We have studied the effects of planar, lamellar, and random nanostructured metal-dielectric environments on spontaneous emission and energy transfer concentration quenching of HITC laser dye. We found an inhibition of the concentration quenching in vicinity of metal, which was stronger in nanostructured substrates than in plain geometries. It was shown that the same substrates, which boosted spontaneous emission, also inhibited the concentration quenching. The effect is discussed in terms of the Förster radius affected by losses.
Work at LLNL was performed under the auspices of the U.S. Department of Energy by LLNL under Contract DE-AC52-07NA27344
We report on the optical properties of nanoporous gold leaf (NPGL) metamaterials consisting of single and multiple-fold layers, and having different pore and ligament sizes. Due to their complex structure, such metamaterials have a unique optical property, which distinguishes them from homogeneous gold films. Thus, the transmission spectra of NPGLs feature two characteristic peaks positioned at ~ 490 nm and ~560 nm to 605 nm. The most notable result of this study is that the optical properties of NPGLs can be tuned by changing the dielectric environment and by applying voltage in an electrochemical cell.
We have studied the effects of metallic substrates, lamellar metal-dielectric stacks and Fabry-Perot cavities on spontaneous emission and concentration quenching of luminescence of the HITC laser dye. Among the most intriguing results of this research are: (1) The long-range (~50 nm) inhibition of the concentration quenching (Förster energy transfer to quenching centers) by metallic and lamellar metal-dielectric substrates and (2) Enhancement of the spontaneous emission quantum yield with reduction of the cavity size below 100 nm.
Conjugated oligomers and polymers are very attractive for potential future plastic electronic and opto-electronic device applications such as plastic photo detectors and solar cells, thermoelectric devices, field effect transistors, and light emitting diodes. Understanding and optimizing charge transport between an active polymer layer and conductive substrate is critical to the optimization of polymer based electronic and opto-electronic devices. This study focused on the design, synthesis, self-assembly, and electron transfers and transports of a phosphonic acid end-functionalized polyphenylenevinylene (PPV) that was covalently attached and self-assembled onto an Indium Tin Oxide (ITO) substrate. This study demonstrated how atomic force microscopy (AFM) can be an effective characterization technique in conjunction with conventional electron transfer methods, including cyclic voltammetry (CV), towards determining electron transfer rates in polymer and polymer/conductor interface systems. This study found that the electron transfer rates of covalently attached and self-assembled films were much faster than the spin coated films. The knowledge from this study can be very useful for designing potential polymer based electronic and opto-electronic thin film devices.
We study the influence of metallic and dielectric films as well as lamellar multilayered stacks on the emission kinetics of Poly(methyl methacrylate) (PMMA) polymer doped with HITC dye at various concentrations. The two factors affecting the emission kinetics are the Purcell enhancement and self-quenching, which is particularly strong at high dye concentrations. Both effects are modified in different ways in the vicinity of metal/dielectric substrates) and appear to interfere with each other. The detailed account of the experimental results and analysis will be presented at the conference.
CMOS logic circuits have entered the sub-100nm regime, and research is on-going to investigate the quantum effects that are apparent at this dimension. To avoid some of the constraints imposed by fabrication, entropy, energy, and interference considerations for nano-scale devices, many have begun designing hybrid and/or photonic integrated circuits. These circuits consist of transistors, light emitters, photodetectors, and electrical and optical waveguides. As attenuation is a limiting factor in any communications system, it is advantageous to integrate a signal amplifier. There are numerous examples of electrical amplifiers, but in order to take advantage of the benefits provided by optically integrated systems, optical amplifiers are necessary. The erbium doped fiber amplifier is an example of an optical amplifier which is commercially available now, but the distance between the amplifier and the device benefitting from amplification can be decreased and provide greater functionality by providing local, on-chip amplification. Zinc oxide is an attractive material due to its electrical and optical properties. Its wide bandgap (≈3.4 eV) and high refractive index (≈2) make it an excellent choice for integrated optics systems. Moreover, erbium doped zinc oxide (Er:ZnO) is a suitable candidate for optical waveguide amplifiers because of its compatibility with semiconductor processing technology, 1.54 μm luminescence, transparency, low resistivity, and amplification characteristics. This research presents the characterization of radio frequency magnetron sputtered Er:ZnO, the design and fabrication of integrated waveguide amplifiers, and device analysis.
