Electrically Tunable Hot-Silicon Terahertz Attenuator*

We have developed a continuously tunable, broadband terahertz attenuator with a transmission tuning range greater than 10$^{\mathrm{3}}$. Attenuation tuning is achieved electrically, by simply changing the DC voltage applied to a heating wire attached to a bulk silicon wafer, which controls its temperature between room temperature and 550 K, with the corresponding free-carrier density adjusted between 10$^{\mathrm{11}}$ cm$^{\mathrm{-3}}$ and 10$^{\mathrm{17}}$ cm$^{\mathrm{-3}}$. This `hot-silicon'-based terahertz attenuator works most effectively at 450-550 K (corresponding to a DC voltage variation of only 7 V) and completely shields terahertz radiation above 550 K in a frequency range of 0.1-2.5 THz. Both intrinsic and doped silicon wafers were tested and demonstrated to work well as a continuously tunable attenuator, but they exhibited slightly different behaviors before a dramatic transmission drop at 450-550 K: intrinsic silicon wafers showed a monotonic transmission decrease with temperature while doped wafers showed a slight increase in transmission before the drop. All behaviors can be understood quantitatively via the free-carrier Drude model taking into account thermally activated intrinsic carriers.