Photonic Crystal-Based Optical Devices
Paul V. Braun
pbraun@uiuc.edu
Department of Materials Science and Engineering,
Frederick Seitz Materials Research Laboratory and
Beckman Institute for Advanced Science and Engineering
University of Illinois at Urbana-Champaign, Urbana, IL
June 2004
Funding: NSF, DOE, ARO – MURI, Beckman Foundation, 3M
Photonics Today: Interesting, but Exciting?
It’s hard to get excited about 2-D
Current 2-D Optical Network Devices
“Innovate to manipulate photons in a flexible, compact way.”
Lucent's (canceled) WaveStar™ LambdaRouter
2.5 dimensional?
Close-up of single mirror.
Array of microscopic mirrors, each able to tilt in
various directions, to steer light.
So, how to get to 3-D?
Colloidal self-assembly
Multiphoton polymerization
3-D Applications
• Low-loss waveguides
• Optical cavities
• Zero-threshold microlasers
• Light-emitting diodes
• All-optical transistors
• Improved photoreactors
• Tunable filters
Ref: many, many groups!
Cumpston et al. Nature 1999, 398, 51.
4-beam holography
Lithography
2 μm
Prof. John Joannopoulos
http://ab-initio.mit.edu/photons/index.html
2 µm
S. Y. Lin, et al. Nature 1998, 394, 251.
Turberfield A. J., et al., Nature 2000
Wiltzius, P. et al., Chem. Mater. 2002
Photonic Crystal Primer
Requirements for a Photonic Crystal: 1) Periodicity in the dielectric constant; 2) Domain sizes ~ λ
1D
ε1
2D
3D
ε2
~λ
Figures modified from: http://www.elec.gla.ac.uk/groups/opto/photoniccrystal/Welcome.html
Properties of a Photonic Crystal:
Bragg Diffraction
Photonic Band Gap
Light Modulation
weak
strong
↑ index contrast
→ appropriate geometry
2
2
mλ = 2 d (neff – sin Φ)
1/2
J. Joannopoulos et al. Photonic Crystals, 1995, p. 82
Example PBG Application: Waveguiding
Current Principle:
PBG-Based:
Total Internal Reflection
Frequency Confinement
λ
Io
2D simulation
total internal
reflection
Io
λ
light escapes
at kink
I f < Io
I f = Io
Inherent losses typically > ~ 0.2 db / km
Defects create states in the bandgap
Cannot tolerate bend radii < 5 cm
Forbidden frequencies are confined within defects
Require periodic amplification of signal
Not suitable for small bend radii
100% transmission around bend radii ~ λ!
http://ab-initio.mit.edu/photons/index.html
3-D Self-Assembly: Colloidal Crystals (Opals)
Opal
SEM of opal cross-section
synthetic “opal”
formed from ~500 nm silica spheres
80
J.V. Sanders, Phil. Mag. A. 1980
• Natural Opals consist of periodically arranged
silica spheres in a matrix
• The colors of an opals are due to Bragg
diffraction of light by planes of silica spheres
• Synthetic Opals are formed by careful assembly
of silica spheres from solution
Reflectance / %
60
40
20
0
400
500
600
700
800
Wavelength / nm
900
1000
Colloidal Crystals – Diffraction Yields Color
White light illumination at varying angles
Pierre Wiltzius
Effect of particle diameter
100
Blue: 570 nm diameter colloids
Red: 590 nm diameter colloids
90
Reflectance (%)
80
70
60
50
FWHM ~100nm
Peak Reflectance ~70-75%
40
30
20
10
0
950
1050
1150
1250
1350
Wavelength (nm)
1450
1550
1650
2 µm
Defects in Colloidal Crystals?
Image courtesy of Satoshi Takeda, Pierre Wiltzius
5μ
m
A Better Colloidal Crystal – Nanoparticle Mediated Colloidal Epitaxy
A. van Blaaderen, R. Ruel, P.
Wiltzius, Nature 1997, 385, 321.
