Optical mode control in VCSEL-based photonic crystal heterostructures

Abstract

The field of photonic crystals is a rapidly developing branch of modern optics. Photonic crystals are believed to be the cornerstone for a wide range of new photonic devices controlling the confinement and propagation of light. These artificial structures have greatly benefited from far-reaching analogies with semiconductor crystals. Following the successful path taken by science and technology in that field, one can expect that novel functionality will emerge by assembling photonic crystals with dissimilar properties into heterostructures. The main theme of this thesis is to explore novel methods for transverse mode control in arrays of vertical cavity surface emitting lasers (VCSELs) using the concept of photonic crystal heterestructures. Starting from the new confinement effects of photonic envelope functions offered by these structures, the extension to multiple heterostructure domains was made in this thesis work. Insight into the complex modal behaviour of such structures was obtained with the aid of model calculations. A theoretical framework, formally identical to coupled mode theory, was devised to enable efficient modelling of arbitrary heterostructure designs. Active photonic crystals were implemented using metal-patterned VCSEL-arrays. For optical characterization of the devices, a novel aperture generation technique using a spatial light modulator in the diffractive mode was employed. This technique was utilized to study the degree of spatial coherence via Young's interference experiments, which yields detailed information on the coupling between VCSELs across the lattice. Two closely separated domains of photonic crystal heterostructure, or islands, were investigated in detail, theoretically and experimentally. Coupling of the photonic envelope wavefunctions confined to each island is brought about by tunneling across the heterobarrier separating them. This splits the lowest loss mode of a single photonic island into bonding and anti-bonding modes. Numerical simulations predict the bonding state to have the lowest modal losses. The experimental observations of lasing supermodes confirm this prediction, evidencing island coupling in the bonding state of the coupled envelope functions. Device applications making use of the two-island system would require dynamic control of the inter-island coupling strength. Investigations of different methods to deliberately break or induce coupling were carried out. One possibility to modify the properties of the active photonic crystal is to locally alter the gain. In this way, it is possible to compensate for inadvertent variations between the islands, which allows for controlled mode switching in the transversal plane. The gain variations can be introduced by controlling the current injected into each island. To this end, photonic crystal heterostructures including two separate electrical contacts were fabricated. Control of the injected currents allows bringing the photonic islands in and out of mutual coherence. The demonstrated switching of the transverse lasing mode in the near-field is accompanied by corresponding variations in the far-field pattern and the emission wavelength. Proper adjustment of the driving currents also allows steering of the beam of the ensemble of coupled lasers. As a natural extension to the two-island system, devices including multiple photonic islands were fabricated. This provides a means for scaling up the size of the lasing mode by controlled coupling between the individual domains. Such structures point towards more advanced photonic crystal superlattices, possibly candidates for the realization