Smaller, Cheaper, Better Lasers
Affordable HD-DVD players are one potential payoff from a simpler process for making semiconductor lasers.
Anyone thinking of buying an HD-DVD player will confront sticker shock: the gadgets cost around $500. But a new design for semiconductor lasers could ultimately cut costs not just for these players but for any device that uses the ubiquitous components.
Researchers at the University of California, Berkeley, overhauled one of the main laser elements, the mirror that makes up the cavity where light is produced. The result: a thinner mirror that can be made in fewer steps, simplifying the fabrication process and lowering costs. The new mirror is also more reflective than previous designs, which could make electronics more energy efficient. That would be particularly helpful in displays or projectors used in handheld devices.
Almost any optoelectronic device that uses mirrors–from sensors to solar cells–could benefit from an efficient, thinner, cheaper mirror. “We feel this is a technology that can be widely used,” says Connie Chang-Hasnain, professor of electrical engineering and computer sciences at Berkeley and lead researcher on the project. The details are published in a recent issue of Nature Photonics.
Most recently, the researchers used their mirror in a type of semiconductor laser called a vertical-cavity surface-emitting laser, or VCSEL. These are commonly used in the telecommunications industry, producing the light that carries packets of Internet information around the globe. High-power VCSELs are being used as the light source for vivid displays that could compete with plasma TVs. (See “Ultra-Colorful TV.”) Although HD-DVDs do not use VCSELs specifically, Chang-Hasnain says the group’s technology would still be applicable.
A typical VCSEL is made of two sets of mirrors that sandwich an active region. When an electric current is applied to the active region, electrons gain or lose energy, producing photons in the process. These photons are contained by the semitransparent mirrors, and their intensity is amplified as they bounce around in the active region. But once they are sufficiently amplified, they pass through the mirrors, producing a beam of coherent, single-color light.
To Chang-Hasnain, the problem with this design is the number of layers that are required to build a mirror. A typical mirror in a VCSEL is made of about 80 layers. So many layers are needed because the materials in the active region and the most reflective material for the mirror have incompatible crystalline structures. To get around this problem, engineers grow numerous thin layers with slightly different structures for each single layer, making the total structure consist of tens of layers, each with precise thickness and composition.
Chang-Hasnain realized that a novel mirror structure could yield benefits with only one layer. In the paper, her group reports a mirror that consists of thin pieces of aluminum gallium arsenide, separated by air, with an air gap between the laser’s active region and the mirror.
The photons from the active region enter the aluminum gallium arsenide pieces; then, because of the optical interaction between the material and air, they take a 90º turn, reflect off the other pieces, come back, and make another 90º turn into the active region. “It’s so simple, one must think it’s been invented before,” says Chang-Hasnain, “but it was not.”
In this first prototype, her group was able to reduce the thickness of the typical laser cavity mirror from five micrometers to less than a quarter of a micrometer. The researchers also found that the mirror worked well for a variety of wavelengths of light, which increases its range of applications. And because photons don’t have to travel through as many layers, fewer photons are lost. This means that laser efficiencies can be improved.
“It’s an interesting and somewhat surprising solution in this field after so many years of research in the VCSEL research community,” says Shun Lien Chuang, professor of electrical and computer engineering at the University of Illinois in Urbana.
The immediate applications, says Chang-Hasnain, would be in telecommunications. The lasers could be useful in bringing fiber optics to people’s homes for ultrafast Internet connections. If the appropriate materials were used, however, the mirrors could also be incorporated into HD-DVD players. The researchers are currently filing a patent. Although she doesn’t yet have a time line for commercialization, she believes the technology is “very close to realization.”
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