Daniel Wigger1
Trinity College Dublin1
Over the last several years, two-dimensional semiconductors, especially in the form of transition metal dichalcogenides, have gained considerable attention in several areas of physics. One of the reasons for this is their strong excitonic optical response, which makes them attractive for future optoelectronic or quantum applications. Their remarkable brightness allows to efficiently study them with ultrafast nonlinear spectroscopy to learn about their fundamental excitonic properties and the excitons’ coupling to other quasi-particles. We have recently studied how the spectral dynamics of pump-probe [1] and four-wave mixing signals [2,3] can be interpreted by an exactly solvable model that extends the basic optical Bloch equations by a local field effect [4]. This local field coupling takes into account the exciton-exciton interaction between the optically generated excitons in the 2D systems on a mean field level and leads to spectral shifts depending on the excitonic occupations.<br/>We then went a step further and studied six-wave mixing (SWM) signal dynamics and discovered a peculiar temporary signal depression, depending on the considered delay between the two applied laser pulses [5]. In this contribution, we will report on this experimental finding and demonstrate that the observed signal dynamics can be understood as a new destructive photon echo. With our local field model we are able to attribute this effect to the interaction between the excitons. We show that the two main contributions to the SWM signal interfere destructively for certain times. Already in his first report of the spin echo effect in 1950, E. Hahn has used the Bloch vector description to illustrate his newly discovered phenomenon [6]. Inspired by this, we developed a similar Bloch vector description for the destructive photon echo. Interestingly, we found that the Bloch vectors contributing to the SWM signal form Lissajous figures that get distorted with progressing time [5]. The new destructive photon echo effect allows to efficiently and systematically study the exciton-exciton interaction across 2D semiconductor samples with a spatial resolution which is only restricted by the diffraction limit of the applied laser pulses. Thereby we will be able to learn more about the potential interplay between exciton-exciton interaction and for example local strain distributions.<br/><br/>[1] A. Rodek et al., Local field effects in ultrafast light-matter interaction measured by pump-probe spectroscopy of monolayer MoSe2, Nanophotonics 10, 2717 (2021).<br/>[2] T. Hahn et al., Influence of local fields on the dynamics of four-wave mixing signals from 2D semiconductor systems, New J. Phys. 23, 023036 (2021).<br/>[3] A. Rodek et al., Controlled coherent-coupling and dynamics of exciton complexes in a MoSe2 monolayer, 2D Mater. 10, 025027 (2023).<br/>[4] M. Wegener et al., Line shape of time-resolved four-wave mixing, Phys. Rev. A 42, 5675 (1990).<br/>[5] T. Hahn et al., Destructive photon echo formation in six-wave mixing signals of a MoSe2 monolayer, Adv. Sci. 9, 2103813 (2022).<br/>[6] E. Hahn, Spin echoes, Phys. Rev. 80, 580 (1950).