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Abstract
In this paper we propose a new strategy towards simultaneous co- and cross-polarized measurements of Raman spectra in a confocal microscope. One of the advantages of this strategy is that it is immune to polarization-dependent efficiency of diffraction gratings. It is shown via linear angle-resolved and circular polarization measurements that the accuracy of these snapshot polarization measurements on solid and liquid samples are in good agreement with available models and data. The interest of simultaneous acquisition of the total Raman response and the degree of polarization is discussed as well.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Raman microscopy is a powerful technique that provides structural and chemical information with the resolution of an optical microscope. It is an important observation technique in fields such as low dimensional materials [1], semiconductors, chemistry, geology, and most recently in bioscience.
Useful information is mainly derived from the position and intensity of the Raman peaks, and sometimes also from their shift or broadening. Modern Raman microscopes and software make it rather simple to get high resolution and informative Raman microscopy images.
It is possible to get further information about the sample by exploiting light degrees of freedom such as the polarization of the scattered Raman signal for various configurations of the excitation laser polarization. In many applications, linear angle-resolved polarization measurements provide useful information on the local orientation of molecules or crystals [2–7]. In other fields, such as “valleytronics”, the Raman and photoluminescence polarized response of materials, when illuminated by a circularly polarized light, provides useful indication on their band structure [8–11]. Polarized Raman investigations associated to Raman theory provide insights on advanced materials [12–15]. Raman polarization is also useful in biology to study the fingerprints of cells [16,17]. In some other cases, polarization effects may be considered as an hindrance since they affect the shape of the Raman spectrum and may render chemical identification and quantification more difficult [18]. In that case, it is useful to be able to get the complete Raman response without any polarization bias of the instrument.
Despite the ever-increasing interest in such polarized studies, several persisting issues in Raman instruments affect the precision and feasibility of such measurement. Many Raman instruments have no or limited polarization capabilities and little assurance about polarimetric accuracy. First, both laser and Raman optical paths include several mirrors, filters, and focusing optics affecting and changing the polarization of the signal. Second, the response of a spectrometer is generally polarization-dependent because gratings efficiency depends on polarization [19]. And finally, polarized measurements generally involve the sequential acquisition of Raman signal with two analyzer positions and take at least twice the time of standard measurements.
However, significant instrumental advances have been reported in the recent years. The polarization transport issue has been largely addressed by preserving the laser polarization (as vertical (V) or horizontal (H) linear) through all the mirrors and filters, until the vertical optical path above the microscope objective, and inserting a polarization transformer in this zone. Depending on the polarization transformer, the laser polarization can be linear at any angle [20–23], circular [20,24], or even normal to the sample plane [25,26]. The Raman light scattered with the same polarization is then converted back into the H or V polarization and transported to the entrance of the spectrometer. The driving idea is that H and V polarization are invariant during the laser and Raman transport, since mirrors manipulate the beam in both horizontal and vertical directions in the plane of incidence. In both cases, it is known from Fresnel equations that both H and V polarizations are unaffected.
Different strategies have been proposed towards mitigating the polarization-dependent response of diffractive gratings such as inserting either a “depolarizer” before the spectrometer [21], or using a half waveplate retarder to transform H and V polarization into +45°and -45° linear states [23], or a quarter waveplate to transform the linear polarization into a circular one [6].
Some previous experiments use the strategy of splitting the light coming from the sample in two orthogonal polarization components either by using a beam displacer [24], or a Wollaston prism [16]. The most spectacular achievement in terms of simultaneous acquisition may be the setup allowing the simultaneous detection of 12 simultaneous Raman spectra corresponding to different polarization illumination and detection conditions reported in [3].
In this paper, we present an evolution of a conventional Raman microscope where laser polarization on the sample is controlled, and Raman response is measured simultaneously for two relevant polarizations. It is based on a Snapshot assembly made of a beam displacer and a polarization-balancing plate. Unlike other spectroscopic devices with polarimetric separation, the assembly presented here is placed inside the spectrometer. Another novelty is that we rigorously derive a constraining condition applied on the polarization-balancing plate that is necessary and sufficient to cancel the grating anisotropic response toward polarization. The broadband efficiency of the polarization-balancing plate designed using this result is verified experimentally.