The behavior of spontaneous emission of emitters embedded inside metamaterials with hyperbolic dispersion has
been investigated. A simple technique has been developed to fabricate lamellar metal-dielectric hyperbolic
metamaterials on substrates which can be flat, flexible or curvilinear in geometry. Moreover, this method opens up the
possibility of functionalizing the dielectric layers by dye molecules. Utilizing this technique, we study the spontaneous
emission kinetics of emitters placed either on top, or embedded inside hyperbolic metamaterials. While we observe a
reduction in the radiative lifetimes in both cases, owing to the singularity in the density of photonic states, the effect is
much stronger when the dye molecules are inside the metamaterial, rather than on its surface.
Chemical engineering of metamaterials to reduce optical losses is studied by first principle density functional
theory. Contribution of the surface states to optical losses is studied by calculations of the imaginary part of the
dielectric function for several organic molecules (water, methanol, and ethanol) adsorbed on the (111) surface of
Ag nano-slabs. Substantial modifications of optical functions of metallic nano-slabs in near infrared and visible
spectral regions, caused by surface states and molecular adsorption, are predicted, discussed, and compared to
experimental data.
A comparison study between a block copolymer and blend samples (D/A;
donor/acceptor) revealed that their optoelectronic properties change significantly in different
morphologies due to different processing conditions. The study shows that the photovoltaic
performance of a block copolymer is better than that of corresponding donor/acceptor simple
blend devices due to smaller scale (5-10 nanometers) donor/acceptor phase separation in the
block copolymer, and that thermal annealing generally improves OE property due to potential
better molecular packing.
Optical absorption spectra of poly(thienylenevinylene) (PTV) conjugated polymers are measured at room temperature
in spectral range 400 to 800 nm. A dominant peak located at 575 nm and a prominent shoulder at
614 nm are observed. Equilibrium atomic geometries of PTV conjugated polymers are studied by first principles
density functional theory (DFT). Electron energy structure is obtained through self-consistent solution of eigen
energy problem using ab initio ultrasoft pseudopotentials and generalized gradient approximation method. This
is a non traditional approach for complex organic systems which is shown to be very promising especially for
optical simulations. Linear optical absorption is calculated within Random Phase Approximation (RPA) picture.
By comparative analysis of experimental and theoretical data it is demonstrated that dominant contribution to
the optical excitations of PTV in visible spectral range are related to the delocalized electrons within the polymer
chains. Obtained optical data together with equilibrium geometry analysis indicate that interchain interactions
substantially effect electronic structure and optical absorption of PTV conjugated polymers.
We report here the synthesis of ZnO films by the pulsed-laser deposition technique using various novel conditions. The
dopants are As, Ga, Al and N. The films show excellent crystalline quality with atomically smooth surface morphology.
The electrical resistivity was found to be close to 2 x~10-4 ohm-cm and transmittance >85% with both Ga and Al doping.
Doping with As shows several distinct transitions in their electrical resistivity and strong aging effects. On the other
hand, doping with Mn in ZnO reduces the grain size. On the other hand, doping with trivalent Er ions in ZnO films
causes two effects: for high doping (>8 wt%), a substantial enhancement of diagonal piezo-optic effect (up to 3.7*10-13
m2/N at &lgr;=633 nm) was observed due to creation of additional dipole moments at the interface of the film and the
substrate, and higher electrical conductivity with enhanced 1.54 &mgr;m emission was demonstrated at room temperature for
low concentration (<2 wt%) of Er. Furthermore, no quenching effects in emission characteristics at 1.54 &mgr;m were
observed up to 2 wt % of Er-doping in ZnO at room-temperature.
The semiconducting conjugated polymers, poly-phenylene-vinylene (PPV), are studied using linear and non-linear optical spectroscopy methods: ultra-fast optical spectroscopy, optical absorption, and luminescence. Study of polarization resolved absorption and excitation spectra at room temperature revealed strong anisotropy of the optical responses. Microscopic interpretation of the results is based of the first principal analysis of the electronic structure of PPV using the density functional theory (DFT). The dominant contribution of the optical excitations in visible spectral range is related to the delocalized pi-electronics of the PPV chains. Time-resolved luminescence measurements indicate the separate contributions from electronic transitions associated to the localized and delocalized electrons in PPV.
