The world of semiconductor nanocrystals, often referred to as colloidal quantum dots (QDs), has opened up a new realm of possibilities in the field of quantum physics. While the idea of size-dependent quantum effects has been a topic of interest among physicists for a long time, it was not until the discovery of QDs that these theories could be sculpted into real nanodimensional objects. The fascinating aspect of QDs lies in their size-dependent colors, which serve as a visual representation of the quantum size effect that can be seen with the naked eye under ambient conditions.
In recent years, researchers worldwide have been delving into the realm of quantum effects and phenomena using QDs as a material platform. These include groundbreaking discoveries such as single-photon emission and quantum coherence manipulation. One such phenomenon is the concept of Floquet states, which are photon-dressed states that explain coherent interactions between light fields and matter. Despite the theoretical understanding of Floquet states, direct observation has proven to be a significant experimental challenge.
Traditionally, the observation of Floquet states has been limited to low-temperature, high-vacuum environments using infrared, terahertz, or microwave regions to prevent sample damage. However, a recent study published in Nature Photonics by Prof. Wu Kaifeng and his colleagues from the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences marks a significant milestone in the direct observation of Floquet states in semiconductors. The researchers utilized all-optical spectroscopy in the visible to near-infrared region under ambient conditions, a groundbreaking approach that defies conventional experimental settings.
The researchers employed quasi-two-dimensional colloidal nanoplatelets, known for their strong, atomically-precise quantum confinement properties. This confinement in the thickness dimension led to interband and intersubband transitions in the visible and near-infrared regions, respectively. The transitions involved in these processes naturally formed a three-level system, allowing for the observation of Floquet states. By utilizing visible and near-infrared photons, the researchers were able to probe the Floquet states and observe their dynamics in real-time.
The direct observation of Floquet states in semiconductor materials not only provides valuable insights into the quantum world but also uncovers the rich spectral and dynamic physics of these states. This breakthrough opens up new avenues for dynamically controlling optical responses and coherent evolution in condensed-matter systems. Prof. Wu highlighted the significance of this study by stating, “Not only does this study provide an all-optical direct observation of Floquet states in semiconductor materials, but it also uncovers the rich spectral and dynamic physics of Floquet states that can be harnessed to dynamically control the optical responses and coherent evolution in condensed-matter systems.”
With this groundbreaking demonstration achieved for colloidal materials under ambient conditions, the field of Floquet engineering is poised to expand its reach. While the current focus has been on tailoring the quantum and topological properties of solid-state materials, this breakthrough opens up the possibility of coherently controlling surface/interfacial chemical reactions through nonresonant light fields. The implications of this study extend far beyond the realm of quantum physics, offering new avenues for manipulating materials at the nanoscale level.