We propose and experimentally demonstrate a 2×2 thermo-optic (TO) crossbar switch implemented by dual photonic crystal nanobeam (PCN) cavities within a silicon-on-insulator (SOI) platform. By thermally tuning the refractive index of silicon, the resonance wavelength of the PCN cavities can be red-shifted. With the help of the ultrasmall mode volumes of the PCN cavities, only ～0.16 mW power is needed to change the switching state. With a spectral passband of 0.09 nm at the 1583.75 nm operation wavelength, the insertion loss (IL) and crosstalk (CT) performances were measured as IL(bar)= 0.2 dB, CT(bar)= 15 dB, IL(cross)= 1.5 dB, and CT(cross)= 15 dB. Furthermore, the thermal tuning efficiency of the fabricated device is as high as 1.23 nm/mW.

New designs are proposed for 2×2 electro-optical switching in the 1.3–12 μm wavelength range. Directional couplers are analyzed using a two-dimensional effective-index approximation. It is shown that three or four side-coupled Si or Ge channel waveguides can provide complete crossbar broad-spectrum switching when the central waveguides are injected with electrons and holes to modify the waveguides’ core index by an amount Δn+iΔk. The four-waveguide device is found to have a required active length L that is 50% shorter than L for the three-waveguide switch. A rule of ΔβL>28 for 3w and ΔβL>14 for 4w is deduced to promise insertion loss <1.5dB and crosstalk <?15dB at the bar state. At an injection of ΔNe=ΔNh=5×1017cm?3, the predicted L decreased from ～2 to ～0.5mm as λ increased from 1.32 to 12 μm. Because of Ge’s large Δk, the Ge bar loss is high in 4w but is acceptable in 3w.