Propagative Oscillations in Codirectional Polariton Waveguide Couplers

We report on novel exciton-polariton routing devices created to study and purposely guide light-matter particles in their condensate phase. In a codirectional coupling device, two waveguides are connected by a partially etched section that facilitates tunable coupling of the adjacent channels. This evanescent coupling of the two macroscopic wave functions in each waveguide reveals itself in real space oscillations of the condensate. This Josephson-like oscillation has only been observed in coupled polariton traps so far. Here, we report on a similar coupling behavior in a controllable, propagative waveguide-based design. By controlling the gap width, channel length, or propagation energy, the exit port of the polariton flow can be chosen. This codirectional polariton device is a passive and scalable coupler element that can serve in compact, next generation logic architectures.

Figure 1

Device layout, SEM images and corresponding experimental as well as theoretical dispersions of a codirectional coupler. (a) Device schematic with indicated laser excitation (red), incoupler region (orange), and the coupling region (blue) along the $x$ axis. (b) Top view SEM image of a codirectional polariton coupler. (c) Enlargement highlighting the coupling region (blue) and the gap between the two waveguides. Here, the cavity and a varying number of mirror pairs are still intact. (d) Measured waveguide dispersion below and (e) above polariton condensation threshold at the input port region (orange) parallel to the waveguide, angled 45° to the $x$ axis. (f) Calculated dispersion below the condensation threshold at the coupling region (blue).

Figure 2

Propagative oscillation of a exciton-polariton condensate in a codirectional coupler. (a) Energy-resolved real space PL for a coupler device with a gap size of 200 nm and a coupler region length of $100\mu \mathrm{m}$ at $E=1.5924–1.5930\mathrm{eV}$. Polaritons are injected nonresonantly in the lower left incoupler and exhibit a distinct oscillatory behavior between the two waveguides in the coupler region (right of the black line). (b) Polariton PL distribution $\mathrm{\Delta }I=(I_{\mathrm{top}}-I_{\mathrm{bot}})/I_{\mathrm{tot}}$. The red dashed line represents the results of the theoretical fitting by Eq. (5).

Figure 3

Propagation dynamics of polaritons calculated within the model Eqs. (6) and (7) for coupled waveguides with separation gap sizes of (a),(b) 200 nm, (c) 300 nm, and (d) 500 nm. While the dynamics for (a) is at an energy of 1.5955 eV, (b)–(d) are at 1.5925 eV. The insets on the right-hand side show the respective distribution in momentum space (on the parameter plane $k_y$ and $k_x$). Antisymmetric and symmetric modes of the coupler are visible.

Figure 4

Time-resolved propagation of the polariton propagation in coupler devices with a gap size of (a)–(c) 200 nm and (d)–(f) 500 nm for three different times during the propagation. While (a)–(c) show a clean oscillation between the two waveguides, no oscillation is visible in (d)–(f). The pump area is indicated by a blue dashed circle.

Figure 5

Routing of a polariton condensate via a different coupling length (a)–(c) and different propagation energy (d)–(g). (Note that the plot orientation has been turned by 90° for visual clarity). A coupler with a length of the coupling region of $20\mu \mathrm{m}$ and a gap size of (a) 200 nm and (b) 300 nm is shown. (c) Line profiles of the normalized intensities at $x=28\mu \mathrm{m}$ for (a) and (b). The PL emission for different energies (d)–(f) in a fixed region of the coupler is shown. (g) Normalized PL intensity line profiles of the emission at $x=13\mu \mathrm{m}$. (h) Experimentally extracted coupling lengths as a function of the propagation energy and numerically calculated values.