Ultrafast Photoswitching Research Articles

Iminothioindoxyl Donors with Exceptionally High Cross Section for Protein Vibrational Energy Transfer.

Various protein functions are related to vibrational energy transfer (VET) as an important mechanism. The underlying transfer pathways can be experimentally followed by ultrafast Vis-pump/IR-probe spectroscopy with a donor-sensor pair of non-canonical amino acids (ncAAs) incorporated in a protein. However, so far only one donor ncAA, azulenylalanine (AzAla), exists, which suffers from a comparably low Vis extinction coefficient. Here, we introduce two novel donor ncAAs based on an iminothioindoxyl (ITI) chromophore. The dimethylamino-ITI (DMA-ITI) and julolidine-ITI (J-ITI) moieties overcome the limitation of AzAla with a 50 times higher Vis extinction coefficient. While ITI moieties are known for ultrafast photoswitching, DMA-ITI and J-ITI exclusively form a hot ground state on the sub-ps timescale instead, which is essential for their usage as vibrational energy donor. In VET measurements of donor-sensor dipeptides we investigate the performance of the new donors. We observe 20 times larger signals compared to the established AzAla donor, which opens unprecedented possibilities for the study of VET in proteins.

Iminothioindoxyl Donors with Exceptionally High Cross Section for Protein Vibrational Energy Transfer

AbstractVarious protein functions are related to vibrational energy transfer (VET) as an important mechanism. The underlying transfer pathways can be experimentally followed by ultrafast Vis‐pump/IR‐probe spectroscopy with a donor‐sensor pair of non‐canonical amino acids (ncAAs) incorporated in a protein. However, so far only one donor ncAA, azulenylalanine (AzAla), exists, which suffers from a comparably low Vis extinction coefficient. Here, we introduce two novel donor ncAAs based on an iminothioindoxyl (ITI) chromophore. The dimethylamino‐ITI (DMA‐ITI) and julolidine‐ITI (J‐ITI) moieties overcome the limitation of AzAla with a 50 times higher Vis extinction coefficient. While ITI moieties are known for ultrafast photoswitching, DMA‐ITI and J‐ITI exclusively form a hot ground state on the sub‐ps timescale instead, which is essential for their usage as vibrational energy donor. In VET measurements of donor‐sensor dipeptides we investigate the performance of the new donors. We observe 20 times larger signals compared to the established AzAla donor, which opens unprecedented possibilities for the study of VET in proteins.

Space-charge limited and ultrafast dynamics in graphene-based nano-gaps

We show that nano-gaps formed in graphene by utilizing a focused helium ion beam can act as ultrafast photoswitches. By temperature-dependent, time-integrated, and ultrafast photocurrent measurements, we demonstrate that the optoelectronic dynamics across such nano-gaps are dominated by a space-charge limited current in combination with the ultrafast dynamics of hot electrons. The demonstrated methodology allows the creation of ultrafast photoswitches with an amplification gain exceeding the ones as formed by pristine graphene.

Anharmonic Exciton‐Phonon Coupling in Metal‐Organic Chalcogenides Hybrid Quantum Wells

AbstractIn contrast to inorganic quantum wells, hybrid quantum wells (HQWs) based on metal‐organic semiconductors are characterized by relatively soft lattices, in which excitonic states can strongly couple to lattice phonons. Therefore, understanding the lattice's impact on exciton dynamics is essential for harnessing the optoelectronic potential of HQWs. Beyond 2D metal halide perovskites, layered metal‐organic chalcogenides (MOCs), which are an air‐stable, underexplored material class hosting room‐temperature excitons, can be exploited as photodetectors, light emitting devices, and ultrafast photoswitches. Here, the role of phonons in the optical transitions of the prototypical MOC [AgSePh]∞ is elucidated. Impulsive stimulated Raman scattering (ISRS) allows the detection of coherent exciton oscillations driven by Fröhlich interaction with low‐energy optical phonons. Steady state absorption and Raman spectroscopies reveal a strong exciton‐phonon coupling (Huang‐Rhys parameter ≈1.7) and its anharmonicity, manifested as a nontrivial temperature‐dependent Stokes shift. The ab initio calculations support these observations, hinting at an anharmonic behavior of the low‐energy phonons <200 cm−1. These results untangle complex exciton‐phonon interactions in MOCs, establishing an ideal testbed for room‐temperature many‐body phenomena.

