Nonpolar orientations

For all Azo-PURs, the quantum yields of the forth, i.e., trans—>cis, are small compared to those of the back, i.e., cis—>trans, isomerization—a feature that shows that the azo-chromophore is often in the trans form during trans<->cis cycling. For PUR-1, trans isomerizes to cis about 4 times for every 1000 photons absorbed, and once in the cis, it isomerizes back to the trans for about 2 absorbed photons. In addition, the rate of cis—>trans thermal isomerization is quite high 0.45 s Q 1 shows that upon isomerization, the azo-chromophore rotates in a manner that maximizes molecular nonpolar orientation during isomerization in other words, it maximizes the second-order Legendre polynomial, i.e., the second moment, of the distribution of the isomeric reorientation. Q 1 also shows that the chromophore retains full memory of its orientation before isomerization and does not shake indiscriminately before it relaxes otherwise, it would be Q 0. The fact that the azo-chromophore moves, i.e., rotates, and retains full orientational memory after isomerization dictates that it reorients only by a well-defined, discrete angle upon isomerization. Next, I discuss photo-orientation processes in chromophores that isomerize by cyclization, a process that differs from the isomeric shape change of azobenzene derivatives. 

PI-1 and PI-2 films exhibit dichroic absorbance after polarized irradiation. Figure 4.15 shows polar plots of the absorbance of linearly polarized probe light (at 488 nm) as a function of the angle, between the polarization of the probe and irradiation light (532 nm 30 mW/cm ). Nonpolar orientation is clearly shown for both PI-1 (left) and PI-2 (right). The highest absorption is observed when the probe and irradiation beams have perpendicular polarizations,... 

Similar PID results have been reported in very high Tg (up to SSO C) azo-polyimides. It has been shown previously that irradiation of the donor-embedded polyimide derivatives with polarized light alone at room temperature induces a quasi-permanent nonpolar orientation, which can be thermally erased only by heating the polymer above Tg. While, the lifetimes of the polar order generated by thermal poling of the donor-embedded polyimides were found to be on the order of tens of years to centuries at room temperature, photoisomerization can efficiently depole these polymer films in a matter of minutes at room temperature. Indeed, Figure 8.6 shows the effect of p-polarized irradiation on the SH signal of PI-2, which had been previously thermally... 

The contribution of photo-induced nonpolar orientation through even order parameters (see Equation 8A.9 in the appendix, page 286) to the effects observed in the EFISH decrease is negligible. The same decrease is observed for a TE- or TM-polarized probe regardless of the pump polarization (not shown). The photochemically induced molecular shape change of the NLO dye blows out the strong optical field driven anharmonic movement of the... 

Due to possible utilization of photoinduced orientation in polymeric films in optical data storage, this phenomenon and the quadratic nonlinear optical effects were extensively investigated in the last few years. It was reported, for instance, that to study photoisomerization in a polymeric environment, a series of polymers containing azo dyes with large differences in the second order transition temperature were compared. Particular emphasis was placed on the relationship between photoisomerization, Tgof the polymers, and their molecular structure. As a result, it was shown that light-induced nonpolar orientation in very high Tg polyimides (Tg up to 350 °C) can take place even at room temperature. The polymers used in one of these studies can be illustrated as follows... 

The first planar nonpolar a-plane InN grown on r-plane sapphire using an AlN nucleation layer or GaN buffer layer by plasma-assisted MBE was reported by the research group from Cornell University in 2003 [38]. With this, all the nitrides with nonpolar orientations were completed, thus paving the way toward a whole range of nitride-based nonpolar heterostructures and devices. 

The other well-known emission bands in GaN, in lower energy regions such as donor-acceptor pair emission together with its phonon replicas, as well as the blue-green, yellow, and red emission bands, have also been observed in GaN films with nonpolar orientations and their origin was, in principle, related to the same defects as the respective emissions in polar materials [95, 99]. 

The emission properties of nonpolar a-plane AlN and InN films grown on r-plane sapphire are the least explored. Only a few papers present the initial data on this topic. The NBE emissions were confirmed very close to that in polar AlN [103,104] and InN [38,105] materials with no detailed studies of the effect of strain on them. Significant improvement of the emission intensity was observed in nonpolar AlN [104] as a result of microstructure improvement in case of ELOG [104] or Pendeo [106] template employment. Similar to the observation for GaN, the AlN bulk material with nonpolar orientations sliced from boules grown in the [0001] direction was found to possess superior emission quality [103]. 

While considerable progress was made in thin-fihn growth of nonpolar GaN from 2000-2002, thick-film or bulk growth of nonpolar orientations continued to be elusive until late 2002. The performance of nonpolar GaN-based devices would be limited by the lack of low-defect density film and substrate options. This chapter describes the progress achieved in thick-film nonpolar GaN growth via hydride vapor phase epitaxy (HVPE) toward the goal of producing low-defect density nonpolar GaN thick-films and substrates. 

Because of the high structural quality of the high-pressure polar substrates, the multilayer structures of polar orientation were also almost defect free. The ones of nonpolar orientation contained areas with structural defects induced by imperfections in the substrate. 

Study of group III nitride heterostructures with nonpolar orientation [13-23]. Recently, ultraviolet LEDs based on GaN/(Al,Ga)N(1120) quantum wells with nonpolar orientation have been realized [24]. 

The in-plane polarization anisotropy can be enhanced by anisotropic strain. This can be achieved by choosing a nonpolar orientation with an appropriate substrate. In the extreme case of M-plane GaN on liAl02, the degree of linear polarization can be increased to its maximum value of one for all three transitions between the three uppermost valence bands (VBs) and the conduction band (CB), corresponding to complete linear polarization for all three transitions. This optical anisotropy can be observed in transmission (absorption) and reflection, as well as photoluminescence (PL) and photoreflectance (PR) spectroscopy. It can therefore be used for polarization filtering, polarization-sensitive photodetectors (PSPDs), and polarized light emitters. For anisotropically strained C-plane GaN films on (1120) sapphire, the in-plane polarization properties have been previously reported in Refs. 1-3. 

For nonpolar orientations, the GaN films are usually grown on LiAl02, l -plane sapphire, and A- or M-plane SiC so that the strain due to the lattice mismatch... 

Strained CaN Films with DifTerent Nonpolar Orientations... 

The optical polarization anisotropy in GaN films with nonpolar orientations can be used for static as well as dynamic polarization filtering. Furthermore, a PS PD has been realized, which has been extended to a narrow-band photodetector by combining it with a polarization filter having the in-plane crystal structure rotated by 90°. The concept of polarization anisotropy has also been applied to achieve polarized emission in (In,Ga)N/GaN LEDs grown along nonpolar orientations. 

In this chapter, we review recent studies of optical phonons in relation to the anisotropic strain in GaN heteroepitaxial layers and quantum dots (QDs) with nonpolar orientations. For reasons of comparison, results on anisotropically strained c-plane GaN films grown on a-plane sapphire is also presented where appropriate. The application of GIRSE and Raman scattering to the studies of optical phonon frequencies of anisotropic wurtzite GaN is described to reveal the phonon mode behavior. The assessment of anisotropic strain components in a-plane GaN films and their evolution with thickness... 

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