Experimental Coherent Laser Control of Physicochemical Processes

Determining Key Factors for the Open-Loop Control of Molecular Fragmentation Using Shaped Strong Fields

Pulse shaping has long been employed for tailoring femtosecond laser pulses to study and control the fragmentation of polyatomic molecules. In many cases, a physical explanation connecting the properties of the field to the observed control is difficult to ascertain. We utilized 80 bit binary spectral phase functions to parametrize and map the search space, gaining insight into which pulse parameters most impact the ion yield and fragmentation pattern for the relatively large triethylamine [N(C2H5)3] molecule. Pulse structures used to control the m/z 86 branching ratio beyond a simple intensity dependence are identified and compared to pump-probe results. All of these findings are explained in terms of control via a dissociative Rydberg state in the neutral molecule. This methodology may be used to discover new control mechanisms and shed light onto which pulse parameters most influence the interaction between strong field lasers and matter.

Strong-field quantum control in the extreme ultraviolet domain using pulse shaping

Tailored light-matter interactions in the strong coupling regime enable the manipulation and control of quantum systems with up to unit efficiency1,2, with applications ranging from quantum information to photochemistry3-7. Although strong light-matter interactions are readily induced at the valence electron level using long-wavelength radiation8, comparable phenomena have been only recently observed with short wavelengths, accessing highly excited multi-electron and inner-shell electron states9,10. However, the quantum control of strong-field processes at short wavelengths has not been possible, so far, because of the lack of pulse-shaping technologies in the extreme ultraviolet (XUV) and X-ray domain. Here, exploiting pulse shaping of the seeded free-electron laser (FEL) FERMI, we demonstrate the strong-field quantum control of ultrafast Rabi dynamics in helium atoms with high fidelity. Our approach reveals a strong dressing of the ionization continuum, otherwise elusive to experimental observables. The latter is exploited to achieve control of the total ionization rate, with prospective applications in many XUV and soft X-ray experiments. Leveraging recent advances in intense few-femtosecond to attosecond XUV to soft X-ray light sources, our results open an avenue to the efficient manipulation and selective control of core electron processes and electron correlation phenomena in real time.

Optical Quantum Control of the Electron Transfer Reactions in Protein Flavodoxin

The optical quantum control has been successfully applied in modulating biological processes such as energy transfer and bond isomerization. Among the reactions in realizing biological functions, the electron transfer (ET) process is fundamental; hence, the quantum control over such an ET reaction is of far-reaching significance. Here, we realized optical quantum control over ultrafast ET processes in a protein, flavodoxin, by applying various chirped excitation pulses. We observed the wavepacket dynamics within a dephasing time of less than 1 ps. Within this time window, we found that the ultrafast photoinduced ET reaction can be controlled by different chirped excitations with a rate change by a factor of about 2. Furthermore, the control effect is propagated into the subsequent ultrafast back ET reaction, showing a variation of the BET dynamics with different excitation chirps. The underlying mechanism is the initial wavepacket dynamics; the differently prepared wavepackets with chirped excitation evolve along various pathways, resulting in the changes of ET rates. The successful demonstration of optical quantum control of ultrafast biological ET is significant and opens a new avenue to explore the quantum control of real biological ET reactions.

Coherence in Chemistry: Foundations and Frontiers

Coherence refers to correlations in waves. Because matter has a wave-particle nature, it is unsurprising that coherence has deep connections with the most contemporary issues in chemistry research (e.g., energy harvesting, femtosecond spectroscopy, molecular qubits and more). But what does the word "coherence" really mean in the context of molecules and other quantum systems? We provide a review of key concepts, definitions, and methodologies, surrounding coherence phenomena in chemistry, and we describe how the terms "coherence" and "quantum coherence" refer to many different phenomena in chemistry. Moreover, we show how these notions are related to the concept of an interference pattern. Coherence phenomena are indeed complex, and ambiguous definitions may spawn confusion. By describing the many definitions and contexts for coherence in the molecular sciences, we aim to enhance understanding and communication in this broad and active area of chemistry.

Weak-Field Coherent Control of Ultrafast Molecule Making

Coherent control of ultrafast molecule making from colliding reactants is crucial for realizing coherent control of binary photoreactions (CCBP). To handle diverse excitation scenarios, feasibility with both weak and strong fields is essential. We experimentally demonstrate here the weak-field feasibility, achieving it even under thermally hot conditions typical of chemical reactions. The making of KAr complexes from hot pairs of colliding K and Ar atoms via resonance-mediated two-photon excitation is controlled by weak linearly chirped femtosecond pulses. Negative chirps enhance the yield. Our experimental and ab initio theoretical results are in excellent agreement. New routes to CCBP are opened.

