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Parametric polarization pulse shaping methods and control of excitation dynamics in ultracold rubidium

Fabian Weise

Published Jan 1, 2010• 0 citations
Publisher

Abstract

This work focuses on the developments of ultrafast laser pulse shaping techniques and their application to diatomic molecular systems in order to reveal fundamental effects in light matter interaction. It describes the development of a new pulse shaper consisting of a sequence of four liquid crystal arrays and a polarizer. This pulse shaper is the first non- interferometric setup for unrestricted phase, amplitude, and polarization shaping. Moreover, a parametric encoding of the electrical field was developed which allowed for generating pulse sequences consisting of several sub-pulses. Each sub-pulse can be controlled in its physically apparent parameters: energy, position in time, phase, and chirps as well as state of polarization with ellipticity, orientation, and helicity. The feasibility of the setup is illustrated by systematic variations of single and double pulses, which are experimentally generated and measured. In these series of pulses, one of the pulses parameters is varied while the other parameters are kept constant. This proves the precise control of the pulse shape. Further, more complex pulses are shown to give an impression of the capabilities of this setup in combination with the parameterization. For some fields of application, it could be desirable to guide the phase, amplitude and polarization shaped pulses through an optical fiber. For example, life science could benefit from shaped pulses in the area of imaging and photodynamic therapy. In order to make the shaped pulses available at the distal end of an optical fiber, the effects of the fiber have to be compensated. In this work, the generation of phase, amplitude, and polarization shaped pulses after transmission trough an optical fiber is presented. This is demonstrated for two types of optical fibers – a standard single-mode fiber and a microstructured hollow core photonic crystal fiber. The procedure for the determined generation of parametrically shaped pulses differs for both types of optical fibers. The capabilities of parametrically shaped pulse sequences are – again – illustrated by a series of example pulses. The parametrically shaped pulses transmitted through the hollow core fiber are implemented in a coherent control experiment. For this purpose, the potassium dimer produced in a molecular beam serves as a test system. The multi-photon ionization is studied by a closed feedback loop optimization and shaper-assisted pump-probe spectroscopy. These experiments revealed the excitation path including the vibrational dynamics in the first and second excited state. Further, the relevance of the polarization control in the excitation process is highlighted. Moreover, coherent control techniques are applied to investigate the interaction of shaped femtosecond laser pulses with ultracold atoms and molecules. The focus of these experiments is the control of the photoassociation of a colliding atom pair to a bound molecule. This process is investigated using two-color pump-probe spectroscopy. The molecular transients are compared to theoretical calculations. The analysis of this data reveals the interaction process which is discussed in the time and frequency domain. In a second experiment, the multi-photon ionization of ultracold rubidium dimers is optimized in a closed feedback loop. In this optimization, a parameterization in the frequency domain is employed which extracts the relevant transition frequencies. In combination with a complementary experiment the excitation pathway was revealed.

