Milan Milosevic

Advances in Optical Spectroscopy: The Ultra-Small Sample Analyzer

A new reflectance accessory has been developed for the spectroscopic analysis of physically small samples and small areas of large samples. This accessory, known as the “Split Pea” accessory (SPA), has several advantages over those currently used for microsampling, i.e., high-pressure diamond cells, microscopes, and beam condensers. The SPA enables nondestructive, internal reflectance studies of microgram and nanogram samples with little or no sample preparation. Its specially configured pressure plate permits the application of high clamping pressures between the sample and the internal reflection element, simplifying the analysis of hard surface solids such as paint chips. The SPA can also be configured for either external or in-line diffuse reflectance, depending on the sample under investigation. The spectra presented here demonstrate the versatility and wide range of applications for this new accessory.

Grazing angle attenuated total reflection spectroscopy: Fields at the interface and source of the enhancement

With the tremendous growth in the semiconductor and coatings industries, spectroscopic methods of examining extremely thin films on high refractive index substrates have become increasingly important. One infrared method for analyzing monolayers on substrates such as silicon and gold that has recently gained popularity is ‘grazing’ or high angle of incidence attenuated total reflection (ATR) spectroscopy. This paper investigates the directional electric field strengths and the extraordinary sensitivity achieved by using the grazing angle ATR method for analyzing monolayers on silicon substrates.

The Seagull®: A Multifunctional Variable-Angle Reflection Attachment

Variable-angle reflectance is an important spectroscopic technique. Certain samples such as opaque substances, films on opaque substrates, and films on liquids are tedious or practically impossible to analyze via conventional transmission spectroscopy equipment. The analyses of such samples by reflection spectroscopy, however, are straightforward.

On the nature of the evanescent wave.

This is an unusual paper in that it does not address a particular research topic or present a novel experimental method or a new theoretical result. This paper addresses our basic understanding of the nature of the evanescent wave, the wave that is the basis of the entire field of Attenuated Total Reflection (ATR) spectroscopy. I recently had the opportunity to reexamine the foundations of ATR spectroscopy and was surprised to have had to change my own mental picture of the evanescent wave that I have built over the last 25 years. Over the years I have had numerous discussions with a large number of workers in the field as well as with my former mentor, and one of the originators and the principal developer of ATR spectroscopy, the late N.J. Harrick. Everything brought up in all these discussions was perfectly consistent with my old mental picture of the evanescent wave. Thus, I believe that the picture of the evanescent wave that I had is virtually universally held by workers in the field. This paper describes the new picture of the evanescent wave that emerged from said reexamination process.

Using the Kramers-Kronig method to determine optical constants and evaluating its suitability as a linear transform for near-normal front-surface reflectance spectra

In this paper, the suitability of using the Kramers-Kronig transform to routinely extract optical constants from near-normal incidence reflectance spectra of solids and liquids is demonstrated. In addition, the possibility of utilizing the Kramers-Kronig transform as a linearizing transform for near-normal incidence reflectance spectra is investigated. Also, several commercial Kramers-Kronig software packages were utilized in determining the optical constants from the near-normal incidence reflectance of Plexiglas. Unexpectedly, the results produced by the various packages differed significantly. The near-normal reflectance of water was measured, the Kramers-Kronig transform was applied to extract the optical constants of water, and the result was compared to values found in the literature. Furthermore, the Kramers-Kronig transforms of near-normal incidence reflectance spectra of various concentrations of sugar in water were calculated to evaluate its use as a linearizing transform for quantitative applications.

The “Refractor”: A Refractive Grazing Incidence Specular Reflection Attachment

External Reflection Spectroscopy (ERS) is one of the six established spectroscopic techniques for recording optical spectra. In this technique, light—usually polarized—is specularly reflected from samples having a smooth surface. External reflection spectroscopy is particularly useful for recording spectra of thin films on metal surfaces. External reflection spectra of a thin (~ 500 A) of SiO2 film on an Al substrate are shown in Fig. 1. These spectra can be recorded only with light having parallel polarization and reflected from the substrate at or near Brewsters angle. Brewsters angle is approximately 89° and, hence, the grazing incidence, for highly conductive metals such as Al, Au, Ag, etc. It is impractical to try to attain such a high angle with conventional spectrometers where the angular light beam spread is typically ±5°. Furthermore, the light beam width striking the sample increases with both increase in angle of incidence (θ) and increase in angular beam spread (±φ). Because of this beam spread, external reflection spectroscopy, in spite of its sensitivity, is not recommended for probing small (micrometer size) areas of surfaces. Under the right conditions (viz., use of polarized light at angles near the Brewsters angle, as shown in Fig. 1), high sensitivity is attained. Because the reflectivity of Al is low (~70%) under these conditions, little is gained by employing many reflections. In most cases a single reflection is adequate—which also simplifies the instrumentation and limits the required size of the sample. Spectra of films less than one monolayer thick have been recorded via external reflection spectroscopy.

Spectroscopic method for measuring refractive index

A method for routine, but precise measurements of refractive index is described. The method is fast and accurate. It is based on the analysis of interference fringes, and uses positions of the fringe maxima and/or minima and a precise measurement of sample thickness to extract refractive index. An extremely dense dataset of refractive index values over the entire spectral range of interest can be routinely obtained.

Extracting infrared absolute reflectance from relative reflectance measurements.

Absolute reflectance measurements are valuable to the optics industry for development of new materials and optical coatings. Yet, absolute reflectance measurements are notoriously difficult to make. In this paper, we investigate the feasibility of extracting the absolute reflectance from a relative reflectance measurement using a reference material with known refractive index.