Development of a scanning angle total internal reflection Raman spectrometer

Abstract

A scanning angle total internal reflection (SATIR) Raman spectrometer has been developed for measuring interfacial phenomena with chemical specificity and high axial resolution perpendicular to the interface. The instrument platform is an inverted optical microscope with added automated variable angle optics to control the angle of an incident laser on a prism/sample interface. These optics include two motorized translation stages, the first containing a focusing lens and the second a variable angle galvanometer mirror. The movement of all instrument components is coordinated to ensure that the same sample location and area are probed at each angle. At angles greater than the critical angle, an evanescent wave capable of producing Raman scatter is generated in the sample. The Raman scatter is collected by a microscope objective and directed to a dispersive spectrometer and charge-coupled device detector. In addition to the collected Raman scatter, light reflected from the prism/sample interface is collected to provide calibration parameters that enable modeling the distance over which the Raman scatter is collected for depth profiling measurements. The developed instrument has an incident angle range of 25.5 degrees-75.5 degrees, with a 0.05 degrees angle resolution. Raman scatter can be collected from a ZnSe/organic interface over a range of roughly 35-180 nm. Far from the critical angle, the achieved axial resolution perpendicular to the focal plane is approximately 34 nm. This is roughly a 30-fold improvement relative to confocal Raman microscopy.

Scanning Angle Total Internal Reflection Raman Spectroscopy of Thin Polymer Films

“…In a standard PWR Raman spectroscopy experiment the illuminating light is directed onto a prism/metal/dielectric/bulk interface at a specific incident angle. Raman scatter and reflectivity data are collected simultaneously for each incident angle above the critical angle for total internal reflection [3]. Conventional Raman spectroscopy, when the illuminating light is not under total internal reflection, suffers in studying interfaces and thin films due to a lack of interfacial selectivity and the small signals that result from reduced analysis volumes.…”

Optical-based spectroscopic methods for measuring chemical, optical, and physical properties of thin polymer waveguide films

“…It consists of all the parts described in detail above; however, the excitation source is coupled into the microscope for epi-illumination or into an optical fiber for scanning angle Raman 97 or the excitation source can be coupled to a fiber optic which is mounted on a rotational stage (as shown in Figure 3B). …”

Total internal reflection Raman spectroscopy

“…90 It should be noted that this latter study mostly exploited the position of waveguide modes below the critical angle, rather than the changing penetration depth above the critical angle. A recent development is the production of a scanning angle Raman spectrometer, 91 allowing greater automation of the measurement of Raman spectra at a range of angles. For a homogeneous benzonitrile sample the measured Raman signal agreed well with the expected angular variation of signal, providing the angular spread of the converging incident light was accounted for.…”