Christoph Lienau

Cryptochromes are blue-light-sensitive flavoproteins that play central roles in biological function. In European robin (Erithacus rubecula) ErCry4a proteins, optical excitation of their flavin chromophore forms a long-lived radical pair through a sequence of electron-transfer steps across a tetradic chain of tryptophan residues, making them primary candidates for magnetoreception in night-migratory songbirds. Recent quantum chemical calculations indicate that nonadiabatic couplings play a central role in the energy- and charge-transfer processes initiated by optical excitation. Here, we study these dynamics in ErCry4a using ultrafast transient absorption spectroscopy with 10 fs time resolution in the 450 nm spectral range. We uncover a rapid, sub-50 fs red shift in stimulated emission, quenched within 360 fs by electron transfer from a nearby tryptophan moiety. While high-frequency excited-state vibrations are rapidly damped, coherent motion involving several low-frequency vibrations persists during both the initial energy relaxation and the subsequent electron transfer. This is evidenced by probing the coherent vibrational motion of the formed FAD•– radical anion and is independently validated by blocking the electron transfer through site-selective tryptophan mutation. Our results provide insight into the role of nonadiabatic couplings for the initial steps of cryptochrome photoactivation and suggest a general strategy for redox-state-specific monitoring of charge transfer dynamics by probing coherent vibrational motion.

Photoactivated intermolecular electron transfer (ET) in cryptochromes proceeds along chains of aromatic residues and creates a spatially separated pair of radical electrons. Ultrafast time-dependent spectroscopy can provide experimental insight into this process and theoretical estimates of charge transfer rates are commonly obtained via Marcus theory. Here, we present a new perspective on the ET in European robin cryptochrome 4a (ErCry4a) that synthesizes insights from real-time ET calculations, ultrafast spectroscopic measurements and analytical derivations. The simulations exemplify that molecular vibrations play an essential role in enabling the ET dynamics, which was further rationalized through analytical derivations. Ultrafast pump-probe spectroscopy provided experimental access to the first 1.5 ns of the ET cascade, where multiple radical pair recombination rates arise due to the dynamic equilibrium along the ET chain. We show that the motions of the protein environment and the ET dynamics are inseparably coupled, violating the timescale separation required for Marcus theory. The presented results highlight that non-equilibrium coupling between electronic and nuclear motion dominates ET kinetics in ErCry4a during the first nanosecond after photo-excitation. The findings exemplify the limits of Marcus theory and refine the interpretation of ultrafast spectroscopic signatures in cryptochromes.

Flavins are the chromophores in several blue-light-sensitive photoreceptor proteins and act as redox cofactors in many enzymes that are relevant for biological processes. Despite their biological relevance and numerous detailed optical investigations of their photophysical properties, the ultrafast nonequilibrium dynamics of their elementary optical excitations are not yet fully known. Here, we use ultrafast coherent vibrational spectroscopy with a 10-fs time resolution in the 450-nm spectral range to study their excited state coherent vibrational dynamics. We observe that coherent wavepacket motion along high-frequency C-C stretching modes with a ∼20-fs period is rapidly damped on a 20-fs time scale. In contrast, coherent motion along several low-frequency modes persists for much longer. We attribute this to a mode-selective intramolecular vibrational energy redistribution driven by nonadiabatic couplings between the optical bright state and a close-lying dark electronic state, in accordance with model calculations. Our results may be of relevance for the formation of long-lived radical pair states in magnetic-field sensitive proteins.

Plasmonic nanogaps confine light to dimensions in the nanometer or even subnanometer range while simultaneously enhancing the local electromagnetic field strength. This spatial light confinement has been key for the development of nano-optics. So far, the temporal dynamics of such nanoconfined fields has received comparatively little attention, in particular in the visible spectral range. Here we measure the amplitude and phase of the electric field of visible to near-infrared light pulses scattered from the gap between a sharp gold tip and a metal surface. We retrieve the time structure of the field with subcycle precision. We provide evidence for a complex-valued local near-field enhancement and demonstrate that the spatial confinement of a few-cycle pulse in the investigated nanogap is correlated with a substantial reduction in its pulse duration. Our results pave the way for probing the linear and nonlinear electric field dynamics of single quantum emitters in nanogaps.

The development of experimental methods monitoring the generation of photo‐excitations and their dynamics on the natural temporal and spatial scales of the electrons has opened up fascinating perspectives to better understand fundamental processes driven by light–matter couplings and use them to design new functional materials. This horizon, however, is shadowed by challenges related to the interpretation and rationalization of the observables. Theory can help to overcome these issues only if the available approaches – both ab initio and based on model Hamiltonians – are complementary to experiments and not merely ancillary to them. In this perspective, we present state‐of‐the‐art experimental and theoretical methods to investigate photo‐excitations and their dynamics in complex materials, taking bulk CsPbBr ₃ as a prototypical example. Experimentally, we show the ability of two‐dimensional electronic spectroscopy to shed light onto the electronic and vibronic excitation landscape of this system, in particular giving access to exciton–phonon coupling mechanisms. From the theory side, we discuss the advantages and drawbacks of first‐principles calculations and effective methods based on the semiconductor Bloch equations. By unveiling strengths and bottlenecks of the presented approaches, we suggest viable strategies to make simulations and experiments finally join hands.

