The Fundamentals of Raman Polarization Microscopy

This article focuses on the fundamental mechanism behind polarized Raman measurements. Polarized Raman spectroscopy offers important data about a sample’s molecular structure and also about its inherent functioning as an effective probe to study orientation in carbon nanotubes, oriented polymers, crystals, and other highly ordered systems. In the past Raman point measurements were typically carried out on bulk liquids and solids. Here, the polarized Raman measurements elucidated were obtained on a Thermo Scientific™ DXR™2 Raman microscope and a Thermo Scientific™ DXR™2xi Raman imaging microscope.

With the help of the DXR2 Raman microscope (Figure 1), polarized Raman point measurements can be carried out on features of micron sizes. The DXR2xi Raman imaging microscope is the newest fast imaging Raman microscope, making it possible to extend polarized Raman point measurements to polarized Raman images with submicron spatial resolution across large areas. This article presents results of polarized Raman point measurements performed on isotropic and anisotropic materials.

Figure 1. Thermo Scientific DXR2 and DXR2xi Raman microscopes.

Spectroscopy is capable of determining light-matter interactions. Under standard circumstances, electromagnetic vectors in light rotate freely as light propagates. When matter is impinged with light, the resulting interactions are attributed to the summation of a mixture of electromagnetic polarization states. Light traveling via a polarizing optic is converted to a polarized beam, leading to propagation of the electromagnetic vector along a single plane or axis. This phenomenon is referred to as linearly polarized light, as opposed to depolarized or freely rotating light.

On exposing a material to linearly polarized light, the material may interact in a different way to the way it interacts to depolarized light, based on how the material is directed in relation to the polarized light axis. These differences in orientation provide more data. Polarization measurements are even utilized to determine stress in materials, including cast materials, doped silicon structures, and stretched polymer films.

In a standard polarization experiment, linearly polarized light is used to interrogate a sample. This is followed by collecting a spectrum with light polarized along a single axis or plane and then collecting a second spectrum with the linearly polarized light oriented in a perpendicular direction to the initial axis.

For these measurements, different terms are utilized, but they all mean the same thing: perpendicular and parallel, horizontal and vertical, 0 and 90 degrees, X and Y, and so on. In this article, basic measurements are discussed, although comprehensive polarization studies are also possible.

For polarization measurements, Raman spectroscopy presents a suitable method as the lasers utilized for excitation are linearly polarized by nature. Natural polarization can be beneficial, however if laser polarization is not considered then Raman results can be rather misleading. Due to this, the laser excitation beam is deliberately depolarized by general purpose instruments to provide uniform results, independent of sample orientation. Depolarized Raman spectra are referred to spectra that use depolarized laser excitation.

DXR2 and DXR2xi Raman microscopes are sophisticated Raman instruments that can be configured to offer polarized as well as depolarized Raman measurements. Through an intuitive software interface, measurements can be easily set up for polarized or depolarized Raman on a sample.

Porto Notation

Whenever polarization measurements are made, the polarization conditions should be monitored so that the outcomes can be interpreted consistently. Porto notation provides a way to record sample orientation with regard to outgoing, post sample, analyzer orientation and incident light polarization. The notation includes four terms as follows:

  a (b c) d

where:

• a represents the direction of propagation of the incident laser
• b is the direction of polarization of the incident laser
• c stands for the direction of polarization of the scattered Raman
• d refers to the direction of propagation of the scattered Raman.

The terms beyond the parentheses describe the propagation direction of the outgoing Raman scattering path and the incident laser beam path. The terms within the parentheses define the direction of polarization of the two light paths.

A 180 degree backscatter configuration is used by the DXR2 and DXR2xi Raman microscopes, where the microscope objective directs the laser onto the sample and at the same time collects the Raman scattering. It is observed that the propagation axes are same, but with a varied direction. An overbar designates this opposite direction. Figure 2 shows the orientation of polarization axes in the DXR2 Raman microscope at the sample as well as the corresponding Porto notation for non-oriented samples.