Polarization control at sample is based on a polarization transformer inserted just above the microscope objective as presented in several previous works [21]. This approach assumes that H and V polarization are transported without modification in the Raman path. A model based on Jones matrices is derived to describe the effect of small deviations from this assumption. It is shown that this model accounts for our experimental results related to polarimetric accuracy.
The paper is organized as follows: in the first part we present the design of a Raman microscope with enhanced polarization analysis capability. Then, in the experimental detail section, we describe its implementation and the setup used for assessing its accuracy. The experimental result section provides results useful to establish the validity of the method and to illustrate its capabilities. The discussion section provides an analysis of the results, with a discussion on the accuracy, and a comparison with well-established Raman polarization results from the literature.
2. Design of a Raman microscope with flexible polarization illumination and Snapshot dual polarization acquisition
We will first describe the design of a spectrometer that can measure both horizontal (H) and vertical (V) polarizations simultaneously. A noticeable feature of this design is the solution implemented to compensate for any polarization-dependent response of the spectrometer grating. Then, we will describe how to perform the analysis with any laser polarization, including angle-resolved linear and circular, and obtain Raman co-polarization and cross-polarization simultaneously. The sketch of the whole setup is presented on Fig. 1. The laser beam has a vertical polarization PV, then P’L after being reflecting by a mirror redirecting it towards the axis of the microscope. It is transformed into polarization PL when passing through a waveplate orientated at an angle C and focused on the sample by a microscope objective. PL is linear if a half-waveplate is used, and circular, if a quarter-waveplate with proper orientation is used. The Raman scattered light follows the same vertical optical path as the laser, but in opposite direction. It has parallel and perpendicular polarization components relative to the PL, termed PCo-Pol and PX-Pol respectively. These two components are transformed by passing through the waveplate into two linear polarization components termed P’Co-Pol, P’X-Pol. A mirror reflects the beam and further transforms the two components into vertical PV and horizontal PH polarizations respectively. These two Raman components are separated from the laser beam by a dichroic mirror (not shown) and transported to the spectrometer. In the spectrometer, a beam displacer spatially separates these two polarization components. A waveplate with Mueller matrix element m1,1 = 0 transforms the V and H polarizations into two polarizations diffracted with the same efficiency by the vertically grooved grating. The two diffracted beams are focused on a CCD into two separate spectra.
Fig. 1. Diagram of the modified Raman microscope including a vertical common path assembly, and a Snapshot assembly. Many optical elements, such as mirrors, Raman filter, and focusing optics are not represented in this diagram.
2.1 Design of a dual polarization Snapshot spectrometer with balanced response
Several spectrometry devices allowing for simultaneous measurement of two polarizations have been described in [3,24,27]. While in most existing designs polarization separation is performed before the spectrometer, in our design it is performed inside the spectrometer. The entrance of the spectrometer is a pinhole, which is adequate for confocal microscopy. A beam displacer separates the V and H component of the polarization (see Fig. 2) in a way that H component seems to come from a separated virtual pinhole. This separation is equal to the beam displacer separation and is given by its length [28].
Fig. 2. Modified Raman spectrometer to achieve Snapshot dual polarization measurements (top). Polarization evolution at every key stage after each optical element at four planes, namely A-A’, B-B’, C-C’, and D-D’ (bottom). Various collimating or focusing elements have not been represented including the focal length correcting lens which doesn’t affect the polarization.
With this design, the CCD camera of the spectrometer provides both the V and H spectra. However, it is well known that diffraction gratings have an efficiency that differs significantly for polarization states parallel or perpendicular to the groove directions [19]. For gratings with large groove densities commonly used to obtain high resolution spectra, the ratio of diffraction efficiencies for these two components can be greater than 4 depending on the grating architecture, orientation, and the wavelength used, thus, resulting in an elusive calibration.
