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International Conference on Applications of Optics and Photonics

Control of population inversion and coherence generation in Rb and Cs atoms

Iduabo John Afa, Benjamin Anwasia

Published Oct 3, 2019• 0 citations
Publisher

Abstract

Control of population inversion and coherence generation between the ground and excited states in Hydrogen–like alkali atoms is presented in this work using multiphoton π-pulse scheme with pulse durations between 100 fs to 10 ps. Density matrix (DM) equations of 4–level Rb and Cs systems are numerically solved beyond the rotating wave approximation (RWA) taking detuning and spectral chirp into account. An extension of the study is done to nP3/2 (n = 12) and nF5/2 (n = 8) of Cs atoms demonstrating the applicability of the current scheme in selective Rydberg excitation and potential extendibility to numerous configurations.

DOI
10.1117/12.2525289

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The coherent control of atomic energy level population is important for several areas physics and engineering including fluorescent spectroscopy, resonantly enhanced multiphoton ionization, harmonic generation, remote sensing and lasing. At low intensities where the Stark shift can be neglected, coherent control mechanisms usually exploit resonances created from stationary energy states dressed by integer multiples of the photon energy. However, in the strong field limit where large laser pulse intensities lead to significant Stark shifts, the resonant conditions are dynamic both in time and space and will likely not be satisfied for the entire laser pulse or beam.\n\nIn this research, we experimentally and theoretically demonstrate coherent control of atomic energy level population in sodium vapor as well as the selective excitation of sodium Rydberg levels using high intensity laser pulses. We also make a direct measurement of the ponderomotive shift of the continuum. While techniques based on resonance interferences and rapid adiabatic passage have been introduced for selective excitation, the conditions that must be satisfied for these phenomena can be hampered by short femtosecond pulse durations and large Stark shifts. In contrast, a technique referred to here as Resonance Sampling, uses the AC Stark shifts to selectively induce Freeman resonances and is resilient to changes in the spectral phase. Freeman resonances established in this way can result in population inversion of the excited state, and may lead to new lasing mediums and remote sensing applications.\n\n\nIn this research, we experimentally and theoretically demonstrate coherent control of atomic energy level population in sodium vapor as well as the selective excitation of sodium Rydberg levels using high intensity laser pulses. While techniques based on resonance interferences and rapid adiabatic passage have been introduced for selective excitation, the conditions that must be satisfied for these phenomena can be hampered by short femtosecond pulse durations and large Stark shifts. In contrast, a technique referred to here as Resonance Sampling, uses the AC Stark shifts to selectively induce Freeman resonances and is resilient to changes in the spectral phase. Freeman resonances established in this way can result in population inversion of the excited state, and may lead to new lasing mediums and remote sensing applications.

