The recent development of a double gyroid structure with a
length scale sufficient to have photonic properties provides
for novel 3D self assembled photonic crystals with
topologically complex structures. A PS/PI diblock of 750K
microphase separates into two PS networks in a PI matrix. UV
degradation selectively removes the PI component of the
structure providing a high dielectric contrast photonic
crystal. The photonic band structure of the resulting
material is computed and compared to experimental
measurements.
[Y18.002] Metallo-Dielectric Photonic Crystals
Based on Self-Assembled Diblock Copolymers
Michael Bockstaller, Edwin L. Thomas (Dept. Materials Science and Engineering M.I.T.)
Photonic crystals are periodic dielectric structures that
display a stopband in their electromagnetic transmission
characteristic. Metallo-dielectric photonic crystals (MDPC)
offer the opportunity for the development of novel optical
devices that combine the low-loss characteristics of
all-dielectric systems with the high reflectivity of metals
at low frequencies. In our contribution we present the
preparation of MDPC’s by preferentially sequestering of
tailored nanoparticles into self-assembled diblock copolymer
structures. Poly(styrene) coated gold nanoparticles have
been synthesized by phase transfer reduction. We present a
detailed analysis of the sequestering process of the
nanocrystals into the self-assembled
poly(styrene-b-isoprene) structures using TEM, AFM as well
as SAXS. The influence of the confinement of the metal
clusters into the diblock copolymer morphology on the
photonic properties of the sample is determined by combined
reflectance and transmittance measurements and compared to
model calculations using the matrix transfer as well as the
finite element approach.
[Y18.003] Near-Field Optical Imaging of Photonic Block Copolymer Morphology
Michael J. Fasolka, Jeeseong Hwang, Lori S. Goldner (Optical Technology Division - NIST, Gaithersburg, MD 20899), Augustine Urbas, Peter DeRege, Edwin L. Thomas (Dept. of Materials Science and Engineering, MIT)
Due to their microphase separated morphology, block
copolymers (BC) can exhibit photonic behavior if their
domain periodicity is sufficiently large (>100nm) [1].
Classical spectroscopic characterization of these optical
properties is routine, but only offers an ensemble picture
of the material's behavior. Incorporation of these materials
into optical devices, especially in the form of thin films
where 2D in-plane photonic structure is possible, will
require characterization which probes the optical properties
associated with single microphase domains and defect
structures. In anticipation of these needs, we apply
Near-Field Scanning Optical Microscopy (NSOM) to the study
of ultrahigh molecular weight photonic BC thin films. The
sub-diffraction limit resolution of NSOM (\approx50nm)
allows for the direct optical imaging of BC microphase
morphology; providing insight into the local optical
structure of thin film specimens. We present here the first
such optical images of BC microphase separation, collected
via transmission aperture NSOM and NSOM polarimetry. Effects
of the various scanning and collection parameters as well as
input-light polarization on the contrast and detail seen in
these micrographs are discussed. [1] Fink, Y. et al. 1999.
J. Lightwave Technol. 17: 1963.
[Y18.004] Reyleigh-Brilluoin Scattering from Block Copolymer Photonic Crystals
Augustine Urbas, Peter DeRege, Edwin Thomas (MIT), George Fytas (F.O.R.T.H.)
Self assembled block copolymer structures have been shown to
act as photonic band gap materials for visible light. Due to
their physical properties these materials also have
interesting acoustic properties. We explore photon-phonon
interactions in mesostructured concentrated block copolymer
solutions with photonic properties using Reyleigh-Brilluoin
scattering. The combination of the photonic bandgap from the
mesostructure and the physical properties of the component
polymers lead to interesting interactions between light and
medium. Experimental results probing the dual nature of
these materials will be presented and compared to
theoretical calculations.
