Research Projects

Absorption of light in materials has been the driving force of non-destructive probing of experiments in condensed matter and materials sciences. Some of the latest techniques that has surfaced to observe ultrafast optical carrier excitations/generation in materials are the angle resolved photoemission spectroscopy (ARPES) and the electron energy loss spectroscopy (EELS) [Rev. Mod. Phys. 75, 473, (2003)]. These experimental techniques essentially captures the single-particle spectral function inside which information about the electronic and hole energies; and their lifetimes are encoded. Accessing this spectral function is therefore a great deal to understand how light is absorbed and scattered by materials at the microscopic level. Excitons in materials are the electron-hole pairs which are bounded together via a Coulombic force of attraction and are often generated through an external perturbation for example, light. The binding energies of these correlated pairs signatures how strong the light absorption will be. In fact, excitons in two-dimensional (2D) semiconductors possess rather larger binding energies and oscillator strengths due to an in-plane quantum confinement than their bulk structures. In the latter case, the electric field is properly screened out by the surrounding charges, while in atomic layers, it is not. Therefore, an intense Coulombic interaction develops, leading to a strongly bounded pair with enhanced optical absorption; observed in photoluminescence, ARPES or EELS experiments. Many body perturbation theory (MBPT) approach is a simile to these expensive experiments in the sense that the former exactly captures the single-particle spectral function. Ab-initio methods based on the many body Bethe-Salpeter equation (BSE) [Rev. Mod. Phys. 74, 601 (2002)] are most reliable theoretical approach to investigate exciton-driven optical spectra in bulk and low dimensional materials [Nature 513, 214 (2014), Nat. Mat. 13, 1091 (2014), ACS Nano 12, 10880 (2018)]. Within this cutting-edge theoretical framework, the PI and his team has demonstrated that an additional electron-phonon coupling plays a fundamental role for the exciton physics of 2D materials and that temperature effects cannot be neglected if one aims at a realistic description of optical properties [Phys. Rev. B 98, 045143 (2018); Phys. Rev. B, 99, 165201, (2019); ACS JPCC, 125, 12738, (2021)]; together with the exciton-driven various nonlinear optical responses. Here, we extend these understandings for the finite temperature exciton characters in 2D materials that goes beyond the limiting case of optical spectroscopy and to obtain excitonic spectra at finite wavevector. A rigorous excitonic-phonon couplings will therefore be solved that will shed light on the excitonic scatterings. The proposal also goes beyond the linear regime to probe various optical nonlinear coefficients together with the strong pulse excitations by solving a more generalized time-dependent BSE.