By Cameron Chai
A research team led by Stephan Link from Rice University has demonstrated that tiny channels of gold nanoparticles are capable of transmitting electromagnetic energy that begins as light and transmits through ‘dark plasmans.’
A scanning electron microscope image, left, shows a 15-micron line of 50-nanometer spherical gold nanoparticles. At right is a fluorescence image of the same chain, coated with a thin film of Cardiogreen dye using 785 nm laser excitation. (Credit Link Lab/Rice University)
The research team also demonstrated that even disordered nanoparticles in arrays that have a thickness of 150 nm are capable of transmitting signals better than the results of earlier experiments, thus opening the door to advance optoelectronic devices.
In Stephan Link’s laboratory, the research team has developed a technique to print thin lines of nanoparticles over glass substrate. It cut microscopic channels into a polymer over the glass substrate utilizing an electron beam to provide the shape to the nanoparticle lines. It then used capillary forces to deposit the gold nanoparticles into the channels. After washing away the remaining stray nanoparticles and the polymer, the team obtained the lines with particles that were at a distance of a few nanometers.
The resulting gold nanoparticle lines are capable of propagating a signal from one particle to the next one over several microns, a distance higher than earlier efforts and nearly comparable to results demonstrated utilizing gold nanowires.
Plasmons are electron waves capable of travelling through a metal surface when disturbed by an external electromagnetic source like light. Dark plasmons are not able to interact with light, as they do not have net dipole moment. Link explained that these modes are not completely dark, particularly in the existence of disorder. A tiny dipole oscillation exists even for the subradiant modes and when these are coupled, cause minimal scattering loss and sustain plasmon transmission over longer distances.
To measure the distance, the research team used a fluorescent dye to coat the 15-micron-long gold nanoparticle lines and then measured the propagation distance of the plasmons that are excited by a laser, using a photobleaching technique.
According to Link, silver nanowires are better plasmon wave carriers when compared to gold counterpart. The team knows that it may transmit a lot longer if it uses silver nanoparticles and it will hopefully do that in more intricate structures. These silver nanoparticles can be coupled to other components like nanowires in structures that would not be achievable otherwise, Link concluded.