Physicists have been working to establish reliable methods for manipulating quantum states in solid-state materials, cold atoms, and other systems, since this might aid in the creation of new technologies. Floquet engineering, which involves the periodic drive of quantum states of matter, is one of these methodologies.
Chinese researchers from Tsinghua University, Beihang University, and the Chinese Academy of Sciences have revealed the experimental realisation of Floquet band engineering in a model semiconductor, namely black phosphorus. Their findings, published in Nature, might help guide future research into the Floquet engineering of semiconducting materials and the realisation of light-induced emergent phenomena including light-induced topological phase transitions.
“Light-matter interaction plays critical roles in experimental condensed matter physics and materials sciences, not only as experimental probes for revealing the underlying physics of low-dimensional quantum materials, but also as effective control knobs for manipulating the electronic structures and quantum states in the non-equilibrium state,” said Shuyun Zhou, the research’s initiator and director.
“Such nonequilibrium control offers intriguing chances to produce novel physical phenomena that are not present in the equilibrium state. Along these lines, modifying quantum states of matter using time-periodic fields (i.e., Floquet engineering) has piqued the curiosity of many during the last several decades.”
Floquet engineering has previously been used to condensed matter systems, cold atoms, and optical lattices. Theoretical efforts based on Floquet engineering have also predicted fascinating phenomena like as light-induced topological phase transitions. However, experimental proof of Floquet engineering is currently limited.
“Experimental findings have yet to resolve many basic problems,” Zhou remarked. “Can Floquet engineering, for example, be accomplished in a semiconductor under realistic experimental conditions? This subject is critical since semiconductors are extensively employed in electrical and optoelectronic devices.”
Zhou and his colleagues have been working for many years to establish suitable methodologies and experimental settings for exploring light-induced emerging phenomena and implementing Floquet engineering in semiconductors. This may be especially difficult since Floquet engineering requires low photon energy and a high peak electric field.
The researchers created equipment that use high-intensity mid-infrared pumping pulses to suit these needs. They paired these technologies with a cutting-edge measurement known as time- and angle-resolved photoemission spectroscopy in their research (TrARPES).
“We started with an almost-ideal semiconductor sample—high-quality black phosphorus with a tiny band gap and good mobility, which might be advantageous for implementing Floquet engineering,” Zhou said. “The most difficult element of our research is that this is still a relatively unknown field, and it is unclear which experimental settings (pump photon energy, pump polarisations, etc.) are optimal for causing light-induced alteration of the electronic structure. It’s like seeking in the dark, and it took us a long time before we noticed anything.”
By consistently fine-tuning the photon energy, polarisation, and time delay in their sample, Zhou and his colleagues were eventually able to witness the light-driven transient Floquet band structure modification in black phosphorus. This is the first time Floquet band engineering has been shown in a semiconductor.
“Our study sheds light on the Floquet engineering of semiconductors, emphasising the necessity of resonance pumping,” Zhou added. “While optical transitions have traditionally been thought to be deleterious to Floquet states, our findings indicate that for a semiconductor, resonance pumping may be beneficial, if not necessary, for Floquet band engineering. This unexpected discovery opens the door to the hunt for Floquet engineering in quantum materials.”
This team of academics’ current study is a major step towards attaining light-induced topological phase transitions, a fundamental aim in quantum physics. Their discoveries might pave the way for future research into transiently modifying topological states at ultrafast timeframes.
Zhou and his colleagues’ experimental approaches are particularly promising for attaining lattice symmetry-enforced Floquet band engineering with higher pump polarisation selectivity. These techniques can be used to reliably switch on and off the Floquet band in semiconductors, potentially enabling the creation of novel high-speed devices.
“This study clearly indicates that the Floquet engineering physics may be further enhanced by pseudospin, a quantum degree of freedom analogous to spin,” said Peizhe Tang, one of the theorists who worked out the theory underpinning the pseudospin selection criteria of the Floquet engineering in this work.
“Through Floquet engineering, our study takes a critical step towards topological phase transition,” Zhou remarked. “The next step would be to use Floquet engineering to achieve light-induced topological phase transitions or even to produce nontrivial topology in a topological trivial material on ultrafast timeframes. Furthermore, we would want to apply Floquet engineering to a broader range of solid-state materials.”
You might also be interested n reading, When the light in the nanoworld is neither ‘on’ nor ‘off.’