Simulating the Excited-State Dynamics of Polaritons with Ab Initio Multiple Spawning

Over the past decade, there has been a growth of interest in polaritonic chemistry, where the formation of hybrid light-matter states (polaritons) can alter the course of photochemical reactions. These hybrid states are created by strong coupling between molecules and photons in resonant optical cavities and can even occur in the absence of light when the molecule is strongly coupled with the electromagnetic fluctuations of the vacuum field. We present a first-principles model to simulate nonadiabatic dynamics of such polaritonic states inside optical cavities by leveraging graphical processing units (GPUs). Our first implementation of this model is specialized for a single molecule coupled to a single-photon mode confined inside the optical cavity but with any number of excited states computed using complete active space configuration interaction (CASCI) and a Jaynes-Cummings-type Hamiltonian. Using this model, we have simulated the excited-state dynamics of a single salicylideneaniline (SA) molecule strongly coupled to a cavity photon with the ab initio multiple spawning (AIMS) method. We demonstrate how the branching ratios of the photodeactivation pathways for this molecule can be manipulated by coupling to the cavity. We also show how one can stop the photoreaction from happening inside of an optical cavity. Finally, we also investigate cavity-based control of the ordering of two excited states (one optically bright and the other optically dark) inside a cavity for a set of molecules, where the dark and bright states are close in energy.

Optical Control of Crossing the Conical Intersection in β-Carotene

Optical control of dynamic processes has been challenging yet has only been demonstrated in several chemical and biological systems. The control of a reaction passing the widely present conical intersection has not been realized. Here, we modulated the phase of the excitation pulse to control the dynamics of β-carotene through accessing the conical intersection (CI). We observed different dynamics in 110-220 fs into the CI and the consecutive process in 400-600 fs through another CI by various chirped excitation pulses. We successfully controlled those ultrafast wavepacket dynamics passing the CIs on the femtosecond time scales. The method developed here can be used to control a various of ultrafast chemical and biological reactions through the CI(s).

Scoring molecular wires subject to an ultrafast laser pulse for molecular electronic devices

A nonionizing ultrafast laser pulse of 20-fs duration with a peak amplitude electric-field ±E = 200 × 10^-4 a.u. was simulated. It was applied to the ethene molecule to consider its effect on the electron dynamics, both during the application of the laser pulse and for up to 100 fs after the pulse was switched off. Four laser pulse frequencies ω = 0.2692, 0.2808, 0.2830, and 0.2900 a.u. were chosen to correspond to excitation energies mid-way between the (S₁ ,S₂ ), (S₂ ,S₃ ), (S₃ ,S₄ ) and (S₄ ,S₅ ) electronic states, respectively. Scalar quantum theory of atoms in molecules (QTAIM) was used to quantify the shifts of the C1C2 bond critical points (BCPs). Depending on the frequencies ω selected, the C1C2 BCP shifts were up to 5.8 times higher after the pulse was switched off compared with a static E-field with the same magnitude. Next generation QTAIM (NG-QTAIM) was used to visualize and quantify the directional chemical character. In particular, polarization effects and bond strengths, in the form of bond-rigidity vs. bond-flexibility, were found, for some laser pulse frequencies, to increase after the laser pulse was switched off. Our analysis demonstrates that NG-QTAIM, in partnership with ultrafast laser irradiation, is useful as a tool in the emerging field of ultrafast electron dynamics, which will be essential for the design, and control of molecular electronic devices.

Intense femtosecond optical pulse shaping approaches to spatiotemporal control

For studying any event, measurement can never be enough; "control" is required. This means mere passive tracking of the event is insufficient and being able to manipulate it is necessary. To maximize this capability to exert control and manipulate, both spatial and temporal domains need to be jointly accounted for, which has remained an intractable problem at microscopic scales. Simultaneous control of dynamics and position of an observable event requires a holistic combination of spatial and temporal control principles, which gives rise to the field of spatiotemporal control. For this, we present a novel femtosecond pulse-shaping approach. We explain how to achieve spatiotemporal control by spatially manipulating the system through trapping and subsequently or simultaneously exerting temporal control using shaped femtosecond pulses. By leveraging ultrafast femtosecond lasers, the prospect of having temporal control of molecular dynamics increases, and it becomes possible to circumvent the relaxation processes at microscopic timescales. Optical trapping is an exemplary demonstration of spatial control that results in the immobilization of microscopic objects with radiation pressure from a tightly focused laser beam. Conventional single-beam optical tweezers use continuous-wave (CW) lasers for achieving spatial control through photon fluxes, but these lack temporal control knobs. We use a femtosecond high repetition rate (HRR) pulsed laser to bypass this lack of dynamical control in the time domain for optical trapping studies. From a technological viewpoint, the high photon flux requirement of stable optical tweezers necessitates femtosecond pulse shaping at HRR, which has been a barrier until the recent Megahertz pulse shaping developments. Finally, recognizing the theoretical distinction between tweezers with femtosecond pulses and CW lasers is of paramount interest. Non-linear optical (NLO) interactions must be included prima facie to understand pulsed laser tweezers in areas where they excel, like the two-photon-fluorescence-based detection. We show that our theoretical model can holistically address the common drawback of all tweezers. We are able to mitigate the effects of laser-induced heating by balancing this with femtosecond laser-induced NLO effects. An interesting side-product of HRR femtosecond-laser-induced thermal lens is the development of femtosecond thermal lens spectroscopy (FTLS) and its ability to provide sensitive molecular detection.

Harnessing deep reinforcement learning to construct time-dependent optimal fields for quantum control dynamics

We present an efficient deep reinforcement learning (DRL) approach to automatically construct time-dependent optimal control fields that enable desired transitions in dynamical chemical systems. Our DRL approach gives impressive performance in constructing optimal control fields, even for cases that are difficult to converge with existing gradient-based approaches. We provide a detailed description of the algorithms and hyperparameters as well as performance metrics for our DRL-based approach. Our results demonstrate that DRL can be employed as an effective artificial intelligence approach to efficiently and autonomously design control fields in quantum dynamical chemical systems.

