This thesis is about coherent control on disordered samples. Coherent control is the steering of processes with light of different frequencies. The relative phases between the frequencies determines in which di- rection the process is steered. When many frequencies are used, the light composes a short pulse. Changing the relative phases of the fre- quencies changes the shape of this pulse. We used such shaped pulses to guide processes on our samples. Pulse shaping and the different disordered samples are introduced in chapter 1, after which chapter 2 proceeds with the experimental details behind our studies. To perform a measurement with a shaped pulse, a nonlinear pro- cess is required. Second-harmonic generation (SHG) is one of the most straightforward nonlinear processes and plays an important role in this thesis. In chapter 3 theory of SHG is developed. Specifically, we deter- mine how to calculate the SHG intensity from shaped pulses on sam- ples with a resonance near the frequency of the illuminating light. We show that two mechanisms that broaden the line width of a resonance can not be distinguished by measuring the SHG signal. In the absence of resonances the maximum SHG intensity is produced by a transform- limited pulse. This pulse shape is required for many nonlinear optical spectroscopies and maximizing the SHG intensity is a useful method to obtain it. For such optimizations, algorithms are used that tune the coefficients for basis functions of the spectral phase. We looked at the fitness landscape of the SHG when a common basis set is used and by inspecting projections of the landscapewe constructed a more efficient basis set, that will make optimizations of SHG more efficient. In chapter 4 we describe simulations of the electric field in a ran- dom electrical network. The electrical network serves as a model for a film of randomly positioned gold islands, which we studied as re- ported in chapter 5. The simulations show that a metal density close to the percolation threshold results in a structure that has favourable properties for selective local excitation of the surface. In chapter 5 we proceed with this experiment on a random gold structure. The structure was coated with a layer of quantum dots. We imaged the fluorescence from this layer with a microscope and in this way were able to detect the local field intensities on the surface. We found that some control on the field intensity is possible, but the effect is minimal compared to the results of the simulations. The reason for this discrepancy is in the radiative damping of the oscillations, which was not included in the simulation. The radiative damping of plas- mons causes dephasing within �20 fs, making control with shaped pulses difficult. We further studied SHG from arrays of sub-wavelength holes in a gold film. The results of this study are presented in chapter 6. The amount of disorder in the positions of the holes in the arrays was var- ied. We found that the the SHG is produced in a resonance with a long lifetime of 55 fs. This lifetime is longer than most plasmonic resonances and shows that the effect of radiative damping is decreased on this structure. Furthermore, the SHG from all the structures has the same features. The disorder does not influence the SHG and the same reso- nance was found on all structures. Thus, it is likely that the resonance is a property of a single hole. Finally, we studied the coherent control of a dye in solution. Here the disorder is not in the structure, but in the transition frequency of the dye. Due to the interaction of the dye with the solvent, this fre- quency is constantly changing. These changes cause coherence to be lost in the system in a short time. We ran a closed-loop optimization to maximize the stimulated emission from the dye. All the pulse shapes tried in the optimization were saved and subsequently tried with the dye dissolved in other solvents. We found that the yield of the opti- mization varied greatly with the used solvent. These results formed a trend with the viscosity of the particular solvents. The viscosity can be related directly to the dephasing time, the time in which coherence is lost in the system. The trend of the optimization yield versus the vis- cosity shows directly that the fluctuations of the solvent are limiting the coherent control of the dye.
