Bragg-filter

This dependence of nPS on p can be used to fabricate optical superlattices, e.g. Bragg filters, by modulating the porosity of a PS layer, as discussed in Section 10.5. 

Fig. 10.10 SEM micrograph of a tunable IR Bragg filter (1.1 xl. 85 mm) of density-modulated PS, suspended at two microactuator arms. Its rest position is more than 180°...

Fig. 1 Types of PSi sensors, (a) single-layer (b) Bragg filter (c) Thue-Morse filter (d) Microcavity...

To summarize Filtering is an effective way of producing intense monochromatic beams, but it is severely limited because it cannot be used at all wavelengths and cannot achieve high spectral purity at any wavelength. The analysis of a spectrum, that is, the selection of a line and the measurement of its intensity, requires Bragg reflection. 

Bragg reflection (1.14) can accomplish, as filtering (4.6) cannot, both monochromatization and spectral analysis. With a well-collimated beam, this can be done with a flat crystal, though not without further losses in intensity. 

For the purposes of analytical chemistry, four kinds of monochromatic beams need to be considered. (The quotation marks are to remind the reader that the beams under discussion are not always truly monochromatic.) Three kinds of beams—those produced by Bragg reflection (4.9), filtered beams (4.6), beams in which characteristic lines predominate over a background that can be neglected— will be discussed later ( 6.2). The fourth kind of beam contains monochromatic x-rays that are a by-product of our atomic age and that promise to grow in importance they are given off by radioactive isotopes. These x-rays must not be confused with 7-rays (11.1), which also originate from radioactive atoms but which differ from x-rays because the transformation that leads to radiation involves the nucleus. 

A series of powder photographs was taken in a Norelco camera 360 mm. in circumference which holds the film in the Straumanis arrangement and permits a maximum Bragg angle of 87-6°. The copper K radiation was filtered through 0-001 in. nickel foil. Eastman No-Screen X-ray film was used throughout. All photographs were taken at room temperature, 26 + 2° C. 

The electrochemical cell used by Flcischmann and co-workers (1986) employing the Bragg configuration is shown in Figure 2.67(b). The source is a copper anode X-ray tube employing a Ni filter to select out the Cu Ka line the detector is a PS PD. 

In conclusion, it has been demonstrated that the manufacture of waveguides, Bragg reflectors, Fabry-Perot filters and anti-reflective coatings can be based on PS thin films. Such filters are applied for example in gas sensors, as described in the preceding section. 

Anyway, due to the limited precision of the EDP - intensity measurement, especially for the (100) reflection, which is too close to the central beam - this argument is not very strong and cannot reject the 1/3 substitution at all. To confirm the hypotheses of the proposed 2/3 substitution HREM observation in [001] and [021] zones were performed in parallel with multislice calculations, based on the 2/3 substitution. These projections are used because the fourfold modulation due to the Mo atoms is well expressed in them. The observed images are enhanced by a Fourier filter with Bragg mask to eliminate the noise frequencies. 

So far a modulator device has been described, not a filter. In order to understand how an AOTF device goes further than this, it is necessary to look at the effect of Bragg regime diffraction and the impact of the anisotropy of the AOTF crystal medium. 

The Bragg-Brentano type of diffractometer is composed of an x-ray tube with a metallic anode that supplies x-rays that are scattered from the sample and focused at the slit before hitting the detector. In some cases, a monochromator capable of yielding a monochromatic x-ray beam is added. The sample is rotated, relative to the x-ray at angles from 0° to 90° with the help of a goniometer, where the powdered sample is placed on the sample holder. Electronic equipment is used to amplify and filter signal pulses from the detector. 

---