High-Sensitive and Background-Free Coherent Anti-Stokes Raman Scattering Microscopy Using Delay Modulation

Coherent anti-Stokes Raman scattering (CARS) microscopy has demonstrated a powerful platform for label-free, non-invasive, and chemically specific imaging of biological samples. However, the non-resonant background limits its ability to detect weak Raman signals. Here, an approach to eliminate the non-resonant background in CARS using delay modulation (DM) is presented, which is enabled by an acousto-optic modulator and spectral focusing. The results demonstrate that DM-CARS reduces the non-resonant background by 10 times and improves detection sensitivity by 100-fold over normal CARS. It also shows that DM-CARS has potential applications in tracking heavy water metabolism in bacteria for antimicrobial susceptibility testing and imaging liver tumor tissues. Furthermore, a handheld DM-CARS device has also demonstrated for in vivo imaging, which can be beneficial in various applications.