Optomechanical crystals are planar, periodic nanostructures on a silicon chip that strongly couple optical and mechanical modes. The study presents the design and experimental realization of such a crystal, where 200 THz photons are co-localized with GHz mechanical modes and 100 femtogram mass. The effective coupling length, L_OM, is as small as 2.9 µm, enabling near-quantum-limited sensitivity for optomechanical transduction. These crystals have applications in RF-over-optical communication and quantum mechanics studies. Periodic structures can form photonic and phononic crystals, which manipulate light and mechanical vibrations. The optomechanical crystal combines both, allowing simultaneous confinement of optical and mechanical modes, enhancing light-matter interactions. The structure is a silicon nanobeam with periodic holes and cross-bars, featuring a defect to localize optical and mechanical modes. The geometry is optimized for strong coupling, with mechanical modes classified as "pinch," "accordian," and "breathing." The optical and mechanical modes are characterized by their frequencies and displacement profiles. The mechanical mode volume, V_m, describes strain energy-averaged localization, while the optical mode volume, V_0, describes electromagnetic localization. Both mode volumes are less than a cubic wavelength. The effective motional mass, m_eff, is between 50 and 1000 femtograms for the mechanical modes. The optomechanical coupling is described by an effective coupling length, L_OM, which determines the strength of the photon-phonon interaction. The coupling is calculated using perturbative Maxwell equations, considering material boundary shifts. The coupling length is inversely proportional to the force per-photon applied to the mechanical system. The experimental setup uses a tapered optical fiber to probe optical modes and measure mechanical vibrations. The RF spectrum of the transmitted light provides information about mechanical modes, with sidebands indicating optomechanical coupling. The study demonstrates the ability to measure mechanical motion with high sensitivity, showing a factor of ~7.5 times the standard quantum limit. The optomechanical crystal's mechanical Q is high, with a room temperature Q of 1300 for the fundamental breathing mode. The structure is ideal for studying mechanical loss mechanisms, with potential applications in high-frequency mechanics and quantum mesoscale oscillators. The crystal can also serve as a high-spatial resolution mass sensor, detecting changes in mechanical frequency due to mass variations. The study provides insights into the design and characterization of optomechanical crystals, demonstrating their potential for advanced optical and mechanical applications. The results highlight the importance of precise engineering in achieving strong optomechanical coupling and high sensitivity in mechanical transduction.Optomechanical crystals are planar, periodic nanostructures on a silicon chip that strongly couple optical and mechanical modes. The study presents the design and experimental realization of such a crystal, where 200 THz photons are co-localized with GHz mechanical modes and 100 femtogram mass. The effective coupling length, L_OM, is as small as 2.9 µm, enabling near-quantum-limited sensitivity for optomechanical transduction. These crystals have applications in RF-over-optical communication and quantum mechanics studies. Periodic structures can form photonic and phononic crystals, which manipulate light and mechanical vibrations. The optomechanical crystal combines both, allowing simultaneous confinement of optical and mechanical modes, enhancing light-matter interactions. The structure is a silicon nanobeam with periodic holes and cross-bars, featuring a defect to localize optical and mechanical modes. The geometry is optimized for strong coupling, with mechanical modes classified as "pinch," "accordian," and "breathing." The optical and mechanical modes are characterized by their frequencies and displacement profiles. The mechanical mode volume, V_m, describes strain energy-averaged localization, while the optical mode volume, V_0, describes electromagnetic localization. Both mode volumes are less than a cubic wavelength. The effective motional mass, m_eff, is between 50 and 1000 femtograms for the mechanical modes. The optomechanical coupling is described by an effective coupling length, L_OM, which determines the strength of the photon-phonon interaction. The coupling is calculated using perturbative Maxwell equations, considering material boundary shifts. The coupling length is inversely proportional to the force per-photon applied to the mechanical system. The experimental setup uses a tapered optical fiber to probe optical modes and measure mechanical vibrations. The RF spectrum of the transmitted light provides information about mechanical modes, with sidebands indicating optomechanical coupling. The study demonstrates the ability to measure mechanical motion with high sensitivity, showing a factor of ~7.5 times the standard quantum limit. The optomechanical crystal's mechanical Q is high, with a room temperature Q of 1300 for the fundamental breathing mode. The structure is ideal for studying mechanical loss mechanisms, with potential applications in high-frequency mechanics and quantum mesoscale oscillators. The crystal can also serve as a high-spatial resolution mass sensor, detecting changes in mechanical frequency due to mass variations. The study provides insights into the design and characterization of optomechanical crystals, demonstrating their potential for advanced optical and mechanical applications. The results highlight the importance of precise engineering in achieving strong optomechanical coupling and high sensitivity in mechanical transduction.