Triple jump lights up laser

A highly efficient laser based has been demonstrated by Guang
He and colleagues at the State University of New York at
Buffalo. The device emits visible light when it is excited by
infrared radiation of the frequency used in optical
communications. Besides improvements in fibre optics
technology, the effect could lead to new techniques for data
storage and medical imaging (G He et al 2002 Nature 415
767).

In a conventional ‘optically pumped’
laser, the atoms or molecules in an
optically active material are excited to
higher energy levels by single photons
from another laser. When the atoms or
molecules fall to a lower energy level,
they emit light with a well-defined
wavelength. Mirrors reflect this light
back and forth through the material
stimulating further emission, which
leads to an intense beam of coherent
light.

He and colleagues have now shown
that the molecules in a laser material can be excited by absorbing
three photons at once. Such ‘multiphoton absorption’ was first
predicted in 1931, and two-photon absorption is already used in
many applications.

Processes based on two-photon absorption are efficient because
the level of excitation of the medium is related to the square of
the ‘pump’ light intensity. This relationship becomes cubic if three
photons are absorbed, leading to even greater efficiency. A laser
based on three-photon absorption can emit extremely intense
light from a very small area, and can be excited by lower-energy
photons – that is, longer-wavelength radiation.

He and co-workers used pulses from a ‘pump’ laser with a
wavelength of 1.3 micrometres – the main wavelength used in
optical communications – to stimulate an optically active organic
solution. This solution then emitted yellowy-green light with a
wavelength of 550 nanometres. This frequency-shifting effect –
known as up-conversion – arises because three-photon
absorption is so efficient.

According to the team, this effect could have many uses in
fibre-optic communications systems, including frequency shifting.
“Shorter wavelengths are also desirable for data transmission
because they allow for higher density data storage and higher
resolution of that data”, says He.

The sensitivity of the organic solution to infrared radiation – which
travels further though human tissue than visible light – could also
lead to new medical imaging techniques that are less harmful than
X-rays.

Author
Katie Pennicott is Editor of PhysicsWeb