## 3 Experimental Results and Discussion
### Chiral Recognition of R/S Chiral Ibuprofen Based on THz Sensing  
Next, we performed sensing experiments on (R)-(\(-\))-ibuprofen and (S)-(+)-ibuprofen at the same concentration to study whether higher-order resonance could be applied in the recognition of chiral enantiomers. Although the structure we designed was not a chiral metasurface, from previous experiments we found that the higher-order resonance generated inside the structure had better sensing performance than the lower-order resonance. It was particularly sensitive to different refractive indices and absorption. Different molecular structures of chiral molecules will cause slight differences in response to THz waves, such as in refractive index and absorption. Therefore, higher-order resonance is used to enhance the response of these slight differences. Additionally, to further test the performance of our sensor in the trace amount of the sample, the sample volume of each measurement was only 5 \(\mu\)L in this experiment.  
The experimental results are shown in Figure 8, including the cases of two different polarization directions. Figure 8b,d shows detailed information for the higher-order resonance. Although the amount of the sample was reduced, our device still showed good sensing performance. The transmittance magnitude at the higher-order resonance valley gradually decreased with increasing concentrations of both (R)-(\(-\))-ibuprofen and (S)-(+)-ibuprofen. In particular, when the incident THz wave was \(x\)-polarized, with the increase in concentration, an obvious blue shift could still be observed. To measure the sensor's ability to recognize R/S chiral ibuprofen, it is important to calculate the degree of differentiation, which can be expressed as \(D_{\Delta f}=\Delta f/m\), \(D_{\Delta T}=\Delta T/m\), where \(\Delta f=f_{\text{R-Ibu}}-f_{\text{S-Ibu}}\) and \(\Delta T=T_{\text{R-Ibu}}-T_{\text{S-Ibu}}\), with the same concentration; \(m\) is the mass of ibuprofen. For the \(x\)-polarized wave, R/S chiral ibuprofen could be distinguished not only by frequency shift but also by different transmittance magnitudes. The detailed results are shown in Table 3.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Polarization** & \(\Delta m\)**(mg)** & \(\Delta f\)**(GHz)** & \(\Delta T\)**(dB)** & \(S_{\Delta f}\)**(GHz/mg)** & \(S_{\Delta T}\)**(dB/mg)** \\ \hline \(x\)-polarization & 3 & 181.25 & 18.81 & 60.42 & 6.27 \\ \(y\)-polarization & 3 & 93.75 & 18.7 & 31.25 & 6.23 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The calculated sensitivity of the higher-order resonance with respect to different polarization directions.  
Figure 8: Transmitted THz spectra of the R/S chiral ibuprofen at different concentrations. (**a**) Transmitted spectra for \(y\)-polarized incident THz wave and (**b**) for the details. (**c**) Transmitted amplitude for \(x\)-polarized incident THz wave and (**d**) for the details.  
When the incident THz wave was \(y\)-polarized, it was difficult to distinguish (R)-(\(-\))-ibuprofen and (S)-(+)-ibuprofen by frequency shift, as shown in Figure 8b. Fortunately, the different transmittance magnitude between the two types of ibuprofen could still help us to recognize them. For example, when the ibuprofen concentration was 0.4 g/mL for (R)-(\(-\))-ibuprofen, the transmittance magnitude reached about \(-27\) dB; in the same case, the transmittance magnitude for (S)-(+)-ibuprofen was around \(-24\) dB, and the degree of differentiation is 1.545 dB/mg in Table 3. According to our experimental results at different concentrations, the transmittance at the higher-order resonance of (S)-(+)-ibuprofen is always higher than that of (R)-(\(-\))-ibuprofen. Therefore, our experiments demonstrated that the slight differences of R/S chiral ibuprofen can be distinguished through THz higher-order resonance in 5 \(\mu\)L trace amounts of samples on this crescent cylinder metasurface sensor.