DOI
10.17169/refubium-5496

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This work focuses on the developments of ultrafast laser pulse shaping techniques and their application to diatomic molecular systems in order to reveal fundamental effects in light matter interaction. It describes the development of a new pulse shaper consisting of a sequence of four liquid crystal arrays and a polarizer. This pulse shaper is the first non- interferometric setup for unrestricted phase, amplitude, and polarization shaping. Moreover, a parametric encoding of the electrical field was developed which allowed for generating pulse sequences consisting of several sub-pulses. Each sub-pulse can be controlled in its physically apparent parameters: energy, position in time, phase, and chirps as well as state of polarization with ellipticity, orientation, and helicity. The feasibility of the setup is illustrated by systematic variations of single and double pulses, which are experimentally generated and measured. In these series of pulses, one of the pulses parameters is varied while the other parameters are kept constant. This proves the precise control of the pulse shape. Further, more complex pulses are shown to give an impression of the capabilities of this setup in combination with the parameterization. For some fields of application, it could be desirable to guide the phase, amplitude and polarization shaped pulses through an optical fiber. For example, life science could benefit from shaped pulses in the area of imaging and photodynamic therapy. In order to make the shaped pulses available at the distal end of an optical fiber, the effects of the fiber have to be compensated. In this work, the generation of phase, amplitude, and polarization shaped pulses after transmission trough an optical fiber is presented. This is demonstrated for two types of optical fibers – a standard single-mode fiber and a microstructured hollow core photonic crystal fiber. The procedure for the determined generation of parametrically shaped pulses differs for both types of optical fibers. The capabilities of parametrically shaped pulse sequences are – again – illustrated by a series of example pulses. The parametrically shaped pulses transmitted through the hollow core fiber are implemented in a coherent control experiment. For this purpose, the potassium dimer produced in a molecular beam serves as a test system. The multi-photon ionization is studied by a closed feedback loop optimization and shaper-assisted pump-probe spectroscopy. These experiments revealed the excitation path including the vibrational dynamics in the first and second excited state. Further, the relevance of the polarization control in the excitation process is highlighted. Moreover, coherent control techniques are applied to investigate the interaction of shaped femtosecond laser pulses with ultracold atoms and molecules. The focus of these experiments is the control of the photoassociation of a colliding atom pair to a bound molecule. This process is investigated using two-color pump-probe spectroscopy. The molecular transients are compared to theoretical calculations. The analysis of this data reveals the interaction process which is discussed in the time and frequency domain. In a second experiment, the multi-photon ionization of ultracold rubidium dimers is optimized in a closed feedback loop. In this optimization, a parameterization in the frequency domain is employed which extracts the relevant transition frequencies. In combination with a complementary experiment the excitation pathway was revealed.
The development of the field of the science of ultra-cold matter has opened some exciting possibilities in exploring the quantum-mechanical processes which dominate matter interactions at the sub-microscopic scale. Although methods of cooling atoms are well established, molecular cooling is made difficult by molecules’ additional vibrational and rotational degrees of freedom. It was the goal of the research in this work to approach molecular cooling indirectly, by using broadband shaped-pulse photoassociation for the generation of tightly bound ultracold Rb2 molecules. The experiments towards this goal conducted by our group included a pumpdecay experiment to observe the generation of ground state singlet or triplet molecules. However, attempts to observe an increase in ground state population have been unsuccessful. A pump-probe study of wavepacket dynamics in the 5s+5p electronic state was conducted in order to determine the appropriate timing for the application of an additional pulse to dump population into the ground state. Although the attempt to observe wavepacket oscillations has been unsuccessful, pump-probe studies have yielded the observation of loosely bound excited state molecules as a result of the photoassociation pulse. These results are promising as a first stage in a fully coherent pump-dump approach to stabilisation into the lowest vibrational ground state. This thesis will provide an introduction and overview to the concerns involved in addressing the problem of molecular cooling and generation. Experimental techniques will be discussed including pulsed laser systems, optical parametric amplifi- cation, and the presentation of an original design for pulse shaping with an acoustooptic modulator. The emphasis of these discussions will be on the principles and operating procedures required for the use of these devices as home-built systems. The thesis will conclude with the results of pump-probe experiments utilising the pulse shaper as a spectral cutting device.
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.
