Passive signal increase with improved signal-to-background ratio in nonlinear light microscopy through time stretching

This work presents a method to enhance signal intensity (SI) and improve the signal-to-background ratio (SBR) in nonlinear light microscopy (NLM) by employing a simple time-stretching technique applied to nonlinear signal pulses. In our experiments, femtosecond second harmonic generation (SHG) signals were temporally broadened to the nanosecond scale through an absorption-and-remission process facilitated by a highly efficient quantum dot material positioned in front of a photomultiplier tube. This process yielded a 77% increase in SHG SI, along with improved image quality. Furthermore, the enhancement in both SI and SBR was consistently observed throughout the full imaging depth of three-dimensional collagen fiber samples. This method not only boosts signal strength and SBR simultaneously but also reduces the risk of photodamage and enables faster imaging. Owing to its simplicity, compatibility with other intensity-enhancing techniques, and ease of integration, the proposed time-stretching strategy holds significant potential for advancing both fundamental research and industrial applications in NLM.