Capability of shifted excitation Raman difference spectroscopy under ambient daylight.

We present the capability of shifted excitation Raman difference spectroscopy (SERDS) under ambient daylight. A dual-wavelength diode laser emitting at 785 nm is used as the excitation light source. The monolithic diode laser provides more than 110 mW in cw operation. Both excitation lines show an emission width ≤0.2 cm(-1) and a spectral distance of 10 cm(-1) as targeted for SERDS. Polystyrene (PS) is used as the test sample and ambient daylight to generate real-world background interference. Here, a broadband background signal with narrowband absorption lines from water vapor and Fraunhofer lines from singly ionized calcium (Ca II) obscure the Raman lines of PS. SERDS clearly separates the Raman signals from the background signals with a 13-fold improvement in signal-to-background noise.