Photoelastic modulators PEMs

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. 

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. 

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... 

Figure 1 Block diagram of the solid-state dedicated CD spectrophotometer (J-800KCM). P polarizer M photoelastic modulator (PEM) S sample and rotation stage LI, L2 lenses A analyzer (Glan-Tayler) D photomultiplier (R376) MINI 12P sample stage controller lock-in ampl JASCO lock-in amp2 Stanford SRS 830 Recorder SEKO-NIC SS-250F PC personal computer.

Birefringence can be produced by mechanical stress exerted on optically isotropic media like glass and cubic crystals. Devices based on this phenomenon are used for polarization modulators. The so called photoelastic modulators (PEM) are widely used for measuring circular dichroism. The mechanical stress in the PEMs is produced by... 

Figure lib shows the optical apparatus for PM-IRAS with the high-pressure cell and the photoelastic modulator (PEM) (Hinds-PEM-90) located between the vacuum FTIR spectrometer (Bruker IFS66v/S) and the detector (113,171). By the use of recessed windows, the IR path length inside the high-pressure cell is reduced to minimize gas-phase absorption. To avoid interference from atmospheric H2O and CO2, the IR spectrometer and beam path are evacuated, and only the PEM is purged with dry nitrogen. The IR spectrometer produces a parallel beam of about 40 mm diameter, which is focused on the sample by a parabolic mirror (250 mm... 

A p - s modulation is performed by placing a photoelastic modulator (PEM) in the optical path of the spectrometer. Thus the difference spectrum at a given potential calculated as... 

Figure 17.2.5 Block diagram for an IRRAS instrument with an FT spectrometer. The photoelastic modulator (PEM) is a crystal, such as ZnSe, whose refractive index can be changed by application of a strain by a piezoelectric transducer, thus modulating the radiation between s- and p-polarization. [Reprinted from J. K. Foley, C. Korzeniewski, J. L. Daschbach, and S. Pons, Electroanal Chem., 14, 309 (1986), by courtesy of Marcel Dekker, Inc.]...

All commercially available CD instrumentation uses the modulation technique introduced by Grosjean and Legrand. Light from a monochromator is linearly polarized, then passes through a modulating device, today a photoelastic modulator (PEM), which converts the plane-polar-... 

The main difficulty of the PM IRRAS technique for quantitative analysis of ultrathin films at metal electrodes stems from the broad-band background that is introduced to the spectra by the photoelastic modulator (PEM), the so called PEM response functions. Recently, Buffeteau et al. described methods to remove... 

Specac) and the photoelastic modulator (PEM) (Hinds Instruments PM-90 with II/ZS50 (ZnSe 50 kHz) optical head). The beam, exiting the cell, is refocused by a ZnSe lens (f= 1 in, diameter 1.5 in, with an anti-reflective coating - Janos Technology) onto an MCT-A detector (TRS 50 MHz bandwidth, Nicolet). A bandpass optical filter (3-4 pm pass, Specac) is placed at the entrance window of the detector when the spectra of the hydrocarbon stretching modes are acquired. 

A classical elhpsometer, where the analyzer has to be rotated to record the whole ellipse of the reflected fight wave, can be improved by using a photoelastic modulator (PEM) [43]. In this case, the polarizer and analyzer are crossed in a fixed position and a photoelastic modulator is inserted behind the polarizer as shown in Fig. 4.2. The measurements with a photoelastic modulator are faster whereas the sensitivity is slightly worse compared to rotating analyser method. 

In the first two Sections, the basics of the Brewster angle ellipsometry and the operation principles of the photoelastic modulator (PEM) based ellipsometer will be overviewed. In the following, three different ellipsometric studies of the solid-liquid crystal interface wiU be presented. 

In a PM-IRRAS experiment (Fig. 5.9), an IR beam is directed to the sample surface after passing through a polarizer and a photoelastic modulator (PEM). The reflected... 

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