Integrated organic photonics one step closer

Organic semiconductors—materials combining opto-electronic functions with plastics processing—will have a large impact on consumer electronics. Organic LEDs are now widely used in cell-phone displays and have potential for use in solid-state lighting. Organic solar cells recently entered the market in consumer products such as Konarka's Power Plastic^® for solar bags. Following this first wave of organic photonics products, there is significant research interest to develop future technologies, such as organic lasers and optical amplifiers. These next-generation devices will offer coherent light emission across the visible spectrum for applications ranging from chemical sensing to data communications.

A major hurdle in the application of organic lasers has been the difficulty in achieving an electrically driven device.¹ This unsolved problem is currently blocked by the slow transport of charges through the materials used. While work continues to address this challenge, we recently demonstrated an alternative, indirect route to electrically driven organic lasers. We used a hybrid opto-electronic approach in which an electrically driven inorganic LED optically pumped the organic laser.² This exploits the fast charge transport and efficient light emission of indium gallium nitride (InGaN) LEDs, combined with the simple processing and excellent photonic properties of the organic semiconductor. We are currently working with partners in the UK-funded HYPIX project (hybrid organic semiconductor/gallium nitride/CMOS smart-pixel arrays) to develop highly integrated versions of this concept: InGaN micro-LEDs, directly integrated on CMOS electronics, will power organic photonic circuits. Figure 1 shows the schematic structure of these hybrid smart-pixel arrays.

 

Figure 1. Concept of integrated organic photonic devices pumped by indium gallium nitride (InGaN) LEDs on a CMOS electronics backplane. Si: Silicon. HYPIX: Hybrid organic semiconductor/gallium nitride/CMOS smart-pixel arrays.

Two challenges in developing these systems are the vertical integration of organic photonics with the pump source and the horizontal integration of different organic devices. InGaN LEDs are limited in terms of the maximum power density they can deliver to pump the organic laser. It is also difficult to form well-defined facets in solution-processed organic films to allow in-plane integration of lasers, optical amplifiers, and waveguides. To address the first challenge, we have developed fluorescent optical concentrators to intensify the LED light emission and, thus, achieve a higher power density for pumping an organic laser.³ The fluorescence concentrator is based on a dye-doped plastic waveguide (see Figure 2). The latter is optically excited on the surface, and much of the light that is absorbed and re-emitted within the film is waveguided to the edge. The ratio of the illuminated surface area to the edge-emission area allows a significant light concentration.

Figure 2. Schematic of a fluorescence concentrator for optical pumping of a polymer-distributed-feedback laser.

This idea has been studied for a number of years in the form of luminescent solar concentrators for photovoltaic panels. We have now adapted it to pumping lasers. In view of the limited size of LEDs and the need for the highest possible intensity, we have scaled down all dimensions by a factor of 1000 to concentrate the square-millimeter emission typical of high-power LEDs to an output stripe ~30μm wide. Luminescent concentrators can increase the power density of a 450nm light source by a factor of 10. When butt-coupled to an organic distributed-feedback laser, they can reduce the required power density of the primary pump source by a factor of five.

High-quality end facets are clearly desirable for integrating optical concentrators with lasers, but these are even more important for in-plane integration of organic lasers with optical amplifiers and waveguides. This is, however, a challenging problem for solution-processed organic semiconductors. When one tries to cleave a semiconducting polymer waveguide, the resulting film edge is usually very ragged and not at all suitable for optical coupling. To address this problem, we developed a process that encapsulates the semiconducting polymer layer within a more brittle, photo-patternable polymer.⁴ The semiconducting polymer is sandwiched between two layers of the negative photoresist SU8, the top layer of which is photo-patterned to define a waveguide. Upon development of the SU8 top layer, the unprotected semiconducting polymer is removed using a solvent wash before the full structure is encapsulated with a further SU8 layer (see Figure 3). We find that these layered structures can be successfully cleaved, yielding good-quality optical facets. We have demonstrated their usefulness as end-fired organic-semiconductor optical amplifiers with 18dB of gain in a 750μm-long waveguide.

Figure 3. (left) Schematic of a cleaved semiconducting polymer waveguide. F8BT, poly(9,9'-dioctylfluorene-co-benzothiadiazole), is the particular polymer used. (right) Scanning-electron-microscope image of the edge of a cleaved polymer waveguide. SiO₂: Silicon dioxide. SU8: Epoxy-based negative photoresist.

Integration of organic lasers with CMOS-controlled nitride LEDs offers a promising route to electrically controlled organic photonics. We have shown how fluorescent organic waveguides can be designed to reduce the external pumping requirements of polymer lasers and developed structures that can allow in-plane coupling to active organic waveguides. Our next step will be integration of these elements into photonic circuits, opening up new routes to on-chip visible-light sources for data communications, spectroscopy, and sensing.

This research is funded by the UK's Engineering and Physical Sciences Research Council's HYPIX project.