We present a theoretical investigation of stimulated emission pumping to achieve sub-diffraction-limited spatial resolution in coherent anti-Stokes Raman scattering (CARS) microscopy. A pair of control light fields is used to prepopulate the Raman state involved in the CARS process prior to the CARS signal generation. Efficient prepopulation is achieved by employing a path via an electronic or vibrational state. Thereby, the buildup of a coherence between the ground and Raman state during irradiation by the light fields for CARS signal generation is prevented, resulting in the suppression of the CARS intensity by more than 99.8
. Two-dimensional spatial excitation profiles have been calculated using donut-shaped spatial profiles for the control light fields, thus, an intensity-dependent narrowing of the CARS excitation profiles below the diffraction limit. Using computer-generated test images we demonstrate a resolution beyond the diffraction limit for CARS microscopy, which is scalable by the control light field intensity, similar to stimulated emission depletion microscopy.
Benefitting from the sub-Poissonian statistics, squeezed light has been applied to push the shot-noise-limited sensitivity of stimulated Raman scattering spectroscopy and microscopy to a sub-shot-noise level [5]. However, the balanced detection regime requires a high squeezing level to counteract the 3-dB signal-to-noise ratio (SNR) drawback. In this presentation, we introduce dual-polarization quantum-enhanced stimulated Raman scattering (QESRS), which is also suitable for high-power operation [6]. This approach utilizes a symmetric beamsplitter but detects squeezed light on two independent photodetectors, thus avoiding the 3-dB decrease in SNR.
Photon statistics of stimulated Raman scattering and degenerate hyper-Raman scattering in the cavity are investigated using the method of small operator corrections to a c-number solution. Weak non-classical effects and super-Poissonian behaviour of all modes are derived in the case of Raman scattering in stationary regime. Higher-order nonlinearity of hyper-Raman scattering leads to more significant non-classical effects—considerable quadrature squeezing and sub-Poissonian behaviour of laser and Stokes modes (Fano factor lower than 0·8) were obtained. A negative quantum noise component is manifested in oscillations of photon-number distribution and in negative values of Wigner distribution of the integrated intensity. The influence of external driving amplitude and thermal bath is examined.