Holography and thermalization in optical pump-probe spectroscopy

TL;DR

This paper uses holography to model optical pump-probe experiments in a strange metal, revealing how different pulse frequencies affect the thermalization process of optical conductivity, with implications for real-world experiments.

Contribution

It introduces a holographic model of pump-probe spectroscopy in strange metals, predicting frequency-dependent thermalization behaviors of optical conductivity.

Findings

Low-frequency pulses cause slow exponential relaxation.

High-frequency pulses lead to near-instantaneous thermalization.

Predicted effects are testable in laboratory experiments.

Abstract

Using holography, we model experiments in which a 2+1D strange metal is pumped by a laser pulse into a highly excited state, after which the time evolution of the optical conductivity is probed. We consider a finite-density state with mildly broken translation invariance and excite it by oscillating electric field pulses. At zero density, the optical conductivity would assume its thermalized value immediately after the pumping has ended. At finite density, pulses with significant DC components give rise to slow exponential relaxation, governed by a vector quasinormal mode. In contrast, for high-frequency pulses the amplitude of the quasinormal mode is strongly suppressed, so that the optical conductivity assumes its thermalized value effectively instantaneously. This surprising prediction may provide a stimulus for taking up the challenge to realize these experiments in the laboratory. Such experiments would test a crucial open question faced by applied holography: Are its predictions artefacts of the large $N$ limit or do they enjoy sufficient UV independence to hold at least qualitatively in real-world systems?