The combination of resonance Raman with deep UV excitation, DUVRR, gives greater selectivity and eliminates background fluorescence, enabling sensitive detection of UV absorbing nucleotide bases and amino acids. We demonstrate this combination with our 3D nanopore structure design. Resonance Raman is specific to a molecule absorbing at the excitation, while plasmon resonance of a small, shape-, index- and size- tuned metal dramatically increases the electric field strength in the active region. The 3D nanostructure exploits nanopores that retain the advantages of small-gap antennas but increases the ease of fabrication, availability, and detection volume compared to conventional plasmon-based designs, such as gaps between two particles, by being inherently single particle, with edge enhancement open to diffusion, and by possessing a large number of pores per particle. We show the large local field enhancement (hot spots) of the pores. Comparisons with an Al and silica coated/uncoated microsphere template with/without nanopores clearly show a significant blue shift of the 280 nm peak to (the more useful) 265 nm, in the presence of a hollow sphere with nanopores. Raman measurement of Tryptophan on an aluminum nanopore structure with excitation from our tunable OPO system in the visible and deep UV region indicate visible excitation causes more fluorescence and is less specific for the tryptophan, even displaying a Raman peak at the silicon substrate, while the deep-UV Raman spectra, at an energy close to the nanopore resonance, shows no substrate signal and peaks with close correlation to the known tryptophan vibrations.
Resonance Raman offers a significant increase in Raman signal levels. We show how this can be used to select a specific molecule within a complex biosystem to study, in our case to determine if hemoglobin survives in ancient fossils. Key to this ability is the fact that the vibration must be on the same molecule as the absorption. Further, we show that the Raman fingerprint, or changes to it, can provide further selectivity or identify changes in that molecule based upon the particular sample. In our case, we find that the iron in the hemoglobin has oxidized into FeOOH, but still attached to both its porphyrin-like heme group and the protein network that gives the hemoglobin absorption. Very narrow Raman resonances are found in molecules with symmetry-forbidden, phonon-allowed absorptions. We show several in biologically relevant materials including that methylated-DNA (m-DNA) can be distinguished from non-methylated (n-DNA) with nano-bowtie- and resonance-enhanced Raman spectra. These efiects are retained when plasmon resonances are used to enhance a local region of the sample, but find that the overall signal from a uniformly distributed specimen is not increased significantly by the enhancement of a small region, so is not recommended unless the sample can be concentrated into that region.
Methylation in DNA is a controlling factor in gene expression, embryonic development, and has been found to be important in infections and cancer. From a basic biology point of view, great heterogeneity has been found in methylation levels within tissues, so questions arises as to how and why. We show that methylated-DNA (m-DNA) can be distinguished from non-methylated (n-DNA) with nano-bowtie- and resonance- enhanced Raman spectra. By tuning the bowtie antenna to the resonance wavelength, both gains can be realized. Two additional Raman peaks in the 1200 – 1700 cm-1 band appear with methylation: one at 1239 cm<sup>-1</sup> and the other at 1639 cm<sub>-1</sub>; a weak peak near 1000 cm-1 also appears with methylation. We also find that the two spectral features, although the latter with slight modification, can be used to distinguish the methylation state even when the DNA is denatured, as we show when we induce crystallization of the salts in the solution with increased excitation power, or allow it to happen naturally via solvent evaporation, and the DNA is trapped within the salt crystals. A comparison between liquid/solution to dried/denatured state m-DNA shows a general broadening of the larger lines and a transfer of spectral weight from the ~1470 cm<sup>-1</sup> vibration to two higher energy lines. The applicability of the resonance-Raman in these spectra is shown by demonstrating that the Raman spectral characteristics hardly change as the Raman resonance in excitation wavelength is approached. Finally, we comment on real signal gain in this double-resonance system.
