Ultrafast Time Scale Research Articles

Engineering the temporal dynamics of all-optical switching with fast and slow materials

All-optical switches control the amplitude, phase, and polarization of light using optical control pulses. They can operate at ultrafast timescales – essential for technology-driven applications like optical computing, and fundamental studies like time-reflection. Conventional all-optical switches have a fixed switching time, but this work demonstrates that the response-time can be controlled by selectively controlling the light-matter-interaction in so-called fast and slow materials. The bi-material switch has a nanosecond response when the probe interacts strongly with titanium nitride near its epsilon-near-zero (ENZ) wavelength. The response-time speeds up over two orders of magnitude with increasing probe-wavelength, as light’s interaction with the faster Aluminum-doped zinc oxide (AZO) increases, eventually reaching the picosecond-scale near AZO’s ENZ-regime. This scheme provides several additional degrees of freedom for switching time control, such as probe-polarization and incident angle, and the pump-wavelength. This approach could lead to new functionalities within key applications in multiband transmission, optical computing, and nonlinear optics.

Primary Photophysics of Nicotinamide Chromophores in Their Oxidized and Reduced Forms.

Nicotinamide adenine dinucleotide (NADH) is an important enzyme cofactor with emissive properties that allow it to be used in fluorescence microscopies to study cell metabolism. Its oxidized form NAD+, on the other hand, is considered to produce negligible fluorescence. In this contribution, we describe the photophysics of the isolated nicotinamidic system in both its reduced and oxidized states. This was achieved through the study of model molecules that do not carry the adenine nucleotide since its absorbance would overlap with the absorption spectrum of the nicotinamidic chromophores. We studied three model molecules: nicotinamide (niacinamide, an oxidized form without nitrogen substitution), the oxidized chromophore 1-benzyl-3-carbamoyl-pyridinium bromide (NBzOx), and its reduced form 1-benzyl-1,4-dihydronicotinamide (NBz). For a full understanding of the dynamics, we performed both femtosecond-resolved emission and transient absorption experiments. The oxidized systems, nicotinamide and NBzOx, have similar photophysics, where the originally excited bright state decays on an ultrafast timescale of less than 400 fs. The depopulation of this state is followed by excited-state positive absorption signals, which evolve in two timescales: the first one is from 1 to a few picoseconds and is followed by a second decaying component of 480 ps for nicotinamide in water and of 80-90 ps for nicotinamide in methanol and NBzOx in aqueous solution. The long decay times are assigned as the S1 lifetimes populated from the original higher-lying bright singlet, where this state is nonemissive but can be detected by transient absorption. While for NBzOx in aqueous solution and for nicotinamide in methanol, the S1 signal decays to the solvent-only level, for the aqueous solutions of nicotinamide, a small transient absorption signal remains after the 480 ps decay. This residual signal was assigned to a small population of triplet states formed during the slower S1 decay for nicotinamide in water. The experimental results were complemented by XMS-CASPT2 calculations, which reveal that in the oxidized forms, the rapid evolution of the initial π-π* state is due to a direct crossing with lower-energy dark n-π* singlet states. This coincides with the experimental observation of long-lived nonemissive states (80 to 480 ps depending on the system). On the other hand, the reduced model compound NBz has a long-lived emissive π-π* S1 state, which decays with a 510 ps time constant, similarly to the parent compound NADH. This is consistent with the XMS-CASPT2 calculations, which show that for the reduced chromophore, the dark states lie at higher energies than the bright π-π* S1 state.

Geometric magnetism and anomalous enantio-sensitive observables in photoionization of chiral molecules

Chiral molecules are instrumental for molecular recognition in living organisms. Distinguishing between two opposite enantiomers, the mirror twins of the same chiral molecule, is both vital and challenging. Geometric magnetism enables a broad class of phenomena in solids including the anomalous electron velocity, the Hall effect, and related topological observables. Here we show that ultrafast electron currents excited in chiral molecules can generate geometric fields which enable a class of anomalous enantio-sensitive observables in photoionization. Next, we introduce the first member of this class: enantio-sensitive orientation of chiral molecules via photoionization. This effect opens unexplored routes to both enantio-separation and imaging of chiral dynamics on ultrafast time scales. Our work suggests that geometric magnetism in photoionization provides the bridge between the two geometrical properties, chirality and topology.

