Revealing the Hidden Stress Field and Its Thermal Input Dependence in Femtosecond Laser-Processed Silicon Photonic Chip Substrates via Weak Measurement.

With the widespread adoption of silicon photonic chips in optical-electrical co-packaging, residual stresses induced by femtosecond laser processing in thick bulk silicon substrates have become a critical factor affecting device performance and reliability. However, existing stress characterization techniques struggle to simultaneously achieve high spatial resolution, high sensitivity, and environmental robustness, leaving the subtle stress fields within the heat-affected zone largely "hidden." To address this challenge, we propose a noncontact stress detection method based on frequency domain weak value amplification. The approach leverages the photoelastic effect to convert localized stress into birefringence-induced phase shifts, which are then read out with high sensitivity via weak measurement. Experiments demonstrate that the method achieves a pascal-level stress detection limit of 0.37 kPa, reveals a quantitative relationship between laser-deposited energy and residual stress magnitude, and further resolves the weak stress gradients and local anisotropy within the heat-affected zone of femtosecond laser-processed silicon samples. Its spatial resolution is nearly an order of magnitude higher than that of confocal laser scanning microscopy. This work provides a new tool for high-precision, in situ stress monitoring in semiconductor manufacturing.