Reflecting on super-skinny mirrors

Engineers at the University of California, Berkeley, have created a high-performance mirror that could dramatically improve the design and efficiency of the next generation of laser optic devices.

The new mirror has the same 99.9 per cent reflective ability as current high-grade mirrors, called distributed Bragg reflectors (DBRs), but is at least 20 times thinner, functional in a considerably wider spectrum of light frequencies, and easier to manufacture.

The super-thin mirror is known as high-index contrast sub-wavelength grating (HCG). Its goal is to lower the mass of devices, which translates into lower power consumption by delivering high reflectivity without the extra thickness.

Instead of multiple levels of alternating refractive-index layers, the HCG mirror developed by the UC Berkeley engineers contains only one pair. The engineers used aluminium gallium arsenide for the high refractive index layer, coupled with a layer of air, which has a very low refractive index of 1. In addition, the high refractive index layer contained grooves spaced by a distance that is less than a wavelength of light.

In this configuration, light hitting the mirror surface was directed over the grooves. As the light waves passed each semiconductor-air interface, they were strongly reflected back in the opposite direction. The researchers noted that other materials could replace air as the low refractive index material. Silicon dioxide, for instance, has a refractive index of 1.5.

To demonstrate the reflectivity of the HCG, the researchers replaced one of the two DBRs in a vertical-cavity surface-emitting laser with the new mirror. They confirmed that the HCG is capable of providing reflectivity greater than 99.9 per cent, equivalent to the DBR.

The engineers are also studying applications for the mobile HCG mirror in micro-electromechanical systems (MEMS), such as wavelength tuneable lasers, which are used in broadband communications.

The researchers added that it may be possible to print this mirror on various surfaces, and that it could one day be used to create organic, plastic displays that can be rolled up for easy transport.