DOI: https://doi.org/10.29363/nanoge.emlem.2022.019
Publication date: 15th July 2022
Layered metal-dielectric-metal (MDM) cavities sustain resonances with vanishing dielectric permittivity that can couple to the light emission of fluorophores positioned either on their surface [1] or within the cavity [2]. The wavelength of the cavity resonance depends on the polarization, the angle, and the thickness of the dielectric layer.[3] In the weak coupling regime, the polarization and angular properties of the MDM cavity resonances are transferred to the emission of the fluorophores inside the dielectric layer of the cavity. [2] In the strong coupling regime, the interaction of the cavity modes with the excitonic emission of the fluorophores leads to the formation of polaritons and Rabi splitting of the resonance modes.[4] Typically, the dispersion of the Rabi splitting is mapped by varying the k-vector that is in-plane with the emitter and mirror layers, for example by imaging the angle-dependent signal in the back focal plane of the objective. [4] Another option to tune the cavity resonance is to vary the length of the photonic cavity (thereby varying the out-of-plane k-vector), however, this is difficult to achieve, for example via the tedious fabrication of several samples with different dimensions.
Here we present an elegant method to vary the thickness of the dielectric layer (that corresponds to the length of the cavity) in situ by using a surface-forces apparatus (SFA). [5] In this experiment, two cylindrical and semitransparent mirrors are separated by a dielectric liquid in which the light emitting nanocrystals or molecules are dispersed. The distance of the mirrors can be controlled by a cantilever spring over the range from large (mm) separation to almost close contact. Here a crossed configuration of the cylindrical mirrors results in a well-defined contact region with sub-millimeter dimension. Such a SFA therefore acts as a tunable MDM cavity that allows to detect the strong coupling of multiple cavity harmonics with the dye emission in a single, rapid, and continuous sweep of cavity thickness. We observed Rabi splitting exceeding 100 meV, and we analyze the Rabi splitting of the different cavity harmonics, finding a square root dependence with respect to the number of the involved emitters.
Furthermore, we prepared planar MDM cavities with fixed dimension and investigated the Rabi splitting via the angle-dependence with spectroscopic ellipsometry. These experiments demonstrated that the polaritons inherit the ENZ character of the cavity modes. The complementarity of this experiment with the SFA study allows to investigate the impact of the in-plane and out-of-plane k vector on the Rabi splitting.
References
[1] V. Caligiuri, M. Palei, M. Imran, L. Manna, R. Krahne, ACS Photonics 2018, 5, 2287-2294.
[2] V. Caligiuri, G. Biffi, M. Palei, B. Martín-García, R. D. Pothuraju, Y. Bretonnière, R. Krahne, Adv. Opt. Mater. 2020, 8, 1901215.
[3] V. Caligiuri, M. Palei, G. Biffi, S. Artyukhin, R. Krahne, Nano Lett. 2019, 19, 3151-3160.
[4] A. Patra, V. Caligiuri, R. Krahne, A. De Luca, Adv. Opt. Mater. 2021, 9, 2101076
[5] A. Fieramosca, et al., Sci Adv. 2019;5(5):eaav9967.
[6] B. Zappone, V. Caligiuri, A. Patra, R. Krahne, A. De Luca, ACS Photonics 2021.
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