[Y18.005] Plasma Polymerized Multilayer Optical Interference Thin Films
Hao Jiang (Anteon Co., Dayton), W.E. Johnson (Air Force Research Laboratory/MLP, WPAFB), J.T. Grant (University of Dayton Research Institute, Dayton), K.G. Eyink (Air Force Research Laboratory/MLP, WPAFB), E.M. Johnson (U.Cincinnati, MSE), D. Tomlin (TMCI, Dayton), J Brown, T.J. Bunning (Air Force Research Laboratory/MLP, WPAFB)
The ability to spatially grade the refractive index profile
is the basis for photonic band gap structures. The ability
to control the refractive index profile in a
non-conventional manner, i. e. a sinusoidal profile, expands
the design space typically associated with conventional ABAB
stack designs. These types of optical films, rugates, have
been pioneered using inorganic materials, such as SiO2 and
TiO2. The first step in our development of a methodology to
fabricate rugate-like structures of polymeric materials
using plasma enhanced chemical vapor deposition (PECVD),
namely the formation of ABAB stacks, is presented here.
Polymer films with low and high refractive indices have been
alternatively deposited on different substrates using
precursor monomers in the flowing afterglow of a low
pressure argon plasma. The refractive index (n) of films
made from octafluorocyclobutane is approximately 1.35, while
a value of n=1.65 is achieved using benzene. The PECVD films
possess a highly cross-linked and dense internal structure
with a smooth and pin-hole free surface. Spectroscopy,
ellipsometry, atomic force microscopy and scanning electron
microscopy have been applied to explore relationships among
the processing parameters, the physical structure and the
optical properties of the resulting thin films. We also
discuss the initial attempts to spatially grade the
refractive index profile by simultaneously varying the
deposition rate of the two precursors
[Y18.006] Tunable Lasing in Cholesteric Liquid Crystalline Elastomers
Peter Palffy-Muhoray, Antonio Munoz, Bahman Taheri (Liquid Crystal Institute, Kent State University, Kent, Ohio 44242-0001, USA.), Heino Finkelmann, Sung Tae Kim (Institut für Makromolekulare Chemie, Albert-Ludwigs-Universität, Freiburg, 79104)
Recently synthesized cholesteric liquid single crystal
elastomers show strong opto-mechanical coupling, where
uniaxial compression or biaxial stretch results in a change
of pitch with a concomitant shift in the reflection band.
Cholesteric elastomer samples prepared with a small amount
of dissolved dye show, under ps excitation at 532nm,
fluorescence line narrowing as function of pump input power.
Above the threshold of ~28 mJ, mirrorless lasing is
observed, with peak emission in the range of 544-630 nm. The
emission can be tuned by applying mechanical strain to the
sample. The observed linewidths are the 3.5A range.
[Y18.007] Spectrally narrowed laserlike emission in a novel organic salt, DEST: cooperative emission
Shida Tan, Alpana Mishra, Ayayi Ahyi, Achintya Bhowmik, Aditya Dharmadhikari, Mrinal Thakur (Photonic Materials Research Lab, 202 Ross Hall, Auburn University, Auburn, AL 36849)
We have synthesized a novel organic salt, 4'-diethylamino-N-methyl-4-stilbazolium p-toluenesulfonate (DEST). Frequency-doubled pulses (55 ps) from a Nd:YAG laser at 10 Hz repetition rate were used to pump DEST solution in methanol and a 20% conversion efficiency in laserlike emission was observed without external mirrors. The low energy PL quantum efficiency of DEST is very low. The peak of the emission spectrum was at 617 nm and the threshold pump energy for spectral-narrowing was less than 1 \muJ. Beyond the threshold, the FWHM of the spectrum was found to have reduced from 70 nm to 14 nm The characteristics are similar to that of another organic salt, SPCD^1, which has been recently reported. Cooperative emission appears to play a dominant role in this emission process.
1. A. K. Bhowmik, A. Dharmadhikari, and M. Thakur, OSA
Technical Digest, 467, CLEO (1999).
