The paper presents the design and experimental realization of a strongly coupled optical and mechanical system in a planar, periodic nanostructure on a silicon chip. The system co-localizes 200-terahertz photons with 2 gigahertz mechanical modes, achieving an effective coupling length (LOM) of 2.9 μm. This allows for all-optical actuation and transduction of nanomechanical motion with near quantum-limited sensitivity. The optomechanical crystal, which combines photonic and phononic crystals, has potential applications in RF-over-optical communication and the study of quantum effects in mesoscopic mechanical systems. The structure consists of a silicon nanobeam with rectangular holes and thin cross-bars, creating a quasi-harmonic potential that localizes optical modes and mechanical modes. The coupling between these modes is described by the effective coupling length, which is crucial for understanding the system's behavior. The experimental results show that the optomechanical coupling is strong enough to achieve significant shifts in the mechanical mode frequencies and to transduce mechanical motion optically. The system's high sensitivity and low thermal noise make it suitable for studying quantum effects in mesoscopic mechanical systems and for developing high-spatial resolution mass sensors.The paper presents the design and experimental realization of a strongly coupled optical and mechanical system in a planar, periodic nanostructure on a silicon chip. The system co-localizes 200-terahertz photons with 2 gigahertz mechanical modes, achieving an effective coupling length (LOM) of 2.9 μm. This allows for all-optical actuation and transduction of nanomechanical motion with near quantum-limited sensitivity. The optomechanical crystal, which combines photonic and phononic crystals, has potential applications in RF-over-optical communication and the study of quantum effects in mesoscopic mechanical systems. The structure consists of a silicon nanobeam with rectangular holes and thin cross-bars, creating a quasi-harmonic potential that localizes optical modes and mechanical modes. The coupling between these modes is described by the effective coupling length, which is crucial for understanding the system's behavior. The experimental results show that the optomechanical coupling is strong enough to achieve significant shifts in the mechanical mode frequencies and to transduce mechanical motion optically. The system's high sensitivity and low thermal noise make it suitable for studying quantum effects in mesoscopic mechanical systems and for developing high-spatial resolution mass sensors.