Polarization-modulation measurement

Polarization modulation ellipsometers use a photo-elastic modulator to modulate the state of polarization of the incident beam. Polarizer and analyzer are fixed during the measurement. Eourier analysis of the time dependent signal gives the ellipse-... 

Recent work in our laboratory has shown that Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) can be used routinely to measure vibrational spectra of a monolayer on a low area metal surface. To achieve sensitivity and resolution, a pseudo-double beam, polarization modulation technique was integrated into the FT-IR experiment. We have shown applicability of FT-IRRAS to spectral measurements of surface adsorbates in the presence of a surrounding infrared absorbing gas or liquid as well as measurements in the UHV. We now show progress toward situ measurement of thermal and hydration induced conformational changes of adsorbate structure. The design of the cell and some preliminary measurements will be discussed. 

Polarization modulation can be done by rotating the polarizer, as is done in rotating-element ellipsometers [339], or by using photoelastic devices [337]. Data acquisition nowadays is fast, which makes real-time measurements of film and interface formation possible a full spectrum ranging from 1.5 to 5 eV can be measured in less than a second [340-342]. 

A more complex but faster and more sensitive approach is polarization modulation (PM) IRLD. For such experiments, a photoelastic modulator is used to modulate the polarization state of the incident radiation at about 100 kHz. The detected signal is the sum of the low-frequency intensity modulation with a high-frequency modulation that depends on the orientation of the sample. After appropriate signal filtering, demodulation, and calibration [41], a dichroic difference spectrum can be directly obtained in a single scan. This improves the time resolution to 400 ms, prevents artifacts due to relaxation between measurements, and improves sensitivity for weakly oriented samples. However, structural information can be lost since individual polarized spectra are not recorded. Pezolet and coworkers have used this approach to study the deformation and relaxation in various homopolymers, copolymers, and polymer blends [15,42,43]. For instance, Figure 7 shows the relaxation curves determined in situ for miscible blends of PS and PVME [42]. The (P2) values were determined... 

Vibrational spectra were recorded using the polarization-modulated infrared reflection absorbance technique (PM-IRRAS). The spectrometer, the electrochemical cell, and the sample preparation and cleaning procedures are all described elsewhere (1 7) All of the measurements were performed using 0.5 M SO solutions, either with or without an added nitrile compound or SnCl,. The solutions were saturated with CO by bubbling the gas through their storage reservoirs before admitting them into the sample cell. 

Instead of recording separately the decays of the two polarized components, we measure the differential polarized phase angle A (co) = — i between these two components and the polarized modulation ratio A (co) = mfm . It is interesting to define the frequency-dependent anisotropy as follows ... 

This restilt was confimed by other groupsi2.i3, using Fourier transform infrared spectroscopy. Kunimatsu and Kita made further progress using polarization modulation to enable quantitative measurements and showed... 

An alternative configuration is the polarizer-modulator placed before the detector rather than the sample. This might be useful for measurements in the atmosphere for example, a searchhght as the Hght source with the modulator at or near the focus of a portable telescope followed by a detector. For laboratory investigations it is also sometimes convenient to place the modulator before the detector. 

Bell (1981) (see also Bell and Bickel, 1981) measured all matrix elements for fused quartz fibers of a few micrometers in diameter with a photoelastic polarization modulator similar to that of Hunt and Huffman (1973) the HeCd (441.6 nm) laser beam was normal to the fiber axes. Advantages of fibers as single-particle scattering samples are their orientation is readily fixed and they can easily be manipulated and stored. Two of the four elements for a 0.96-jtim-radius fiber are shown in Fig. 13.16 dots represent measurements and solid lines were calculated using an earlier version of the computer program in Appendix C. Bell was able to determine the fiber radius to within a few tenths of a percent by varying the radius in calculations, assuming a refractive index of 1.446 + iO.O, until an overall best fit to the measured matrix elements was obtained. 

A variant of IRRAS is polarization modulation IR reflection absorption spectroscopy (PM-IRRAS). In this method, the polarization of the IR beam incident on the sample is modulated between parallel and perpendicular polarization. When the sample is metallic, only the parallel-polarized light yields signals from adsorbed molecules, because the electric field amplitude of perpendicular-polarized light vanishes at the metal surface. This statement is the basis for the metal surface selection rule 100,109). When the medium above the sample (gas or liquid phase) is isotropic, both polarizations are equivalent. The PM-IRRAS method thus enables the measurement of signals from adsorbates on a metal surface in the presence of an absorbing gas or liquid phase. 

Over conventional reflectance photometry, IRES retains advantages 1) and 2), with 3) being partially lost (the substrate contribution to A only is measurable) however, sensitivities are a factor of 10 or more lower. Polarization modulation techniques that determine only Ae, and not the corresponding dispersion property (6,J), retain advantage 1) over wavelength modulation photometry,but not advantage 2),and retain advantage 3) only in part, as above. 

For optimum performance, CD measurements require a polarization modulated source. In principle, any of the polarization-selective optical devices discussed earlier could be mechanically moved to create the required modulation. However, this approach is problematic in that it is difficult to implement physically, the mechanical movement may introduce noise into the measurement situation, and there are limitations to the rate at which the polarization can be modulated. A preferable approach is to use an electronic device to effect the required phase retardation. Although a number of devices have been used for this purpose (e.g. magneto-optical, Kerr effect, etc.), modern CD instruments rely upon either the Pockels effect, or photoelastic modulation for this function. 

As discussed earlier, the PEM is, in many ways, an ideal modulation device for polarization-selective measurements. Thus, the optimum simultaneous multiwavelength CD instrument should incorporate both the PEM and CCD, and at the same time allow the PEM to operate at its resonant frequency. One way to overcome the basic incompatibility of the PEM and CCD is to use an optical demodulation scheme in which the two oppositely polarized components would be directed by a polarizing beamsplitter, to either different areas of the same CCD, or to different CCDs [14]. Since two distinct detector areas would be accessed by the oppositely polarized light, pixel-to-pixel sensitivity variations may be a significant source of noise. 

The improvement in the instrumental SNR afforded by the use of polarization modulation has permitted CD detection to be applied to stopped-flow studies of biological reactions. The important information which can be obtained from such an approach has served as an impetus for the development of new instrumental approaches for the measurement of CD. These new approaches have allowed CD measurements to be extended to the time domain below that available with stopped-flow techniques. Presently, nanosecond, and even picosecond CD techniques have been developed, and it seems clear that extension to the sub-picosecond regime will follow. 

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