Sum Frequency Generation (SFG) spectra of nanocrystalline porous silicon (por-Si) exposed to different chemical treatments are studied. We report the first SFG studies of por-Si in direct contact with a liquid. SFG is excited by a regeneratively amplified Ti:sapphire system (787 nm, 120 fs, 1 kHz). The sum frequency is generated by combining this light with infrared that is generated with an optical parametric amplifier (OPA) that delivers 100-200 μJ pulses at 1370-1770 nm. Por-Si is made from a 10-20 Ω cm p-type Si(001) wafer. Comparisons are made to planar Si(001) as well as GaAs(001). First principle electronic structure theory based on density functional theory (DFT) is used to study the adsorption and optical response functions from the system of ethanol molecule adsorbed on Si(001) and Si(111) surfaces. Equilibrium atomic geometries are obtained through molecular dynamics and total energy minimization methods. Electron energy structure and optical properties are calculated using generalized gradient approximation method with ab initio pseudopotentials. Predicted differential optical absorption spectra for chemisorbed Si(001) and Si(111) surfaces are analyzed in comparison with SFG data measured on differently treated porous silicon. Substantial modifications of the surface atomic and electron energy structures of silicon surfaces due to chemisorption are shown to provide the dominant contributions to the SFG response.
Electron energy structure and optical response functions of semiconducting conjugated polymers, Poly-Phenylene-
Vinylenes (PPV) are studied by first principle modeling based on the density functional theory (DFT). Equilibrium
atomic geometries are obtained through total energy minimization. Electron energy structure and optical properties
are calculated using generalized gradient approximation method with ab initio pseudopotentials. Predicted data are
compared with measured optical absorption spectra of PPV. Dominant contribution of the optical excitations related
to the delocalized π-electrons within the PPV chains is demonstrated by comparative analysis of experimental and
theoretical data. Optical absorption spectra of acceptor type PPV chains show substantial blue shift with respect to
the donor type of the PPV polymer. Comparison between calculated and measured optical spectra demonstrates that
the shift could be attributed to the structural deformations of the benzene rings of the host PPV unit due to the
interaction with the atoms of the derivative groups.
In EO polymer materials, the second-order nonlinear optic chromophores must be oriented in one direction in order to be electro-optically active. Interchromophore electrostatic interactions, which encourages the formation of non-active and light-scattering crystalline domains through the antiparallel stacking of dipoles, have long been an obstacle to the translation of large molecular optical nonlinearity into corresponding bulk nonlinearity. Great progresses have been made in the design and synthesis of chormophores with reduced electrostatic interactions. New effort toward the complete elimination of the destructive effect of the electrostatic forces is on the way. On the contrary, strong intermolecular electrostatic interaction (e.g. the force responsible for π-π stacking of conjugated systems) is often desired in LED, transistor and photovoltaic (PV) devices since high mobilities of the charge carriers are the key to their high performances and often come from ordered stacking of pi-conjugated systems. For PV applications, bi-continuous "bulk heterojunction" of electron donor (D) and acceptor (A) is ideal for efficient charge carrier generation, transport and collection. Differential electrostatic interactions between D and A is the key to the formation of such morphology. We have synthesized a novel type of block copolymer having a basic unit of D-B-A (B is a non-conjugate bridge) for solar cell application. D and A can be designed in such a way that D-D and A-A interactions are stronger than the D-A interaction, and therefore, have a strong tendency to phase separate.
Organic/polymeric solar cells can be optimized in both space (morphology) and energy regimes to minimize the 'photon loss', the 'exciton loss' and the 'carrier loss'. In spatial regime optimization, for instance, a set of thin film solar cell devices fabricated from a -donor-bridge-acceptor-bridge- (-DBAB-) type block copolymer containing polyphenylenevinylene (PPV) conjugated donor and acceptor segments exhibited open circuit voltages (Voc) up to 1.1 volt, which is very impressive for organic/polymeric photovoltaics, though the photoelectric quantum and power conversion efficiencies are still very low due to energy gap mismatch and very small and not yet optimized short circuit current (in micro Amps regime).
The optical properties of previously synthesized sulfone and methoxy substituted block co-polymers of poly-phenlyenevinylene (PPV) have been examined. An internal space charge field is formed which has been used to quench the luminescence intensity in these materials by separating optically generated excitons and electron-hole pairs. The absorption and emission spectra and the time dependence of the emission of donor and acceptor derivatized block co-polymers was measured and the quenching of the luminescence was observed and quantified. PPV materials with this internal field have potential applications as solar energy converters and photodetectors.
The optical and electro-optic properties and the temperature stability of those properties has been measured for DR-19 doped polymer films with a range of cross-linking agents. The films show good transmission in the optical communication range due to the lack of OH and NH bonds in the material. The refractive index and field dependent indices are comparable with those previously reported. However, the temperature stability of the films is high. It is this temperature stability that makes the material attractive for electro-optic switching applications.