Figure 2. Orientation of DXR2xi Raman microscope polarization axes at the sample and the corresponding Porto notation for non-oriented samples.

The Cartesian coordinate system, as shown in Figure 2 inset right, reveals the axis notation employed by the Thermo Scientific™ OMNIC™ xi software to choose polarization optic for laser excitation as well as post sample analyzer. Laser excitation arising from the microscope objective travels along the z-axis. Using the same microscope objective, Raman scattering is collected along the same z-axis, but in the reverse direction. This approach is termed 180-degree backscattering configuration, and the required convention is to utilize an overbar on the z-axis notation (Z) to designate the opposite direction.

The Horizontal axis or x-axis or 0-degree polarization axis is parallel to the front edge of the stage going left-to-right, and the Vertical or y-axis or 90-degree polarization axis is parallel to the edge of the stage (front-to-back) and perpendicular to the horizontal x-axis.

Experimental Procedure

Experiments were carried out on a DXR2xi or DXR2 Raman microscope. Using a fully automated polarization option, each instrument was suitably configured. This option makes it possible to operate the instrument in a number of different modes: depolarized Raman, polarized Raman, and angle resolved polarized Raman.

Results and Discussion

Isotropic and Anisotropic Samples

All materials are not sensitive to the orientation of polarized light. Isotropic refers to the sample’s properties that are insensitive to sample orientation. Powders, liquids, and randomly oriented polymers are some of the examples of Isotropic samples. Despite being insensitive to orientation, these samples can still interact with polarized light and allow the rotation of the polarization of the scattered Raman beam corresponding to the incident laser polarization.

Strained films and crystals are extremely sensitive to the orientation of sample and are known as anisotropic. Anisotropic or oriented samples possess an optical axis or axis of symmetry. The alignment of this axis with respect to the incident laser polarization can lead to different spectra.

Just two polarization measurements can be made in the case of isotropic samples: analyzer aligned parallel to the laser polarization axis, Z(XX)Z or Z(YY)Z, and analyzer perpendicular to the laser polarization, Z(XY)Z or Z(YX)Z. In the case of anisotropic or oriented samples such as fibers, stretched films, and crystals, the alignment of the sample axis with the instrument polarization axis should be documented.

The convention is to utilize the sample’s coordinate system for the Porto notation instead of the instrument’s coordinate system. Through this convention, researchers can align the oriented sample on another instrument, and eventually determine the same polarization results. If sample properties are not known or if sample axis is not identified, then the sample’s micrograph can be captured to record its orientation. With the help of a rotatable sample holder, a known axis of the sample can be easily aligned with the instrument polarization axis.

The following are four polarization tensors that can be examined with a Raman microscope.

• Z ( X X ) Z — laser polarized parallel to an x-axis; analyzer set to pass x-axis polarized light
• Z ( Y Y ) Z — laser polarized parallel to a y-axis; analyzer set to pass y-axis polarized light
• Z ( X Y ) Z — laser polarized parallel to an x-axis; analyzer set to pass y-axis polarized light
• Z ( Y X ) Z — laser polarized parallel to a y-axis; analyzer set to pass x-axis polarized light

For unknown or anisotropic samples, all four of these measurements should be made. It would also be useful to determine the depolarized spectrum to fully document the sample.

Raman Polarization for Isotropic Samples

Carbon tetrachloride (CCl₄) is a standard example for Raman polarization measurements. It is an isotropic liquid with bands at 218, 314, and 459 cm^-1 Raman shift. The band at 459 cm^-1 is the result of the symmetric vibrational mode A1 and is designated as totally polarized, which means it produces Raman scattering in the same polarization orientation as the incident light. The highest Raman scattering is seen on aligning the analyzer with the laser polarization, and the intensity of the linearly polarized Raman scattering is quenched when the analyzer is crossed. Depolarization ratio is referred to the ratio of the crossed intensity divided by the aligned intensity and will approach zero. In contrast to the band at 459 cm^-1, the 314 and 218 cm^-1 bands are not as sensitive to polarization.