The discovery of soft structures in dinosaur bone with the morphological and molecular characteristics of blood vessels in extant vertebrates was both surprising and controversial. Mounting evidence suggests that these soft tissues are blood vessels, their preservation driven in part by reactive oxygen species derived from hemoglobin degradation. More data are needed to support this hypothesis. Raman spectroscopy, and resonance Raman in particular, can provide detailed information as to the chemical makeup of these samples. We used two different excitation wavelengths in microscale Raman measurements to look for lines characteristic of degraded heme molecules, both in ancient vessels and modern analogues taken from semi-fossilized, hemoglobin-soaked ostrich bones. In both samples, we observed two regimes: dark colored, stiff regions and more transparent, elastic regions. We discovered that the two apparent regimes in the samples had different strengths of Raman returns, and that resonance effects greatly affected the Raman intensity. In all cases, there was some evidence of degraded heme spectra, though the increased returns indicated that the dark regimes had reacted more strongly with the heme specie. The modern vessels displayed a resonance Raman intensity consistent with hemoglobin molecular structures, which indicated resonance spectra would provide understanding of the ancient heme molecule. To investigate the two regimes more thoroughly, we acquired Raman spectra over areas where the sample transitioned from one regime to another. Variable wavelength resonance Raman measurements over the whole sample were used to give more information about the heme species present, in both ancient and modern samples.
The major unknown in the global climate radiation balance calculations is the effect of aerosols. The extinction of aerosols depends upon the wavelength, size, concentration, composition, and to a lesser extent, shape of the aerosols. Thus, methods are needed to determine and model these quantities. The size distribution of larger aerosols can be monitored with multistatic lidar, at least in the spherical approximation. We can use this approximation in humid environments, and for old desert dusts in which the aspect ratio is typically below two. Aerosols that are small compared to the incident wavelength present a Rayleigh-like scattering dependence, and the size cannot be determined using multistatic lidar techniques. We discuss the analysis of true extinction from Raman lidar measurements at several wavelengths for determining the size distribution of aerosols. The Angstrom ratio, which is the natural log of the extinction ratio divided by the natural log of the wavelength ratio, has been used in column-integrated measurements to classify aerosols. Lidar backscatter Angstrom ratio measurements have also been used to classify aerosols as a function of range. However, the use for aerosol size distribution has not been investigated in detail before this work. We find, from Raman lidar measurements, Mie models of extinction and backscatter Angstrom ratios, that small aerosols make a significant contribution to optical scattering, and find that size information can be extracted from the lidar data.
Raman lidar measurements provide profiles of several different tracers of spatial and temporal variations, which are excellent signatures for studies of dynamical processes in the atmosphere. An examination of Raman lidar data collected during the last four decades clearly show signatures of atmospheric planetary waves, gravity waves, low-level jets, weather fronts, turbulence from wind shear at surfaces and at the interface of the boundary layer with the free troposphere. Water vapor profiles are found to be important as a tracer of the sources of turbulence eddies associated with thermal convection, pressure waves, and wind shears, which result from surface heating, winds, weather systems, orographic forcing, and regions of reduced atmospheric stability. Examples of these processes are selected to show the influence of turbulence on profiles of atmospheric properties. Turbulence eddies generated in the wind shear region near the top of the boundary layer are found to mix into the atmospheric boundary layer. Results from several prior research projects are examined to gain a better understanding of processes impacting optical propagation through the many sources of turbulence observed in the lower atmosphere. Advances in lasers, detectors, and particularly in high-speed electronics now available are expected to provide important opportunities to improve our understanding of the formation processes, as well as for tracking of the sources and dissipation of turbulence eddies.
The Raman scattering of several liquids and solid materials has been investigated near the deep ultraviolet absorption features corresponding to the electron energy states of the chemical species present. It is found to provide significant enhancement, but is always accompanied by absorption due to that or other species along the path. We investigate this trade-off for water vapor, although the results for liquid water and ice will be quantitatively very similar. An optical parametric oscillator (OPO) was pumped by the third harmonic of a Nd:YAG laser, and the output frequency doubled to generate a tunable excitation beam in the 215-600 nm range. We use the tunable laser excitation beam to investigate pre-resonance and resonance Raman spectroscopy near an absorption band of ice. A significant enhancement in the Raman signal was observed. The A-term of the Raman scattering tensor, which describes the pre-resonant enhancement of the spectra, is also used to find the primary observed intensities as a function of incident beam energy, although a wide resonance structure near the final-state-effect related absorption in ice is also found. The results suggest that use of pre-resonant or resonant Raman LIDAR could increase the sensitivity to improve spatial and temporal resolution of atmospheric water vapor measurements. However, these shorter wavelengths also exhibit higher ozone absorption. These opposing effects are modeled using MODTRAN for several configurations relevant for studies of boundary layer water and in the vicinity of clouds. Such data could be used in studies of the measurement of energy flow at the water-air and cloud-air interface, and may help with understanding some of the major uncertainties in current global climate models.
