Multi-Pulse Terahertz Spectroscopy Unveils Hot Polaron Photoconductivity Dynamics in Metal-Halide Perovskites

The behavior of hot carriers in metal-halide perovskites (MHPs) present a valuable foundation for understanding the details of carrier-phonon coupling in the materials as well as the prospective development of highly efficient hot carrier and carrier multiplication solar cells. Whilst the carrier population dynamics during cooling have been intensely studied, the evolution of the hot carrier properties, namely the hot carrier mobility, remain largely unexplored. To address this, we introduce a novel ultrafast visible pump - infrared push - terahertz probe spectroscopy (PPP-THz) to monitor the real-time conductivity dynamics of cooling carriers in methylammonium lead iodide. We find a decrease in mobility upon optically depositing energy into the carriers, which is typical of band-transport. Surprisingly, the conductivity recovery dynamics are incommensurate with the intraband relaxation measured by an analogous experiment with an infrared probe (PPP- IR), and exhibit a negligible dependence on the density of hot carriers. These results and the kinetic modelling reveal the importance of highly-localized lattice heating on the mobility of the hot electronic states. This collective polaron-lattice phenomenon may contribute to the unusual photophysics observed in MHPs and should be accounted for in devices that utilize hot carriers.Comment: 28 pages, 4 figures, 77 reference

Interplay Between Mixed and Pure Exciton States Controls Singlet Fission in Rubrene Single Crystals

Singlet fission (SF) is a multielectron process in which one singlet exciton S converts into a pair of triplet excitons T+T. SF is widely studied as it may help overcome the Shockley-Queisser efficiency limit for semiconductor photovoltaic cells. To elucidate and control the SF mechanism, great attention has been given to the identification of intermediate states in SF materials, which often appear elusive due to the complexity and fast timescales of the SF process. Here, we apply 10fs-1ms transient absorption techniques to high-purity rubrene single crystals to disentangle the intrinsic fission dynamics from the effects of defects and grain boundaries and to identify reliably the fission intermediates. We show that above-gap excitation directly generates a hybrid vibronically assisted mixture of singlet state and triplet-pair multiexciton [S:TT], which rapidly (<100fs) and coherently branches into pure singlet or triplet excitations. The relaxation of [S:TT] to S is followed by a relatively slow and temperature-activated (48 meV activation energy) incoherent fission process. The SF competing pathways and intermediates revealed here unify the observations and models presented in previous studies of SF in rubrene and propose alternative strategies for the development of SF-enhanced photovoltaic materials

Comparison of nonlinear properties of monomer and dimer of bacterial phytochrome from Deinococcus radiodurans

Current medicine might be greatly enhanced by the ability to in vivo control and monitor neurons using opsins/phytochromes expressed in neural cells. The fundamental challenge with non-invasive neural cell activity regulation is a high absorption of visible light into biological tissues. This drawback could be mitigated by the photoconversion of phytochromes in spectral ranges with higher tissue transparency. In this study, we first demonstrated two-photon Pr-Pfr conversion of monomeric phytochrome at 1.2 µm wavelength. We did a comparison of linear and nonlinear conversion of truncated DrBphP bacterial phytochromes. This work provides a structured understanding of the optical properties of the dimer and monomer of phytochrome as well as their potential for use in optogenetics

Few-femtosecond soft X-ray transient absorption spectroscopy with tuneable DUV-Vis pump pulses

Achieving few-femtosecond resolution for a pump-probe experiment is crucial to measuring the fastest electron dynamics and for creating superpositions of valence states in quantum systems. However, traditional UV-Vis pump pulses cannot achieve few-fs durations and usually operate at fixed wavelengths. Here, we present, to our knowledge, an unprecedented temporal resolution and pump tuneability for UV-Vis-pumped soft X-ray transient absorption spectroscopy. We have combined few-fs deep-UV to visible tuneable pump pulses from resonant dispersive wave emission in hollow capillary fiber with attosecond soft X-ray probe pulses from high harmonic generation. We achieve sub-5-fs time resolution, sub-fs interferometric stability, and continuous tuneability of the pump pulses from 230 to 700 nm. We demonstrate that the pump can initiate an ultrafast photochemical reaction and that the dynamics at different atomic sites can be resolved simultaneously. These capabilities will allow studies of the fastest electronic dynamics in a large range of photochemical, photobiological and photovoltaic reactions