Ultrafast Changes Research Articles

Protein Dynamics Preceding Photoisomerization of the Retinal Chromophore in Bacteriorhodopsin Revealed by Deep-UV Femtosecond Stimulated Raman Spectroscopy.

Bacteriorhodopsin is a prototypical photoreceptor protein that functions as a light-driven proton pump. The retinal chromophore of bacteriorhodopsin undergoes C13═C14 trans-to-cis isomerization upon photoexcitation, and it has been believed to be the first event that triggers the cascaded structural changes in bacteriorhodopsin. We investigated the protein dynamics of bacteriorhodopsin using deep-ultraviolet resonance femtosecond stimulated Raman spectroscopy. It was found that the stimulated Raman signals of tryptophan and tyrosine residues exhibit significant changes within 0.2 ps after photoexcitation while they do not noticeably change during the isomerization process. This result implies that the protein environment changes first, and its change is small during isomerization. The obtained femtosecond stimulated Raman data indicate that ultrafast change is induced in the protein part by the sudden creation of the large dipole of the excited-state chromophore, providing an environment that realizes efficient and selective isomerization.

Interference and polarization beating of independent arbitrarily polarized polychromatic optical waves

We study the properties of optical fields created by interfering polychromatic stationary waves that have different spectra and polarizations. Such fields can exhibit both deterministic and random intensity and polarization beatings, where the latter stands for a periodic variation of the field polarization state. For visible light, the beating period enters the femtosecond scale already when the central wavelengths of the waves differ by 10 nm. If the bandwidth of at least one of the waves is also on the order of 10 nm, the periodic variations are accompanied by ultrafast random changes which cannot be measured directly. We propose a set of statistical characteristics for such rapidly varying vector fields and practical methods to determine them in terms of fully time-averaged quantities. Our results may have impact on a variety of fundamental and applied aspects of optical polarimetry and interferometry.

Design and modeling of subpicosecond all-optical modulator using the nonlinear response of indium tin oxide

We report the design of an all-optical modulator based on the ultrafast nonlinear optical response of indium tin oxide (ITO) combined with the sharp spectral features of a high-Q resonant grating structure. Under optical excitation, a thin layer of ITO exhibits an ultrafast, reversible, and large intensity-dependent change in its refractive index. The grating structure provides rapid variation of the transmission coefficient, which is tuned to the epsilon-near-zero spectral region of ITO. The proposed design achieves all-optical modulation with a response time of 650 fs (speed >1.5 THz) using a simple design with a compact footprint.

Ultrafast reflectivity change of vanadium dioxide induced by THz field enhanced by a metallic structure

We studied the transient reflectivity change of vanadium dioxide that is induced by terahertz (THz) pulse excitation, whose electric field is enhanced by a metallic dipole antenna. The tightly focused probe pulse in the near-infrared region allows one to monitor the reflectivity change between the antenna gap and also quantitative evaluation of the field dependence of reflectivity change. The observed steep rising of the dependence shows that the insulator-to-metal transition occurs with the threshold electric field strength at around 2 MV cm−1. Our result is explained by the model taking into account the carrier generation via the Poole–Frenkel effect induced by THz electric field.

Imaging an isolated water molecule using a single electron wave packet.

Observing changes in molecular structure requires atomic-scale Ångstrom and femtosecond spatio-temporal resolution. We use the Fourier transform (FT) variant of laser-induced electron diffraction (LIED), FT-LIED, to directly retrieve the molecular structure of H2O+ with picometer and femtosecond resolution without a priori knowledge of the molecular structure nor the use of retrieval algorithms or ab initio calculations. We identify a symmetrically stretched H2O+ field-dressed structure that is most likely in the ground electronic state. We subsequently study the nuclear response of an isolated water molecule to an external laser field at four different field strengths. We show that upon increasing the laser field strength from 2.5 to 3.8 V/Å, the O-H bond is further stretched and the molecule slightly bends. The observed ultrafast structural changes lead to an increase in the dipole moment of water and, in turn, a stronger dipole interaction between the nuclear framework of the molecule and the intense laser field. Our results provide important insights into the coupling of the nuclear framework to a laser field as the molecular geometry of H2O+ is altered in the presence of an external field.

Femtosecond Laser-Excitation-Driven High Frequency Standing Spin Waves in Nanoscale Dielectric Thin Films of Iron Garnets.

We demonstrate that femtosecond laser pulses allow triggering high-frequency standing spin-wave modes in nanoscale thin films of a bismuth-substituted yttrium iron garnet. By varying the strength of the external magnetic field, we prove that two distinct branches of the dispersion relation are excited for all the modes. This is reflected in particular at a very weak magnetic field (∼33 mT) by a spin dynamics with a frequency up to 15GHz, which is 15 times higher than the one associated with the ferromagnetic resonance mode. We argue that this phenomenon is triggered by ultrafast changes of the magnetic anisotropy via laser excitation of incoherent and coherent phonons. These findings open exciting prospects for ultrafast photo magnonics.