To obtain meaningful values of the polarization ratio between V and H, our design includes a plate placed after the beam displacer (as sketched on Figs. 1, 2) which role is to transform both V and H polarizations into states that have equal H and V projections. As the polarization properties of an optical component are fully described through its Mueller matrix [29], one should expect that this condition can be expressed as a constraint on the Mueller matrix. We have established this condition to be m1,1 = 0 which is demonstrated rigorously in Supplement 1, section S.1. Perfect depolarizers verify this condition, but real depolarizers may not behave as expected, especially when used with a high-resolution spectrometer. A quarter waveplate oriented at 45° and a half waveplate oriented at 22.5° also verify this condition. However, the design of a broadband plate with m1,1 = 0 is much less stringent than the design of a broadband half or quarter waveplate.
A small defocus induced by the introduction of the beam displacer inside the spectrometer has been corrected via a large focal length lens placed on the optical path of the diverging beam.
2.2 Creating and measuring any polarization on the sample: the common path reciprocal polarization transformer
Reference [21] presents in detail a clever approach to perform angle-resolved polarized Raman measurements. The approach is to insert a half waveplate (HWP) in the optical path common to the laser and Raman beams. More precisely, the HWP is placed before the microscope objective lens, and any mirror, beam splitter, or any component that can affect the polarization. A rotation of the waveplate by an angle C (referred to the linear polarization of the incoming laser, denominated V) creates a rotation of the linear polarization V by an angle P = 2C (see Fig. 1). The Raman component, with linear polarization angle P, is transformed back by the HWP into polarization V and analyzed via a fixed linear vertical polarizer (termed “analyzer”) and register by a CCD. The same acquisition is done by rotating the analyzer by 90°. The validity of this approach has been demonstrated theoretically and verified experimentally. It is a popular approach to perform angle-resolved polarization Raman imaging [20,23,30–32].
In the following, we will term “Co-polarization” (Co-Pol) the configuration of the Raman spectrum acquisition when the laser polarization (on the sample) and the Raman polarization have the same directions, and consequently “Cross-polarization” (X-Pol) when they are orthogonal. This denomination is frequently used in the field of Radar [33] and it is also adequate in optics. It applies not only to linear polarization, but also to circular or any kind of elliptical polarization.
Other groups have reported a similar approach to perform circular polarization analysis by placing a quarter waveplate (QWP) oriented at 45° with respect to the incoming laser polarization, in the vertical common path [10,31,34]. The V polarization of the laser is transformed into circular right by the QWP. The circular right and left components of the Raman signal emitted by the sample are transformed back into V and H by the QWP and analyzed using the analyzer.
While the principle of this common path polarization assembly has been fully established when inserting a HWP and a QWP, we think it is useful to establish two more general properties of this system, even if the waveplate is not HWP or QWP:
- A) The converter is reciprocal: if the laser polarization before the converter is V, the Raman co-polarization is detected through a V analyzer
- B) The orthogonality is conserved through the converter
This establishes that the V and H channel of our spectrometer equipped with the Snapshot assembly measures respectively the Raman co- and cross-polarization spectra. This holds within the following approximation: the retardance variation of the waveplate between Raman and laser wavelengths are negligeable; the effect of the objective lens on polarization is negligeable. The full demonstration of these properties is provided in the Supplement 1, section S.2.
3. Experimental details
We implemented the Snapshot assembly and the V common path assembly on a LabRAM Soleil Raman microscope. It is equipped with a 532nm and a 638nm laser. Its spectrometer is Czerny-Turner type, with 4 gratings. The detector is a CCD Sincerity OE. The pinhole diameter can be adjusted. We use a 50µm diameter pinhole and 1800groove/mm grating to obtain the best resolution spectra, and a 100µm diameter pinhole and 600 groove/mm grating in other cases.
