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Researchers from Skoltech and the University of Southampton, U.K., have used all-optical methods to create an artificial lattice whose nodes house polaritonsquasiparticles that are half-light and half-matter excitations in semiconductors. The team was able to show breakthrough results in condensed matter Physics thanks to the Lieb lattice. This is a phenomenon that rarely occurs in nature. The laser-generated polariton lattice, which was published in Nature Communications, can also be used to design next-generation optical computers that rely on guided light and dispersion management.
The strong light-matter coupling regime allows electronic excitations within a semiconductor to be placed between two mirrors, which form a microcavity. These photons then influence the semiconductor's electrical excitations. This creates new quantum modes, called exciton-polaritons or simply polaritons. They allow for the investigation of photonic and hybrid matter-wave phenomena at the microscale. If the conditions are right, polaritons may form coherent many-body states similar to BoseEinstein condensates. This allows for access to dissipative, nonlinear dynamics.
Researchers decided to investigate how condensates behaved in artificial optical lattices that are not normally found in nature. They used a programmable spatial modulator to create a laser beam inside the cavity. This was similar to the laser pointer caps that project fancy patterns onto distant surfaces. The polaritons that were generated increased in number and became energetic at the most intense areas of the laser field. The polaritons began to form condensates at the potential maxima of lattice when they had sufficient laser power. This is the so-called ballistic regime. High-energy polariton waveforms escaping condensates scatter and diffuse across the lattice in this so-called ballistic mode.
Researchers observed that condensates undergo a phase transition between the ballistic regime and the opposite situation of condensates trapped deep within the lattice. The system was unable to decide whether the polariton wave should be delocalized and localized at intermediate lattice constants. Instead, condensates were fractured across multiple energies. This transition was not seen in polariton lattices before.
Researchers then showed that they could create one of the most unusual features in solid-state Physics: flatbands, or dispersionless crystal band. This is where particle mass becomes practically infinite. They created an optical Lieb lattice that is not commonly found in nature and which has flatbands.
This story features a study co-authored and written by young researchers from Hybrid Photonics Lab, led by Professor Pavlos Lamoudakis. These results will be of great value to a wide scientific community that includes nonlinear optics and condensed matter Physics, cold atoms, light matter Physics, polaritonics, and cold atoms. This is the first demonstration that nontrivial phases and flatband engineering can be found in optically generated, polariton lattices. Flatband states in polariton system had previously been only shown in lithographically-written structures.
The paper's first author, Dr. Sergey Alyatkin, an experimental physicist from Skoltech, said that he and his colleague, Dr. Helgi Sigurdsson, a theoretical physicist from the University of Southampton, had "a very nice demonstration of advances in optical control and richness within the field of Polaritonics." We are seeing more interesting effects as we continue to study microcavity-polaritons in lattices. We have discovered new physics in nonstationary lattices with matter-wave quasiparticles. Our findings are not limited to one type of lattice.
Continue reading Researchers create artificial solid-state crystal structures by using laser light
Additional information: S. Alyatkin and al, Quantum fluids in all-optical scatterer lattices, Nature Communications (2021). Information from Nature Communications: S. Alyatkin and al., Quantum fluids in all-optical scatterer lattices, (2021). DOI: 10.1038/s41467-021-25845-4