Field-free Molecular Orientation Research Articles

Field-free orientation of 7LiH steered by a few-cycle nonlinearly chirped pulse

We demonstrate theoretically that an efficient field-free molecular orientation of 7LiH molecule steered by a few-cycle nonlinearly chirped laser pulse with frequency in the terahertz regime can be achieved. The frequency modulation parameter β plays an important role in the molecular orientation, the maximum molecular orientation |〈cosθ〉|max is 0.848 and the orientation duration is 150 fs when β=7.25. In the range of electric-field peak intensity E0=[0.1,4.0] MV/cm, we can find the maximum orientation degree in both positive and negative directions. Compared to the scheme steered by a normal laser pulse, we can find that a more efficient molecular orientation and a smaller difference in population between the adjacent states can be realized by using the nonlinearly chirped laser pulse under the same near-single-period conditions. In addition, at the same temperature, a higher molecular orientation and a smaller decrease rate of orientation with temperature can be obtained by the nonlinearly chirped laser pulse.

Quantum control of field-free molecular orientation.

Generating field-free (non-stationary) orientation of molecules in space has been a longstanding goal in the field of quantum control of molecular rotation, which has significant applications in physical chemistry, chemical physics, strong-field physics, and quantum information science. In this Perspective, we review and examine several representative control schemes developed in recent years and implemented in theoretical and experimental areas for generating field-free orientation of molecules. By conducting numerical simulations of different control schemes on the same molecular system, we demonstrate that quantum coherent control, specifically targeting a limited number of the lowest-lying rotational levels to achieve an optimal superposition, can result in a high degree of orientation. To this end, we provide an overview of our latest developed analytical method, which enables the precise design of terahertz field parameters through resonant excitation. This design approach facilitates the attainment of desired field-free orientations by optimizing the amplitudes and phases of rotational wave functions for the selected rotational levels. Finally, we outlook the significance of such progress in multiple frontier research fields, highlighting its potential applications in ultracold physics, quantum computation, quantum simulation, and quantum metrology.

Revival structure of rotational wave packets in the process of the field-free molecular orientation

We investigate theoretically the relationship between the revival structure of rotational wave packets and the orientation of the LiH molecule induced by the half-cycle THz laser pulse (HCP). The revival structure of oriented rotational wave packets can be further controlled through the choice of the delay time and the phase difference between two HCPs. We found the higher absolute values of the orientation corresponding to the higher asymmetries and more centralization of the time-dependent probability density ψθ,t2. We also found a similar population of the adjacent rotational states gives a higher ceiling for the absolute value of molecular orientation. Finally, the effect of the system temperature is discussed, the higher temperature makes a lower degree of orientation steered by two HCPs.

Generation of maximal three-state field-free molecular orientation with terahertz pulses

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.

Evolution of the rotational quantum state in field-free molecular alignment and orientation

Evolution of the rotational quantum state in field-free molecular alignment and orientation

Field-free molecular orientation induced by a four-color laser pulse

The mechanisms of four-color (ω ± 2 ω ± 3 ω ± 4 ω) laser-induced field-free molecular orientation based on the polarizability and hyperpolarizability are explored. The CO molecule is used as a computational example. It is shown that the degree and direction of the molecular orientation can be coherently manipulated by changing the polarization direction of the odd- and even-order harmonics of the four-color laser pulse. When relative amplitude σ1 = σ2 = σ3 = σ4 = 1, the molecular orientation reaches the maximum. By modulating the phase of the odd- and even-order harmonics of the four-color laser pulse, enhancement and suppression of molecular orientation can be effectively controlled. The laser field peak amplitude is an important parameter to control the molecular orientation. Moreover, the combination of two four-color pulses can significantly increase the degree of molecular orientation.

Optimal field-free molecular orientation with nonresonant two-color adiabatic-turn-on and sudden-turn-off laser pulses

Field-free molecular orientation, achieved by using two-color slow turn-on and rapid turn-off laser pulses, is theoretically investigated for linear molecules. We derive analytical results for the maximum degree of field-free orientation for a given number of populated rotational states. We show that numerical results with adiabatic-turn-on and sudden-turn-off laser pulses attain the optimal orientation. Based on the adiabatic theorem, the effects of laser and molecular parameters on both the adiabatic process and the degree of molecular orientation are analyzed. The adiabaticity is well reflected through the temporal evolution of the phase difference between two adjacent rotational states.

Field-free molecular orientation by delay- and polarization-optimized two fs pulses

Unless the molecular axis is fixed in the laboratory frame, intrinsic structural information of molecules can be averaged out over the various rotational states. The macroscopic directional properties of polar molecules have been controlled by two fs pulses with an optimized delay. In the method, the first one-color laser pulse provokes molecular alignment. Subsequently, the molecular sample is irradiated with the second two-color laser pulse, when the initial even—J states are aligned, and the odd—J states are anti-aligned in the thermal ensemble. The second pulse selectively orients only the aligned even—J states in the same direction, which results in significant enhancement of the net degree of orientation. This paper reports the results of simulations showing that the two-pulse technique can be even more powerful when the second pulse is cross-polarized. This study shows that the alignment and orientation can be very well synchronized temporally because the crossed field does not disturb the preformed alignment modulation significantly, suggesting that the molecules are very well confined in the laboratory frame. This cross-polarization method will serve as a promising technique for studying ultrafast molecular spectroscopy in a molecule-fixed frame.

