Resonant cavity LEDs

The emission linewidth of LEDs is typically 10 nm, which is quite broad in comparison to that achievable with laser diodes. Narrower linewidths down to ca. 0.9 nm can be obtained using resonant cavity LEDs.(61) Superluminescent LEDs have been produced with low spectral ripple (less than 10% at wavelengths down to 670 nm) by suppressing the optical feedback in laser diode junctions.(62)... 

R. Bockstaele, T. Coosemans, C. Sys, L. Vanwassenhove, A. Van Hove, B. Dhoedt, I. Moerman, P. Van Daele, R.G. Baets, R. Annen, ReaUzation and characterization of 8 x 8 resonant cavity LED arrays mounted onto CMOS drivtns for POF-based interchip interconnections. IEEE J. Sel. Top. (Quantum ElectroiL 5(2), 224—235 (1999)... 

Dorsaz, (., Carlin, J.-F., Zellweger, C.M., Gradecak, S. and Ilegems, M. (2004) InGaN/GaN resonant-cavity LED including an AIInN/GaN Bragg mirror. Physica Status Solidi a Applied Research, 201, 2675. 

Semiconductor laser diodes are widely used in CD players, DVDs, printers, telecommunication or laser pointers. In the structure, they are similar to LEDs but they have a resonant cavity where laser amplification takes place. A Fabry-Perot cavity is established by polishing the end facets of the junction diode (so that they act as mirrors) and also by roughening the side edges to prevent leakage of light from the sides of the device. This structure is known as a homojunction laser and is a very basic one. Contemporary laser diodes are manufactured as double heterojunction structures. 

Another major application for microresonators is in development and fabrication of novel light sources such as resonant-cavity-enhanced light-emitting diodes (LEDs), low-threshold microlasers, and colour flat-panel displays. In wavelength-sized microresonator stmctures, semiconductor material luminescence can be either suppressed or enhanced, and they also enable narrowing of the spectral linewidth of the emitted light (Haroche, 1989 Yokoyama, 1992 Yamamoto, 1993 Krauss, 1999 Vahala, 2003). 

A.M. Green, D.G. Gevaux, C. Roberts, C.C. Phillips, Resonant-cavity-enhanced photodetectors and LEDs in the mid-infrared. Phys. E 20(3), 531-535 (2004)... 

LEDs can produce infrared or visible light from the recombination of carriers in the junction regions. By inverting the electron population and providing a resonant cavity, stimulated emission may be produced making solid-state lasers possible. 

Figure 141 shows the EL spectra from a microcavity (a) and conventional LED (b) based on the emission from an NSD dye forming a thin emitting layer of a three-organic layer device. It is apparent that the half-width of emission spectra from the diode with microcavity is much narrower than those from the diode without cavity. With 0 = 0°, for example, the half-width of the spectrum of the diode with cavity is 24 nm whereas that of the sample without cavity increases to 65 nm. According to Eq. (275), the resonance wavelength, A, decreases with an increase of 0 in agreement with the experimental data of Fig. 141. We note that no unique resonance condition in the planar microcavity is given due to broad-band emission spectrum of the NSD emission layer. Multiple matching of cavity modes with emission wavelengths occurs. Thus, a band emission is observed instead a sharp emission pattern from the microcavity structure as would appear when observed with a monochromator the total polychromic emission pattern is a superposition of a range of monochromatic emission patterns. The EL spectra...

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%. 

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