Ultrafast Spin Dynamics and Photoinduced Insulator-to-Metal Transition in α-RuCl3.

Laser-induced ultrafast demagnetization is a phenomenon of utmost interest and attracts significant attention because it enables potential applications in ultrafast optoelectronics and spintronics. As a spin-orbit coupling assisted magnetic insulator, α-RuCl3 provides an attractive platform to explore the physics of electronic correlations and unconventional magnetism. Using time-dependent density functional theory, we explore the ultrafast laser-induced dynamics of the electronic and magnetic structures in α-RuCl3. Our study unveils that laser pulses can introduce ultrafast demagnetizations, accompanied by an out-of-equilibrium insulator-to-metal transition in a few tens of femtoseconds. The spin response significantly depends on the laser wavelength and polarization on account of the electron correlations, band renormalizations, and charge redistributions. These findings provide physical insights into the coupling between the electronic and magnetic degrees of freedom in α-RuCl3 and shed light on suppressing the long-range magnetic orders and reaching a proximate spin liquid phase for two-dimensional magnets on an ultrafast time scale.

Energy level offsets determine the interplay between charge and energy transfer in all-small-molecule organic solar cells

All-small-molecule organic solar cells (ASM OSCs) hold great promise in OSCs owing to their defined structures, simple purification, and good reproducibility, but are challenging for further improved efficiency. The energy level strategy has been broadly applied to obtain a better performance; however, a comprehensive understanding of the effects of energy level offset on photoexcitation dynamics in ASM OSCs is rarely studied. Herein, for Y-series molecules (Y6, Y10, Y5, and BTP-4F-12) based ASM OSCs, the effect of energy level offset on charge photogeneration was investigated using steady-state and time-resolved spectroscopies. We found that both energy and charge transfer could occur in blend films. A method to quantitatively analyze the contribution of charge and energy transfer processes was developed. For BTR-Cl:Y6 with the highest LUMO level offset, ∼ 23% of photogenerated excitons in donor dissociated via “energy transfer and the subsequent charge transfer” pathway, suggesting that the energy transfer in blend films should also be considered. And for the hole transfer, the excitons in Y-series molecules can only be effectively dissociated when the HOMO energy level offset is higher than 0.11 eV. Besides, a higher energy level offset would also suppress carrier recombination in ultrafast timescale. These results may shed light on the design of ASM OSCs.

Ultrafast control over chiral sum-frequency generation.

We introduce an ultrafast all-optical approach for efficient chiral recognition that relies on the interference between two low-order nonlinear processes that are ubiquitous in nonlinear optics: sum-frequency generation and third-harmonic generation. In contrast to traditional sum-frequency generation, our approach encodes the medium's handedness in the intensity of the emitted harmonic signal, rather than in its phase, and it enables full control over the enantiosensitive response. We show how, by sculpting the sub-optical-cycle oscillations of the driving laser field, we can force one molecular enantiomer to emit bright light while its mirror twin remains dark, thus reaching the ultimate efficiency limit of chiral sensitivity via low-order nonlinear light-matter interactions. Our work paves the way for ultrafast and highly efficient imaging and control of the chiral electronic clouds of chiral molecules using lasers with moderate intensities, in all states of matter: from gases to liquids to solids, with molecular specificity and on ultrafast time scales.