This thesis is about coherent control on disordered samples. Coherent control is the steering of processes with light of different frequencies. The relative phases between the frequencies determines in which di- rection the process is steered. When many frequencies are used, the light composes a short pulse. Changing the relative phases of the fre- quencies changes the shape of this pulse. We used such shaped pulses to guide processes on our samples. Pulse shaping and the different disordered samples are introduced in chapter 1, after which chapter 2 proceeds with the experimental details behind our studies. To perform a measurement with a shaped pulse, a nonlinear pro- cess is required. Second-harmonic generation (SHG) is one of the most straightforward nonlinear processes and plays an important role in this thesis. In chapter 3 theory of SHG is developed. Specifically, we deter- mine how to calculate the SHG intensity from shaped pulses on sam- ples with a resonance near the frequency of the illuminating light. We show that two mechanisms that broaden the line width of a resonance can not be distinguished by measuring the SHG signal. In the absence of resonances the maximum SHG intensity is produced by a transform- limited pulse. This pulse shape is required for many nonlinear optical spectroscopies and maximizing the SHG intensity is a useful method to obtain it. For such optimizations, algorithms are used that tune the coefficients for basis functions of the spectral phase. We looked at the fitness landscape of the SHG when a common basis set is used and by inspecting projections of the landscapewe constructed a more efficient basis set, that will make optimizations of SHG more efficient. In chapter 4 we describe simulations of the electric field in a ran- dom electrical network. The electrical network serves as a model for a film of randomly positioned gold islands, which we studied as re- ported in chapter 5. The simulations show that a metal density close to the percolation threshold results in a structure that has favourable properties for selective local excitation of the surface. In chapter 5 we proceed with this experiment on a random gold structure. The structure was coated with a layer of quantum dots. We imaged the fluorescence from this layer with a microscope and in this way were able to detect the local field intensities on the surface. We found that some control on the field intensity is possible, but the effect is minimal compared to the results of the simulations. The reason for this discrepancy is in the radiative damping of the oscillations, which was not included in the simulation. The radiative damping of plas- mons causes dephasing within �20 fs, making control with shaped pulses difficult. We further studied SHG from arrays of sub-wavelength holes in a gold film. The results of this study are presented in chapter 6. The amount of disorder in the positions of the holes in the arrays was var- ied. We found that the the SHG is produced in a resonance with a long lifetime of 55 fs. This lifetime is longer than most plasmonic resonances and shows that the effect of radiative damping is decreased on this structure. Furthermore, the SHG from all the structures has the same features. The disorder does not influence the SHG and the same reso- nance was found on all structures. Thus, it is likely that the resonance is a property of a single hole. Finally, we studied the coherent control of a dye in solution. Here the disorder is not in the structure, but in the transition frequency of the dye. Due to the interaction of the dye with the solvent, this fre- quency is constantly changing. These changes cause coherence to be lost in the system in a short time. We ran a closed-loop optimization to maximize the stimulated emission from the dye. All the pulse shapes tried in the optimization were saved and subsequently tried with the dye dissolved in other solvents. We found that the yield of the opti- mization varied greatly with the used solvent. These results formed a trend with the viscosity of the particular solvents. The viscosity can be related directly to the dephasing time, the time in which coherence is lost in the system. The trend of the optimization yield versus the vis- cosity shows directly that the fluctuations of the solvent are limiting the coherent control of the dye.
The experimental work in this thesis is divided into two distinct parts. In both parts, broadband femtosecond laser pulses are "shaped" by adjusting the relative phase and amplitude of spectral components. In the first set of experiments, time-dependent perturbation theory is used to show that the probability of a quantum transition in atomic rubidium can be substantially enhanced or suppressed using pulse shaping, compared to the probability of transition observed when a transform-limited or "flat phase" optical pulse is used. These enhancement or suppression effects are also demonstrated experimentally. As quantum interference (the material phase having been transferred from the optical phase) is used to enhance or diminish a particular final quantum state, this can be classified as a quantum coherent control experiment. In the second set of experiments, an optical pulse is scattered into a train of pulses by a layered structure. The layered structure is used to simulate the effect of optical pulses travelling through certain types of complex media. One consequence of the disruption of a single pulse into a train of pulses is lower per-pulse peak intensity, and thus a greatly diminished nonlinear signal. It is shown that spectral pulse shaping (in phase only) is sufficient to pre-compensate for the scattering structure, allowing a single transform-limited pulse to be obtained at the output.
