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Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society

mirror, and also quenches excitation by energy transfer to surface plasmon modes in the silver layer [8]. We have recently developed microcavities which use only dielectric stack mirrors, using evaporated layers of ZnS and MgF2 deposited on top of the PPV layer [9], and we have reported that these structures show much lower lasing thresholds. We report here two recent studies we have conducted which advance our understanding of the issues that determine conditions for lasing. In the first, we summarise results obtained on microcavities which are arranged to be wavelength-tunable, by introduction of a liquid crystal layer within the cavity [10]. In the second, we report the response of polymer LEDs driven to high current densities [11]. RESULTS Tunable Microcavity Structure The planar microcavity is a Fabry-Perot resonator with a mirror separation of the order of the optical wavelength which contains a photon emitting medium. The value of this structure is that the spontaneous emission rate and the emission spectrum of an optical emitter can be modified, since the cavity enhances the rate of emission at the resonance wavelengths of the cavity and suppresses the rate of emission at other wavelengths [12-14]. Structures such as the one investigated here affect the total emission rate only marginally. However, the spectral and spatial dependence of the radiative rate is still strongly modified by the microcavity. At the resonance wavelengths of the cavity the radiative rate for emission in the forward direction can easily be enhanced by more than two orders in magnitude, and large enhancements are found experimentally [15]. In the same fashion it will be suppressed at other wavelengths. This leads to a spatial and spectral redistribution of emission as a result of channeling of radiation power into the modes for which the radiative rate is enhanced. For an efficient broad band emitter inside a tunable microcavity this effect can be used to control the spectral and spatial distribution of emission without the losses of radiative power associated with filters. Three factors determine the effective length, Leff, and thus the resonance wavelengths of a

microcavity, the separation of the mirrors from the mirrors Leff = n

(Lphase change) and

(L...or separaon),the phase

change on reflection of light

the refractive indices (n) of the materials inside the cavity:

Li.r.or separation + Lph.s change

(1)

and

"ARes = 2Leff

(2)

q where XRes are the resonance wavelengths and q is an integer. In our devices, changes in the effective refractive index of the materials inside the cavity are used to control the effective length of the microcavity. Although many materials display refractive index changes in response to applied electric fields, the magnitude of these changes is too small to shift significantly the resonance wavelengths of a microcavity. We have therefore used highly birefringent liquid crystalline materials. Liquid crystalline materials have pr