Ground based lidar techniques using Raleigh and Raman scattering, differential absorption (DIAL), and supercontinuum sources are capable of providing unique signatures to study dynamical processes in the lower atmosphere. The most useful profile signatures of dynamics in the lower atmosphere are available in profiles of time sequences of water vapor and aerosol optical extinction obtained with Raman and DIAL lidars. Water vapor profiles are used to study the scales and motions of daytime convection cells, residual layer bursts into the planetary boundary layer (PBL), variations in height of the PBL layer, cloud formation and dissipation, scale sizes of gravity waves, turbulent eddies, as well as to study the seldom observed phenomena of Brunt–Väisälä oscillations and undular bore waves. Aerosol optical extinction profiles from Raman lidar provide another tracer of dynamics and motion using sequential profiles atmospheric aerosol extinction, where the aerosol distribution is controlled by dynamic, thermodynamic, and photochemical processes. Raman lidar profiles of temperature describe the stability of the lower atmosphere and measure structure features. Rayleigh lidar can provide backscatter profiles of aerosols in the troposphere, and temperature profiles in the stratosphere and mesosphere, where large gravity waves, stratospheric clouds, and noctilucent clouds are observed. Examples of several dynamical features are selected to illustrate interesting processes observed with Raman lidar. Lidar experiments add to our understanding of physical processes that modify atmospheric structure, initiate turbulence and waves, and describe the relationships between energy sources, atmospheric stability parameters, and the observed dynamics.
We have measured UV resonance Raman near and at the resonance phonon-allowed absorption lines of several liquid species. Resonance absorption with excitation on the symmetry-forbidden but strongly phonon coupled bands in the 230- 290 nm spectral band present enhancement corresponding to the vapor phase absorptions rather than those of the liquid phase. This effect is related to the coherence forced by the internal molecular resonance required to absorb light at this energy. Large resonance gains (~3500x) reflect the narrower vapor phase lines. At the low laser fluence used, bubble formation is observed when the excitation energy corresponds to the maximum in Raman signal generation, not at the wavelength of maximum absorption in the liquid sample, which is several nanometers away.
We study systems in which the resonance Raman process is fast due to the requirement for phonon involvement in the
absorption. The resonance enhancement is found to track the isolated molecule, or vapor phase, absorption since the
molecule does not have time to exchange energy with its neighbors. This corroborates with studies of pre-resonance,
where Heisenberg’s uncertainty principle enforces a rapid process, but differs from resonance on electronically allowed
transitions, where the resonance allows a relatively prolonged interaction. High resolution excitation spectroscopy
reveals large gains and narrow features usually associated with the isolated molecule. Vibration energies shift as the
resonance is approached and the excited state vibration levels are probed. Several multiplets and overtone modes are
enhanced along with the strongly coupled ring-breathing mode in aromatic molecules.
A review of current lidar techniques summarizes present capabilities to: (1) measure atmospheric concentrations of most major and several minor molecular species using Raman scattering and DIAL techniques, (2) detect and measure concentrations of certain trace level species, (3) characterize active dynamical processes in the troposphere based upon using water vapor as a tracer, and (4) describe interesting thermodynamic properties based upon rotational Raman temperature profiles, multi-wavelength aerosol distributions, and changes in the phase states of water. Advances in lasers and detectors have extended the range of wavelengths available through the ultraviolet, visible, and infrared spectrum by using tunable laser techniques and supercontinuum broad spectrum lasers. Prior studies are reviewed, several applications for the technology are suggested which extend the techniques proposed to future investigations. In particular, the extension of tunable laser sources into the ultraviolet region has opened opportunities to use resonance Raman techniques, which provide greatly increased sensitivity for certain molecular species, such as hydrocarbons. The developments of supercontinuum lasers and tunable OPO lasers has enabled long-path trace concentration measurements of molecular spectra lines to detect and measure the concentrations of many species, as well as to distinguish any interfering species.
Identification of atmospheric aerosol species and their chemical composition may help to trace their source and better estimate their impact on climate and environment. Optical scattering of aerosols depends primarily on aerosol chemical composition, size distribution, particle shape and the wavelength used. Extraction of features due to the aerosol complex refractive index from scattering spectroscopy at a single angle of observation allows composition identification via the spectral fingerprint, as shown computationally with Mie calculations of the optical scattering. Size-dependent scattering effects are eliminated by using near-forward scattering, such as in the scattering aureole. The only features of the aerosol aureole scattering spectra that very rapidly with wavelength are associated with the composition, so the aureole can give a reliable identification of aerosol composition.