Abstract The first step in the coherent control of a photoinduced binary reaction is bond making or photoassociation. We have recently demonstrated coherent control of bond making in multi-photon femtosecond photoassociation of hot magnesium atoms, using linearly chirped pulses (Levin et al 2015 Phys. Rev. Lett. 114 233003 ). The detected yield of photoassociated magnesium dimers was enhanced by positively chirped pulses which is explained theoretically by a combination of purification and chirp-dependent Raman transitions. The yield could be further enhanced by pulse optimization resulting in pulses with an effective linear chirp and a sub-pulse structure, where the latter allows for exploiting vibrational coherences. Here, we systematically explore the efficiency of phase-shaped pulses for the coherent control of bond making, employing a parametrization of the spectral phases in the form of cosine functions. We find up to an order of magnitude enhancement of the yield compared to the unshaped transform-limited pulse. The highly performing pulses all display an overall temporally increasing instantaneous frequency and are composed of several overlapping sub-pulses. The time delay between the first two sub-pulses fits very well the vibrational frequency of the generated intermediate wavepacket. These findings are in agreement with chirp-dependent Raman transitions and exploitation of vibrational dynamics as underlying control mechanisms.
Coherent control of quantum phenomena can be achieved by using phase- and amplitude modulated laser pulses. With a self-learning loop, which combines a femtosecond pulse shaper, an optimization algorithm and an experimental feedback signal, it is possible to automatically steer the interaction between system and electric field. This approach allows control even without any knowledge of the Hamiltonian. We have successfully implement this learning loop for the coherent control of the nonresonant two-photon excitation in sodium. New solutions for 'bright' and 'dark' pulses could be designed for this two-photon resonance without previous knowledge of the resonances. In a further experiment we combined the self-learning loop with degenerate four-wave-mixing spectroscopy in order to study the influence of phase and amplitude modulated pulses on the molecular FWM response of the prototype system K2. The obtained control pulses typically show a complex structure which hides the responsible physical mechanism. In order to extract the applied mechanism the concept of parameterization has been introduced which allows the learning algorithm to optimize already known control processes. Therefore the control of vibrational wavepacket motion in the electronic ground and excited state could be attributed to chirped pulse excitation, impulsive Raman excitation by pulse trains and phase-related double pulses representing the well-known Tannor-Rice-Kosloff control scheme.
This work presents the results of an effort to use unique forms of optical radiation to better probe and control matter. Results are presented of a study of intense field photo-ionization of krypton and xenon in a two-color field. The use of a two-color field provides a valuable probe, the relative optical phase, into the dynamics of the ionization process. It is found that phase dependent tunneling character is preserved even though the photoelectron spectra indicate that the experiments performed were well into the multi-photon regime of ionization. Evidence for core scattering of the departing electrons is seen in the changes to the phase dependent spectra as the polarization of the exciting light is varied from linear to slightly elliptical. To further control the optical field, a pulse shaper was constructed using liquid crystal modulators that allowed either spectral phase or spectral amplitude shaping of a short pulse. The results were characterized using cross-correlations. The shaped light was then subsequently amplified in a chirped pulse amplifier. This light was characterized using Frequency Resolved Optical Gating, a newly developed technique for the complete determination of the optical field in a short pulse. The shaped pulses were then used to tailor atomic radial wavepackets in cesium. The evolution of the wavepackets was monitored by measuring atomic auto-interferograms for the case of amplitude shaping, which was used to control the atomic states excited. Cross-interferograms were used for phase shaping, which was used to select the initial phase of the atomic states. The cross-interferograms required the simultaneous amplification of a shaped and an unshaped pulse in our amplifier.
Calculations relating to two experiments that demonstrate coherent control of preformed rubidium-85 molecules in a magneto-optical trap using ultrafast laser pulses are presented. The two experiments are a step toward the stabilization of ultracold rubidium dimers using ultrafast lasers. In the first experiment, it is shown that preassociated molecules in an incoherent mixture of states can be made to oscillate coherently using a single ultrafast pulse. A mechanism that can transfer molecular population to more deeply bound vibrational levels is used in the second. Optimal parameters of the control pulse are presented for the application of the mechanism to molecules in a magneto-optical trap. The calculations make use of an experimental determination of the initial state of molecules photoassociated by the trapping lasers in the magneto-optical trap and use shaped pulses consistent with a standard ultrafast laser system. The experiment's purpose is to demonstrate and evaluate the use of ultrafast shaped pulses to manipulate ultracold rubidium dimers with a view to the eventual stabilization of the molecules.