Exciton polaritons based on atomically thin semiconductors are essential building blocks of quantum optoelectronic devices. Their properties are governed by an ultrafast and oscillatory energy transfer between their excitonic and photonic constituents, resulting in the formation of polaritonic quasiparticles with pronounced nonlinearities induced by the excitonic component. In metallic, plasmonic nanoresonators, dissipation phenomena limit the polariton lifetime to a few tens of femtoseconds, so short that the role of these polaritons for the nonlinear response of such hybrids is yet unexplored. Here we use ultrafast two-dimensional electronic spectroscopy (2DES) to uncover coherent polariton dynamics in a hybrid monolayer (1L) WS ₂ /plasmonic nanostructure. With respect to an uncoupled WS ₂ flake, we observe an over 20-fold, polarization-dependent enhancement of the optical nonlinearity and a rapid evolution of the 2DES spectra within ~70 fs. We relate these dynamics to a transition from coherent polaritons to incoherent excitations, unravel the microscopic origin of the optical nonlinearities and show the potential of coherent polaritons for ultrafast all-optical switching.

Antisymmetric molecular vibrations are central to ultrafast, nonadiabatic photophysical and photochemical processes such as conical intersection dynamics, Herzberg-Teller couplings and, potentially, singlet fission. Direct spectroscopic identification of such vibrations is, however, challenging, since they are typically Raman inactive and affect optical transitions only weakly. Here, we report experimental signatures of vibronic couplings to a high-frequency antisymmetric vibration in the excited state dynamics of a quasi-quadrupolar molecule by ultrafast two-dimensional electronic spectroscopy (2DES). The early time, sub-50 fs 2DES maps reveal an asymmetric peak pattern with characteristic low-energy cross-peaks. We show that these peaks arise from stimulated emission transitions from an anharmonic, double-minimum excited state potential energy surface formed by vibronic coupling to a high-frequency antisymmetric mode. Simulations based on a phenomenological essential state model support the results. Our findings offer a new approach for identifying antisymmetric vibrations in ultrafast 2DES and track excited state wavepacket motion before relaxation washes out the spectra.

Quadrupolar dyes, such as acceptor–donor–acceptor molecules, are highly relevant for applications in nonlinear optics and photovoltaics. They are also versatile models for exploring photoinduced charge-transfer dynamics. The interplay between electronic and vibronic couplings in these molecules may break excited-state symmetry, resulting in intramolecular charge separation and pronounced solvatochromism. Experimentally, distinguishing the roles of intramolecular vibronic coupling and solvent reorganization for the initial charge-transfer dynamics has been challenging so far. Here we investigate a prototypical quadrupolar dye in polar and non-polar solvents using ultrafast pump–probe and two-dimensional electronic spectroscopy. Our results reveal that vibronic couplings initiate excited-state symmetry breaking during the first ~50 fs of the photoinduced charge transfer, whereas solvent-induced charge localization sets in at later times. Quantum dynamics and electronic structure simulations support our experimental findings. Our results reveal the details of solvation dynamics in photoexcited molecules and suggest strategies for their manipulation through vibronic couplings.

Triggered by advances in atomic-layer exfoliation and growth techniques, along with the identification of a wide range of extraordinary physical properties in self-standing films consisting of one or a few atomic layers, two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and other van der Waals (vdW) crystals now constitute a broad research field expanding in multiple directions through the combination of layer stacking and twisting, nanofabrication, surface-science methods, and integration into nanostructured environments. Photonics encompasses a multidisciplinary subset of those directions, where 2D materials contribute remarkable nonlinearities, long-lived and ultraconfined polaritons, strong excitons, topological and chiral effects, susceptibility to external stimuli, accessibility, robustness, and a completely new range of photonic materials based on layer stacking, gating, and the formation of moiré patterns. These properties are being leveraged to develop applications in electro-optical modulation, light emission and detection, imaging and metasurfaces, integrated optics, sensing, and quantum physics across a broad spectral range extending from the far-infrared to the ultraviolet, as well as enabling hybridization with spin and momentum textures of electronic band structures and magnetic degrees of freedom. The rapid expansion of photonics with 2D materials as a dynamic research arena is yielding breakthroughs, which this Roadmap summarizes while identifying challenges and opportunities for future goals and how to meet them through a wide collection of topical sections prepared by leading practitioners.