We have measured UV resonance Raman scattering at and near the resonance absorption lines of liquid benzene and toluene. Resonance occurs for excitation on the symmetry-forbidden but strongly phonon coupled states in the <sup>1</sup>B<sub>2u</sub> band, ~230-270 nm, resulting in enhancements corresponding to the vapor phase absorptions rather than those of the liquid phase. This effect is related to the coherence forced by the internal molecular resonance required to absorb light at this energy. The resonance gains (~1000x) are larger than expected due to the narrower vapor phase lines. Several multiplet and overtone modes are enhanced along with the strongly coupled ring-breathing mode. A contrasting case of resonance Raman of ice is also discussed; in this case resonance is observed for excitation energy corresponding to absorptions that depend upon the final state shielding by the neighbors, and corresponds with the solid phase absorption. This typifies the more common, slow, time dependence of the resonance Raman process.
The optical scattering from laser beams propagating through atmospheric aerosols has been shown to be very useful in
describing air pollution aerosol properties. This research explores and extends that capability to particulate matter. The
optical properties of Arizona Road Dust (ARD) samples are measured in a chamber that simulates the particle dispersal
of dust aerosols in the atmospheric environment. Visible, near infrared, and long wave infrared lasers are used. Optical
scattering measurements show the expected dependence of laser wavelength and particle size on the extinction of laser
beams. The extinction at long wavelengths demonstrates reduced scattering, but chemical absorption of dust species
must be considered. The extinction and depolarization of laser wavelengths interacting with several size cuts of ARD are
examined. The measurements include studies of different size distributions, and their evolution over time is recorded by
an Aerodynamic Particle Sizer. We analyze the size-dependent extinction and depolarization of ARD. We present a
method of predicting extinction for an arbitrary ARD size distribution. These studies provide new insights for
understanding the optical propagation of laser beams through airborne particulate matter.
Aerosol optical scattering experiments are often large, expensive, and provide poor control of dust uniformity and size
distribution. The size distribution of such suspended atmospheric aerosols varies rapidly in time, since larger particles
settle quickly. Even in large chambers, 10 micron particles settle in tens of seconds. We describe lab-scale experiments
with stable particle distributions. A viscous colloidal solution can stabilize the particles for sufficient time to measure
optical scattering properties. Colloids with different concentrations or size distributions enable nearly time independent
studies of prepared distributions. We perform laser aureole scattering from a colloid containing a few percent by volume
of Arizona Road Dust (ARD) in mineral oil and glycerin, and 1-micron polystyrene spheres in water. We discuss aureole
analysis, the differences expected in scattering properties due to the index of refraction of the mineral oil medium versus
air, and the impact of non-spherical shape on the scattering. This research demonstrates that particles suspended in a
viscous medium can be used to simulate aerosol optical scattering in air, while enabling signal averaging, offering
reproducibility, and easing problems resulting from parameter variations in studies of dust properties.
Lidar is a powerful tool for measuring the vertical profiles of aerosols. Dusts are irregularly-shaped particles with varied
composition and strong index of refraction variations in the LWIR. We measure dust indices using ellipsometry and
transmission through KBr pellets. Milling makes the ellipsometry data less dependent on incidence angle, and the results
of measurements on milled materials agree with those from transmission measurements. Measurements show that the
spectrum of a milled Arizona Road Dust (ARD) approaches that of pure quartz, indicating a decrease of absorption
efficiency for particles larger than the absorption length. These indices of refraction will be used in the future to simulate
extinction for the beam of a LWIR lidar.
Lidar is a powerful tool for measuring the vertical profiles of aerosols in the atmosphere using Rayleigh and Raman lidar
techniques. Bistatic lidar can be used to obtain the angular structure of the scattered light. When the aerosols are
uniformly distributed, this information can be analyzed to provide particle size distribution information. However, dusts
tend to be irregularly shaped particles with varied composition. We investigate the impact of the irregular shape using
optical scattering at several wavelengths, scanning electron microscopy, and T-matrix calculations. In particular, we
study the rapid loss of Mie scattering resonances as the particle shape departs from spherical. Different size distributions
produced by different size-cuts of Arizona Road Dust (ARD) are studied.
