Using ultrafast broadband optical spectroscopy, we measured the transient electronic structure and charge dynamics of an off-resonantly pumped Mott insulator Ca₂RuO₄. We observe coherent bandwidth renormalization and nonlinear doublon-holon pair production occurring in rapid succession within a sub-100-fs pump pulse duration. By sweeping the electric field amplitude, we demonstrate continuous bandwidth tuning and a Keldysh crossover from a multiphoton absorption to quantum tunneling dominated pair production regime. Our results provide a procedure to control coherent and nonlinear heating processes in Mott insulators, facilitating the discovery of novel out-of-equilibrium phenomena in strongly correlated systems.

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See more details in the article:  X. Li*, H. Ning*, O. Mehio*, B. Hu, M. C. Lee, K. W. Kim, T. W. Noh, G. Cao, D. Hsieh✉️, Keldysh space control of charge dynamics in a strongly driven Mott insulator, Phys. Rev. Lett. 128, 187402 (2022).​​

Keldysh Space Control of Charge Dynamics in a Strongly Driven Mott Insulator

The response of a Mott insulator to a strong electric field is a fundamental question in the study of nonequilibrium correlated many-body systems. There is growing interest to understand doublon-holon (d-h) pair production and their nonthermal dynamics in the strong field ac regime. Notably, d-h pairs are primarily produced through two nonlinear mechanisms: multiphoton absorption and quantum tunneling. The two regimes are characterized by distinct electric field scaling laws and momentum space distributions of d-h distributions. A crossover between the two regimes has been theoretically predicted. However, direct experimental tests are lacking owing to the challenging need to combine strong tunable low frequency pumping fields with sensitive ultrafast probes of nonequilibrium distribution functions.

Keldysh tuning of photoluminescence in a lead halide perovskite crystal

Over the past few decades, lead halide perovskites have emerged as unconventional semiconductors with exceptional functionalities, finding broad applications ranging from photonics and optoelectronics  to electronics. The large quantum yield of perovskite luminescence is exploited in applications such as multiphoton absorption for photon upconversion and electroluminescence in light-emitting diodes enabled by quantum tunneling under a strong DC electric field. There exists a crossover from photon-induced incoherent light emission to electric-field-induced emission. However, the evolution of a photoluminescence response from a multiphoton process
to quantum electron tunneling remains to be established. Here, we investigate the origin of photoluminscence in the archetypal lead halide perovskite CsPbBr₃ under sub-bandgap radiation. By varying the frequency and amplitude of the driving AC fields, we demonstrate the ability to tune the photoluminescence across multiphoton and electron tunneling processes on demand. Our findings, explained by Keldysh theory through the Landau-Dykhne formalism, offer new insights for developing highly efficient photon conversion schemes in semiconductors.