Deep Silicon Amorphization Induced by Femtosecond Laser Pulses up to the Mid-Infrared

Direct laser writing of amorphous lines in crystalline silicon has the potential for becoming a flexible alternative to silicon-on-insulator technology for photonic integrated circuits. Yet, the maximum amorphous layer thickness achieved is 60 nm, which is below the requirements for waveguiding at telecom wavelengths. Here, the authors report on different strategies to push the layer thickness beyond today's limit. To this end, irradiation with femtosecond laser pulses covering an extremely broad wavelength range (515 nm–4 µm) up to the yet unexplored near- and mid-infrared region of silicon transparency is investigated. The results show that much thicker amorphous layers can be obtained upon multipulse irradiation at 3-µm wavelength. The deepest amorphization is achieved in silicon wafers covered with a thick silicon dioxide layer that strongly assists the heat extraction, yielding steep index profiles with a maximum amorphous layer thickness of 128 nm. This superior thickness is compatible with single mode waveguiding for a symmetric waveguide configuration. This study also contributes to a better understanding of the mechanisms involved in laser-induced amorphization.