Photonic crystals enable ultra-small organic surface-emitting lasers

Increasingly-high-bandwidth data transmission requires novel and innovative hardware concepts and devices, because today's technologies will soon be facing physical limits. Reaching this performance ceiling could be postponed significantly by application of optical interconnects on short distances, such as card-to-card, on-card, and ultimately on-chip scales. In recent years, organic semiconductor materials have attracted increasing interest in this context, owing to their tailor-made, excellent optical properties, their enormous potential for display, sensing, and solar-cell applications, and their compatibility with CMOS technology. These materials are also characterized by low lasing thresholds and a spectrally broad gain, allowing for the emission wavelength to be tuned across the entire visible spectrum. Ladder-type poly- para-phenylenes (LPPPs) are among the materials with the highest optical gain, of about 2000cm⁻¹. Typical resonator designs for organic lasers consist of linear and circular distributed-feedback structures (DFBs), as well as microrings and microcavities, thus facilitating optical pumping.

Figure 1. (a) Concept showing the enclosure of a vertically emitting second-order feedback structure with a first-order ‘mirror.’ (b) Scanning electron microscopy image showing the corner of the device (without organic gain layer; the scale bar represents a size of 2μm). (c) Reduction of the laser threshold enables optically pumped lasing, even in ultra-small-footprint devices.

We have built a vertical surface-emitting organic-laser structure with a 2D photonic-crystal layer made from high-index tantalum pentoxide (Ta₂O₅), which exhibits a reduced lasing threshold. The basic idea is to enclose a second-diffraction-order vertically emitting laser lattice by mirrors, to reflect light back into the active region otherwise lost due to leakage.¹ This way, more light is confined within the second-order region, resulting in a dramatically reduced lasing threshold. Additionally, the construction of much smaller device footprints becomes feasible. Here, we choose first-diffraction-order photonic crystals as mirrors, with their band gap centered around the lasing wavelength, and no perpendicular outcoupling. Since the mirror material is integrated into the same planar Ta₂O₅ layer as the second-order lasing structure, the layer thicknesses of both the Ta₂O₅ and the LPPP-gain material are fixed. Therefore, only the lattice constant and the hole radii forming the structure are free parameters. The former can be determined to select one mode at the desired wavelength. For a lattice constant of 300nm and a hole radius of 105nm for the second-order structure, lasing occurs around 494nm, close to the gain maximum of our polymer material. To center the band gap of the first-order structure around the lasing wavelength, a hole radius of 50nm and a lattice constant of 150nm are needed. The first-order mirror structure consists of 55 periods.

Figure 1 shows the emission intensity as a function of the pump-pulse energy, in addition to a conceptual overview of the device's operation and a scanning-electron-microscopy image of its structure. Lasing occurs for both the second-order-only and the mixed-order structure of our devices. However, the 25×25μm² and the 15×15μm² second-order-only structures (without first-order mirrors) show no lasing within the pump-power range where the gain polymer is not degraded. In contrast, all mixed-order photonic-crystal structures, even the smallest ones, exhibit lasing with a considerably reduced threshold compared with their second-order-only equivalents. The threshold pump energy per pulse is a record low ≃40pJ. Here, the first-order photonic-crystal structure reflects the propagating laser mode back into the second-order structure, enhancing the propagation length and thus satisfying the requirements for a certain interaction length for lasing. By using the mixed-order approach, all structures provide the necessary coupling length due to the mirror effect of the first-order structure. One can decrease the area of the vertically emitting laser by an order of magnitude and still work at the same lasing threshold level.

In summary, we have investigated mixed-order 2D photonic-crystal laser structures based on a thin film of high-index Ta₂O₅, in combination with a high-gain organic polymer. We designed an optimized mixed-order structure by means of finite-difference-time-domain simulations and showed that the mixed-order structures are superior to the second-order-only lattice in terms of lasing threshold and device footprint. Such small-footprint structures are key components of future integrated photonic devices for both communication and sensing applications. They also offer an attractive packaging possibility for light-emitting arrays coupled to high-density optical interconnects.

Future improvements will include optimization of the distance between first- and second-order structures, as well as application of thicker Ta₂O₅ layers. Additionally, including defects into the periodic structure will offer a controlled output coupling.

We gratefully acknowledge financial support from the European Commission as part of the 'OLAS' project (FP6-015034).