Exploring, tailoring, and traversing the solution landscape of a phase-shaped CARS process

Pulse shaping techniques are used to improve the selectivity of broadband CARS experiments, and to reject the overwhelming background. Knowledge about the fitness landscape and the capability of tailoring it is crucial for both fundamental insight and performing an efficient optimization of phase shapes. We use an evolutionary algorithm to find the optimal spectral phase of the broadband pump and probe beams in a background-suppressed shaped CARS process. We then investigate the shapes, symmetries, and topologies of the landscape contour lines around the optimal solution and also around the point corresponding to zero phase. We demonstrate the significance of the employed phase bases in achieving convex contour lines, suppressed local optima, and high optimization fitness with a few (and even a single) optimization parameter

An introduction into optimal control for quantum technologies

In this series of lectures, we would like to introduce the audience to quantum optimal control. The first lecture will cover basic ideas and principles of optimal control with the goal of demystifying its jargon. The second lecture will describe computational tools (for computations both on paper and in a computer) for its implementation as well as their conceptual background. The third chapter will go through a series of popular examples from different applications of quantum technology.Comment: Lecture notes for the 51st IFF Spring Schoo

Coherent Control of Molecular Dynamics with Shaped Femtosecond Pulses

Coherent control of molecular dynamics deals with the steering of quantum mechanical systems with suitably shaped ultrashort laser fields. The coherence properties of the laser field are exploited to achieve constructive interference for a predefined target wave function via a phasecorrect superposition of wave functions. The goal of coherent control, the selective preparation of a target state, is an important prerequisite for mode-selective chemistry. The laser pulse tailored to drive the system from the initial to the target state as a perturbation can in general not be determined by a quantum-mechanical calculation, since usually even the Hamiltonian of the system is unknown. A practical alternative is to determine the required shape of the laser field in a feedback-controlled regulation loop which uses a signal derived from the experiment as feedback. The loop is repeated until a pulse that suits the requirements is obtained. Experiments in this area have until recently mostly been limited to the wavelength regime of Ti:Sa lasers and their fundamentals. This work deals with the fundamentals of feedback-controlled shaping of ultrashort laser pulses with respect to both establishment of its technical prerequisites and its application to suitable model systems. The feedback loop has been tested using a simple optimization experiment with known outcome; then it was applied to experiments of progressively increasing complexity. From the optimized pulses, physical insight into the optimization process has been gained. In the first part of this work, the required technology has been implemented and standardized such that control experiments might employ it as a standard tool. One of the technical prerequisites was the frequency conversion of the 800 nm Ti:Sa laser pulses to a wavelength range suited to the particular systems. To this end, non-collinear optical parametric amplifiers have been built in different designs that routinely produce tunable sub-20 fs pulses in the visible. The characterization techniques for ultrashort pulses have been implemented as well. Pulse shapers with cylindrical instead of spherical mirrors have been implemented for the modulation of broadband pulses, and their functionality has been explained both theoretically and experimentally. A new liquid crystal device, the core of our pulse shapers, has been developed in cooperation with the group of Thomas Feurer at the Universit¨at Jena and the Jenoptik GmbH which allows for the generation of more complex pulse shapes than with other commercially available devices to date. Using a pulse shaper to modulate the white light continuum that serves as the seed for the non-collinear optical parametric amplifier, generation of phase-locked two-color double pulses has been achieved, with tunable wavelengths, delays, and relative carrier phases between the single pulses. The basic principle, phase conservation during optical parametric amplification, has been demonstrated. With this setup, control experiments which require pulses with the above described attributes in electronically controllable form are possible for the first time. An evolutionary strategy used as the optimization