Reconfigurable infrared hyperbolic metasurfaces using phase change materials.

Metasurfaces control light propagation at the nanoscale for applications in both free-space and surface-confined geometries. However, dynamically changing the properties of metasurfaces can be a major challenge. Here we demonstrate a reconfigurable hyperbolic metasurface comprised of a heterostructure of isotopically enriched hexagonal boron nitride (hBN) in direct contact with the phase-change material (PCM) single-crystal vanadium dioxide (VO2). Metallic and dielectric domains in VO2 provide spatially localized changes in the local dielectric environment, enabling launching, reflection, and transmission of hyperbolic phonon polaritons (HPhPs) at the PCM domain boundaries, and tuning the wavelength of HPhPs propagating in hBN over these domains by a factor of 1.6. We show that this system supports in-plane HPhP refraction, thus providing a prototype for a class of planar refractive optics. This approach offers reconfigurable control of in-plane HPhP propagation and exemplifies a generalizable framework based on combining hyperbolic media and PCMs to design optical functionality.

Electronic Stopping of Slow Protons in Oxides: Scaling Properties.

Electronic stopping of slow protons in ZnO, VO_{2} (metal and semiconductor phases), HfO_{2}, and Ta_{2}O_{5} was investigated experimentally. As a comparison of the resulting stopping cross sections (SCS) to data for Al_{2}O_{3} and SiO_{2} reveals, electronic stopping of slow protons does not correlate with electronic properties of the specific material such as band gap energies. Instead, the oxygen 2p states are decisive, as corroborated by density functional theory calculations of the electronic densities of states. Hence, at low ion velocities the SCS of an oxide primarily scales with its oxygen density.

Probing plasmons in three dimensions by combining complementary spectroscopies in a scanning transmission electron microscope.

The nanoscale optical response of surface plasmons in three-dimensional metallic nanostructures plays an important role in many nanotechnology applications, where precise spatial and spectral characteristics of plasmonic elements control device performance. Electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) within a scanning transmission electron microscope have proven to be valuable tools for studying plasmonics at the nanoscale. Each technique has been used separately, producing three-dimensional reconstructions through tomography, often aided by simulations for complete characterization. Here we demonstrate that the complementary nature of the two techniques, namely that EELS probes beam-induced electronic excitations while CL probes radiative decay, allows us to directly obtain a spatially- and spectrally-resolved picture of the plasmonic characteristics of nanostructures in three dimensions. The approach enables nanoparticle-by-nanoparticle plasmonic analysis in three dimensions to aid in the design of diverse nanoplasmonic applications.

Ultrafast Mid-Infrared Nanoscopy of Strained Vanadium Dioxide Nanobeams.

Long regarded as a model system for studying insulator-to-metal phase transitions, the correlated electron material vanadium dioxide (VO2) is now finding novel uses in device applications. Two of its most appealing aspects are its accessible transition temperature (∼341 K) and its rich phase diagram. Strain can be used to selectively stabilize different VO2 insulating phases by tuning the competition between electron and lattice degrees of freedom. It can even break the mesoscopic spatial symmetry of the transition, leading to a quasiperiodic ordering of insulating and metallic nanodomains. Nanostructuring of strained VO2 could potentially yield unique components for future devices. However, the most spectacular property of VO2--its ultrafast transition--has not yet been studied on the length scale of its phase heterogeneity. Here, we use ultrafast near-field microscopy in the mid-infrared to study individual, strained VO2 nanobeams on the 10 nm scale. We reveal a previously unseen correlation between the local steady-state switching susceptibility and the local ultrafast response to below-threshold photoexcitation. These results suggest that it may be possible to tailor the local photoresponse of VO2 using strain and thereby realize new types of ultrafast nano-optical devices.

Plasmon-exciton hybridization in ZnO quantum-well Al nanodisc heterostructures.

We demonstrate the formation of a hybridized plasmon-exciton state exhibiting strong exciton-plasmon coupling in ZnO/Zn(0.85)Mg(0.15)O single quantum wells capped with arrays of Al nanodiscs. Tuning the quantum-well width and the diameter and pitch of the Al nanodisc arrays facilitates a transition from the weak-coupling regime into the strong coupling regime. Finite-difference time-domain simulations substantiate the localization of the plasmonic quadrupole moment within the ZnO quantum-well layer, resulting in a hybridized plasmonexciton state demonstrating a Rabi splitting of roughly 15 meV in heterostructures that exhibit a prominent plasmon quadrupole mode. The significant tunability offered by quantum-well heterostructures like those discussed here provides a flexible system for controlling exciton plasmon coupling in a device-compatible thin-film architecture.

Selective Purcell enhancement of defect emission in ZnO thin films.

