Description
In contrast to conventional semiconductors, two-dimensional transition metal dichalcogenide (TMD) monolayers allow both permanent as well as tunable control of their band structure. For instance, alloying or inducing strain during growth [1,2] can permanently alter their direct, energy-degenerate band gaps at the ±K valleys of the Brillouin zone. While this approach leads to substantial shifts of several 100 meV, it lacks the possibility for active control.
Therefore, dynamic, all-optical methods have recently gained attention due to their ability to induce changes to the band structure on ultrafast timescales. This approach involves perturbing the TMD system with an oscillating driving field to create a modified nonequilibrium state.
Here, we demonstrate the influence of ultrafast coherent effects, in particular the intensity-dependent optical Stark (OS) and Bloch-Siegert (BS) shifts, on the band structure of a tungsten diselenide (WSe$_{\mathrm{2}}$) monolayer. In particular, we identify two regimes in which OS and BS shifts induce either a symmetric or an asymmetric modulation of the ±K valleys depending on the pump polarization. A circularly polarized pump leads to an asymmetric bandgap opening in ±K, breaking time-reversal symmetry (TRS). We probe this valley imbalance by polarization-resolved second-harmonic (SH) measurements as the altered symmetry also influences the nonlinear susceptibility tensor by changing the effective point group from D$_{\mathrm{3h}}$ to C$_{\mathrm{3h}}$ [3]. In contrast, a linearly polarized pump field leads to a symmetric shift in ±K, preserving TRS and tensor structure but inducing a blue-shift of the excitonic resonance. We observe this as modulated SH signal depending on the pump detuning and intensity [4]. Our experimental data are fully explained by an analytical model based on the semiconductor-Bloch equations, which allows us to estimate transition dipole moment, dephasing time, and the induced bandgap modulation.
$\small{\mathrm{[1]~S.~Susarla~\textit{et}~\textit{al.},\mathrm{~Adv.~Mater.}~\textbf{29},~\mathrm{35},~\mathrm{1702457} ~(2017).}}$
$\small{\mathrm{[2]~M.~Zeng~\textit{et}~\textit{al.},\mathrm{~Nat.~Mater.}~\textbf{19},~\mathrm{528\text{-}533} ~(2020).}}$
$\small{\mathrm{[3]~P.~Herrmann~\textit{et}~\textit{al.},\mathrm{~Nat.~Photon.}~\textbf{19},~\mathrm{300\text{-}306} ~(2025).}}$
$\small{\mathrm{[4]~S.~Klimmer~\textit{et}~\textit{al.},\mathrm{~Adv.~Opt.~Mater.}~\textbf{14},~\mathrm{5},~\mathrm{e03236} ~(2026).}}$
| I am the presenting author | Yes |
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