Optical Control of Non-Equilibrium Phonon Dynamics.

The light-induced selective population of short-lived far-from-equilibrium vibration modes is a promising approach for controlling ultrafast and irreversible structural changes in functional nanomaterials. However, this requires a detailed understanding of the dynamics and evolution of these phonon modes and their coupling to the excited-state electronic structure. Here, we combine femtosecond mega-electronvolt electron diffraction experiments on a prototypical layered material, MoTe2, with non-adiabatic quantum molecular dynamics simulations and ab initio electronic structure calculations to show how non-radiative energy relaxation pathways for excited electrons can be tuned by controlling the optical excitation energy. We show how the dominant intravalley and intervalley scattering mechanisms for hot and band-edge electrons leads to markedly different transient phonon populations evident in electron diffraction patterns. This understanding of how tuning optical excitations affect phonon populations and atomic motion is critical for efficiently controlling light-induced structural transitions of optoelectronic devices.

Dynamics of Magnetization in Multilayer TbCo / FeCo Structures under the Influence of Femtosecond Optical Excitation

The need to study ultrafast processes in magnetism is due to the prospects for creating ultrafast magnetic recording and ultrafast spintronic devices. In order to excite the magnetic subsystem femtosecond optical pulses are used. The excitement is manifested as in spin precession. In metals, the material is heated first due to significant optical absorption, and significant Joule losses occur. The most important task is to search for materials in which spin processes are excited without heating. Obvious candidates are weakly absorbing materials, such as ferrite garnets. However, the range of such materials and the range of their functionality are limited.The purpose of this work is to study the dynamics of systems with nonthermal mechanisms of spin precession excitation. Such excitation is possible in ferromagnetic / antiferromagnetic heterostructures with exchange interaction, provided that the recombination time of photocarriers is shorter than the time of heat diffusion. Multilayer TbCo / FeCo structures of the near IR range were investigated for a femtosecond optical pulse. The spin dynamics are compared with the direction of the wave vector of the exciting pulse along and perpendicular to the axis of easy magnetization of the structures (“easy axis” and “hard axis” geometry, respectively). It is shown that in case of “easy axis” geometry the determinative mechanism is the thermal interaction. When the system is exposed to an excitation pulse, this mechanism leads to a decrease in the projection of magnetization on the direction of propagation of the test beam. In case of “hard axis” geometry, the magnetization turns to the magnetic field at the initial stage. Then it precesses and relaxes to an equilibrium angular orientation. Such dynamics indicate a rapid recovery of the uniaxial anisotropy field after laser irradiation. The presented results demonstrate an ultrafast change in the magnetic anisotropy induced during the fabrication of the heterostructure under study, which may be of interest for optical control of the orientation of the magnetization.

Ultrafast anisotropic disordering in graphite driven by intense hard X-ray pulses

We present results from the SPring-8 Angstrom Compact free electron LAser (SACLA) X-ray free electron laser (XFEL) facility, using an X-ray pump, X-ray probe scheme to observe ultrafast changes in the structure of heated graphite. The 9.8 keV XFEL beam was focused to give an intensity on the order of ∼ 1019 W/cm2, and the evolution of the diffraction pattern observed up to delays of 300 fs. The interplanar diffraction peaks weaken significantly within 10s of femtoseconds, but in-plane diffraction orders i.e. those with Miller Index (hk0), persist up to 300 fs, with the observed signal increasing. We interpret this as nonthermal damage through the breaking of interplanar bonds, which at longer timescales leads to ablation by removal of intact graphite sheets. Post-experiment examination of the graphite samples shows damage which is comparable in size to the range of the excited photoelectrons. These results highlight the challenges of accurately modelling X-ray driven heating, as it becomes a routine approach to generating high energy density states.

Resolving the Ultrafast Changes of Chemically Inequivalent Metal–Ligand Bonds in Photoexcited Molecular Complexes with Transient X-ray Absorption Spectroscopy

Photoactive transition-metal complexes that incorporate heteroleptic ligands present a first coordination shell, which is asymmetric. Although it is generally expected that the metal–ligand bond lengths respond differently to photoexcitation, resolving these fine structural changes remains experimentally challenging, especially for flexible multidentate ligands. In this work, ultrafast X-ray absorption spectroscopy is employed to capture directly the asymmetric elongations of chemically inequivalent metal–ligand bonds in the photoexcited spin-switching FeII complex [FeII(tpen)]2+ solvated in acetonitrile, where tpen denotes N,N,N′,N′-tetrakis(2-pyridylmethyl)-1,2-ethylenediamine. The possibility to correlate precisely the nature of the donor/acceptor coordinating atoms to specific photoinduced structural changes within a binding motif will provide advanced diagnostics for optimizing numerous photoactive chemical and biological building blocks.

