Long-range evanescent coupling through photonic molecules

Abstract

Photonic molecules support the excitation of higher-order states, which are otherwise hard to access at individual waveguides. In this work, we demonstrate the resonant excitation of photonic molecular states which evanescently couple to single-mode waveguides. We implement the experiments on femtosecond laser written photonic structures and demonstrate an efficient resonant excitation of higher-orbital states, optimized at specific wavelengths and propagation distances. We suggest the use of long photonic molecules as long-distance photonic links, and demonstrate strong coupling for very distant waveguides separated by 127 μm. We apply this concept to a one-dimensional lattice and demonstrate the excitation of topological edge states emerging due to the third-order next-neighbour interactions. Our findings demonstrate effective long-range evanescent coupling which could be a concrete solution for fiber-based photonic chips, topological physics emerging from long-range interactions, or fundamental studies of initially uncoupled systems.

Figures (3)

• Figure 1: (a) Orbital states for different photonic configurations. (b) Excitation picture of a photonic molecule via a single mode waveguide. (c) Mismatch in the propagation constants for a molecule and single waveguide $\Delta k_z$ versus the excitation wavelength $\lambda$. The red line indicates $\Delta k_z=0$. (d) Normalized power at the molecule vs $z$ and $\lambda$, obtained from continuous modeling. (e) Intensity profiles at $z=30$ mm in the range $600-860$ nm, every $10$ nm.
• Figure 2: (a) Sketch for the fs laser writing technique. Inset: microscope image of a fabricated system. (b) Output intensity profiles for $d=16\ \mu$m and $z=30$ mm, at the indicated $\lambda$. (c) and (d) Extracted normalized power at the photonic molecule vs $\lambda$, for different distances $d$ (at $z=30$ mm) and for different propagation lengths $z$ (for $d=16\ \mu$m), respectively.
• Figure 3: (a1) Sketch of two waveguides separated a distance $d=127\ \mu$m and (a2) output image at $\lambda=820$ nm. (b1) Sketch of a coupler including a photonic molecule as a link. (b2) Extracted normalized power vs $\lambda$ at the non-excited waveguide (red) and the molecule (black). (b3) Output images at the indicated wavelengths. (c1) A 1D lattice with horizontal $V_h$, diagonal $V_d$, and long-range $V_{lr}$ couplings. (c2) Linear spectra vs $V_{lr}/V$ (colors indicate the state's participation ratios). (c3) Microscope images of trivial and non-trivial fabricated lattices. (d1) and (d2) Output images, at the indicated wavelengths, after exciting the left edge site (see yellow ellipses) of a trivial and a non-trivial lattice, respectively.