Ultrafast pulses allow observation of molecular dynamics with femtosecond time resolution through pump probe experiments. However, averaging over an ensemble of molecules tends to wash out phase sensitive information, necessary to probe quantum effects, due to the intrinsic inhomogeneity in molecular conformations, orientations and interactions that lead to unique potential energy landscapes for each molecule. It is therefore important to go beyond the ensemble average when looking at quantum dynamics of organic systems at room temperature, and resolve the behaviour of specific molecules on an individual basis. In this thesis, we show the creation, manipulation and observation of ultrafast coherent effects in single molecules at room temperature, and resolve a certain measure of environmental influence on the specific dynamics of each molecule. Moreover, we apply this insight to investigate a functional light harvesting biosystem, and lay the basis for a technique that has the time and space resolution required to observe these systems in vivo. In chapter 1, we introduce the concepts and techniques the research in this thesis is built on. In chapter 2, we treat the possibility of controlling ultrafast pulses at the high-NA diffraction limit, and come to conclusions about the procedure to follow there that hold for all pulse-shaping experiments. We show in proof of principle experiments that we can control the ultrafast characteristics of optical pulses in nanometric excitation volumes. In chapters 3 and 4 we report the creation, detection and control of ultrafast quantum dynamics in single organic molecules at room temperature. We show that manipulation of superposition states is possible in these systems within a coherence dephasing time of ~50 fs. This leads to the first observation of rabi-oscillations in room temperature single molecules, to ultrafast operation of an organic qubit, and to the creation of non-stationary superposition states (vibrational wavepackets). We probe the influence of the local environment on the composition and dynamics of these wavepackets and show we can optimize the state preparation protocol for each individual molecule in its own nanoenvironment, leading to high fidelity coherent control. In these chapters we lay out the proof of principle work of detecting the quantumdynamics of a complex system in interaction with its environment at room temperature. In chapter 5 we discuss application of these techniques to the investigation of long lived coherence in photosynthetic systems. We show that electronic coherence between different rings of the LH2 system persists to time scales of 100s of femtoseconds at room temperature. Moreover we show that the energy transfer pathways in LH2 adapt to environmentally induced changes in the molecule and that the nature of the transfer remains coherent for each pathway, providing strong evidence that coherent energy transfer is the optimum process for energy transfer in photosynthesis. Finally, in chapter 6 we take the technical development one step further and report on the creation of a framework based on plasmonic antennas that allows for control of the amplitude-phase characteristics in nanometric sized hotspot fields. We show for the first time that the ultrafast characteristics of plasmonic hotspots can directly be engineered through design of the plasmonic system and experimentally demonstrate two much-anticipated examples: a sub-diffraction resolution phase shaper and an ultrafast plasmonic switch for pump probe experiments. The results presented in this thesis form the first creation and observation of ultrafast coherent dynamics in individual molecular systems at room temperature. This is a necessary step to be able to do true quantum tomography in complex systems, resolve the influence of the environment on molecular dynamics, and investigate the physics that determines evolutionary optimization and functionality in biomolecules.
Whenever scientific tools are significantly advanced; new discoveries soon follow. My thesis work relies on advances in ultrafast laser technology that allow precise control over the shape of sub 20-fs laser pulses. This laser system enabled many new experiments that yielded significant advances in attosecond science, atomic physics; extreme nonlinear optics, quantum chemistry, coherent control, molecular physics, ultrafast optical phase modulation, EUV coherent imagines, and EUV radiation sources. By using an automated learning control algorithm coupled to our deformable mirror pulse shaper; we have demonstrated the control of a process on an attosecond time-scale for the first time. We demonstrate the manipulation of the phase of an electron wave packet, allowing us to tailor the EUV spectrum generated by high-order harmonic generation. These optimal fields were discovered by a learning algorithm that both found an unknown optimal solution; but also discovered an unknown phase-matching mechanism that occurs in the interaction of a single atom and light pulse. We control molecular vibrational and rotational coherences with the same learning algorithm; demonstrating selective control in room temperature molecules at atmospheric pressures. Moreover, we demonstrate that modifications to the algorithm can help automatically uncover the physics of the control mechanism found by the learning algorithm. We also demonstrate a new pulse-compression technique using rotational phase modulation, where optical pulses were compressed by an order of magnitude after phase modulation by a molecular rotational coherence and subsequent propagation through a transparent window. This technique will enable efficient pulse compression in the deep-UV. Finally, we measure the coherence of EUV light generated by high-harmonic upconversion of a femtosecond laser. In phase-matched hollow-fiber geometry, the EUV light exhibits the highest inherent spatial coherence of any source in this region of the spectrum. We use this source to demonstrate the coherent image with first table-top EUV radiation. Since this source exhibits full spatial coherence at very short wavelength, this light source represents has smallest inherent effective source-size of any light source yet created. We also demonstrate that the spectrum of an optical field can be determined by measuring the interference of a double-slit pattern.
