Ultrafast Coherent Imaging Spectroscopy of Van-der-Waals Materials

Abstract

Transition metal dichalcogenides (TMDs) have received considerable attention in the past decade for their optoelectronic applications in photovoltaics, lasers, and quantum information. In the monolayer limit, these materials exhibit extraordinary properties, including efficient light-matter coupling, ultrafast charge transfer, long-lived interlayer excitons with high binding energies, and many-body excitonic interactions.

In this thesis, we present the development and application of multidimensional coherent imaging spectroscopy (MDCIS), a four-wave mixing (FWM) based nonlinear spectroscopic imaging technique, to TMD monolayers and heterostructures. Based on multidimensional coherent spectroscopy (MDCS), MDCIS allows us to distinguish between homogeneous and inhomogeneous contributions to the material linewidths and distinguish coherent and incoherent coupling mechanisms in TMD heterostructures. The imaging aspect allows us to capture the spatial variation of the aforementioned physical processes across TMDs.

We first discuss our results applying MDCS to an MoSe2/WSe2 heterostructure, for which we characterize the coherent and incoherent coupling mechanisms present in these materials. We quantify the timescales of rapid electron (91+/-9 fs) and hole (148+/-28 fs) transfer between the two materials. Furthermore, we visualize strong coherent coupling between excitons in the MoSe2 and WSe2 layers by observing oscillations of the coupling peaks in one-quantum MDCS and measuring a mixing energy of 73 meV in zero-Quantum MDCS. We also observe many-body signatures of the interlayer excitons and, in conjunction with photoluminescence measurements, measure their binding energy to be 254 meV.

To accelerate nonlinear imaging, we develop a lock-in amplifier that uses a box-weighted instead of an exponentially-weighted lowpass filter. The transfer function of the box lock-in has tunable notches in the frequency domain that enable sufficient suppression of adjacent modulations present on the detector. We use Monte-Carlo simulations to quantify the signal-to-noise ratio and suppression of adjacent modulations, demonstrating the superiority of the box lock-in over the conventional exponential lock-in at short pixel-dwell times. We further experimentally demonstrate this advantage by imaging a monolayer of MoSe2 on a distributed Bragg reflector.

Furthermore, we present results using MDCIS to study the potential of MoSe2 monolayers and MoSe2/WSe2 heterostructures for quantum information applications. We map the distribution of homogeneous and inhomogeneous linewidths across an MoSe2 monolayer, identifying promising areas with low inhomogeneity and long dephasing times that bear the potential for qubits. We also visualize the strain across the MoSe2 monolayer and comment on the detrimental effects strain may have in device applications. Similarly, we map strain across an MoSe2/WSe2 heterostructure and quantify the spatial homogeneity of coherent coupling (81 % of the sample) and charge transfer (91 % of the sample). We further map the distribution of interlayer exciton lifetimes. These quantities display a surprising robustness in the presence of strain, strengthening the case for TMD heterostructures as an applications platform for quantum information and photovoltaics.

Lastly, we demonstrate how to further accelerate the nonlinear imaging techniques in this thesis by smart scanning and sampling schemes in the time-domain. We obtain FWM images, dephasing maps, and decay maps within minutes, opening the avenue for moving these techniques out of the lab and into a fabrication/manufacturing setting for advanced materials characterization.