Optoelectronic application through nonli

Optoelectronic application through nonlinear carrier generation

* Denotes equal contribution ✉️ Denotes corresponding author

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* denotes equal contribution; ✉️ denotes corresponding author

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 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

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 subbandgap 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, superfluidity, and Bose–Einstein condensation. A closely related analogue in light–matter interactions is superradiance, in which an ensemble of emitters prepared with phase coherence couples collectively to the radiation field and emits cooperatively. Another similar but distinct phenomenon is superfluorescence (SF): starting from an initially incoherent population of uncorrelated dipoles, phase correlations are seeded and grow cooperatively until a macroscopic dipole forms. After a characteristic buildup delay, the ensemble emits a short, highly directional, spectrally narrow burst. Extending superfluorescence to upconversion —— emission at photon energies exceeding those of the pump —— would broaden both fundamental reach and applications. While upconverted SF has been reported in nanoparticles, it exploits discrete, localized electronic manifolds and near-resonant energy transfer that tightly constrain pump photon energies. Whether upconverted SF can occur in bulk semiconductors —— where pump photon energies can be chosen flexibly and the implications are broader —— 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 the defining cooperative signatures: (i) abrupt spectral narrowing above threshold, (ii) intensity scaling that exceeds linear behavior and approaches N^2 with the number N of excited emitters, (iii) a buildup delay and burst width that shorten with increasing excitation density, and (iv) Burnham–Chiao time-domain ringing at high pump fields. A minimal Maxwell–Bloch framework with non-perturbative excitation reproduces the threshold behavior, nonlinear scaling, delay shortening, and ringing, 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 long-wavelength, 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, in prep (2025)