Microcavity devices

A semiconductor MC is very similar to a simple Fabry-Perot resonator with planar mirrors. However, due to the penetration of the cavity field into DBRs, the depth is given by [110] 

Compared to semiconductor DBRs, dielectric DBRs possess a number of advantages such as high-refractive index mismatch (e.g., 0.6 at 400 nm for Si02/SiNx 

A planar cavity provides no in-plane confinement (perpendicular to the growth axis) and just as for electronic states in QWs photons have only in-plane dispersion. Photons are quantized along the cavity as the mirrors force the axial wave vector kz in the medium to be 2n/L. Hence the cavity photon energy is approximately 

For small k//, the in-plane dispersion is parabolic (as depicted in the above equation) and therefore it can be described by a cavity photon effective mass M = iTi hnJcLc. This effective mass is very small, 10 me, and the dispersion can be measured directly in angle-resolved experiments allowed by the introduction of an in-plane component to the photon wave vector. Experiments involving off-normal incidence can also be modeled by including an appropriate in-plane wave vector for the field. 

The Rabi splitting in MCs is enhanced significantly compared to that in bulk making it easier to achieve the desired coupling at practical temperatures. The cavity polaritons feature large and unique optical nonlinearities not achievable in a bulk 

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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). 

When designing microcavity devices, the three most important considerations are the following mirror properties, the length of the cavity... 

Comparison of optical characteristics of a mlcrocavity device and a noncavity device, (a) Measured EL spectra with relative intensities at 0° 30° and 60° off the surface normal of the microcavity device, and at 0° of the noncavity device, (b) cdA efficiencies for both devices, (c) 1976 CIE coordinates of 0°-80° EL of the microcavity device and 0° EL of the noncavity device, (d) Polar plots of measured EL intensities (normalized to the 0° intensity of the noncavity device) for both devices. 

Figure 9.18a and b shows the cdA efficiencies and the external quantum efficiencies, respectively, of the three devices [36]. The conventional bottom-emitting device, the microcavity device without microlenses, and the microcavity device with microlenses show efficiencies up to (14.5 cd/A,... 

The first microcavity devices and distributed feedback (DFB) laser with spiro compounds were realized by Benstem et al. [92]. For a microcavity with Spiro-60 between a semitransparent silver mirror and a multistack of Ta205/Si02, a spectral halfwidth (FWHM) of 7.2 nm was achieved. The fluorescence lifetime within the microcavity (240 ps) was distinctly lower than without the cavity (345 ps). The emission of the DFB laser (250-nm-... 

Vandaveer, W. R., Woodward, D. J., and Fritsch, I. 2003. Redox cycling measurements of a model compound and dopamine in ultrasmall volumes with a self-contained microcavity device. Electrochim. Acta 48 3341-3348. 

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