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Optoelectronic engineering through
nonlinear carrier generation
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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. While a crossover between the two regimes has been theoretically predicted, 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.
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.
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, Physical Review Letters 128, 187402 (2022).
Keldysh tuning of photoluminescence in a lead halide perovskite crystal
In 1964, Keldysh laid the groundwork for strong-field physics in atomic, molecular, and solid-state systems by delineating a ubiquitous transition from multiphoton absorption to classical field-driven electron tunneling under intense electromagnetic waves. While both processes in semiconductors can generate carriers and result in photon emission through electron–hole recombination, the low quantum yields in most materials have hindered direct observation of the Keldysh crossover.
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. Leveraging the large quantum yields of photoluminescence in lead halide perovskites, we show that we can not only induce bright light emission from extreme sub-bandgap excitation but also distinguish between photon-induced and electric-field-induced processes. Our results span the transition between quantum-mechanical and classical field effects of the light and provide generalizable insights into the nonequilibrium dynamics that result from strong-field light–matter interactions. Supported by Keldysh theory through the Landau-Dykhne formalism, the findings open avenues for light upconversion and subbandgap photon detection, highlighting the potential of lead halide perovskites in advanced optoelectronic applications.
See more details in the article: Z. Zhang*, H. Ning*, Z.-J. Liu*, J. Hou, A. D. Mohite, E. Baldini, N. Gedik, K. A. Nelson✉️, Keldysh tuning of photoluminescence in a lead halide perovskite crystal, Proceedings of the National Academy of Sciences 122, e2426253122 (2025)
Strong-field-driven
upconverted superfluorescence
at room temperature
Stabilizing collective quantum states is essential for realizing macroscopic quantum phenomena such as superconductivity. A closely related analogue in light–matter interactions is superfluorescence (SF): starting from an initially incoherent population of uncorrelated dipoles, phase correlations grow cooperatively until a macroscopic dipole forms. Extending superfluorescence to upconversion with emission at photon energies exceeding those of the pump would broaden both fundamental reach and applications. However, whether upconverted SF can occur in bulk semiconductors has remained largely uncharted.
Here, we demonstrate strong-field-driven upconverted SF at room temperature, with pump photon energies as small as one-tenth of the emission photon energy. Pumping a CsPbBr₂ single crystal with intense mid-infrared pulses, the emission evolves with increasing field from upconverted photoluminescence to amplified spontaneous emission and, beyond a critical pump-field threshold, to a superfluorescent burst that is only accessible in the strong-field regime. The SF emission exhibits several defining signatures including spectral narrowing, intensity N² scaling, characteristic buildup delay and burst shortening, Burnham–Chiao ringing, and long coherence. A minimal Maxwell–Bloch framework with non-perturbative excitation reproduces the experimental behavior, identifying strong-field synchronization followed by cooperative re-emission as the operative mechanism. These results constitute a rare solid-state realization of room-temperature SF enabled by tailored strong fields and open avenues to engineer collective emission pathways using nonresonant excitation.
See more details in the article: Z. Zhang*✉️, H. Ning*✉️, Z.-J. Liu, J. Hou, X. Zhang, A. D. Mohite, N. Gedik, K. A. Nelson✉️, Strong-field-driven upconverted superfluorescence at room temperature, under review at Nature (2025).