We consider the rotational excitation of molecules induced by asymmetric (``half-cycle'') as well as symmetric single-cycle THz pulses, leading to field-free time-dependent orientation. We show that the basic excitation mechanisms are very similar for the two types of pulses; i.e., the frequency distributions of the pulses at the rotational resonance frequencies play an important role. Furthermore, we investigate the interference between multiple rotational excitation pathways following prealignment with a nonresonant 800-nm femtosecond pulse. It is shown that such interference can lead to an enhancement of the orientation of the linear HCN molecule by a factor close to 2.

We demonstrate theoretically an efficient field-free orientation in LiH and LiCl driven by available terahertz few-cycle pulses (TFCPs). Exact results by numerically solving the time-dependent Schrödinger equation including the vibrational and rotational degrees of freedom are compared to the rigid-rotor approximation (RRA) as well as to the impulsive approximation (IA), and the effect of rotational-vibrational coupling on the both RRA and IA is examined in detail. We find that the current available TFCPs may overcome the technical limitation of terahertz half-cycle pulse for enhancing the field-free molecular orientation.

We present a combined analytical and numerical study for coherent terahertz control of a single molecular polariton, formed by strongly coupling two rotational states of a molecule with a single-mode cavity. Compared to the bare molecules driven by a single terahertz pulse, the presence of a cavity strongly modifies the postpulse orientation of the polariton, making it difficult to obtain its maximal degree of orientation. To solve this challenging problem toward achieving complete quantum coherent control, we derive an analytical solution of a pulse-driven quantum Jaynes-Cummings model by expanding the wave function into entangled states and constructing an effective Hamiltonian. We utilize it to design a composite terahertz pulse and obtain the maximum degree of orientation of the polariton by exploiting photon blockade effects. This Letter offers a new strategy to study rotational dynamics in the strong-coupling regime and provides a method for complete quantum coherent control of a single molecular polariton. It, therefore, has direct applications in polariton chemistry and molecular polaritonics for exploring novel quantum optical phenomena.

The possibility to manipulate quantum coherence and interference, apart from its fundamental interest in quantum mechanics, is essential for controlling nonlinear optical processes such as high harmonic generation, multiphoton absorption, and stimulated Raman scattering. We show, analytically and numerically, how a nonlinear optical process via resonance Raman scattering (RRS) can be manipulated in a four-level double-$\mathrm{\Lambda }$ system by using pulsed laser fields. We find that two simultaneously excited RRS paths involved in the system can generate an ultimately destructive interference in the broad-bandwidth-limit regime. This, in turn, reduces the four-level system to an equivalent three-level system in a $V$ configuration capable of naturally vanishing RRS effects. We further show that this counterintuitive phenomenon, i.e., the RRS vanishing, can be prevented by transferring a modulated phase of the laser pulse to the system at resonance frequencies. This work demonstrates a clear signature of both quantum destructive and constructive interference by actively controlling resonant multiphoton processes in multilevel quantum systems, and it therefore has potential applications in nonlinear optics, quantum control, and quantum information science.