algorithm in the feedback loop has been programmed and characterized both in simulation and experiment using a simple optimization experiment, namely pulse recompression by phase compensation. In the second part of this work, pulse recompression of ultra-broadband spectra in the sub-20fs regime serves as an example of utility of feedback-controlled optimization. This experiment simultaneously served as a further test of the feedback loop in the limit of a physically unreachable optimization goal. It has been demonstrated that a suitable parameterization of the electric field, implemented by a mapping of the optimization parameters adjusted by the algorithm to the physical parameterscontrolling the liquid crystal mask affords a means of acquiring physical knowledge from the retrieved optimal electric fields. A parameterization helps to dissect the physical processes mediating the control process, thereby assuring fast, secure convergence and robustness against signal noise. So-called ”bright” and ”dark” pulses, i.e. pulses that are absorbed by a medium or transmitted, respectively, have been demonstrated for the case of the two-photon transition Na(3s→→5s). The physical constraints responsible for pulses being either ”bright” or ”dark”, namely a symmetric or anti-symmetric spectral phase, have been incorporated in the parameterization with the purpose of testing the concept of parameterization for such studies. An example of mode-selective preparation of vibrational states in a polyatomic molecule is the control of the ground state dynamics in polydiacetylene. In a Raman step with a shaped Stokes pulse, the population of the backbone vibrations of polydiacetylene in its ground state could be controlled. A consecutive probe pulse in a CARS (coherent anti-Stokes Raman scattering) arrangement generates an anti-Stokes signal which, once frequency-resolved, served as feedback. Of the three or four modes, respectively, accessible within the pulse bandwidth, single modes as well as combinations of modes could be excited with high selectivity. Again, suitable parameterizations helped to identify one of the processes responsible for the control as a Tannor-Rice scheme. Since both the amplitude and the phase of each mode could be influenced, the focusing of a wave packet at a predefined time, or, equivalently, the generation of local modes represents the control of a unimolecular reaction. Starting from the control of a unimolecular reaction, the possibilities of controlling a bimolecular reaction were addressed. The NaH2 collision complex was chosen as a suitable system for the control of bimolecular reactions generally and a conical intersection in particular. First timeresolved experiments have been presented.Koh¨arente Kontrolle von Molek¨uldynamik befaßt sich mit der Steuerung quantenmechanischer Systeme mittels ultrakurzer, zeitlich geeignet geformter Laserfelder. Dabei wird die Koh¨arenz des Laserfeldes ausgenutzt, um durch phasenrichtige ¨Uberlagerung von Wellenfunktionen konstruktive Interferenz f¨ur eine definierte Ziel-Wellenfunktion zu erreichen. Das Ziel koh¨arenter Kontrolle, die selektive Pr¨aparation eines Zielzustandes, ist eine der Hauptvoraussetzungen f¨ur modenselektive Chemie. Der auf eine spezifische Anregung hin maßgeschneiderte Laserpuls, welcher als St¨orung das System vom Anfangs- in den Zielzustand treibt, kann bei komplexen Systemen in der Regel nicht mehr vorab durch quantenmechanische Rechnungen bestimmt werden, da oftmals nicht einmal mehr der Hamilton-Operator des Systems bekannt ist. Ein Ansatz ist, das erforderliche Laserfeld in einer Regelschleife zu bestimmen, welche ein aus dem Experiment gewonnenes Signal als R¨uckkopplung benutzt. Diese Optimierungsschleife wird solange durchlaufen, bis ein den Anforderungen gen¨ugender Puls gefunden wurde. Bisherige Experimente auf diesem Gebiet beschr¨ankten sich zu Beginn dieser Arbeit gr¨oßtenteils auf den Wellenl¨angenbereich von Ti:Sa Lasern und deren Harmonischen. Diese Arbeit befaßt sich mit Grundlagen der r¨uckgekoppelten Formung ultrakurzer Laserpulse im Hinblick sowohl auf die Etablierung ihrer technischen Voraussetzungen in geeigneten Wellenl ¨angenbereichen als auch der Anwendung auf geeignete Modellsysteme. Die R¨uckkopplungsschleife wurde zun¨achst eingehend an einem einfachen Optimierungsexperiment mit bekanntem Ergebnis getestet, und erst dann wurden Kontrollexperimente mit steigender Komplexit¨at durchgef¨uhrt. Aus den optimierten Pulsen wurde ein physikalisches Verst¨andnis des Optimierungsvorganges abgeleitet. Im ersten Teil dieser Arbeit wurde die erforderliche Technik so weit implementiert und standardisiert, daß weiterf¨uhrende Kontrollexperimente auf die Module der Regelschleife ohne weiteres zur¨uckgreifen k¨onnen. Zu den