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20 Feb 2007
US scientists have designed an ultrathin mirror with a reflectivity of 99.5% that could replace distributed Bragg reflectors as the mirror of choice in laser optics.
The new mirror, developed by researchers at the University of California, Berkeley, US, exploits the large difference in refractive index between AlGaAs and air to achieve a reflectivity that matches that of distributed Bragg reflectors (DBRs). According to Constance Chang-Hasnain, director of UCB's Center for Optoelectronic Nanostructured Semiconductor Technologies (CONSRT), the work could have important implications for vertical-cavity surface-emitting lasers (VCSELs).
All lasers exploit mirrors at opposite ends of a photon source to produce coherent, single-wavelength light. Photons bounce back and forth between the two mirrors, building up their energy with each pass, and a laser beam is formed once they gain enough energy to compensate for the mirror losses.
Vertical-cavity lasers exploit the same idea, but require highly reflective mirrors because the gain medium is so short. DBRs, which are made of alternating layers of AlGaAs and GaAs, achieve a reflectivity of more than 99.5%. However, DBRs typically require some 80 layers to achieve the required reflectivity – which makes them quite thick (5 µm wide) and expensive to manufacture.
In a departure from this multiple-layer design, the mirror developed by the UCB team contains only one layer of high-index AlGaAs. This layer was patterned with nanoscale, sub-wavelength grooves on its surface to couple this high-index layer with an air-cladding layer, which has a refractive index of 1.0. "We chose air as the low-index cladding layer, because it yields a large refractive index contrast with AlGaAs," said Chang-Hasnain.
"Surface-normal incident light hitting the mirror surface is directed over the grooves," she continued. "As the lightwave travels along the grating and passes each semiconductor–air interface, it is strongly reflected in the opposite direction." In the future, a dielectric-like silicon dioxide or alumina could also be used instead of air.
To demonstrate its effectiveness, the team replaced one of the DBRs in a vertical-cavity laser with a mirror based on the new high-index contrast sub-wavelength grating (HCG) design. Experimental results confirmed that it is capable of providing a reflectivity of greater than 99.5%. The HCG laser source also exhibited good optical performance, with a low lasing threshold current of 0.8 mA and an output power of 1 mW.
According to Chang-Hasnain, lasers fabricated using these HCG mirrors are likely to have switching speeds in the order of 10–20 GHz. And that is not all. "The physical movement of the HCG mirror can result in wavelength variation (tuning) and this speed will be 30–100 times faster than other lasers," she said. "We are very excited about its potential, and are now looking for ways to use it in MEMS devices."
By making lasers thinner and lighter, this mirror could open up applications that were previously not possible with vertical-cavity lasers. The researchers claim that LEDs, photovoltaic devices, sensors, computer chips and telecommunications equipment could all become a lot smaller.
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