As the final step of our work and guided by the PSO results, we experimentally realized the optimized sensor design using electron beam lithography (Fig. -b, Supplementary Figure , and Methods) and assessed its detection limit to hydrogen. To this end, we exposed the sensor to pulses of gradually decreasing H 2 concentration in Ar carrier gas (1000 ppm to 250 ppb, the lowest attainable concentration in our setup) at room temperature and plot its associated Δλ peak , which is obtained through a Lorentzian fit 2 (Supplementary Figure and Methods). As depicted in Fig. , the sensor responds positively to different H 2 concentrations, with a signal noise, σ sensor , of 0.01 nm (Supplementary Figure ). Due to this small noise, the sensor is able to measure even the lowest 250 ppb pulse, making it the first optical hydrogen sensor to achieve sub-ppm detection (Supplementary Table ). Recalling LoD as the lowest hydrogen pressure measurable with a signal larger than 3σ, we extrapolate it to be ~200 ppb (Fig. ). We expect such sensitivity to also hold in air, thanks to the excellent O 2 sieving provided by PMMA. comparably to the periodic array sensor but suffers from its higher noise. Hence, its LoD is ~8 times higher at 1.5 ppm.