The Snapshot assembly consisted of a lens with a focal length of 2m; a 10mm long calcite beam displacer; and a home -designed and -made polarization-balancing plate with m1,1∼0 [35], where m1,1 is a Mueller matrix component. This waveplate transforms incoming linear polarizations into circular polarizations at 520nm, like a QWP oriented at 45°. At 720nm, it transforms H and V polarizations into +45° and -45° polarizations, like an HWP oriented at 22,5° thus acting as a dual-wavelength waveplate. These are two particular cases that are well described in the literature to provide immunity to grating anisotropy, but the present work shows that the less restrictive “|m1,1| close to zero” condition is sufficient to obtain good balancing (see the Supplement 1, section S.1). The elements of the first column of the Mueller matrix of our waveplate measured using an ellipsometer are displayed on Fig. 3. It is seen that |m1,1| is less than 0.1 for nearly all the 350-1000 nm wavelength range, hence this plate is expected to provide an optimal performance towards cancelling the grating anisotropic polarization response.
Fig. 3. Measured values of the Mueller matrix elements mj,1 for the polarization balancing plate placed after the beam displacer.
The Snapshot assembly was placed 50mm after the pinhole of the Raman spectrometer.
The vertical common path assembly consisted of a Newport rotating stage, with a 3D-printed interface that can accommodate a linear polarizer, a QWP and a HWP. The QWP and HWP are achromatic in the 450 to 700nm range. The waveplates and the polarizer can be mounted with a reproducible orientation. The whole Raman setup including the Newport rotating stage is controlled by the LabSpec software (version 6.7).
A Neon spectral lamp was placed on the sample stage to perform spectral resolution experiment. A fibered halogen broadband light source was used to investigate the response of Co-Pol and X-Pol channels.
Raman experiments were performed on a Si (100) wafer, on a Si (111) sample, and on a tert-butanol liquid sample contained in a sealed glass vial.
4. Experimental results
4.1 Method qualification
First series of experiments were carried out to investigate the spectroscopic and polarimetric performance of the Snapshot assembly.
Figure 4(a) shows an image obtained on the Raman CCD (Syncerity OE) when observing a neon spectral lamp with a polarizer in the vertical common path assembly set at P = 45° and a 10x objective lens. If the Snapshot system is working correctly, it should separate such polarization state into equal V and H channels. As it can be observed, two vertically separated spectral traces are observed, corresponding to V and H polarizations. The vertical binning of the pixel values in each channel can be performed directly by the LabSpec software to provide the H and V spectra. The intensities of three peaks of the neon lamp (N76280) acquired by optimizing the grating position for each wavelength are displayed at high resolution on Fig. 4(b). The intensity of the pic is equally distributed in both channels with a slight decline towards the NIR wavelengths. Thus, our system is capable of separation of V and H channels.
Fig. 4. Experimental results recorded on the Raman spectrometer, with the 1800gr/mm grating, when observing a Neon spectral lamp with 10x objective lens; a) image on the spectrometer CCD; b) Co-polarization and cross-polarization spectra obtained in 2 different wavelength ranges.
Next, it is important to verify the capability of our Snapshot system to direct the given polarization (V or H) into its proper channel. Experiments were performed using a broadband halogen lamp (unpolarized source) on the sample stage. When setting the common path polarizer at P = 0° (V polarization), a near extinction of the H channel is observed. When setting the polarizer at 90° (H polarization), a near extinction of the V channel is observed. Figure 5 shows that the intensity residue is < 2% in both cases.
Fig. 5. Extinction ratio (in %) as a function of wavelength, measured by placing a white light source on the sample holder, and a polarizer at P = 0° and 90° in the vertical common path assembly. The 10x objective lens and 1800gr/mm grating were used.
A linear polarizer with P = 45° orientation was placed after the halogen lamp on the sample stage, to further investigate the performance of the system. A system with perfect accuracy would yield a ratio V/H = 1 for all gratings. Figure 6 shows the V/H response with the Snapshot assembly. It shows a robust performance for both gratings, with measured V/H values within 40% from unity. After removing the Snapshot assembly, the response at P = 0° and P = 90° were sequentially measured. The corresponding ratio are also represented on Fig. 6. This conventional system fails to provide V/H close to unity in the case of highly dispersive grating (black curve). This is attributed mainly to the dependence of the diffraction efficiency of gratings with polarization, often termed “grating anisotropy” [19].
