Giorgia Sparapassi

We report here an experimental setup to perform three-pulse pump–probe measurements over a wide wavelength and temperature range. By combining two pump pulses in the visible (650 nm–900 nm) and mid-IR (5 μm–20 μm) range, with a broadband supercontinuum white-light probe, our apparatus enables both the combined selective excitation of different material degrees of freedom and a full time-dependent reconstruction of the non-equilibrium dielectric function of the sample. We describe here the optical setup, the cryogenic sample environment, and the custom-made acquisition electronics capable of referenced single-pulse detection of broadband spectra at the maximum repetition rate of 50 kHz, achieving a sensitivity of the order of 10⁻⁴ over an integration time of 1 s. We demonstrate the performance of the setup by reporting data on a mid-IR pump, optical push, and broadband probe in a single crystal of Bi₂Sr₂Y_0.08Ca_0.92Cu₂O_8+δ across the superconducting and pseudogap phases.

I will review the possibility to retrieve nonlinear responses in complex materials by measuring noise correlations of classical and quantum nature.

The interaction between ultrashort light pulses and non-absorbing materials is dominated by impulsive stimulated Raman scattering (ISRS). The description of ISRS in the context of pump&probe experiments is based on effective classical models describing the interaction between the phonon and pulsed electromagnetic fields. Here we report a theoretical description of ISRS where we do not make any semi-classical approximation and we treat both photonic and phononic degrees of freedom at the quantum level. The results of the quantum model are compared with semiclassical results and validated by means of spectrally resolved pump&probe measurements on α-quartz.

Significance: 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.

Noise Correlation Spectroscopy (NCS) is a technique exploiting statistical correlations imprinted onto spectral components of shaped ultrashort light pulses by a light-matter inelastic scattering process, in order to measure low energy eigenmodes of a material.