Recently, Terahertz time domain spectroscopy (THz-TDS) has emerged as a successful method to measure the refractive index of thin solid films without sample perturbation. However, a need remains for a new non-invasive biosensor technology having application, for example, in healthcare, food monitoring, and weapon detection. This invention is directed to a method for detecting specific associations between a tethered molecule and an untethered target molecule, thus providing a method of probing bimolecular interactions using THz radiation. The biomolecular interactions that may be detected include non-covalent and covalent molecular interactions such as ligand and receptor binding, antigen and antibody binding, and DNA and RNA hybridization interactions. Specific interactions that may be examined include enzyme and coenzyme binding, as well as cell surface binding to cell-specific signaling agents. This invention could be used in several fields: assaying molecules for potential drug targets, detecting and purifying genetic material, and detection of small amounts of contaminants in water or food. Similarly, blood samples may be tested for the presence of disease-indicating agents.