mesopores
Macropores
Effective Medium Approximation
Air in pores, l = 1500 nm
Bruggeman 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
Effective index
3.5
3
2.5
2
1.5
1
0
20
40
60
Porosity (%)
80
100
Porous Silicon PBG Microcavity

Investigate temperature dependence of two
different size scale porous silicon microcavities
Mesopores
Macropores
LP = 50%, n ~ 2.16
LP = 70%, n ~ 1.60
HP = 80%, n ~ 1.34
HP = 75%, n ~ 1.44
Porous Silicon PBG Microcavity

Investigated temperature dependence of porous
silicon microcavity (1-D PBG with defect)
Mesopores
Macropores
40
Reflectance (%)
Reflectance (%)
100
80
60
40
20
0
1000
1400
1800
Wavelength (nm)
near IR
30
20
10
0
650 700 750 800 850
Wavelength (nm)
visible
How Serious is the Problem?
Resonance redshift (nm)
Mesoporous silicon microcavity temperature dependence
3.0
2.5
experiment
2.0
simulation
~ 3 nm redshift for 100°C
1.5
dn
dT
1.0
0.5
cannot be neglected
silicon
0.0
30 40 50 60 70 80 90 100
Temperature (ºC)
How Serious is the Problem?
Mesoporous silicon microcavity
Reflectance (%)
100
95
90
Q ~ 1700
Dl ~ 2.8 nm
85
> 10 dB
80
25ºC
80ºC
1320
1340
75
70
1300
Wavelength (nm)
1360
How Serious is the Problem?
Resonance shift (meV)
Mesoporous and macroporous silicon microcavity
temperature dependence
1
0
-1
-2
-3
-4
-5
-6
-7
Mesopores (near IR)
dn/dT ~ 2 x 10-4 K-1
Experiment
Simulation
Experiment
30 40 50 60 70 80 90 100
Temperature (ºC)
Macropores (visible)
dn/dT ~ 4 x 10-4 K-1
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 Shift
Resonance permanently shifts to shorter wavelengths
when silicon converted into silicon dioxide
Reflectance (%)
100
Native oxide
90
80
400ºC
70
900ºC
60
Active shift
50
1100
1200
1300
1400
Wavelength (nm)
1500
Experimental Setup for Active Tuning
Al
back
PSi
front

Resistors = Heat Source

Thermistor = Temperature Measurement Tool
Temperature Effect on Reflectance
0.004
900ºC
0.003
400ºC
0.002
300ºC
native oxide
0.001
0.000
-0.001
-0.002
20 30 40 50 60 70 80 90 100
Temperature (ºC)
Note: oxidation time ~ 10 min
Oxide thickness
Reflectance shift (eV)
Mesoporous Silicon Microcavities
Temperature Effect on Reflectance
0.004
0.002
Oxide thickness
Reflectance shift (eV)
Macroporous Silicon Microcavities
0.000
-0.002
1100ºC
-0.004
1000ºC
-0.006
900ºC
native oxide
-0.008
20
40
60
80
Temperature (ºC)
Note: oxidation time ~ 10 min
100
Porous Silicon PBG Microcavity
Mesopores
Macropores
Pore size ~ 20 nm
Silicon walls ~ 5 nm
Pore size ~ 150 nm
Silicon walls ~ 50 nm
Understanding Temperature Insensitivity
Reflectance shift due to refractive index change
n
n
Dn 
DT 
DP
T
P
+
For silicon:
dn/dP = -10-5 MPa-1
+/-
X-ray analysis to determine pressure change
Pressure Increase
Strain Increase
Intensity (a.u.)
X-ray Analysis of Strain
Unoxidized
mesoporous silicon
microcavity
1.0
85°C
0.5
45°C
Increasing temperature
25°C
(P)
0.0
34.78
(S)
34.82
w (degrees)
34.86
Decreasing strain
(decreasing pressure)
Temperature Effect on Strain
Mesoporous silicon microcavity
D(Da/a) (x10-4)
10
8
6
4
2
0
-2
-4
20
Dn 
Native oxide
30
n
n
DT 
DP
T
P
40
50
60
70
Temperature (°C)
80
90
X-ray Analysis of Strain
Slightly oxidized
mesoporous silicon
microcavity
80C
Intensity (a.u.)
0.6
40C
0.3
Increasing temperature
25C
(P)
0.0
34.75
(S)
34.80
w (degrees)
34.85
Increasing strain
(increasing pressure)
Temperature Effect on Strain
Mesoporous silicon microcavity
D(Da/a) (x10-4)
10
8
Slightly oxidized
6
4
2
Temperature insensitive
0
-2
-4
20
Dn 
More heavily oxidized
Native oxide
30
n
n
DT 
DP
T
P
40
50
60
70
Temperature (°C)
80
90
Pressure & Temperature Effect on Refractive Index
10
8
6
4
2
0
-2
-4
20
Reflectance shift (meV)
D(Da/a) (x10-4)
Mesoporous silicon microcavity
More heavily oxidized
Slightly oxidized
Temperature insensitive
as-anodized
30
40
50
60
70
Temperature (°C)
80
90
4
900ºC
3
400ºC
2
300ºC
native oxide
1
0
-1
-2
20
40
60
80
100
Temperature (ºC)
Pressure effect compensates temperature effect on refractive index
400°C in O2 leads to temperature insensitive mesoporous silicon PBG
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