A zinc interstitial defect present but unobservable in ZnO thin films annealed at 500 °C in oxygen or in atmosphere was selectively detected by interaction of the film with an Ag surface-plasmon polariton. The time-dependent differential reflectivity of the ZnO near the ZnO/MgO interface exhibited a subpicosecond decay followed by a several nanosecond recovery, consistent with the Purcell-enhanced Zn interstitial luminescence seen in Ag-ZnO heterostructures. Heterostructures annealed at other temperatures showed significantly greater band-edge photoluminescence and no evidence of the Zn interstitial defect.

Ultrafast changes in lattice symmetry probed by coherent phonons.

The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. Here we show that a time-dependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, thus providing an all-optical probe of structural transitions. We examine the photoinduced structural phase transition in VO(2) and show that, above the phase transition threshold, photoexcitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibrium-phase phonon modes occurs promptly, indicating a non-thermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice potential changes its symmetry before ionic rearrangement has occurred.

Enhancement of ZnO photoluminescence by localized and propagating surface plasmons.

Insulating spacer layers of MgO were used to identify the enhancement mechanisms of the ZnO band-edge and visible luminescence in ZnO-MgO-Ag and ZnO-MgO-Au multilayers. Purcell enhancement of the ZnO band-edge emission by both Ag and Au surface plasmon polaritons is confirmed by demonstrating that the exponential decay of this emission as a function of increasing MgO thickness is consistent with the Ag and Au SPP evanescent decay lengths. Local surface plasmons excited in Ag and Au nanoparticles and rough films are also shown to enhance the ZnO visible donor-acceptor-pair photoluminescence by dipole-dipole scattering, again with an appropriate dependence on the thickness of the MgO spacer layer. We also confirm that both Ag and Au nanoparticles enhance the ZnO band-edge emission by charge transfer when the MgO spacer layer is absent.

Coherent structural dynamics and electronic correlations during an ultrafast insulator-to-metal phase transition in VO2.

We directly trace the multi-THz conductivity of VO2 during an insulator-metal transition triggered by a 12-fs light pulse. The femtosecond dynamics of lattice and electronic degrees of freedom are spectrally discriminated. A coherent wave packet motion of V-V dimers at 6 THz modulates the lattice polarizability for approximately 1 ps. In contrast, the electronic conductivity settles to a constant value already after one V-V oscillation cycle. Based on our findings, we propose a qualitative model for the nonthermal phase transition.

Applications of free-electron lasers in the biological and material sciences.

Free-Electron Lasers (FELs) collectively operate from the terahertz through the ultraviolet range and via intracavity Compton backscattering into the X-ray and gamma-ray regimes. FELs are continuously tunable and can provide optical powers, pulse structures and polarizations that are not matched by conventional lasers. Representative research in the biological and biomedical sciences and condensed matter and material research are described to illustrate the breadth and impact of FEL applications. These include terahertz dynamics in materials far from equilibrium, infrared nonlinear vibrational spectroscopy to investigate dynamical processes in condensed-phase systems, infrared resonant-enhanced multiphoton ionization for gas-phase spectroscopy and spectrometry, infrared matrix-assisted laser-desorption-ionization and infrared matrix-assisted pulsed laser evaporation for analysis and processing of organic materials, human neurosurgery and ophthalmic surgery using a medical infrared FEL and ultraviolet photoemission electron microscopy for nanoscale characterization of materials and nanoscale phenomena. The ongoing development of ultraviolet and X-ray FELs are discussed in terms of future opportunities for applications research.

Size-dependent optical properties of VO2 nanoparticle arrays.

The size effects on the optical properties of vanadium dioxide nanoparticles in ordered arrays have been studied. Contrary to previous VO2 studies, we observe that the optical contrast between the semiconducting and metallic phases is dramatically enhanced in the visible region, presenting size-dependent optical resonances and size-dependent transition temperatures. The collective optical response as a function of temperature presents an enhanced scattering state during the evolving phase transition. The effects appear to arise because of the underlying VO2 mesoscale optical properties, the heterogeneous nucleation behind the phase transition, and the incoherent coupling between the nanoparticles undergoing an order-disorder-order transition. Calculations that support these interpretations are presented.

Temperature-controlled surface plasmon resonance in VO (2) nanorods.

The optical properties of VO(2) nanoparticles formed in an amorphous SiO(2) host by stoichiometric ion implantation of vanadium and oxygen and thermal annealing have been determined and correlated with the particle size and morphology. The results show that that the temperature-controlled semiconductor-to-metal phase transition of the VO(2) nanophase precipitates turns on the classical surface plasmon resonance, with specific features that depend on the size and aspect ratio of the VO(2) particles. This effect improves the optical contrast between the metallic and semiconducting states in the near-IR region of the spectrum as a result of dielectric confinement that is due to the SiO(2) host. A fiber-optic application is demonstrated, as is the ability to control the characteristics of the phase transition by using ion implantation to dope the VO(2) nanoparticles with tungsten or titanium ions.

