Sapphire polarized spectra

Material like sapphire, which is a uniaxial crystal, does not show nonlinear fluorescence, and direct visualization of filaments is, consequently, not an option. However, the effect of rotation of plane polarization can also be probed in such materials by monitoring the spatial size of the white light disc, and of the spectrum of the super continuum that is produced. Figure 5.3 depicts some typical results obtained with 3 mm long sapphire crystals. The extent of the supercontinuum spectrum is seen to vary as a function of polarization angle. The supercontinuum spectrum has two components [39] symmetric broadening about the incident wavelength that is essentially ascribable to... 

The crystal structures of transition metal compounds and minerals have either cubic or lower symmetries. The cations may occur in regular octahedral (or tetrahedral) sites or be present in distorted coordination polyhedra in the crystal structures. When cations are located in low-symmetry coordination environments in non-cubic minerals, different absorption spectrum profiles may result when linearly polarized light is transmitted through single crystals of the anisotropic phases. Such polarization dependence of absorption bands is illustrated by the spectra ofFe2+ in gillespite (fig. 3.3) and of Fe3+in yellow sapphire (fig. 3.16). 

In the / -spectrum of the ZnO thin film, a similar plateau as in the 3 -spectrum of the ZnO bulk sample is present. However, the phonon modes of the sapphire substrate introduce additional features, for example atw 510, 630, and "-900 cm 1 [38,123]. The spectral feature at w 610 cm-1 is called the Berreman resonance, which is related to the excitation of surface polari-tons of transverse magnetic character at the boundary of two media [73]. In the spectral region of the Berreman resonance, IRSE provides high sensitivity to the A (LO)-mode parameters. For (OOOl)-oriented surfaces of crystals with wurtzite structure, linear-polarization-dependent spectroscopic... 

The photomodulation (PM) spectrum of trans- CH) at 4 K is shown in Fig. 22.8 the trans- CH).x sample used here was a thin film d - 1000 A) polymerized on a sapphire substrate by the Shirakawa method. The PM spectrum consists of two main PA bands with Ja > 0 and bleaching (J a < 0) for h(o > 1.65 eV. The low energy (LE) PA band peaks at 0.43 eV, and it originates from photoinduced charged defects this has been concluded from its correlation with the photoinduced IRA Vs (Fig. 22.6). The intensity of the LE band easily saturates with increased excitation power and is sample-dependent. Moreover, as was demonstrated in oriented (CH), films, the LE band is induced preferentially with light polarized perpendicular to the chain s direction (27J. Therefore it seems that the LE band is stabilized by extrinsic defects of the polymer chain. The LE band is now considered to be due to charged soliton 5S transitions, which are pushed away from midgap (predicted by the SSH theory) due to electron correlation (18], as discussed in Section II.A. 

Figure 8.3 Low-temperature (2 K) PL spectrum of a 1-pm-thick a-plane GaN layer grown by MOCVD on r-plane sapphire with low-temperature GaN nucleation layer. The inset shows the high-resolution spectra in the exciton region for E c and E c polarizations.

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