technischen Voraussetzungen geh¨orte unter anderem die Frequenzkonvertierung der Ti:Sa Laserpulse bei 800 nm in einen f¨ur die zu untersuchenden Systeme geeigneten Wellenl¨angenbereich. Dazu wurden nichtkollineare optisch-parametrische Verst¨arkerstufen im Rahmen dieser Arbeit in verschiedenen Ausf¨uhrungen gebaut. Sie erzeugen routinem¨aßig im sichtbaren Wellenl¨angenbereich durchstimmbare Pulse mit sub-20 fs Zeitdauer. Die erforderlichen Nachweismethoden zur Analyse ultrakurzer Pulse wurden ebenfalls implementiert. Pulsformer mit zylindrischen statt sph¨arischen Spiegeln wurden im Rahmen dieser Arbeit zur Modulierung ultrakurzer Pulse aufgebaut und in ihrer Funktionsweise in Theorie und Experiment erkl¨art. Die in Zusammenarbeit mit der Arbeitsgruppe von Thomas Feurer an der Universit ¨at Jena und der Jenoptik GmbH entstandene Fl¨ussigkristallmaske, das zentrale Element unserer Pulsformer, welche die Erzeugung komplexerer Pulsformen als mit bisher erh¨altlichen Masken erlaubt, wurde vorgestellt. Durch Implementierung eines Pulsformers in eine nichtkollineare optisch-parametrische Verst¨arkerstufe zur Formung desWeißlichts, welches als Seed f¨ur den Verst¨arkungsprozeß dient, konnten u. a. phasenkoh¨arente Zweifarb-Doppelpulse mit einstellbaren Wellenl¨angen, Zeitabst¨anden und relativen Phasen zwischen den beiden Pulsen demonstriert werden. Es wurde nachgewiesen, daß eine Phase, welche dem Seed aufgepr¨agt wird, w¨ahrend des Verst¨arkungsprozesses erhalten bleibt. Kontrollexperimente, welche Pulse mit den obigen Eigenschaften in elektronisch ansteuerbarer Form ben¨otigen, werden mit diesem Aufbau erstmals m¨oglich. Eine evolution¨are Strategie, welche als Optimierungsalgorithmus in der R¨uckkopplungsschleife diente, wurde entwickelt und anhand eines einfachen Optimierungsexperimentes, der Pulskompression durch PhasenkomIm zweiten Teil der Arbeit wurde als Anwendungsbeispiel f¨ur r¨uckgekoppelte Optimierungen die Pulskompression von breitbandigen Spektren im sub-20fs-Bereich gew¨ahlt. Dieses Experiment diente gleichzeitig als ein weiterer Test f¨ur das Verhalten der Regelschleife im Grenzfall eines physikalisch unerreichbaren Optimierungszieles. Es wurde gezeigt, daß eine geeignete Abbildung zwischen den dem Algorithmus zug¨anglichen Optimierungsparametern und den Steuerparametern des Pulsformers ein Instrument darstellt, um aus den optimalen elektrischen Feldern R¨uckschl¨usse auf die physikalischen Eigenschaften des Systems ziehen zu k¨onnen. Eine solche Parametrisierung unterst¨utzt eine Herauspr¨aparation des gesuchten Effektes, der die Optimierung letztendlich bewerkstelligt, und beeinflußt Konvergenzgeschwindigkeit und Rauschunempfindlichkeit der Optimierung. Sogenannte ,,bright” und ,,dark pulses”, d. h. Pulse, die in einem Medium absorbiert bzw. ungehindert transmittiert werden, wurden am Zweiphotonen-¨Ubergang 3s→→5s in Natrium demonstriert. Mit einer Parametrisierung der Phasenfunktion der Pulse wurden die f¨ur die Eigenschaften ,,bright” und ,,dark” verantwortlichen, bereits bekannten physikalischen Prozesse, n¨amlich symmetrische bzw. antisymmetrische spektrale Phase, im Optimierungsprozeß implementiert, und das Konzept der Parametrisierung daran getestet. Ein Beispiel f¨ur die modenselektive Pr¨aparation von Vibrationszust¨anden in einem vielatomigen Molek¨ul ist die Kontrolle der Grundzustandsdynamik in Polydiazetylen. In einem Raman- Schritt, bei welchem der Stokes-Puls geformt wird, konnte die Besetzung der Ger¨ustschwingungen von Polydiazetylen im Grundzustand kontrolliert werden. Dabei wurde neben dem Anregungs- und Stokes-Puls ein Abtast-Puls eingestrahlt, der in dieser CARS-Anordnung (coherent anti-Stokes Raman scattering) ein Anti-Stokes-Signal erzeugte, welches frequenzaufgel¨ost als R¨uckkopplung diente. Von drei bzw. vier innerhalb der Laserbandbreite anregbaren Moden konnten einzelne Moden sowie Kombinationsmoden mit hoher Selektivit¨at angeregt werden. Auch hier halfen spezielle Parametrisierungen, einen der f¨ur die Kontrolle zust¨andigen Prozesse, ein Tannor-Rice-Schema, zu identifizieren. Da sowohl die Amplituden als auch die Phasen der einzelnen Moden beeinflußt werden konnten, ist eine Wellenpaketfokussierung zu einer vorgegebenen Zeit m¨oglich, was gleichbedeutend mit der Erzeugung von Lokalmoden und somit der Kontrolle einer unimolekularer Reaktion ist. Ausgehend von der Kontrolle einer unimolekularen Reaktion wurden die M¨oglichkeiten der Kontrolle einer bimolekularen Reaktion diskutiert. Der als Beispiel gew¨ahlte NaH2 - Stoßkomplex stellt ein geeignetes Objekt f¨ur zuk¨unftige Kontrollexperimente an bimolekularen Reaktionen und speziell konischen Durchschneidungen dar. Erste zeitaufgel¨oste Experimente wurden vorgestellt.pensation, sowohl im Experiment als auch in der Simulation getestet

