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Over the past decade, the proliferation of pulsed laser sources with high repetition rates has facilitated a merger of ultrafast time-resolved spectroscopy with imaging microscopy. In transient absorption microscopy (TAM), the excited-state dynamics of a system are tracked by measuring changes in the transmission of a focused probe pulse following photoexcitation of a sample. Typically, these experiments are done using a photodiode detector and lock-in amplifier synchronized with the laser and images highlighting spatial heterogeneity in the TAM signal are constructed by scanning the probe across a sample. Performing TAM by instead imaging a spatially defocused widefield probe with a multipixel camera could dramatically accelerate the acquisition of spatially resolved dynamics, yet approaches for such widefield imaging generally suffer from reduced signal-to-noise due to an incompatibility of multipixel cameras with high-frequency lock-in detection. Herein, we describe implementation of a camera capable of high-frequency lock-in detection, thereby enabling widefield TAM imaging at rates matching those of high repetition rate lasers. Transient images using a widefield probe and two separate pump pulse configurations are highlighted. In the first, a widefield probe was used to image changes in the spatial distribution of photoexcited molecules prepared by a tightly focused pump pulse, while in the second, a widefield probe detected spatial variations in photoexcited dynamics within a heterogeneous organic crystal excited by a defocused pump pulse. These results highlight the ability of high-sensitivity lock-in detection to enable widefield TAM imaging, which can be leveraged to further our understanding of excited-state dynamics and excitation transport within spatially heterogeneous systems. 

Time-resolved spectroscopy of plasmonic nanoparticles is a vital technique for probing their ultrafast electron dynamics and subsequent acoustic and photothermal properties. Traditionally, these experiments are performed with spectrally broad probe beams on the ensemble level to achieve high signal amplitudes. However, the relaxation dynamics of plasmonic nanoparticles is highly dependent on their size, shape, and crystallinity. As such, the inherent heterogeneity of most nanoparticle samples can complicate efforts to build microscopic models for these dynamics solely on the basis of ensemble measurements. Although approaches for collecting time-resolved microscopy signals from individual nanoparticles at selected probe wavelengths have been demonstrated, acquiring time-resolved spectra from single objects remains challenging. Here, we demonstrate an alternate method that efficiently yields the time-resolved spectra of a single gold nanodisk in one measurement. By modulating the frequency-doubled output of a 96 MHz Ti:sapphire oscillator at 8 kHz, we are able to use a lock-in pixel-array camera to detect photoinduced changes in the transmission of a white light continuum probe derived from a photonic crystal fiber to produce broadband femtosecond transmission spectra of a single gold nanodisk. We also compare the performance of the lock-in camera for the same single nanoparticle to measurements with a single-element photodiode and find comparable sensitivities. The lock-in camera thus provides a major advantage due to its ability to multiplex spectral detection, which we utilize here to capture both the electronic dynamics and acoustic vibrations of a single gold nanodisk following ultrafast laser excitation. 

Materials that undergo singlet fission are of interest for their use in light-harvesting, photocatalysis, and quantum information science, but their ability to undergo fission can be sensitive to local variations in molecular packing. Herein we employ transient absorption microscopy, molecular dynamics simulations, and electronic structure calculations to interrogate how structures found at the edges of orthorhombic rubrene crystals impact singlet fission. Within a micrometer-scale spatial region at the edges of rubrene crystals, we find that the rate of singlet fission increases nearly 4-fold. This observation is consistent with formation of a region at crystal edges with reduced order that accelerates singlet fission by disrupting the symmetry found in rubrene’s orthorhombic crystal structure. Our work demonstrates that structural distortions of singlet fission materials can be used to control fission in time and in space, potentially offering a means of controlling this process in light harvesting and quantum information applications. 

ABSTRACT We report single-shot transient absorption (SSTA) measurements of an organic film of 3,3’-Diethyloxatricarbocyanine iodide (DOTCI). In SSTA, the pump-probe time delay is spatially encoded by using a tilted pump pulse. Translation of the sample during SSTA measurements averages over any spatial heterogeneity in the film. We demonstrate that exciton dynamics measured with the single-shot technique agrees with traditional transient absorption measurements of the same film. A signal-to-noise ratio of ∼40 is achieved in 10 s. The ability to measure exciton dynamics in organic films will enable future SSTA measurements of exciton dynamics during the molecular aggregation events that result in film formation.