Optical control of ultrafast structural dynamics in a fluorescent protein

The photoisomerization reaction of a fluorescent protein chromophore occurs on the ultrafast timescale. The structural dynamics that result from femtosecond optical excitation have contributions from vibrational and electronic processes and from reaction dynamics that involve the crossing through a conical intersection. The creation and progression of the ultrafast structural dynamics strongly depends on optical and molecular parameters. When using X-ray crystallography as a probe of ultrafast dynamics, the origin of the observed nuclear motions is not known. Now, high-resolution pump–probe X-ray crystallography reveals complex sub-ångström, ultrafast motions and hydrogen-bonding rearrangements in the active site of a fluorescent protein. However, we demonstrate that the measured motions are not part of the photoisomerization reaction but instead arise from impulsively driven coherent vibrational processes in the electronic ground state. A coherent-control experiment using a two-colour and two-pulse optical excitation strongly amplifies the X-ray crystallographic difference density, while it fully depletes the photoisomerization process. A coherent control mechanism was tested and confirmed the wave packets assignment.

Imaging and Controlling Ultrafast Electron Pulses Emitted from Plasmonic Nanostructures.

We experimentally study photoemission from gold nanodisk arrays using space-, time-, and energy-resolved photoemission electron microscopy. When excited by a plasmonic resonant infrared (IR) laser pulse, plasmonic hotspots are generated owing to local surface plasmon resonance. Photoelectrons emitted from each plasmonic hotspot form a nanoscale and ultrashort electron pulse. When the system is excited by an extreme ultraviolet (EUV) laser pulse, a uniformly distributed photoelectron cloud is formed across the sample surface. When excited by the IR and EUV laser pulses together, both the photoemission image and kinetic energy vary significantly for the IR laser-generated electrons depending on the time delay between the two laser pulses. These observations are well explained by the Coulomb interaction with the EUV laser-generated electron cloud. Our study offers a feasible approach to manipulate the energy of electron pulse emitted from a plasmonic nanostructure on an ultrafast time scale.

Theoretical insights into ultrafast excited state proton transfer coupled with twisted intramolecular charge transfer mechanism in 7-(2ʹ-pyridyl)indole: Effect of hydrogen bonding

The ultrafast proton transfer (PT) process in the excited state of 7-(2′-pyridyl)indole (abbreviated as 7PyIn) has been elucidated by on-the-fly dynamics simulations using the algebraic-diagrammatic construction scheme of second order (ADC(2)) method under the resolution of identity (RI) approximation. The tautomers of 7PyIn alone and associated with an acetonitrile molecule in the 7PyIn-CH3CN complex are most likely found after the excited state intramolecular proton transfer (ESIntraPT) within an ultrafast time scale between 40 and 60 fs. While a methanol molecule in the complex of 7PyIn-CH3OH could establish both ESIntraPT and excited-state intermolecular proton transfer (ESInterPT). ESInterPT of 7PyIn-CH3OH possibly occurs through a concerted process via the intermolecular hydrogen bonding bridge of the methanol molecule almost twice faster than that of ESIntraPT. This slower circumstance of the ESIntraPT process found in 7PyIn-CH3OH may be caused by the competition between formations of the intramolecular and intermolecular hydrogen bonds between 7PyIn and methanol. In addition, after the accomplishment of excited state proton transfer (ESPT), the initiation of conical intersection could be taking place by the structural twist of 7PyIn. The geometries, energies, proton probabilities, and provided mechanisms via the dynamics simulations of 7PyIn and its solvent clusters exhibiting ESIntraPT and ESInterPT are explored to provide the dynamic pictures of the ESPT processes at the molecular level.

Stark control of electrons across the molecule-semiconductor interface.

Controlling matter at the level of electrons using ultrafast laser sources represents an important challenge for science and technology. Recently, we introduced a general laser control scheme (the Stark control of electrons at interfaces or SCELI) based on the Stark effect that uses the subcycle structure of light to manipulate electron dynamics at semiconductor interfaces [A. Garzón-Ramírez and I. Franco, Phys. Rev. B 98, 121305 (2018)]. Here, we demonstrate that SCELI is also of general applicability in molecule-semiconductor interfaces. We do so by following the quantum dynamics induced by non-resonant few-cycle laser pulses of intermediate intensity (non-perturbative but non-ionizing) across model molecule-semiconductor interfaces of varying level alignments. We show that SCELI induces interfacial charge transfer regardless of the energy level alignment of the interface and even in situations where charge exchange is forbidden via resonant photoexcitation. We further show that the SCELI rate of charge transfer is faster than those offered by resonant photoexcitation routes as it is controlled by the subcycle structure of light. The results underscore the general applicability of SCELI to manipulate electron dynamics at interfaces on ultrafast timescales.