Study of the effect of higher-order dispersions on photoionisation induced by ultrafast laser pulses applying a classical theoretical method

We investigated the effect of higher order dispersion on ultrafast photoionisation with Classical Trajectory Monte Carlo (CTMC) method for hydrogen and krypton atoms. In our calculations we used linearly polarised ultrashort 7 fs laser pulses, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$6.5 \times 10^{14} \mathrm {W/cm^{2}}$$\end{document}6.5×1014W/cm2 intensity, and a central wavelength of 800 nm. Our results show that electrons with the highest kinetic energies are obtained with transform limited (TL) pulses. The shaping of the pulses with negative second- third- or fourth- order dispersion results in higher ionisation yield and electron energies compared to pulses shaped with positive dispersion values. We have also investigated how the Carrier Envelope Phase (CEP) dependence of the ionisation is infuenced by dispersion. We calculated the left-right asymmetry as a function of energy and CEP for sodium atoms employing pulses of 4.5 fs, 800 nm central wavelength, and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$4 \times 10^{12}\mathrm {W/cm^{2}}$$\end{document}4×1012W/cm2 intensity. We found that the left-right asymmetry is more pronounced for pulses shaped with positive Group Delay Dispersion (GDD). It was also found that shaping a pulse with increasing amounts of GDD in absolute value blurs the CEP dependence, which is attributed to the increasing number of optical cycles.

Laser control of a dark vibrational state of acetylene in the gas phase—Fourier transform pulse shaping constraints and effects of decoherence

We propose a methodology to tackle the laser control of a non-stationary dark ro-vibrational state of acetylene (C2H2), given realistic experimental limitations in the 7.7 μm (1300 cm−1) region. Simulations are performed using the Lindblad master equation, where the so-called Lindblad parameters are used to describe the effect of the environment in the dilute gas phase. A phenomenological representation of the parameters is used, and they are extracted from high-resolution spectroscopy line broadening data. An effective Hamiltonian is used for the description of the system down to the rotational level close to experimental accuracy. The quality of both the Hamiltonian and Lindblad parameters is assessed by a comparison of a calculated infrared spectrum with the available experimental data. A single shaped laser pulse is used to perform the control, where elements of optics and pulse shaping using masks are introduced with emphasis on experimental limitations. The optimization procedure, based on gradients, explicitly takes into account the experimental constraints. Control performances are reported for shaping masks of increasing complexity. Although modest performances are obtained, mainly due to the strong pulse shaping constraints, we gain insights into the control mechanism. This work is the first step toward the conception of a realistic experiment that will allow for population characterization and manipulation of a non-stationary vibrational “dark” state. Effects of the collisions on the laser control in the dilute gas phase, leading to decoherence in the molecular system, are clearly shown.

Resolving plasmonic hotspots by label-free super-resolution microscopy

The plasmonic hotspot of metal nanostructures has small dimension far beyond the optical diffraction limit. When trying to locate the hotspot using fluorescent probes, the localization is significantly distorted due to the coupling of emission and surface plasmon. A label-free technique can solve the problem, which uses hotspot emission as the native probe. We demonstrate a super-resolution microscopy investigation based on this idea. By modulating hotspot emission of crossed silver nanowires, which have a pair of plasmonic hotspots approximately 100 nm apart at the intersection, we precisely locate and separate them with nanometer precision. This label-free technique could be applied for analyzing hotspot distribution with high efficiency and precision.

Phase-only femtosecond optical pulse shaping based on an all-dielectric polarization-insensitive metasurface

Recently, metasurfaces capable of manipulating the amplitude and the phase of an incident wave in a broad frequency band have been employed for femtosecond optical pulse shaping purposes. In this study, we introduce a phase-only pulse shaper based on an all-dielectric CMOS-compatible polarization-insensitive metasurface, composed of Si nano cylinders sitting on a fused silica substrate. The required phase profile of the metasurface for desired waveforms are calculated using an iterative Fourier transform algorithm, and the performance of the pulse shaper metasurface in implementing the phase masks was assessed using full-wave simulations. Such approach for realizing a polarization-insensitive metasurface-based phase-only pulse shaper has never been investigated to the best of our knowledge. It is demonstrated that the simulated results of the proposed metasurface-based pulse shaper is in great agreement with the results of the algorithm, while exhibiting a very high transmission efficiency. This work indicates yet another exciting but not fully examined application of meta-structures that is the optical pulse shaping.

Investigation of photoassociation with full-dimensional thermal-random-phase wavefunctions

By taking the femtosecond two-photon photoassociation (PA) of magnesium atoms as an example, we propose a method to calculate the thermally averaged population, which is transferred from the ground X¹Σ_g ⁺ state to the target (1)¹Π_g state, based on the solution of full-dimensional time-dependent Schrödinger equation. In this method, named as method A, we use thermal-random-phase wavefunctions with the random phases expanded in both the vibrational and rotational degrees of freedom to model the thermal ensemble of the initial eigenstates. This method is compared with the other two methods (B and C) at different temperatures. Method B is also based on thermal-random-phase wavefunctions, except that the random-phase expansion is merely used for the vibrational degree of freedom. Method C is based on the independent propagation of every initial eigenstate, instead of the thermal-random-phase wavefunctions. Taking the (1)¹Π_g state as the target state, it is found that although these three methods can present the same population on the (1)¹Π_g state, the computation efficiency of method A increases dramatically with the increase in temperature. With this efficient method A, we find that the PA process at 1000 K can also induce rotational coherence, i.e., the molecular field-free alignment in the excited electronic states.

