Transient absorption (TA) is a well-known tool for understanding the dynamical response of materials following the absorption of a photon. TA allows the dynamics of photoinduced energy flow and energy conversion to be monitored, along with other competing dissipative processes. With current technologies, TA can provide dynamical information over the ultraviolet, visible, near-infrared, mid-infrared, and terahertz spectral ranges, which allows the study of a concomitant range of photoinduced processes of relevance to photocatalysis. However, since TA usually involves very small changes in absorption, the approach can be quite time consuming and, as a result, signal averaging for hours is not uncommon. In addition to time limitations, long signal averages are challenging for many materials, including photocatalytic systems, because many materials degrade under long periods of laser illumination. Longer experiments also place greater demands on the stability of lasers, usually leading to the need for environmental controls, which adds cost and complexity to TA spectroscopies. Thus, there are several important reasons to increase the speed of TA as long as accuracy and sensitivity can be maintained.

In this talk, I present recent efforts to dramatically decrease the time needed to perform TA through compressive sensing, whereby a smaller number of data points are randomly collected over the chosed dynamical range of the TA experiment.[1] The general principle works well when the transformation of the optical signal to another domain has a low density of TA features. Studies and applications to colloidal plasmonic nanoparticles, such as nanostructures made of refractory materials, and energy conversion materials are shown. I also present imaging efforts using femtosecond optical pulses in scattering environments where conventional imaging is difficult.[2] By utilizing enhanced second harmonic generation from high peak-power ultrashort pulses, combined with compressive sensing approaches, successful reconstruction of optical images can be undertaken.

Work performed at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, was supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This material is based on work supported by Laboratory Directed Research and Development (LDRD) funding from Argonne National Laboratory, provided by the Director, Office of Science, of the U.S. Department of Energy under Contract No. DE-AC02-06CH11357.

References

1. Adhikari, S., Cortes, C. L., Wen X., Panuganti, S., Gosztola, D. J., Schaller, R. D., Wiederrecht, G. P., Gray, S. K., “Accelerating ultrafast spectroscopy with compressive sensing,” Phys. Rev. Appl., Vol. 15, 024032, 2021.
2. Wen, X., Adhikari, S., Cortes, C. L., Gosztola, D. J., Gray, S. K., Wiederrecht, G. P., “Ghost imaging second harmonic generation microscopy,” Appl. Phys. Lett., Vol. 116, 191101, 2020.