Dot by dot sharp
used scanning single-element dipole antennas to strongly modify the emission pattern, which was dominated by the resonant dipolar modes of the antenna and was thus neither highly directional nor very tunable 6, 17. used a multi-element Yagi-Uda antenna to generate highly directional, strongly polarized emission from a single QD, but because that QD was painstakingly positioned lithographically, it was difficult to study the emission properties as a function of the QD position. Other work falls mostly into two general categories: non-scannable but highly directional antennas 12 and scannable resonant antennas, which include both linear 6, 17 and spherical 5, 18, 19 geometries. used the annular disk of an aperture-type near-field probe to control the emission pattern from single molecules 3 and later this group used a similar setup with polarization analysis to demonstrate reversible polarization control 7. In one of the earliest demonstrations, Gersen et al. When separated by an intermediate distance, the emitter and tip contribute roughly equally to the emission properties and we show that maximum control over the emission direction and polarization is achieved in this intermediate zone, with performance similar to resonant antennas but with much better tunability. In contrast to resonant antennas where the large local density of optical states (LDOS) leads to emission properties dominated by the antenna modes, the much smaller LDOS values for nonresonant tips leads to an emission pattern and polarization that depend very sensitively on the position of the tip and the orientation of the emitter.
Here, we demonstrate that the sharp, nonresonant tip of an atomic force microscope (AFM) can be used to simultaneously control the polarization and direction of emission from individual quantum emitters. Optimally, antennas would provide simultaneous control over the direction, phase and polarization of individual photons emitted from a source, requiring a comprehensive understanding of the emitter-antenna coupling. This phenomenon is fundamentally interesting and forms a basis for developing single photon sources for quantum information applications 14, 15, 16. A consequence of the coupling between an antenna and an emitter is that the radiation pattern and/or polarization can be strongly modified 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13. The function of an optical antenna is to reduce the impedance mismatch between propagating (far-field) radiation modes and non-propagating (near-field) evanescent modes, which allows an emitter to absorb and/or emit light more efficiently 1, 2. Due to the relatively weak emitter-tip coupling, the tip must be positioned very precisely near the emitter, but this weak coupling also leads to highly tunable emission properties with a similar degree of polarization and directionality compared to resonant antennas. Together, the measurements and simulations demonstrate that interference between light emitted directly into the far field with that elastically scattered from the tip apex in the near field is responsible for this control over polarization and directionality. Here we show experimentally that the emission polarization can be manipulated using a simple, nonresonant scanning probe consisting of the sharp metallic tip of an atomic force microscope finite element simulations reveal that the emission simultaneously becomes highly directional. Usually, these metallic nanostructures utilize localized plasmon resonances to generate highly directional and strongly polarized emission, which is determined predominantly by the antenna geometry alone and is thus not easily tuned.
Optical antennas can be used to manipulate the direction and polarization of radiation from an emitter.