Field-free molecular orientation steered by combination of super-Gaussian and THz half-cycle laser pulses**Project supported by the National Natural Science Foundation of China (Grant Nos. 11674198 and 11874241) and the Taishan Scholar Project of Shandong Province, China.

The molecular orientation created by laser fields is important for steering chemical reactions. In this paper, we propose a theoretical scheme to manipulate field-free molecular orientation by using an intense super-Gaussian laser pulse and a time-delayed terahertz half-cycle pulse (THz HCP). It is shown that the degree of field-free orientation can be doubled by the combined pulse with respect to the super-Gaussian pulse or THz HCP alone. Moreover, different laser intensities, carrier envelop phases, shape parameters, and time delays have great influence on the positive and negative orientations, with other conditions unchanged. Furthermore, it is indicated that the maximum degree and direction of molecular orientation can be precisely controlled by half of the duration of the super-Gaussian pulse. Finally, by adjusting the laser parameters of the super-Gaussian laser pulse and THz HCP, the optimal results of negative orientation and corresponding rotational populations are obtained at different temperatures of the molecular system.

Field-free orientation of LiH molecule induced by two-color femtosecond and terahertz laser pulses

We theoretically study the field-free molecular orientation by the combination of a two-color femtosecond laser pulse and a time-delayed terahertz (THz) laser pulse, with LiH as the prototype molecule. It is shown that the molecular hyperpolarizability is a vital parameter in achieving molecular orientation according to the pre-excitation created by an intense two-color femtosecond laser pulse. The relative amplitude of the two-color femtosecond pulse is discussed. In addition, the center frequency of the THz laser pulse is chosen on the basis of the pre-excited population distributions. The carrier-envelope phase of the THz laser pulse as well as the delay time between the femtosecond laser pulse and THz laser pulse are adjusted to achieve a high orientation degree. In addition, the effect of temperature on molecular orientation is also discussed. Finally, according to the population distributions at room temperature, the parameters of the THz laser pulse are re-optimized, and the achieved maximal orientation degree is 0.221 at 300 K.

Shaping of the time evolution of field-free molecular orientation by THz laser pulses

We present a theoretical study of the shaping of the time-evolution of field-free orientation of linear molecules. We show the extend to which the degree of orientation can be steered along a desired periodic time-dependent signal. The objective of this study is not to optimize molecular orientation but to propose a general procedure to precisely control rotational dynamics through the first moment of the molecular axis distribution. Rectangular and triangular signals are taken as illustrative examples. At zero temperature, we compute the quantum states leading to such field-free dynamics. A TeraHertz laser pulse is designed to reach these states by using optimal control techniques. The investigation is extended to the case of non-zero temperature. Due to the complexity of the dynamics, the control protocol is derived with a Monte-Carlo simulated annealing algorithm. A figure of merit based on the Fourier coefficients of the degree of orientation is used. We study the robustness of the control process against temperature effects and amplitude variations of the electric field.

Dynamical analysis of the effect of super-Gaussian laser pulses on molecular orientation

In this paper, through the investigation of the field-free orientation dynamics driven by super-Gaussian laser pulses using a quantum mechanics method, we show that compared with the molecular orientation driven by Gaussian laser pulses, it is hard for super-Gaussian laser-induced orientation to reach the purely adiabatic regime at the same pulse duration. By analyzing the varying of the maximal orientation degree with the pulse, it is found not only that using super-Gaussian laser pulses can give us more pulse durations to choose from for field-free molecular orientation, but that increasing the shape parameter N and choosing an appropriate electric field intensity can also be beneficial to enhance the orientation degree. The temperature effect on the maximal orientation degree is also discussed, and the super-Gaussian laser pulse is found to be more feasible at higher temperatures.

Influence factor analysis of field-free molecular orientation**Project supported by the National Natural Science Foundation of China (Grant No. 11674198), the Taishan Scholar Project of Shandong Province, China, and the Natural Science Foundation of Shandong Province, China (Grant No. ZR2014AM002).

The effects of the characteristics of molecules and external fields on field-free molecular orientation are investigated through the comparison of HBr with LiH driven by the combination of a two-color laser pulse and a time-delayed THz laser pulse. It is shown that the dipole interaction has greater influence on field-free orientation than the hyperpolarizability interaction. In addition to the temperature dependence of orientation degree, the effects of the amplitudes of the two-color laser pulse and THz laser pulse, rising time, and THz laser frequency on molecular orientation are also discussed.