On the quantum and classical control of laser-driven isomerization in the Wigner representation

We investigate the validity of the classical approximation to the numerically exact quantum dynamics for infrared laser-driven control of isomerization processes. To this end, we simulate the fully quantum mechanical dynamics both by wavepacket propagation in position space and by propagating the Wigner function in phase space employing a quantum-mechanical correction term. A systematic comparison is made with purely classical propagation of the Wigner function. On the example of a one-dimensional double well potential, we identify two complementary classes of pulse sequences that invoke either a quantum mechanically or a classically dominated control mechanism. The quantum control relies on a sequence of excitations and de-excitations between the system's eigenstates on a time scale far exceeding the characteristic vibrational oscillation periods. In contrast, the classical control mechanism is based on a short and strong few-cycle field exerting classical-like forces driving the wavepacket to the target potential well where it is slowed down and finally trapped. While in the first case, only the quantum mechanical propagation correctly describes the field-induced population transfer, the short pulse case is also amenable to a purely classical description. These findings shed light on the applicability of classical approximations to simulate laser-controlled dynamics and may offer a guideline for novel control experiments in more complex systems that can be analyzed and interpreted utilizing efficient state-of-the-art classical trajectory simulations based on ab initio molecular dynamics.

Selective Enhancement of Spectroscopic Features by Quantum Optimal Control

Tailored light can be used to steer atomic motions into selected quantum pathways. In optimal control theory (OCT), the target is usually expressed in terms of the molecular wave function, a quantity that is not directly observable in experiment. We present simulations using OCT that optimize the spectroscopic signal itself. By shaping the optical pump, the x-ray stimulated Raman signal, which occurs solely during the passage through conical intersections, is temporally controlled and amplified by up to 2 orders of magnitude. This enhancement can be crucial in order to bring small coherence-based signatures above the detectable threshold. Our approach is applicable to any signal that depends on the expectation value of a positive definite operator.

Harnessing deep neural networks to solve inverse problems in quantum dynamics: machine-learned predictions of time-dependent optimal control fields

Inverse problems continue to garner immense interest in the physical sciences, particularly in the context of controlling desired phenomena in non-equilibrium systems. In this work, we utilize a series of deep neural networks for predicting time-dependent optimal control fields, E(t), that enable desired electronic transitions in reduced-dimensional quantum dynamical systems. To solve this inverse problem, we investigated two independent machine learning approaches: (1) a feedforward neural network for predicting the frequency and amplitude content of the power spectrum in the frequency domain (i.e., the Fourier transform of E(t)), and (2) a cross-correlation neural network approach for directly predicting E(t) in the time domain. Both of these machine learning methods give complementary approaches for probing the underlying quantum dynamics and also exhibit impressive performance in accurately predicting both the frequency and strength of the optimal control field. We provide detailed architectures and hyperparameters for these deep neural networks as well as performance metrics for each of our machine-learned models. From these results, we show that machine learning, particularly deep neural networks, can be employed as cost-effective statistical approaches for designing electromagnetic fields to enable desired transitions in these quantum dynamical systems.

Next-generation QTAIM for scoring molecular wires in E-fields for molecular electronic devices

The effect of a varying, directional E _x , E _y , and E _z electric field on the ethene molecule was investigated using next-generation quantum theory of atoms in molecules (QTAIM). Despite using low E-field strengths that are within the realm of experiment and do not measurably alter the molecular geometry, significant changes to the QTAIM properties were observed. Using conventional QTAIM, the shifting of the C─C and C─H bond critical points (BCPs) demonstrates polarization through an interchange in the size of the atoms involved in a bond, since a BCP is located on the boundary between a pair of bonded atoms. Next-generation QTAIM, however, demonstrates the polarization effect more directly with a change in morphology of the 3-D envelope around the BCP. Modest increases of ≈ 2% in the ellipticity ε of the BCP were uncovered when the C─C bond was aligned parallel or anti-parallel to the applied E _x -field. Significant asymmetries were found in the response of the next-generation QTAIM 3-D paths of the C─H bonds to the applied E-field. When the E-field coincided with the C─C bond, the BCP moved in response and was accompanied by the envelope constructed from 3-D next-generation paths. The response displayed a polarization effect that increased with increasing magnitude of the E _x -field parallel and anti-parallel to the C─C bond. Our analysis demonstrates that next-generation QTAIM is a useful tool for understanding the response of molecules to E-fields, for example, for the screening of molecular wires for the design of molecular electronic devices. © 2019 Wiley Periodicals, Inc.

Super-resolution nanoscopy by coherent control on nanoparticle emission

Super-resolution nanoscopy based on wide-field microscopic imaging provided high efficiency but limited resolution. Here, we demonstrate a general strategy to push its resolution down to ~50 nm, which is close to the range of single molecular localization microscopy, without sacrificing the wide-field imaging advantage. It is done by actively and simultaneously modulating the characteristic emission of each individual emitter at high density. This method is based on the principle of excited state coherent control on single-particle two-photon fluorescence. In addition, the modulation efficiently suppresses the noise for imaging. The capability of the method is verified both in simulation and in experiments on ZnCdS quantum dot-labeled films and COS7 cells. The principle of coherent control is generally applicable to single-multiphoton imaging and various probes.

