arXiv subjects

Valery Yu. Davydov

Publications and source records attributed to Valery Yu. Davydov.

3 recordsLinked to original sources

arXiv2022

Twisted Nanotubes of Transition Metal Dichalcogenides with Split Optical Modes for Tunable Radiated Light Resonators

Synthesized micro- and nanotubes composed of transition metal dichalcogenides (TMDCs) such as MoS$_2$ are promising for many applications in nanophotonics, because they combine the abilities to emit strong exciton luminescence and to act as whispering gallery microcavities even at room temperature. In addition to tubes in the form of hollow cylinders, there is an insufficiently-studied class of twisted tubes, the flattened cross section of which rotates along the tube axis. As shown by theoretical analysis, in such nanotubes the interaction of electromagnetic waves excited at opposite sides of the cross section can cause splitting of the whispering gallery modes. By studying micro-photoluminescence spectra measured along individual MoS$_2$ tubes, it has been established that the splitting value, which controls the energies of the split modes, depends exponentially on the aspect ratio of the cross section, which varies in "breathing" tubes, while the relative intensity of the modes in a pair is determined by the angle of rotation of the cross section. These results open up the possibility of creating multifunctional tubular TMDC nanodevices that provide resonant amplification of self-emitting light at adjustable frequencies.

physics.optics↗

arXiv2022

Strong photoluminescence enhancement in indirect bandgap MoSe$_2$ nanophotonic resonator

Transition metal dichalcogenides (TMDs) is a promising platform for new generation optoelectronics and nanophotonics due to their unique optical properties. However, in contrast to direct bandgap TMDs monolayers, bulk samples have an indirect bandgap that restricts their application as light emitters. On the other hand, the high refractive index of these materials seems ideal for creating high-quality nanophotonic resonators with a strong Purcell effect. In this work, we fabricate Whispering-gallery mode (WGM) resonators from bulk (i.e., indirect bandgap) MoSe$_2$ using resistless scanning probe lithography and study their optical properties. Micro-photoluminescence($μ$-PL) investigation revealed WGM spectra of resonators with an enhancement factor of 100 compared to pristine flake. Scattering experiments and modeling also confirm the WGM nature of spectra observed. Temperature dependence of PL revealed two components of photoluminescence. The first one quenches with decreasing temperature, the second one does not and becomes dominant. Therefore, this suggests that resonators amplify both direct and temperature-activated indirect PL. Thus, here we demonstrated the novel approach to fabricating nanophotonic resonators from bulk TMDs and obtaining PL from indirect bandgap materials. We believe that the suggested approach and structures have great prospects in nanophotonics.

cond-mat.mes-hall↗

arXiv2021

Photoluminescence kinetics of dark and bright excitons in atomically thin MoS$_2$

The fine structure of the exciton spectrum, containing optically allowed (bright) and forbidden (dark) exciton states, determines the radiation efficiency in nanostructures. We study time-resolved micro-photoluminescence in MoS$_2$ monolayers and bilayers, both unstrained and compressively strained, in a wide temperature range (10-300 K) to distinguish between exciton states optically allowed and forbidden, both in spin and momentum, as well as to estimate their characteristic decay times and contributions to the total radiation intensity. The decay times were found to either increase or decrease with increasing temperature, indicating the lowest bright or lowest dark state, respectively. Our results unambiguously show that, in an unstrained monolayer, the spin-allowed state is the lowest for a series of A excitons (1.9 eV) with the dark state being < 2 meV higher, and that the splitting energy can increase several times at compression. In contrast, in the indirect exciton series in bilayers (1.5 eV), the spin-forbidden state is the lowest, being about 3 meV below the bright one. The strong effect of strain on the exciton spectrum can explain the large scatter among the published data and must be taken into account to realize the desired optical properties of 2D MoS$_2$.

cond-mat.mes-hall↗