Cooling molecular vibrations with shaped laser pulses: Optimal control theory exploiting the timescale separation between coherent excitation and spontaneous emission

TL;DR

This paper develops an optimal control method using shaped laser pulses to enhance molecular laser cooling by exploiting the timescale separation between excitation and spontaneous emission, even under unfavorable Franck-Condon conditions.

Contribution

It introduces two new optimization functionals for laser cooling that leverage timescale separation, enabling effective cooling despite non-ideal Franck-Condon maps.

Findings

Optimal control can improve molecular laser cooling efficiency.

Cooling is achievable even with heating-favoring Franck-Condon maps.

The method applies to various degrees of freedom with separated timescales.

Abstract

Laser cooling of molecules employing broadband optical pumping involves a timescale separation between laser excitation and spontaneous emission. Here, we optimize the optical pumping step using shaped laser pulses. We derive two optimization functionals to drive population into those excited state levels that have the largest spontaneous emission rates to the target state. We show that, when using optimal control, laser cooling of molecules works even if the Franck-Condon map governing the transitions is preferential to heating rather than cooling. Our optimization functional is also applicable to the laser cooling of other degrees of freedom provided the cooling cycle consists of coherent excitation and dissipative deexcitation steps whose timescales are separated.