Any real system, no matter how well isolated, unavoidably interacts with the environment. This interaction makes the evolution of a quantum system nonunitary and destroys the coherence of quantum superpositions. This process, known as decoherence, is widely regarded as the most important and fundamental obstacle to the practical realization of quantum information processing. On the other hand, the objectives of quantum control are predicated on the ability to manipulate the system coherently over a period of time. Quantum control has been applied successfully to several experimental systems and coupling such a controlled system to an environment makes the study of coherence loss possible. The standard model of decoherence is formulated in terms of a system, defined by the observables that can be measured, and a reservoir, de¯ned by those degrees of freedom that are not accessed by observation. The vibrational and rotational degree of freedom of diatomic molecules form an experimental model of such a system and bath 1. The nature of the ro-vibrational coupling does not allow a Decoherence Free Subspace, instead we try to identify a Decoherence Resistant Subspace using closed loop control. We set up a learning control loop experiment to monitor the coherence in a vibrational wavepacket, consisting of about 6 vibrational states of the A(1Σ+g ) electronic state, excited by a shaped, ultrashort optical pulse, and then to determine the pulse shape that yielded the longest coherence lifetime. The coherence of the vibrational wavepacket was monitored by measuring the fluorescence quantum beats from the outer turning point of the wave packet's trajectory. The closed loop controller altered the shape of the driving optical pulse to maximize the the depth of modulation of the quantum beats at a prescribed time after excitation, within the constraints of the laser system. The outcome of this closed loop control experiment shows an improvement of the beat decay constant of about 60%, as shown in Fig. (1), a result which occurs for several di®erent pulse shapes. Our experiments show that a number of interesting decoherence-resistant excitations exist. The nature of the dephasing was examined theoretically. Using measured optical pulse shapes, we simulated numerically the molecular wavepacket in the limit of perturbation theory. The experiment and simulations show that there are di®erent mechanisms contributing to the decoherence resistant excitation. The shaped pulse reduces the °uctuations in the wave packet spatial extent, which in turn changes the system reservoir coupling. Other components of the optimal pulse are a sequence of pulses that creates constructive interference of the vibrational wavepacket at the desired time, and chirp which compensates for the anharmonicity of the adiabatic potential. The experimental results show general agreement with numerical simulations. We conclude that wavepackets resistant to dephasing do exist,and that learning algorithms provide an effcient means for finding them.
As a succession of two pulses propagates through a medium which exhibits a resonance (single or multiphoton), the resonant parts of the susceptibility will depend on the relative phase of the two pulses, provided the delay between the pulses is shorter than the phase relaxation time of the interaction. As a result of this "coherent interaction", the reradiation of the sample (the fluorescence, the absorption and the generation of harmonics) will be a periodic function of the phase difference and delay between the two pulses of the sequence 1. The study of these coherent effects is important, because one expect to find conditions under which the maximum harmonic conversion to much shorter wavelengths can be greatly enhanced 2. The presence of the multiphoton resonance enhances the nonlinear susceptibility responsible for the harmonic generation, but reduces also the maximum achievable conversion efficiency because of resonant losses (depletion of the fundamental by multiphoton absorption). The latter effect can be eliminated by a judicious choice of the pulse sequencing and phasing. In addition to demonstrating, for the first time, coherent interaction enhancement in harmonic generation, the data of third harmonic versus relative phase of the pulse sequences enable us to measure the phase angle of the third order susceptibility (hence the contribution of the continuum relative to that of the resonance).
In this thesis, the coherent interaction of single semiconductor quantum dots and ultrafast optical pulses is studied. Under certain conditions, localized exciton transitions in quantum dots can be seen as semi-isolated two-level systems. While this description is sufficient for the explanation of some observations in coherent experiments, it is sometimes necessary to explicitly consider coupling of the discreet quantum states confined to the dot with the environment. We start out from simple, classical examples of coherent spectroscopy and then turn towards experiments where the interaction with the vicinity of the dot becomes an important factor. First, a novel method for transient differential reflectivity spectroscopy of single quantum systems is introduced. It is a pure far-field optical technique which does not require any sophisticated sample preparation steps which makes it applicable to a broad range of structures. Pump pulses excite the sample structure and probe pulses read out the pump-induced changes in the system after a variable delay time. In the case of a single dipole, the signal is given in the form of the spectral inteferogram between the backscattered wave from the particle and the probe light which is reflected at the sample surface. This form of homodyne detection amplifies the weak scattered wave from the particle and thus makes this kind of spectroscopy for single quantum dots feasible. In the remainder of this thesis our spectroscopic method is applied to either characterize the coherent properties of single quantum dots, to prepare and read-out a desired quantum state or to deliberately manipulate them. Coherence times and oscillator strengths are determined for localized exciton transitions. Arbitrary population states can be written by driving coherent population oscillations using resonant pulses, while entangled superpositions of two exciton states in a single dot are investigated by quantum beats on transient differential spectra. We finally exploit the interaction between the dot and a nearby absorbing layer to switch the dot's absorption spectrum on ultrafast timescales via light-induced transient electric fields.