Visualizing surface plasmons with photons, photoelectrons, and electrons.

Both photons and electrons may be used to excite surface plasmon polaritons, the collective charge density fluctuations at the surface of metal nanostructures. By virtue of their nanoscopic and dissipative nature, a detailed characterization of surface plasmon (SP) eigenmodes in real space-time ultimately requires joint nanometer spatial and femtosecond temporal resolution. The latter realization has driven significant developments in the past few years, aimed at interrogating both localized and propagating SP modes. In this mini-review, we briefly highlight different techniques employed by our own groups to visualize the enhanced electric fields associated with SPs. Specifically, we discuss recent hyperspectral optical microscopy, tip-enhanced Raman nano-spectroscopy, nonlinear photoemission electron microscopy, as well as correlated scanning transmission electron microscopy-electron energy loss spectroscopy measurements targeting prototypical plasmonic nanostructures and constructs. Through selected practical examples from our own laboratories, we examine the information content in multidimensional images recorded by taking advantage of each of the aforementioned techniques. In effect, we illustrate how SPs can be visualized at the ultimate limits of space and time.

Structural characterization of epitaxial Cr(x)Mo(1-x) alloy thin films.

To develop a model system containing regularly spaced misfit dislocations for studies of the radiation resistance of nanoscale defects, epitaxial thin films of Cr, Mo, and Cr(x)Mo(1-x) alloys were deposited on MgO(001) by molecular beam epitaxy. Film compositions were chosen to vary the lattice mismatch with MgO. The film structure was investigated by x-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS) and scanning transmission electron microscopy (STEM). Epitaxial films with reasonably high crystalline quality and abrupt interfaces were achieved at a relatively low deposition temperature, as confirmed by STEM. However, it was found by XRD and RBS in the channeling geometry that increasing the Mo content of the CrMo alloy films degraded the crystalline quality, despite the improved lattice match with MgO. XRD rocking curve data indicated that regions of different crystalline order may be present within the films with higher Mo content. This is tentatively ascribed to spinodal decomposition into Cr-rich and Mo-rich regions, as predicted by the Cr(x)Mo(1-x) phase diagram.

High-pressure effects on the electronic structure of anthracene single crystals: role of nonhydrostaticity.

Optical spectroscopy methods were used to examine the effect of nonhydrostaticity on the electronic structure of anthracene single crystals compressed statically to 9 GPa. Two pressure-transmitting media, nitrogen (hydrostatic) and water (nonhydrostatic above approximately 5.5 GPa), were utilized. It was found that nonhydrostatic compression generates several new features both in the absorption and fluorescence spectra: (i) formation of new absorption and fluorescence bands, (ii) deviations in pressure shift of fluorescence peaks, (iii) extensive broadening of vibrational peaks, and (iv) irreversible changes in the spectra shape upon pressure unloading. Furthermore, the time-resolved fluorescence decay curves measured at the wavelength corresponding to the new fluorescence band show clear initial increase. These new features are accompanied by inhomogeneous color changes and macroscopic lines on the (001) plane of the crystal. All of the changes are discussed and correlated with microscopic transformations in the crystal. It is demonstrated that nonhydrostatic compression in anthracene crystal introduces inelastic changes in the form of dislocations along [110] and [110] directions. These dislocations lead to the development of dimeric structures and, consequently, to various changes in the electronic response of the compressed anthracene crystal.

A fluorescence spectroscopic study of phenanthrene sorption on porous silica.

Fluorescence spectroscopic characteristics of sorbed phenanthrene in porous silica provide information about its chemical state such as monomer vs dimer or higher aggregates, as well as a basis for high sensitivity detection. In this study, the chemical state and distribution of phenanthrene sorbed in two types of porous silica particles, mesoporous silica (365 microns particle diameter, 150 A average pore diameter) and microporous silica (custom synthethized, 1 micron particle diameter, 20 A pore diameter), is determined by fluorescence spectroscopy, fluorescence lifetime measurements, and scanning two-photon excitation fluorescence profiling. From the characteristic fluorescence emission spectra, it is found that at loading levels of < or = 4.7 mg/g (phenanthrene/silica) phenanthrene exists as monomers in both meso- and microporous silica particles for phenanthrene loaded from super critical CO2 (SCF). Two-photon excitation fluorescence intensity distribution profiles indicate that for the mesoporous silica particles phenanthrene is adsorbed throughout the entire silica particle. Introduction of water into phenanthrene-loaded mesoporous silica particles causes instantaneous conversion of phenanthrene from monomer to crystalline form at phenantherene loading levels > or = 4.7 micrograms/g due to hydration of the silica surface. In this process, sorption of water molecules expels phenanthrene from the surface sorption sites and causes localized phenanthrene concentration beyond its solubility limit, resulting in crystallization. In comparison this fast conversion is not observed for phenanthrene-loaded microporous silica particles that show extremely slow conversion even for phenanthrene loading levels as high as 4.7 mg/g. This difference is interpreted as reflecting hindered diffusion of phenanthrene in the nearly monodispersed micropores with pore sizes close to the molecular diameter of phenanthrene.

Temperature dependence of up-conversion luminescence and photoluminescence of Mn2+ in ZnS:Mn2+ nanoparticles.

The photoluminescence (excited at both 300 nm and 383.5 nm) and up-conversion luminescence (excited at 767 nm) of the Mn2+ 4T1-->6A1 transition in both bulk and ZnS:Mn2+ nanoparticles have been measured as a function of temperature. The Mn2+ emission spectra shift monotonically to longer wavelengths at lower temperatures, whereas the intensity change of the luminescence is more complex. The complicated temperature behavior is explained by considering the processes of nonradiation relaxation via phonon coupling, exciton thermal dissociation (binding energy), energy transfer, carrier trapping, and the temperature change of the absorption spectra. The fact that the temperature dependence of the 767 nm excited up-conversion luminescence is the same as the 383.5 nm excited photoluminescence in both bulk and nanoparticles supports the conclusion that the up-conversion luminescence is due to two-photon absorption.

Pulse compression with a high-energy Nd:YAG regenerative amplifier system.

We describe a method for the generation of readily synchronizable, near-transform-limited, 1064-nm, 6-mJ pulses with <20-ps duration at a repetition rate of 20 Hz. The method employs chirped pulse amplification of spectrally broadened and temporally stretched pulses from a cw mode-locked Nd:YAG laser in a commercial Nd:YAG regenerative amplifier followed by pulse compression with a grating pair. Linear amplification subsequent to regenerative amplification is not required with this method, although higher energies would be easily obtained.