Colloidal epitaxy Æ low defect density & defined orientation with respect to the substrate
NH3
Gravity Driven Nanoparticle Mediated Colloidal Epitaxy
Objective lens
10 µm
Nanoparticle volume fraction ( φnano )
Silica (φ=1.18μm, 0.5vol%)
Zirconia (φ~ 3nm, 0.03vol%)
10
-1
10
-2
10
-3
10
-4
10
-5
10
-6
Phase behavior
microsphere-nanoparticle size ratio 197
Gel
Fluid
Gel
0
0
0.1
0.2
0.3
0.4
0.5
Microsphere volume fraction ( φmicro )
Tohver, V. PNAS 2001, 98, 8950
Tohver, V. Langmuir 2001, 17, 8414
Crystal Engineering through Substrate Engineering
Vacancy concentration (#/microsphere)
Vacancy concentration ~1 per 200 particles
0.07
Patterned substrate
0.06
φmicro = 10-3
φmicro = 2.5x10-3
0.05
0.04
0.03
0.02
0.01
0
0
5
10
15
20
Layer number (#)
25
substrate
2 µm
Langmuir, In press, 2004
1st layer (40x40)
2nd layer (39x39)
3rd layer
10th layer
25th layer
39th layer (2x2)
Vacancy concentration (#/microsphere)
Flat substrate
0.07
0.06
φmicro = 10-3
φmicro = 2.5x10-3
0.05
Patterned Substrate
0.07
0.06
0.06
1.18 µm pitch
1.21 µm pitch
1.26 µm pitch
0.05
0.04
0.04
0.03
0.03
0.03
0.02
0.02
0.02
0.01
0.01
0.01
0
5
10
15
20
Layer number (#)
1.18 µm pitch
20 µm
25
0
0
0
5
10
15
20
Layer number (#)
1.21 µm pitch
φmicro = 10-3
φmicro = 2.5x10-3
0.05
0.04
0
Patterned substrate
0.07
25
0
1.26 µm pitch
5
10
15
20
Layer number (#)
25
But, defined defects in colloidal crystals?
10 µm
Optical cavities & Waveguides?
Two-Photon Polymerization (TPP)
Motivation: Need method for generation of embedded
3D defect features in self-assembled photonic crystals
Absorption
Probability
α Pn
P = Laser intensity
n = Number of photons involved
in the excitation process
Multi-Photon
Excitation Volume
Photopolymerization
of high-resolution
three-dimensional
free-form structures
2 µm
S. Kawata, et al, Nature
2001, 412, 697.
α λ3
System Characteristics
Beam:
Intitiator:
Ti:Sapphire
Pulsed, mode-locked
λ = 780 nm
τ ~ 100 fs
F = 82 MHz
P ~ 20-200 mW
N.A. ~ 1.32
σ ~ 9 x 10-47 cm4 s / photon molecule
λmax ~ 780 nm for two photon excitation
Monomer:
Trimethylolpropane
triacrylate (TMPTA)
AF-240*
AF-350*
AF-270*
*Courtesy of Air Force Research Laboratory (e.g. R. Kannan et al. Chem. Mater. 2001, 13, 1896-1904)
Optics for Multiphoton Polymerization
Galvometer
z
Stage
Spectrum Analyzer
Nd:YAG
Ti:Sapphire
Beam
Expander
Objective
y
x
y
x
Region of Interest
Pulse Compression Sequence
Shutter
EOM
High
Low
Scanning
Mirrors
(defined in software)
Pulse Amplifier
3-D Pattern Formation in Colloidal Crystals – Procedure
Settling of colloids
onto template
Formation of
colloidal crystal
Formation of
colloidal crystal
Image using confocal
Fill with dye for
imaging
Drying and stabilization
of colloidal crystal
1) vacuum
2) heat
invert and
photopolymerize
Rinse away
unpolymerized
material
objective
Addition of monomer
and photoinitiator
Imaging of Templated Multiphoton Written Polymers
Sedimentation
of colloidal
crystal
5 µm
Multi-photon
polymerization
LSCM,
top view
y
z
x
Remove
unpolymerized
materials
SEM
Edge resolution
≤ 100nm
LSCM,
side view
W. Lee, S. A. Pruzinsky, P. V. Braun,
Adv. Mater. 2002, 14, 271.
