Radiative Higgs mode in photoinduced $\eta$ pairs

Abstract

We demonstrate that the $\eta$ symmetry of the pump-induced $\eta$-pairing state in the Hubbard chain can be transiently broken by an additionally applied probe pulse. This leads to characteristic dynamic Higgs oscillations of the $\eta$-pair correlations during the probe pulse and a sharp dynamic negative optical conductivity $\sigma(\omega;t)$, which appears above the equilibrium Mott gap $\omega>\Delta_{\rm c}$, long after the pump pulse is finished. This negative peak in the real part of $\sigma(\omega;t)$ is a distinctive hallmark of the photoinduced $\eta$-pairing state, distinguishing it from parameter regimes dominated by incoherent doublon formation, which do not exhibit this feature. Remarkably, the broadband probe pulse then leads to the emission of photons with the Higgs frequency.