Metallic　resonators　allow　the　manipulation　of　electromagnetic　fields　in　subwavelength　volumes　for　strong　lightmatter　interaction,　underpinning　emerging　flat-optics　devices.　Metamaterials　designed　with　optimized　effective　mode　volume　(Veff)　for　stimulating　and　maximizing　the　utilization　of　high-density　photons　are　highly　desirable　for　sensing　applications.　Here,　we　demonstrate　a　bowtie-type　terahertz　(THz)　metallic　aperture　metamaterial　structure　with　low　Veff　for　enhanced　biosensing.　The　device　leverages　the　inherent　characteristic　of　the　femtosecond　laser　fabrication　process　enabling　wedge　cavities　down　to　3　mu　m　by　tailoring　the　pulse　energy,　whose　sensing　performance　is　superior　to　trapezoidal　ones.　Such　a　substrate-free,　microcavity　metallic　metamaterial　sensor　with　extremely　strong　interactions　achieves　an　experimentally　normalized　sensitivity　of　0.654　RIU-1,　and　demonstrates　the　detection　of　L-proline　down　to　0.87　nmol.　These　findings　provide　new　insights　into　the　design　and　optimization　of　efficient　THz　metamaterials　with　tightly　confined　fields　for　ultrasensitive　biomolecular　detection.