A D(donor)-B(bridge)-A(acceptor)-B(bridge)-type block copolymer system has been developed and preliminarily examined for potential opto-electronic photovoltaic functions. The unique feature of the device includes a primary DBAB-type block copolymer backbone, where D and A are conjugated donor and acceptor polymer blocks, and B is a
non-conjugated and flexible chain, a π orbital stacked and conjugated chain self-assembled and ordered "secondary structure", and a donor/acceptor asymmetric layers sandwiched D/A columnar "tertiary structure". This structure is expected to improve photovoltaic power conversion efficiency significantly in comparison to most existing organic or polymeric donor/acceptor binary photovoltaic systems due to the reduction of "exciton loss", the "carrier loss", as well as the "photon loss" via three-dimensional space and energy level optimizations. Preliminary experimental results revealed better morphology and opto-electronic properties of DBAB vs. D/A blends.
The intensity dependent index of refraction and themolecular two-photon absorptivity (TPA) of 3,3'-dielthylthia-cyanine iodide (DETCI) has been measued in a range of solvents by the femtosecond z-scan technique. In DMSO, where DETCI is quite soluble, both the two-photon absorptivity and the nonlinear index of refraction are linear functions of concentration. In contrast, the two photon absorptivity and non-olniear index change of DETCI in MeOH is an order of magnitude lower and shows signs of saturation at concentrations well below the saturation limit. In high index solvents such as MeOH, the TPA of DETCI is much smaller in lower index solvents such as DMSO. The intensity dependent index chagne, n2 is large and relatively insensitive to the index of the solvent.
Sulfone and methoxy substituted block co-polymers of poly-phenlyenevinylene (PPV) have been synthesized and the optical properties of both the oligomers and the combined block co-polymers have been examined. The substitution on the phenylene ring leads to small energy shifts in both the optical band gap as well as shifts in the valence and conduction band energies. When the oligomers are combined in a block co-polymer either with or without an aliphatic bridge unit, a space charge field is formed which can be used to quench the luminescence intensity in these materials by separating optically generated excitons and electron-hole pairs. PPV materials with this internal field have potential applications as solar energy converters and photodetectors. The absorption and emission spectra and the time dependence of the emission of donor and acceptor derivatized block co-polymers was measured and the quenching of the luminescence was observed and quantified.
The intensity dependent index of refraction and the molecular two-photon absorptivity (TPA) of 3,3'-dielthylthia-cyanine iodide (DETCI) has been measured in a range of solvents by the femtosecond z-scan technique. In DMSO, where DETCI is quite soluble, both the two-photon absorptivity and the nonlinear index of refraction are linear functions of concentration. In contrast, the two photon absorptivity and non-linear index change of DETCI in MeOH is an order of magnitude lower and shows signs of saturation at concentrations well below the saturation limit. In high index solvents such as MeOH, the TPA of DETCI is much smaller in lower index solvents such as DMSO. The intensity dependent index change, n2 is large and relatively insensitive to the index of the solvent.
With the availability of terawatt laser systems with subpicosecond pulses, laser damage to optical components has become the limiting factor for further increases in the output peak power. Evaluation of different material structures in accordance to their suitability for high-power laser systems is essential. Multi-shot damage experiments, using 110 fs laser pulses at 800 nm, on polycrystalline single layer gold films and multi-layer (gold-vanadium, and gold-titanium) films were conducted. The laser incident fluence was varied, in both cases, from 0.1 to 0.6 J/cm2. No evidence of surface damage was apparent in the gold sample up to a fluence of 0.3 J/cm2. The multilayer sample experienced the onset of surface damage at the lowest fluence value used of 0.1 J/cm2. Damage results are in contrast with the time resolved ultrafast thermoreflectivity measurements that revealed a reduction of the thermoreflectivity signal for the multilayer films. This decrease in the thermoreflectivity signal signifies a reduction in the surface electron temperature that should translate in a lower lattice temperature at the later stage. Hence, one should expect a higher damage threshold for the multilayer samples. Comparison of the experimental results with the predictions of the Two-Temperature Model (TTM) is presented. The damage threshold of the single layer gold film corresponds to the melting threshold predicted by the model. In contrast to the single layer gold film, the multi-layer sample damaged at almost one third the damage threshold predicted by the TTM model. Possible damage mechanisms leading to the early onset of damage for the multilayer films are discussed.
