Effect of nonresonant frequencies on the enhancement of quantum beat amplitudes in rovibrational states of Li2: The role of state spacing

Abstract

Optical phase manipulation of nonresonant frequencies is investigated as a method of achieving optimal population transfer during resonant impulsive stimulated Raman scattering. Wave packets containing quantum beats between an initially prepared rovibrational level in the A(1Σu+) electronic state of Li2 and states populated via a resonance-enhanced rotational Raman process are created using a shaped ultrafast pulse centered near 800 nm. Study of these wave packets allows a quantitative comparison of population transfer as a function of applied phases in the ultrafast pulse. Two cases are explored to determine the ability to enhance population transfer: one with a wide state spacing [A(νA=11, JA=28)-A(11,30) at 50.1 cm−1] and one with a narrow spacing [A(11,8)-A(11,10) at 16.6 cm−1]. In both cases, several different phase masks are applied to the wave packet preparation pulse to enhance the population transferred to the newly formed state of interest. One phase mask involves the application of a −90° phase shift to the nonresonant optical frequencies that lie between the resonant transition frequencies, resulting in an optimal phase relationship between pairs of nonresonant frequencies contributing to the Stokes–Raman excitation. Another extends the phase modification to the nonresonant frequencies lying outside the two resonant transitions to allow constructive enhancement from a larger range of frequencies. Significant population enhancements, up to a factor of ∼12, of the newly formed A(11,30) and A(11,10) states are demonstrated. In addition, the dependence on the state spacing and therefore the extent to which nonresonant frequencies affect the population transferred in the stimulated Raman process are demonstrated.

Control of Li2 wave packet dynamics by modification of the quantum mechanical amplitude of a single state

“…Each pulse creates a wave packet that includes states prepared by one-photon excitation and by two photons through a resonant Raman transition, as described previously. 20,21 The excitations share a common final-state for the Raman transition and interference of the final state amplitudes is observed. Previous wave packet interference schemes use identical pulses to launch wave packets that differ only in phase.…”

“…Lithium dimer has been a successful candidate for coherent control experiments [16][17][18][19][20][21] and is an ideal system in which to demonstrate the multiple-pulse control presented here. The rotational transitions in Li 2 are well separated and the vibrational periods of different electronic states are on the order of the spectral bandwidth of commercially available ultrafast laser a͒ systems.…”

“…Pertinent to the present work, the role of phase in Raman wave packet excitation of Li 2 was previously investigated. 20,21 First-and second-order perturbation theories were used to predict the phases of specific wave packet states excited through stimulated Raman transitions. 20 The laser pulse phase was then altered to enhance certain quantum beats.…”

“…20 The laser pulse phase was then altered to enhance certain quantum beats. 21 Here, interference is used to eliminate the quantum mechanical amplitude of specific individual rotational states to design wave packets with precise population distributions. The technique changes the quantum mechanical amplitudes of particular rotational states by addressing the states through two interfering pathways.…”

Energy efficient method for two-photon population transfer with near-resonant chirped pulses

“…In particular, control of population transfer in multiphoton transitions with laser pulses has been achieved through several methods. A common strategy for ultrashort pulses is the manipulation of the spectral phases and amplitudes of the frequency components of the fields exciting the medium, resulting in pulses or sequences of pulses with very diverse temporal shapes which enhance or suppress the coupling between selected states by exploiting quantum interference effects [7][8][9][10][11][12][13][14]. Other techniques using longer pulses such as adiabatic methods [15][16][17][18] and π-pulse polychromatic control [19][20][21][22][23] can be implemented to induce complete population transfer between a pair of quantum states.…”