Tailored dissipation for directional transport in plasmonic ratchets

Abstract

We present a joint experimental and theoretical study of a ratchet implemented in arra ys of evanescently coupled plasmonic waveguides with tailored losses. In this setup the time-periodic dissipation is the only active mechanism and notably, we find better rectified transport and lower losses in the transmitted signal with increased local dissipation. Using Floquet theory, we uncover a driving regime that allows efficient directional tr ansport for suitable driving frequencies and loss rates, which are linked to linear qu asienergy bands with minimal losses. These regions are separated from non-resonant beh avior by sharp transitions with characteristic exceptional points in the spectrum. Direct experimental observation of the Floquet-dissipative ratchet effect using a comb ination of real- and Fourier-space leakage radiation microscopy is provided.

Figures (9)

• Figure 1: Top - Schematic of the lattice with periodically driven damping for three equidistant times during a driving cycle of period $T$. Lossy sites with dissipation rate $\gamma$ are depicted in red, while non-lossy sites are depicted in blue. $J$ is the hopping constant. Bottom - Sketch of waveguide array corresponding to the last time step of the lattice above.
• Figure 2: Frequency-dependent transmission of the ratchet as defined in Eq. \ref{['eq:tau']} for different losses $\gamma$ calculated after propagation time of 3T. The vertical lines indicate the first three resonance frequencies for the infinite damping case in Eq. \ref{['eq:res_con']}.
• Figure 3: Simulated temporal evolution of $\vert\psi(x,t)\vert^2$ (left) and the Fourier transform in Eq. (\ref{['fourier']}) $\vert\psi(k_x,\omega_t)\vert$ (middle) using $\gamma\!=\!5\,J$, $a_0\!=\!3\upmu$m, length $L\!=\!61 a_0$, $t_{max}\!=\!30T$, and an initial single-site excitation $|B,j\!=\!0\rangle$. The lines show the numerically calculated quasienergies separated in real part (middle) and imaginary part (right).
• Figure 4: Scanning electron micrograph of the ratchet realization in a plasmonic waveguide array. The red dashed line marks the boundary of the excitation region, $a_0$ - the size of the unit cell, $A$, $B$, $C$ indicate sites within the unit cell - possible inputs, $T$ is the period of modulation.
• Figure 5: Measured real-space (left) and Fourier-space (right) SPP intensity distributions for different dissipation rates and single-site excitation at site $B$ for driving period $P\!=\!35\upmu$m corresponding frequency $\omega\!=\!2.02J$ (a) and (b) show results for no loss, (c) and (d) - $\gamma\!=\! 1.96J$, where arrow highlights Floquet replica, (e) and (f) - $\gamma\!=\! 4.88J$.