Fig. 6. Ratio V/H of the polarization response in various configurations, with a white light source placed on the sample holder: polarizer is set at P = 45° with Snapshot assembly (yellow: with 600gr/mm grating; blue 1800gr/mm grating); without the Snapshot assembly, polarizer is set at 0° and 90°, and intensity ratio is represented for both 600gr/mm grating: brown; and 1800gr/mm gratings: black.
4.2 Raman experiments
In the Raman experiments, the laser polarization at the sample is controlled by the vertical common path assembly. It is adequate to describe the polarization configuration by the polarization state at the sample, and by the Co-Pol or X-Pol indication of the analyzed channel. The Co-Pol channel is the channel labelled “V” on Fig. 4(a), as the laser polarization is V.
4.2.1 Snapshot angle-resolved linear polarization Raman measurements on silicon
A first series of Raman measurements were performed with the HWP inserted in the vertical common path assembly, as a function of its orientation C. A Si(100) sample with [110] flat positioned along the X axis was investigated. Figure 7 provides some CCD spectrometer images corresponding to the 520 cm-1 peak of silicon. When the HWP is rotated, one polarization channels faints and the other gets brighter.
Fig. 7. Images from the spectrometer CCD corresponding to the Raman peak of Si(100), for different orientations of the HWP in the vertical common path. The 532nm laser was used, with 100x objective lens and 1800gr/mm grating.
Spectra were recorded with 2.5° steps of angular orientation C of the HWP, from 0 to 90°. This corresponds to a linear polarization on the sample ranging from P = 0° to P = 180° with 5° steps. The intensity map of the 37 pairs of spectra where each line corresponds to a single acquisition with the give linear polarization is displayed on Fig. 8. The intensity map of a similar investigation for Si(111) sample is displayed on Fig. 9.
Fig. 8. Intensity map of the Raman signal as a function of Raman shift and the orientation of the HWP placed in the common vertical path, measured on Si(100), with a 10x objective lens, and 532nm laser.
Fig. 9. Intensity map of the Raman signal as a function of Raman shift and the orientation of the HWP placed in the common vertical path, measured on Si(111), with a 10x objective lens, and 532nm laser.
The spectra were processed using LabSpec software to extract the amplitude of the Raman 520 cm-1 Raman peak. The amplitude of the peak as a function of the orientation of the laser polarization for both Si(100) and Si(111) samples is represented in Fig. 10. It can be observed that the intensity of the peak at 520cm-1 is changing for the sample with (100) orientation (Fig. 10(a)-(c)) for both 532nm and 638nm lasers while it stays constant for the one with (111) orientation (Fig. 10(d)).
Fig. 10. Angular-resolved study of the silicon Raman peak at 520cm-1 using our Snapshot polarization system and a 1800gr/mm grating; a) Si(100) observed with a 10x objective lens, 532nm laser; b) same sample and laser but changing the objective lens to 100x; c) same sample, 10x objective lens, 638nm laser; d) Si(111) sample, observed with 10x objective lens and 532nm laser.
4.2.2 Snapshot circular polarization Raman measurements on silicon
Circular polarization snapshot measurements performed on Si(100) and Si(111) using 1800gr/mm grating are shown on Fig. 11. Such polarization state is created by inserting a quarter waveplate in the common path of the microscope and selecting its angle either P = 45°(right) or P = 135°(left). Circularly polarized Raman signal passing through the quarter waveplate is transformed into a linear polarization and directed into Co-pol or X-pol channels. We checked that changing the polarization from circular right to circular left (by changing C from 45° to 135°) has no effect on the Co-Pol and X-Pol measurement which is inherently expected owing to the symmetry of the circular state. Intensity ratios of both channels with 10x and 100x objectives shows that highly focusing optics slightly affects the polarization performance (polarization transformation due to non-paraxial effects).
Fig. 11. Raman spectra of silicon wafers under circularly polarized illumination; a) Si(100); b) Si(111).
