Generating Chirped Nanosecond Laser Pulses for Rapid Adiabatic Passage Experiment

Abstract

A chirped pulse is a short optical or electromagnetic pulse in which the frequency and phase change over time. The use of chirped laser pulses, where the laser frequency is dynamically varied during the pulse duration, has been shown to enhance optical forces significantly, leading to applications in quantum control, laser cooling, and atomic state preparation. In this presentation, we will talk about how radio-frequency driven Acousto-Optic Modulator (AOM) generates controlled frequency sweeps (chirped) optical pulses. The chirped laser pulse will be used to sweep the frequency of the laser across resonance slowly enough to maintain adiabatic evolution. Then, the system is robustly transferred from ground state to excited state if frequency sweeps (chirp) is slow compared to the Rabi frequency. By sweeping the laser frequency dynamically, we aim to achieve robust and efficient population transfer in atoms and molecules.

Fig. 1 – Energy levels of sodium corresponding to the 3s²S1/2, 3p²P1/2, 3p²P3/2, and 5s²S1/2 states. The detuning of the pump (p) and Stokes (s) laser pulses are shown for resonance with the 3p²P1/2 state. The labeled frequencies (508.332 THz, 486.990 THz) correspond to the approximate optical frequencies of the transitions between the energy levels, while the 516.250 GHz offset highlights the fine structure splitting between the 3p²P1/2 and 3p²P3/2 states.

Fig. 2 – Experimental Setup for Rapid Adiabatic Passage Experiment. Two laser beams are counter propagating.

Rapid Adiabatic Passage Experiment

Population transfer will be detected by monitoring the fluorescence doublet from the 5s state to the 3p states (616.07 and 615.42 nm). The fluorescence signal will be collected perpendicular to the laser propagation direction and imaged onto the entrance slit of the Horiba spectrometer. The laser must sweep across resonance slowly enough that the population can follow the dressed states, but fast enough to avoid decoherence. If the pulse is properly shaped (chirped via EOM), a nanosecond pulse can meet these conditions. In Na vapor at high temperatures, Doppler width is significant (~1 GHz at 300 K), which can limit adiabaticity unless Doppler-free configuration (e.g., counter-propagating beams) is used.

Optical Delay Stages & Acousto-Optic Modulators

To generate chirped pulses for rapid adiabatic passage, we can utilize optical delay stages or radio-frequency driven Acousto-Optic Modulators (AOMs). Optical delay stages, shown in Fig. 3, allow us to precisely control the relative timing between the pump and Stokes pulses, enabling temporal overlap necessary for excitation. Meanwhile, AOMs, illustrated in Fig. 4, are used to modulate the frequency of the beam in real-time. By applying a time-varying RF signal to the AOM, we sweep the laser frequency across resonance, producing a chirped pulse. This controlled frequency sweep ensures a transition between atomic energy levels, satisfying the adiabatic condition.

Fig. 3 – Optical Delay Stage, used to control the timing between laser pulses.

Fig. 4 – Acousto-Optic Modulator (AOM), which modulates frequency using sound waves in a crystal medium

Expected Outcomes

[1] Demonstration of ARP transfer efficiency from 3s to 5s state as a function of time delay between pump and Stokes pulses (chirped pulses) in Na atomic vapor.

[2] Identification of optimal experimental conditions for efficient population transfer.

[3] Comparison of experimental results with theoretical predictions.

References

Jim L. Hicks, Chakree Tanjaroon, Susan D. Allen, Matt Tilley, Steven Hoke, and J. Bruce Johnson. Stimulated Raman adiabatic passage in sodium vapor with picosecond laser pulses. Phys. Rev. A, 96:023803 (2017).

Acknowledgements

NSF (PHY-2309340), MU CAS and OARS are acknowledged.

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