Terahertz spectroscopy technology has widespread applications in public safety, precision instruments, biomedicine, etc. However, due to the mismatch between the terahertz wavelength and the absorption cross-section of trace analytes, the interaction between them is very weak, making it difficult to directly detect the terahertz absorption spectrum of trace analytes. Therefore, researchers use waveguides, graphene, and metasurfaces to generate local electromagnetic fields to enhance the interaction between trace analytes and electromagnetic waves. To solve the problem of detecting the broadband terahertz absorption spectrum of trace analytes, researchers propose using the angle-multiplexing or geometric-multiplexing method of metasurface to obtain multiple resonance modes, thereby enhancing the broadband terahertz absorption spectrum. However, there are various shortcomings in the angle-multiplexing and geometric-multiplexing of metasurfaces. One-dimensional photonic crystal defect modes, as one of the important means of electromagnetic field localization, have the advantages of simple structure and good enhancement effect. To further improve the terahertz absorption spectrum enhancement effect based on the photonic crystal defect-mode structure, we design a one-dimensional photonic crystal defect-mode enhancement structure for the terahertz absorption spectrum based on a metal-dielectric periodic structure. By changing the width of the defect cavity, we obtain a series of resonance peaks, thus achieving the broadband terahertz absorption spectrum of trace analytes in the 0.46?0.6 THz wide frequency range. For the 0.2 μm α-lactose thin film, an absorption enhancement factor of about 101.2 can be obtained, while the all-dielectric one-dimensional photonic crystal defect-mode structure with the same geometric parameters can only gain an absorption enhancement factor of 76.2. The enhancement effect is significantly improved. This structure provides a new research direction for improving the terahertz absorption spectrum of trace analytes.

We design a one-dimensional photonic crystal defective-mode-enhanced terahertz absorption spectroscopy structure based on a metal-dielectric periodic structure with a high-density polyethylene (HDPE) dielectric material, a polytetrafluoroethylene (teflon) substrate for the thin-film lactose to be measured, and an Au metal material. The refractive indices of an HDPE plastic layer, teflon, and air are 1.525, 1.43, and 1, respectively. We assume them to be lossless in the simulation as the loss of the materials is very low. We simulate the electromagnetic response of the structure using the finite-element method with transmission and reflection ports in the x-direction and Floquet periodic boundary conditions in the y-direction. The resonant frequency of the defect mode will vary with the number of layers N, the thickness of the dielectric material, the thickness of the air, and the width of the defect cavity. The defect cavity width d_f, the thickness of the alternating HDPE-metal layer d_t, and the thickness of the air layer d_a can be optimized for different analytical materials to match the characteristic absorption of the corresponding object to be measured. Next, we investigate the specific properties of one-dimensional photonic crystal defect-mode-enhanced terahertz absorption spectral structures based on metal-dielectric periodic structures and the influences of varying each parameter on the enhancement effect.

Firstly, we investigate the transmission spectrum of the structure when the teflon substrate is loaded with or without α-lactose film under the condition of the number of layers N=6, the dielectric model of α-lactose, and the absorption spectrum at this time (Fig.2). We find that the enhancement factor of the designed structure for 1.0 μm α-lactose is 41.5. After that, to show that the enhanced terahertz absorption spectra of the designed one-dimensional photonic crystal defective cavity composed of dielectric-metal are better than those of the one-dimensional photonic crystal defective cavity with pure dielectric, the enhancement effects of the two structures for 1.0 μm-thick α-lactose are investigated for different numbers of layers N (Fig. 3). We observe that the enhancement abilities of the metal-dielectric periodic structure are more than 25.4%, 19.9%, and 7.2%, respectively. To further discuss the enhancement mechanisms, we give the electric field distributions of these two structures at N=6 and d_f=260 μm (Fig. 4) and the electric field (E_x) distributions of the structure in the x-y plane when the d_f value is varied from 240 μm to 280 μm in steps of 10 μm for a blank and loaded 1.0 μm α-lactose film (Fig.5). We find that without the metal structure, the maximum intensity of the localized electromagnetic field is relatively weaker than that with the metal structure by 4.1%. Also, the intensity of the electric field with lactose decreases by 28.1%, 40.2%, 44.0%, 35.4%, and 25.0% compared to that of the sample without lactose, respectively. Next, the effects of the number of photonic crystal layers N (Fig. 6), α-lactose thickness t (Fig. 7), and height h (Fig. 8) on the enhanced absorption spectra are investigated at a metal structure thickness of 1.0 μm, respectively. Finally, we investigate the effect of the metal thickness w on the enhanced absorption spectrum in the metal-dielectric periodic structure for N=6 and t=1.0 μm (Fig. 9). We note that saturation occurs if the local electromagnetic field exceeds the upper absorption limit of the object to be measured, which should be avoided in practical design. Table 1 lists the performance comparison among some dielectric hypersurface enhancement structures, existing dielectric one-dimensional photonic crystal defect-mode enhancement structures, and the designed structures.

We propose a one-dimensional photonic crystal defect-mode enhanced terahertz absorption spectrum structure based on a metal-dielectric periodic structure. By introducing a metal periodic structure into the all-dielectric one-dimensional photonic crystal defect-mode structure, the electric field intensity of the Q resonance structure can be effectively enhanced. Then, by adjusting the width of the defect cavity, the terahertz absorption spectrum of the thin-film sample can be enhanced within a wide band. When applied to a 0.2 μm α-lactose thin-film sample, the enhancement factor of this structure is approximately 101.2, while the enhancement factor of the all-dielectric one-dimensional photonic crystal defect-mode structure with the same geometric parameters is only 76.2. The improvement effect is obvious, and the defect cavity structure is convenient for detecting thin-film samples. Currently, the designed structure needs the precise adjustment of the defect cavity width using a precision translation stage to cover a certain frequency range. This brings certain measurement difficulties, but it does achieve wide-band coverage, which is worthwhile for the specific non-labeled detection of the broadband absorption spectrum of the sample. In the future, we can achieve parallel measurements of multiple different resonant cavity structures to simplify the measurement process and increase the measurement speed. Of course, this structure will increase the difficulty of processing and manufacturing, but for better absorption enhancement effects, the manufacturing cost is worth it. In summary, this structure will contribute to the further development of high-sensitivity terahertz detection technology for trace samples.