Transcript Atomic Layer Deposition for Photonic Crystal Devices

PHOTONIC CRYSTALS
@ GEORGIA TECH
E. Graugnard, J. S. King, Curtis Neff, Davy Gaillot,
Tsuyoshi Yamashita, D. Heineman, and C. J. Summers
School of Materials Science and Engineering,
Georgia Institute of Technology, Atlanta, GA, USA
Photonic Crystals
1D
z
2D
3D
y
x
Periodic in
one direction
Periodic in
two directions
Periodic in
three directions
(Joannopoulos)
• Photonic Crystal – periodic modulation of dielectric constant
• Exhibits a “Photonic Band Gap” (PBG) where propagation of a range of
photon energies is forbidden.
• For visible wavelengths, periodicity on order of 150 – 500 nm.
• Introduction of “dielectric defects” yield modes within the PBG.
• Luminescent 2D & 3D PC structures offer the potential for controlling
wavelength, efficiency, time response and threshold properties (phosphors,
displays, solid state lighting, etc.).
Photonic Crystal Properties
•
Density of states of radiation field in free space & photonic crystal
(Sakoda)
• Photonic band gap and associated defect mode are used to create
waveguides, microcavities, resonators, couplers and filters.
• Luminescent 2D & 3D PC microcavity structures offer the potential for
controlling the wavelength, efficiency, time response and threshold
properties by embedding a defect in a photonic crystal structure. (LEDs,
Lasers, Phosphors)
Photonic Crystals:
Dimensionality Defined
• 1-, 2-, & 3-D photonic
crystals are all 3-D
structures
• Dimensions refer to number
of dimensions in which the
photonic bandgap exists
• Dielectric constant
modulated in 1, 2, or 3
directions.
• Modulation of dielectric
constant on the order of the
wavelength of illumination
source.
Bragg
stack
Square
lattice of
rods
Inverse
opal
1D
2D
3D
Real Photonic Crystals:
Applications for thin films
1-D
2-D
3-D
The Bragg Stack:
‘1D Photonic Crystal’ Treatment
a
• Treat structure with
periodicity in order to
cast into reciprocal space.
• a = lattice constant
• b = reciprocal lattice
constant
 ( x)   ( x  a )
• Also, plane waves can be
represented by k vector in
l
reciprocal space
b
0
2/a
k
2
l
4/a
Result of the Bragg Stack
• Dispersion lines: plot of the
frequency vs. k vector
considering the given
structure.
• Similar result to 1-D multiple
quantum well problem in
solid state physics
• The ‘Photonic Band Gap’ is a
range of frequencies where a
solution does not exist.
wa
wn 
2c
w nk
Photonic
Band Gap
2

a

a
0

a
2
a
k
Results Compared:
Photonic Crystal vs. Traditional Optics
• Reflected waves
interfere constructively
• Band gap corresponds
to high reflectivity
• Thickness of each
layer:
llayer
4
or
l0
n
llayer = wavelength in medium
l0 = free space wavelength
n = refractive index of layer
wn