Multidimensional engineered metasurface for ultrafast terahertz switching at frequency-agile channels

Abstract The ability to actively manipulate free-space optical signals by using tunable metasurfaces is extremely appealing for many device applications. However, integrating photoactive semiconductors into terahertz metamaterials still suffers from a limited functionality. The ultrafast switching in picosecond timescale can only be operated at a single frequency channel. In the hybrid metasurface proposed here, we experimentally demonstrate a dual-optically tunable metaphotonic device for ultrafast terahertz switching at frequency-agile channels. Picosecond ultrafast photoswitching with a 100% modulation depth is realized at a controllable operational frequency of either 0.55 THz or 0.86 THz. The broadband frequency agility and ultrafast amplitude modulation are independently controlled by continuous wave light and femtosecond laser pulse, respectively. The frequency-selective, temporally tunable, and multidimensionally-driven features can empower active metamaterials in advanced multiplexing of information, dual-channel wireless communication, and several other related fields.

Spatiotemporal Terahertz Metasurfaces for Ultrafast All‐Optical Switching with Electric‐Triggered Bistability

AbstractOptoelectronic terahertz switching achieved by dynamically tuning metamaterials is viewed as a major breakthrough in promoting the advancement of terahertz technology. However, the main thrust toward the development of ultrafast switchable components and optical logic operations is still in a catch‐up stage for progressively increasing information and communication demands. Here, a novel spatiotemporal metadevice is proposed for terahertz wave steering in multidimensional domains with ultrahigh spatial and ultrafast temporal manipulations. By spatially changing the interconnect architecture via the embedded vanadium dioxide bridges, the state of electromagnetically induced transparency resonance is switched between “1” and “0” states under electrical stimuli, exhibiting a typical 1 bit coding bistability with a long retention time. Furthermore, with the characteristic of writable/erasable logic operation, ultrafast photoswitching of each memorized state is manifested by delivering optical excitations. The high‐speed temporal response of terahertz radiation exhibits an on–off–on switching cycle within 16 ps, owing to the defect‐rich germanium photoactive layer. By leveraging both degrees of freedom in space and time domains, this multifunctional spatiotemporal metaphotonic device can upgrade and improve the current capabilities of optical computing, data storage, and ultrahigh‐speed information processing, paving the way for a highly unexplored territory toward manipulations of light.

Spatial and spectral mode mapping of a dielectric nanodot by broadband interferometric homodyne scanning near-field spectroscopy

We investigate the optical properties of nanostructures of antimony sulfide (Sb2S3), a direct-bandgap semiconductor material that has recently sparked considerable interest as a thin film solar cell absorber. Fabrication from a nanoparticle ink solution and two- and three-dimensional nanostructuring with pattern sizes down to 50 nm have recently been demonstrated. Insight into the yet unknown nanoscopic optical properties of these nanostructures is highly desired for their future applications in nanophotonics. We implement a spectrally broadband scattering-type near-field optical spectroscopy technique to study individual Sb2S3 nanodots with a 20-nm spatial resolution, covering the range from 700 to 900 nm. We show that in this below-bandgap range, the Sb2S3 nanostructures act as high-refractive-index, low-loss waveguides with mode profiles close to those of idealized cylindrical waveguides, despite a considerable structural disorder. In combination with their high above-bandgap absorption, this makes them promising candidates for applications as dielectric metamaterials, specifically for ultrafast photoswitching.