Intensity dependence of cation kinetic energies from 2,5-dihydroxybenzoic acid near the infrared matrix-assisted laser desorption/ionization threshold.

The mechanisms responsible for matrix-assisted laser desorption/ionization (MALDI) are far from being well understood, particularly where infrared laser irradiation is used to initiate the process. We measured the emission yields and kinetic energy distributions of positive ions emitted from 2,5-dihydroxybenzoic acid loaded with angiotensin II in a standard MALDI preparation during irradiation with an infrared free-electron laser tuned to 2.94 microm. As the laser intensity is scanned through the MALDI threshold, we see a marked change in the energy distributions of the matrix ion. Above threshold, the energy distributions of both analyte and matrix cations are constant over a broad range of laser intensities. This behavior does not appear to be consistent with any extant model of the MALDI mechanism.

Effect of smear layer and direction of dentinal tubules on osteoblast adhesion to human dentin tissue.

The purpose of this study was to investigate the effect of smear layer and direction of dentinal tubules on osteoblast adhesion to human dentin tissue in vitro. Dentin disks were made from human premolars extracted for orthodontic reasons. Dentin disks were cut either perpendicularly to the long axis of the tooth or at 45 degrees to the long axis of tooth. The smear layer was removed by 34% phosphoric acid gel from half of the dentin disk surface. Human osteoblast-like Saos-2 cells were grown in RPMI medium with 10% fetal bovine serum and 1% antibiotic/antimycotic cocktail under standard cell culture conditions. Cells were seeded into Nunc four-well culture plates at 1.5 x 10(5) cells per well with dentin disks in the bottom of each well. After 1 day in culture the dentin disks along with cells grown on their surface were examined with a scanning electron microscopy. Osteoblasts attached and spread on the dentin surface and formed a monolayer in the presence and absence of a smear layer. Cells spread over the dentinal tubules despite their direction. These results suggest that cell adhesion and spreading of osteoblasts is not influenced either by the existence of a smear layer or the direction of the dentinal tubules on the dentin surface.

Adhesion of human osteoblasts on root-end filling materials.

Adhesion of human osteoblasts to root-end filling materials (mineral trioxide aggregate (MTA), IRM, composite, and amalgam) was observed by scanning electron microscopy. Root-end filling materials were inserted into 96-well flat-bottomed plates and condensed to disks of approximately 1 mm thick and the same diameter as the wells. After the disks were set, they were placed in the bottom of Nunc four-well culture plates at one disk per well. Then human osteoblasts were seeded into the wells at 1.5 x 10(5) cells per well. After 1 day in culture the disks of root-end filling materials along with cells grown on their surface were examined with a scanning electron microscopy. Results showed that osteoblasts attached and spread on MTA and composite by forming a monolayer. Osteoblasts also attached on amalgam, but with few cells spreading. In the presence of IRM, osteoblasts appeared rounded with no spreading. These results indicate that osteoblasts have a favorable response to MTA and composite resin compared with IRM and amalgam.

Infrared matrix-assisted laser desorption and ionization by using a tunable mid-infrared free-electron laser.

Initial results of infrared matrix-assisted laser desorption/ionization (IR-MALDI) mass spectrometry of proteins by using the Vanderbilt free-electron laser as the source of selective vibrational excitation are reported. The ability of this laser to initiate desorption and ionization by excitation of specific vibrational modes is demonstrated. For the first time it is shown that IR-MALDI mass spectrometry at wavelengths other than those available from conventional fixed-frequency IR lasers, that is, 2.79 (Er:YSGG), 2.94 (Er:YAG), and 9.3-10.6 µm (CO2), is feasible and exhibits similar performance. IR-MALDI mass spectra were taken in the wavelength ranges 2.8-4 and 5.5-6.5 µm, covering the absorption bands of the O-H and C=O stretch vibrations typical of many organic compounds such as succinic acid, fumaric acid, or nicotinic acid, which were used as matrices in these studies. A comparison between these results and Er:YAG/YSGG MALDI data are given. The potential of IR-MALDI at wavelengths near the C=O stretch vibration and the possibilities for studies of the IR-MALDI mechanisms by using this kind of tunable source are discussed.

Picosecond nonlinear optical response of a Cu:silica nanocluster composite.

