Bragg mode

Fleischmann and co-workers have carried out a number of in-situ x-ray diffraction studies at roughened electrodes.Their experiments are different from those previously discussed in that instead of a synchrotron source they employ a conventional x-ray tube ( 1.5 kW) with a position-sensitive proportional counter for the measurement of the diffracted intensity. They have carried out experiments in both transmission (Laue) and reflection (Bragg) modes. 

Kakiuchida, H., Tazawa, M., Yoshimura, K., Ogiwara, A. Optical dillractometry of anisotropic holographic structure composed of liquid crystal and polymta phases with extended Bragg modes. Thin Solid Films 571, 431-436 (2013)... 

A design for Bragg mode experiments is shown in Figure 7. This cell is again made of glass and is fairly similar to the transmission mode one, except that the incident and scattered X-rays pass through the same window and the... 

The observation of the departure from cubic symmetry above Tm co-incident with the appearance of the central peak scattering serves to resolve the conflict between dynamic and lattice strain models. The departure from cubic symmetry may be attributed to a shift in the atomic equilibrium position associated with the soft-mode anharmonicity. In such a picture, the central peak then becomes the precusor to a Bragg reflection for the new structure. 

Such off-zone-centre, soft-mode systems offer the most favourable conditions for a test of the hypothesis that the central peak is a precursor to a Bragg reflection in the transformation phase. Zone-centre softening, such as occurs in NbaSn, results in the central mode scattering emerging from an existing Bragg peak, which ultimately splits in the lower symmetry transformation structure, which presents a problem with resolution. 

In situ XRD spectra were collected on beam line X18A at the National Synchrotron Light Source (NSLS) located at Brookhaven National Laboratory (BNL). The X-ray wavelength (X) was 1.195 A. The step size of the 29 scan was 0.02° in the regions with Bragg reflections and 0.05° in the regions without reflections. The XRD spectra were collected in the transmission mode (Liu et al., 2004). 

The Bragg confinement mechanism allows great flexibility in engineering the radial mode profile. For example, it is possible to design a resonator in which the light is confined within a defect composed of low refractive index material or even air. By contrast, this would be impossible in the case of conventional TIR-based resonators. Such configuration is useful for sensing applications as discussed later. 

Fig. 12.10 (a) SEM image of the circular Bragg nanocavity designed to support the m 0 mode in the 300 nm wide central pillar, (b) The evolution of the emitted spectrum from the device shown in Fig. 12.9a as a function of the pump intensity. Inset L L curve, indicating a lasing threshold of Pth 900 pW. (c) Calculated modal intensity profile of the nanocavity, (d) IR image of the emitted beam profile... 

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