arXiv subjects

Nihit Saigal

Publications and source records attributed to Nihit Saigal.

4 recordsLinked to original sources

arXiv2026

Resonant Raman signatures of bright and momentum-dark exciton coupled by intervalley phonon scattering in monolayer WSe2

Exciton-phonon coupling in atomically thin transition metal dichalcogenides governs key processes such as exciton thermalization and intervalley scattering and remains challenging to access directly by optical spectroscopy. Here, we employ resonance Raman spectroscopy at cryogenic temperatures to probe exciton-phonon coupling in hBN-encapsulated WSe_2 monolayers. Tuning the excitation laser across the bright exciton X_KK resonance, we observe rich Raman spectra and focus on the resonance profile of the degenerate A_1'/E' optical phonon mode. The profile exhibits two asymmetric resonance peaks whose energetic separation significantly exceeds the phonon energy - a feature that cannot be explained by first-order Raman scattering alone. We demonstrate that this discrepancy is resolved by including third-order Raman scattering, in which intervalley scattering enabled by a finite-momentum phonon couples the bright exciton X_KK to a momentum-dark exciton X_d. Fitting the experimental resonance profiles of two independent samples with a model comprising coherent first- and third-order scattering yields consistent exciton energies and linewidths, with a momentum-dark exciton X_d approximately 45meV to 55meV below the bright exciton X_KK. The results indicate efficient bright-to-dark exciton coupling. Our findings provide a microscopic framework for understanding the anomalously bright emission spectra of WSe_2 monolayers despite its spin-forbidden lowest exciton transition and highlight the role of momentum-dark excitons in resonant light-matter interaction.

cond-mat.mtrl-sci↗

arXiv2024

Collective charge excitations between moiré-minibands in twisted WSe2 bilayers from resonant inelastic light scattering

We establish low-temperature resonant inelastic light scattering (RILS) spectroscopy as a tool to probe the formation of a series of moiré-bands in twisted WSe_{2} bilayers by accessing collective intermoiré-band excitations (IMBE). We observe resonances in RILS spectra at energies in agreement with inter-moiré band transitions obtained from an ab-initio based continuum model. Transitions between the first and second inter-moiré band for a twist angle of about 8° are reported and between first and second, third and higher bands for a twist of about 3°. The signatures from IMBE for the latter highlight a strong departure from parabolic bands with flat minibands exhibiting very high density of states in accord with theory. These observations allow to quantify the transition energies at the K-point where the states relevant for correlation physics are hosted.

cond-mat.str-el↗

arXiv2022

Exciton manifolds in highly ambipolar doped WS2

The disentanglement of single and many particle properties in 2D semiconductors and their dependencies on high carrier concentration is challenging to experimentally study by pure optical means. We establish an electrolyte gated WS2 monolayer field-effect structure capable to shift the Fermi level from the valence into the conduction band suitable to optically trace exciton binding as well as the single particle band gap energies in the weakly doped regime. Combined spectroscopic imaging ellipsometry and photoluminescence spectroscopies spanning large n- and p-type doping with charge carrier densities up to 10^14 cm-2 enable to study screening phenomena and doping dependent evolution of the rich exciton manifold whose origin is controversially discussed in literature. We show that the two most prominent emission bands in photoluminescence experiments are due to the recombination of spin-forbidden and momentum-forbidden charge neutral excitons activated by phonons. The observed interband transitions are redshifted and drastically weakened under electron or hole doping. This field-effect platform is not only suitable for studying exciton manifold but is also suitable for combined optical and transport measurements on degenerately doped atomically thin quantum materials at cryogenic temperatures.

cond-mat.mes-hall↗

arXiv2022

Spectroscopic imaging ellipsometry of two-dimensional TMDC heterostructures

Semiconducting two-dimensional materials and their heterostructures gained a lot of interest for applications as well as fundamental studies due to their rich optical properties. Assembly in van der Waals heterostacks can significantly alter the intrinsic optical properties as well as the wavelength-dependent absorption and emission efficiencies making a direct comparison of e.g. photoluminescence intensities difficult. Here, we determine the dielectric function for the prototypical MoSe2/WSe2 heterobilayer and their individual layers. Apart from a redshift of 18 meV - 44 meV of the energetically lowest interband transitions, we find that for larger energies the dielectric function can only be described by treating the van der Waals heterobilayer as a new artificial homobilayer crystal rather than a stack of individual layers. The determined dielectric functions are applied to calculate the Michelson contrast of the individual layers and the bilayer in dependence of the oxide thickness of often used Si/SiO2 substrates. Our results highlight the need to consider the altered dielectric functions impacting the Michelson interference in the interpretation of intensities in optical measurements such as Raman scattering or photoluminescence.

cond-mat.mtrl-sci↗