Valery Milner
Relevant Thesis-Based Degree Programs
Graduate Student Supervision
Doctoral Student Supervision
Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
The major purpose of this work is the experimental study of the applicability of an optical centrifuge - a novel tool, utilizing non-resonant broadband laserradiation to excite molecular rotation - to produce and control moleculesin extremely high rotational states, so called molecular "super rotors", andto study their optical, magnetic, acoustic, hydrodynamic and quantum mechanicalproperties.
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The objective of this dissertation is the experimental study and control of laser-kicked molecular rotors. Nonresonant rotational Raman excitation of linear molecules by periodic sequences of ultra-short laser pulses allows for the realization of a paradigm system - the periodically kicked rotor. This apparently simple physical system has drawn much interest within the last decades, especially due to its role in the field of quantum chaos. This thesis presents an experimental apparatus capable of producing long sequences of high-energy femtosecond pulses. Rotation of diatomic molecules, the most basic version of quantum rotors, is investigated under multi-pulse excitation. In the case of periodic kicking, the wave function of the quantum rotor dynamically localizes in the angular momentum space, similarly to Anderson localization of the electronic wave function in disordered solids. We present the first direct observation of dynamical localization in a system of true rotors. The suppressed growth of rotational energy is demonstrated, as well as the noise-induced recovery of diffusion, indicative of classical dynamics. We examine other distinct features of the quantum kicked rotor and report on quantum resonances, the phenomena of rotational Bloch oscillations and Rabi oscillations. In addition, multi-pulse excitation is investigated in the context of creating broad rotational wave packets. Another goal of the reported study is the coherent control of quantum chaos. We demonstrate that the relative phases in a superposition of rotational states can be used to control the process of dynamical localization. We specify the sensitivity to external parameters and illustrate the loss of control in the classical limit of laser-molecule interaction. Our work advances the general understanding of the dynamics of laser kicked molecules and complements previous studies of the quantum kicked rotor in a system of cold atoms. The results encourage further studies, e.g. of quantum phenomena which are unique to true rotors. The possibility of control in classically chaotic systems has far reaching implications for the ultimate prospect of using coherence to control chemical reactions.
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Wave-mechanical phenomena such as resonance and interference, in both light and matter, are central to the principles of quantum coherent control over molecular processes. Focusing on the dynamical aspects, this dissertation is a compilation of studies on the interaction physics involving wave packets in molecules, the driving light field, and the underlying coherence and control. In each work, we will demonstrate interesting correlations between the properties of a carefully designed excitation light field and desirable outcomes of the molecules quantum dynamics. We will analyze the dynamical effect of a Feshbach resonance in the adiabatic Raman photoassociation for ultracold diatomic molecule formation from ultracold atoms. A narrow resonance is shown to be able to increase the effective number of collisions, in an ultracold atomic gas, that are available for photoassociation. This results in an optimal resonance width much smaller than the atomic collision energy bandwidth, due to the balance between the effective collision rate and single-collision transfer probability. Next, we demonstrate the linear molecular response to high-intensity, broadband, shaped optical fields. We show that this originates from interferences based on intra-pulse Raman excitations, and thus response linearity is not unique to the first-order perturbative limit and can not be used to infer the strength of the field. In the last study, we simulate the stochastic vibrational wave packet and dissociation-flux dynamics in a molecule excited by light with temporal and spectral incoherent properties. Between this case and that using a coherent pulse with the same spectral profile, we compare the vibrational wave functions and the loss of electronic and vibrational coherence, and demonstrate the qualitative difference between coherently and incoherently driven dynamics in molecules.
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This thesis explores new ways with which to utilize molecular spectroscopic data in both the time and frequency domain. Operating within the Born-Oppenheimer approximation (BOA), we show how to obtain the signs of transition-dipole amplitudes from fluorescence line intensities. Using the amplitudes thus obtained we give a method to extract highly accurate excited state potential(s) and the transition-dipole(s) as a function of the nuclear displacements. The procedure, illustrated here for the diatomic and triatomic molecules, is in principle applicable to any polyatomic system. We, also, extend this approach beyond the BOA and demonstrate applications involving bound-continuum transition, and double-minimum potentials.Furthermore, by using as input these measured energy level positions and the transition dipole moments (TDMs), we derive a scheme that completely determines the non-adiabatic coupling matrix between potential energy surfaces and the coordinate dependence of the coupling functions. We demonstrate results in a diatomic system with two spin-orbit coupled potentials, whereby experimentally measured information along with TDMs computed for two corresponding diabatic potentials to the fully spin-orbit coupled set of eigenstates, are used to extract the diagonal and off-diagonal spin-orbit coupling functions.Using time-resolved spectra, we show that bi-chromatic coherent control (BCC) enables the determination of the amplitudes (=magnitudes+phases) of individual transition-dipole matrix elements (TDMs) in these non-adiabatic coupling situation. The present use of BCC induces quantum interferences using two external laser fields to coherently deplete the population of different pairs of excited energy eigenstates. The BCC induced depletion is supplemented by the computation of the Fourier integral of the time-resolved fluorescence at the beat frequencies of the two states involved. The combination of BCC and Fourier transform enables the determination of the complex expansion coefficients of the wave packet in a basis of vibrational energy eigenstates, from simple spontaneous fluorescence data.
