Ultrafast Laser Science at the Nanoscale: Pump-Probe Spectroscopy of MoSe2
Ethan Murray ’21, Jacob Stuligross ’22, Madelaine Pelletier ’22, and Professor
Transition metal dichalcogenides (TMDs) are semiconductor materials that consist of one atom from the transition metal group and two atoms from the chalcogenide group; they exhibit different properties when found in thin film form as opposed to their bulk form. One of these properties is the direct band gap found in monolayer TMDs, which makes these materials ideal for the development of smaller and faster optical electronic devices like solar cells and photodetectors. We studied the elastic properties of TMDs, particularly those of molybdenum diselenide (MoSe2), with pump-probe spectroscopy. In this form of spectroscopy, an ultrafast laser generates pump and probe pulses of light. The pump pulse energizes the TMD sample, while the probe pulse measures the change in reflectivity of the sample over time. This reflectivity measurement can tell us about the underlying structure of our sample. The measurement has sub-picosecond (10-12 seconds) resolution, which allows us to study extremely fast physical phenomena in our TMD samples. With pump-probe spectroscopy, we found that ultrafast pulses of laser light excite our samples and produce measurable acoustic waves in bulk MoSe2 crystals. By measuring the transit time of these waves through the crystals, and combining that with an atomic force microscope measurement of the thickness of the crystals, the elastic properties of MoSe2 can be determined. The values we obtained through experimentation can then be compared to the results of computer simulations of bulk crystals, which can validate our measurements.