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The integrable substrate for THz modulation directly influences both the quality of films and THz absorption. Currently, the available THz substrate candidate library is still not clear. Here, we have carried out a systematic investigation of commonly used commercial substrates, including Si, quartz SiO₂, MgO, Al₂O₃, GdScO₃ and TbScO₃ in the range of 0.4–1.6THz. It is found that low resistance Si, TSO and GSO are certainly not appropriate for THz light modulation due to their relatively higher absorption and dielectric constant, while the rest show better THz transmittance, low refractive index and loss. However, the dielectric constant and refractive index of high resistance Si are generally two times larger than quartz SiO₂, Al₂O₃ and MgO. Compared with Al₂O₃ and MgO, quartz SiO₂ shows at least 50% lower dielectric constant, refractive index and absorption, making it the best candidate. Our research is believed to build the rich substrate candidate library for THz range light modulation.

Low-temperature co-fired ceramics (LTCC) applied in millimeter/microwave and terahertz frequencies (5G/6G) have attracted a lot of attention recently. In this study, MgO-based dielectric ceramics were successfully sintered at 950°C with the sintering aids: x wt.% of LiF fluoride ($x=2$, 4, 6, 8, 10) and 0.5wt.% of BBSZ (Bi₂O₃–B₂O₃–SiO₂–ZnO) glass. BBSZ glass was introduced as another sintering aid to facilitate the sintering and densification. Crystalline structure and micro-morphology were investigated and analyzed. Dielectric properties (${\epsilon }_r$, $Q\times f$, ${\tau }_f$) at millimeter/microwave and terahertz wave frequencies were also studied. The ionic characteristics of Mg–O bond ($f_i$), the lattice energy (U) and the bond energy (E) were calculated and analyzed. It is suggested that the optimal $x=4$, where ${\epsilon }_r=10.5$, $Q\times f=120,000$GHz (@12GHz) and ${\tau }_f=-26$ppm/°C at millimeter/microwave range. When the frequency was up to terahertz (1.0THz), the ${\epsilon }_r$ values were 8.8–9.35 and the tan$\delta$ were $5.6\times 10^{-3}$–$8.7\times 10^{-3}$. The experimental results indicated that the low-temperature sintered MgO-based ceramics have potential for millimeter/microwave and terahertz communication applications.