[dba-Tech] Laser Puts New Spin on Light

This from physicsworld:

Laser puts a new spin on light

Nov 7, 2011

A new type of pulsed laser that modulates the polarization of its emitted
light very rapidly has been created by researchers in Germany and Scotland.
The polarization modulation occurs much faster than the intensity
modulation normally used in optical telecoms systems – and the resulting
polarization pulses could increase dramatically the speed of fibre-optic
communications.
[image: Schematic diagram showing spin-polarized electrons in a
laser]<http://images.iop.org/objects/phw/news/15/11/7/spin1.jpg>
Spin boosts laser <http://images.iop.org/objects/phw/news/15/11/7/spin1.jpg>

Modern telecoms systems encode information in pulses of laser light that
are then sent along optical fibres. This is an incredibly efficient method
of transmitting information, but its speed is ultimately limited by the
rate at which the intensity of the laser can be modulated, since this
dictates how long it takes to encode data into a train of pulses. With a
traditional, intensity-modulated laser, the maximum achievable modulation
rate is about 40 GHz.

Now Nils Gerhardt and colleagues at the Ruhr-Universitat Bochum, together
with a colleague at the University of Strathclyde in Glasgow, have worked
out a way to use electron spin to boost the modulation speed of a
semiconductor laser – something that physicists have been working on for
several years.
Lowering the energy threshold

In a standard semiconductor laser there are equal numbers of spin-up and
spin-down electrons, so spin plays no part in its operation. However,
physicists know that if the relative proportion of charge carriers in
either spin state can be increased, this lowers the amount of energy that
must be put into the laser before it starts emitting light – called the
lasing threshold.

But sustaining this spin imbalance – or polarization – at room temperature
has proven extremely difficult because thermal energy will randomize the
spin in a few picoseconds. So Gerhardt and colleagues created waves of spin
polarization in the semiconductor by blasting it with very short pulses of
polarized light from another laser. While the electrons themselves still
lose their spin polarization rapidly, some is passed on to photons, which
then re-polarize the charge carriers. Such spin oscillations between
photons and electrons last about 200 times longer than the electron spin
polarization itself.

And there was a more interesting feature to the Bochum group's laser. In
contrast to the light from a standard semiconductor laser, which has no net
polarization, the polarization of the light oscillated rapidly because of
the coupling of the photon spins with the electron spins. While this
switching had been demonstrated before, the polarization modulation rate
had always been pegged to the intensity modulation rate.
To 100 GHz and beyond

Gerhardt and colleagues used their technique to modulate the polarization
of the light from a 4 GHz laser at 11.6 GHz. This is still slower than
state-of-the-art intensity-modulated lasers, but the researchers believe
that it should be possible to improve on this. "Principally, you can go to
over 100 GHz," explained Gerhardt. "We've shown it theoretically, but first
we have to develop a device and that's what we're currently doing."

Physicist Igor Zutic of the State University of New York at Buffalo is
impressed. "I would say this is a very exciting proof of principle and
probably showing us the tip of the iceberg of what may be possible with
these spin lasers," he says.

However, Zutic and Gerhardt both agree that before such a spin laser can be
commercially viable, it must be possible to excite the spin oscillations
without another laser. This would involve injecting spin-polarized
electrons into the laser – a process so far realized only at cryogenic
temperatures. "Some of the advances that are now pursued in very different
areas – such as magnetic hard drives and magnetic random-access memory –
are based on better magnets and better methods of electrical spin
injection," concludes Zutic. "A broader view of these developments may
allow useful transfer of knowledge."

The work is described in *Appl. Phys. Lett.* *99*
151107<http://apl.aip.org/resource/1/applab/v99/i15/p151107_s1>
.
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