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International Journal of Quantum Chemistry

Internal state cooling with a femtosecond optical frequency comb

Svetlana Malinovskaya, V. Patel, Tom Collins

Published Sep 8, 2010• 1 citation
Publisher

Abstract

Abstract A single femtosecond optical frequency comb is implemented to induce two‐photon transitions between molecular vibrational levels to form ultracold molecules. The phase across an individual pulse in the pulse train is sinusoidally modulated with a carefully chosen modulation amplitude and frequency. Piecewise population transfer is fulfilled to the final state by each pulse in the applied pulse train providing a controlled population accumulation in the final, ultracold state. The dynamics of rovibrational cooling is discussed on an example of the KRb molecule. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

DOI
10.1002/qua.22896

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Abstract A single femtosecond optical frequency comb is implemented to induce two‐photon transitions between molecular vibrational levels to form ultracold molecules. The phase across an individual pulse in the pulse train is sinusoidally modulated with a carefully chosen modulation amplitude and frequency. Piecewise population transfer is fulfilled to the final state by each pulse in the applied pulse train providing a controlled population accumulation in the final, ultracold state. The dynamics of rovibrational cooling is discussed on an example of the KRb molecule. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010
We show that a single femtosecond optical frequency comb may be used to induce two-photon transitions between molecular vibrational levels to form ultracold molecules (e.g., KRb). The phase across an individual pulse in the pulse train is sinusoidally modulated with a carefully chosen modulation amplitude and frequency. Piecewise adiabatic population transfer is fulfilled to the final state by each pulse in the applied pulse train, providing a controlled population accumulation in the final state. Detuning the pulse train carrier and modulation frequency from one-photon resonances changes the time scale of molecular dynamics but leads to the same complete population transfer to the ultracold state. A standard optical frequency comb with no modulation is shown to induce similar dynamics, leading to rovibrational cooling.
We discuss femtosecond Raman-type techniques to control molecular vibrations, which can be implemented for internal-state cooling from Feshbach states with the use of optical frequency combs (OFCs) with and without modulation. The technique makes use of multiple two-photon resonances induced by optical frequencies present in the comb. It provides us with a useful tool to study the details of molecular dynamics at ultracold temperatures. In our theoretical model we take into account decoherence in the form of spontaneous emission and collisional dephasing in order to ascertain an accurate model of the population transfer in the three-level system. We analyze the effects of odd and even chirps of the OFC in the form of sine and cosine functions on the population transfer. We compare the effects of these chirps to the results attained with the standard OFC to see if they increase the population transfer to the final deeply bound state in the presence of decoherence. We also analyze the inherent phase relation that takes place owing to collisional dephasing between molecules in each of the states. This ability to control the rovibrational states of a molecule with an OFC enables us to create deeply bound ultra-cold polar molecules from the Feshbach state.
We report on a theoretical study of implementation of a single femtosecond optical frequency comb to perform an efficient transfer of Feshbach molecules to the ground electronic and rovibrational state. A standard optical frequency comb is considered with zero offset frequency and a sinusoidally phase modulated frequency comb, having modulation frequency in the IR region. A stepwise population transfer to the ultracold molecular state is observed, with a negligible population of the excited state in the case of the sinusoidally modulated frequency comb.
Abstract. Some of us have recently reported in Science [1] vibrational cooling of translationally cold Cs2 molecules into the lowest vibrational level v = 0 of the singlet X1Σg ground electronic state. Starting from a sample of cold molecules produced in a collection of vibrational levels of the ground state, our method was based on repeated optical pumping by laser light with a spectrum broad enough to excite all populated vibrational levels but frequency-limited in such a way to eliminate transitions from v = 0 level, in which molecules accumulate. In this paper this method is generalized to accumulate molecules into an arbitrary selected ”target ” vibrational level. It is implemented by using ultrashort pulse shaping techniques based on Liquid Crystal spatial light modulator. In particular a large fraction of the initially present molecule is transferred into a selected vibrational level such as v = 1, 2 and 7. Limitations of the method as well as the possible extension to rotational cooling are also discussed. PACS numbers: 06.60.Jn; 32.80.Qk; 33.20.Tp; 37.10.Mn; 42.65.Re; 42.79.Kr 1.
AbstractA method is proposed to create molecules in the ultracold state from the Feshbach molecules by stepwise adiabatic passage using an optical frequency comb without losses due to decoherence. An emphasis is made on the impact of the vibrational state manifold on controllability of the coherent dynamics by including five excited states into the model. The results are compared with recently reported results on a three-level λ system. Sinusoidal modulation across an individual pulse in the pulse train is applied, leading to the creation of a quasi-dark state, which minimizes population of the transitional, vibrational state manifold, and efficiently mitigates decoherence in the system. The parity of the temporal chirp is shown to be an important factor in designing population dynamics in the system.