We have investigated theoretically the applicability of phase-fluctuating laser fields in saturated absorption spectroscopy. The fluctuations of pump and probe fields are fully correlated if they are derived from the same laser source. Inside the saturable medium, phase fluctuations are converted into intensity noise. This nonlinear mixing modifies the statistics of the transmitted fields. By measuring higher-order correlations one can deduce additional spectroscopic information. Apart from the mean intensity, we have examined the intensity noise and intensity power spectrum of the weak probe field. The resonances of these correlation functions are also unaffected by large inhomogeneous broadening since they are inherently related to the usual saturated absorption dip. We find qualitative agreement with results of a recent experiment employing this technique [D. H. McIntyre et al., Opt. Lett. 18, 1816 (1993)], which demonstrates the advantages of noise spectroscopy using spectrum analyzers.
Abstract Nonlinear optics plays a crucial role in science and technology, with applications ranging from spectroscopy to quantum information processing. Therefore, precise estimation of their nonlinear optical parameters is of fundamental importance. Here, we investigate quantum-enhanced sensitivity in the estimation of nonlinear absorption parameters, including saturable absorption (SA) and reverse saturable absorption (RSA). By employing the quantum Fisher information, we quantify the enhancement in sensitivity for estimating transmission ( T ), linear absorption coefficient (
α0 ), saturable intensity (
Is ), and two-photon absorption coefficient (
β ) using bright two-mode squeezed states (bTMSS) and applied it to analyze the nonlinear parameters of Pt nanoparticle samples as an example. In addition, we propose an experimental scheme based on an open-aperture Z-scan with bTMSS to realize quantum-enhanced sensitivity. Our findings establish a pathway toward quantum-enhanced estimation of nonlinear optical parameters.