In this work, the Fourier transform infrared spectroscopy (FTIR) and Terahertz time domain spectra (THz-TDS) were employed to study Pu’er white tea with different oxidation levels by changing the withering time. The strongest absorption peak of 3359 cm-1 in FTIR spectra comes from the proteins and polysaccharides. Several intensity ratios of the characteristic absorption peaks with different oxidation levels were analyzed. The white tea with different oxidation levels can be distinguished from the absorbance of THz-TDS in the range of 0.5–1.5 THz. The refractive index calculated from THz-TDS was between 1.42 and 1.44. Principal component analysis (PCA) using FTIR absorbance data showed that the scattered points of PC1 and PC2 can be well distributed in different districts. Hierarchical cluster analysis (HCA) helped to carry out the clustering results of different white tea samples. The sample with overoxidation level can be distinguished by PCA and HCA using THz-TDS absorbance data. Therefore, FTIR might be a convenient, fast, and nondestructive strategy for identifying white tea with different oxidation levels. THz-TDS combined with some software algorithms can be applied to distinguish the different oxidation-processed tea.

We demonstrate the bias-selectable mid- or long-wave infrared detectors composed by back-to-back pin-nip diodes with “M” barrier inserted in two diodes to suppress the dark current. Mid-wave infrared (MWIR) and long-wave infrared (LWIR) active region (π layer) are composed by 8 ML/8 ML and 12.5 ML/7 ML InAs/GaSb superlattices. The cutoff wavelength is 4.8 μm under forward bias called MWIR channel and expands to 10 μm under reverse bias called LWIR channels. The responsivities are 0.27 A/W at 4.2 μm at forward 300 mV bias, while 0.31 A/W at 9 μm at reverse 400 mV bias at 77 K. The dependence of the response on the bias is due to the competition of the built-in electric field from the two junctions. The current–voltage curve and the quantum efficiency were measured at 77 K. The dark current densities are 22.7×10−5 A/cm2 and 4.51×10−2 A/cm2 for two channels under different bias. The peak detectivity of MWIR diode is calculated to 2.0×1012 cm·Hz1/2/W at 4.2 μm and 1.0×1010 cm·Hz1/2/W at 9 μm for LWIR channel.