Optical gain in plasmonic waveguides opens door to subwavelength applications

Jörg Schwartz, joerg.schwartz@photonics.com

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.