a
0

a
2D Photonic Crystals
Real Space
Reciprocal Space
M
wn
X
Band Diagram
G
a
Square Lattice
G
M
K
X
M
G
K M
G
0.7
0.6
r
G
a
wn
0.5
0.4
0.3
0.2
0.1
Triangular Lattice
0
G
2D Photonic Crystals:
Methods of Visualizing Properties
M
G
• Band Surface
–
–
Plot of eigen-solutions
in the irreducible
Brillouin zone
Complete information
but difficult to analyze
• Band Diagram
– Plot of the boundaries
of the band surface
– Useful for identifying
band gaps and general
band shifts
C
• Dispersion Curve
– Iso-frequency contours
of the band surface
– Useful for identifying
refraction and
propagation effects
Dispersion Curve Analysis:
Refraction Effects
• The dispersion curve can be used to predict
the refraction effects of a photonic crystal.
k0n1
k0n1
n1
O
n1
Interface
n2(q)
O
k0n2(q)
k0n2(q)
Conventional Materials
Photonic Crystals
n2(q)
Principle of Self-Collimated
Beams
• Conservation of the
Isotropic Media
transverse component
of the wave vector
• Group velocity is
normal to the
4/w0
dispersion curve
• Possible to achieve
nearly parallel beam
propagation
Photonic Crystal
Nearly Parallel
Beam
2/a
2D Virtual Waveguides
Beam spreading in an isotropic material
Sharp Turns
Beam spreading in a photonic crystal virtual
waveguide
No cross talk
Virtual Waveguide System
Simulation using FDTD
• Photonic band gap
perfect mirrors
• Signals can cross with
no interference
• Small deviations in
beam width and
wavelength can be
accommodated
1555nm
9.97mm
1550nm
8.55mm
1545nm
7.12mm
2D Superlattice
2D Superlattice
• Based on triangular
lattice but with two
different hole sizes.
0.5
wn
0.4
0.3
0.2
0.1
M
X
G
X’
0
G
M
X
G
X’
M
2D Superlattice:
Dispersion Curve
30
20
Refracted Angle
• Large refraction
characteristics with
small change in
incident beam angle
• Effect does not
require a band gap
• Effect can be ‘tunable’
by using electro-optic
materals
10
0
-18
-13
-8
-10
-20
-30
-40
-50
Isofrequency
contour
-60
Incident Angle
Application: Beam steering/rastering in optical
communications or displays
-3
3D Photonic Crystals:
Opals & Inverse Opals
• For 3D PC’s: “top-down” approaches are difficult.
– “Bottom-up” approach: self-assembly
• Most common 3D photonic crystal is the opal.
– Close-packed silica spheres in air
• Opal is used as a template to create an inverse opal.
– Close-packed air spheres in a dielectric material
ALD
3D-PC
Opal
26% air
Inverse Opal
74% air for high
dielectric contrast
SiO2 Opal Films
• Opal films are polycrystalline, 10 mm thick, FCC films
with the (111) planes oriented parallel to the surface.
• For visible spectrum, lattice constant ~ 140 – 500 nm.
1 µm
300 nm
Challenge: growth of uniform films within a dense,
highly porous, high surface-area, FCC matrix
Inverse Opal:
Fabrication
• Self-assembled silica opal template
– 10 μm thick FCC polycrystalline film, (111) oriented.
• Infiltration of opal with high index materials
– ZnS:Mn n~2.5 @ 425 nm (directional PBG)
– TiO2 (rutile) navg~ 3.08 @ 425 nm (omni-directional PBG)
Self
Assembly
ALD
Sintered Opal
Etch
Infiltrated Opal
Inverted Opal
ALD of TiO2 at 100ºC
(111)
Cross-sections
300 nm
433 nm opal infiltrated
with 20 nm of TiO2
433 nm opal infiltrated
with TiO2
433 nm TiO2 inverse opal
• TiO2 infiltration at 100ºC produces very smooth and
conformal surface coatings with rms roughness ~2Å.
• Heat treatment (400C, 2 hrs.) of infiltrated opal converts it to
anatase TiO2, increasing the refractive index from 2.35 to 2.65,
with only a 2Å increase in the rms surface roughness.
Optimized TiO2 Infiltration
• Pulse and purge times were increased to optimize
infiltration in opals with small sphere sizes.
2 µm
433 nm TiO2 inverse opal
Specular Reflectivity
• Measurements: 15° from normal
• Probes changes in G-L photonic band structure (111)
TiO2
ZnS:Mn
Flat band peaks
G-L PPBGs
G-L PPBGs
Flat band peaks
(a) (c)
Normalized Reflectivity
Normalized Reflectivity
1.0
(b)
(a) (c)
200
300
400
500
600
Wavelength (nm)
200 nm opal
700
800
(b)
0.8
0.6
0.4
0.2
0.0
400
600
800
Wavelength (nm)
330 nm opal
(a) sintered, (b) as-infiltrated, and (c) inverse opals
1000
Photonic Crystal Properties
•
•
Photonic band diagrams: ω vs. k (reciprocal space representation)
Calculated from wave form of Maxwell’s equations.
• Plane wave expansion (PWE)
• Finite-difference time domain (FDTD)
Photonic Bands of inverted 3D Si-air PC
U
1.0
# ixmax= 20
# iymax= 20
# izmax= 20
# dx=1
# ikmax= 16
# nmax =32768
# n_pts_store=16
# radius=0.5
# eps_b=11.9
# eps_s=1
# sig_b=0
# sig_s=0
FPBG ratio=3.5%
