Abstract We theoretically investigate molecular vibrational dynamics driven by intense and broadband terahertz (THz) pulses. The light–nuclei interaction is derived from a fully quantum-mechanical minimal coupling Hamiltonian to analyse the direct excitation of vibrational modes by the THz pulses. We show that for diatomic molecules a ‘vibrational moment’ can be defined in a manner similar to the dipole moment and that it has non-zero values only for heteronuclear diatomic molecules. Taking ultra-cold heteronuclear diatomic molecules as examples, we show that vibrational-mode excitation by THz pulse irradiation occurs in an almost stepwise manner, especially for low vibrational modes. A coherent control method using a frequency-controlled THz pulse sequence is also proposed.
Abstract We theoretically investigate molecular vibrational dynamics driven by intense and broadband terahertz (THz) pulses. The light–nuclei interaction is derived from a fully quantum-mechanical minimal coupling Hamiltonian to analyse the direct excitation of vibrational modes by the THz pulses. We show that for diatomic molecules a ‘vibrational moment’ can be defined in a manner similar to the dipole moment and that it has non-zero values only for heteronuclear diatomic molecules. Taking ultra-cold heteronuclear diatomic molecules as examples, we show that vibrational-mode excitation by THz pulse irradiation occurs in an almost stepwise manner, especially for low vibrational modes. A coherent control method using a frequency-controlled THz pulse sequence is also proposed.
The exquisite control of diluted atomic vapors that can be achieved using electromagnetic (em) fields triggered the overwhelming development of researches on ultracold (T<<1millikelvin) quantum gases through laser cooling and trapping, opening ways to amazing applications in quantum technologies. For a composite system like a molecule under such ultracold conditions, the additional control of the internal degrees of freedom has also been recently demonstrated [1] with cw visible, microwave, and radio radiation frequencies for electronic, rotational, and hyperfine transitions, respectively. Vibrational transitions could be addressed with THz transitions, as an attractive path for controlling the interatomic interactions and creating ultracold gases of a wide class of diatomic molecules in their absolute ground state. More specifically, THz radiation can induce a coupling between the bound level of a pair of trapped ultracold ground-state atoms and a loosely-bound vibrational level of the related molecular electronic ground state. This coupling, generated by the intrinsic permanent electric dipole moment of a heteronuclear atomic pair (RbSr in the present work), is related to a specific spectroscopic feature of the atomic pair dressed by the THz field, the Laser Assisted Self-Induced Feshbach Resonance (LASIFR) [2] . In analogy with magnetic Feshbach resonance, the scattering length can be controlled by tuning the THz radiation frequency around a LASIFR (see Fig (a)).
In the present work we analyze the dipole response of a diatomic heteronuclear molecule on the nonperturbative action of the external low-frequency laser field. Main results are obtained by numerical solution of the time-dependent Schrodinger equation for the nuclear subsystem of the molecule. We study the spectrum of polarization response and the influence of the coherent repopulation of the ro-vibrational states on it. Generally, we consider two cases: repopulation due to Raman-type transitions leading to the interference stabilization of the molecule and repopulation arising as a result of extremely short pulse action. We also study the transformation of a weak probe pulse propagating through the highly excited medium of the studied molecules.
We explore the possibility of using shaped infrared laser pulses to deexcite a homonuclear diatomic molecule from its highest vibrational state down to its ground vibrational state. The motivation for this study arises from the need to deexcite alkali metal dimers in a similar way so as to stabilize molecular Bose-Einstein condensates. We demonstrate that for the case of the H(2) molecule, where it is possible to evaluate all the necessary high accuracy ab initio data on the interaction of the molecule with an electric field, we are able to successfully design a sequence of infrared laser pulses to accomplish the desired deexcitation process in a highly efficient manner.
In this work, difference-frequency generation is theoretically analysed for the three-level quantum system, having forbidden in dipole approximation electronic and optically active vibration transitions from the ground state. Excitation of coherent phonon polaritons wave in the terahertz frequency range by an ultrashort optical pulse, being in the visible or near-infrared spectral regions, and injected pulse with frequency almost twice that of the pump is considered. Dynamics of terahertz radiation arising during excitation of both electronic and vibrational coherences at the two-photon and combined resonances in the ensemble of three-level quantum systems was evaluated.
Terahertz radiation has unique potential for applications to material diagnostics including semiconductors, chemical compounds, biomolecules and biotissues, imaging for medical and security applications, remote atmospheric sensing and monitoring, astronomy, etc. The search for efficient, high-power, inexpensive, portable and room temperature suitable methods of generation of THz radiation is one of the main topics in modern optoelectronics and photonics. Specifically, coherent solid-state compact efficient THz sources are desirable. Recently we suggested a new approach to the problem of generation of short coherent THz pulses in molecular gases taking advantage of the dramatic enhancement of a nonlinear response of a medium via quantum coherence. Stimulated Raman adiabatic passage (STIRAP) technique was proposed for preparation of maximal molecular coherence by two optical pump fields, which is further used to efficiently scatter off an IR probe beam and generate a Stokes THz field in a double Λ scheme. The central idea of the crystal-based technique is to generate THz radiation by resonantly inducing coherence at the corresponding transition by coherent optical fields. Electronic levels of the THz transition are coupled by a pair of optical fields to a common ground state forming a V scheme of energy levels. Coherence induced by the optical fields at the THz transition gives rise to polarization provided that the corresponding dipole moment is not zero, which will radiate out a THz pulse. We show that coherently driven molecular (such as methanol, H2F2, CH3Cl, etc.) and solid-state (ruby, alexandrite) media can potentially produce strong, controllable, short pulses of THz radiation. The pulses have energies ranging from hundreds pJ to J and time durations from several fs to ns at room temperature.
A review of our recent experiments on broadband vibrational cooling of cold cesium molecules and of the related theory is presented. Our method is based on repetitive optical pumping cycles driven by laser light which is broad enough to excite all populated vibrational levels. Originally, the accumulation of molecular population in a particular, pre-selected vibrational level was achieved by removing from the broadband light all frequencies that could excite that vibrational level and thus making it a ‘dark state’ of the system. Here, we focus onto an additional, more sophisticated shaping method, which consists of selecting only specific frequency components that excite molecules into vibrational levels that favourably decay into the pre-selected level. The population transfer to any desired state can thus be optimised, i.e. the total population transfer to the desired vibrational level is maximised while the number of absorption–emission cycles required for the vibrational cooling is minimised. Finally, we apply this optimised technique to some more complex and still experimentally open cases: the pumping into the ground state for the case of Cs2 homonuclear molecules, the rotational pumping into a pre-selected ro-vibrational level and the NaCs as an example for heteronuclear molecules.