Encryption Using Optical Pseudo-Random Binary Sequence Based on Optical Logic Gate

In this paper, we propose a scheme for high-speed all-optical Pseudo-Random Binary Sequence (PRBS) generator and use it for generating keystream for encryption. This PRBS generator design is based on Linear Feedback Shift Registers (LFSR) and optical XOR and AND gates. The optical logical gates are based on quantum dot-semiconductor optical amplifier Mach-Zehnder interferometer (QD-SOA-MZI). With two photon absorption (TPA) in quantum dot-semiconductor optical amplifier (QD-SOA), this kind of optical logic gates performs well when processing data in an ultra-fast timescale and therefore able to function as high speed PRBS generator. Result shows that it’s possible for this scheme to realize all-optical encryption and decryption at high process rate up to 320 Gb/s. We simulated different ways of generating keystream with schemes such as cascaded generator, parallel generator and alternating step generator. These generators use more than one LFSR. Result shows that the schemes we use can function as stable and complex keystream generators.

Modeling the Unimolecular Decay Dynamics of the Fluorinated Criegee Intermediate, CF3CHOO

When volatile alkenes are emitted into the atmosphere, they are rapidly removed by oxidizing agents such as hydroxyl radicals and ozone. The latter reaction is termed ozonolysis and is an important source of Criegee intermediates (CIs), i.e., carbonyl oxides, that are implicated in enhancing the oxidizing capacity of the troposphere. These CIs aid in the formation of lower volatility compounds that typically condense to form secondary organic aerosols. CIs have attracted vast attention over the past two decades. Despite this, the effect of their substitution on the ground and excited state chemistries of CIs is not well studied. Here, we extend our knowledge obtained from prior studies on CIs by CF3 substitution. The resulting CF3CHOO molecule is a CI that can be formed from the ozonolysis of hydrofluoroolefins (HFOs). Our results show that the ground state unimolecular decay should be less reactive and thus more persistent in the atmosphere than the non-fluorinated analog. The excited state dynamics, however, are predicted to occur on an ultrafast timescale. The results are discussed in the context of the ways in which our study could advance synthetic chemistry, as well as processes relevant to the atmosphere.

Light-Induced Metastable Hidden Skyrmion Phase in the Mott Insulator Cu2 OSeO3.

The discovery of a novel long-lived metastable skyrmion phase in the multiferroic insulator Cu2 OSeO3 visualized with Lorentz transmission electron microscopy for magnetic fields below the equilibrium skyrmion pocket is reported. This phase can be accessed by exciting the sample non-adiabatically with near-infrared femtosecond laser pulses and cannot be reached by any conventional field-cooling protocol, referred as a hidden phase. From the strong wavelength dependence of the photocreation process and via spin-dynamics simulations, the magnetoelastic effect is identified as the most likely photocreation mechanism. This effect results in a transient modification of the magnetic free energy landscape extending the equilibrium skyrmion pocket to lower magnetic fields. The evolution of the photoinduced phase is monitored for over 15 min and no decay is found. Because such a time is much longer than the duration of any transient effect induced by a laser pulse in a material, it is assumed that the newly discovered skyrmion state is stable for practical purposes, thus breaking ground for a novel approach to control magnetic state on demand at ultrafast timescales and drastically reducing heat dissipation relevant for next-generation spintronicdevices.