4.2.2 Snapshot polarized Raman measurements on liquids
We performed polarized Raman snapshot measurement on tert-butanol, with a 10x objective lens, the 600gr/mm grating and the 532nm laser. Linear polarization measurements were performed without any waveplate in the vertical common path assembly since the laser is already linearly polarized. As expected, the polarized peak of tert-butanol at 753cm-1 is almost completely directed into Co-Pol channel. Measurements are shown on Fig. 12(a).
Circular polarization measurements which are rarely conducted in the framework of polarized Raman studies were also carried out on tert-butanol for the first time to our knowledge. Circular state is created in the same configuration explained in the previous section are shown on Fig. 12(b).
5. Discussion
5.1 Spectral and polarimetric precision and accuracy
The spectral atomic lines observed with the Snapshot assembly have a Full Width at Half Maximum (FWHM) that is about 2 to 3 pixels for both polarization channels, as shown on Fig. 4(b). This is very close to the 2pixels/FWHM that is a commonly considered as the resolving power limit for a spectrometer [36].
Figure 5 shows that the polarimetric precision is excellent in nulling conditions since the measured extinction ratio is less than 2%.
Figure 6 provides indications on polarimetric precision in cases where the ratio between the two polarization channels is expected to be unity. For light polarized at P = 45°, the yellow and blue curve show that the error is nearly independent on the grating, while the 1800gr/mm grating efficiency between H and V differs by a factor of more than 4 at some wavelengths, as shown on Fig. 6 (black curve). This proves the efficiency of the polarization-balancing plate based on the condition m1,1 = 0.
The quadratic error is 13% for the 600gr/mm grating, and 16% for the 1800gr/mm grating, but the maximum error can be up to 40%. We have looked for reasons why the observed V/H ratio can be significantly different from unity for P = 45°. One possible reason is that the V and H polarization components are not transported with the same efficiency. Another possible reason is that H and V polarizations are not fully conserved through the Raman path. While it may seem against Curie’s Principle [37] that H and V polarization are not transported without modification, it is conceivable that some imperfection in the mirror orientation break the H and V symmetry of along the Raman path, or that the multilayer dielectric mirrors have some built-in strain, causing a slight birefringence that breaks the system symmetry. The calculations using Jones formalism [29] are detailed in Supplement 1. We find that our experimental results are fully consistent with some slight modification of the H and V polarization due to imperfection in the Raman path. We have established that a 7° ellipticity or rotation of the H and V polarization due to such imperfections would account for our observations.
As a conclusion, the polarimetric accuracy of the system is excellent in the case the ratio X-Pol/Co-Pol is significantly smaller or significantly larger than unity. In the case where X-Pol/Co-Pol is close to unity, the polarimetric accuracy depends on wavelength and polarization, it has a quadratic error of about 15%, a maximum error of 40%. The polarimetric accuracy is not affected by the grating anisotropy.
The confocality of the microscope is also preserved in this system, as polarization separation is performed inside the spectrometer, after the confocal pinhole. Confocality is essential in obtaining good spatial resolution.
5.2 Benchmark on the Raman polarization experiments on silicon
The Raman polarized response of silicon wafers is well known, both for linear and circular polarization [38–40]. For Si(100), the theory predicts that Raman intensity of the 520cm-1 line is related to the angle P between polarization and [110] axis by [41]:
The results on Fig. 10(a) to 10(c) are fully consistent with the above equations. There is a small discrepancy in the amplitude of co-polarization and cross-polarization, namely 12% error with the 532nm laser and 18% with the 638nm laser. The nulling at certain angles predicted by Eq. (1) is verified within 1% of Raman intensity scale for the observations made with the 10x objective lens. This supports our previous observation that our instrument has excellent sensitivity to detect cancelation of either co-polarization or cross-polarization.
For Si(111), theory predicts [39] that IX-Pol/ICo-Pol = 0.66 for any value of P, and our observations on Fig. 10(d) agree within 10% with this prediction: we find IX-Pol/ICo-Pol = 0.61 with a standard deviation of 2%.
