Tip-enhanced Raman Scattering (TERS) can be used to image plasmon-enhanced local optical fields on the nanoscale. In the few molecule regime where the tensorial nature of Raman scattering is operative, this results in TERS images that directly reflect the local field characteristics. For a well-defined substrate, we can numerically simulate TERS spectral images to identify the effective molecular orientation on the tip that maps any experimentally encountered combination of local electric field components. For a corrugated surface where both the vector components of the local electric fields and the molecular orientation are unknown, we can simulate many spectral features.
We study plasmonic nanostructures in single-crystal gold with scanning electron and femtosecond photoemission electron microscopies. We design an integrated laser coupling and nanowire waveguide structure by focused ion beam lithography in single-crystal gold flakes. The photoemission results show that the laser field is efficiently coupled into a propagating surface plasmon by a simple hole structure and propagates efficiently in an adjacent nano-bar waveguide. A strong local field is created by the propagating surface plasmon at the nano-bar tip. A similar structure, with a decreased waveguide width and thickness, displayed significantly more intense photoemission indicating enhanced local electric field at the sharper tip.
For Raman spectroscopy the ability to detect is often limited by the existence and quality of the reference library to
which field spectra are compared. Developing such databases is often labor- and resource-intensive; typically the
generated data are not transferred to other instruments. Still other considerations may exist for comparing data at visible
and ultraviolet excitation wavelengths such as resonance enhancement. However, for the common near-infrared
wavelengths of 785, 830, 960, 1047 and 1064 nm where this is normally of a lesser concern, it is logical to consider
whether data can be ported from one spectrometer to another so as to obviate the expensive and time-consuming process
of generating reference data for each system. The present experiment generated a list of 125 chemical and common
substances and formed a database from their corresponding 1064 nm spectra. The same molecules were then measured
using a 785 nm system. The new spectra were treated as "unknowns" and compared to the 1064 nm database using a
commercial search algorithm. We found that at least 108 of the 125 spectra recorded at 785 nm were correctly identified
using the search algorithm. For the few that were incorrectly identified, in most cases the spectra were extremely similar
or the 785 nm signal was degraded due to fluorescence, as would occur regardless of reference data. Our results indicate
that if the spectrometers are properly calibrated on both their wavelength and intensity axes, "foreign" data recorded at a
different NIR wavelength can be successfully used as reference libraries.
Coatings of conducting gold-black nano-structures on commercial thin-film amorphous-silicon solar cells enhance the
short-circuit current by 20% over a broad spectrum from 400 to 800 nm wavelength. The efficiency, i.e. the ratio of the
maximum electrical output power to the incident solar power, is found to increase 7% for initial un-optimized coatings.
Metal blacks are produced cheaply and quickly in a low-vacuum process requiring no lithographic patterning. The
inherently broad particle-size distribution is responsible for the broad spectrum enhancement in comparison to what has
been reported for mono-disperse lithographically deposited or self-assembled metal nano-particles. Photoemission
electron microscopy reveals the spatial-spectral distribution of hot-spots for plasmon resonances, where scattering of
normally-incident solar flux into the plane increases the effective optical path in the thin film to enhance light harvesting.
Efficiency enhancement is correlated with percent coverage and particle size distribution, which are determined from
histogram and wavelet analysis of scanning electron microscopy images. Electrodynamic simulations reveal how the
gold-black particles scatter the radiation and locally enhance the field strength.
The positive ion yield as a function of delay between ultraviolet femtosecond pulse pairs for four alkali halide single crystals has been measured. Two-pulse correlation allows direct observation of solid state and surface dynamics on an ultrafast timescale. The ion yield from 265 nm irradiated NaBr, KCl, KBr, and KI depends critically on the time delay between the two sub-threshold pulses. Following irradiation of single crystal NaBr and KCl, the positive ion desorption yield displays three distinct features; a coherence peak, followed by rise, and decay features. In contrast, the yield of K+ from KBr displays only the coherence peak and picosecond decay features while the yield from KI shows only the coherence feature. The data suggest that although the nanosecond ion desorption mechanism may be dominated by defect photoabsorption, significant electron-hole pair production may contribute to the desorption mechanism following femtosecond excitation.
Recent results on the structure and luminescence enhancement of Eu2O3, EuS, and ZnS:Mn2+as well as photo-stimulated luminescence of Ag and AgI nanoparticles encapsulated in porous hosts are presented. Eu2O3 nanoparticles encapsulated in MCM-41 display different structures depending on the temperature used to form the guest-host material. Particles formed following heat treatment at 140° C show monoclinic structure with enhanced luminescence efficiency. This increased efficiency may result from a decrease in the radiative lifetime of the emitter within the host cavity. Similarly, EuS and ZnS:Mn2+ show increased luminescence when encapsulated in zeolite-Y. Ag and AgI nanoparticles encapsulated in zeolite-Y show significant photostimulated luminescence with very short lifetimes. The appearance of strong photostimulated luminescence with short decay times demonstrates that nanoparticles encapsulated in porous host materials have potential for digital storage and medical radiology applications.
We have used femtosecond laser pulse pairs to measure the positive ion yield, from wide band-gap single crystals, as a function of time-delay between pulses. Two pulse correlation allows direct observation of solid state and surface dynamics on an ultrafast timescale. The ion yield, from 265 nm irradiated MgO and KBr, depends critically on the time delay between two sub- threshold pulses. For example, the Mg+ desorption yield displays three distinct features; a coherence peak, followed by rise, and decay features. In constrast, the yield of K+ from KBr displays only the coherence peak and picosecond decay features. The data suggest, that although the nanosecond ion desorption mechanism is dominated by defect photoabsorption, significant electron-hole pair production may contribute to the desportion mechanism following femtosecond excitation. Nanosecond photoexcitation of KBr near 64 eV leads to desorption of hyperthermal neutral bromine atoms without a significant thermal velocity component. Two-photon femtosecond excitation at 3.2 eV produces very similar results. Multiphoton femtosecond excitation provides an efficient excitation mechanism of the wide-gap material. There results are likely general for ionic crystals and are consistent with a recently described theoretical model.
We have compared the desorption of positive ions, including Mg+ and MgO+, form ionic magnesium oxide single crystals following pulsed laser excitation using either nanosecond or femtosecond sources. Following optical excitation, desorbed ions are rapidly extracted and mass analyzed using standard time-of-flight techniques. Ion yields and velocities are determined as a function of laser fluence. The threshold similarity is a surprising result, as sub-band gap nanosecond pulses are only likely to excite defect states efficiently, while the ultrahigh peak-power femtosecond pulses could in principle induce multiphoton and avalanche excitation. We argue that at least in this specific case, the important factor appears to be merely the number of photons and not the pulse duration. However, it is observed that femtosecond excitation yields considerable H+ and less interference from impurity alkali ions than does nanosecond excitation. The source of the protons is presumably the hydroxylated MgO surface.
We report the first application of a multiphoton ionization based, mass selective implementation of psec time resolved rotational coherence spectroscopy (RCS) to molecular clusters. This implementation of rotational coherence spectroscopy retains all of the advantages of fluorescence based implementations of RCS and also allows determination of the moments of inertia of molecular species which cannot be size selected via excitation energy or which do not fluoresce. The method is described and rotational coherence transients obtained are presented.