1 µm
2-photon Polymerization in and out of Colloidal Crystals
xz slice
xy slice
Colloidal
Crystal
4 µm
2 µm
polymer
150 nm
Embedded Waveguide Structure Fabrication
5 µm
Successful fabrication of embedded
waveguide structures in self-assembled
photonic crystals!
Press Reports:
R.F. Service, Science. 2002, 295, 2399.
T.A. Taton, D.J. Norris, Nature. 2002, 416, 685.
W. Roush, Technology Review. 2002, 105, 22.
Selenium – a High Refractive Index Filler
Selenium photonic crystal
Results in high refractive index contrast,
highly oriented photonic band gap materials
True fcc colloidal crystal created by settling on a
patterned substrate. The colloidal crystal nucleates
and grows perpendicular to the 001 face, therefore
no stacking faults form.
5m
m
011 face
single crystal
po
(n lycr
o
te ysta
m
pl lline
at
e)
After selenium
imbibing and removal
of colloidal template
1.6 μm silica colloid settled on a 1.66 μm template. Index
matched with DMF (n ~ 1.43). White light illumination
001 face
P. V. Braun, et al. Adv. Mater.
13, 721-724 (2001)
Dielectric contrast enhancement: Melt Imbibing of Selenium
LSCM image of
polymer feature
Melt
10 μm
selenium
(~250 C)
Air
Pressurize
Etch silica
(HF)
3 μm
Selenium
Polymer
Next Step
Characterization of transmission
through embedded waveguides
Inserted Planar Defects in Colloidal Crystals
Defect thickness
130 nm
230 nm
280 nm
C. López, et al., Adv. Mater. 2004
N. Tétreault, et al.,
Adv. Mater. 2004
Integrated Photonics?
reflectance
FWHM = 1 nm (0.25 nm possible)
FWHM =
50 nm
48-channel echelle grating demultiplexer chip.
C. López, et al., Adv. Mater. 2004
Siegfried Janz, Topics Appl. Phys. 94, 323 (2004)
Metallic Photonic Crystals
Enhance blackbody emission?
S. Y. Lin, Nature 2002
Electrodeposited Ni inverse Opal
Templated by 466 nm PS spheres
Yun-Ju Lee, P. V. Braun unpublished
Electrodeposition of Photonic Crystals
CdS
After semiconductor electrodeposition, the
colloidal particles are removed via solvent
CdSe
Braun and Wiltzius, Nature 1999
Self-Assembled Chemical Sensors: Polymeric Photonic Crystals
Note: See lectures by Prof. Sandy Asher
Pioneered the field
Possible Stimulus
pH
Ionic strength
Solvent
Binding
d
white
light
diffracted
light
Δλ a function of
Δd
Δn
ΔV
white
light
diffracted
light
Because Δλ ∼ Δd, swelling enables sensing
1/ 2
⎛
⎞
λ = 2dneff ≅ 2d ⎜ ∑ ni2Vi − sin 2 φ ⎟
⎝
i
⎠
d = interlayer distance
ni = refractive index of component i
Vi = volume fraction of component i
φ = angle between incident beam and sample normal
Polymerization of Templated Hydrogels
1. Assemble colloidal crystal in flow cell
2. Infiltrate with monomer mixture
3. UV irradiate (356 nm, 50 min)
4. CHCl3 etch (24 hours)
5. Solvent exchange
6. Structural and optical characterization
Inflow from
syringe pump
170μm
Outflow
Flow cell
Hydrogel
10x Objective
Beamsplitter
Photodiode
Array