Field-free molecular orientation of nonadiabatically aligned OCS

We investigate an enhancement of the orientation of OCS molecules by irradiating them with a near IR (ω) ultrashort laser pulse for alignment followed by another ultrashort laser pulse for orientation, which is synthesized by a phase-locked coherent superposition of the near IR laser pulse and its second harmonic (2ω). On the basis of the asymmetry in the ejection direction of S3+ fragment ions generated by the Coulomb explosion of multiply charged OCS, we show that the extent of the orientation of OCS is significantly enhanced when the delay between the alignment pulse and the orientation pulse is a quarter or three quarters of the rotational period. The recorded enhanced orientation was interpreted well by a numerical simulation of the temporal evolution of a rotational wave packet prepared by the alignment and orientation pulses.

Dynamical analysis of the effect of elliptically polarized laser pulses on molecular alignment and orientation

In this Letter, we study the molecular alignment and orientation driven by two elliptically polarized laser pulses. It is shown that the field-free molecular alignment can be achieved in a three-dimensional (3D) case, while the field-free molecular orientation is only along the x and y directions, and that the field-free alignment and orientation along different axes are related to the populations of the rotational states. It is demonstrated that changing the elliptic parameter is efficient for controlling both in-pulse and post-pulse molecular alignment and orientation. The delay time also has an influence on the field-free molecular alignment and orientation.

Field-free molecular orientation by delayed elliptically polarised laser pulses

A theoretical model of NAREX (non-adiabatic rotational excitation) and field-free molecular orientation by a short specific elliptically polarised laser pulses (EPLPs) driving a polar molecule is presented. By choosing the proper value of elliptically polarised field parameters, efficient field-free orientation could be achieved. It is demonstrated that NAREX can be controlled by various laser parameters, out of which pulse shape plays the most significant role. The effect of elliptic parameter on the rotational excitation and orientation dynamics is also under concern.

Field-free molecular orientation enhanced by tuning the intensity ratio of a three-color laser field**Project supported by the National Natural Science Foundation of China (Grant Nos. 61221064, 61078037, 11134010, 61205208, and 61521093), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB16), and the International S & T Cooperation Program of China (Grant No. 2016YFE0119300).

We theoretically study the field-free molecular orientation induced by a three-color laser field. The three-color laser field with a large asymmetric degree can effectively enhance the molecular orientation. In particular, when the intensity ratio of the three-color laser field is tuned to a proper value of , the molecular orientation can be improved by % compared with that of the two-color laser field at intensity ratio for the same total laser intensity of 2×10 W/cm2. Moreover, we investigate the effect of the carrier-envelope phase (CEP) on the molecular orientation and use the asymmetric degree of the laser field to explain the result. We also show the influences of the laser intensity, rotational temperature, and pulse duration on the molecular orientation. These results are meaningful for the theoretical and experimental studies on the molecular orientation.

Field-Free Molecular Orientation Induced by a Single-Cycle THz Laser Pulse Train

AbstractWe investigate theoretically the field-free orientation of CO molecules induced by a single-cycle THz laser pulse train. It is shown that the molecular orientation can be obviously enhanced by applying the pulse train. The laser intensity and pulse number have some effects on the molecular orientation. The high degree of field-free molecular orientation |⟨cosθ⟩|max=0.9246 is obtained at temperature T=0 K. The variations of the maximum orientation degree with the experimentally available pulse number and peak intensity are given. Temperature T has a considerable influence on the field-free molecular orientation. The maximal field-free molecular orientation at T=0, 10, 20 and 30 K for N=14 and E0=1.8 MV/cm are |⟨cosθ⟩|max=0.9188, 0.7338, 0.6055 and 0.5154 in order, and the corresponding effective duration times of molecular orientation are Δt=0.759, 0.432, 0.297 and 0.117 ps.

Enhancement and suppression of field-free molecular orientation with two ultrashort pulses

The field-free molecular orientation induced by two half-cycle pulses is investigated systematically. In addition to the structure parameters of the pulses, the polarization direction and the delay time are modulated to influence the rotational dynamics. The degree of orientation is effectively enhanced by tailoring the parameters of the pulses. The enhancement occurs cyclically, depending on the delay time. In particular, the degree of orientation can be greatly suppressed for various initial states when the pulses are polarized in opposite directions. The pronounced effect becomes weak in the case of the two differently shaped pulses. The feature of orientation can also be modulated by altering the field strength of one pulse. Furthermore, the effect of temperature on the orientation is analyzed.

Field-free molecular orientation by two-color femtosecond laser pulse and time-delayed THz laser pulse

We propose a scheme for achieving molecular orientation steered by the combination of a nonresonant two-color femtosecond laser pulse and a time-delayed THz laser pulse. It is shown that the molecular negative and positive orientation can be significantly enhanced by applying this scheme compared to the single femtosecond or THz laser pulse. The effects of the intensity of the two-color femtosecond laser pulse and the delay time between the femtosecond laser pulse and the THz laser pulse on molecular orientation are discussed. The molecular orientation also depends on the carrier-envelope phase of the THz laser pulse and the initial rotational temperature of the molecule. The maximal negative and positive orientation degrees calculated are and , respectively.