Developing new, multidimensional pump-probe spectroscopies for investigating semiconductors

Optical harmonic generation and sum-frequency methods are largely used to provide
insight into the vibrational structure and symmetry group of analytes. These methods
rely on quantifying a system’s ability to create new electric fields from intense, applied
electric fields. Work in the molecular spectroscopy community has demonstrated the
strength of using a multi-resonant approach with multiple, tunable lasers in a sum-
frequency geometry to specifically access coupled electronic and vibrational modes.
In this talk, I will describe the experimental and theoretical extension of these
multidimensional methods to semiconductor microstructures with a focus on triple sum-
frequency (TSF) spectroscopy and transition metal dichalcogenide semiconductors. I
will show that TSF is susceptible to group and phase velocity mismatch artifacts, and
that these artifacts can be mitigated.[1] I will show how TSF can assess the electronic
structure of a polycrystalline MoS 2 thin film and experimentally confirm predictions of
band nesting contributions to MoS 2 ’s optical joint density of states.[2] Then I will show
that TSF, when preceded by a pump, can probe the ultrafast dynamics of MoS 2 and
WS 2 microstructures without suffering from sensitivity losses due to low surface
coverage like the more common transient-reflectance spectroscopy.[3] Finally, I will
show how the optical Stark effect manifests in this pump-probe method via coherent
photon exchange between the pump and sum-frequency generation fundamental
fields.[4]
[1] Morrow et al. Phys. Rev. A 96, 063835. 2017
[2] Morrow et al. J. Chem. Phys. 149, 091101. 2018
[3] Morrow et al. Phys. Rev. B 100, 235303. 2019
[4] Morrow et al. Phys. Rev. B 102, 161401(R). 2020