Supersymmetry in the time domain and its applications in optics

Supersymmetry is a conjectured symmetry between bosons and fermions aiming at solving fundamental questions in string and quantum field theory. Its subsequent application to quantum mechanics led to a ground-breaking analysis and design machinery, later fruitfully extrapolated to photonics. In all cases, the algebraic transformations of quantum-mechanical supersymmetry were conceived in the space realm. Here, we demonstrate that Maxwell's equations, as well as the acoustic and elastic wave equations, also possess an underlying supersymmetry in the time domain. We explore the consequences of this property in the field of optics, obtaining a simple analytic relation between the scattering coefficients of numerous time-varying systems, and uncovering a wide class of reflectionless, three dimensional, all-dielectric, isotropic, omnidirectional, polarisation-independent, non-complex media. Temporal supersymmetry is also shown to arise in dispersive media supporting temporal bound states, which allows engineering their momentum spectra and dispersive properties. These unprecedented features may enable the creation of novel reconfigurable devices, including invisible materials, frequency shifters, isolators, and pulse-shape transformers.

The destabilization of hydrogen bonds in an external E-field for improved switch performance

The effect of an electric field on a recently proposed molecular switch based on a quinone analogue was investigated using next-generation quantum theory of atoms in molecules (QTAIM) methodology. The reversal of a homogenous external electric field was demonstrated to improve the "OFF" functioning of the switch. This was achieved by destabilization of the H atom participating in the tautomerization process along the hydrogen bond that defines the switch. The "ON" functioning of the switch, from the position of the tautomerization barrier, is also improved by the reversal of the homogenous external electric field: this result was previously inaccessible. The "ON" and "OFF" functioning of the switch was visualized in terms of the response of the most preferred directions of motion of the electronic charge density to the applied external field. All measures from QTAIM and the stress tensor provide consistent results for the factors affecting the "ON" and "OFF" switch performance. Our analysis therefore demonstrates use for future design of molecular electronic devices. © 2019 Wiley Periodicals, Inc.

Pathways to New Applications for Quantum Control

In 1998, the first successful quantum control experiment with application to a molecular framework was conducted with a shaped laser pulse, optimizing the branching ratio between different organometallic reaction channels. This work induced a vast activity in quantum control during the next 10 years, and different optimization aims were achieved in the gas phase, liquid phase, and even in biologically relevant molecules like light-harvesting complexes. Accompanying and preceding this development were important advances in theoretical quantum control simulations. They predicted several control scenarios and explained how and why quantum control experiments work. After many successful proofs of concept in molecular science, the big challenge is to expand its huge conceptual potential of directly being able to steer nuclear and/or electronic motion to more applied implementations. In this Account, based on several recent advances, we give a personal evaluation of where the field of molecular quantum control is at the moment and especially where we think promising applications can be in the near future. One of these paths leads to synthetic chemistry. The synthesis of novel pharmaceutical compounds or natural products often involves many synthetic steps, each one devouring resources and lowering the product yield. Shaped laser pulses can possibly act as photonic reagents and shorten the synthetic route toward the desired product. Chemical synthesis usually takes place in solution, and by including explicit solvent molecules in our quantum control simulations, we were able to identify their highly inhomogeneous influence on chemical reactions and how this affects potential quantum control. More important, we demonstrated for a synthetically relevant example that these complications can be overcome in theory, and laser pulses can be optimized to initiate the desired carbon-carbon bond formation. Putting this into context with the recently emerging concept of flow chemistry, which brings several practical advantages to the application of laser pulses, we want to encourage experimental groups to exploit this concept. Another path was opened by several additions to the commonly used laser pulse optimization algorithm (optimal control theory, OCT), several of which were developed in our group. The OCT algorithm as such is a purely mathematical optimization procedure, with no direct relation to experimental requirements. This means that usually the electric fields obtained out of OCT optimizations do not resemble laser pulses that can be achieved experimentally. However, the previously mentioned additions are aimed at closing the gap toward the experiment. In a recent quantum control study of our group, these algorithmic developments came to fruition. We were able to suggest a shaped laser pulse which can induce a long-living wave packet in the excited state of uracil. This might pave the way for novel experiments dedicated to investigating the formation of biological photodamage in DNA and RNA. The pulse we suggest is surprisingly simple because of the extended OCT algorithm and fulfills all criteria to be experimentally accessible.

Temporally shaped Laguerre-Gaussian femtosecond laser beams

Femtosecond vortex beams with adjustable temporal pulse shapes are generated. These shaped laser pulses are characterized in the spectral domain by determination of the spectral amplitude and phase as well as in the spatial domain by expansion of the beam profile in a superposition of Laguerre-Gaussian transversal laser modes. The experiments demonstrate that the temporal pulse shapes impressed with a pulse shaper based on a programmable liquid-crystal spatial light modulator are basically unaltered by subsequent transmission through a spiral phase plate, while a high-quality optical vortex is imposed. The combination of programmable pulse shapes and optical vortices in femtosecond laser beams opens new possibilities for applications in micromachining, high harmonic generation, and microscopy.

