Tailoring Optical Response of 2D Photonic Materials via Nanoscale Strain Gradients

We present an experimental and computational study of engineering in-situ strain in two-dimensional materials aiming to unlock and enhance their fundamental magnetic, electronic, and optical properties. The materials under study include semiconducting metal thio(seleno)phosphates, which have been synthesized by vapor transport techniques reported in previous literature [1] and exhibit a wealth of promising nonlinear optoelectronic and magnetic properties, including bulk photovoltaic effect, second harmonic generation, and (anti)ferromagnetic response [1]. These single crystalline materials complete the multi-spectral library of 2D photonic materials as they possess bandgaps between 1.6 and 4 eV, which provide the opportunity to fill the void between narrow band gap materials such as TMDs and wide band gap hBN.<br/>Our prior work has shown that accidental structural defects in these materials can enhance and modulate photoluminescence from spatially localized areas via a combined effect of strain and optical confinement [2]. Here, we describe the process of tailored engineering of the in-situ strain fields with large, localized strain gradients via a custom-built robotic transfer setup that is loaded with micron-scale PDMS stamps. The setup picks up and transfers nanocrystalline flakes onto substrates with pre-engineered nano-pillars and gratings, which impart strong localized strain gradients to the materials. We have developed a protocol of strain gradient mapping with Raman spectroscopy, which is used to reveal the limits and advantages of different strain-engineering strategies. Strain-induced linear and nonlinear optical responses are characterized by imaging ellipsometry, atomic force microscopy, photoluminescence spectroscopy [2], and second harmonic generation experiments, revealing opportunities for engineering strain-enhanced optical responses.<br/><br/>This research has been supported by the MIT Lincoln Laboratory Advanced Concepts Committee Award (ACC-777), along with a Draper Fellowship to Morgan Blevins, as well as the Siebel Scholarship and MIT MathWorks Fellowship to Abhishek Mukherjee.<br/><br/><br/>References:<br/>1. Susner, Michael A., et al. "Metal thio-and selenophosphates as multifunctional van der Waals layered materials." Advanced Materials 29.38 (2017): 1602852.<br/>2. Mukherjee, Abhishek, et al. "Defect-Induced Strain-Tunable Photoluminescence in AgScP2S6." arXiv preprint arXiv:2403.01581 (2024).