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1. Spatiotemporal analysis of a final-state shape resonance in interferometric photoemission from Cu(111) surfaces

   https://doi.org/10.1103/PhysRevA.100.043412  

   Ambrosio, M ; Thumm, U. ( October 2019 , Physical review and Physical review letters index)

   Photoemission from solid targets includes the excitation and motion of electrons inside the substrate, followed by their propagation in vacuum and detection. It thus depends on the electronic band structure of the solid in the two distinct spectral domains of bound initial and continuum final states. While the imprint of the static (initial-state) valence electronic structure of solids on photoemission spectra is routinely examined in standard photoemission spectroscopy in the energy domain, state-of-the-art time-resolved photoelectron spectroscopy allows, in addition, the scrutiny of photoelectron propagation in the electronic continuum. Within a quantum-mechanical model for attosecond time-resolved interferometric photoelectron emission from solids, we calculated photoemission spectra as a function of the delay between the exciting primary attosecond pulse train and assisting infrared (IR) laser pulse. Accounting for final-state interactions of the photoelectron with the IR laser electric field and the periodic substrate, our numerical results for interferometric photoemission from the 3d-valence band of Cu(111) surfaces show a striking resonantly enhanced sideband yield at photoelectron kinetic energies near 24 eV, in conjunction with a pronounced increase of the photoelectron wave-function amplitude inside the solid on a length scale of a few nanometers. This resonant shift of final-state photoelectron-probability density towards the bulk can be interpreted as an increase in the photoelectron propagation time in the solid and is commensurate with the resonantly enhanced spectral sideband-phase shifts observed in recent two-pathway two-photon interference spectra by Kasmi et al. [Optica 4, 1492 (2017)]. 

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2. Optical Momentum Alignment Effect in WSe 2 Phototransistor

   https://doi.org/10.1002/adom.202002243  

   Zhang, Yuchen ; Xu, Ya‐Qiong ( April 2021 , Advanced Optical Materials)

   The optical momentum alignment effect is demonstrated in WSe₂phototransistors . When the photon energy is above the A exciton energy, the maximum photocurrent response occurs for the light polarization direction parallel to the metal electrode edge, suggesting that electrons in the valence band of WSe₂prefer to absorb photons with the polarization direction perpendicular to their momentum direction. Further studies indicate that the anisotropic distribution of photo‐excited carriers is likely due to the pseudospin‐induced optical transition selection rules. If the photon energy is below the A exciton energy, the photocurrent signals are maximized when the incident light is polarized in the direction perpendicular to the electrode edge, which is mainly attributed to the polarized absorption of the plasmonic gold electrodes. Moreover, the photocurrent peak can be controlled by an electric field via the quantum confined Stark effect. This resonance peak can also be shifted by adjusting environmental temperatures due to the temperature‐dependent nature of the WSe₂band gap. These experimental studies shed light on the knowledge of photocurrent generation mechanisms, opening the door for engineering future anisotropic optoelectronics.

    

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3. Manipulating long-lived topological surface photovoltage in bulk-insulating topological insulators Bi2Se3 and Bi2Te3

   https://doi.org/10.1038/s41535-020-0218-4  

   Ciocys, Samuel ; Morimoto, Takahiro ; Mori, Ryo ; Gotlieb, Kenneth ; Hussain, Zahid ; Analytis, James G. ; Moore, Joel E. ; Lanzara, Alessandra ( March 2020 , npj Quantum Materials)

   The appearance of topologically protected spin-momentum locked surface states in topological insulators gives rise to robust room temperature spin currents making them ideal candidates for the realization of spintronic devices. New methods are needed to access and manipulate such currents with timescales that are compatible with modern electronics. Here we reveal that an optically induced long-lived (~10 ns), spin-polarized surface state excitation in topological insulators can be easily tuned in both magnitude and duration. Time-resolved angle-resolved photoemission spectroscopy, together with a quantitative model, reveals the ideal conditions for a surface photovoltage in two different topological insulators. Our model predicts that the reported effects are an intrinsic property of topological insulators, as long as the chemical potential falls within the band gap. This work demonstrates that persistent excited topological surface states are photon-accessible and easily tuned in both magnitude and duration, merging photonics- and spintronics-based devices in the same material.