Multi-mW, few-cycle mid-infrared continuum spanning from 500 to 2250 cm-1

The demand for and usage of broadband coherent mid-infrared sources, such as those provided by synchrotron facilities, are growing. Since most organic molecules exhibit characteristic vibrational modes in the wavelength range between 500 and 4000 cm^-1, such broadband coherent sources enable micro- or even nano-spectroscopic applications at or below the diffraction limit with a high signal-to-noise ratio^{1, 2, 3}. These techniques have been applied in diverse fields ranging from life sciences, material analysis, and time-resolved spectroscopy. Here we demonstrate a broadband, coherent and intrinsically carrier-envelope-phase-stable source with a spectrum spanning from 500 to 2250 cm^-1 (-30 dB) at an average power of 24 mW and a repetition rate of 77 MHz. This performance is enabled by the first mode-locked thin-disk oscillator operating at 2 μm wavelength, providing a tenfold increase in average power over femtosecond oscillators previously demonstrated in this wavelength range⁴. Multi-octave spectral coverage from this compact and power-scalable system opens up a range of time- and frequency-domain spectroscopic applications.

Coherent control of an opsin in living brain tissue

Retinal-based opsins are light-sensitive proteins. The photoisomerization reaction of these proteins has been studied outside cellular environments using ultrashort tailored light pulses^1-5. However, how living cell functions can be modulated via opsins by modifying fundamental nonlinear optical properties of light interacting with the retinal chromophore has remained largely unexplored. We report the use of chirped ultrashort near-infrared pulses to modulate light-evoked ionic current from Channelrhodopsin-2 (ChR2) in brain tissue, and consequently the firing pattern of neurons, by manipulating the phase of the spectral components of the light. These results confirm that quantum coherence of the retinal-based protein system, even in a living neuron, can influence its current output, and open up the possibilities of using designer-tailored pulses for controlling molecular dynamics of opsins in living tissue to selectively enhance or suppress neuronal function for adaptive feedback-loop applications in the future.

Controlled modification of biomolecules by ultrashort laser pulses in polar liquids

Targeted chemical modification of peptides and proteins by laser pulses in a biologically relevant environment, i.e. aqueous solvent at room temperature, allows for accurate control of biological processes. However, the traditional laser methods of control of chemical reactions are applicable only to a small class of photosensitive biomolecules because of strong and ultrafast perturbations from biomolecule-solvent interactions. Here, we report excitation of harmonics of vibration modes of solvent molecules by femtosecond laser pulses to produce controlled chemical modifications of non-photosensitive peptides and proteins in polar liquids under room conditions. The principal modifications included lysine formylation and methionine sulfoxidation both of which occur with nearly 100% yield under atmospheric conditions. That modification occurred only if the laser irradiance exceeded certain threshold level. The threshold, type, and extent of the modifications were completely controlled by solvent composition, laser wavelength, and peak irradiance of ultrashort laser pulses. This approach is expected to assist in establishing rigorous control over a broad class of biological processes in cells and tissues at the molecular level.

Rapid and robust control of single quantum dots

The combination of single particle detection and ultrafast laser pulses is an instrumental method to track dynamics at the femtosecond time scale in single molecules, quantum dots and plasmonic nanoparticles. Optimal control of the extremely short-lived coherences of these individual systems has so far remained elusive, yet its successful implementation would enable arbitrary external manipulation of otherwise inaccessible nanoscale dynamics. In ensemble measurements, such control is often achieved by resorting to a closed-loop optimization strategy, where the spectral phase of a broadband laser field is iteratively optimized. This scheme needs long measurement times and strong signals to converge to the optimal solution. This requirement is in conflict with the nature of single emitters whose signals are weak and unstable. Here we demonstrate an effective closed-loop optimization strategy capable of addressing single quantum dots at room temperature, using as feedback observable the two-photon photoluminescence induced by a phase-controlled broadband femtosecond laser. Crucial to the optimization loop is the use of a deterministic and robust-against-noise search algorithm converging to the theoretically predicted solution in a reduced amount of steps, even when operating at the few-photon level. Full optimization of the single dot luminescence is obtained within ~100 trials, with a typical integration time of 100 ms per trial. These times are faster than the typical photobleaching times in single molecules at room temperature. Our results show the suitability of the novel approach to perform closed-loop optimizations on single molecules, thus extending the available experimental toolbox to the active control of nanoscale coherences.

Order of Magnitude Dissociative Ionization Enhancement Observed for Pulses with High Order Dispersion

While the interaction of atoms in strong fields is well understood, the same cannot be said about molecules. We consider how dissociative ionization of molecules depends on the quality of the femtosecond laser pulses, in particular, the presence of third- and fourth-order dispersion. We find that high-order dispersion (HOD) unexpectedly results in order-of-magnitude enhanced ion yields, along with the factor of 3 greater kinetic energy release compared to transform-limited pulses with equal peak intensities. The magnitude of these effects is not caused by increased pulse duration. We evaluate the role of pulse pedestals produced by HOD and other pulse shaping approaches, for a number of molecules including acetylene, methanol, methylene chloride, acetonitrile, toluene, and o-nitrotoluene, and discuss our findings in terms of processes such as prealignment, preionization, and bond softening. We conclude, based on the quasi-symmetric temporal dependence of the observed enhancements that cascade ionization is likely responsible for the large accumulation of charge prior to the ejection of energetic fragments along the laser polarization axis.

