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Introduction (T.S., G.B., C.F., G.F. and M.M.); Experimentation (T.S., G.B., C.F., G.F. and M.M.); Results (T.S., G.B., C.F., G.F. and M.M.); Applications (T.S., G.B., C.F., G.F. and M.M.); Reproducibility and data deposition (T.S., G.B., C.F., G.F. and M.M.); Limitations and optimizations (T.S., G.B., C.F., G.F. and M.M.); Outlook (T.S., G.B., C.F., G.F. and M.M.); overview of the Primer (T.S.).

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There are various types of reflective materials, each with different properties and applications. Retroreflective materials are most common; they are designed to reflect light directly back to its source using tiny glass beads, prisms, or other specialized materials. Traffic signs and safety vests are examples. Diffusely reflective materials reflect light in a wider, more scattered pattern, making objects visible from different angles. Street paint and bike reflectors are often made up of these materials. Fluorescent reflective materials combine reflection with fluorescence to bounce light back to its source while also emitting their own light when illuminated for even greater visibility. Examples include construction cones and emergency signs.

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Amplification of a desired, weak, optical signal (ES) by its mixing with a strong field (EL), leading to the measured intensity \(I={{\rm{| }}{E}_{{\rm{L}}}{\rm{| }}}^{2}+{{\rm{| }}{E}_{{\rm{S}}}{\rm{| }}}^{2}+{E}_{{\rm{L}}}{E}_{{\rm{S}}}^{* }+{E}_{{\rm{S}}}{E}_{{\rm{L}}}^{* }\).

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Batignani, G., Mai, E., Fumero, G., Mukamel, S. & Scopigno, T. Absolute excited state molecular geometries revealed by resonance Raman signals. Nat. Commun. 13, 7770 (2022).

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Barak, A. et al. Solvent polarity governs ultrafast structural dynamics: a case study of 4-dimethylamino-4′-carbomethoxydiphenylacetylene. J. Phys. Chem. C 127, 5855–5865 (2023).

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Bera, K., Kwang, S. Y., Cassabaum, A. A., Rich, C. C. & Frontiera, R. R. Facile background discrimination in femtosecond stimulated Raman spectroscopy using a dual-frequency Raman pump technique. J. Phys. Chem. A 123, 7932–7939 (2019).

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The stimulated Raman scattering time-domain analogue, in which a full scan of the temporal delay between two ultrashort pulses is required to record a single Raman spectrum in the time domain. The addition of an actinic pump turns impulsive stimulated Raman into a time-resolved technique capable of probing excited-state dynamics, similarly to femtosecond stimulated Raman scattering.

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A signal in which various frequencies arrive at different time delays. In optical pulses, chirp commonly stems from the chromatic dispersion caused by transmitting optics, leading to red-shifted spectral components arriving earlier (positive chirp) or later (negative chirp) than the blue-shifted ones.

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Pastorczak, M., Nejbauer, M. & Radzewicz, C. Femtosecond infrared pump-stimulated Raman probe spectroscopy: the first application of the method to studies of vibrational relaxation pathways in the liquid HDO/D2O system. Phys. Chem. Chem. Phys. 21, 16895–16904 (2019). This article describes the use of FSRS with an infrared actinic pump.

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Molesky, B. P., Guo, Z. & Moran, A. M. Femtosecond stimulated Raman spectroscopy by six-wave mixing. J. Chem. Phys. 142, 212405 (2015).

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The authors are grateful to G. Cerullo, P. Kukura, S. Mukamel and M. H. Vos for several inspiring discussions. They acknowledge early contributions by E. Pontecorvo to the planning and development of their first FSRS prototype. G.B. acknowledges funding from the PRIN 2022 Project (Dynamat) (grant number 2022PR7CCY).

Kuramochi, H., Takeuchi, S. & Tahara, T. Ultrafast structural evolution of photoactive yellow protein chromophore revealed by ultraviolet resonance femtosecond stimulated Raman spectroscopy. J. Phys. Chem. Lett. 3, 2025–2029 (2012).

First demonstrated in 1994, femtosecond stimulated Raman scattering (FSRS) has gained popularity since the early 2000s as an ultrafast pump–probe vibrational spectroscopy technique with the potential to circumvent the time and energy limitations imposed by the Heisenberg uncertainty principle. This Primer explores whether, why, when and how the temporal precision and frequency resolution of traditional time-resolved spontaneous Raman spectroscopy can be surpassed by its coherent counterpart (FSRS), while still adhering to the uncertainty principle. We delve into the fundamental concepts behind FSRS and its most common experimental implementations, focusing on instrumentation details, measurement techniques, data analysis and modelling. This includes discussions on synthesizing the Raman pump beam, artificial intelligence (AI)-assisted baseline removal methods and analytical expressions for reproducing experimental data and extracting key parameters such as relaxation times and out-of-equilibrium temperature profiles. Recent applications of FSRS from physics, chemistry and biology are showcased, demonstrating how this approach has facilitated cross-disciplinary studies. We also address the technical and conceptual limitations of FSRS to aid in designing optimal experiments based on specific goals. Finally, we explore future directions, including multidimensional extensions to address vibrational couplings and the use of quantum light to untangle temporal and spectral resolution.

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Light reflection material is any substance or surface that reflects light back to its source, making it highly visible even in low-light conditions. Essentially, it bounces light back toward the observer, making it appear brighter than its surroundings.

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