The computational inverse design has paved the way for the design of highly efficient, compact, and novel nanophotonic structures beyond human intuition and trial-and-error approaches. Consequently, with this new design power, the exploration and implementation of multi-objective, complex, and functional nanophotonic devices become feasible. Herein, we used a recently emerged inverse design framework to demonstrate the design of a 1 × 2 polarization-insensitive wavelength division multiplexer (PIWDM) made of a low-refractive-index material with an index of 1.55. The designed PIWDM structure successfully steers toward the targeted channels for 1.30 µm and 1.55 µm with TE and TM polarizations, respectively. The transmission values were -2.42 and -2.18 dB for TE and -2.19 and -2.23 dB for TM polarization at the upper and lower waveguides, respectively. Taking advantage of the design with a low refractive index material, we scaled the structural dimensions corresponding to the microwave region, fabricated the compact device using a 3D printer, and conducted an experiment as a proof of concept. The experimentally verified PIWDM structure shows a power transmission efficiency of over -2.42 dB and a crosstalk value of less than -11.45 dB for the targeted wavelengths.

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