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arXiv:0802.2986 · Journal of Physics B Atomic Molecular and Optical Physics

Coherent control of spin–orbit precession with shaped laser pulses

B. Chatel, Damien Bigourd, S. J. Weber, B. Girard

Published Mar 26, 2008• 13 citations
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Abstract

Spin precession in rubidium atoms is investigated through a pump–probe technique. The excited wave packet corresponds to a precession of spin and orbital angular momentum around the total angular momentum. We show that using shaped laser pulses allows us to control this dynamics. With a Fourier transform limited pulse, the wave packet is initially prepared in the bright state (coupled to the initial state) whereas a pulse presenting a π step in the spectral phase prepares the wave packet in the dark state (uncoupled to the initial state).

DOI
10.1088/0953-4075/41/7/074023

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Spin precession in rubidium atoms is investigated through a pump–probe technique. The excited wave packet corresponds to a precession of spin and orbital angular momentum around the total angular momentum. We show that using shaped laser pulses allows us to control this dynamics. With a Fourier transform limited pulse, the wave packet is initially prepared in the bright state (coupled to the initial state) whereas a pulse presenting a π step in the spectral phase prepares the wave packet in the dark state (uncoupled to the initial state).
A sequence of two identical ultrashort laser pulses creates quantum interferences in an atom. In the weak-field regime, the observed beats between interferences arising from various transitions are not a conclusive signature of wave-packet creation. On the contrary, in the saturation regime, the appearance of slow modulation is. We report on the observation of such modulations in atomic potassium in this field regime. The results are explained within the bright-state--dark-state framework. The wave packet corresponds to the precession of the electronic spin and orbital angular momentum.
Experimental and theoretical work is presented on the control of wave packet dynamics in atomic and molecular systems. Using sequences of phase-locked optical pulses, the link between optical phase and quantum mechanical phase is explored in the Na atom and Naj dimer, representing a step towards the logical engineering of quantum states in more complicated systems. A novel apparatus, constructed to study and control the vibrational dynamics of vibrational wave packets on the Na ionic potential surface, is described in chapter 3. Theoretical simulations for proposed experiment are presented in chapter 4. Control is achieved by exploiting the phase-evolution of the constituent vibrational quantum states within the wave packet superposition. The phase relationship and the accumulated phase difference between the various components of the wave packet is determined, and a sequence of phase-locked optical pulses is employed to selectively enhance or depopulate specific vibrational states, or sets of vibrational states. The quantum state composition of the resulting wave packet, and the efficiency of the control scheme, is determined by calculating the multi-pulse response of the time-dependent vibrational state populations. In chapter 5, the quantum interference between Rydberg electron wave packets in the Na atom is investigated using pairs of phase-locked wave packets, allowing manipulation of the total orbital angular momentum of Na Rydberg atoms. Initially the wave packet is composed of a superposition of s and d Rydberg series. Exploitation of the difference between the quantum defects of the two series allows specific angular momentum compositions within the resultant wave packet to be engineered. Experimentally, this final quantum state distribution is analysed in the frequency domain using state selective field ionisation, and in the time domain using the optical Ramsey method. The theoretical calculations show how the phase difference between pairs of optical pulses is linked to the corresponding Rydberg frequency spectrum, therefore enabling the control of the quantum state composition of the wave packets.
Using techniques of ultrafast coherent control, we propose a method to produce high-purity high-l\mathcal{l}l (angular momentum) states in Rydberg Stark wave packets. Two time-delayed phase-locked laser pulses excite the atom from a low-lying ``launch state'' into a low-l\mathcal{l}l Rydberg wave packet, in the presence of a static electric field. By choosing the time delays between the pulses, and the static electric field, we find that a high-l\mathcal{l}l state with high purity can be created at the target time.
The absorption of traveling photons resonant with electric dipole transitions of an atomic gas naturally leads to electric dipole spin-wave excitations.For a number of applications, it would be highly desirable to shape and coherently control the spatial waveform of the spin waves before spontaneous emission can occur.This paper details a recently developed optical control technique to achieve this goal, where counterpropagating, shaped subnanosecond pulses impart subwavelength geometric phases to the spin waves by cyclically driving an auxiliary transition.In particular, we apply this technique to reversibly shift the wave vector of a spin wave on the D2 line of laser-cooled 87 Rb atoms by driving an auxiliary D1 transition with shape-optimized pulses, so as to shut off and recall superradiance on demand.We investigate a spin-dependent momentum transfer during the spin-wave control process, which leads to a transient optical force as large as ∼1hk/ns, and study the limitations to the achieved 70 ∼ 75% spin-wave control efficiency by jointly characterizing the spin-wave control and matter-wave acceleration.Aided by numerical modeling, we project potential future improvements of the control fidelity to the 99% level when the atomic states are better prepared and by equipping a faster and more powerful pulse shaper.Our technique also enables a background-free measurement of the superradiant emission to unveil the precise scaling of the emission intensity and decay rate with optical depth.
The precession of orbital and spin angular momentum vectors has been observed in a pump-probe study of the 4P fine-structure states of atomic potassium. A femtosecond pump pulse prepared a coherent superposition of the two fine-structure components. A time-delayed probe pulse then ionized the system after it had been allowed to evolve freely. Oscillations recorded in the ion signal reflect the evolution of the orientation of the orbital and spin angular momentum due to spin-orbit coupling. This interpretation gives physical insight into the cause of the half-period phase shift observed when the relative polarizations of the laser pulses were changed from parallel to perpendicular. Finally, it is shown that these changes in the orientation of the spin momentum vector of the system can be utilized to produce highly spin-polarized free electrons on the femtosecond scale
A method to optically control a dark transition, for instance, the coupling between different spin states, is proposed. The control is achieved by manipulating the direction, amplitude, and duration of dynamic Stark shifts. Laser-driven spin switches can be prepared by conveniently generalizing different optical techniques, such as $\ensuremath{\pi}$-pulse schemes and adiabatic passage schemes. The efficiency and robustness of the schemes is analyzed for both two-level and multilevel systems, implying quantum state selective wave packet transfer between states of different multiplicity.