Dynamic pigmentary and structural coloration within cephalopod chromatophore organs

Chromatophore organs in cephalopod skin are known to produce ultra-fast changes in appearance for camouflage and communication. Light-scattering pigment granules within chromatocytes have been presumed to be the sole source of coloration in these complex organs. We report the discovery of structural coloration emanating in precise register with expanded pigmented chromatocytes. Concurrently, using an annotated squid chromatophore proteome together with microscopy, we identify a likely biochemical component of this reflective coloration as reflectin proteins distributed in sheath cells that envelop each chromatocyte. Additionally, within the chromatocytes, where the pigment resides in nanostructured granules, we find the lens protein Ω- crystallin interfacing tightly with pigment molecules. These findings offer fresh perspectives on the intricate biophotonic interplay between pigmentary and structural coloration elements tightly co-located within the same dynamic flexible organ - a feature that may help inspire the development of new classes of engineered materials that change color and pattern.

Optical and Physical Probing of Thermal Processes in Semiconductor and Plasmonic Nanocrystals.

This article reviews thermal properties of semiconductor and emergent plasmonic nanomaterials, focusing on mechanisms through which hot carriers and phonons are produced and dissipated as well as the related impacts on optoelectronic properties. Elevated equilibrium temperatures, of particular relevance for implementation of nanomaterials in devices, affect absorptive and radiative transitions as well as emission efficiency that can present reversible and irreversible changes with temperature. In noble metal or doped semiconductor/insulator nanomaterials, hot carriers and lattice heating can substantially influence localized surface plasmon resonances and yield large ultrafast changes in transmission or strongly oscillatory coherences. Transient optical and diffraction characterizations enable nonequilibrium investigations of phonon dynamics and cooling such as lattice expansion and crystal phase stability. Timescales of nanoparticle thermalization with surroundings and transport of heat within films of such materials are also discussed.

Ultrafast broadband circular dichroism in the deep ultraviolet

The measurement of chirality and its temporal evolution are crucial for the understanding of a large range of biological functions and chemical reactions. Steady-state circular dichroism (CD) is a standard analytical tool for measuring chirality in chemistry and biology. Nevertheless, its push into the ultrafast time domain and in the deep-ultraviolet has remained a challenge, with only some isolated reports of subnanosecond CD. Here, we present a broadband time-resolved CD spectrometer in the deep ultraviolet (UV) spectral range with femtosecond time resolution. The setup employs a photoelastic modulator to achieve shot-to-shot polarization switching of a 20 kHz pulse train of broadband femtosecond deep-UV pulses (250–370 nm). The resulting sequence of alternating left- and right-circularly polarized probe pulses is employed in a pump-probe scheme with shot-to-shot dispersive detection and thus allows for the acquisition of broadband CD spectra of ground- and excited-state species. Through polarization scrambling of the probe pulses prior to detection, artifact-free static and transient CD spectra of enantiopure [Ru(bpy)3]2+ are successfully recorded with a sensitivity of <2×10−5 OD (≈0.7 mdeg). Due to its broadband deep-UV detection with unprecedented sensitivity, the measurement of ultrafast chirality changes in biological systems with amino-acid residues and peptides and of DNA oligomers is now feasible.

Crystallization Properties of Mg35Sb65/Sb Nanocomposite Multilayer Films for Phase Change Memory Application

The phase transition properties of Mg35Sb65/Sb multilayer thin films were studied, including the crystallization mechanism, surface morphology, atomic bonding mode and adhesion strength. With the increase of the thickness of Mg35Sb65 interlayers, Mg35Sb65/Sb thin film had better thermal stability and lower electrical conductivity. The growth-dominated crystallization mechanism made Mg35Sb65/Sb have ultra-fast phase change speed. The subtle change on surface morphology was ascribed to the grain growth and interface stress during crystallization. The scratch test revealed the adhesion strength contrast before and after crystallization. This work showed that Mg35Sb65/Sb multilayer film was a potential material with fast speed and good thermal stability for phase change memory application.

How optical excitation controls the structure and properties of vanadium dioxide

We combine ultrafast electron diffraction and time-resolved terahertz spectroscopy measurements to link structure and electronic transport properties during the photoinduced insulator-metal transitions in vanadium dioxide. We determine the structure of the metastable monoclinic metal phase, which exhibits antiferroelectric charge order arising from a thermally activated, orbital-selective phase transition in the electron system. The relative contribution of the photoinduced monoclinic and rutile metals to the time-dependent and pump-fluence-dependent multiphase character of the film is established, as is the respective impact of these two distinct phase transitions on the observed changes in terahertz conductivity. Our results represent an important example of how light can control the properties of strongly correlated materials and demonstrate that multimodal experiments are essential when seeking a detailed connection between ultrafast changes in optical-electronic properties and lattice structure.

