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Physical Review A

Efficient photoassociation with a train of asymmetric laser pulses

Wei Zhang, Gao‐Ren Wang, Shu‐Lin Cong

Published Apr 18, 2011• 24 citations
Publisher

Abstract

The photoassociation (PA) dynamics implemented by a train of asymmetric slowly turned-on and rapidly turned-off (STRT) laser pulses is investigated theoretically with Cs${}_{2}$ as an example. A higher PA efficiency is achieved by optimizing the parameters of the STRT pulse train. The PA reaction goes partly beyond the scope of the PA window. Numerical calculations show that an efficient population accumulation in the PA process can be realized with the STRT laser-pulse train which is available in the current experiment based on laser mode-lock and shaping technology.

DOI
10.1103/physreva.83.045401

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The photoassociation (PA) dynamics implemented by a train of asymmetric slowly turned-on and rapidly turned-off (STRT) laser pulses is investigated theoretically with Cs2{}_{2}2​ as an example. A higher PA efficiency is achieved by optimizing the parameters of the STRT pulse train. The PA reaction goes partly beyond the scope of the PA window. Numerical calculations show that an efficient population accumulation in the PA process can be realized with the STRT laser-pulse train which is available in the current experiment based on laser mode-lock and shaping technology.
We present a theoretical scheme used for investigating the formation of ultracold molecules via photoassociation with a slowly-turned-on and rapidly-turned-off (STRT) laser pulse, with Cs2{}_{2}2​ as an example. As compared with an unshaped laser pulse, this kind of shaped laser pulse has a prominent advantage by which we can prevent the associated molecules from returning to the initial scattering state. Numerical calculations show that an efficient photoassociation can be realized with the STRT laser pulse, which is available in the current experiment as an optical tool.
We investigate theoretically the photoassociation (PA) of two Cs\mathrm{Cs}Cs atoms using a slowly-turned-on and rapidly-turned-off (STRT) laser pulse at the temperature of $54\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$ by taking the thermal average of the initial continuum states into account. For comparison, the PA probabilities under the action of the STRT and typical Gauss-type pulses are calculated, respectively. The rising and falling times of the STRT pulse are set to be ${\ensuremath{\tau}}_{r}=10$ ps and ${\ensuremath{\tau}}_{f}=0.2$ ps, and the two Gauss-type pulses are set to be ${\ensuremath{\tau}}_{r}={\ensuremath{\tau}}_{f}=10$ ps and ${\ensuremath{\tau}}_{r}={\ensuremath{\tau}}_{f}=5.1$ ps, respectively. Compared to the calculation considering a single initial continuum state (non-thermal-average calculations), the PA probabilities with the consideration of the thermal average are reduced to different extents according to different laser pulses. The PA process induced by the STRT pulse is more robust than that by the two Gauss-type pulses. For the STRT pulse, the PA probability is reduced by roughly 30%, while for the two Gauss-type pulses, it is reduced by roughly 97% and 64%, respectively. Because of the larger bandwidth and the asymmetric time profile of the STRT pulse, the PA probability induced by this pulse is related to both the off-resonant and the near-resonant transitions in a much broader phase space formed by the initial continuum states and the final vibrational states. Consequently, the thermal average of the initial continuum states has relatively smaller influence on the PA probability induced by the STRT pulse. By changing the pulse shape of the standard Gaussian to the sin2{\mathrm{sin}}^{2}sin2 and the Lorentz ones, the STRT pulse still results in the highest PA probability and is the most robust to the thermal average.
We investigate theoretically the formation of ultracold Cs2 molecules via photoassociation (PA) with three kinds of pulses (the Gaussian pulse, the asymmetric shaped laser pulse SL1 with a large rising time and a small falling time and the asymmetric shaped laser pulse SL2 with a small rising time and a large falling time). For the three kinds of pulses, the final population on vibrational levels from v′ = 120 to 175 of the excited state displays a regular oscillation change with pulse width and interaction strength, and a high PA efficiency can be achieved with optimised parameters. The PA efficiency in the excited state steered by the SL1-pulse (SL2-pulse) train with optimised parameters which is composed of four SL1 (SL2) pulses is 1.74 times as much as that by the single SL1 (SL2) pulse due to the population accumulation effect. Moreover, a dump laser is employed to transfer the excited molecules from the excited state to the vibrational level v″ = 12 of the ground state to obtain stable molecules.
The formation of ultracold Cs 2 molecules via photoassociation (PA) with a spectral π-jump laser pulse is theoretically investigated by solving the time-dependent Schrödinger equation using the mapped Fourier grid method. Numerical calculations show that the population distributions of associated molecules mainly result from the off-resonant excitation of the π-jump laser pulse. Compared with the transform-limited pulse, an efficient population accumulation in the PA process is achieved with the π-jump pulse. An oscillation pattern appears in the population on the vibrational levels of the state owing to the action of the post-half-pulse in the π-jump pulse. The PA efficiency is related to the Rabi angle of the laser pulse. A π-jump pulse can be used to enhance more effectively the PA efficiency than a transform-limited pulse with the same Rabi angle.
Abstract Photoassociation (PA) of 85Rb atoms from the ground X 1 Σ g + state to the excited A 1 Σ g + and b 3 Π u states by two-color laser pulses at 100 μ K × k B is investigated by using quantum wavepacket method. Two pulses with detunings δ 1 and δ 2 varying from −1.0 to 1. cm−1 are taken into account. The specific case δ 1 = − δ 2 is generalized to the condition of arbitrary combination of δ 1 and δ 2 for both the Gaussian-pulse field and the slowly-turned-on and rapidly-turned-off (STRT) field. It is found that the introduce of the detunings in the two pluses can enhance the asymmetry of the time profile and broaden the linewidth of the laser field. The two factors can further enhance the PA probability for both resonant and nonresonant regions. Compared to the Gaussian field, the STRT field which is even more asymmetric in time profile and more broaden in linewidth can significantly enhance the PA probability, with the nonresonant transition dominating the PA process. It can be expected that once the two detunings are both positive, which is a relatively loose restriction for coherent control, a considerable PA probability can be obtained in a two-color STRT field.
The photoassociation (PA) dynamics of ultracold cesium atoms steered by the picosecond laser pulse with cubic-phase modulation has been investigated theoretically by solving numerically the time-dependent Schr\"odinger equation using the mapped Fourier grid method. The temporal duration of the pulse is prolonged, and the pulse area is enlarged by modulating the cubic phase. The PA efficiency is related to the pulse area. When the pulse area is larger than a certain value, the cubic-phase pulses noticeably enhance the PA efficiency, although they have lower energies.