Rotating analyzer with compensator

Fig. 2.15 Schematic diagram of a few possible configurations of automated ellipsometers. From the top to the bottom, a rotating analyzer ellipsometer (RAE), a rotating compensator ellipsometer (RCE), and a rotating analyzer with compensator ellipsometer (RACE)...

A standard polarized-light microscope equipped with a Senarmont compensator, having magniflca tions up to 400X, and a rotatable analyzer with a graduated scale are used by the writer. All the obser vations are made with the use of the upper element of the substage condensing lens on the microscope. 

A polarizing microscope for transmitted light usually has a polarizer at a fixed azimuth beneath the substage condenser, and an analyzer in the microscope tube above the objective. Typically, a compensator is inserted at 45° beneath the analyzer. Some compensators (such as the de Senarmont) are fixed in azimuth, and measurements are made with a rotary analyzer with a variable azimuth. For other compensators, the analyzer is fixed and the compensator is rotated or tilted to make measurements in the axis of the microscope. 

When the sample is introduced between the polarizer and analyzer prism, optical rotation is produced, i.e., the plane polarized light emitted by the polarizer is rotated. The extent of rotation can be measured by rotating the analyzer with respect to the polarizer till the rotation is fully compensated. In more sophisticated instruments, a quartz-wave compensator is introduced. In these instruments, the polarizer and the analyzer are left permanently crossed... 

In 1990, Y. T. Kim et al. realized a real time spectrum measurement with the combination of RPE, prism spectrometer, and optical multichannel analyzer (OMA) [32]. It took 40 ms to finish the measurement of 128 sets of ellipsometry parameters over the whole spectrum. In 2003, this group developed a generalized ellipsometer with multichannel detecting using combination of RCE and OMA [33], The measurement time depends on the rotating frequency of compensation device, and it can take 150 sets of ellipsometry parameters in the energy range of 2 - 5 eV in 0.25 seconds. 

The Elliptic (or Brdce-Kohler) compensator is used for measuring the smallest retardations, with a maximum range of 20 or 50 nm and accuracy to 0.1 nm [57]. It has a birefringent plate that revolves in the horizontal plane. The Senar-mont compensator has an intermediate range, to 150 nm. It uses a X/4 plate and a rotating analyzer. For an explanation of how these work, see [19, 51, 52, 55]. 

With the compensator set for no compensation and (depending on the age of the microscope) and the polarizer and analyzer set for extinction, rotate the microscope stage, observing the intensity of light transmitted through the sample. A point of minimum intensity should easily be found (see Note 5). 

The detailed compensation mechanism is explained using Figure 8.26(b). When the unpolarized light from backlight passes through the polarizer (point P), it becomes linearly polarized and its polarization state is located at point T, which deviates fi om the absorption axis of the analyzer (point A). Afterwards, the linearly polarized light (point T) traverses the positive a film, whose position on the Poincare sphere overlaps with point A, and its polarization state is rotated clockwise from point T to point E around the AO axis. Point E is the first intermediate elliptical polarization state. 

Textile test specimens vary widely in both physical and chemical characteristics however, most samples can be characterized without any special preparation. Usual samples include fibers, yarns, and fabrics that can be presented directly to most commercial spectrophotometers. It is important to realize that variability within the textile sample is fairly high. A statistical sampling scheme is necessary to achieve a fair representation of a production lot. In the case of fabric samples, several layers of fabric or an appropriate background, such as ceramic or Teflon tile, will be necessary to create diffuse reflectance or transreflectance signals. Textile yarns and fabrics have bidirectional orientations and, hence, either a rotational sample cup or at least three rotations of the stationary cup are necessary to compensate for the differences owing to the sample orientation. Another difficulty with the textile fabrics is that they are usually dyed with various dark or pastel colors. Black and gray samples are more difficult to measure however, using appropriate procedure dyed fabrics are analyzed quite frequently. It is not uncommon to use separate calibrations for (i) white or pastel, (ii) medium shade, and (iii) dark color samples. 

In 1937, C. V. Kent and J. Lawson [18] first reported the photometric instrument with the polarizer and compensator adjusted to produce circularly polarized light on reflection from the sample. The analyzer was rotated at 40 Hz, and the pseudonull condition detected by amplifying the output of the photocell detector and listening to the 80 Hz components on a pair of headphones. 

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