Depolarized bands possess a ratio Z(XY)Z /Z(XX)Z of 0.75. This data can be leveraged to establish which bands are depolarized within the sample spectrum. Figure 3 depicts an example of how to measure the depolarization ratios from the four polarized Raman spectra of CCl₄.

Figure 3. Example showing the calculation of depolarization ratio (ρ) from spectra of CCl4 collected on a DXR2xi Raman imaging microscope with 785 nm laser excitation. Values are baseline corrected peak area measurements.

Although the ratio can be based on a single pair of measurements (XY/XX) or (YX/YY), it would be useful to determine both and utilize the average. This reduces any bias which may be caused by the polarization optics because of the difference between instruments.

Raman Polarization for Anisotropic Solids

Detecting spectral features relevant to a target sample property is one of the major challenges in spectroscopic analysis. For this type of application, Raman polarization can be quite useful. For instance, the depolarization ratio for a spectral peak could suggest a particular sample morphology, and this would not have been evident from the depolarized Raman spectra alone.

LiNbO₃ Example

CCl₄ was used as an example for isotropic Raman polarization in the above section. Lithium niobate (LiNbO₃) is utilized as an example of an anisotropic sample with known crystallographic symmetry. A Cartesian coordinate system is employed to define the LiNbO₃ crystal with the z-axis kept parallel to the optical c axis. Figure 4 shows this vertical axis.

Figure 4. Cartesian coordinate system of a LiNbO₃ crystal showing surfaces planes when cut perpendicular to each axis and a slide with LiNbO₃ oriented in two different directions.

In the LiNbO₃ example, the outer terms of the Porto notation are parallel to the crystal x-axis, and the inner terms denote the polarized light orientation in terms of the crystal axes. Mounting the chip with the long axis front-to-back on the sample stage and setting the laser polarization to Vertical, the initial inner term will be Z, suggesting that the polarization of the incident light is aligned with the crystal z-axis. For the second inner term, a value of Y means that the analyzer polarizer is either perpendicular to the polarization of the incident light or oriented along the crystal y-axis.

X (Z Y)Z

Eight polarized LiNbO₃ spectra can be collected (Figure 5), if the x cut LiNbO₃ sample is sequentially oriented along the x and y axes and the four combinations of laser and analyzer polarizer positions are determined for individual orientations.

Figure 5. Polarized Raman spectra of LiNbO₃ samples mounted on the slide vertically (left side with_vert in title) and horizontal (right) with their Porto notation.

It should be noted that the spectra with the same Porto notation are similar, but they posses different polarizer optics settings and physical orientation. By plotting the spectra on the same vertical axis scale and offset (Figure 6), depolarized peaks can be observed as was done for CCl₄.

Figure 6. LiNbO₃ spectra plotted as cross-polarized pairs. All spectra are on the same Y-axis scale and offset for comparison. Spectra were measured over the full range (50–3300 cm^-1 Raman shift); the only bands observed are the ones shown.

Different polarizations emit a set of spectral features, with all bands remaining sensitive to polarization. In this case, the bands can be designated to particular Raman polarization tensors as a known sample and crystal axis are aligned in a certain orientation to the polarization axes of the instrument. In the case of unknown samples, it is possible to determine and compare the same set of spectral measurements. If the sample is consistently oriented, it would be easy to correlate the spectral variations with sample properties.

Conclusion

This article has shown the fundamental basis for Raman polarization measurements. Porto notation provides a suitable means to record a sample orientation with regard to the polarization of the incident light as well as the outgoing post sample analyzer orientation, which enables reliable interpretation of Raman polarization measurements. Anisotropic (LiNbO₃) and isotropic (CCl₄) samples were used to demonstrate Raman point polarization results. Raman polarization measurements give data, which reveals the nature of the vibrations and also aids in assigning these bands.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

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