Electronic properties and lattice vibrations are expected to be strongly correlated in metal-halide perovskites, due to the soft fluctuating nature of their crystal lattice. Thus, unveiling electron–phonon coupling dynamics upon ultrafast photoexcitation is necessary for understanding the optoelectronic behavior of the semiconductor. Here, we use impulsive vibrational spectroscopy to reveal vibrational modes of methylammonium lead-bromide perovskite under electronically resonant and non-resonant conditions. We identify two excited state coherent phonons at 89 and 106 cm−1, whose phases reveal a shift of the potential energy minimum upon ultrafast photocarrier generation. This indicates the transition to a new geometry, reached after approximately 90 fs, and fully equilibrated within the phonons lifetime of about 1 ps. Our results unambiguously prove that these modes drive the crystalline distortion occurring upon photo-excitation, demonstrating the presence of polaronic effects.

Spontaneous Raman spectroscopy is a formidable tool to probe molecular vibrations. Under electronic resonance conditions, the cross section can be selectively enhanced enabling structural sensitivity to specific chromophores and reaction centers. The addition of an ultrashort, broadband femtosecond pulse to the excitation field allows for coherent stimulation of diverse molecular vibrations. Within such a scheme, vibrational spectra are engraved onto a highly directional field, and can be heterodyne detected overwhelming fluorescence and other incoherent signals. At variance with spontaneous resonance Raman, however, interpreting the spectral information is not straightforward, due to the manifold of field interactions concurring to the third order nonlinear response. Taking as an example vibrational spectra of heme proteins excited in the Soret band, we introduce a general approach to extract the stimulated Raman excitation profiles from complex spectral lineshapes. Specifically, by a quantum treatment of the matter through density matrix description of the third order nonlinear polarization, we identify the contributions which generate the Raman bands, by taking into account for the cross section of each process.

Manipulating the macroscopic phases of solids using ultrashort light pulses has resulted in spectacular phenomena, including metal-insulator transitions 1-3 , superconductivity 4 and subpicosecond modification of magnetic order 5 . The development of this research area strongly depends on the understanding and optical control of fundamental interactions in condensed matter, in particular the exchange interaction. However, disentangling the timescales relevant for the contributions of the exchange interaction and spin dynamics to the exchange energy, E ex , is a challenge. Here, we introduce femtosecond stimulated Raman scattering to unravel the ultrafast photoinduced dynamics of magnetic excitations at the edge of the Brillouin zone. We find that femtosecond laser excitation of the antiferromagnet KNiF 3 triggers a spectral shift of the two-magnon line, the energy of which is proportional to E ex . By unravelling the photo-induced modification of the twomagnon line frequency from a dominating nonlinear optical effect, we find that E ex is increased by the electromagnetic stimulus.Magnetic order is a macroscopic manifestation of the quantum phenomenon of exchange coupling between spins. Under thermodynamic equilibrium conditions, the energy of the interaction is conventionally described in the Heisenberg-Dirac formwhere E ex is the exchange energy, J ab is the exchange interaction, and the last term is the correlation function between spins on neighbouring sites a and b.Despite the enormous number of experiments reporting on the optical control of spins 5 , there are very few studies of the ultrafast photo-induced dynamics of the exchange interaction 6,7 . One of the reasons for the popularity of spin dynamics studies is the possibility to use all-optical techniques, which can be applied easily to a broad class of materials. However, the studies that have succeeded in revealing the dynamics of the exchange energy E ex make use of the rather demanding and less flexible technique of time-and spinresolved photoelectron spectroscopy. Despite its complexity, photoelectron spectroscopy has proven to be very powerful when applied to metal surfaces 6,7 . In these materials, however, ultrafast laser excitation also triggers electronic dynamics, which defines the dynamics of J, and demagnetization 8,9 , which is related to the spin correlation function, both on the femtosecond timescale. So, disentangling the dynamics of the two contributions appearing in equation (1) is challenging. In some semiconductors, the overall dynamics of the d-f exchange energy has been monitored by measuring the transient optical properties 10 . On the other hand, the scenario in dielectrics is markedly different. Although the timescale of the charges' response is still basically limited by the duration of a femtosecond stimulus, demagnetization occurs over a timescale as long as 100 ps after photoexcitation 11 . Assessing the capability of light to probe the exchange energy at the femtosecond timescale in such cases would therefore allow the dy...