Free-electron lasers are the must-have gadgets for all self-respecting
modern laboratories. They work by sending a beam of electrons into an
undulating magnetic field, called a wiggler. This changes the trajectory of the
electrons, forcing them to emit coherent photons. That’s cool, but their real
flexibility comes from their tunability. Change the energy of the incident
electrons or fiddle with the wiggler, and you can change the wavelength of the
laser light they produce.
There’s one problem though. Wigglers are big, complex devices that are
expensive to build and difficult to operate and maintain. And that means that
only the biggest and best-funded labs can afford to run them.
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Now Nikolay Zheludev at the University
of Southampton in the U.K. and a few
mates are doing for the free-electron laser what Shockley, Bardeen, and
Brattain did for the vacuum tube: they’re turning it into a solid-state device
that can be built into a single chip.
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Here’s how. They first create a slab of material consisting of alternating
layers of gold (a metal) and silicon dioxide (a dielectric). They then drill a
tiny hole–just 700 nanometers across–through the slab. Finally, they fire a
beam of electrons through the hole. Instead of experiencing an undulating
magnetic field, the electrons experience an alternating dielectric environment that
has the same effect–it forces them to emit photons. And crucially, the team can
tune the wavelength of the photons by changing the energy of the incident
electrons.
Zheludev and his pals have already built a prototype that produces light in
the range of visible to infrared with an emission intensity equivalent to 200 watts
per square centimeter.That’s an impressive device that could make a big impact
in nanophotonic devices, in optical memory, and in next-generation displays.
But there’s work to do yet. The light emitted by this device, which Zheludev
calls a light well, is incoherent because the photon conversion process is
relatively inefficient (the number of photons produced per incident electron is
just 10^-5).So Zheludev’s next job will be to improve this efficiency by
reducing the energy lost to surface plasmons and the light lost inside the
material slab. If he can improve the efficiency enough, lasing may become
possible.
When that happens, Zheludev and his colleagues will have a device on their
hands that could revolutionize lasing. In the meantime, they’ll just have to
settle for a revolution in nanophotonics.
Ref: arxiv.org/abs/0907.2143: The Light-Well: A Tuneable Free-Electron Light Source on a Chip
