Institut für Theoretische Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany.
Faraday Discuss. 2009;142:389-402; discussion 429-61. doi: 10.1039/b818458e.
Photoassociation with short laser pulses has been proposed as a technique to create ultracold ground state molecules. A broad-band excitation seems the natural choice to drive the series of excitation and deexcitation steps required to form a molecule in its vibronic ground state from two scattering atoms. First attempts at femtosecond photoassociation were, however, hampered by the requirement to eliminate the atomic excitation leading to trap depletion. On the other hand, molecular levels very close to the atomic transition are to be excited. The broad bandwidth of a femtosecond laser then appears to be rather an obstacle. To overcome the ostensible conflict of driving a narrow transition by a broad-band laser, we suggest a two-photon photoassociation scheme. In the weak-field regime, a spectral phase pattern can be employed to eliminate the atomic line. When the excitation is carried out by more than one photon, different pathways in the field can be interfered constructively or destructively. In the strong-field regime, a temporal phase can be applied to control dynamic Stark shifts. The atomic transition is suppressed by choosing a phase which keeps the levels out of resonance. We derive analytical solutions for atomic two-photon dark states in both the weak-field and strong-field regime. Two-photon excitation may thus pave the way toward coherent control of photoassociation. Ultimately, the success of such a scheme will depend on the details of the excited electronic states and transition dipole moments. We explore the possibility of two-photon femtosecond photoassociation for alkali and alkaline-earth metal dimers and present a detailed study for the example of calcium.
利用短激光脉冲进行光缔合已被提议作为一种产生超冷基态分子的技术。宽带激发似乎是自然的选择，可以驱动一系列激发和去激发步骤，从而将两个散射原子形成其振动态基态中的分子。然而，飞秒光缔合的最初尝试受到消除导致陷阱耗尽的原子激发的要求的阻碍。另一方面，非常接近原子跃迁的分子能级需要被激发。飞秒激光的宽带宽似乎是一个障碍。为了克服用宽带激光驱动窄跃迁的表面冲突，我们建议采用双光子光缔合方案。在弱场情况下，可以采用光谱相位模式来消除原子线。当通过多个光子进行激发时，场中的不同路径可以建设性或破坏性地干涉。在强场情况下，可以施加时间相位来控制动态斯塔克位移。通过选择使能级失谐的相位，可以抑制原子跃迁。我们在弱场和强场两种情况下推导出原子双光子暗态的解析解。因此，双光子激发可以为光缔合的相干控制铺平道路。最终，这样的方案的成功将取决于激发电子态和跃迁偶极矩的细节。我们探索了双光子飞秒光缔合用于碱金属和碱土金属二聚体的可能性，并以钙为例进行了详细研究。