Chemically selective microspectroscopy with broadband shaped femtosecond laser pulses

This doctoral thesis presents a new, unified approach to nonlinear microspectroscopy employing tailored broadband femtosecond laser radiation. The key concept is to functionalize the femtosecond excitation in order to implement a series of multiphoton spectroscopy techniques, especially for microscopic imaging. The most important application is coherent anti-Stokes Raman scattering (CARS) spectroscopy, which allows chemical identification of untreated samples in microscopy due to their characteristic vibrational spectra. The presented approach allows huge experimental simplifications of CARS, schemes for very rapid spectral acquisition and determination of the chemical composition (based on the quantitative analysis of entangled multiplex spectra by evolutionary algorithm fitting), as well as new methods for microscopic CARS measurements in the time-domain, resolving molecular vibrations temporally. This is possible, because coherent control of the signal generation is applied, manipulating the quantum mechanical processes of the underlying light-matter interaction by shaping the excitation light field in phase, amplitude and polarization. Thus, spectroscopic function and even molecular control is imprinted on the excitation pulses. It is shown that this idea of functional “photonic integration” can be pursued even further by incorporating an interferometric detection scheme in the same pulses without any additional optical elements in the experimental setup, drastically improving the measurement sensitivity by more than three orders of magnitude. In addition to these novel conceptual findings, new technological developments have been invented and pushed forward. These include the generation of ultrabroadband femtosecond radiation in microstructured optical fibres and its precise phase measurement and management, which is a prerequisite for coherent control. In this context, a new pulse-shaper enabled variant of SPIDER was developed, allowing very rapid compression in collinear beam geometry in the microscope. Employing the developed set of tools and concepts, application examples are given ranging from quantitative chemical imaging of polymer blend samples, to the chemical identification of potentially hazardous powdery substances and the microanalytical sensing of the chemical composition in a microfluidic device

4-Dimensional Tracking with Ultra-Fast Silicon Detectors

The evolution of particle detectors has always pushed the technological limit in order to provide enabling technologies to researchers in all fields of science. One archetypal example is the evolution of silicon detectors, from a system with a few channels 30 years ago, to the tens of millions of independent pixels currently used to track charged particles in all major particle physics experiments. Nowadays, silicon detectors are ubiquitous not only in research laboratories but in almost every high-tech apparatus, from portable phones to hospitals. In this contribution, we present a new direction in the evolution of silicon detectors for charge particle tracking, namely the inclusion of very accurate timing information. This enhancement of the present silicon detector paradigm is enabled by the inclusion of controlled low gain in the detector response, therefore increasing the detector output signal sufficiently to make timing measurement possible. After providing a short overview of the advantage of this new technology, we present the necessary conditions that need to be met for both sensor and readout electronics in order to achieve 4-dimensional tracking. In the last section we present the experimental results, demonstrating the validity of our research path.Comment: 72 pages, 3 tables, 55 figure

