Proceedings of nanoGe Fall Meeting 2021 (NFM21)
DOI: https://doi.org/10.29363/nanoge.nfm.2021.159
Publication date: 23rd September 2021
Two-dimensional (2D) semiconductors are primed as excellent optoelectronic materials to realize a variety of photonic devices which rely on the absorption of light and the consequent transient properties of photo-generated charges and excitons. Surprisingly, very little is known about the changes in the 2D material's refractive index upon excitation. The associated optical phase changes when light propagates through the 2D material, can be beneficial or undesired depending on the application at hand, but clearly require proper quantification. Measuring optical phase modulation of dilute 2D materials is however not trivial with common ultrafast methods. In this work, we first demonstrate that 2D colloidal CdSe quantum wells, a useful model system, can modulate the phase of light across a broad spectrum using an experimental ultrafast interferometry method. Next, we proceed to develop a toolbox to calculate the time-dependent refractive index of colloidal 2D materials from more widely available broadband transient absorption data using a modified effective medium algorithm. We confirm the experiments and show that the pronounced room temperature excitonic features found in 2D materials result in broadband, ultrafast and sizable phase modulation, even extending sub-band gap to the near-infrared where modulation is associated with well defined intraband transitions.
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