[Y18.008] Ultrafast Optical Kerr Effect Measurements of Third-Order Nonlinearities in Novel Polydiacetylene-Based Organic Chromophores
Aaron Slepkov, Frank A. Hegmann (Department of Physics, University of Alberta), Rik R. Tykwinski (Department of Chemistry, University of Alberta)
The third-order nonlinear optical behaviour for a new class
of organic chromophores based on the cross-conjugated,
iso-polydiacetylene framework is investigated. An optical
Kerr gate setup utilizing amplified Ti:sapphire femtosecond
pulses is used to time resolve the optical nonlinearities.
The structure-property relationships for these novel
semiconducting materials, as well as comparisons to related
compounds such as polydiacetylene, are discussed.
[Y18.009] On the Prediction of the Nonlinear Absorption in Reverse Saturable Absorbing Materials
Ruth Pachter, Kiet A. Nguyen, Paul N. Day, Joshua C. Kennel (Air Force Research Laboratory, Materials amp; Manufacturing Directorate, Wright-Patterson AFB, OH 45433-7702)
In our continuing efforts to design materials that exhibit
reverse saturable absorption (RSA), we systematically
examine the ability of the time-dependent density functional
theory (TDDFT) method using local, nonlocal, and hybrid
functionals, to predict the experimental nonlinear
absorption for a variety of organic and organometallic
molecular systems, including a number of free-base
porphyrins, phthalocyanine and their metal complexes. The
ground and triplet-triplet excitation energies, as well as
the oscillator strengths are calculated, indicating good
agreement with experiment. We conclude that the TDDFT
approach with a hybrid functional provides good estimates
for the nonlinear absorption of RSA materials.
[Y18.010] Measurement of the specific heat of a copolymer film of vinylidene fluoride and trifluoroethylene at low temperatures
R. W. Newsome Jr., E. Y. Andrei (Dept. of Physics, Rutgers University, NJ)
The specific heat, \rmC_\rmp(T), of a 10
\mu-thick, copolymer film of vinylidene fluoride
(\rmVF_2) and trifluoroethylene (\rmVF_3)
was measured from 4 to 8 K with a heat-pulse calorimeter.
The analysis algorithm (J. S. Hwang, K. J. Lin, and
C. Tien, Rev. Sci. Instr. \bf68) (1), 94 (1997) utilizes
comprehensive least-squares fits to the temperature and
power responses produced by square-wave voltage pulses
applied to a resistive heater, which also serves as a
thermometer. Preliminary results are consistent with a
distinct discontinuity, at approximately 6 K, in the
temperature dependence of the slope of
\rmC_\rmp(T). There is also strong evidence for a
pure cubic temperature dependence of \rmC_\rmp(T)
from \sim 5 to \sim 6 K. A similar temperature
dependence was observed in measurements of the pyroelectric
coefficient of this copolymer. ( R. W. Newsome, Jr.
and E. Y. Andrei, Phys. Rev. B \bf55) (11), 7264 (1997)
Similar behavior has also been reported for the specific
heat of polyethylene, \rmCH_2, in this temperature
regime. (B. Wunderlich, J. Chem. Phys. \bf37)
(6), 1203 (1962)
[Y18.011] INFRARED SPECTROSCOPIC ELLIPSOMETRY STUDY ON PVDF COPOLYMER LANGMUIR-BLODGETT THIN FILMS
Mengjun Bai (University of Nebraska-Lincoln), Jaewu Choi (Center for Advanced Microstructure and Decices, Louisiana, University of Nebraska-Lincoln), Daniel W. Thompson, P. A. Dowben, Stephen Ducharme (University of Nebraska-Lincoln), V. M. Fridkin (Institute of Crystallography, Russian Academy of Sciences,University of Nebraska-Lincoln), S. P. Palto, N. N. Petukora, S. G. Yudin (Institute of Crystallography, Russian Academy of Sciences)
We report the investigation of the ferroelectric films of
vinylidene fluoride (70copolymer made by Langmuir-Blodgett deposition. Infrared
spectroscopic (IR) ellipsometry yields useful spectra as
thin as a few nanometers. A uniaxial model and multi-sample
analysis allowed us to extract the film thickness and
optical constants. The IR spectra, which probe the bulk
modes, permit detailed assignment of vibrational mode
symmetries identifying of conformation changes at 80 ^o C
bulk phase transtion. Electron Energy Lose Spectroscopy
(EELS), which probes the surface vibration modes, identified
the changes at surface phase transition at 20 ^o C. IR
spectroscopic ellipsometry revealed three peaks, at 846
cm^-1, 1186 cm ^-1 and 1294 cm ^-1 specific to
the all-trans ferroelectric phase at 25 ^o C, and two
peaks, at 1238 cm ^-1 and 1376 cm ^-1 specific to
the gauche-trans conformation of the paraelectric phase at
100 ^o C. The EELS spectra revealed two peaks from
all-trans conformation, at 500 cm ^-1 and 850 cm ^-1
that vanish above the surface phase transition at 20 ^o C.
