Terahertz time-domain spectroscopy (THz-TDS) combining with chemometrics methods was proposed for the identification of hepatic tumors. Two linear compression methods, principle component analysis and locality preserving projections (LPPs), and a nonlinear method, Isomap, were used to reduce the dimensionality of the measured dataset. Comparing two-dimensional (2-D) data reduced by these three dimensionality reduction techniques, only 2-D Isomap plot could separate the distances between two classes for the THz time-domain data and LPP had capacity of distinguishing two types of samples building on frequency-domain data. The best classification accuracies from 2-D time-domain data were ${\text{99.81}}\,\pm \,{\text{0.30}}\% $ and ${\text{99.69}}\,\pm \,{\text{0.61}}\% $ given by Isomap probabilistic neural network (PNN) and Isomap support vector machine (SVM), respectively, while the best classification results of 2-D frequency-domain data were ${\text{100.00}}\,\pm \,{\text{0.00}}\% $, ${\text{99.75}}\,\pm \,{\text{0.32}}\% $ provided by LPP-PNN, LPP-SVM. The results showed that Isomap and LPP are appropriate techniques to reflect the nonlinear manifold of the THz data. The THz technology either in time-domain or frequency-domain coupled with Isomap-PNN or LPP-PNN could offer a potential procedure to identify hepatic tumors.

We report high-resolution THz-TDS characterization of the transmission and reflection of a thin metal hole-array in optical contact with a 10 mm thick high-resistivity Si plate. It was found that the red-shift of the [±1, 0] spoof surface-plasmon (SSP) transmission peak from the Wood's anomaly valley indicates the coupling strength of the SSP wave to the metal surface due to the filling factor and shape of the holes, and that the red-shift provides a measurement of the SSP wave velocity. Our measurements of the amplitude transmission peak, and the corresponding reflection dip, of the hole-array agreed with numerical simulation to an accuracy of ±2 GHz, and the measured phase was in good agreement with simulation. However, our reflection measurements showed a second unexplained dip at the frequency of Wood's anomaly.

In recent years, metamaterials (MMs) have been widely investigated for their exotic electromagnetic characteristics which cannot be achieved in nature. However, one of the main limitations in traditional metallic-film MMs is a high level of radiation loss in metal and insertion loss of the high-permittivity or thick substrate. Fortunately, all-dielectric MMs with high refractive-index dielectric structures show significantly less material loss than their metallic counterparts. In this paper, an all-dielectric grating is fabricated on a 100-m-thick silicon wafer by using direct-laser-writing technique, and the properties of its Mie resonances are investigated by THz time-domain spectroscopy. Then we measure the spectral response of the all-dielectric grating under the optical modulation by a near-infrared pump-THz probe method. The modulation light source is an 808 nm continuous semiconductor laser with a maximum power (10 W). To give an insight into the underlying mechanisms of the Mie-type resonance effects on the arrayed, silicon pillars, the transmission of the all-dielectric grating is investigated numerically by the finite-element simulations through using CST Microwave Studio. In our experiment, the incident THz magnetic field is along the grating lines. The research results show that three typical Mie resonances are excited from 0 to 1 THz in the all-dielectric structure, and all the three resonant modes are different in the distributions of electric field and magnetic field. Furthermore, it is found that the resonance intensities of these three resonance peaks appear to be weakened variously with the increase of the optical power, and the first resonant peak modulation amplitude maximally reaches more than 50%. Combining the simulation results, we prove that the decrease of Mie resonance intensity under photo-excitation is caused by the absorption and the scattering of the incident THz wave by photo-generated carriers. Besides, we estimate the conductivity values of the all-dielectric grating under different optical excitations and find that the conductivity values reach 1000 S/m and 1500 S/m corresponding to 5 W and 10 W optical excitation, respectively. The estimated conductivity data will play an important role in the prospective optical modulation simulation. All the results mentioned above will provide an important reference for researches on the resonance properties of the all-dielectric metamaterials and the development of related functional devices.