Although waveguides are essential in guiding light along nanoscale photonic devices, they are plagued by optical losses caused by the coupling of light into the waveguide as well as inefficient light propagation along their lengths. An elegant solution to these problems may come from organic microtube waveguides now fabricated by researchers from the Institute of Chemistry, Chinese Academy of Sciences in Beijing.1

Typically, structures used to guide light are made from solid materials. Optical fibers for example rely on the internal reflections of light at the interface between the fiber core and its cladding materials. However, although such schemes work very well for the relatively large optical fibers, they are less advantageous on the scale of integrated photonic circuits, where the light reflected from the internal surface of the waveguide interacts with the device substrate, significantly increasing losses.

Therefore, rather than using solid waveguides, hollow microtubes may serve as better alternatives because they offer a tighter confinement of light. “The air medium inside the tubes plays an important role in reducing optical losses,” says Jiannian Yao from the research team. To this end, the researchers have now developed a synthesis process that achieves the self-assembly of tubular waveguide structures made from a light-emitting dye compound. The length and diameter of the tubes can be controlled during the preparation.

Fig. 1: Light emission from isolated microtubes. The scale bar is 10 µm.

The tubes absorb light in the blue region of the spectrum and emit yellow light (Fig. 1). To couple light efficiently into the waveguides, a blue laser beam is used to excite the waveguide material. The laser light is absorbed and consequently re-emitted as yellow luminescence, which then propagates along the waveguide.

The losses in light propagating along the tubes are found to be much smaller than for comparable solid microrods. Unlike for solid waveguides the main losses are not related to the substrate but are due to the continued absorption and reemission of the yellow light along the path.

The tubular waveguides offer the additional benefit that light can even cross over from one tube to the other, offering valuable functionality that establishes these structures as valuable components in future devices. Indeed, according to Yao, “their integration into photonic devices is one of our next tasks."