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Direct Observation of Floquet States in Colloidal Quantum Dots

Colloidal quantum dots (QDs), often referred to as solution-processed semiconductor nanocrystals, have revolutionized the exploration of quantum mechanics at the nanoscale. This innovative material platform has enabled researchers to visualize size-dependent quantum effects through the distinct colors produced by varying the size of these nanocrystals. These colors offer an immediate, visual representation of the quantum size effect, a phenomenon long studied by physicists but not fully realized in practical applications until the advent of QDs.

In recent years, scientists worldwide have intensified their efforts to uncover intriguing quantum phenomena using QDs, focusing on aspects such as single-photon emission and the manipulation of quantum coherence. Among these phenomena, Floquet states—photon-dressed states resulting from interactions between light and matter—have garnered particular interest. However, experimentally verifying the existence of these states has proven quite challenging.

Previous research has only recently begun to reveal experimental signatures of Floquet-Bloch bands in materials like black phosphorus, utilizing time- and angle-resolved photoemission spectroscopy. These studies typically occurred under stringent conditions that included low temperatures and high vacuum to protect samples from damage, while the probing light sources were carefully chosen from the infrared, terahertz, or microwave regions.

A new study published in Nature Photonics marks a significant breakthrough. Led by Prof. Wu Kaifeng and his team from the Dalian Institute of Chemical Physics at the Chinese Academy of Sciences, this research represents the first direct observation of Floquet states in semiconductors using all-optical spectroscopy within the visible to near-infrared spectrum and under ambient conditions.

Advances with Colloidal Nanoplatelets

The study utilized quasi-two-dimensional colloidal nanoplatelets, a class of materials that has emerged over the past decade. The design of these nanoplatelets allows for strong quantum confinement, which leads to unique interband and intersubband transitions visible in the near-infrared region. This mechanism effectively sets up a three-level quantum system necessary for exploring Floquet states.

Specifically, the research observed a scenario where a sub-bandgap visible photon interacts with a heavy-hole state, transforming it into a Floquet state with characteristics similar to the first quantized electron state. This transition can then be accessed via near-infrared photons, providing compelling insights into the dynamics at play.

Interestingly, while it has been conventionally suggested that Floquet states diminish following the temporal overlap of the pump and probe pulses, this study demonstrated a direct observation of the dephasing process, revealing that the Floquet state transitions into a real population state within a timeframe of hundreds of femtoseconds. The findings are corroborated by comprehensive quantum mechanical simulations.

As highlighted by Prof. Wu, this research not only successfully establishes an all-optical observation method for Floquet states in semiconductor materials but also unveils a rich array of spectral and dynamic properties associated with these states. The implications of this work are profound, suggesting potential applications in dynamically controlling optical responses and the coherent evolution of various systems across condensed matter physics.

Given that these observations were made under ordinary atmospheric conditions, this breakthrough could significantly broaden the application of Floquet engineering, allowing for the management of quantum and topological characteristics in solid-state materials and enabling coherent control over surface and interfacial reactions driven by nonresonant light fields.

More information:
Observation of Floquet states and their dephasing in colloidal nanoplatelets driven by visible pulses. Nature Photonics (2024). DOI: 10.1038/s41566-024-01505-z

Source
phys.org