Quantum spectroscopy, for the measurements of dynamical current current thermalization

Nearly all spectroscopic measurements deals with the measurements of the average properties of the material. As an example, the reflectivity of a material is simply defined by the ratio between the number of photons which are reflected by the sample divided by the number of those arriving on it. The interest in measuring mostly average properties is the main drive of the standard scientific practice of repeating the measurements a lot of times so that the error made in one single measurements is averaged out by the repetition of the measurements. In this context the noise which determines fluctuation of the repeated measurements have always been considered as an impediment to a good quality measurements which needs to be mitigated by careful experimentalists.
The approach of repeated measurements is employed conspicuously in pump-probe experiments which are the prime way to study condensed matter out of its equilibrium state. In standards optical pump-probe experiments, ultrashort pulses are always used in pairs. The pump triggers the dynamical response and the probe is used to detect changes in the optical properties of the sample.
By and large, the experiments performed to date measure the intensity variation of probe pulses after their interaction with the sample for each pump-probe delay, following the protocol of averaging over many (sometimes millions of) stroboscopically repeated experiments and little attention has been given to the accurate measurement of the fluctuation of the intensity of reflected (or transmitted) probe pulses. A collaboration between different groups from the University of Trieste, Elettra - Sincrotrone Trieste and the University of Hamburg and Erlangen propose the change of paradigm of using the noise itself as a spectroscopic mean.
 In details, with this work we set the bases for the implementation of a statistical pump-probe set-up (Fig. 1), where the intensity of every single probe pulse is separately acquired with low-electronic-noise detectors, for every pump-probe delay.

Figure 1. Schematic view of the pump-probe set-up used for the experiments. The intensity of every single probe pulse was separately acquired with low-electronic-noise detectors for every pump-probe delay.

This enables the measurement of both the average of the intensity, which gives the usual pump-probe signal (e.g. the relative variation of the reflectivity as a function of pump-probe delay tp), and its statistical distribution to all orders. This allows the authors to tackle the interesting, yet largely unexplored, question: What is the spectroscopic information carried by the fluctuations of the intensity of ultrashort light pulses reflected (or transmitted) by complex materials out of equilibrium? Can the fluctuations of the measurement be connected to microscopic properties of the material?
Proof of principle experiments in this direction performed in the TRex laboratories within the Fermi project have enabled the authors to benchmark a formalism capable of linking the intensity fluctuations of the probe pulses to the spectrum of current fluctuations, which is not accessible via the average intensity in a pump-probe experiment.In detail, the transient noise spectroscopy allowed the authors measure to what extent electronic degrees of freedom dynamically obey the fluctuation-dissipation theorem, and how well electrons thermalize during a coherent lattice vibrations (see Fig. 2). They showed therefore that in optical pump-probe experiments on bulk samples, the statistical distribution of the intensity of ultrashort light pulses after the interaction with a non-equilibrium complex material can be used to measure the time-dependent noise of the current in the system. The general argument is illustrated with experiments and theory for a photo-excited Peierls material.
The statistical measurement developed in this work provides a new general framework to retrieve dynamical information on the excited distributions in nonequilibrium experiments which could be extended to other degrees of freedom of magnetic or vibrational origin.

Figure 2. Evidence for the violation of the Fluctuation Dissipation theorem. Transient reflectivity measurements in different photoexcitation regimes (a,b) compared to the model (c) allowed the authors to measure to what extent electronic degrees of freedom dynamically obey the fluctuation-dissipation theorem, and how well electron thermalize during the coherent lattice vibrations. 


This research was conducted by the following research team:

Francesco Randi1, Martina Esposito1, Francesca Giusti1, Oleg Misochko2, Fulvio Parmigiani1,3, Daniele Fausti1,3 and Martin Eckstein4

Università degli Studi di Trieste, Italy;
Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow Region, Russia and Moscow Institute of Physics and Technology (State University), Dolgoprudny, Moscow Region, Russia
Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
Department of Physics, University of Erlangen-Nürnberg, Erlangen, Germany and Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany

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Francesco Randi, Martina Esposito, Francesca Giusti, Oleg Misochko, Fulvio Parmigiani, Daniele Fausti, and Martin Eckstein Probing the Fluctuations of Optical Properties in Time-Resolved Spectroscopy” Phys. Rev. Lett. 119, 187403 (2017), DOI:10.1103/PhysRevLett.119.187403
Last Updated on Monday, 20 November 2017 17:13