斯坦福科学家发明纳米级非线性光源

Not long after the development of the first laser in 1960 scientists discovered that shining a beam through certain crystals produced light of a different color; more specifically, it produced light of exactly twice the frequency of the original. The phenomenon was dubbed¹ second harmonic generation（倍频效应） . The green laser pointers in use today to illustrate² presentations are based on this science, but producing such a beautiful emerald beam is no easy feat³. The green light begins as an infrared⁴ ray that must be first processed through a crystal, various lenses and other optical elements before it can illuminate⁵ that PowerPoint on the screen before you. #

It was later discovered that applying an electrical field to some crystals produced a similar, though weaker, beam of light. This second discovery, known as EFISH -- for electric-field-induced second harmonic light generation -- has amounted mostly to an interesting bit of scientific knowledge and little more. EFISH devices are big, demanding high-powered lasers, large crystals and thousands of volts⁶ of electricity to produce the effect. As a result, they are impractical⁷ for all but a few applications. #

In a paper published September 22 in Science, engineers from Stanford have demonstrated a new device that shrinks EFISH devices by orders of magnitude to the nanoscale. The result is an ultra-compact light source with both optical and electrical functions. Research implications for the device range from a better understanding of fundamental science to improved data communications.

Spring-loaded electrons

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The device is based on the physical forces that bind⁸ electrons in orbit around a nucleus⁹. #

"It's like a spring," said Mark Brongersma, an associate professor of materials science and engineering at Stanford.

In most cases, when you shine a light on an atom, the added energy will pull the electron away from the positively¹⁰ charged nucleus very predictably, in a linear（线的） fashion, so that when the light is turned off and the electron springs back to its original orbit, the energy released is the same as the light that displaced it. #

The key phrase here being: "in most cases." When the light source is a high-intensity laser shining on a solid, researchers discovered that the farther the electrons are pulled away from the nuclei¹¹ the less linearly the light interacts with the atoms.

"In other words, the light-matter interaction becomes nonlinear," said Alok Vasudev, a graduate student and co-author of the paper. "The light you get out is different from the light you put in. Shine a strong near-infrared laser on the crystal and green light exactly twice the frequency emerges."#p#分页标题#e# #