Coherent anti-Stokes Raman scattering (CARS) optimized by exploiting optical interference

The purpose of this work is to study the interference between the coherent nonresonant four-wave-mixing (FWM) background and the Raman-resonant signal in the coherent anti-Stokes Raman spectroscopy (CARS). The nonresonant background is usually considered as a detriment to CARS. We prove that the background can be exploited in a controllable way, through the heterodyne detection due to the interference, to amplify the signal and optimize the spectral shape of the detected Raman signal, and hence enhance the measurement sensitivity.

Our work is based on an optimized CARS technique which combines instantaneous coherent excitation of multiple characteristic molecular vibrations with subsequent probing of these vibrations by an optimally shaped, time-delayed, narrowband laser pulse. This pulse configuration mitigates the nonresonant background while maximizing the resonant signal, and allows rapid and highly specific detection even in the presence of multiple scattering.

We investigate the possibility of applying this CARS technique to non-invasive monitoring of blood glucose levels. Under certain conditions we find that the measured signal is linearly proportional to the glucose concentration due to optical interference with the residual background light instead of a quadratic dependence, which allows reliable detection of spectral signatures down to medically-relevant glucose levels.

With the goal of making the fullest use of the background, we study the interference between an external local oscillator (nonresonant FWM field) and the CARS signal field by controlling their relative phase and amplitude. Our experiment shows that this control allows direct observation of the real and imaginary components of the third-order nonlinear susceptibility (?(3)) of the Raman sample. In addition, this method can be used to amplify the signal significantly.

Furthermore, we develop an approach by femtosecond laser pulse shaping to precisely control the interference between the Raman-resonant signal and its intrinsic nonresonant background generated within the same sample volume. This technique is similar to the heterodyne detection with the coherent background playing the role of the local oscillator field. By making fine adjustments to the probe field shape, we vary the relative phase between the resonant signal and the nonresonant background, and observe the varying spectral interference pattern. These controlled variations of the measured pattern reveal the phase information within the Raman spectrum, akin to holographic detection revealing the phase structure of a source.