Optimal three-state field-free molecular orientation with terahertz pulses

TL;DR

This paper combines analytical and numerical methods to design optimal terahertz control fields for maximum field-free molecular orientation across three rotational states, demonstrated on ultracold HCN molecules.

Contribution

It derives analytical conditions for optimal control fields and applies a quantum control scheme with two terahertz pulses to achieve maximum molecular orientation.

Findings

Maximum orientation achieved when control fields meet specific amplitude and phase conditions.

Multiple optimal solutions exist for the control fields.

Sensitivity analysis of rotational state populations and phases to control parameters.

Abstract

We present a combined analytical and numerical investigation to show how an optimal control field can be designed to generate maximum field-free orientation of molecules for three populated rotational states. Based on a model involving pure rotational ladder-climbing excitation between rotational states, a set of optimal amplitude and phase conditions are analytically derived for the applied control fields. The maximum degree of orientation can be achieved when the field satisfies amplitude and phase conditions at the two transition frequencies. Multiple optimal solutions exist and to examine these conditions, we devise a quantum coherent control scheme using two terahertz pulses and successfully apply it to the linear polar molecule HCN at ultracold temperature. The sensitivity of both populations and phases of rotational states to control field parameters, i.e., the detuning, bandwidth, and time delay, is analyzed for understanding the optimal orientation mechanism. This work thus examines the frequency domain landscape belonging to optimal pulses.