Photoelastic modulators

In a second kind of infrared ellipsometer a dynamic retarder, consisting of a photoelastic modulator (PEM), replaces the static one. The PEM produces a sinusoidal phase shift of approximately 40 kHz and supplies the detector exit with signals of the ground frequency and the second harmonic. From these two frequencies and two settings of the polarizer and PEM the ellipsometric spectra are determined [4.316]. This ellipsometer system is mainly used for rapid and relative measurements. 

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... 

The heart of the polarization-modulated nephelometer is a photoelastic modulator, developed by Kemp (1969) and by Jasperson and Schnatterly (1969). The latter used their instrument for ellipsometry of light reflected by solid surfaces (the application described here could be considered as ellipsometry of scattered light). Kemp first used the modulation technique in laboratory studies but soon found a fertile field of application in astrophysics the modulator, coupled with a telescope, allowed circular polarization from astronomical objects to be detected at much lower levels than previously possible. 

A photoelastic modulator mated to a nephelometer is shown schematically in Fig. 13.12 (Hunt and Huffman, 1973, 1975 Perry et al., 1978). The transmission axis of the linear polarizer P is fixed at 45° relative to the stress... 

Plane-polarized radiation comprises two circularly polarized vectors of equal intensity, one right-handed and the other left-handed (Fig. B3.5.3A), which are separately measured in the CD spectrometer by means of a photoelastic modulator. A chromophore situated... 

Photoelastic modulation in CD analysis of. see also Circular dichroism proteins, 219, 221-222 Photosynthetic pigment, see Carotenoids Chlorophylls... 

The application of an external field onto many materials will induce optical anisotropy. If the applied field oscillates, a time-dependent modulation of the polarization of the light transmitted by the device will result. Modulators of this sort include photoelastic modulators (PEM) [30,31], Faraday cells [32], Kerr cells [32], and Pockel cells. 

The Pockel s effect [3] refers to an electro-optical process wherein the application of large electric fields onto crystals lacking a center of symmetry can lead to nonlinear polarization effects and optical rotation. Pockel cells can be used in place of photoelastic modulators and can achieve very high modulation frequencies but often have the undesirable property of a nonzero birefringence in the absence of an applied field. 

Figure 8.12 Full Mueller matrix polarimeter using photoelastic modulators.

A variety of choices are available for the PSG section of this experiment. As before, the selection is based on the Mueller matrix components of the sample that are sought. A convenient arrangement that has been used for samples subject to uniaxial deformation is described in detail in reference 22 and collects the Raman scattered light in the forward direction. The PSG section of the instrument consists of a polarizer oriented at zero degrees, and a photoelastic modulator at 45°. Following the sample, the PSA section consists of a polarizer oriented at 45°. The signal measured at the photomultiplier tube was shown to have the form ... 

To add further credence to these critical assignments, the polarization-depen-dent OODR(Si) spectrum was recorded for the Ojj and 6j bands with implement of photoelastic modulator (PEM). This device alters the polarizations of the probe and pump beams with each laser pulse in a shot-by-shot fashion. For the 0q transition, the two beams would have parallel polarizations (pump and probe transition are both y (B2) polarized). For the 6j transition, the two photons would be perpendicularly polarized to each other, with y and z polarization, respectively. The temporal profiles of polarization-dependent OODR spectra in Figure 2.25 show the expected behavior, where the signals of the parallel and perpendicular polarizations for the S2(0°) < S [ (41) < S0(Oo) and S2(61) <— SjtT1) <— So(0o) are plotted against the delay time. The effects of reversing the polarizations is not too pronounced in these experiments, as the rotational coherence lifetimes are very short and the overall rotation of the molecule quickly scrambles the polarizations of the signals. At very short delay... 

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. 

The next class of VCD instruments to be developed was centered around a Fourier transform infrared (FT-IR) spectrometer. The idea was to design the sample compartment to be the same as in a dispersive VCD instrument, including a photoelastic modulator. To measure VCD, the detector signal is first sent to a lock-in amplifier to demodulate the high-frequency polarization modulation. The output of the lock-in is a VCD interferogram which is Fourier transformed in much the same way as the ordinary transmission interferogram. 

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