PT - JOURNAL ARTICLE AU - Tollerud, Jonathan Owen AU - Sparapassi, Giorgia AU - Montanaro, Angela AU - Asban, Shahaf AU - Glerean, Filippo AU - Giusti, Francesca AU - Marciniak, Alexandre AU - Kourousias, George AU - Billè, Fulvio AU - Cilento, Federico AU - Mukamel, Shaul AU - Fausti, Daniele TI - Femtosecond covariance spectroscopy AID - 10.1073/pnas.1821048116 DP - 2019 Mar 19 TA - Proceedings of the National Academy of Sciences PG - 5383--5386 VI - 116 IP - 12 4099 - //www.hmclip.net/content/116/12/5383.short 4100 - //www.hmclip.net/content/116/12/5383.full SO - Proc Natl Acad Sci USA2019 Mar 19; 116 AB - Here we establish femtosecond covariance spectroscopy as a technique that uses ultrashort stochastic light pulses to measure nonlinear material responses. By using pulses with spectrally uncorrelated fluctuations we can leverage on the noise and consider each repetition of the experiment as a measurement under different conditions. In this limit we demonstrate that nonlinear processes in the sample can be retrieved by measuring the spectral correlations in different pulses. We validate the approach by studying stimulated Raman scattering in α-quartz. This concept can be applied to reveal low-energy modes of electronic, spin, and vibrational origin and adapted to different techniques and wavelength ranges, from optical to X-ray free-electron lasers, where strong stochastic fluctuations are unavoidable.The success of nonlinear optics relies largely on pulse-to-pulse consistency. In contrast, covariance-based techniques used in photoionization electron spectroscopy and mass spectrometry have shown that a wealth of information can be extracted from noise that is lost when averaging multiple measurements. Here, we apply covariance-based detection to nonlinear optical spectroscopy, and show that noise in a femtosecond laser is not necessarily a liability to be mitigated, but can act as a unique and powerful asset. As a proof of principle we apply this approach to the process of stimulated Raman scattering in α-quartz. Our results demonstrate how nonlinear processes in the sample can encode correlations between the spectral components of ultrashort pulses with uncorrelated stochastic fluctuations. This in turn provides richer information compared with the standard nonlinear optics techniques that are based on averages over many repetitions with well-behaved laser pulses. These proof-of-principle results suggest that covariance-based nonlinear spectroscopy will improve the applicability of fs nonlinear spectroscopy in wavelength ranges where stable, transform-limited pulses are not available, such as X-ray free-electron lasers which naturally have spectrally noisy pulses ideally suited for this approach.