A Raman spectrometer technique is described that aims at suppressing the fluorescence background typical of Raman spectra. The sample is excited with a high power (65W), short (300ps) laser pulse and the time position of each of the Raman scattered photons with respect to the excitation is measured with a CMOS SPAD detector and an accurate time-to-digital converter at each spectral point. It is shown by means of measurements performed on an olive oil sample that the fluorescence background can be greatly suppressed if the sample response is recorded only for photons coinciding with the laser pulse. A further correction in the residual fluorescence baseline can be achieved using the measured fluorescence tails at each of the spectral points.
The confocal Raman technique is widely used for the depth profiling of thin transparent polymer films. Reported depth resolutions are on the order of two micrometers. The depth resolution is worsened and the actual measurement depth is changed by the use of metallurgical “dry” objectives. Also, if the sample is strongly light scattering, the measurement depth is reduced drastically. In this work, we demonstrate how these problems can be circumvented by using an immersion technique in confocal Raman depth profiling. In the method, two different immersion fluid layers and a cover glass, which separates the two fluid layers, are used. This configuration allows the fluid that is in contact with the sample to be selected with respect to the requirements dictated by the refractive index of the sample, sample–immersion fluid interaction, Raman spectra overlapping, or fluorescence quenching properties. The use of the immersion technique results in major improvements in the depth resolution and in the depth profiling capability of the confocal Raman technique when applied to strongly light scattering materials.
The nominal depth resolution achieved in confocal Raman microscopy is on the order of a few micrometers. Often, however, the depth resolution is decreased by light refraction at the sample surface. The problem can be avoided with the use of an immersion objective and index matching oils. Through this intervention the instrument point-spread function (PSF) can be assumed to be independent of the depth of focus in the sample, and spatially invariant depth profiles can be acquired. In this work the instrument PSF was determined by measuring a depth profile of a thick uniform sample and calculating the first derivative of the depth profile curve. The first-derivative method was also used to determine sample thickness. Convolution with the PSF makes it possible to simulate the behavior of the instrument with different sample functions. It is also possible to use the instrument PSF to deconvolve depth-profiling data. Deconvolution reduces the blurring effect of the instrument and increases the depth resolution. Deconvolution can also be used in analysis of the sample surface position and in layer structure analysis. In this paper we show how the convolution integral can be used with the immersion sampling technique to determine the PSF and how the sample thickness can be determined.
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