Colloquium: On the Development of Repetition-rate Scalable Multidimensional Spectroscopy

Pump-probe(PP) spectroscopy is a powerful and now routine time-resolved spectroscopic technique that probes the non-equilibrium electronic and nuclear motions routinely 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 spectroscopy¹(MES) 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} requires development of multidimensional spectroscopy methods that can scale equally well with diffraction-limited spatial resolution. Pushing multidimensional spectroscopy 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 scalablePP⁶ and MES^7,8spectrometer 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 approaches⁹ 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 lead to enhancement of electrical conductivity – enhanced polymer backbone planarity leading to enhanced interchain coupling and hole delocalization on the backbone.

We first demonstrate a PP spectrometer that takes in a white-light supercontinuum (WLC) input and works at100kHz shot-to-shot detection⁶ with speed only limited by the camera frame rate. The simplicity of WLC generation using non-linear crystal comes at the cost of only 5pJ/nm pulse energy and poor shot-to-shot stability. The implementation of shot-to-shot detection suppress the 1/f noise by fully utilising the correlations between laser shots. The additional suppression of 1/f experimental noise component and better averaging is achieved with the rapid scanning¹⁰ of T delay. Electronically synchronized laser, optical chopper, CCD line camera, and the mechanical delay stage demonstrates the combination of shot-to-shot detection and rapid scanning that works at any input repetition rate with rapid scanning being 17x faster simultaneously with 2x better signal-to-noise ratio compared to traditional step scanning method, obtained on a scattering nanotube sample.

Next we extend the above ideas to a new MES approach^7,8 that uses conventional optics, electronics and mechanical delay lines, yet is superior to the complex and artifact probe acousto-optic pulse shaping (AOPS) approaches⁹ which are intrinsically constrained by the optical dispersion and the limited acoustic velocity in the crystal. Our spectrometer combines the shot-to-shot rapid scanning method with a common path interferometer¹¹ for generating delayed pump pulse pairs required for MES. We demonstrate a maximum throughput of 700ms/2D spectra and sensitivity down to 0.05OD with11.6s averaging, obtained over a 200 nm supercontinuum bandwidth with sub-10 fs temporal resolution. The WLC-MES approach was further extended to collect 2D quantum beat maps on a typical pentacene sample with vibrational coherences upto ~1500 cm^-1. Together these specifications represent the best-in-class 2D spectrometer.

We then utilize our PP and MES spectrometers to understand the observation¹² of ~30-340x enhancement of electrical conductivity of polythiophene (P3HT) films when doped with tetracyanoquinodimethane (TCNQ) based do pants for doping concentrations as low as 3 wt%.Enhancing the electrical conductivity of organic semiconducting polymer thin films at low dopant concentrations while maintaining film morphology remains the key challenge to overcome before their potential for solution processed and tunable electronics can be realized. Understanding polymer-dopant interactions and their optical signatures at low concentration doping may be key in this regard. The enhancement observed in TCNQ dopants is similar or higher than that observed for the higher electron affinity fluorinated archetypal dopant F4TCNQ. Our impulsive optical probes provide interesting insights into the polymer-dopant interactions responsible for these observations. These include uniform film morphology with no preferential doping in the crystalline domains, absence of strongly bound ion-pair complexes between the P3HT chain and the dopant, and enhancement of interchain interactions induced by the dopant. These features are reported by the observation of excitation wavelength independent dynamics, no 2D cross-peaks, rapid (sub-50 fs) generation of polaron pairs and <200 fs exciton quenching through exciton-hole interactions. Taken together, the presented evidence points towards dopant induced enhanced interchain interactions as the reason behind highly delocalized holes in the crystalline P3HT domains that ultimately enable higher electrical conductivity at low doping concentrations. Our findings provide new optical signatures that directly report on the polymer-dopant interactions responsible for enhanced conductivity as lower doping concentrations.

Overall, this thesis has introduced a powerful new approach to multidimensional spectroscopy 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.

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[3] Schnedermann,C., Lim,J., Wende,T., Duarte, A., Ni, L., Gu,Q., Sadhanala,A., Rao, A., Kukura, P.,Sub-10 fs Time-Resolved Vibronic Optical Microscopy.J. Phys. Chem. Lett., 7, 4854−4859 (2016).

[4]Sung, J.,Schnedermann, C., Ni, L., Sadhanala,A., Chen, R., Cho, C., Priest, L., Lim, J., Kim, H., Monserrat, B.,Kukura, P., and Rao, A.,Long-range ballistic propagation of carriersin methylammonium lead iodide perovskitethin films. Nature Physics, 16, 171–176 (2020).

[5] BlachD. D., Lumsargis-Roth, V., Chuang, C., Clark,D. E.,  Deng, S., Williams, O.,  Li, C., Cao, J., Huang, L., Environment-assisted quantum transport of excitons in perovskite nanocrystal superlattices. Nat Commun 16, 1270 (2025).

[6] Bhat, V.*, Thomas, A.*, Bhattacharyya, A., and Tiwari, V., Rapid scan white light pump-probe spectroscopy with 100 kHz shot-to-shot detection. Opt. Contin. 2, 1981–1995 (2023). *Denotes equal authors.

[7] Thomas, A.*, Bhat, V.*,and Tiwari, V., Rapid scan white light two-dimensional electronic spectroscopy with 100 kHz shot-to-shot detection. J. Chem. Phys. 159, 244202 (2023).*Denotes equal authors.

[8] Tiwari, V., Bhat, V. N.and Thomas, A. S. WO2024134663 – system and method for variable repetition rate shot-to-shot rapid scan pump-probe and 2d electronic spectroscopy. (2024), doi: https://patentscope.wipo.int/search/en/detail.jsf?docId=WO2024134663.

[9] Kearns, N. M., Mehlenbacher, R. D., Jones, A. C. and Zanni, M. T., Broadband 2D electronic spectrometer using white light and pulse shaping: noise and signal evaluation at 1 and 100 kHz. Opt. Express, 25, 7869–7883 (2017).

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[12] In collaboration with Karandeep Singh and Prof. Satish A. Patil, SSCU. Manuscript in preparation.