Quantum Control Landscapes

Numerous lines of experimental, numerical and analytical evidence indicate that it is surprisingly easy to locate optimal controls steering quantum dynamical systems to desired objectives. This has enabled the control of complex quantum systems despite the expense of solving the Schrodinger equation in simulations and the complicating effects of environmental decoherence in the laboratory. Recent work indicates that this simplicity originates in universal properties of the solution sets to quantum control problems that are fundamentally different from their classical counterparts. Here, we review studies that aim to systematically characterize these properties, enabling the classification of quantum control mechanisms and the design of globally efficient quantum control algorithms.Comment: 45 pages, 15 figures; International Reviews in Physical Chemistry, Vol. 26, Iss. 4, pp. 671-735 (2007

Computational and Theoretical Developements for (Time Dependent) Density Functional Theory

En esta tesis se presentan avances computacionales y teoricos en la teoria de funcionales de la densidad (DFT) y en la teoria de funcionales de la densidad dependientes del tiempo (TDDFT). Hemos explorado una posible nueva ruta para la mejora de los funcionales de intercambio y correlacion (XCF) en DFT, comprobado y desarrollado propagadores numericos para TDDFT, y aplicado una combinacion de la teoria de control optimo con TDDFT.En los ultimos anos, DFT se ha convertido en el metodo mas utilizado en el area de estructura electronica gracias a su inigualable relacion entre coste y precision. Podemos usar DFT para calcular multitud de propiedades fisicas y quimicas de atomos, moleculas, nanoestructuras, y materia macroscopica. El factor principal que determina la precision que podemos alcanzar usando DFT es el XCF, un objeto desconocido para el cual se han propuesto cientos de aproximaciones distintas. Algunas de estas aproximaciones funcionan correctamente en ciertas situaciones, pero a dia de hoy no existe un XCF que pueda aplicarse con certeza sobre su validez a un sistema arbitrario. Mas aun, no hay una forma sistematica de refinar estos funcionales. Proponemos y exploramos, para sistemas unidimensionales, una nueva manera de estudiarlos y optimizarlos basada en establecer una relacion con la interaccion entre electrones.TDDFT es la extension de DFT a problemas dependientes del tiempo y problemas conestados excitados, y es tambien uno de los metodos mas populares (a veces el unico metodo que se puede poner en practica) en la comunidad de estructura electronica para tratar conellos. De nuevo, la razon detras de su popularidad reside en su relacion precision/coste computacional, que nos permite tratar sistemas mayores y mas complejos. Puede usarse en combinacion con la dinamica de Ehrenfest, un tipo de dinamica molecular no adiabatica.Hemos ido mas alla y hemos combinado TDDFT y la dinamica de Ehrenfest con la teoria de control optimo, creando un instrumento que nos permite, por ejemplo, predecir la forma de los pulsos laser que inducen una explosion de Coulomb en clusters de sodio. A pesar del buen rendimiento computacional de TDDFT en comparacion con otros metodos, hallamos que el coste de estos calculos era bastante elevado.Motivados por este hecho, tambien dedicamos una parte del trabajo de la tesis a la investigacion computacional. En particular, hemos estudiado e implementado familias de propagadores numericos que no se habian examinado en el contexto de TDDFT. Mas concretamente, metodos con varios pasos previos, formulas Runge-Kutta exponenciales, y las expansiones de Magnus sin conmutadores. Finalmente, hemos implementado modificaciones de estas expansiones de Magnus sin conmutadores para la propagacion de las ecuaciones clasico-cuanticas que resultan de la combinacion de la dinamica de Ehrenfest con TDDFT.In this thesis we present computational and theoretical developments for density functional theory (DFT) and time dependent density functional theory (TDDFT). We have explored a new possible route to improve exchange and correlation functionals (XCF) in DFT, tested and developed numerical propagators for TDDFT, and applied a combination of optimal control theory with TDDFT. In recent years, DFT has become the most used method in the electronic structure field thanks to its unparalleled precision/computational cost relationship. We can use DFT to accurately calculate many physical and chemical properties of atoms, molecules, nanostructures, and bulk materials. The main factor that determines the precision that we can obtain using DFT is the XCF, an unknown object for which hundreds of different approximations have been proposed. Some of these approximations work well enough for certain situations, but to this day there is no XCF that can be reliably applied to any arbitrary system. Moreover, there is no clear way for a systematic refinement of these functionals. We propose and explore, for one-dimensional systems, a new way to optimize them, based on establishing a relationship with the electron-electron interaction. TDDFT is the extension of DFT to time-dependent and excited-states problems, and it is also one of the most popular methods (sometimes the only practical one) in the electronic structure community to deal with them. Once again, the reason behind its popularity is its accuracy/computational cost ratio, which allows us to tackle bigger, more complex systems. It can be used in combination with Ehrenfest dynamics, a non-adiabatic type of molecular dynamics. We have furthermore combined both TDDFT and Ehrenfest dynamics with optimal control theory, a scheme that has allowed us, for example, to predict the shapes of the laser pulses that induce a Coulomb explosion in different sodium clusters. Despite the good numerical performance of TDDFT compared to other methods, we found that these computations were still quite expensive. Motivated by this fact, we have also dedicated a part of the thesis work to computational research. In particular, we have studied and implemented families of numerical propagators that had not been tested in the context of TDDFT. More concretely, linear multistep schemes, exponential Runge-Kutta formulas, and commutator-free Magnus expansions. Moreover, we have implemented modifications of these commutator-free Magnus methods for the propagation of the classical-quantum equations that result of combining Ehrenfest dynamics with TDDFT.<br /

