Hybridization of photon-plasmon modes in metal-coated microtubular cavities

The coupling of resonant light and surface plasmons in metal layer–coated optical microcavities results in the formation of hybrid photon-plasmon modes. Here, we comprehensively investigate the hybridization mechanism of photon-plasmon modes based on optoplasmonic microtubular cavities. By changing the thicknesses of both the cavity wall and the metal layer, weakly, moderately, and strongly hybridized resonant modes are demonstrated depending on the photon-plasmon coupling strength. An effective potential approach is applied to illustrate the hybridization of photon-plasmon modes relying on the competition between light confinement by the cavity wall and the potential barrier introduced by the metal layer. Our work reveals the basic physical mechanisms for the generation of hybrid modes in metal-coated whispering-gallery-mode microcavities, and is of importance for the study of enhanced light-matter interactions and potential sensing applications.

Figure 1

(a) Sketch of a WGM microtubular cavity. $R$, $r$, and $T$ are the outer radius, inner radius, and cavity wall thickness, respectively. (b) WGM resonant wavelengths (with azimuthal mode number $M=30$, 31, and 32) as a function of $r/R$. Here, the outer radius $R$ is fixed at 2.5 μm. The left and right insets indicate the corresponding mode profiles in the cavity with $r/R=0.52$ and 0.92, respectively. The yellow dashed line indicates the critical value of the relative inner radius where the resonant wavelengths start to shift. (c) Effective potential distributed along the relative radial distance $(d/R)$ for a representative microtubular cavity with $r/R=0.6$ for $M=30$. The inset shows the confined optical field distribution along the radial direction $(d/R)$. The arrows denote the tunneling of confined photons to the classically forbidden region. The red horizontal line denotes the normalized energy level of confined photons.

Figure 2

Energy fraction in the outer surrounding of the cavity as a function of $r/R$. The left and right insets show the field distributions along the radial direction in the cases of $r/R=0.52$ and 0.92, respectively.

Figure 3

Field distributions and effective potentials along the radial direction of (a) weakly, (b) moderately, and (c) strongly hybridized photon-plasmon modes. The thicknesses of the cavity wall ($T$) and the metal coating layer ($t$) are set as $T/R=0.48$ and $t/R=0.032$, $T/R=0.48$ and $t/R=0.008$, and $T/R=0.08$ and $t/R=0.008$, respectively.

Figure 4

Diagram of three types of hybrid modes in microtubular cavities with different thickness of cavity wall and metal coating. The triangles, squares, and circles represent the weakly, moderately, and strongly hybridized photon-plasmon modes, respectively. The typical field distributions of three hybrid modes are shown in the right insets.

Figure 5

The field distributions (for mode $M=30$) of two sets of representative data in Fig. 4 are extracted out to show in Figs. 6 and 7, respectively.

Figure 6

The plasmon-type field (for mode $M=30$) gradually transfers from the inner surface to the outer surface of a metal coating layer when the thickness of the metal layer decreases from (a) $t=0.032R$ (weakly hybridized) to (b) $t=0.028R$ (weakly hybridized), (c) $t=0.024R$ (moderately hybridized), (d) $t=0.02R$ (moderately hybridized), (e) $t=0.016R$ (moderately hybridized), and (f) $t=0.012R$ (moderately hybridized). The cavity wall thickness is fixed at $T=0.48R$.

Figure 7

The transition of field distribution can also be clearly seen by changing the cavity wall thickness from (a) $T=0.48R$ (moderately hybridized) to (b) $T=0.32R$ (moderately hybridized), (c) $T=0.24R$ (moderately hybridized), (d) $T=0.16R$ (moderately hybridized), (e) $T=0.12R$ (strongly hybridized), and (f) $T=0.08R$ (strongly hybridized). The thickness of the metal coating layer is fixed at $t=0.008R$.