Crystalline Colloidal Array Crystalline Colloidal Array Optical Devices Optical Devices Sanford A. Asher Sanford A. Asher Dept. of Chemistry Dept. of Chemistry University of Pittsburgh University of Pittsburgh Pittsburgh, PA 15260 Pittsburgh, PA 15260 412 412 - - 624 624 - - 8570 8570 [email protected][email protected]
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OutlineOutlineCCA and PCCA Photonic Crystal CCA and PCCA Photonic Crystal FabricationFabricationNanosecond Photonic Crystal SwitchesNanosecond Photonic Crystal SwitchesHydrogelHydrogel Volume Phase Transition Volume Phase Transition Refractive Index SwitchingRefractive Index Switching–– Thermally activatedThermally activated–– Chemically activatedChemically activated–– PhotochemicallyPhotochemically activatedactivated
Sanford A. Asher, Department of Chemistry
Holtz, Asher et al J. Am. Chem. Soc. 1994, 116, 4497
Crystalline Colloidal Arrays Self-Assemblyfabricated from monodisperse, highly charged colloidal particles
~ 1013 spheres/cm3
spacing dependent only upon particle number density and crystalline structure
-
Dialysis /Ion Exchange Resin
Self-assembly
Crystalline Colloidal Array
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Preparing ~ 100 nm Polystyrene Colloids
160 ml Water 60 ml Styrene (monomer) 2.00 g MA-80-1 (surfactant) 2.90 g COPS -1 (ionic co-monomer)2.00 g Divinyl Benzene (crosslinker)0.20 g Sodium Bicarbonate (buffer)0.70 g Ammonium Persulfate (initiator)
* Dynamical Diffractionstrong scatteringmust consider coupledincident and diffracted wave
* Theoretical Foundation Based on Work in1930-1940
W.H. Zachariasen, The Theory of X-ray Diffractionin Crystals, Wiley, 1945.
3-D Photonic Bandgap Crystals-for much larger modulations of refractive index
Dynamical Bragg Diffraction From Crystalline Colloidal Arrays, P. A. Rundquist, P. Photinos, S. Jagannathan, and S. A. Asher, J. Chem. Phys. 91, 4932-4941 (1989).
Ultra Efficient Diffraction
0 200 400 600 800 1000
0
2
4
6
8
10
-Log
T91 nm PS CCA
100 μm = 400 layers
Number of fcc (111) layers
A B
C
FCC: ABCABCABC…
FCC Twin: ABCABCBACBA…
Twin Planes
HCP: ABABABAB…
Stacking of closest-packed layers:
FCC Twin: ABCABCBACBACBABCABC…Lamellar
A B
C
FCC: ABCABCABC…
FCC Twin: ABCABCBACBA…
Twin Planes
HCP: ABABABAB…
Stacking of closest-packed layers:
FCC Twin: ABCABCBACBACBABCABC…Lamellar
FCC Crystal
0
.5
1
1.5
2
2.5
3
400 600 800 1000 1200 1400
L U
X
W
K
L
X
Wavelength (nm)
-log
10T
U
σ polarization
CCA in the middle: ABCABC layers. Reciprical lattice and unit cell.
Sanford A. Asher, Department of Chemistry
(-200)(0-20)
(-2-20)
Incident light
0 5 10 15 200
0.1
0.2
0.3
0.4
0.5
% stacking faults
Diff
ract
ed in
tens
ity ra
tio(200)/(111)(220)/(111)(311)/(111)
Dependence of Diffraction Efficiency on Number of Stacking Faults Along 111
Conclusions
• Diffraction from FCC 111 planes independent of stacking faults
• Diffraction from higher Miller index planes severely attenuated by stacking faults
• Stacking faults increase with particle spacing
• Stacking faults will be the major difficulty for fabricating 3-D photonic bandgapmaterials
High intensity illumination causes the sphere refractive indexto mismatch that of the medium - Bragg diffraction occurs.
no transmission
CCA
100% transmission
CCA
index matched (np = nm)
≠index mismatched (np nm)
Transmission Through CCA as a Function of the Refractive Index Between the
Spheres and the Medium
Nanosecond PhotothermalDynamics in Colloidal Suspension
R. Kesavamoorthy, M. S. Super, and S. A. Asher,
J. Appl. Phys. 71, 1116-1123 (1992).
Nanosecond Photothermal Dynamics in Colloidal SuspensionR. Kesavamoorthy, M. S. Super, and S. A. Asher, J. Appl. Phys. 71, 1116-1123 (1992).
Refractive Index Dependence on Temperature
Photothermal Effect: Δn = (dn/dT) · ΔT
Example: PMMA, dn/dT = -1.1 X 10-4 / 0° C×
Requirements:(1) To index match the particles to the aqueous medium (nH2O = 1.33), we need to use a low refractive index polymer to synthesize the particles.(2) We need to chemically bond a dye to the particles in order to prepare transparent, absorbing CCA.
OCH2CF2CF2CF3
OC
CH2 CHn( )
Wavele ngth / nm
Ext
inct
ion,
-log
(It/I
0)
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
500 550 600 650 700 750 800
medium refractive index, nm
Ext
inct
ion
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.34 1.36 1.38 1.40 1.42 1.44 1.46
np > nm = 1.333
np > nm = 1.437
Transmission Spec tra of PFB MA Pol ymerize d CCA at Di fferent Me dium Re frac ti ve Inde x
A ~ ( n p /n m )2 - 1
Dyed PFBMA CCA Hydrogel
0
0.2
0.4
0.6
0.8
1.0
400 450 500 550 600 650 700 750 800
Wavelength (nm)
Ext
inct
ion
dye absorption band
diffraction band
Covalently attached absorbing dye to the CCA in the hydrogel.
Chemical Structure of NIPAMPoly(N-isopropylacrylamide) (PNIPAM) undergoes a reversible phase transition when heated above 32.1 oC. This coil-globule transition is analogous to a liquid-vapor phase transition. The recipe and synthesis conditions determine the extent of volume changes and whether they are continuous or discontinuous.
ON
ON
Sanford A. Asher, Department of Chemistry
Thermally Switchable Periodicities from Novel Mesocopically Ordered Materials J. M. Weissman, H. B. Sunkara, A. S. Tse , and S. A. Asher
Marta Kamenjicki, Igor K. Lednev, Sanford A. Asher
Department of ChemistryUniversity of Pittsburgh
Pittsburgh, PA 15260
N
N
N
N
Photochemically Controlled Photonic Crystals, M. Kamenjicki, I. Lednev, A. Mikhonin, R. Kesavamoorthy and S.A. Asher, Advanced Functional Materials, 13, 774-780 (2003).