Normalized Frequency
0.9
PBG
0.8
0.7
0.6
0.5
• Photonic band gaps (PBG)
• Pseudo-photonic band gaps (PPBG)
• Flat bands, low group velocity
• Superprism and giant refraction
PPBG
mse6059
Source-->Exec.
metalpc.f90 --> metalpc
This file:
\My documents\GT project\PcP
project\FDTD\defect\band
diagram\band diagram air_Si.xls
0.4
0.3
0.2
0.1
0.0
X
U
L
W
k-vector
FCC Brillouin zone
K
Inverse Opal Reflectivity:
Theoretical Comparison
Normalized Reflectivity
300
• Agreement: full index attained!
Flat band peaks
400
Wavelength (nm)
• TiO2 infiltration of 330 nm opal.
• ~88% filling fraction
• 2.65 Refractive Index
Band Diagram
500
600
700
2-3 PPBG
800
Fabry-Perot
fringes
900
1000
1100
G
L
Sintered Opal
Band Diagram
300
500
400
600
Wavelength (nm)
Wavelength (nm)
Normalized Reflectivity
400
Flat band
peaks
700
800
2-3 PPBG
900
1000
1100
Normalized Intensity
Band Diagram
10-11
8-9
5-6
PPBG's
500
600
700
2-3 PPBG
800
900
L
Infiltrated Opal
G
1000
L
Inverse Opal
G
Opal Defect Engineering
Silica Opal with Defect
Inverted Opal Structure
Infiltrated Opal
(With Defect – soon!)
Inverse Opal:
Defect Mode Calculations for PcP
• What is the main idea behind Photonic Crystal Phosphor ?
– Combining a 3D inverse opal with nanophosphors as a local defect in the Pc
lattice
10
20
30
40
50
60
70
10
20
30
40
50
60
70
Si-air Pc slice
•
•
Luminescent nanocrystal
Specific frequencies in the Photonic Band-Gap of the inverse structure are
inhibited except for the defect modes
A broad luminescent material spectrum within this band-gap would be filtered by
the resonant frequency and therefore tuned up
Photonic Band-Gap Analysis
Defect
mode
0.7
0.65
Frequency
0.6
0.55
0.5
0.45
0.4
0.35
0.3
0
0.5
1
1.5
2
Spectrum analysis for
PcP
Regular spectrum of a green
phosphor
Regular spectrum of a defect
mode
Both spectrum combine and
Emission Energy of phosphor is totally
quenched into the defect mode
Main Characteristics of PcP:
Field of applications
• The cavity mode spectrum lies into the phosphor emission spectrum
– A matching nanophosphor would spontaneously emit in by the confined
defect mode in the ultra-high Q-factor cavity
• The nature of the resonant spectrum acts as an optical amplifier and filter
and allows Static Tunability of luminescent properties.
– The position and peak cavity spectrum controls the color, luminous
intensity and decay time of structure
• Intrinsic properties are therefore controlled by the geometry of the host
• Ultimate tunability would be achieved by optically or electrically biasing
materials such as respectively Liquid-crystal or PLZT (instead of air)
– Changing dynamically the refractive index of host materials would
affect both position and peak of cavity mode
• The amplified mode leaks upon near-UV pumping and then propagates out
PcPs are perfect candidates for High-Definition Display devices !!!!
Three-Layer Inverse Opal:
PcP
• SEM of TiO2/ZnS:Mn/TiO2
inverse opal
330 nm sphere size
Luminescent multi-layered inverse opals
fabricated using ALD: PcP
Photoluminescence:
ZnS:Mn/TiO2 Composite PcP
(a) 2-layer TiO2/ZnS:Mn/air
(14 nm/20 nm) inverse opal
(b-f) 3-layer TiO2/ZnS:Mn/TiO2
inverse opal after backfilling
with TiO2 by
(b) 1 nm
(c) 2 nm
(d) 3 nm
(e) 4 nm
(f) 5 nm
Relative Intensity
• 433 nm opal
• 337 nm N2 laser excitation
• Detection normal to surface
Cl
-
Mn
2+
108%
(a)
(b)
(c)
(d)
400
(e)
(f)
500
600
700
Wavelength (nm)
800
Using ALD of TiO2 to create
novel 2D Photonic Crystals.
X. D. Wang, E. Graugnard, J. S. King,
C. J. Summers, and Z. L. Wang
TiO2 Coated ZnO Arrays
Aligned ZnO nano-rods in a
hexagonal matrix on a
sapphire substrate.
Aligned ZnO nano-rods coated
with 100 nm of TiO2 at 100°C.
Summary
• Precise control of thin film growth enables novel photonic
crystal structures:
– Inverse opals with void space air pockets (enhanced PBG)
– Achieved maximum infiltration of 86%
– Perfect match between reflectivity and calculated band
structure
– Multi-layered luminescent inverse opals: PcP
• Modification of photoluminescence by precise infiltration
– Increased Mn2+ peak intensity by 108%
• Pathway for photonic crystal band gap engineering.
• Novel 2D PCs created with ALD
– TiO2/ZnO aligned nano-rod arrays
Acknowledgments
• US Army Research Lab: S. Blomquist, E.
Forsythe, D. Morton
• Dr. Won Park, U. Colorado
• Dr. Mike Ciftan, US Army Research Office:
MURI “Intelligent Luminescence for
Communication, Display and
Identification”