Ultrafast Terahertz Frequency and Phase Tuning by All‐Optical Molecularization of Metasurfaces

AbstractThe integration of photoactive semiconductors exhibiting strong light–matter interactions into functional unit meta‐atoms facilitates effective approaches to dynamically manipulate terahertz (THz) waves. Here, a new metaphotonic modulator is proposed and comprehensively studied, which demonstrates extensive tunability of the resonant frequency and phase with the merit of ultrafast photoswitching. Specifically, parallel silicon (Si) bridges are embedded in metasurfaces to reinforce the connection ability, achieving ultrafast optical molecularization from a magnetic quadrupole into an electric dipole. Under femtosecond pulse excitation, the demonstrated resonant frequency tuning range is as high as 40% (from 1.16 to 0.7 THz) and can be further promoted up to 48% (from 1.56 to 0.81 THz) by varying the Si bridge length. Meanwhile, the phase delay at given frequencies can be controlled up to 53.3° without significantly changing the high transmission. Furthermore, the transient frequency switching and phase shifting dynamics are systematically investigated for the first time, showing a full recovery time within 2 ns. By optically molecularizing metasurfaces, extended tuning ranges with regard to the resonant frequency and phase, as well as an ultrafast switching speed, are simultaneously acquired in the proposed metamodulator, which provides deeper insight into the multifunctional active‐tuning systems.

Ultrafast Photodetection in the Quantum Wells of Single AlGaAs/GaAs-Based Nanowires.

We investigate the ultrafast optoelectronic properties of single Al0.3Ga0.7As/GaAs core-shell nanowires. The nanowires contain GaAs-based quantum wells. For a resonant excitation of the quantum wells, we find a picosecond photocurrent which is consistent with an ultrafast lateral expansion of the photogenerated charge carriers. This Dember-effect does not occur for an excitation of the GaAs-based core of the nanowires. Instead, the core exhibits an ultrafast displacement current and a photothermoelectric current at the metal Schottky contacts. Our results uncover the optoelectronic dynamics in semiconductor core-shell nanowires comprising quantum wells, and they demonstrate the possibility to use the low-dimensional quantum well states therein for ultrafast photoswitches and photodetectors.

Diverse photoinduced dynamics in an organic charge-transfer complex having strong electron-phonon interactions.

CONSPECTUS: Phenomena that occur in nonequilibrium states created by photoexcitation differ qualitatively from those that occur at thermal equilibrium, and various physical theories developed for thermal equilibrium states can hardly be applied to such phenomena. Recently it has been realized that understanding phenomena in nonequilibrium states in solids is important for photoenergy usage and ultrafast computing. Consequently, much effort has been devoted to revealing such phenomena by developing various ultrafast observation techniques and theories applicable to nonequilibrium states. This Account describes our recent studies of diverse photoinduced dynamics in a strongly correlated organic solid using various ultrafast techniques. Solids in which the electronic behavior is affected by Coulomb interactions between electrons are designated as strongly correlated materials and are known to exhibit unique physical properties even at thermal equilibrium. Among them, many organic charge-transfer (CT) complexes have low dimensionality and flexibility in addition to strong correlations; thus, their physical properties change sensitively in response to changes in pressure or electric field. Photoexcitation is also expected to drastically change their physical properties and would be useful for ultrafast photoswitching devices. However, in nonequilibrium states, the complicated dynamics due to these characteristics prevents us from understanding and using these materials for photonic devices. The CT complex (EDO-TTF)2PF6 (EDO-TTF = 4,5-ethylenedioxytetrathiafulvalene) exhibits unique photoinduced dynamics due to strong electron-electron and electron-phonon interactions. We have performed detailed studies of the dynamics of this complex using transient electronic spectroscopy at the 10 and 100 fs time scales. These studies include transient vibrational spectroscopy, which is sensitive to the charges and structures of constituent molecules, and transient electron diffraction, which provides direct information on the crystal structure. Photoexcitation of the charge-ordered low-temperature phase of (EDO-TTF)2PF6 creates a new photoinduced phase over 40 fs via the Franck-Condon state, in which electrons and vibrations are coherently and strongly coupled. This new photoinduced phase is assigned to an insulator-like state in which the charge order differs from that of the initial state. In the photoinduced phase, translations of component molecules proceed before the rearrangements of intramolecular conformations. Subsequently, the charge order and structure gradually approach those of the high-temperature phase over 100 ps. This unusual two-step photoinduced phase transition presumably originates from steric effects due to the bent EDO-TTF as well as strong electron-lattice interactions.

