On the Development of Repetition-Rate Scalable Multidimensional Spectroscopy

Abstract

Pump-probe (PP) spectroscopy is a powerful and now routine time-resolved spectroscopic technique that routinely probes the non-equilibrium electronic and nuclear motions with sub-10 fs temporal resolution. PP spectroscopy carries a significant limitation of no excitation frequency information and a significantly more complicated technique, multidimensional electronic spectroscopy (MES)[1] overcomes this limitation by resolving the pump-probe dynamics along a waiting time T as a contour map of detection versus excitation frequency. Femtosecond multidimensional spectroscopy has been extended to wavelength ranges from UV to THz and has revolutionized our understanding of ultrafast phenomena at the interface of physics, chemistry, and biology, such as natural photosynthesis, photocatalysis, photovoltaics, and layered quantum materials. However, increasingly important physics, such as energy/charge transfer across grain boundaries in photovoltaic thin films[2, 3] and exciton diffusion across light harvesting assemblies[4, 5], require the development of MES methods that can scale equally well with diffraction-limited spatial resolution. Pushing MES towards a viable imaging technique requires overcoming significant challenges imposed by small signal sizes and sample photodamage while simultaneously ensuring broadest possible spectral throughput and few-cycle temporal resolution. This is where the frontier of MES currently lies at.

This thesis outlines the development of a repetition rate scalable PP[6] and MES[7] spectrometer which takes a supercontinuum input, provides sub-10 fs temporal resolution 2D spectra in as fast as 700ms, sample exposure of 4.8 secs/432ps time window. Vitally, the setup works, in principle, at any repetition-rate between Hz-MHz while maintaining the above specifications and limited only by the camera frame rate. Together, these specifications beat the state-of-the-art acousto-optic pulse shaping approaches[8] by using mechanical delay lines with conventional optics and electronics. We demonstrate applications of this method in deciphering some crucial details of polymer-dopant interactions in doped organic polymers that enhance electrical conductivity, enhanced polymer backbone planarity, and hole delocalization on the backbone. In general, this thesis has introduced a powerful new approach to MES that is scalable with repetition-rate, maintains spectral throughput and temporal resolution, minimizes sample exposure and fully leverages shot-to-shot correlations to enhance sensitivity. Future developments include coupling this approach to a microscope for micro-spectroscopy applications.

References:

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[3] Christoph Schnedermann, Jong Min Lim, Torsten Wende, Alex S Duarte, Limeng Ni, Qifei Gu, Aditya Sadhanala, Akshay Rao, and Philipp Kukura. Sub-10 fs time resolved vibronic optical microscopy. The Journal of Physical Chemistry Letters, 7:4854–4859, 2016. PMID: 27934055.

[4] Jooyoung Sung, Christoph Schnedermann, Limeng Ni, Aditya Sadhanala, Richard Y. S. Chen, Changsoon Cho, Lee Priest, Jong Min Lim, Hyun-Kyung Kim, Bartomeu Monserrat, Philipp Kukura, and Akshay Rao. Long-range ballistic propagation of carriers in methylammonium lead iodide perovskite thin films. Nature Physics, 16:171– 176, 2020.

[5] Daria D. Blach, Victoria A. Lumsargis-Roth, Chern Chuang, Daniel E. Clark, Shibin Deng, Olivia F. Williams, Christina W. Li, Jianshu Cao, and Libai Huang. Environment-assisted quantum transport of excitons in perovskite nanocrystal superlattices. Nature communications, 16:1270, 12 2025.

[6] Vivek N Bhat, Asha S Thomas, Atandrita Bhattacharyya, and Vivek Tiwari. Rapid scan white light pump–probe spectroscopy with 100 khz shot-to-shot detection. Optics Continuum, 2:1981–1995, 2023.

[7] Asha S. Thomas, Vivek N. Bhat, and Vivek Tiwari. Rapid scan white light twodimensional electronic spectroscopy with 100 khz shot-to-shot detection. Journal of Chemical Physics, 159, 12 2023.

[8] Nicholas M Kearns, Randy D Mehlenbacher, Andrew C Jones, and Martin T Zanni. Broadband 2d electronic spectrometer using white light and pulse shaping: noise and signal evaluation at 1 and 100 khz. Optics Express, 25:7869–7883, 2017.