Forgotten interaction is effective guard for confining light
Researchers from the FOM Institute AMOLF, the University of Kyoto (Japan) and the Sungkyunkwan University (South Korea) have discovered a way of changing the behaviour of light at the nanoscale. They were the first to observe that with the help of a metal nanoring, light confined in a nanocavity can be influenced: the colour of the light confined changes and the period of the light's confinement can be increased. The researchers have demonstrated that the interaction between the nanocavity and the ring is caused by the light´s magnetic field, an interaction that is completely negligible under normal circumstances. The researchers published their findings this week in the renowned journal Physical Review Letters.
In a photonic crystal nanocavity, light can be confined in a volume smaller than its wavelength (see Figure 1a). As the light in such a nanocavity can be confined for a long time, these photonic structures are viewed as important components for future quantum communication networks. The light in the nanocavity is also highly sensitive for minuscule disruptions effected in the vicinity of the nanocavity. Although that might sound like a disadvantage, the opposite is in fact true. If a nanoparticle or a molecule comes close to the nanocavity, the resonant frequency of the nanocavity will measurably shift. In other words, the nanocavity will let light of a different colour pass through it. That makes these nanocavities ideal for use as extremely sensitive biosensors.
To date the observed shift has always been the result of the interaction with the electric field of light. As a result of this interaction, the proximity of a nanoparticle or molecule ensures that the nanocavity becomes larger because the light interacts with more matter: the resonant frequency shifts as a result of this to longer wavelengths (red shift), just like a longer guitar string gives a lower tone. Yet light has a magnetic field as well. The interaction between this rapidly vibrating magnetic field and matter is usually negligible. However, if the magnetic field of light comes into contact with a small metal ring, a tiny current will start to flow in the ring (see Figure 1d). This current in turn generates a new magnetic field that will counteract the original magnetic field. This is exactly what happens in the coils of an electric motor, only in this case it happens at a frequency 1 billion times higher. As a result of this so-called magnetic induction, the metal ring locally suppresses the magnetic field of the light. Due to the light being put under pressure, the nanocavity effectively becomes smaller and the resonant frequency of the nanocavity shifts to shorter wavelengths (blue shift).
Besides a shift in the resonant frequency, one would expect that the light in the nanocavity would dissipate quicker due to the presence of the ring, as the current that passes through the ring is subject to electrical resistance. This means that the light field should lose extra energy due to the ring. Interestingly the researchers measured the exact opposite: the dissipation of the light field decreased and the lifespan of the light in the nanocavity increased by up to 50%. This contraintuitive effect is still not fully understood and shall be the subject of a follow-up study.
As the researchers can precisely determine the position of the nanoring in relation to the nanocavity, they now have a tool to actively control confinement of light and can even to switch this on and off.
Reference
'Magnetic light-matter interactions in a photonic crystal nanocavity', Matteo Burresi, Tobias Kampfrath, Dries van Oosten, Jord C. Prangsma, Bongshik Song, Susumu Noda and L. (Kobus) Kuipers, Physical Review Letters (PRL). The article was published on the PRL website on 14 September 2010.
Link: http://prl.aps.org/abstract/PRL/v105/i12/e123901
Highlight in Physical Review Focus
This research is currently (end of September 2010) mentioned in 'Physical Review Focus' (PRF). The researchers are honoured by the publication, only a few PRL-publications are mentioned in PRF. PRF is an online service that provides brief explanations of selected research papers from APS journals at a level accessible to students and researchers in all fields of physics.
Link: http://focus.aps.org/story/v26/st13
Contact
For further information please contact:
Prof. Kobus Kuipers, FOM Institute AMOLF, telephone +31 20 754 71 94.