The circular polarization experiment on Si(100) displayed in Fig. 11(a) shows that if a Si(100) sample is excited with a circularly polarized laser, the Raman emission is also circularly polarized, as predicted and observed in [39]. We did not manage to find other experimental report than [39] on the Raman response of Si(100) to circularly polarized excitation, while this is very easily evidenced with our setup. The fact that very few experimental investigations on the Raman response of Si to circular polarization exist may also reflect limitations in the capabilities of commonly used Raman microscopes.
The polarization ratio observed in the circular polarization experiment on Si(111) reported in Fig. 11(b) is 24%, which is close to the 27% ratio reported in [39].
In all our experiments, the results obtained with 100x objective lens (Numerical Aperture NA = 0,9) show larger discrepancies with the model than those obtained with the 10x objective lens (NA = 0,3). This can be easily understood for two reasons. For high Numerical Aperture objectives lenses, the polarization is no longer in-plane, it has a significant z component, and therefore the models should be adapted [25,42]. Another reason is that the polarization is affected by the transmission through microscope objectives lens, since Fresnel transmission coefficients are highly dependent on polarization for large incidence.
5.3 Analysis of the Raman polarization experiments on tert-butanol
The experimental results displayed on Fig. 12 show that several Raman peaks, including the strongest one, have a purely co-polarization response to a linear polarized excitation, and a circular polarization response that is also peak-dependent.
5.4 Importance of “unpolarized” measurements for spectral identification
Measuring only one component of polarization can be misleading when performing chemical identification based on Raman peak identification. As an example, if Si(100) is observed by detecting only one linear or circular polarization, the 520cm-1 peak may completely vanish, as can be seen on Fig. 8 and Fig. 11(a). In a similar manner, the observation of tert-butanol under linear laser polarization will miss some peaks if only one polarization is detected. The total Raman response, obtained by summing the Co-Pol and the X-Pol channels and designated by Sum-Pol, is shown on Fig. 13 for these two materials. It shows that Sum-Pol has very little dependence on the laser polarization. Sum-Pol appears to be the most adequate quantity for Raman spectral identification. The system presented here has the advantage of measuring simultaneously both the “unpolarized” information useful for spectral identification, and the polarized information useful for further understanding. The “unpolarized” signal provided by our system also has the advantage of being immune to grating anisotropy effects. In some cases, the determination of polarization-insensitive spectra is more sophisticated [18], but would benefit from a dual-polarization acquisition capability.
Fig. 13. Sum of the two polarization components; a) for various Si wafers, as a function of the orientation of the linear polarization on the sample; b) for tert-butanol.
6. Conclusion
We have presented a Raman microscopy setup [35] that simultaneously acquires conventional “unpolarized” Raman spectra, and polarization information. The polarization information is the cross-polarization over co-polarization ratio, related to the excitation laser polarization.
We have demonstrated Raman acquisition with linear at any angle and circular polarizations in the sample plane. Placing an adequate polarization transformer as shown in [25,26] in the vertical common path assembly would allow simultaneous in-plane and out-of-plane polarization measurements. We have shown that our system is immune to grating anisotropy response effects, and we have assessed experimentally the polarimetric accuracy of our system.
Our work introduced several novelties, such as a dual polarization spectrometer with internal polarization separation; establishing the necessary and sufficient conditions for a waveplate to balance grating anisotropy, expressed as a constraint on its Mueller matrix; the introduction of a model based on Jones formalism to describe slight imperfection in H and V conservation through transport along the Raman path.
Significant theoretical work has been published that show how Raman polarization information can be used to bring additional information to conventional Raman microscopy investigations. Present work may pave the way to turn the routine acquisition of Raman polarization information as a “no effort” addition to conventional Raman acquisition, into a very powerful source of additional information, and enrich Raman microscopy images.
Acknowledgements
The authors thank Céline Eypert and Thibault Brulé for help in Labspec data treatment; Cécile Lerondeau for help in designing the 3D-printed waveplate attachments; Philippe de Bettignies, Dominique Verreault and William Watkins for helpful discussions.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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