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The main goal of this PhD work is an experimental study of coherent excitation of atomic and molecular wavepackets, i.e. superpositions of many quantum eigenstates, by shaped femtosecond pulses. Approaches allowing nearly complete population transfer between quantum eigenstates were well studied in the past within the two level approximation. In this work we focus on adiabatic and non-adiabatic methods of population transfer beyond the two-level approximation.Excitation of multi-level target states is possible due to broad spectrum of an ultrashort pulse which contains frequencies needed for multiple transitions to different states in the final superposition. At the same time, the spectrum of an ultrashort pulse can be modified, or ``shaped'', in order to affect the excitation process and control the amplitudes in the final superposition. Both non-adiabatic and quasi-adiabatic methods were first implemented and studied in electronic wavepackets in alkali atoms. The non-adiabatic approach revealed features linked to the strong-field perturbations of the energy level structure of the quantum system. An adiabatic method was implemented for the first time on a femtosecond time scale, and was thoroughly characterized. The control over complex amplitudes in the target superposition was demonstrated as well as completeness of the population transfer. In the second part of this work, we focused on coherent control of rotational wavepackets in diatomic molecules. Rotational excitation by a periodic train of femtosecond pulses was investigated in the context of ``delta-kicked'' rotor - a paradigm system for studying quantum chaos, and the effect of quantum resonance was demonstrated for the first time in a system of true quantum rotors. Control of uni-directional molecular rotation was proposed and demonstrated with a novel ``chiral pulse train'' - a sequence of femtosecond pulses with polarization rotating from pulse to pulse by a predefined angle. All the developed techniques offer new tools in coherent control of atomic and molecular wavepackets on an ultrashort time scale.
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Master's Student Supervision
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
The feasibility of using rotating molecules as “nanoprobes” for testing the superfluidity of helium nanodroplets is explored in this thesis. Helium nanodroplets have an internal temperature of 0.37 K and are below the superfluid transition temperature in bulk helium of 2.17 K. The onset of superfluidity in this microscopic environment will be explored by rotationally exciting molecules using a tool called an optical centrifuge. This tool affords a high degree of precision in the final rotational frequency that the molecule will reach and makes it useful in probing the coupling between the rotor and helium. A unique helium nanodroplet vacuum chamber system was characterized for the range of operation possible, especially with focus on the signal to background detection conditions. Two techniques were explored to characterize the dynamical rotational behaviour of the molecules in these conditions: direct measurement of the molecular orientation and direct measurement of the angular momentum state. The molecular orientation of a molecule is characterized by it’s confinement to the rotational plane using ⟨cos² θ2D⟩ as the metric. A ⟨cos² θ2D⟩ measurement of ≈ 0.7 was successfully recovered from background for carbon disulfide doped helium droplets using an alignment probe pulse, however, with the centrifuge it was ultimately unclear if the molecule was rotating or simply aligning to the plane of rotation. The angular momenta of a molecule was characterized via its ion signal from a Resonance Enhanced Multiphoton Ionization (REMPI) scheme. The feasibility of measuring a transition in oxygen at the low signal to background densities was studied and would be promising to use with oxygen doped helium droplets. In order to continue the research, improvements need to be made to the set up and the two techniques should be used in tandem so that rotation can be better detected and characterized.
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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.
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The main goal of this work is the detection of the directionality of molecular rotation and the characterization of two experimental approaches to controlling the directionality of molecular rotation with ultrashort pulses. Control of the directionality of molecular rotation is desired in order to learn more about the internal properties of molecular systems as well as for studying and controlling molecular interactions. Further, the techniques for generating unidirectional molecular rotation must be studied to understand the properties of the molecular ensembles that are generated. In order to detect the directionality of molecular rotation, we use circular polarization sensitive resonance-enhanced multiphoton ionization spectroscopy to allow state-selective directionality detection. In this work we explain this technique and demonstrate its ability to measure the directionality of individual rotational states. The two methods for controlling the directionality of molecular rotation are based on the molecular interaction with either a pair of pulses (a “double-kick” scheme) or a larger sequence of pulses (a “chiral pulse train” scheme). In both cases, rotational control is achieved by varying the polarization of and the time delay between consecutive laser pulses. The double-kick and chiral train methods have demonstrated the ability to control the directionality of molecular rotation but have not been extensively studied. In this work, we perform experiments with both the double-kick and chiral train techniques for thorough comparison and characterization of both methods. We show that both methods produce significant rotational directionality. We also demonstrate that increasing the number of excitation pulses enables one to control the sense of molecular rotation and predominately excite a single rotational state, i.e. quantum state selectivity. To further explore the capabilities of both techniques we perform experiments on selectivity in mixtures of spin isomers and molecular isotoplogues. We demonstrate the ability of both techniques to generate counter-rotation of molecular nuclear spin isomers (here, ortho- and para-nitrogen) and molecular isotopologues.
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