A series of functional and photo-crosslinkable maleate or fumarate type polyesters containing azo-type NLO chromophores were studied. The result demonstrated this is a versatile and convenient method of fabricating crosslinked supramolecular ultra-structure polymer thin films for potential nonlinear optical (NLO) and other photonic applications. The unsaturated PDRMA/PDRFC polyester thin films are capable of crosslinking in air via photo polymerization to form a hardened lattice. A ratio of 1:1 of crosslinker double bonds to polyester double bonds was preferred for optimal crosslinking. Thermal stability of Second Harmonic Generation (SHG) signals for a photo crosslinked polymer thin film reached to about 150°C. Films from photo crosslinking of fumarate or maleate vs. vinyl crosslinkers have the advantages of avoiding NH/OH groups and their vibrational overtones that absorb at telecommunication wavelength of 1550 nm.
A future polymer based electro-optical (EO) modulator has several advantages over a current commercial Lithium Niobate EO modulator, a key device that encodes electronic signals into optical signals. These advantages include larger bandwidth (over 100 GHz), lower drive voltage (less then 1 volt), more cost effective, etc. However, one major challenge for EO polymer development is materials stability, including short term and long term SHG (NLO chromophore orientation) stability. Since the modulator fabrication and packaging processes typically require heating temperatures of 200-250oC for up to 10 minutes, yet the SHG thermal stability of most NLO polymers developed so far only reach about 150oC. In order to overcome this challenge, either modulator processing temperatures are reduced, or SHG thermal stability of EO polymers be increased. Polymer crosslinking technique is one of the most versatile and effective methods for fabricating and stabilizing polymer nano structures at high temperatures. Among various NLO polymer crosslinking schemes developed so far, fumaryl chloride (FC) and maleic anhydride (MA) derived crosslinked polyester system seems to be a versatile and convenient scheme. This scheme also looks attractive for low loss applications at 1550 nm. FC/MA crosslinking systems also offer visible light photolithographic fabrication advantage during waveguide fabrication.
The nonlinear optical absorption of a range of dialkyl- carbothiacyanine dyes has been measured using intensity dependent transmission measurements of the absorption cross- section at 532 nm. The ground and effective excited state absorption cross-section for each of the dyes has been determined. It was observed that with increasing bridge conjugation length between the carbothiacyanine end groups, the absorption cross section of ground state decreases slightly while the absorption cross-section of the excited state increases leading to an overall increase in the absorption cross-section ratio by an order of magnitude from 0.3 to 5.29. The absorption cross-section ratio tracks linearly with the difference between the energy of the pump laser and the energy of the ground state absorption. The counter ions of the dyes also affect the nonlinear absorption significantly. These organic materials exhibit good potential for optical limiting applications.
The synthesis and characterization of maleate type crosslinkable nonlinear optical (NLO) polymers derived form maleic anhydride and fumaryl chloride is described. Preliminary results demonstrated this is a convenient, inexpensive, and versatile method of fabricating crosslinked NLO polymer thin films. These maleate type polyesters containing NLO chromophores such as Disperse Red 19 are capable of crosslinking to form a hardened lattice under thermal or high-energy radiation conditions. Crosslinking is a critical nano-scale technique for second order nonlinear optical as well as other potential photonic applications where molecular orientations need to be aligned and be stabilized against molecular thermal motions. Photolithographic techniques may be readily employed in this system to fabricate patterned polymer waveguide.
A novel phase-matching technique for coherent THz-wave generation was proposed. This approach uses cross- Reststrahlen band dispersion compensation phase-matching in a collinear optical mixing technique using isotropic, semiconductor nonlinear crystals. The pump and signal sources are in the near-IR transmission window of the nonlinear crystal and the generated idler wave is in the far-IR transmission window, on the other side of the crystal's Reststrahlen band. The choice of the nonlinear crystal for frequency conversion from the near-IR to the far-IR is highly dependent on the optical properties of the nonlinear crystal being used as the frequency converter element. We evaluate the use of the proposed phase-matching technique with a variety of III-V and II-VI compounds taking into account their optical dispersion and their phase- matching properties. Theoretical projections of the device performance such as perfect phase-matching conditions, coherence length and wavelength tuning range for different nonlinear crystals are compared.
The vibrations of phosphate (PO4-3) ions in single crystals of hexagonal Ba5(PO4)3F have been investigated by means of polarized Raman scattering and compared to Raman frequencies observed in fluorapatite analogs Sr5(PO4)3F and Ca5(PO4)3F. The Raman vibrational frequencies decrease upon substitution of Ba2+ for Sr2+ or Ca2+ in the lattice, asymptotically approaching the Raman frequencies for vibrations of the free ion. The observed splitting is in accord with predictions made by a group theory analysis of the factor group of the unit cell. The effects of the host lattice environment on the magnitude of the Davydov splittings and frequency shifts of phosphate ions within different crystals with the fluorapatite structure are demonstrated to be inversely proportional to the unit cell volume.
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