Sub-picosecond collapse of molecular polaritons to pure molecular transition in plasmonic photoswitch-nanoantennas

Molecular polaritons are hybrid light-matter states that emerge when a molecular transition strongly interacts with photons in a resonator. At optical frequencies, this interaction unlocks a way to explore and control new chemical phenomena at the nanoscale. Achieving such control at ultrafast timescales, however, is an outstanding challenge, as it requires a deep understanding of the dynamics of the collectively coupled molecular excitation and the light modes. Here, we investigate the dynamics of collective polariton states, realized by coupling molecular photoswitches to optically anisotropic plasmonic nanoantennas. Pump-probe experiments reveal an ultrafast collapse of polaritons to pure molecular transition triggered by femtosecond-pulse excitation at room temperature. Through a synergistic combination of experiments and quantum mechanical modelling, we show that the response of the system is governed by intramolecular dynamics, occurring one order of magnitude faster with respect to the uncoupled excited molecule relaxation to the ground state.

Ultrafast response of Ge2Sb2Te5 nanoparticles: The benefits of low energy amorphization switching with the same read/write speed of bulk memories

We investigate the ultrafast response of crystalline Ge2Sb2Te5 nanoparticles (NPs) below the phase transformation threshold fluence. The observed rapid change of the optical response and the presence of coherent optical phonons are consistent with the relaxation dynamics in bulk Ge2Sb2Te5 films and, more importantly, occur within the same ultrafast timescales. We conclude that the benefit of the lower energy consumption of memories based on Ge-Sb-Te (GST) NPs aggregates, demonstrated by Casarin et al. (2018), occurs with no disadvantage, as the read/write speed can be as fast as in bulk GSTs.

The Role of Energy Offsets on Charge Photogeneration Dynamics in Y‐Series Molecules‐Based Polymer Solar Cells

Recent research has revealed that low‐energy offset polymer solar cells (PSCs) are capable of a power conversion efficiency of over 19%. However, it is unclear how energy offsets and the charge photogeneration process are correlated. Herein, the effect of energy offsets on charge photogeneration dynamics for Y‐series molecules (Y5, Y6, Y10, and BTP‐4F‐12)‐based PSCs with the variations of the lowest unoccupied molecular orbital energy offsets (ΔELUMO) of 0.11–0.42 eV and the highest occupied molecular orbital energy offsets (ΔEHOMO) of 0.08–0.23 eV utilizing steady‐state and time‐resolved spectroscopies is studied. The steady‐state measurement shows that the probability of photoluminescence quenching via energy transfer for the donor exciton reduces with the increasing ΔELUMO. It is found that even in PM6:Y6 with the highest ΔELUMO, ≈18% of PM6 exciton dissociated via the path of “energy transfer first and then hole transfer,” manifesting the energy transfer also plays a vital role in the process of exciton dissociation. Furthermore, it is found that the PM6 exciton can efficiently dissociate under the ΔELUMO of 0.11 eV. After photoexcitation of the Y‐series molecule acceptors, the exciton dissociation efficiency enhances with the increase of ΔEHOMO. Besides, the higher energy offsets, the lower charge recombination rate in the ultrafast timescale has been found from the transient absorption measurement. These findings reveal that energy offsets are important for charge photogeneration and recombination in an ultrafast timescale for Y‐series molecule‐based PSCs, which may shed light on the design of high‐performance PSCs.

Ultrafast Optical Control of Polariton Energy in an Organic Semiconductor Microcavity

AbstractThe manipulation of exciton–polaritons and their condensates is of great interest due to their applications in polariton simulators and high‐speed, all‐optical logic devices. Until now, methods of trapping and manipulating such condensates are not dynamically reconfigurable or resulted in an undesirable reduction in the exciton–photon coupling strength. Here, a new strategy for the ultrafast control of polariton resonances via transient modification of an optical cavity mode is presented. Multilayer organic semiconductor microcavities that contain two absorbers are constructed: one strongly‐coupled to the cavity photon mode and one that is out‐of‐resonance. By selectively exciting the out‐of‐resonance absorber using ultrashort laser pulses, the cavity refractive index is modulated, and fully‐reversible blueshifts of the lower polariton branch by up to 8 meV in sub‐ps timescales with no corresponding reduction in the exciton–photon coupling strength are generated. This work demonstrates the ability to manipulate polariton energy landscapes over ultrafast timescales with important applications in emerging computing technologies.

