## 1 Introduction  
With increasing demands for developing reliable and effective biological sensors [1], terahertz (THz) radiation has attracted great attention as a promising tool in biological and medical investigation in recent years. Because of the unique characteristics of THz waves--which are as follows: (1) non-ionizing and do not damage biological tissues; (2) sensitive to weak resonances [2]; (3) highly sensitive to water--THz sensing could soon become a suitable technology for biological applications. In particular, up to now many reports have demonstrated that THz time-domain spectroscopy (THz-TDS) is a powerful technique for label-free, non-contact, and highly sensitive biomolecule sensing, including the detection of proteins, DNA, or RNA [3, 4, 5]; the detection of cells [6]; and antibiotics [7], etc. However, achieving reliable sensing for trace amounts of sample is still a major challenge. One of the most promising routes to promote THz sensing sensitivities is to use metasurfaces [8, 9, 10, 11, 12, 13, 14]. These artificial structures' electromagnetic properties can be easily controlled by the geometry, shape, or orientation of their constituents, which provide extremely strong confinement of electromagnetic fields to detect small changes in the dielectric environment around the metasurfaces [15, 16, 17], leading to high efficiency and sensitivity. Many reports have successfully demonstrated the high sensitivity of metasurface-based THz biosensors; for example, [PERSON] et al. proposed a THz biosensing metamaterial absorber based on spoof surface plasmon polaritons which showed ultrasensitive sensitivity and high resolution in virus detection [18]. [PERSON] et al. achieved highly sensitive THz sensing of small-volume liquid samples by a multi-microfluidic channel metamaterialbiosensor [19]. However, applications in chiral pharmaceutical analysis have rarely been reported until now. The chiral structure of molecules is one of the basic elements of nature. In the living system, chiral molecules often have extremely important properties and functions. Many important biological macromolecules, such as amino acids, polypeptides, proteins, polysaccharides, nucleic acids, and enzymes, are almost all chiral molecules [20; 21]. Chiral drugs contain chiral centers in drug molecules. They usually achieve their pharmacological effects through strict chiral matching and molecular recognition with biological macromolecules in vivo [22; 23]. Significant differences in pharmacological activity, the metabolic process, and toxicity exist between the chiral enantiomers of the drugs; one enantiomer may be effective, while the other enantiomer may be harmful [24]. For example, ibuprofen (4-isobutyl-2-phenylpropionic acid, C\({}_{13}\)H\({}_{18}\)O\({}_{2}\)), a well-known painkiller and an efficient anti-inflammatory, is widely used in daily life. It contains in its chemical structure a stereogenic carbon, and, thus, it has two enantiomers, called S-(+)-ibuprofen and R-(\(-\))-ibuprofen, respectively. S-(+)-ibuprofen has anti-inflammatory and analgesic effects, while R-(\(-\))-ibuprofen has almost no anti-inflammatory effect but will increase the metabolic burden. Therefore, the identification of enantiomers of chiral substances has always been one of the main problems in life sciences and other fields. Recently, [PERSON]'s group [25] proposed a tunable and controllable graphene chiral metastructure to analyze the collagen biomolecule. Their simulations showed that a refractive index sensitivity value can be achieved as high as 0.96 THz per refractive index unit for the CD spectra. [PERSON] et al. [26] proposed a THz polarization sensing method and a chiral metasurface to improve sensing sensitivity. D- and L- enantiomers of three kinds of chiral amino acids in aqueous solution were successfully distinguished. [PERSON] et al. [27] incorporated graphene with a THz metasurface into a microfluidic cell for sensitive biosensing, and they found that 100 nm DNA short sequences could be detected by this method.