Optical signal enhancement in biomedical, chemical, and environmental analyses typically relies on engineered optical components, which increase system complexity and cost. Bioinspired approaches, such as enriching analytes via droplet evaporation on superhydrophobic surfaces or shaping sessile droplets into microlenses, have been explored as alternatives. However, these platforms generally support only a single spectroscopic modality, depend on evaporation dynamics, and require specialized readout instruments. Here we report a bioinspired, spontaneously formed air-encapsulated cavity (BIOSAC) that overcomes these constraints by combining a superhydrophobic coating with V-shaped geometric confinement inside a sample container. Upon liquid loading, the sample spontaneously forms a stable air-encapsulated droplet that functions as a built-in liquid lens, modifying excitation-light propagation and signal collection without auxiliary optics or additional sample preparation. BIOSAC achieved maximum measured signal enhancements of 40.3-fold for fluorescence, 56.4-fold for surface-enhanced Raman scattering, and 8.6-fold for UV–vis absorbance. Coupled with an amplification-free CRISPR/Cas13a assay, the platform detected SARS-CoV-2 RNA at 78 copies/µL within 30 min using a standard microplate reader. This work shows that interfacial wetting and geometric confinement can be jointly engineered to make the sample itself a built-in optical element, decoupling signal gain from system complexity and providing an adaptable framework for high-sensitivity optical sensing.