Self-Organized Optical Pathways in Optofluidic Photonic Crystals

Abstract

This paper reports FDTD simulations of optofluidic reconfiguration in two-dimensional silicon photonic crystal waveguides, treating structural plasticity (the creation and destruction of optical pathways) via selective fluid infiltration. Using MPB eigenmode analysis, we decouple bandgap narrowing from defect-mode weakening, showing that defect weakening dominates (2.4 times faster transmission decay than bandgap narrowing at CS_2 indices). Infiltration topology controls signal routing (L-bend selectivity S = 0.98), though modulation depth is weak (Delta varepsilon/ varepsilon_ textSi = 11 %). A phenomenological optothermal feedback model produces self-organized pathways that achieve 63 % of a hand-designed waveguide's bandgap transmission (7.6 times the heavily suppressed empty-crystal baseline). Amplitude competition between counter-propagating sources produces strong, monotonic pathway steering (DeltaCOM_x from +0.03 to +4.92 ;a), while pulsed spike-timing-dependent plasticity yields a predictable null result: the timing-sensitive cross-term is suppressed by >10^2 when pulse delays exceed the temporal pulse width. The results provide benchmarks and identify physical limits for bio-inspired reconfigurable optofluidic photonics.

Figures (27)

• Figure 1: Optofluidic structural plasticity concept. Left: empty photonic crystal, where the bandgap blocks light propagation. Center: fluid-infiltrated pathway; filling a line of holes with CS$_2$ creates a defect mode that guides light. Right: biological analogy; filling/flushing holes corresponds to synaptogenesis/pruning of neural connections. The process is fully reversible.
• Figure 2: Weight plasticity vs. structural plasticity in photonic systems. Left: conventional neuromorphic photonics (Tait, Feldmann) uses fixed waveguide connections with tunable weights ($O(N^2)$ elements for $N \times N$ mapping). Right: our optofluidic approach reconfigures which connections exist by filling/flushing holes in a photonic crystal ($2^N$ possible topologies from $N$ holes). The structural approach trades modulation depth for combinatorial richness.
• Figure 3: Simulation geometry (top-down view). Triangular lattice of air holes (white) in silicon (gray), with a line of CS$_2$-infiltrated holes (blue) forming the waveguide channel. A Gaussian source (red) launches TE-polarized light; a flux monitor (green) records transmission. PML absorbing boundaries (hatched) surround the domain. Inset: unit cell with $r/a = 0.3$.
• Figure 4: Photonic band structure along high-symmetry directions ($r/a = 0.3$, $\varepsilon_{\text{Si}} = 11.56$). Blue: TE modes; red dashed: TM modes. (a) Air holes: 39.2% TE bandgap. (b) CS$_2$-infiltrated: 30.0% bandgap.
• Figure 5: Waveguide creation via fluid infiltration. (a) Transmission spectra for selected line lengths. (b) Integrated bandgap transmission vs. number of infiltrated holes showing non-monotonic behavior.