Abstract
Van der Waals materials, existing in a range of thicknesses from monolayer to bulk, allow for interplay between surface and bulk nonlinearities, which otherwise dominate only at atomically-thin or bulk extremes, respectively. Here, we observe an unexpected peak in intensity of the generated second harmonic signal versus the thickness of Indium Selenide crystals, in contrast to the quadratic increase expected from thin crystals. We explain this by interference effects between surface and bulk nonlinearities, which offer a new handle on engineering the nonlinear optical response of 2D materials and their heterostructures.
Highlights
Thin form[26], and is expected to outperform other III-VI van der Waals materials in opto-electronic devices in the visible[27]
The model is in good agreement with experimental data, showing that the unexpected peak in the Second harmonic generation (SHG) is the result of high-contrast interference effects of the surface and bulk contributions to the overall nonlinear signal
We largely focused on the thickness of flakes, ranging from 9 to 25 nm, which were determined by the energy of the photoluminesence peaks[32,33] and confirmed by atomic force microscopy (AFM)
Summary
Thin form[26], and is expected to outperform other III-VI van der Waals materials in opto-electronic devices in the visible[27]. Given the range of potential thicknesses, van der Waals crystals like InSe, which maintain broken inversion symmetry for all layer thicknesses, provide another interesting opportunity. They allow exploration of the interplay between surface and bulk contributions to the nonlinear signal, which otherwise dominate only at the atomic and bulk extremes, respectively. In van der Waals crystals, by controlling the thickness layer-by-layer, one obtains fine control over relative strengths of bulk and surface contributions and explore interesting aspects of the transition of nonlinear optical phenomena from 2D to 3D. The model is in good agreement with experimental data, showing that the unexpected peak in the SHG is the result of high-contrast interference effects of the surface and bulk contributions to the overall nonlinear signal. We describe the experimental setup used for measurement of the SHG response in reflection as shown in the schematic (Fig. 1d)
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