Floquet tape technique in black phosphor

Physicists have tried to identify reliable strategies for manipulating quantum states in solid-state materials, cold atoms and other systems, as this could influence the development of new technologies. One such strategy is Floquet engineering, which involves the periodic propulsion of quantum states of matter.

Researchers from Tsinghua University, Beihang University and the Chinese Academy of Sciences in China recently demonstrated the experimental implementation of the Floquet band technique in a model semiconductor, namely black phosphorus. Her work, published in Naturecould inform future research efforts investigating the floquet technique of semiconductor materials, attempting to realize light-induced emergent phenomena such as light-induced topological phase transitions.

“Light-matter interaction plays a crucial role in experimental solid-state physics and materials science, not only as experimental probes to uncover the underlying physics of low-dimensional quantum materials, but, more importantly, as effective control knobs to manipulate the electronic structures and Nonequilibrium quantum states,” Shuyun Zhou, who initiated and led this research, told Phys.org.

“Such non-equilibrium control offers the fascinating possibilities to induce new physical phenomena beyond the equilibrium state. In this sense, the adjustment of the quantum states of matter by time-periodic fields (i.e., Floquet engineering) has attracted extensive interest in recent decades.”

Previous studies have applied the Floquet technique to condensed matter systems, cold atoms, and optical lattices. Theoretical work has also predicted intriguing phenomena based on the Floquet technique, such as light-induced topological phase transitions. However, experimental evidence for the Floquet technique is still relatively scarce.

“Many fundamental questions have yet to be answered through experimental results,” Zhou said. “Can Floquet engineering, for example, be realized in a semiconductor under realistic experimental conditions? Answering this question is important because semiconductors are widely used for electronic and optoelectronic devices.”

For several years, Zhou and his colleagues have been trying to identify favorable methods and experimental conditions for studying light-induced phenomena and implementing the Floquet technique in semiconductors. This can be particularly challenging as the Floquet technique requires low photon energy and a strong peak electric field.

To meet these requirements, researchers developed instruments using high-intensity pump pulses in the mid-infrared range. In their experiments, they combined these tools with a state-of-the-art method known as time- and angle-resolved photoemission spectroscopy (TrARPES).

“We chose an almost ideal semiconductor sample to start with — high-quality black phosphorus with a small band gap and high mobility, which could be favorable for implementation of the Floquet technique,” Zhou said. “The most challenging aspect of our study is that this is still a largely unexplored area and it is not clear which experimental conditions (pump photon energy, pump polarizations, etc.) are favorable to induce light-induced manipulation of electronic structure such as searching in the dark, and it took a few years for us to observe anything.”

Finally, Zhou and his colleagues were able to observe the light-driven transient modulation of the Floquet band structure in black phosphorus by systematically fine-tuning the photon energy, polarization, and time delay in their sample. This is the first experimental demonstration of the Floquet band technique in a semiconductor.

“Our work provides important insights into the floquet technique of semiconductors and underscores the importance of resonant pumping,” Zhou said. “While optical transitions have traditionally been thought of as detrimental to Floquet states, our work shows that resonant pumping could indeed be beneficial to a semiconductor and even critical to the Floquet band technique. This surprising finding provides an avenue to search for Floquet engineering in quantum materials.”

The recent work by this research team is an important step towards achieving a light-induced topological phase transition, a key goal in the field of quantum physics. Their results could therefore soon pave the way for new studies aimed at transiently manipulating topological states on ultrafast timescales.

The experimental methods used by Zhou and his colleagues are very promising to achieve a lattice symmetry-enforced Floquet band technique with stronger pump polarization selectivity. These methods can be used to reliably switch the floquet band on and off in semiconductors, which could aid in the development of new high-speed devices.

Peizhe Tang, one of the theorists who elaborated the theory behind the pseudospin selection rules of floquet engineering in this work, commented: “This work clearly shows that floquet engineering physics can be further enriched by pseudospin, a quantum degree of freedom Analogy to spiders.”

“This work paves the way for an important step towards a topological phase transition by Floquet engineering,” Zhou added. “The next step would be to achieve a light-induced topological phase transition or even induce a nontrivial topology in a topologically trivial material on ultrafast timescales by the Floquet technique. In addition, we want to extend the Floquet technique to many more solid-state materials.”

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