“Controlling the interaction between light and matter confined in nanoscale”
Área de concentração: Ciências Exatas (Física)
Leonardo de Souza Menezes
Doutor
Departamento de Física, Universidade Federal de Pernambuco
In this contribution, the peculiar optical properties of a microsphere resonator as a confining structure for the electromagnetic field in the optical region are studied and exploited. Such kind of microresonatorscan confine light in a volume V=3003, which in our case (=680 nm) corresponds to ~90 m3. A scanning near-field optical microscope (SNOM) probe was used to manipulate the interaction between single nanoparticles doped with organic dye molecules and the so-called whispering-gallery modes (WGMs) [1], which are eigenmodes of microspherical cavities presenting ultrahigh Q factors (~109), corresponding to photon lifetimes of some s [2]. These enhanced photon lifetimes give rise to the observation of nonlinear optical effects even with extremely low (~W) pump powers coupled into the microcavity, among other interesting effects.
In our experiment, we have attached a 200 nm bead containing ~105organic dye molecules (donor bead) to the tip of a SNOM probe and positioned it at will in the evanescent field of a WGM. The properties of the coupling between these molecules with the WGMs through their fluorescence (when they are optically excited) were studied and characterized in detail. Additionally, we have dip coated a microsphere in a diluted solution of 200 nm beads containing other kind of dye molecules (acceptor bead), different from those which were inside the donor bead, so that after this process one could find at most 10 beads on the sphere surface.
While pumping the donor bead via the optical fiber from which the SNOM probe was fabricated with a laser emitting at 532 nm, the SNOM probe was approached to the sphere surface and kept at a distance of ~10 nm from it using a shear-force feedback loop. The fluorescence of the donor beads coupled into the WGM and started circulating inside the microsphere. A home-made laser scanning confocal optical microscope was used to collect light coming from the acceptor bead, exclusively [3], which was then sent to a spectrometer. The results show that the acceptor particle has efficiently absorbed the fluorescence of the donor one, which coupled to the WGM. It is verified that this incoherent photon transfer mechanism is about 106times more efficient than that via propagation in the free space [4].
[1] Götzinger et al., Applied Physics B 73, 825 (2001).
[2] Braginsky et al., Physics Letters A 137, 393 (1989).
[3] Götzinger et al., Proceedings of SPIE 4969, 207 (2003).
[4] Götzinger et al., Nano Letters 6, 1151 (2006).
Acknowledgments: L. de S. Menezes acknowledges a fellowship from the Alexander von Humboldt Foundation.