Symmetry, Aperiodicity and Ultrafast Photo-Switching in Spin-Crossover Compounds

The optical control of materials and related physical properties (electronic state, magnetic, optical...) by laser irradiation has gained tremendous interest within the emerging field of photoinduced phase transitions. Light-induced changes of molecular systems involve subtle coupling between the electronic and structural degrees of freedom, which are essential to stabilize the photo-excited state, different in nature from the stable state. Therefore the new experimental field of photocrystallography plays a key role. Its outreach goes far beyond simple structural analysis under laser excitation. By playing on different physical parameters and developing the techniques and analysis, one can investigate new out of equilibrium physics through light-driven cooperative dynamics and transformations in materials, or follow a chemical reaction in real time [1]. The use of photo-crystallography allows to investigate the nature, the mechanisms and the dynamics of photoinduced phase transitions. Here we will present photocrystallography studies of the photo-switching process in spin-crossover materials. On the one hand, ultrafast diffraction allows to follow the structural dynamics [2] and to probe the different processes following femtosecond laser excitation. Recent studies performed on the LCLS X-FEL have shown that the structural molecular changes, with the characteristic Fe-N bond elongation, occur within 160 fs. On the other hand, we will present investigations on the effect of light excitation on spin-state concentration waves, which may be of aperiodic nature (Figure, [3]). Time-resolved x-ray diffraction studies reveal that the high symmetry phase is reached only after milliseconds.

Ultrafast dynamics of photoinduced semiconductor-to-metal transition in the optical switching nano-oxideTi3O5

Nanogranular trititanium pentaoxide $({\mathrm{Ti}}_{3}{\mathrm{O}}_{5})$, an oxide showing a photoreversible phase transition at room temperature, has recently attracted considerable attention because of its applicability in optical data storage. We report the dynamics of the photoinduced semiconductor-to-metal transition (\ensuremath{\beta} phase to \ensuremath{\lambda} phase) at room temperature. With time-resolved diffuse reflection spectroscopy conducted over a wide range of time scales, from femtoseconds to microseconds, the overall relaxation behavior was characterized. We found that a transient phase transition occurs within a few hundreds of femtoseconds and that 40% of the converted \ensuremath{\lambda}-phase fractions revert to \ensuremath{\beta} phase within 1 ps. Carriers are generated instantaneously and disappear within 20 ns. The \ensuremath{\lambda} phase increases from 1 to 10 ps, indicating growth of the metallic domain. A slow recovery process, with a duration ranging from ${10}^{\ensuremath{-}9}$ to ${10}^{\ensuremath{-}4}$ s, was found and is ascribed to the shrinking and annihilation of the size-distributed metastable \ensuremath{\lambda}-phase domains. The fast onset, with a duration within a few hundreds of femtoseconds and the subsequent stabilization process of less than 10 ps, signal the applicability of this material to ultrafast photoswitching.

In Vivo Photoswitchable Flow Cytometry for Direct Tracking of Single Circulating Tumor Cells

Photoswitchable fluorescent proteins (PSFPs) that change their color in response to light have led to breakthroughs in studying static cells. However, using PSFPs to study cells in dynamic conditions is challenging. Here we introduce a method for invivo ultrafast photoswitching of PSFPs that provides labeling and tracking of single circulating cells. Using invivo multicolor flow cytometry, this method demonstrated the capability for studying recirculation, migration, and distribution of circulating tumor cells (CTCs) during metastasis progression. In tumor-bearing mice, it enabled monitoring of real-time dynamics of CTCs released from primary tumor, identifying dormant cells, and imaging of CTCs colonizing a primary tumor (self-seeding) or existing metastasis (reseeding). Integration of genetically encoded PSFPs, fast photoswitching, flow cytometry, and imaging makes invivo single cell analysis in the circulation feasible to provide insights into the behavior of CTCs and potentially immune-related and bacterial cells in circulation.