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Control is important for transferring theoretical scientific knowledge into practical technology for applications in numerous fields. This is why coherent control study is significant on every timescale to have a complete understanding of dynamic processes that occur on the electron, atomic and molecular levels. As a result, numerous schemes have been proposed to carry out effective quantum control of diverse systems and study the dynamics of these systems based on their natural timescales from the picoseconds (10-12 s), femtosecond (10-15 s) to attosecond (10-18 s) regimes. The goals of these various studies depend on the desired application, for instance in Photochemistry a long standing objective is achieving selective population transfer from an initial state to a desired target state with little or no diminution in the energy transferred. 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Physical Review A•Feb 1, 2007•11 citations
Control of population inversion and coherence generation between the ground and excited states in Hydrogen–like alkali atoms is presented in this work using multiphoton π-pulse scheme with pulse durations between 100 fs to 10 ps. Density matrix (DM) equations of 4–level Rb and Cs systems are numerically solved beyond the rotating wave approximation (RWA) taking detuning and spectral chirp into account. An extension of the study is done to nP3/2 (n = 12) and nF5/2 (n = 8) of Cs atoms demonstrating the applicability of the current scheme in selective Rydberg excitation and potential extendibility to numerous configurations.
The coherent control of atomic energy level population is important for several areas physics and engineering including fluorescent spectroscopy, resonantly enhanced multiphoton ionization, harmonic generation, remote sensing and lasing. At low intensities where the Stark shift can be neglected, coherent control mechanisms usually exploit resonances created from stationary energy states dressed by integer multiples of the photon energy. However, in the strong field limit where large laser pulse intensities lead to significant Stark shifts, the resonant conditions are dynamic both in time and space and will likely not be satisfied for the entire laser pulse or beam.\n\nIn this research, we experimentally and theoretically demonstrate coherent control of atomic energy level population in sodium vapor as well as the selective excitation of sodium Rydberg levels using high intensity laser pulses. We also make a direct measurement of the ponderomotive shift of the continuum. While techniques based on resonance interferences and rapid adiabatic passage have been introduced for selective excitation, the conditions that must be satisfied for these phenomena can be hampered by short femtosecond pulse durations and large Stark shifts. In contrast, a technique referred to here as Resonance Sampling, uses the AC Stark shifts to selectively induce Freeman resonances and is resilient to changes in the spectral phase. Freeman resonances established in this way can result in population inversion of the excited state, and may lead to new lasing mediums and remote sensing applications.\n\n\nIn this research, we experimentally and theoretically demonstrate coherent control of atomic energy level population in sodium vapor as well as the selective excitation of sodium Rydberg levels using high intensity laser pulses. While techniques based on resonance interferences and rapid adiabatic passage have been introduced for selective excitation, the conditions that must be satisfied for these phenomena can be hampered by short femtosecond pulse durations and large Stark shifts. In contrast, a technique referred to here as Resonance Sampling, uses the AC Stark shifts to selectively induce Freeman resonances and is resilient to changes in the spectral phase. Freeman resonances established in this way can result in population inversion of the excited state, and may lead to new lasing mediums and remote sensing applications.
Control is important for transferring theoretical scientific knowledge into practical technology for applications in numerous fields. This is why coherent control study is significant on every timescale to have a complete understanding of dynamic processes that occur on the electron, atomic and molecular levels. As a result, numerous schemes have been proposed to carry out effective quantum control of diverse systems and study the dynamics of these systems based on their natural timescales from the picoseconds (10-12 s), femtosecond (10-15 s) to attosecond (10-18 s) regimes. The goals of these various studies depend on the desired application, for instance in Photochemistry a long standing objective is achieving selective population transfer from an initial state to a desired target state with little or no diminution in the energy transferred. In quantum computation, a central issue is the excitation of unoccupied Rydberg states with numerous proposals for its use in the design and implementation of robust fast quantum gates. Also, since the advent of the generation of attosecond XUV pulses, doors have been opened for achieving control of atomic-scale electron dynamics and observing them in real-time. This thesis explores the modelling of dynamical light-matter interaction processes, like effective population inversion and generation of vibrational coherences in atoms and molecules, on their fundamental timescales using the density matrix (DM) theory under and beyond the rotating wave approximation (RWA). The thesis begins by introducing the concept of coherent control of simple quantum systems based on the DM formalism and expands the application to a more complex Oxazine system. Multiphoton p-pulse scheme is demonstrated for the control of population transfer in multilevel systems, for example with a trichromatic p-pulse having a set of areas v3 p, 2p and v3 p, complete population transfer in a four level system can be achieved. The aforementioned scheme is used to achieve effective control of low-lying Rydberg states in rubidium atoms, demonstrating how the effective control can be crucially affected by numerous physical processes. One main advantage of the density matrix approach over other theoretical approaches is that it allows the possibility of easily computing relaxation terms and other physical parameters critical to successful coherent control. The DM formalism is shown to be successful in properly describing the enhancement effects in atoms and complex molecular systems. It is robust in coherent control and quantum control spectroscopy (QCS) schemes and is extendable to numerous systems and geometric configurations. In the last part of the thesis, experiments on laser dressing processes in attosecond transient absorption spectroscopy are compared to numerical simulations