Optical structures with periodic variations of the dielectric constant in one or more directions (photonic crystals) have been employed extensively for studying optical diffraction phenomena. Practical interest in such structures arises from the possibilities they offer for tailoring photon modes, and thereby the characteristics of light propagation and light-matter interactions. Photonic resonator crystals comprising two-dimensional arrays of coupled optical microcavities have been fabricated using vertical-cavity surface-emitting laser wafers. In such structures, the light propagates mostly normal to the periodic plane. Therefore, the corresponding lateral Bragg-periodicities are larger, a feature that is advantageous for device manufacture as it allows for larger lattice constants in the lateral direction. Here we investigate strain effects in a photonic resonator crystal by shifting neighbouring lattice rows of microcavities in opposite directions, thereby introducing an alternating square or quasi-hexagonal pattern of shear strain. We find that, for strain values below a critical threshold, the lasing photon mode is virtually locked to the corresponding mode supported by the unstrained photonic crystal. At the critical strain value, we observe a phase-transition-like switching between the square and quasi-hexagonal lattice modes. The tolerance of subcritical strains suggests that the resonator crystal may be useful for applications requiring high spatial coherence across the lattice, while the mode switching could potentially be exploited in free-space optical communications.

CdS microcrystals under weak confinement conditions embedded in a polyvinyl alcohol polymer film have been optically characterized by linear absorption and time-resolved luminescence spectroscopy.They are studied in the presence of an external electrical field at low and room temperature and different densities of photoexcited carriers. By applying a voltage of 400 V corresponding to an external electricfield strength of 5 X 10 V/cm, the observed absorption change in this material is Aa/a =0.07. The electrical field produces a redshift of the absorption band which is explained in terms of the quantumconfined Stark effect. In addition, a restoring of the oscillator strength is observed and explained by screening effects of internal fields in the interface region by the photogenerated electron-hole pairs. At high laser excitation an electric-field-induced change of the nonlinear absorption spectrum of ha/a=0. 25 is achieved, giving a considerable modulation of the absorption edge. The electrical field separates the laser excited carriers and the change in the absorption is attributed to a compensation of the many-particle interaction by the external electrical field.

Coupled arrays of vertical cavity surface emitting lasers with controlled optical disorder were realized by patterning the reflectivity of the top cavity mirrors. The size of each square array element, defined by photolithography, determines the local cavity oscillation frequency and loss. Optical disorder across the array can therefore be introduced by varying the size of the lasing elements. The localization of photon modes and the array coherence were investigated as a function of the degree of disorder and the array dimensionality. We found that a higher degree of disorder leads to more localized coherent modes and consequently to broader far-field patterns. However, the effects of disorder reduce considerably as the array dimensionality increases from one to two by adding array rows, leading to more effective suppression of mode localization.

Arrays consisting of coupled vertical-cavity surface-emitting lasers represent a model system for photonic lattices. Fluctuations among array elements, caused by wafer nonuniformities or processing defects, can lead to photon-mode localization, thus deteriorating the spatial coherence of such devices. The influence of cavity nonuniformities on the near-field and far-field patterns in arrays of different dimensionalities is analyzed in the framework of coupled-mode theory. It is shown that, for the same degree of disorder, photon localization is drastically reduced as the array dimensionality increases.