In this work it was investigated the formation of optical cavities in Li doped ZnO nanostructures, which were grown by the vapor-solid method by which long micro and nanostructures with a hexagonal cross section were obtained.
The incorporation of Li in the growth process improved the quality of the optical cavities and produced morphologies of structures suitable to confine light in the cross section.

These structures have favorable morphologies to form Fabry-Pérot cavities (two reflective surfaces facing each other) and the appearance of whisper gallery modes.
Whisper gallery is the phenomenon by which waves (sound or light waves in this case) can be transmitted with almost no loss when circulating around a structure.

These investigated materials have great potential for the manufacture of integrated optoelectronic devices. The possibility of integrating them into the same material as the optical resonator and the light guide opens the door to the manufacture of very compact devices.

ZnO is a well-known material with applications in many different fields, from photonics to cosmetics.
It has a wide forbidden band (materials with a larger forbidden band width offer better behavior as semiconductors), a relatively high refractive index, high exciton bond energy and non-ohmic behavior are behind many of these applications.

In particular, the capabilities of generating lasers at room temperature have received much attention in recent years, with special attention to the formation of hexagonal structures, rods, plates or nails at micro and nanoscale. Resonant cavities play an important role in a variety of devices, such as musical instruments or microwave generators; however, in recent years, the need for miniaturized devices has focused attention on optical micro and nano-cavities.
Semiconductor resonators with whispering gallery modes are one solution to this need. The hexagonal morphology and size of the structures causes multiple internal reflections. The crystalline structure of wurtzite offers unique properties to create micro cavities for this purpose as it favors the growth of hexagonal sections. These structures lead to high Q-factors and consequently a low threshold of laser emission.

In elongated structures, such as hexagonal rods obtained with ZnO, different resonant modes may appear. The easiest is of course the Fabry-Pérot mode which is set along the structure due to the reflection at both ends of the structure, however these modes are not optimal for laser or resonant cavity applications due to the low reflectivity of the normal incident light of the final facets.
Much better conditions are obtained when confinement occurs in the cross section of the structure. The resonant modes would be set at the perimeter of the rod and propagate along the structure.
In most cases, the observed resonances are compatible with whispering gallery modes.

More information in the article

The work is a collaboration between the Institute of Optics and the Department of Materials Physics of the Faculty of Physics of the Complutense University of Madrid