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We achieve high aspect-ratio laser ablation of silicon with a strong nonlinear dependence on pulse duration while using a power density 10(6) times less than the threshold for typical multiphoton-mediated ablation. This is especially counter-intuitive as silicon is nominally transparent to the modulated continuous wave Yb:fiber laser used in the experiments. We perform time-domain finite-element simulations of thermal dynamics to investigate thermo-optical coupling and link the observed machining to an intensity-thresholded runaway thermo-optically nonlinear process. This effect, cascaded absorption, is qualitatively different from ablation observed using nanosecond-duration pulses and is general enough to potentially facilitate high-quality, high aspect-ratio, and economical processing of many materials.

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BACKGROUND AND OBJECTIVE: During tissue ablation, laser light can be delivered with high precision in the transverse dimensions but final incision depth can be difficult to control. We monitor incision depth as it progresses, providing feedback to ensure that material removal occurs within a localized target volume, reducing the possibility of undesirable damage to tissues below the incision. MATERIALS AND METHODS: Ex vivo cortical and cancellous bone was ablated using pulsed lasers with center wavelengths of 1,064 and 1,070 nm, while being imaged in real-time using inline coherent imaging (ICI) at rates of up to 300 kHz and axial resolution of ∼6 µm. With real-time feedback, laser exposure was terminated before perforating into natural inclusions of the cancellous bone and verified by brightfield microscopy of the crater cross-sections accessed via side-polishing. RESULTS: ICI provides direct information about incision penetration even in the presence of intense backscatter from the pulsed laser and plasma emissions. In this study, ICI is able to anticipate structures 176 ± 8 µm below the ablation front with signal intensity 9 ± 2 dB above the noise floor. As a result, the operator is able to terminate exposure of the laser sparing a 50 µm thick layer of bone between the bottom of the incision to a natural inclusion in the cancellous bone. Versatility of the ICI system was demonstrated over a wide range of light-tissue interactions from thermal regime to direct solid-plasma transition. CONCLUSIONS: ICI can be used as non-contact real-time feedback to monitor the depth of an incision created by laser ablation, especially in heterogeneous tissue where ablation rate is less predictable. Furthermore, ICI can image below the ablation front making it possible to stop laser exposure to limit unintentional damage to subsurface structures such as blood vessels or nervous tissue.

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We observe sample morphology changes in real time (24 kHz) during and between percussion drilling pulses by integrating a low-coherence microscope into a laser micromachining platform. Nonuniform cut speed and sidewall evolution in stainless steel are observed to strongly depend on assist gas. Interpulse morphology relaxation such as hole refill is directly imaged, showing dramatic differences in the material removal process dependent on pulse duration/peak power (micros/0.1 kW, ps/20 MW) and material (steel, lead zirconate titanate PZT). Blind hole depth precision is improved by over 1 order of magnitude using in situ feedback from the imaging system.

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We demonstrate real-time depth profiling of ultrafast micromachining of stainless steel at scan rates of 46 kHz. The broad bandwidth and high power of the light source allows for simultaneous machining and coaxial Fourier-domain interferometric imaging of the ablation surface with depth resolutions of 6 mum. Since the same light is used to machine as to probe, spatial and temporal synchronization are automatic.