Controlling the Excited-State Dynamics of Nuclear Spin Isomers Using the Dynamic Stark Effect

Stark control of chemical reactions uses intense laser pulses to distort the potential energy surfaces of a molecule, thus opening new chemical pathways. We use the concept of Stark shifts to convert a local minimum into a local maximum of the potential energy surface, triggering constructive and destructive wave-packet interferences, which then induce different dynamics on nuclear spin isomers in the electronically excited state of a quinodimethane derivative. Model quantum-dynamical simulations on reduced dimensionality using optimized ultrashort laser pulses demonstrate a difference of the excited-state dynamics of two sets of nuclear spin isomers, which ultimately can be used to discriminate between these isomers.

Controlling S2 Population in Cyanine Dyes Using Shaped Femtosecond Pulses

Fast population transfer from higher to lower excited states occurs via internal conversion (IC) and is the basis of Kasha's rule, which states that spontaneous emission takes place from the lowest excited state of the same multiplicity. Photonic control over IC is of interest because it would allow direct influence over intramolecular nonradiative decay processes occurring in condensed phase. Here we tracked the S2 and S1 fluorescence yield for different cyanine dyes in solution as a function of linear chirp. For the cyanine dyes with polar solvation response IR144 and meso-piperidine substituted IR806, increased S2 emission was observed when using transform limited pulses, whereas chirped pulses led to increased S1 emission. The nonpolar solvated cyanine IR806, on the other hand, did not show S2 emission. A theoretical model, based on a nonperturbative solution of the equation of motion for the density matrix, is offered to explain and simulate the anomalous chirp dependence. Our findings, which depend on pulse properties beyond peak intensity, offer a photonic method to control S2 population thereby opening the door for the exploration of photochemical processes initiated from higher excited states.

Broadband single-molecule excitation spectroscopy

Over the past 25 years, single-molecule spectroscopy has developed into a widely used tool in multiple disciplines of science. The diversity of routinely recorded emission spectra does underpin the strength of the single-molecule approach in resolving the heterogeneity and dynamics, otherwise hidden in the ensemble. In early cryogenic studies single molecules were identified by their distinct excitation spectra, yet measuring excitation spectra at room temperature remains challenging. Here we present a broadband Fourier approach that allows rapid recording of excitation spectra of individual molecules under ambient conditions and that is robust against blinking and bleaching. Applying the method we show that the excitation spectra of individual molecules exhibit an extreme distribution of solvatochromic shifts and distinct spectral shapes. Importantly, we demonstrate that the sensitivity and speed of the broadband technique is comparable to that of emission spectroscopy putting both techniques side-by-side in single-molecule spectroscopy.

Single Broadband Phase-Shaped Pulse Stimulated Raman Spectroscopy for Standoff Trace Explosive Detection

Recent success with trace explosives detection based on the single ultrafast pulse excitation for remote stimulated Raman scattering (SUPER-SRS) prompts us to provide new results and a Perspective that describes the theoretical foundation of the strategy used for achieving the desired sensitivity and selectivity. SUPER-SRS provides fast and selective imaging while being blind to optical properties of the substrate such as color, texture, or laser speckle. We describe the strategy of combining coherent vibrational excitation with a reference pulse in order to detect stimulated Raman gain or loss. A theoretical model is used to reproduce experimental spectra and to determine the ideal pulse parameters for best sensitivity, selectivity, and resolution when detecting one or more compounds simultaneously.

Mitigating self-action processes with chirp or binary phase shaping

We report the use of binary phase shaping to mitigate pulse degradation and self-focusing in fused silica. The results of simulation and estimated mitigation efficiency are supported by experimental results using both chirped and binary phase-shaped pulses. Possible applications are considered.

Shaped and Feedback-Controlled Excitation of Single Molecules in the Weak-Field Limit

Coherent control uses tailored femtosecond pulse shapes to influence quantum pathways and drive a light-induced process toward a specific outcome. There has been a long-standing debate whether the absorption properties or the probability for population to remain in an excited state of a molecule can be influenced by the pulse shape, even if only a single photon is absorbed. Most such experiments are performed on many molecules simultaneously, so that ensemble averaging reduces the access to quantum effects. Here, we demonstrate systematic coherent control experiments on the fluorescence intensity of a single molecule in the weak-field limit. We demonstrate that a delay scan of interfering pulses reproduces the excitation spectrum of the molecule upon Fourier transformation, but that the spectral phase of a pulse sequence does not affect the transition probability. We generalize this result to arbitrary pulse shapes by performing the first closed-loop coherent control experiments on a single molecule.

