Jörg Schwartz, email@example.com
REYKJAVIK, Iceland – Researchers
at the University of Iceland, in collaboration with colleagues at Germany’s
University of Cologne and Fraunhofer Institute for Applied Optics and Precision
Engineering in Jena, have demonstrated amplified propagation of light in plasmonic
waveguides. Such optical gain is critical in making light travel over any sizable
distance when confined in a plasmonic mode, and it paves the way for applications
ranging from supercompact communication and computing devices to the detection and
characterization of cells, virus particles or even single molecules.
Plasmons are quasi-particles resulting from quantization of plasma
oscillations – similar to photons and phonons being quantizations of light
and heat, respectively. As a result, plasmons are collective oscillations of a free
electron gas density that can occur at optical frequencies. They also can couple
with photons to create another quasi-particle, a plasma polariton. Surface plasmons,
also known as surface plasmon polaritons (SPPs), are electromagnetic surface waves
propagating on a metal/dielectric interface in a direction parallel to it.
Because this wave occurs only on the boundary of the metal and
the external medium, it is very sensitive to surface inhomogeneities, such as the
adsorption of molecules to the metal surface.
In theory, SPPs could be used to create planar micro-optical components
and devices, enabling miniaturized integrated optical circuits that manipulate radiation
at the length scale much smaller than the light wavelength; i.e., below the conventional
diffraction limit. In reality, however, they experience dramatic levels of attenuation
because metals naturally impose ohmic losses on electromagnetic fields, which are
largest for wavelengths close to the surface plasmon resonance.
Scattering from surface roughness, grain boundaries and other
imperfections causes further losses, leading to the fact that SPPs cannot travel
significant distances in passive waveguides. SPPs propagating on a metal-dielectric
interface suffer from typical propagation losses of >1000/cm for visible wavelengths.
Although some researchers have shown that clever design can somewhat
increase the useful length, it is gain that is needed to balance the high propagation
loss over any macroscopic distances; i.e., a mechanism that continuously amplifies
the light as it travels along the plasmonic waveguide. This was demonstrated by
the investigators and published in Nature Photonics online in May 2010.
The Icelandic experimentalists, Kristjan Leosson and Malte C.
Gather, used a structure consisting of ultrathin gold film embedded in a highly
fluorescent polymer that was optically pumped by an ultrafast laser source. In the
process, the structure channels light generated by the polymer to the plasmonic
waveguide. As the plasmonic wave travels along the waveguide, its intensity is increased
by stimulated emission of the optical energy stored in the fluorescent polymer,
which was contributed by co-author Klaus Meerholz from the University of Cologne.
Simulations were performed by Norbert Danz of the Fraunhofer institute.
Optical amplification is critical in making light bound in a plasmonic mode travel over
sizable distances. A new plasmonic amplifier consists of an ultrathin gold film
and a highly fluorescent orange-emitting polymer. When the structure is exposed
to green laser light, the intensity of plasmonic waves traveling along the waveguide
is increased by stimulated emission of the optical energy stored in the fluorescent
polymer. Courtesy of the University of Iceland.
Because of the material choice – the semiconducting polymers
have large emission cross sections and quantum efficiencies of >80 percent –
the achieved optical amplification is large enough to provide a net gain of the
plasmon-bound light as it travels along the waveguide.
The researchers reported that the propagation loss issue in plasmonic
waveguides has been a major hurdle in the development of devices that make use of
surface plasmon effects.
“The key to the success of our work was that we found a
way to embed the plasmonic waveguides into an amplifying fluorescent polymer without
affecting the properties of the waveguide too much,” said Gather, who also
is associated with Harvard Medical School’s Wellman Center for Photomedicine
in Boston. The tightly focused optical energy of plasmonic waves can be used as
a “nanoprobe,” providing valuable measurements in fields including solid-state
physics, chemistry and the life sciences.
It also opens the possibility for nano-photonic integrated circuits,
sensors, solar cells and other devices that take advantage of the strong optical
field confinement at the metal/dielectric interface.