[Paper Review] Ultrafast nano-imaging of dark excitons

David Schmitt, Jan Philipp Bange+16 more|arXiv (Cornell University)|May 30, 2023

Quantum Dots Synthesis And PropertiesMaterials Science3 citations

TL;DR

This paper introduces ultrafast dark-field momentum microscopy, a novel technique combining nano-imaging and time-resolved spectroscopy to achieve 55 fs temporal and 500 nm spatial resolution in mapping dark exciton dynamics in twisted WSe2/MoS2 heterostructures. The method reveals nanoscale spatial heterogeneity in excitonic energy landscapes and directly correlates it with ultrafast dark exciton formation and relaxation, overcoming limitations of conventional trARPES by resolving local inhomogeneities such as strain, dielectric disorder, and moiré potential effects.

ABSTRACT

The role and impact of spatial heterogeneity in two-dimensional quantum materials represents one of the major research quests regarding the future application of these materials in optoelectronics and quantum information science. In the case of transition-metal dichalcogenide heterostructures, in particular, direct access to heterogeneities in the dark-exciton landscape with nanometer spatial and ultrafast time resolution is highly desired, but remains largely elusive. Here, we introduce ultrafast dark field momentum microscopy to spatio-temporally resolve dark exciton formation dynamics in a twisted WSe$_2$/MoS$_2$ heterostructure with 55 femtosecond time- and 500~nm spatial resolution. This allows us to directly map spatial heterogeneity in the electronic and excitonic structure, and to correlate these with the dark exciton formation and relaxation dynamics. The benefits of simultaneous ultrafast nanoscale dark-field momentum microscopy and spectroscopy is groundbreaking for the present study, and opens the door to new types of experiments with unprecedented spectroscopic and spatiotemporal capabilities.

Motivation & Objective

• To overcome the limitations of conventional time- and angle-resolved photoelectron spectroscopy (trARPES), which averages over large areas and misses nanoscale heterogeneities in 2D quantum materials.
• To enable direct, spatially resolved observation of ultrafast dark exciton dynamics in transition-metal dichalcogenide (TMD) heterostructures, which are critical for optoelectronic and quantum information applications.
• To correlate local electronic and excitonic inhomogeneities—such as strain gradients, dielectric disorder, and moiré potential variations—with the formation and relaxation of dark excitons.
• To develop a new experimental platform that combines ultrafast nano-imaging with spectroscopic capability for probing quasiparticle dynamics at the femtosecond and nanometer scale.

Proposed method

• The method employs ultrafast dark-field momentum microscopy, a variant of time-resolved photoelectron spectroscopy, adapted for nanoscale spatial resolution.
• It uses a laser pulse to excite the system, followed by a time-delayed probe to measure photoemission with momentum resolution.
• The technique is applied to a twisted WSe2/MoS2 heterostructure, enabling simultaneous mapping of energy landscape and excitonic dynamics at 55 fs time and 500 nm spatial resolution.
• A theoretical model based on the semiconductor Bloch equation is used, incorporating hybrid exciton-phonon and exciton-light coupling via a second-order Born-Markov approximation.
• The model includes a spatially dependent energy shift (ΔE_sp) to account for local variations in dielectric environment or layer distance, without altering tunneling strength.
• The degree of hybridization (DoH) is calculated as a measure of the mixing between intra- and interlayer excitonic character, defined as DoH(Q) = 1 - | |c_intra|^2 - |c_inter|^2 |.

Experimental results

Research questions

• RQ1 How does spatial inhomogeneity in the electronic and excitonic structure of a 2D heterostructure affect the formation and relaxation dynamics of dark excitons?
• RQ2 Can ultrafast nano-imaging with femtosecond and nanometer resolution resolve local variations in excitonic energy landscapes that are averaged out in conventional trARPES?
• RQ3 To what extent do local perturbations—such as strain, dielectric disorder, or moiré potential variations—modify the dynamics of dark excitons in TMD heterostructures?
• RQ4 How does the degree of hybridization between intra- and interlayer excitonic states influence the ultrafast relaxation pathways of dark excitons?
• RQ5 Can the coupling between hybrid excitons and phonons be quantitatively linked to observed relaxation dynamics in the presence of spatial inhomogeneities?

Key findings

• The technique achieves 55 femtosecond time resolution and 500 nanometer spatial resolution, enabling direct observation of ultrafast dark exciton dynamics in a twisted WSe2/MoS2 heterostructure.
• Spatial heterogeneity in the excitonic energy landscape is directly mapped, revealing local variations in exciton formation and relaxation dynamics.
• The degree of hybridization (DoH) analysis shows that hybrid excitonic states exhibit a 50-50% mixture of intra- and interlayer character at maximum hybridization, indicating strong coupling between layers.
• Local variations in the energy landscape, induced by spatially dependent dielectric environment or layer distance, are shown to significantly alter the relaxation dynamics of dark excitons.
• Phonon-mediated scattering processes are identified as key contributors to incoherent relaxation, with scattering tensors W^ηξ_QQ′ derived from the model to quantify these pathways.
• The model successfully accounts for the absence of strong moiré trapping due to the large twist angle, allowing focus on local electronic inhomogeneities as the dominant source of spatial dynamics variation.