We describe the picosecond nonlinear optical response of a metal-dielectric composite made by implanting Cu ions in fused silica. The implanted Cu ions aggregate during implantation to form nanometer-diameter clusters in a dense, thin (~150 nm) layer just beneath the surface of the substrate. The third-order susceptibility X((3)) has an electronic component with a magnitude of the order of 10(-8) esu and is enhanced for laser wavelengths near the surface plasmon resonance of the copper colloids.

Persistence of benzo[a]pyrene and 7,8-dihydro-7,8-dihydroxybenzo[a] pyrene in Fischer 344 rats: time distribution of total metabolites in blood, urine and feces.

A comparison of the rates of elimination of [3H]benzo[a] pyrene (BaP) and 7,8-dihydro-7,8-diol-[3H]benzo[a]pyrene (BPD), after subcutaneous injection into Fischer 344 rats, shows they are both eliminated at about the same rates and with the same pattern over at least 7 days post-exposure. The end-rate of combined urinary and fecal excretion was approximately 40 nmol/day. About 20% of the injected BaP and approximately 3% of the injected BPD remained at the site of injection for at least 9 days. The remainder was distributed throughout the animal. If the rate of excretion continued at the observed steady-state rates, the BaP and BPD could persist for up to 40 days for each milligram of injected substance. The concentration of excretion products were highest during day 1 and day 2 following exposure, decreased exponentially to a concentration of approximately 0.5 microM (mixed metabolites) by day 5 following exposure, and then continued to be excreted at that rate. Feces contained the highest total amounts of radioactivity, which were approximately 2- to 4-fold higher than the amounts in urine and approximately 15- to 50-fold higher than in total blood. The conversion of organic 3H to 3H2O during the experimental period indicates that whole-body phenol(quinone) formation was significant for BaP metabolism, but was much less for BPD metabolism. When BaP was injected, both blood and urine contained water-soluble, volatile tritium counts (3H2O). Injection of BPD resulted in volatile 3H2O in urine but not in blood. The persistence of BaP and BPD metabolites in skin, blood, urine and feces compartments indicates there is a substantial reservoir of the chemical(s) that could be used to replenish repaired or discarded DNA adducts.

Regions upstream from the core promoter of the rat ribosomal gene are required for the formation of a stable transcription initiation complex by RNA polymerase I in vitro.

The sites required for the formation of a stable transcription initiation complex and for the initiation of transcription of rat rDNA in vitro were examined. A series of 5' deletion mutants of the rat transcription initiation region (-167 through +638) were constructed. These mutants were examined for their ability to support the faithful initiation of transcription in vitro. Mutants which contain less than 31 nucleotides upstream of the initiation site (+1) were unable to support detectable initiation of transcription. In this transcription system a series of deletion mutants from -167 to -31 were transcribed with equal efficiency when assayed individually. On the other hand, when the wild-type and mutant templates were compared in order-of-addition assays, they were found to be unequal. The incubation of an extract with a wild-type template, prior to the addition of nucleotides, precluded transcription of any second template added after the preincubation step. However, the preincubation of extract with mutants of the region upstream of the core promoter, from -122 to -31, did not preclude transcription of a wild-type template added after the preincubation step. Formation of the stable preinitiation complex was found to require the region between and -167.

Isolation, fractionation and reconstitution of a nuclear extract capable of transcribing ribosomal DNA.

A procedure for preparing a nuclear extract that efficiently transcribes rat rDNA in vitro has been developed. This procedure, which is based on the protocol described by Dignam et al. (Nucl Acids Res 11:1475, 1983), allows the preparation of extract from large or small amounts of material and requires neither ultracentrifugation nor column chromatography. These extracts were found to be more efficient than other transcription systems. Extract prepared as described routinely synthesize 1-2 transcripts per linear template, and could synthesize upto 6 transcripts per linear template at an elongation rate of 2.1 nucleotides per second. 0.3 M NaCl extracts of nuclei contained RNA polymerase I, but did not transcribe rat rDNA in vitro, whereas extract prepared with 0.42 M NaCl did. The 0.42 M NaCl extract of nuclei was fractionated by chromatography on DEAE-Sephadex and heparin-Sepharose. Two activities were identified that were required for accurate in vitro transcription by endogenous RNA polymerase I. One of these activities was required for accurate initiation, and the second inhibited non-specific transcription. The fraction required for accurate initiation by the endogenous RNA polymerase I is that factor which directs species specific transcription, as it also directed the transcription of rat rDNA by nuclear extracts of HeLa cells. Combining that same chromatographic fraction of the 0.42 M NaCl extract with the 0.3 M NaCl extract resulted in specific transcription. These results suggest that a fraction of the RNA polymerase I molecules may exist in a complex with some, or all, of the factors required for transcription.