Several laser remote sensing techniques are used to characterize the properties of aerosols. The various techniques
include: backscatter, optical extinction using Raman scatter, and bistatic/multistatic scattering using the polarization
ratio of the scattering phase function. The number density, size, and size distribution are obtained under the
assumption of spherical scatterers. Other measurements can be used to describe additional properties, such as
aerosol type based upon approximate refractive index and detected departure from spherical, when simultaneous
measurements at several wavelengths and several angles are analyzed. Examples are shown to demonstrate our
present capability to characterize aerosol particles using recently developed techniques.
Raman scattering techniques have long been used as unique identifiers for spectral fingerprints of chemical and
biological species. Raman lidar has been utilized on a routine basis to remotely measure several constituents in the
atmosphere. While Raman scattering is very reliable in uniquely identifying molecules, it suffers from very small
scattering cross sections that diminish its usefulness at increased ranges and decreased concentrations of the species of
interest. By utilizing a resonance Raman technique, where the laser excitation is tuned near an electronic absorption
band, it is possible to increase the Raman scattering cross section. An optical parametric oscillator (OPO) with a UV
tuning range of ~220 nm - 355 nm has been utilized to explore the wavelength dependence of Raman scattering for
diamond, water, benzene, and toluene. Resonance enhancements of the Raman spectra have been studied.
This paper presents a new method that exploits the interference and polarization properties of light to monitor, in real time, the rapid thermal elongation of near-field optical probes. The typically flat (nanometer in size) morphology of the probe apex serves as one mirror of a Fabry-Perot type cavity; a flat semitransparent metal coated surface constitutes the other mirror. The optical-interferometry set-up permits distance acquisition with a high frequency bandwidth (compared to other methods based on electronic feedback) while control of the light polarization allows an increase of the signal to noise ratio of the measurements.
We have performed Raman spectroscopy using a near-field scanning optical microscope. The small sample volume coupled with the light-starved nature of the Raman effect makes nano-Raman studies difficult. We present results showing near-field effects in an investigation of Rb-doped KTP. These effects include a change in selection rules due to the presence of a z-polarization component in the near-field, a surface-enhancement effect in near-field Raman data, a reduced Rayleigh tail, and simultaneous topography with the near-field probe. An image taken within a Raman feature demonstrates that nano-Raman imaging is indeed possible if the near-field instrument has considerable long-term stability.
In measurements of sample temporal response with a near-field scanning optical microscope, or NSOM, one must account for the temporal response of the probe. The coupling of thermal and temporal effects in an NSOM fitted with a coated tapered fiber probe is considered. Study of the perturbation of cw infrared light by a pulse of visible light simultaneously sent through an illumination mode NSOM allows one to separate the relatively slow thermal response of the probe from the appreciably faster response of a silicon sample imaged with the probe. Temporal and thermal contrast in NSOM imaging are discussed in terms of the results.
KEYWORDS: Infrared detectors, Semiconductors, Infrared imaging, Visible radiation, Silicon, Near field scanning optical microscopy, Infrared radiation, Semiconducting wafers, Signal detection, Near field optics
We demonstrate the ability of near-field scanning optical microscopy (NSOM) technique to detect inhomogeneities of the dynamics of excess carriers in oxidized silicon wafers. NSOM is used to improve the spatial resolution of a standard IR-scattering optical technique, which is carried out in a noncontact fashion. Continuous wave infrared light is used as a detector of the time dependent carrier population produced by a pulsed visible laser. We will show high resolution images of carrier lifetime, and discuss some aspects of the NSOM measurement that differentiate it from its far field counterpart.
A high resolution scanning Hall probe microscope is used to spatially resolve vortices in high temperature superconducting Bi<SUB>2</SUB>Sr<SUB>2</SUB>CaCu<SUB>2</SUB>O<SUB>8+(delta)</SUB> crystals. We observe a partially ordered vortex lattice at several different applied magnetic fields and temperatures. At higher temperatures, a limited amount of vortex re-arrangement is observed, but most vortices remain fixed for periods long compared to the imaging time of several hours even at temperatures as high as 75 degree(s)K (the superconducting transition temperature for these crystals is approximately 84 degree(s)K). A measure of these local magnetic penetration depth can be obtained from a fit to the surface field of several neighboring vortices, and has been measured as a function of temperature. In particular, we have measured the zero temperature penetration depth and found it to be 275 +/- 40 nm.