For specific samples and molecules, traditional Raman spectroscopic information can be optimized by managing the polarization of light that excites the sample and the light that scatters from it.
The investigation of Raman spectra acquired utilizing this method can offer data concerning the sample’s molecular structure.
This data includes the symmetry of the sample’s vibrational modes, along with data for highly-ordered samples, for example, polymers, carbon materials, and crystals.
Fully computer-controlled polarization optics can be supplied with the RM5 Raman Microscope to help the user to simply gather and analyze maps and spectra in several polarized configurations. This information can be collected from features less than 1 µm, due to the entirely confocal pinhole in the RM5.
This article will cover the main concepts regarding polarized Raman spectroscopy and the polarization of light. It will also demonstrate the various sample types that the RM5 can analyze utilizing this method.
What is Polarization of Light?
Light is considered to be a wave comprising of electric and magnetic fields in classical physics. Every electromagnetic wave contains both a magnetic field and an electric field, which are positioned perpendicularly to each other.
Put simply, in the electric field (EF) component, unpolarized light comprises waves with EFs that are positioned in a variety of directions in relation to each other (Figure 1).
Linearly polarized light contains waves with EFs that are located in a single plane across the direction of travel of light. Circularly polarized light includes waves with two EF components of equal amplitudes that are perpendicular to each other but have a phase difference of π/2.
Elliptically polarized light is composed of waves with two EF components that have an unequal phase difference and amplitude and are perpendicular to each other.
These various types of light can be modified to enhance experimental use. There are certain materials that only let electromagnetic waves of a particular orientation to pass through them, for example.
These materials are known as polarizers. They can be utilized to generate various kinds of polarized light. A half wave-plate is employed in the RM5 to alter the polarization direction of linearly polarized light. A quarter wave-plate is utilized for the conversion of linearly polarized light into circularly polarized light.
What is Polarized Raman?
In Raman spectra, the positions of peaks can be allocated to the molecular vibrations of the molecule(s) within the analyzed sample.
Raman spectroscopy under normal conditions can explain the chemical composition of a sample, but polarized Raman spectroscopy can offer more information, like the orientation of the sample and the symmetry of vibrational modes.
The data gathered utilizing polarized Raman spectroscopy is useful for a broad range of industries, such as:
- Electronics and optics, by identifying the orientations of components
- Nanomaterials, by quantifying the alignment of single-walled carbon nanotubes
- Crystallography, by evaluating crystal structure
- Polymers, by uncovering the locations of stress
- Biology, by characterizing the anisotropic and isotropic vibrational reactions in cancerous and noncancerous human breast tissue
- Theoretical studies, by offering values for vibrational modes and experimental evidence
It is beneficial to look at two different kinds of samples, isotropic and anisotropic, to greater understand the hardware, notation, and concepts employed in polarized Raman spectroscopy.
Figure 1. Types of polarization of light (Note: linearly and circularly polarized light can be thought of as special cases of elliptically polarized light).
Isotropic samples (randomly oriented samples, such as microcrystalline powders and liquids).
Raman scattered light includes a mixture of light with a polarization that is parallel to that of the excitation light, and with a polarization perpendicular to it.
Both of these elements are quantified in the same spectrum in conventional Raman measurement. In opposition, polarizing filters are utilized in polarized Raman measurements to quantify the Raman spectra of each distinct component.
Anisotropic samples (directionally oriented samples, such as polymers and crystal lattices).
The molecules are in secure positions in these samples. This means the bonds inside of them can be manipulated into a particular position according to the excitation polarization.
The Raman peaks and their intensity differs according to this alignment, producing very different spectra that offer helpful sample information.
For any given peak, noting the ratio of its perpendicular intensity, against its parallel intensity, provides the ‘depolarization ratio’ (ρ).
The depolarization ratio gives information about the molecule’s symmetry, and the vibrational mode portrayed by the Raman peak.
For an entirely symmetrical vibrational mode, the ρ is less than 0.75. This is known as a polarized band. If ρ is more than or equal to 0.75, then the band is known as a de-polarized band (meaning it is not entirely symmetrical).
This data can be utilized to assist in the determination of the structure and identity of samples, and the vibrational modes.
‘Porto notation’ is utilized to document the polarization conditions in a standardized manner so that many data sets can be compared correctly. Porto notation is found through the following form:
A (B C) D
A is the excitation (laser) propagation direction,
B is the excitation (laser) polarization direction,
C is the scattering (Raman) polarization direction,
D is the scattering (Raman) propagation direction.
