Robert I. Altkorn

Intensity Considerations in Liquid Core Optical Fiber Raman Spectroscopy

This paper develops expressions for the Raman power emitted by liquid core optical fiber (LCOF) sample cells in six simple excitation/collection geometries and the fraction of that power that can be utilized in a conventional Raman spectrometer. From these expressions a “figure of merit” is developed that can be used to predict the relative intensity enhancement provided by LCOFs having different inside diameters and loss characteristics. For the apparatus used here, we show theoretically and experimentally that the figure of merit takes the simple form (αd)−1 where α and d are, respectively, the loss coefficient and inside diameter of the LCOF. Given the varying optical quality of Teflon®-AF LCOFs currently in use, the analysis presented here should be useful in optimizing the performance of LCOF accessories in Raman and related spectroscopic applications.

Raman Performance Characteristics of Teflon®-AF 2400 Liquid-Core Optical-Fiber Sample Cells

Several properties of Teflon®-AF liquid-core optical-fiber (LCOF) sample cells relevant to their utility in analytical Raman spectroscopy have been investigated. These include the nature and extent of cladding-related background and its effect on signal-to-noise ratio, and the dependence of intensity enhancement on LCOF length, core diameter, excitation wavelength, and scattering wavelength. Significant improvements in signal-to-noise ratio relative to conventional sampling techniques were demonstrated in LCOFs filled with dilute (6 × 10−2 M and 6 × 10−4 M) aqueous solutions of phenylalanine. A maximum intensity enhancement factor of approximately 120 was measured by using 50 μm i.d. LCOFs filled with methanol and 532 nm excitation. Enhancement in 785 nm-excited spectra was found to vary with Raman shift in a manner consistent with self-absorption by high-overtone vibrational bands in the core liquid.

Waveguide Capillary Cell for Low-Refractive-Index Liquids

We describe a waveguide capillary cell based on a fused-silica tube coated externally with a thin layer of a low-refractive-index (n = 1.31) fluoropolymer. When filled with a transparent liquid of refractive index greater than that of the fluoropolymer, the cell is capable of transmitting light through total internal reflection. Loss below 1 dB/m is demonstrated throughout much of the visible region for a 530-μm-i.d., 660-μm-o.d. cell filled with water.

Spatially resolved surface enhanced Raman spectroscopy: feasibility, intensity dependence on sampling area and attomole mass sensitivity

Abstract Spatially resolved surface enhanced Raman scattering (SR SERS) from μm sized sampling areas has been observed in the electrochemical environment for the first time. Analysis of the equation for SR SERS intensity reveals that the detected signal should be area independent. This is demonstrated experimentally for the aqueous pyridine/Ag model system using 200 kW cm −2 peak laser irradiance. The mass detection limits for SR SERS are found to be on the order of 10 5 molecules or 0.17 attomoles!

Spatially resolved surface enhanced second harmonic generation: Theoretical and experimental evidence for electromagnetic enhancement in the near infrared on a laser microfabricated Pt surface

Surface enhanced second harmonic generation (SESHG) has been observed for the first time from a Pt surface in a <10−3 Torr vacuum environment. Both ‘‘smooth,’’ mechanically polished Pt surfaces and rippled Pt microstructures prepared by laser microchemical etching in Cl2(g) were studied with the newly developed technique of SESHG imaging. The etching procedure and the behavior of the Pt surface under the SESHG imaging conditions is reported in detail. The rippled/smooth enhancement factor for SHG excited with 80 ps, 1064 nm pulses from a cw, mode‐locked Nd‐YAG laser focused to a 3.1 μm 1/e2 radius ranges from 4 to 17, in qualitative agreement with theoretical calculations for isolated Pt spheroids.

Efficient Elimination of Fluorescence Background from Raman Spectra Collected in a Liquid Core Optical Fiber

Liquid core optical fiber (LCOF) waveguides have been used to enhance the Raman sensitivity of liquid sample measurements by more than two orders of magnitude. These waveguides can also enhance photobleaching of fluorescence in transparent liquids. We report the reduction of fluorescence intensity by more than two orders of magnitude by rapid photobleaching of solutions illuminated in an LCOF. The same solutions showed no detectable photobleaching when illuminated in a cuvette. The observed rates of photobleaching are in good quantitative agreement with theory.

Novel Lightpipes for Infrared Spectroscopy

The fabrication and temperature-dependent mid-infrared transmission characteristics of gold-on-nickel, silver-on-nickel, silica, and sapphire hollow lightpipes are discussed. The metal tubes offer improved low-temperature transmission and more versatile fabrication methods than do conventional FT-IR lightpipes. The dielectric tubes offer extremely low loss in certain spectral regions, outstanding high-temperature performance, and, in some cases, sufficiently low cost to be considered disposable.

Grazing incidence and multilayer x-ray optical systems

The development of x-ray optics for astrophysical investigations in the 40 - 100 keV energy range is extremely important. In this energy range, a focusing system is necessary to resolve crowded regions, to improve sensitivity, and to provide the deep sky images necessary to make the next great step forward in this field. Such a step was ably demonstrated by the Einstein and ROSAT observatories. These systems used grazing incidence optics, and, as is well known, the critical angle of reflectivity decreases linearly with energy for ordinary metal surfaces which adversely impacts on the design of a focusing system for higher energy x rays. At least 3 parameters are negatively affected: (1) the field of view is decreased; (2) the projected area of an individual mirror element is decreased; and, (3) the focal length for a fixed diameter system is increased. In order to counter these effects, mirrors coated with multilayers have been designed. It is theoretically possible to increase the grazing angle by coating the mirror surface with a graded d-spacing. The ability to produce a coated mirror with close to theoretical performance is, however, technically challenging. We describe our approach to the fabrication of a system designed for the 40 - 100 keV range that is based on electroforming technology. We also describe some of the general considerations that must be taken into account when fabricating a viable mirror.

In-situ sensors for intelligent process control for fabrication of polymer-matrix composite materials

The key to reducing processing costs, improving product yield, and optimizing material properties in composite materials fabrication is the use of intelligent process control (IPC). IPC completes the processing feedback loop by obtaining in situ sensor information, analyzing this information to make processing decisions, and then applying control parameters to the process in real-time. Information for IPC sensors must go beyond the traditional measurements of temperature and pressure, and include data such as resin viscocity, resin position within the mold, resin gelation-point, degree-of-cure of the composite, types of polymerization reactions taking place, presence of moisture, and the like. Two light-based sensors are described which have been utilized to obtain this information: (1) a novel infrared (IR) fiber-optic sensor, developed at BIRL, and (2) a commercially available index of refraction sensor. The information provided by these sensors relates directly to material performance. This paper also describes how neural networks were used to interpret the data collected by the IR fiber-optic sensor.

Replication as an alternative approach for large segmented telescopes

The next generation of optical/IR telescopes will require large numbers of co-phased segmented mirrors. Therefore, some form of replication technology is desirable to reduce costs. Electroforming has the advantage that it is a commercially developed technology for replication, and the technology has been widely used for making X-ray mirrors (e.g. XMM-Newton). Composite materials are appealing, since a great deal of development work as been done with composites as well. There are 3 areas that need to be addressed: replication with minimal stress so as to produce a high quality figure; attachment of support of the mirror segment so as to maintain the figure quality; and, thermal control requirements. Here we present a discussion of the requirements that lead us to select replication as the fabrication technology and the advantages of replication. We report on our first results of making a concave mirror and testing support methods of flats.