ERA NanoSci - Nanofibre Optical Interfaces for Ions, Atoms and Molecules
Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
Final Report Abstract
How can a beam of light tell the difference between left and right? At the Vienna University of Technology tiny particles have been coupled to a glass fibre. The particles emit light into the fibre in such a way that it does not go travel in both directions, as one would expect. Instead, the light can be directed either to the left or to the right. This has become possible by employing a remarkable physical effect – the spin-orbit coupling of light. This new kind of optical switch has the potential to revolutionize nanophotonics. When a particle absorbs and emits light, this light is not just emitted into one direction. A particle in free space will always emit as much light into one particular direction as it emits into the opposite direction. This project has succeeded in breaking this symmetry of emission using gold nanoparticles coupled to ultra-thin glass fibres. The incident laser light determines whether the light emitted by the particle travels left or right in the glass fibre. This is only possible because light has an intrinsic angular momentum, the spin. Similar to a pendulum which can swing in one particular plane or move in circles, a light wave can have different directions of oscillation. If it has a well-defined vibrational direction, it is called a “polarized wave”. A simple plane wave has the same polarization everywhere but when the intensity of the light changes locally, the polarization changes too. Usually, the light oscillates in a plane perpendicular to its direction of propagation. If the light then oscillates circularly, this is similar to the motion of an airplane propeller. Its rotational axis – corresponding to the spin – points into the direction of propagation. But light moving through ultrathin glass fibres has very special properties. Its intensity is very high inside the glass fibre, but it rapidly decreases outside the fibre. This leads to an additional field component in the direction of the glass fibre. The rotational plane of the light wave pivots by 90 degrees. Then, the direction of propagation is perpendicular to the spin, just like a bicycle, moving into a direction which is perpendicular to the axes of the wheels. By checking the wheels’ sense of rotation – clockwise or counter-clockwise – one can tell whether a bicycle moves right or left when looking at it from the side. It is exactly the same with the beams of light in the ultra-thin glass fibre. The sense of rotation of the light field is coupled to the direction of motion. This kind of coupling is a direct consequence of the glass fibre geometry and the laws of electrodynamics. The effect is called “spin-orbit-coupling of light”. When a particle that is coupled to the glass fibre is irradiated with a laser in such a way that it emits light of a particular sense of rotation, the emitted light will thus propagate into just one particular direction inside the glass fibre – either to the left or to the right. Here, this effect has been demonstrated using a single gold nanoparticle on a glass fibre. The fibre is 250 times thinner than a human hair; the diameter of the gold particle is even four times less. Both the diameter of the fibre and the particle are even smaller than the wavelength of the emitted light. This new technology should be easily made available in commercial applications. For example, the method could be applied to integrated optical circuits. Such systems may one day replace the electronic circuits we are using today. A very surprising and counterintuitive effect could be demonstrated with a gold nanoparticle deposited on the surface of an optical nanofibre: Due to the strong lateral confinement of the nanofibre modes, the polarisation of the latter exhibits a strong component along the nanofibre axis. This component oscillates in quadrature with the transverse components and thus gives rise to elliptical polarisation where the plane of polarisation contains the nanofiber axis. This means that the ellipticity vector of the field is orthogonal to the propagation direction of the field and changes sign when the direction of propagation is reversed. This link between local polarisation and direction of propagation is referred to as spin-orbit interaction of light and allowed us to control the direction of emission of light by the nanoparticle into the nanofibre. This novel technique for controlling the flow of light at the nanoscale is highly versatile, and we expect it to find application in various scenarios of nanophotonic systems. The result on controlling the flow of light with a gold nanoparticle deposited on the surface of an optical nanofibre was featured in the online version of the Austrian national newspaper Der Standard, see http://derstandard.at/2000006478145/Symmetriebruch-Nanopartikel-schicken-Licht-nach-links-oder-rechts
Publications
- 2010: DPG Physics School 2010 on “Nanophotonics Meets Quantum Optics”, Bad Honnef. Optical Nanofibres in ion-traps
J. Petersen, B. Ames, M. Brownnutt, R. Blatt, A. Rauschenbeutel
- 2010: Spring-Meeting of the German Physical Society, Hannover. Optical nanofibres in ion-traps
Jan Petersen, Michael Brownnutt, Rainer Blatt, Arno Rauschenbeutel
- 2011: Spring-Meeting of the German Physical Society, Dresden. Optical nanofibres in ion-traps
J. Petersen, B. Ames, M. Brownnutt, R. Blatt, A. Rauschenbeutel
- 2012: Spring-Meeting of the German Physical Society, Stuttgart. Interfacing optical nanofibres and ions
J. Petersen, B. Ames, M. Brownnutt, R. Blatt, A. Rauschenbeutel
- "Chiral nanophotonic waveguide interface based on spinorbit coupling of light", Science 346, 67 (2014)
J. Petersen, J. Volz, and A. Rauschenbeutel
(See online at https://doi.org/10.1126/science.1257671)