Control of Optically Induced Currents in Semiconductor Crystals

The generation and control of optically induced currents has the potential to become an important building block for optical computers. Here, shift and rectification currents are investigated that emerge from a divergence of the optical susceptibility. It is known that these currents react to the shape of the impinging laser pulse, and especially to the shape of the pulse envelope. The main goal is the systematic manipulation of the pulse envelope with an optical pulse shaper that is integrated into a standard THz emission setup. The initial approach, the chirping of the laser pulse only has a weak influence on the envelope and the currents. Instead, a second approach is suggested that uses the combined envelope of a phase-stable pulse-pair as a parameter. In a laser pulse, the position of the maxima of the electrical field and the pulse envelope are shifted relative to each other. This shift is known as the Carrier-Envelope Phase (CEP). It is a new degree of freedom that is usually only accessible in specially stabilized systems. It is shown, that in a phase-stable pulse-pair, at least the relative CEP is usable as a new degree of freedom. It has a great influence on the shape of the pulse envelope and thus on the current density. It is shown that this approach enables the coherent control of the current density. The experiments are corroborated by a theoretical model of the system. The potential of this approach is demonstrated in an application. A framework is presented that uses an iterative genetic algorithm to create arbitrarily shaped THz traces. The algorithm controls the optical pulse shaper, and varies the phase of the impinging laser pulses until the desired target trace is found

Quick and specific non-linear microscopy using shaped pulses with durations down to 10fs

Nonlinear optical microscopy is ideally suited for in vivo imaging: it provides label-free contrast revealing intrinsic structural and chemical properties of the sample in a non-invasive way. Successful nonlinear microscopy relies on the use of pulsed lasers to obtain high signal levels at moderate average laser power. In particular, broadband excitation increases the nonlinear generation efficiency as well as the spectral coverage. However, lower photodamage thresholds and less straightforward signal interpretation have prevented its application to sensitive samples. In this thesis, a pulse shaper is used to tailor ultrashort pulses for optimal imaging. This work concentrates on Coherent Anti-Stokes Raman Scattering (CARS) because it provides an access to highly specific vibrational spectra. The main concept is to encode molecule-specific information directly in the excitation. This is realized either by direct tailoring in a shaperassisted variant of a Multiplex CARS setup or by phase shaping of a single ultrashort pulse (<10fs). The photon load reduction and the optimization of the pulse profile achieved by shaping are demonstrated with the imaging of polymer samples and sensitive biological tissue. The flexibility of the setup allows switching between spectrally resolved acquisition for precise chemical mapping and single channel detection for rapid imaging. Further nonlinear effects can likewise be controlled by pulse shaping. In this work, the systematic modification of the relative intensities of Second Harmonic Generation (SHG), Two-Photon Excited Fluorescence (TPEF) and CARS is investigated as well as selective excitation of fluorophores and molecular vibrations. Multimodal imaging with shaped ultrashort pulses proves to be particularly efficient for biological samples as illustrated by the imaging of plant cells and skin biopsies