Primary Photochemistry of the Dark- and Light-Adapted States of the YtvA Protein from Bacillus subtilis

The primary (100 fs to 10 ns) and secondary (10 ns to 100 μs) photodynamics in the type II light-oxygen-voltage (LOV) domain from the blue light YtvA photoreceptor extracted from Bacillus subtilis were explored with transient absorption spectroscopy. The photodynamics of full-length YtvA were characterized after femtosecond 400 nm excitation of both the dark-adapted D447 state and the light-adapted S390 state. The S390 state relaxes on a 43 min time scale at room temperature back into D447, which is weakly accelerated by the introduction of imidazole. This is ascribed to an obstructed cavity in YtvA that hinders access to the embedded FMN chromophore and is more open in type I LOV domains. The primary photochemistry of dark-adapted YtvA is qualitatively similar to that of the type I LOV domains, including AsLOV2 from Avena sativa, but exhibits an appreciably higher (60% greater) terminal triplet yield, estimated near the maximal ΦISC value of ≈78%; the other 22% decays via non-triplet-generating fluorescence. The subsequent secondary dynamics are inhomogeneous, with three triplet populations co-evolving: the faster-decaying (I)T* population (38% occupancy) with a 200 ns decay time is nonproductive in generating the S390 adduct state, a slower (II)T* population (57% occupancy) exhibits a high yield (Φadduct ≈ 100%) in generating S390 and a third (5%) (III)T*population persists (>100 μs) with unresolved photoactivity. The ultrafast photoswitching dynamics of the S390 state appreciably differ from those previously resolved for the type I AcLOV2 domain from Adiantum capillus-veneris [Kennis, J. T., et al. (2004) J. Am. Chem. Soc. 126, 4512], with a low-yield dissociation (Φdis ≈ 2.5%) reaction, which is due to an ultrafast recombination reaction, following photodissociation, and is absent in AcLOV2, which results in the increased photoswitching activity of the latter domain.

Nonadiabatic dynamics and simulation of time resolved photoelectron spectra within time-dependent density functional theory: Ultrafast photoswitching in benzylideneaniline

We present a theoretical approach for the nonadiabatic dynamics "on the fly" based on the combination of the time-dependent density functional theory (TDDFT) with Tully's stochastic surface hopping method. Our formulation is based on localized Gaussian basis sets and is suitable for the simulation of ultrafast processes in complex molecular systems including all degrees of freedom. Our approach is used for the simulation of time resolved photoelectron spectra in the framework of the Wigner distribution approach. In order to illustrate the scope of the method, we study the ultrafast photoswitching dynamics of the prototype Schiff base benzylideneaniline (BAN). The nonradiative lifetime of the S(1) state of BAN is determined to be approximately 200 fs. The mechanism of the photoisomerization has been investigated and a connection between the time resolved photoelectron signal and the underlying nonadiabatic processes has been established.

Semiconductor Materials for Ultrafast Photoswitches

This paper gives a review of semiconductor materials that are used to fabricate ultrafast photoswitches. The optoelectrical response of the switches is first described with simple models, from which the material requirements are deduced. The basic principles of the required material properties - ultrashort free carrier lifetime and high mobility, high dark resistivity, and high field breakdown - are explained. Then, the most popular ultrafast semiconductors are listed, together with their characteristics. A special emphasis is put on low-temperature grown GaAs. Finally, two applications of these ultrafast materials are presented, namely antennae for terahertz radiation and all-optical nonlinear devices.