Femtosecond laser pulse shaping at megahertz rate via a digital micromirror device

In this Letter, we present a scanner and digital micromirror device (DMD)-based ultrafast pulse shaper, i.e., S-DUPS, for programmable ultrafast pulse modulation, achieving a shaping rate of 2 MHz. To our knowledge, the S-DUPS is the fastest programmable pulse shaper reported to date. In the S-DUPS, the frequency spectrum of the input pulsed laser is first spread horizontally, and then mapped to a thin stripe on the DMD programmed with phase modulation patterns. A galvanometric scanner, synchronized with the DMD, subsequently scans the spectrum vertically on the DMD to achieve a shaping rate up to 10 s MHz. A grating pair and a cylindrical lens in front of the DMD compensate for the temporal and spatial dispersion of the system. To verify the concept, experiments were conducted with the DMD and the galvanometric scanner operated at 2 kHz and 1 kHz, respectively, achieving a 2 MHz speed for continuous group velocity dispersion tuning, as well as 2% efficiency. Up to 5% efficiency of S-DUPS can be expected with high efficiency gratings and optical components of proper coatings.

Surface-Assisted Cyclodehydrogenation; Break the Symmetry, Enhance the Selectivity

Selectivity in chemical reactions is a major objective in industrial processes to minimize spurious byproducts and to save scarce resources. In homogeneous catalysis the most important factor which determines selectivity is structural symmetry. However, a transfer of the symmetry concept to heterogeneous catalysis still requires a detailed comprehension of the underlying processes. Here, we investigate a ring-closing reaction in surface-confined meso-substituted porphyrin molecules by scanning tunneling microscopy, temperature-programmed desorption, and computational modeling. The identification of reaction intermediates enables us to analyze the reaction pathway and to conclude that the symmetry of the porphyrin core is of pivotal importance regarding product yields.

One Versus Two Photon Control of Dynamical Tunneling: Influence of the Irregular Floquet States

A useful approach to control quantum processes involves driving systems with two colored laser fields and varying the relative phase between the fields to control the quantum interferences. A particularly interesting class of bichromatic control schemes involves the so-called M versus N-photon control that results in laser-induced symmetry breaking and leads to directed transport; however, recent studies have shown that the mechanism of laser-induced symmetry breaking has a common classical and quantum origin. In this context, a relevant question is the extent to which such a detailed classical-quantum correspondence holds if the process to be controlled involves quantum tunneling. In this work, we address this issue in terms of controlling dynamical tunneling between field-induced islands of stability in the classical phase space of a model system, a periodically driven pendulum. This is also a paradigmatic model for Hamiltonian ratchets wherein the islands of stability, that is, nonlinear resonances, play a crucial role in the observed directed transport. We compute an appropriate control landscape for the process and show that despite breaking the relevant symmetries, there exist regions in the control landscape where the control fails. The lack of control can be understood in terms of the phase-space nature of the quantum Floquet states that participate in the dynamics of the initial wavepacket. We argue that robust regions of no control arise due to the phenomenon of chaos-assisted tunneling and comment on the possible influence of such regions on the directed transport in the model system.

Photochemistry and photophysics at extended seams of conical intersection

The role of extended seams of conical intersection in excited-state mechanisms is reviewed. Seams are crossings of the potential energy surface in many dimensions where the decay from the excited to the ground state can occur, and the extended seam is composed of different segments lying along a reaction coordinate. Every segment is associated with a different primary photoproduct, which gives rise to competing pathways. This idea is first illustrated for fulvene and ethylene, and then it is used to explain more complex cases such as the dependence of the isomerisation of retinal chromophore isomers on the protein environment, the dependence of the efficiency of the azobenzene photochemical switch on the wavelength of irradiation and the direction of the isomerisation, and the coexistence of different mechanisms in the photo-induced Wolff rearrangement of diazonaphthoquinone. The role of extended seams in the photophysics of the DNA nucleobases and the relationship between two-state seams and three-state crossings is also discussed. As an outlook, the design of optical control strategies based on the passage of the excited molecule through the seam is considered, and it is shown how the excited-state lifetime of fulvene can be modulated by shaping the energy of the seam.

Observation and symmetry-based coherent control of transient two-photon absorption: the bright side of dark pulses

We present and experimentally demonstrate for the first time the observation and femtosecond coherent control over the temporal evolution of a transient population that is excited via nonresonant two-photon absorption. Based on symmetry properties of the two-photon absorption process, the exciting femtosecond pulses are phase-shaped to photoinduce different evolutions of the transient excited population for a given final excited population. As a study case, we focus here on the attractive case of two-photon dark pulses that, although inducing zero final population (hence, the terminology of "dark pulses"), they induce a transient excited population during the pulse irradiation that can significantly deviate from zero. This nonzero transient population can be viewed as the bright side of such dark pulses. The symmetry-based coherent control is demonstrated first with dark pulses that we shape to induce transient excited population that at all times is kept below different target levels. Then, it is further demonstrated with pairs of dark pulses where one is rationally shaped to induce temporal evolution of the transient excited population that is the inverse of the evolution induced by the other. The work is conducted in the weak-field regime with the sodium atom as the model system. The approach developed here is general, conceptually simple, and very effective.

Femtosecond pulse shaping using the geometric phase

We demonstrate a femtosecond pulse shaper that utilizes polarization gratings to manipulate the geometric phase of an optical pulse. This unique approach enables circular polarization-dependent shaping of femtosecond pulses. As a result, it is possible to create coherent pulse pairs with orthogonal polarizations in a 4f pulse shaper setup, something until now that, to our knowledge, was only achieved via much more complex configurations. This approach could be used to greatly simplify and enhance the functionality of multidimensional spectroscopy and coherent control experiments, in which multiple coherent pulses are used to manipulate quantum states in materials of interest.