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Associated with the energy-band structure

In the quantum world, time is theoretically malleable and reversible. And it is through these time-reversal dynamics that scientists have attempted to control quantum systems. However, applying such a time-reversal phenomenon becomes difficult in more sophisticated, quantum condensed-matter systems – i.e., quantum systems with infinite degrees of freedom. This is because quantum coherence gets lost quickly when interacting with the environment.

Atsushi Ono, an assistant professor in the physics department at Tohoku University, is the leader of a research team that has discovered a new sort of echo phenomenon associated to the energy-band structure in crystalline solids. After the team started theorizing the ultrafast dynamics of optically generated quasiparticles in crystalline solids, so-called “energy-band echoes” were found.

present echo-generation process
A schematic diagram of the present echo-generation process. (a) An electron-hole pair is created by a photo-excitation pulse. (b) After the excitation, the electron and hole move in opposite directions. (c) A driving electric-field pulse reverses the relative velocity of the electron-hole pairs, resulting in the recombination of the pairs and the emission of echo pulses. ©Atsushi Ono

According to the group’s numerical simulation and analytical formulas, a coherent electric field pulse drives quasiparticles, and the photoexcitation process results in echoes when the quasiparticles recombine. These echo pulses contain data on the quasiparticle dispersion relations.

Moreover, scientists observed energy-band echoes even in strongly correlated systems, where free electrons are not well-defined quasiparticles because of many-body interactions.

Ono said, “Our discovery provides a different perspective of ultrafast dynamics driven and controlled by a lightwave. Energy-band echoes could be used for all-optical momentum-resolved spectroscopy of quasiparticles in both crystalline solids and cold atoms in optical lattices, even when strong many-body correlations are present.”

Journal Reference:

  1. Shohei Imai, Atsushi Ono, and Sumio Ishihara. Energy-band echoes: Time-reversed light emission from optically driven quasiparticle wave packets. Phys. Rev. Research 4, 043155. DOI: 10.1103/PhysRevResearch.4.043155

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