using the DM analysis beyond the RWA. The research in this thesis opens a pathway to numerous studies using the DM formalism for applications in diverse fields of femtochemistry, attophysics, high precision spectroscopy and quantum information processing. El control quàntic coherent és important per transferir coneixement científic teòric a la tecnologia, per a aplicacions en nombrosos camps. Per aquest motiu, l'estudi del control coherent és significatiu a totes les escales de temps, per comprendre de manera completa els processos dinàmics que es produeixen en els nivells electrònics, atòmics i moleculars. Com a resultat, s'han proposat nombrosos esquemes per dur a terme un control quàntic eficient de diversos sistemes i estudiar la dinàmica d'aquests sistemes basant-se en les seves escales de temps naturals, des dels picosegons (10-12 s), femtosegons (10-15 s) fins a règims d'attosegons (10-18 s). Els objectius d'aquests estudis depenen de l'aplicació desitjada, per exemple, en fotoquímica, un objectiu buscat extensivament és aconseguir una transferència selectiva de la població des d'un estat inicial fins a un estat final desitjat, amb poca o cap disminució de l'energia transferida. En computació quàntica, un tema central és l'excitació dels estats desocupats de Rydberg, amb nombroses propostes per al seu ús en el disseny i implementació de portes quàntiques robustes i ràpides. A més, des de l'arribada de la generació de polsos XUV d'attosegons, s'han obert camins per aconseguir el control de la dinàmica electrònica a escala atòmica i observar-la en temps real. Aquesta tesi explora la modelització de processos dinàmics d'interacció llum-matèria, com ara la inversió efectiva de població i la generació de coherències vibracionals en àtoms i molècules, en les seves escales de temps fonamentals, utilitzant la teoria de la matriu densitat (MD), en l'aproximació d'ona rotant (RWA) i més enllà. La tesi comença introduint el concepte de control coherent de sistemes quàntics simples basats en el formalisme de MD i amplia l'aplicació a un sistema més complexes, com ara oxazines. Es demostra l'esquema multifotó amb polsos p per al control de la transferència de població en sistemes multinivell, per exemple, amb un pols p-tricromàtic que té un conjunt d'àrees v3 p, 2p i v3 p, amb el qual la transferència completa de població en un sistema de quatre nivells pot ser aconseguit. L'esquema esmentat s'utilitza per aconseguir un control eficient d'estats de Rydberg en àtoms de rubidi, i es mostra com aquest control pot ser afectat crucialment per nombrosos processos físics. Un avantatge principal de l'estudi de la matriu densitat comparat amb altres enfocaments teòrics és que permet la possibilitat de computar fàcilment els termes de relaxació i altres paràmetres físics crítics per a un control coherent efectiu. Es demostra que el formalisme de la MD és exitós per descriure correctament l'amplificació d'efectes tant en àtoms com en sistemes moleculars més complexos. El formalisme de la MD és robust en esquemes de control coherent i d'espectroscòpia quàntica i és extensible a nombrosos sistemes i configuracions. En la darrera part de la tesi, es comparen experiments realitzats sobre processos de "vestiment" amb làsers intensos en espectroscòpia d'absorció transitòria d'attosegon amb simulacions numèriques utilitzant l'anàlisi de MD més enllà de la RWA. La recerca en aquesta tesi obre un camí a nombrosos estudis utilitzant el formalisme de MD per a aplicacions en diversos camps de femtoquímica, attofísica, espectroscòpia d'alta precisió i processament d'informació quàntica.
We propose an ultrafast femtosecond time scale trichromatic $\ensuremath{\pi}$-pulse illumination scheme for coherent excitation and manipulation of low-lying Rydberg states in rubidium. Selective population of nP3/2{nP}_{3/2}nP3/2​ levels with principal quantum numbers $n\ensuremath{\lesssim}12$ using 75-fs laser pulses is achieved. The density-matrix equations of a four-level ladder system beyond the rotating-wave approximation have to be solved to clarify the balance between the principal quantum numbers, the duration of the laser pulses, and the associated ac-Stark effects for the fastest optimal excitation. The mechanism is robust for femtosecond control using different level configurations for applications in ultrafast quantum information processing and spectroscopy.
Detailed theoretical analysis and numerical simulation indicate that nearly complete electronic population inversion of molecular systems can be achieved with intense positively chirped broadband laser pulses. To provide a simple physical picture, a two-level model is used to examine the condition for the so-called π pulses and a four-level model is designed to demonstrate for molecular systems the correlation between the sign of the chirp and the excited state population. The proposed molecular π pulse is the combined result of vibrational coherence in the femtosecond regime and adiabatic inversion in the picosecond regime. Numerical results for a displaced oscillator, for LiH and for I2, show that the proposed molecular π pulse scheme is robust with respect to changes in field parameters such as the linear positive chirp rate, field intensity, bandwidth, and carrier frequency, and is stable with respect to thermal and condensed phase conditions including molecular rotation, rovibronic coupling, and electronic dephasing.
Coherent control of regions with spatial excitation of populations and coherence between two lower states in three-level $\ensuremath{\Lambda}$-type quantum systems mediated by the self-induced transparency (SIT) phenomenon is theoretically investigated with one- and two-color ultrashort pulses and with pulse sequences beyond the slowly varying envelope (SVEA) and rotating wave (RWA) approximations. The effects of different parameters characterizing the pulses and the medium on the location as well as the width of such excited regions have been studied numerically by means of the finite-difference time-domain (FDTD) method. It has been determined that with a scheme of two-pulse excitation one can effectively control the position at which the region is written and its width. In particular, the position of the excited region can be controlled by the area of the pulses. We find that the maximum value of the population transferred to the lower excited state depends on the detuning of the pulses with respect to the one-photon resonances, and that both position and width of the region also depend on the temporal duration of the pulses. We show how after the excited region is written, its position can be shifted by additional pulses. On the basis of numerical results, scaling laws are formulated for the reported phenomena. With such control, the width of the region excited inside the medium can be reduced to the order of the wavelength of the light.