Figure 2 provides an example of an experimental structure and the related Porto Notation. The z scattering propagation direction shows a 180° back-scattering configuration (as is utilized in the RM5).
As the Raman spectrum from an isotropic sample is independent of the sample orientation, two alternative polarized measurements can be performed. The relationship between Scattering Polarization Direction and Excitation Polarization Direction is as follows:
Parallel, for example z (x x) z
Perpendicular, as demonstrated in Figure 2, for example, z (y x) z
Figure 2. Porto Notation example.
When anisotropic samples are analyzed, it is essential to make a note of how the sample is positioned in relation to the microscope or stage.
As an example, this could be performed by employing the coordinate system of the sample (such as the crystal structure), or stage. If these are not known, then taking a white light image of the sample will suffice.
In the experimental setup in Figure 2, four different scattering polarization and excitation combinations are possible:
z (x x) z
z (x y) z
z (y y) z
z (y x) z
Experimental and Results
To demonstrate how these methods are performed in real life, measurements of anisotropic and isotropic samples will be outlined.
All experimental data presented was acquired employing the RM5 Raman Microscope (Figure 3), fully supplied with automated polarization turrets that are simply managed by the Ramacle software.
A 532 nm excitation laser, a 20x air objective, and a 1800 gr/mm grating were used. A half wave-plate was utilized to modify the polarization direction of linearly polarized light. 400 µL of cyclohexane was introduced to a well in a 96 well plate, and the lithium niobate was inserted on a microscope slide.
Figure 3. The RM5 Raman Microscope.
Isotropic Sample - Cyclohexane
It can be seen in the following spectrum that vibrational modes that are fully symmetric have the most differences in intensity between perpendicular and parallel configurations.
Figure 4. Cyclohexane polarized Raman spectra (532 nm, 1800 gr/mm).
A combination of peaks that are both depolarized and polarized can be seen in the spectrum of cyclohexane (Figure 4).
At 383, 801, and 1157 cm-1 polarized peaks (ρ<0.75) can be observed, which come from fully symmetric modes. To illustrate, the strongest peak in the spectrum, at 801 cm-1 is a result of the ring modes and CH2 deformation.
At 425, 1028, 1266, 1345, and 1444 cm-1, depolarized peaks can be seen (ρ≥0.75) which arise from modes that are not fully symmetric. The peak at 1444 cm-1 for example, comes from the CH2 scissor mode.
Anisotropic Sample – Lithium Niobate
The following spectra show that it is essential to consider and detail the orientation between the Raman spectrometer and the crystal utilizing Porto Notation.
Lithium niobate is a uniaxial ferroelectric material. The characteristics of lithium niobate, for example, its large nonlinear optical coefficients, make it technologically important, particularly in (nonlinear) optical applications.
The sample employed here was cut down the crystal X-axis face, and this face was positioned in a perpendicular manner to the incident laser’s direction (parallel to the surface of the stage). This can be observed in Figure 5, along with the location of the Y- and Z- axes.
Figure 5. Lithium niobate sample orientation example.
The Porto notation for the setup of the Raman instrument used for each spectral measurement can be acquired using Figure 5.
Figure 6 demonstrates the lithium niobate spectra gathered utilizing four alternate polarization configurations. The modes observed, along with their position and intensity, can be seen to greatly differ between the spectra.
Contrast this to the isotropic liquid sample, where the peak intensity was the key variable between a range of polarization configurations.
Comparing the data included in the polarized spectra from an anisotropic sample can assist in determining the material and its crystal orientation and structure.
Figure 6. Lithium niobate polarized Raman spectra (532 nm, 1800 gr/mm).
This article showed how the polarization of light can offer beneficial information that cannot be obtained using conventional Raman spectroscopic investigations.
The depolarization ratio and how it gives information about the symmetry of vibrational modes was discussed, along with Porto notation and how it is used to observe the relationship between spectra, and the orientation of the light, sample, and instrumentation.
For both isotropic (the liquid cyclohexane) and anisotropic (the crystal lithium niobate) samples, how to configure a polarized Raman investigation in the RM5 Raman Microscope was outlined, along with how to analyze the resulting spectra.
Produced from material originally authored by Sam Stanfield from Edinburgh Instruments.
This information has been sourced, reviewed and adapted from materials provided by Edinburgh Instruments.
For more information on this source, please visit Edinburgh Instruments.