Phonon-assisted upconversion in twisted two-dimensional semiconductors

Abstract

Phonon-assisted photon upconversion (UPC) is an anti-Stokes process in which incident photons achieve higher energy emission by absorbing phonons. This letter studies phonon-assisted UPC in twisted 2D semiconductors, in which an inverted contrast between UPC and conventional photoluminescence (PL) of WSe2 twisted bilayer is emergent. A 4-fold UPC enhancement is achieved in 5.5° twisted bilayer while PL weakens by half. Reduced interlayer exciton conversion efficiency driven by lattice relaxation, along with enhanced pump efficiency resulting from spectral redshift, lead to the rotation-angle-dependent UPC enhancement. The counterintuitive phenomenon provides a novel insight into a unique way that twisted angle affects UPC and light-matter interactions in 2D semiconductors. Furthermore, the UPC enhancement platform with various superimposable means offers an effective method for lighting bilayers and expanding the application prospect of 2D stacked van der Waals devices.

Probing the Twist-Controlled Interlayer Coupling in Artificially Stacked Transition Metal Dichalcogenide Bilayers by Second-Harmonic Generation

“…The B 1 2g mode increases from 307 cm –1 in ∼0° stacking and reaches the highest 309 cm –1 in ∼15° stacking. This observation is mainly attributed to the angle-dependent lattice distortion and interlayer separation associated with lattice relaxation, which is consistent with a recent experimental report . The observed angular dependence of the B 1 2g modes indicates that the interlayer coupling in twisted WSe 2 /WSe 2 strongly depends on the stacking angle, leading to the twist-controlled SHG response, as shown in Figure (i).…”

Up-conversion photoluminescence of transition metal chalcogenide monolayers under high pressure

“…When the temperature increases, there are more mediated phonons to participate in UPL, resulting in an increase in UPL intensity. The energy band of 𝑇𝑇+𝛽𝛽 , 𝐸𝐸 𝑔𝑔 (0) , 𝛼𝛼 and 𝛽𝛽 are fitting parameters [26]. Therefore, the peak position of UPL will redshift significantly which prove this is a multi-phonon-assisted UPL process again.…”

“…But the single phonon energy is less than the offset energy. Therefore, the multi-phonon events must be involved in this up-conversion process [26,27]. To understand the multi-phonon effects, we change the excitation energy from 1.81 eV to 1.89 eV with different offset energy at atmospheric pressure.…”

“…Due to the direct-indirect crossover of the band type, the UPL decreases at 1.52 GPa dramatically, as the K-K direct transition is easier compared with the indirect one. Therefore, the number of phonons participating in the UPL process will decrease as the pressure increases [26,35]. For comparison, figure 4b and 4c show the power dependence and excitation energy dependence of the UPL respectively, where the results of 1.52 GPa and atmospheric pressure are compared.…”

Cathodoluminescence Nanoscopy: State of the Art and Beyond

“…As discussed in Time-Resolved CL, CL photon statistics based on time-resolved light detection play an important role in unveiling nanoscale dynamics in luminescent materials. An exciting development is cathodoluminescence excitation spectroscopy (Figure d), relying on a coincidence scheme between inelastic electron scattering and photon emission events that are temporally recorded in a CL-EELS joint setup. , This approach solves a longstanding problem regarding the decomposition of excitation and emission pathways in electron-beam spectroscopy and greatly expands the scope of time-resolved CL, suggesting versatile applications in the future such as the optimization of quantum emitters, studies of energy upconversion, and exciton fission . Moreover, the use of ultrafast electron pulses, polarization-resolved collection, and faster-response detectors may lead to further improvements.…”