Ultrafast Optical Field Metrology via Unbalanced‐Intensity Autocorrelation Enabled by Two‐Photon Absorption in a Nonlinear Photodiode

ABSTRACT Complete characterization of ultrafast optical fields in integrated photonic platforms remains technologically challenging because most existing ultrafast pulse metrology methods require bulk spectrometers and nonlinear crystals for parametric frequency conversion, which are difficult to integrate with compact chip‐scale architectures. This study presents a simple opto‐electronic approach for ultrafast optical field metrology by employing two‐photon absorption (TPA) in a semiconductor photodiode as both the nonlinear medium and detector within a fast‐scanning unbalanced‐intensity nonlinear interferometric autocorrelation (IAC) system. The TPA‐based unbalanced IAC signal preserves spectral phase information of the optical pulse by breaking the delay‐reversal symmetry, which enables field retrieval without spectrally resolved detection or parametric frequency conversion, making this technique inherently scalable to on‐chip integration. As an experimental demonstration, the full electric field of ∼8 fs nJ‐level laser pulses is retrieved directly from the TPA photocurrent, and the results are validated by comparison with the well‐established Frequency‐Resolved Optical Gating method. Additionally, the spectral phase alone can be measured even without any spectral calibration of the photodiode's nonlinear responsivity. This compact, low‐cost, and robust technique enables real‐time ultrafast pulse metrology for interdisciplinary photonics applications without spectrally resolved measurements or nonlinear crystals and establishes a clear pathway toward an implementation with chip‐scale photonic systems.