    

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4. Enhanced photochemical activity and ultrafast photocarrier dynamics in sustainable synthetic melanin nanoparticle-based donor–acceptor inkjet-printed molecular junctions

   https://doi.org/10.1039/d3nr02387g  

   DeMarco, Max ; Ballard, Matthew ; Grage, Elinor ; Nourigheimasi, Farnoush ; Getter, Lillian ; Shafiee, Ashkan ; Ghadiri, Elham ( September 2023 , Nanoscale)

   Cinzia Casiragi (Ed.)

   Melanin is a stable, widely light-absorbing, photoactive, and biocompatible material viable for energy con- version, photocatalysis, and bioelectronic applications. To achieve multifunctional nanostructures, we synthesized melanin nanoparticles of uniform size and controlled chemical composition (dopamelanin and eumelanin) and used them with titanium dioxide to fabricate donor–acceptor bilayers. Their size enhances the surface-to-volume ratio important for any surface-mediated functionality, such as photo- catalysis, sensing, and drug loading and release, while controlling their chemical composition enables to control the film’s functionality and reproducibility. Inkjet printing uniquely allowed us to control the de- posited amount of materials with minimum ink waste suitable for reproducible materials deposition. We studied the photochemical characteristics of the donor–acceptor melanin–TiO2 nanostructured films via photocatalytic degradation of methylene blue dye under selective UV-NIR and Vis-NIR irradiation con- ditions. Under both irradiation conditions, they exhibited photocatalytic characteristics superior to pure melanin and, under UV-NIR irradiation, superior to TiO2 alone; TiO2 is photoactive only under UV irradiation. The enhanced photocatalytic characteristics of the melanin–TiO2 nanostructured bilayer films, particularly when excited by visible light, point to charge separation at the melanin–TiO2 interface as a possible mechanism. We performed ultrafast laser spectroscopy to investigate the photochemical charac- teristics of pure melanin and the melanin–TiO2 constructs and found that their time-resolved photo- excited spectral patterns differ. We performed singular value decomposition analysis to quantitatively deconvolute and compare the dynamics of photochemical processes for melanin and melanin–TiO2 heterostructures. This observation supports electronic interactions, namely, interfacial charge separation at the melanin and TiO2 interface. The excited-state relaxation in melanin–TiO2 increases markedly from 5 ps to 400 ps. The results are remarkable for the future intriguing application of melanin-based con- structs for bioelectronics and energy conversion. 

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5. Excited-state resonance Raman spectroscopy probes the sequential two-photon excitation mechanism of a photochromic molecular switch

   https://doi.org/10.1063/5.0126974  

   Burns, Kristen H. ; Quincy, Timothy J. ; Elles, Christopher G. ( December 2022 , The Journal of Chemical Physics)

   Some diarylethene molecular switches have a low quantum yield for cycloreversion when excited by a single photon, but react more efficiently following sequential two-photon excitation. The increase in reaction efficiency depends on both the relative time delay and the wavelength of the second photon. This paper examines the wavelength-dependent mechanism for sequential excitation using excited-state resonance Raman spectroscopy to probe the ultrafast (sub-30 fs) dynamics on the upper electronic state following secondary excitation. The approach uses femtosecond stimulated Raman scattering (FSRS) to measure the time-gated, excited-state resonance Raman spectrum in resonance with two different excited-state absorption bands. The relative intensities of the Raman bands reveal the initial dynamics in the higher-lying states, Sn, by providing information on the relative gradients of the potential energy surfaces that are accessed via secondary excitation. The excited-state resonance Raman spectra reveal specific modes that become enhanced depending on the Raman excitation wavelength, 750 or 400 nm. Many of the modes that become enhanced in the 750 nm FSRS spectrum are assigned as vibrational motions localized on the central cyclohexadiene ring. Many of the modes that become enhanced in the 400 nm FSRS spectrum are assigned as motions along the conjugated backbone and peripheral phenyl rings. These observations are consistent with earlier measurements that showed higher efficiency following secondary excitation into the lower excited-state absorption band and illustrate a powerful new way to probe the ultrafast dynamics of higher-lying excited states immediately following sequential two-photon excitation. 

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