Thanks to its cell-type specificity, high spatiotemporal precision, and reversibility, optogenetic neuromodulation has been widely utilized in brain mapping, visual prostheses, psychological disorders, Parkinson's disease, epilepsy, and cardiac electrophysiology [1]. While a variety of optical neural interfaces have been developed, most have substantial limitations due to their size and tethering, needed to either deliver light or electricity, which may restrict the animal movements and bias the results, particularly in behavioral studies. In contrast, wirelessly powered optogenetic interfaces improve accuracy, reliability, and validity of the outcomes by eliminating tethers. Recently, a few wirelessly powered optogenetics approaches have been reported with impressive reduction in size of the implant [2]. However, their practical application is impeded by requiring high operating frequencies in GHz range, which increases the risk of exposure to unsafe electromagnetic specific absorption rates (SAR), resulting in excessive heat generation. They also lack proper control over optical stimulus characteristics. Towards this end, we propose a practical mm-sized Free-Floating Wirelessly-powered implantable Optical Stimulating (FF-WIOS) SoC to not only eliminate the tethering effects but also reduce the level of invasiveness and SAR in the tissue.