Ultrafast enhancement of electron-phonon coupling via dynamic quantum well states

The density of states at the Fermi surface controls many material properties by influencing the low-energy interactions of conductive electrons. Typically, external tuning knobs generate only small perturbations to the density of states in crystals, since their band structure depends strongly on the lattice potential. In contrast, quantum well states are contingent on a localized potential extrinsic to the crystal lattice and can be easily modified leading to changes in the density of states at the Fermi level. Here, we are able to control the quantum well potential on the surface of Bi2Se3 with light, driving a density of states singularity below EF at ultrafast timescales, thereby triggering a reversible Lifshitz transition. We reveal a substantial ultrafast enhancement of the electron-phonon coupling with repercussions on the relaxation dynamics in the system. We argue that the relaxation dynamics are governed largely by the interplay of the dynamic density of states of the quantum wells with c-axis scattering from the {A}_{{{{{{{{rm{1g}}}}}}}}}^{2} optical phonon mode. These results demonstrate a powerful way to enhance electron-boson interaction on ultrafast timescales providing new avenues for controlling material properties and driving novel quantum phases of matter.

Deciphering Methylation Effects on S2(ππ*) Internal Conversion in the Simplest Linear α,β-Unsaturated Carbonyl.

Chemical substituents can influence photodynamics by altering the location of critical points and the topography of the potential energy surfaces (electronic effect) and by selectively modifying the inertia of specific nuclear modes (inertial effects). Using nonadiabatic dynamics simulations, we investigate the impact of methylation on S2(ππ*) internal conversion in acrolein, the simplest linear α,β-unsaturated carbonyl. Consistent with time constants reported in a previous time-resolved photoelectron spectroscopy study, S2 → S1 deactivation occurs on an ultrafast time scale (∼50 fs). However, our simulations do not corroborate the sequential decay model used to fit the experiment. Instead, upon reaching the S1 state, the wavepacket bifurcates: a portion undergoes ballistic S1 → S0 deactivation (∼90 fs) mediated by fast bond-length alternation motion, while the remaining decays on the picosecond time scale. Our analysis reveals that methyl substitution, generally assumed to mainly exert inertial influence, is also manifested in important electronic effects due to its weak electron-donating ability. While methylation at the β C atom gives rise to effects principally of an inertial nature, such as retarding the twisting motion of the terminal -CHCH3 group and increasing its coupling with pyramidalization, methylation at the α or carbonyl C atom modifies the potential energy surfaces in a way that also contributes to altering the late S1-decay behavior. Specifically, our results suggest that the observed slowing of the picosecond component upon α-methylation is a consequence of a tighter surface and reduced amplitude along the central pyramidalization, effectively restricting the access to the S1/S0-intersection seam. Our work offers new insight into the S2(ππ*) internal conversion mechanisms in acrolein and its methylated derivatives and highlights site-selective methylation as a tuning knob to manipulate photochemical reactions.

Imaging and controlling coherent phonon wave packets in single graphene nanoribbons

The motion of atoms is at the heart of any chemical or structural transformation in molecules and materials. Upon activation of this motion by an external source, several (usually many) vibrational modes can be coherently coupled, thus facilitating the chemical or structural phase transformation. These coherent dynamics occur on the ultrafast timescale, as revealed, e.g., by nonlocal ultrafast vibrational spectroscopic measurements in bulk molecular ensembles and solids. Tracking and controlling vibrational coherences locally at the atomic and molecular scales is, however, much more challenging and in fact has remained elusive so far. Here, we demonstrate that the vibrational coherences induced by broadband laser pulses on a single graphene nanoribbon (GNR) can be probed by femtosecond coherent anti-Stokes Raman spectroscopy (CARS) when performed in a scanning tunnelling microscope (STM). In addition to determining dephasing (~440 fs) and population decay times (~1.8 ps) of the generated phonon wave packets, we are able to track and control the corresponding quantum coherences, which we show to evolve on time scales as short as ~70 fs. We demonstrate that a two-dimensional frequency correlation spectrum unequivocally reveals the quantum couplings between different phonon modes in the GNR.