Detector
Optical fiber
Pinhole
Light source
Y.-J. Lee and P.V. Braun, Adv. Mater. 2003, 15, 563
Y.-J. Lee , S. A. Pruzinsky, P.V. Braun, Langmuir, in press
Glucose Sensing with Mesoscale Photonic Crystals
Can also do pH sensing (Adv. Mater. 2003)
OH
HO
B
HO
O
OH
R
HO
B
HO
OH
R
HO
PBA (phenylboronic acid)
[Glucose]
0 mM
100 mM
OH
O
O
B
R
Glucose-PBA complex
0 mM glucose
Inflow from
syringe pump
Outflow
Flow cell
Hydrogel
10x Objective
Beamsplitter
1 mM glucose
Photodiode
Array
Detector
Optical fiber
Pinhole
Light source
SEM of templated hydrogel
Y.-J. Lee et al. Langmuir, 2004
Kinetics of Glucose Sensing
Increasing [Glu] Diffraction Shift Kinetics
10 Æ 100 mM
teq ~ 250 s
teq ~ 1100 s
teq > 1500 s
1 Æ 10 mM
0.1 Æ 1 mM
10 Æ 100 mM
pump off
pump off
1 Æ 10 mM
0.1 Æ 1 mM
pump off
Dramatic Decrease in Diffraction Efficiency with Swelling
WHY?
Reflection Spectra of 6.25% APBA Hydrogel
DI
pH
2
7
swelling
?
[Glucose] (in buffer)
0 mM
100 mM
Simple Models for Hydrogel Swelling Æ Diffraction Response
Initial State
100
90
constant
volume
80
Reflectance (%)
70
60
50
pore swelling
40
30
20
experiment
10
0
1.00
1.05
1.10
1.15 1.20
λ / λ0
1.25
1.30
1.35
pore
shrinkage
Confocal Imaging of Inverse Opal Hydrogels
1.
Synthesize Acrylated Rhodamine B
Rhodamine B-ITC + 2-Aminoethylmethacrylate·HCl
2.
Polymerize hydrogel in colloidal crystal (PS, d = 3 μm,
t = 25 μm) HEMA + 5% AA + 0.66% EDGM + ~10 μM
acrylated Rhodamine B
3.
Etch colloids
4.
Image with 2-photon confocal microscopy
0.55 µm
z
x
y
Two-photon Imaging – Bottom Layer
fcc (111), bottom layer
pH 3.4
pH 5.7
pH 4.5 (~ pKa)
pH 6.6
pH 5.0
Pore deformation at pH >= 4.5
Partial pore closure at high pH
- Substrate pinning
Two-photon Imaging Results – Layer 2
fcc (111), 2nd layer
pH 3.4
pH 5.7
pH 4.5 (~ pKa)
pH 6.6
pH 5.0
Pore deformation below pH 4.5
Nearly complete collapse at pH 6.6
Finite Element Analysis
Parameters
- ¼ of an inverse fcc unit cell modeled
- Periodic boundary conditions
- Bottom surface does not move vertically
- Thermal strain applied Æ 59% volume change
- E = 106 N/m2, ν = 0.499
fcc (111)
~50% volume change
fcc (110)
So, how will this impact the optical response?
Conclusions and Acknowledgements
10 µm
Colloidal Epitaxy
Binary nanoparticle-colloid suspensions enable the formation of
crack-free, low defect density dry colloidal crystals
Dr. Wonmok Lee, Dr. Michael Bevan, Prof. Jennifer Lewis
Waveguides
Direct writing of 3-D structures in colloidal crystals
through multiphoton polymerization
Stephanie Pruzinsky, Dr. Wonmok Lee
5 µm
0 mM glucose
1 mM glucose
Chemical Sensors
Optically active structures formed from chemically
responsive inverse opal hydrogels
Yun-Ju (Alex) Lee, Stephanie Pruzinsky, Carla Heitzman,
Walter Frey, Prof. Harley Johnson
Funding: NSF, DOE, ARO – MURI, Beckman Foundation, 3M
Paul Braun: pbraun@uiuc.edu; www.mse.uiuc.edu