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Optical actuation for MEMS takes advantage of directly coupling of energy into selected device areas without any electrical interconnects as required in integrated electrostatic and piezoelectric actuation. In opto-thermo-mechanical actuation, light incident onto the structures is absorbed and converted to heat via photon absorption. Here, we realized linear, high efficiency and broad-band optical actuation for circular Si/SiO2 membrane resonator with integrated vertical silicon nano-pillar photonic crystal arrays. The first-ever nanopillar membrane acoustic speaker, using nanoscale photonic crystal optical absorbers for thermo-mechanical excitation of speaker membrane, is demonstrated. Moreover, our device has a very high surface area-to-volume ratio, enabling DNA sensing of femtomolar concentration. Femtomolar concentration DNA detection is important as this is the concentration needed for early-stage cancer and bacterial infection diagnosis application. Our method represents a mass-based platform technology that can sense molecules at low concentrations.