Polarized Light Emitters

In the same way as for photodetectors, it would be desirable to have polarized light-emitting diodes (LEDs) with a built-in polarization filter so that the emitted light may be completely linearly polarized. This application is particularly useful for flat-panel displays of laptops or notebooks. Gardner etal. [39] fabricated (In,Ga)N/GaN MQW LEDs on (1010) M-plane GaN films by low-pressure MOVPE on (1010) M-plane 4H-SiC substrates. The electroluminescence spectra showed a strong polarization anisotropy, with 

73 times more Kght emitted with polarization perpendicular to the c axis compared to that parallel to the c axis. The polarization ratio exceeds 0.8 and is persistent to a drive current density of 800 A cm and a temperature of 300 °C. An Arrhenius fit of the polarization ratio indicates that there is a 49 meV difference in the energy gap between the two polarization states. This suggests that a high polarization ratio can he maintained even at temperatures higher than 150 °C and drive current densities required for high-power LED applications. 

The transition energies for two M- and two A-plane GaN films, which are all anisotropically strained, have been determined from a line-shape analysis of the polarized PR spectra measured at low and room temperature. In all cases, the anisotropic strain together with the reduced crystal symmetry of the M- and A-planes films results in a strong optical anisotropy. Due to the large strain in at least one of the in-plane directions for the M-plane GaN films, the polarization anisotropy is much more pronounced in the M-plane than in the A-plane films. The polarized PL spectra of M-plane GaN films and M-plane (In,Ga)N/GaN MQWs clearly exhibit a strong polarization anisotropy with degrees of polarization of about 0.90 for the films and 0.96 for the MQW. However, as a result of the large separation of the two uppermost 

the transition related to the second uppermost VB usually exhibits a PL intensity, which is orders of magnitude smaller than the transition related to the uppermost VB. 

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. 

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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. 

Homo-FRET is a useful tool to study the interactions in living cells that can be detected by the decrease in anisotropy [106, 107]. Since commonly the donor and acceptor dipoles are not perfectly aligned in space, the energy transfer results in depolarization of acceptor emission. Imaging in polarized light can be provided both in confocal and time-resolved microscopies. However, a decrease of steady-state anisotropy can be observed not only due to homo-FRET, but also due to rotation of the fluorescence emitter. The only possibility of discriminating them in an unknown system is to use the variation of excitation wavelength and apply the... 

Yellow and Red Dopants Phosphorescent Emitters Creating White Light Creating Polarized Light... 

Even for systems of emitters with isotropic distribution of orientations, the anisotropy of the excitation of emitters by polarized or natural light always occurs, beii caused by the anisotropy of the incident %ht itself. The oscillations of the exciting electric vector always take place in the plane iK>rmal to the direction of the incident light and for linearly polarized light they occur in the direction of the polar-izatvm vector. 

The anisotropic distribution of emitters can be generated even in an isotropic medium by the action of polarized light if this medium exhibits photodiemical... 

Emissivity (e) - Ratio of the radiant flux emitted per unit area to that of an ideal black body atthe same temperature. Also called emittance. [1] Emu - The electromagnetic system of units, based upon the cm, g, and s plus the emu of current (sometimes called the abampere). Enantiomers - A chiral molecule and its non-superposable mirror image. The two forms rotate the plane of polarized light by equal amounts in opposite directions. Also called optical isomers. 

Cholesteric and nematic GLC films following the core-pendant strategy are promising for circular polarizers, optical notch filters and reflectors with spectral position and bandwidth adjustable by chemical and photochemical means circularly polarized fluorescence from a doped light-emitter photonic switching and optical memory with cholesteric and nematic GLC films comprising a dithienylethene core. 

The anisotropic distribution of emitters can be generated even in an isotropic medium by the action of polarized light if this medium exhibits photodiemical activity, i.e. if photochemical reactions can take place in it at a rate dependiiig on the angle between the orientation of a particle and the electrical vector of the incident light wave (Weigert effect). Photoselection takes place in the medium, i.e, the emitters of a certain orientation are selected by the light. 

Light emitters based on nitride semiconductors typically consist of [0001]-oriented quantum wells (QWs) [1] where the quantum confinement Stark effect (QCSE) is caused by piezoelectric and spontaneous polarizations, which lower the optical transition probability [2, 3]. To circumvent this issue, several groups have tried to fabricate InGaN/GaN and GaN/AlGaN QWs on nonpolar planes such as the 1100 plane (m-plane) [4] and the (1120) plane (a-plane) [5-7]. However, owing to the difficulty in growing high-quality crystals in nonpolar directions, the layers contain numerous nonradiative recombination centers. 

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