Free-space emission

Pigure 10 shows the typical commercial performance of LEDs used for optical data communication. Both free-space emission and fiber-coupled devices are shown, the latter exhibiting speeds of <10 ns. Typically there exists a tradeoff between speed and power in these devices, however performance has been plotted as a function of wavelength for purposes of clarity. In communication systems, photodetectors (qv) are employed as receivers rather than the human eye, making radiometric power emitted by the devices, or coupled into an optical fiber, an important figure of merit. 

Figure 15.18 (Top Left) shows the surface plasmon-coupled chemiluminescence (SPCC) and the fiee-space emission from a blue chemiluminescent dye on a 20 nm aluminum ftiin-film layer. It can be seen that the free-space emission is of much higher magnitude than the SPCC signal. This is...

Experiments were also performed to determine the rate of decay of luminescence for blue and green chemiluminescent dyes as a function of time for both the free-space emission as well as the SPCC emission (with p-polarizers so that... 

Fig. 4.3b illustrates the modifications to the free-space emission produced by the planar microcavity device whose structure is shown schematically in Fig. 4.3a. The principal effect is a narrowing of the emission spectmm in most directions. The cavity device spectrum is taken along the cavity axis, and the full width at half-maximum (FWHM) is lowered from 100 nm for a noncavity device to 18 nm. With higher- Q cavities, it is possible to get narrower spectra, and Tokito et al. have reported LEDs with a FWHM of 8 nm.28...

The EL spectrum has multiple peaks when more than one mode of the cavity overlaps the free-space emission spectrum. It is possible to realize a white LED with a single electroluminescent material such as Alq by employing a two-mode microcavity device structure in which one of the modes is centered near 480 nm and the other near 650 nm. Such an electroluminescence spectrum, for which the CIE coordinates are (0.34, 0.386),13 is shown in Fig. 4.6. The approximate spectrum calculated with Eq. (4) is also shown in Fig. 4.6. With very minor changes in the device design, it is easy to achieve (0,33, 0.33). For comparison, the CIE coordinates of a noncavity Alq LED are (0.39, 0.56). 

We have indicated how a modest (1.3) enhancement in angular intensity can be obtained in cavity devices with Alq emissive layers. Further enhancements in angular intensity are possible by choosing emissive layers with narrower free-space emission spectra than Alq. Alq doped with small quantities of the laser dye pyrromethene 580 (PM) results in the emission spectrum of the system becoming narrower than that of Alq. This is result of resonance energy transfer33 from the excited states of Alq to the excited states of PM580. The full width at half-maximum of the luminescence drops from 100 nm to 45 nm. The spectra are shown in Fig. 4.12. The external quantum efficiency of noncavity devices is enhanced in comparison with devices with an undoped Alq emissive layer. For a device with a ITO/TAD/Alq+0.5%PM (20 nm)/Alq/Li (1 nm)/Al(200 m) structure, an external quantum efficiency of 1.8-2% photons/electron was measured. For comparison, equivalent LEDs without the pyromethene dye had an external quantum efficiency (with Li/Al cathodes) of 0.8%. 

For monochrome displays, backlights, and indicators, the Alq+PM system described in the previous section, or its equivalent, would be very suitable. High efficiencies are attainable with this material system and the free-space emission range is centered near 545 nm, which coincides with the maximum sensitivity of... 

The effects produced by a planar microcavity on the electroluminescence characteristics of organic materials have been described. A number of organic and polymeric semiconductors have been employed by various groups in studies on microcavity LEDs. However, for detailed descriptions, three categories of emissive materials have been considered undoped Alq, Alq doped with 0.5% pyrromethene, and Alq+NAPOXA. Alq has a broad free-space emission spectrum spanning the... 

Fig. 1.8 Optical mode density for (a) a short and (b) a long cavity with the same finesse F. (c) Spontaneous free-space emission spectrum of an LED active region. The spontaneous emission spectrum has a better overlap with the short-cavity mode spectrum compared with the long-cavity mode spectrum.

B) of the sample. The intensity observed on the front (F) side of the sample was much lower and not sharply distributed at any particular angle. T o determine the relative intensities of the coupled and free-space emissions, we integrated the intensities observed at all angles on the front side (180° 90°) and the back side (0° 90°). The total back-to-front intensity ratio (Ib... 

In previous studies of the effects of silver particles on fluorophores we observed substantial decreases in lifetime as the intensities increased [6-8]. We interpreted this effect as an increase in the radiative decay rates near the metal particles [32-33]. In an analogous way we expected the radiative decay rate to be increased in the direction of the surface plasmon angle. Stated alternatively, we imagined that a large fraction of the emission appeared as SPCE because the rate of transfer to the surface plasmon was larger than the rate of spontaneous free-space emission. Hence, we expected the lifetime of SPCE to be shorter than the free space emission. 

Figure 18 shows frequency-domain intensity decays for the free-space emission (top) and the surface plasmon-coupled emission (bottom). Overall, the lifetimes of SPCE (bottom) and free-space do not differ significantly. This was an unexpected result which we do not fully understand. We carefully considered possible artifacts and the effects of sample geometry, but can only conclude that our experiments indicate that the component of SPCE that we observe occurs without a substantial change in lifetime. At present we do not understand the origin of this discrepancy. 

Figure 18. Frequency-domain intensity decays of SlOl in PVA. Top, free-space emission. Bottom, surface plasmon-coupled emission. Adopted from [30].

Fig. 29.19 Photoluminescence spectrum for the microcavity structure shown in Fig. 29.18, for a structure made with both aluminum and silver cathodes. The free-space emission from the PPV layer is also shown for comparison (as shown, for example, in Fig. 29.3). (From Ref. 138.)...

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