Excited State Vibrational Spectra of All- trans Retinal Derivatives in Solution Revealed By Pump-DFWM Experiments.

The ultrafast structural changes during the photoinduced isomerization of the retinal-protonated Schiff base (RPSB) is still a poorly understood aspect in the retinal's photochemistry. In this work, we apply pump-degenerate four-wave mixing (pump-DFWM) to all- trans retinal (ATR) and retinal Schiff bases (RSB) to resolve coherent high- and low-frequency vibrational signatures from excited electronic states. We show that the vibrational spectra of excited singlet states in these samples exhibit pronounced differences compared to the relaxed ground state. Pump-DFWM results indicate three major features for ATR and RSB. (i) Excited state vibrational spectra of ATR and RSB consist predominately of low-frequency modes in the energetic range 100-500 cm-1. (ii) Excited state vibrational spectra show distinct differences for excitation in specific regions of electronic transitions of excited state absorption and emission. (iii) Low-frequency modes in ATR and RSB are inducible during the entire lifetime of the excited electronic states. This latter effect points to a transient molecular structure that, following initial relaxation between different excited electronic states, does not change anymore over the lifetime of the finally populated excited electronic state.

Snapshots of a phase transition

Physics![Figure][1] Depiction of the insulator-to-metal transition of vanadium dioxide CREDIT: GREG STEWART/SLAC NATIONAL ACCELERATOR LABORATORY Time-resolved x-ray scattering can be used to investigate the dynamics of materials during the switch from one structural phase to another. So far, methods provide an ensemble average and may miss crucial aspects of the detailed mechanisms at play. Wall et al. used a total-scattering technique to probe the dynamics of the ultrafast insulator-to-metal transition of vanadium dioxide (VO2) (see the Perspective by Cavalleri). Femtosecond x-ray pulses provide access to the time- and momentum-resolved dynamics of the structural transition. Their results show that the photoinduced transition is of the order-disorder type, driven by an ultrafast change in the lattice potential that suddenly unlocks the vanadium atoms and yields large-amplitude uncorrelated motions, rather than occurring through a coherent displacive mechanism. Science , this issue p. [572][2]; see also p. [525][3] [1]: pending:yes [2]: /lookup/doi/10.1126/science.aau3873 [3]: /lookup/doi/10.1126/science.aav2019

Ultrafast changes in optical properties of SiO2 excited by femtosecond laser at the damage threshold and above

Ultrafast changes in optical properties of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>SiO</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math> excited by femtosecond laser at the damage threshold and above

Ultra-fast tuning of refractive index in Lithium Niobate slab by GHz acoustic wave

Refractive index modulation in optical materials facilitates dynamic control on electromagnetic waves. Ultra fast tuning of both refractive index and its spatial distribution in Y−Z Lithium Niobate slab via strain employed through GHz acoustic perturbation has been simulated and studied. To create acoustic disturbance in taken 500 nm thick slab, the fundamental symmetric Lamb mode at 5 GHz acoustic frequency is excited. Displacement field patterns generated by longitudinal and transverse disturbances of perturbing fundamental symmetric Lamb mode are analyzed under reinstate limits of optical material to create elastic strain. Modification in optical index of Lithium Niobate due to strain generated by longitudinal and transverse acoustic disturbance is then examined. The sinusoidal modulation in refractive index with maximum change of 0.070 along nz and 0.036 along ny is observed. The modulated refractive index has a periodic distribution over space with period equals to wavelength of acoustic disturbance created by fundamental Lamb mode. This acoustically induced ultra-fast change in refractive index and in its spatial distribution depends on perturbing acoustic frequency and thickness of slab. Thus, the slab can optimize dynamically, to control electromagnetic wave for various optical applications.

All-electric single-electron spin-to-charge conversion

We examine spin-dependent displacement of a single electron, resulting in separation and relocation of the electron wavefunction components, and thus charge parts, corresponding to opposite spins. This separation is induced by a pulse of an electric field which generates varying Rashba type spin-orbit coupling. This mechanism is next implemented in a nanodevice based on a gated quantum dot defined within a quantum nanowire. The electric field pulse is generated by ultrafast changes of voltages, of the order of several hundred mV, applied to nearby gates. The device is modeled realistically with appropriate material parameters and voltages applied to the gates, yielding an accurate confinement potential and Rashba coupling. At the end, we propose a spin-to-charge conversion device, which with an additional charge detector will allow for electron spin state measurement.