Keio University's "Mosquito method" draws light paths into polymer with a single needle, using a graded-index core to avoid interface scattering for near-material-limit loss — and uses fluid dynamics to beat the needle-size limit for 20 µm pitch. What is the open question for the polymer waveguide route?

DNP brought polymer waveguides on glass down to a 10 µm pitch, but the real point is an iron rule: density is paid for with index contrast. Only Δ=2.3% pushes 10 µm crosstalk down to −32.3 dB. Which three material parameters decide whether polymer waveguides can compete?