[Y18.012] Planarization of polyphenyls under hydrostatic pressure
Kirill Zhuravlev (Washington State University), Matthew McCluskey (Washington State University and Institute for Shock Physics)
The vibrational properties of molecular crystals under
pressure have been a subject of fundamental interest in
solid-state physics and chemistry. In contrast to earlier
Raman-scattering studies, we have used infrared (IR)
spectroscopy to investigate the vibrational spectra of
solids under pressure. To generate pressures up to 5 GPa, we
used a piston-cylinder diamond-anvil cell. Mid-IR absorption
spectra were obtained with a Bomem DA8 vacuum Fourier
transform IR spectrometer with a KBr beamsplitter. The
spectral range was 500 to 5000 cm-1 and the instrumental
resolution was 1 cm-1. The sensitivity of this technique has
enabled us to observe changes in the symmetry of vibrational
modes due to a conformational change of para-quaterphenyl.
We have obtained direct spectroscopic evidence for the
pressure-induced planarization of para-quaterphenyl
molecules. The transformation from a low to a high symmetry
results in the disappearance of five IR-absorption peaks, in
agreement with a group-theoretical analysis. Recent results
on biphenyl and terphenyl will be discussed.
[Y18.013] Semiempirical and first-principles electronic-structure calculations for phases of nitromethane under pressure
Dionisios Margetis (Division of Engineering and Applied Sciences, Harvard University), Marcus Elstner (Division of Engineering and Applied Sciences, Harvard University, and Theoretische Physik, Universitat-GH, Paderborn, Germany), M. Riad Manaa (Chemistry and Materials Science Directorate, Lawrence Livermore National Laboratory), Efthimios Kaxiras (Department of Physics and Division of Engineering and Applied Sciences, Harvard University), Thomas Frauenheim (Theoretische Physik, Universitat-GH, Paderborn, Germany)
The effect of pressure on the structure and electronic properties of nitromethane, a prototypical molecular energetic solid, is simulated at extremely low temperatures. Distortions of the nitromethane molecules are investigated under uniform and uniaxial pressure via compression and expansion of the perfect crystal with four molecules per unit cell. The case of an imperfect crystal with molecular vacancies is also examined. Computations are carried out by two different methods, namely, a semiempirical self-consistent-charge density-functional-theory based tight-binding (SCC-DFTB) approach [1], and a first-principles adaptive-coordinate real-space electronic-structure (ACRES) method [2]. Specifically, the effect of crystal anisotropy and vacancy density on the optical band gap under pressure is studied by following the behavior of the highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO). The effect that high pressure may have on the various chemical bonds existing in the isolated nitromethane molecule is discussed. [1] M. Elstner, D. Porezag, G. Jungnickel, J. Elsner, M. Haugk, Th. Frauenheim, S. Suhai, and G. Seifert, Phys. Rev. B 58, 7260 (1998). [2] N. A. Modine, Gil Zumbach, and Efthimios Kaxiras, Phys. Rev. B 55, 10289 (1997).