This paper presents an energy-efficient mm-scale self-contained bidirectional optogenetic neuro-stimulator, which employs a novel highly-linear LED driving circuit architecture as well as inkjet-printed custom-designed optical lenses for light directivity enhancement. The proposed current-mode LED driver performs linear control of optical stimulation for the entire target range (<10mA) while requiring the smallest reported headroom, yielding a significant boost in the energy conversion efficiency. A 30.46 improvement in the power delivery efficiency to the target tissue is achieved by employing a pair of printed optical lenses. The fabricated SoC also integrates two recording channels for LFP recording and digitization, as well as power management blocks. A micro-coil is also embedded on the chip to receive inductive power and our experimental results show a PTE of 2.24% for the wireless link. The self-contained system including the LEDs, lenses and the capacitors required by the power management blocks is sized 6mm3 and weighs 12.5mg. Full experimental measurement results for electrical and optical circuitry as well as in vitro measurement results are reported.