Broadband terahertz anti-reflective structure fabricated by femtosecond laser drilling technique

• An anti-reflection structure sample with period of 90 μm is fabricated on silicon.
• The maximum of the relative transmission (T=Pstructured/Punstructured) increases 14%.
• Several ideal reverse conical structures are simulated by finite integral method.
• Samples of different hole diameters are obtained by different conditions.
• The anti-reflection mechanism of reverse conical structure is discussed.

We fabricated several reverse conical holes on high-resistivity silicon substrate with different power and pulse number of femtosecond laser, and investigated their patterns and features by using scanning electron microscope (SEM). Then, we chose one of the experimental parameters prepared a reverse conical anti-reflection structure sample with period of 90 μm. Terahertz Time-domain Spectroscopy (THz-TDS) was used to test its properties. Compared with the nonstructural high-resistivity silicon, the transmission of structural high-resistivity silicon increases by the maximum of 14% in the range 0.32–1.30 THz. Furthermore, we simulated the sample by finite integral method (FIM). The simulated results show good consistency with experimental results. The transmission effect of the reverse conical holes were optimized via simulation. Results show that the related transmission effect can be improved by increasing the pulse numbers and decreasing the spot size of the femtosecond laser. The different transmission window can also be tuned by changing the reverse conical structure of different periods.