Organic Molecules in Optical Nanoguides and Microcavities: Engineering Novel States of Light and Matter

Speaker: Professor, Vahid Sandoghdar, Max Planck Institute for the Science of Light, Germany

A current challenge and trend in quantum nano-optics is to extend the toolbox generated from experiments with single emitters and single photons towards interfacing of a well-defined number of photons and quantum emitters. The resulting complex optical network would enable studies of many-body quantum optical phenomena and novel states of light and matter. Here, we present two different systems for such studies.

First, we discuss a chip-based scalable platform, which relies on the evanescent coupling of organic molecules to subwavelength TiO₂ waveguides (nanoguides) embedded in an organic matrix (Fig. 1a). Operation at liquid helium temperature provides access to lifetime-limited linewidths of the order of MHz for individual molecules distributed over an inhomogeneously broadened spectrum of several THz. The dopant concentration determines the density of our system, which can reach up to a few hundred molecules per cubic wavelength. By using integrated microelectrodes, the resonances of the constituent molecules can be Stark-shifted over a large bandwidth of several GHz. The highly confined mode of a nanoguide allows a substantial mode overlap with the emission pattern of a single emitter, resulting in large coupling efficiencies. Therefore, we can observe coherent extinction signals from many different molecules in the transmission signal of a nanoguide [1]. Furthermore, the efficient linear coupling of photons and molecules also paves the way for achieving nonlinear effects such as switching or amplification at very low light power [1, 2]. Our work shows that it is possible to control the optical response of a nanoguide by modifying the spectral properties of a few molecules either via external optical or static electric fields.

In the second project, we perform coherent linear and nonlinear spectroscopy of single molecules coupled to an ultrasmall microcavity [3, 4]. The latter consists of a flat distributed Bragg reflector (DBR) and a micromirror fabricated by focused ion beam milling. The cavity design relies on low-moderate quality factors and, thus, large bandwidth, while a very small mode volume allows achieving considerable Purcell factors for modifying the internal branching ratio of organic molecules. In addition to discussing the subtleties of design and fabrication, we elaborate on future efforts towards the realization of on-chip polaritonic states [5] and the integration of further optical elements such as micoresonators [6], which would enhance and tailor the coupling of molecules to each other via photonic channels.

Fig. 1 (a) Schematics of a TiO₂ nanoguide with integrated grating couplers surrounded by a dye-doped organic matrix. The structure is sealed by a top substrate with integrated reservoir. (b) Schematics of an ultrasmall microcavity with one flat DBR and a curved micromirror fabricated on a silicon tip.

Prof. Vahid Sandoghda is the director of Max Planck Institute for the Science of Light and Alexander von Humboldt Professor at the Department of Physics, Friedrich Alexander University, Erlangen, Germany.

References

[1] P. Türschmann, N. Rotenberg, J. Renger, I. Harder, O. Lohse, T. Utikal, S. Götzinger, and V. Sandoghdar, “On-chip linear and nonlinear control of single molecules coupled to a nanoguide” Submitted; arXiv:1702.05923 (2017).

[2] A. Maser, B. Gmeiner, T. Utikal, S. Götzinger, and V. Sandoghdar, “Few-photon coherent nonlinear optics with a single molecule”, Nature Photonics 10, 450 (2016).

[3] H. Kelkar, D. Wang, D. Martin-Cano, B. Hoffmann, S. Christiansen, S. Götzinger, and V. Sandoghdar, Phys. Rev. Appl. 4, 0504010 (2015).

[4] D. Wang, H. Kelkar, D. Martin-Cano, T. Utikal, S. Götzinger, V. Sandoghdar “Coherent coupling of a single molecule to a scanning Fabry-Perot microcavity” Phys. Rev. X 7, 021014 (2017).

[5] H.R. Haakh, S. Faez, and V. Sandoghdar, “Polaritonic normal-mode splitting and light localization in a one-dimensional nanoguide”, Phys. Rev. A 94, 053840 (2016).

[6] N. Rotenberg, P. Türschmann, H. Haakh, D. Martin-Cano, S. Götzinger, and V. Sandoghdar, “Small slot waveguide rings for on-chip quantum optical circuits” Opt. Express 25, 5397 (2017).