Theoretical studies of molecular machines

Only volume 2 has been digitized.Molecular machines are essential components of living organisms. They are highly efficient and robust, much more than their macroscopic analogs. This stimulated growing interest in construction of artificial molecular machines with a set of functions which may be controlled in a specific way. Such man-made molecular complexes are designed as the building blocks for future nanotechnological devices. During the last decades many new molecular machines have been synthesized and characterized by various experimental techniques. This significantly increased our knowledge about systems of such kind and their functioning. However, there are only a few real applications of molecular machines. This is because the fundamental principles of operation of such single-molecule systems are not well understood. Existing theoretical studies, although very helpful, are still very sparse. This is because the molecular machines are very complex systems, comprising up to thousands atoms. Thus the progress in our understanding of nanoscale materials is tightly related to development of efficient computational and theoretical methodologies. In this work we studied two large classes of molecular machines: surface-moving nanocars and molecular rotors/motors, working on the surfaces and in crystalline state. In particular we studied the role of the internal interactions of these machines as well as their interactions with the environment. This included the flexibility of the molecules, including the rotation of the nanocars' wheels, effects of surface and rotors symmetry, charge transfer effects as well as many other factors. We have found out relations which determine the properties of studied classes of molecular machines. The development of computational and theoretical methods was another essential part of this work. In particular we have developed a family of the surface-molecule interaction potentials, aimed to performing long time scale and molecular simulations of complex systems. We also developed a physics-based model of the charge transfer happening between metals and the nanocars. This opened new ways to control such molecular machines. We also developed a theoretical framework to predict response of molecular rotors on various types of driving. Finally, we developed new and improved existing rigid-body molecular dynamics methods and extensively used them in our studies of molecular machines

Resonant thermal energy transfer to magnons in a ferromagnetic nanolayer

Energy harvesting is a concept which makes dissipated heat useful by transferring thermal energy to other excitations. Most of the existing principles are realized in systems which are heated continuously. We present the concept of high-frequency energy harvesting where the dissipated heat in a sample excites resonant magnons in a thin ferromagnetic metal layer. The sample is excited by femtosecond laser pulses with a repetition rate of 10 GHz which results in temperature modulation at the same frequency with amplitude ~0.1 K. The alternating temperature excites magnons in the ferromagnetic nanolayer which are detected by measuring the net magnetization precession. When the magnon frequency is brought onto resonance with the optical excitation, a 12-fold increase of the amplitude of precession indicates efficient resonant heat transfer from the lattice to coherent magnons. The demonstrated principle may be used for energy harvesting in various nanodevices operating at GHz and sub-THz frequency ranges

Contributions from coherent and incoherent lattice excitations to ultrafast optical control of magnetic anisotropy of metallic films

Spin-lattice coupling is one of the most prominent interactions mediating response of spin ensemble to ultrafast optical excitation. Here we exploit optically generated coherent and incoherent phonons to drive coherent spin dynamics, i.e. precession, in thin films of magnetostrictive metal Galfenol. We demonstrate unambiguously that coherent phonons, also seen as dynamical strain generated due to picosecond lattice temperature raise, give raise to magnetic anisotropy changes of the optically excited magnetic film; and this contribution may be comparable to or even dominate over the contribution from the temperature increase itself, considered as incoherent phonons

Hybrid coherent control of magnons in a ferromagnetic phononic resonator excited by laser pulses

We propose and demonstrate the concept of hybrid coherent control (CC) whereby a quantum or classical harmonic oscillator is excited by two excitations: one is quasiharmonic (i.e., harmonic with a finite lifetime) and the other is a pulsed broadband excitation. Depending on the phase relation between the two excitations, controlled by the detuning of the oscillator eigenfrequencies and the wave forms of the quasiharmonic and broadband excitations, it is possible to observe Fano-like spectra of the harmonic oscillator due to the interference of the two responses to the simultaneously acting excitations. Experimentally, as an example, the hybrid CC is implemented for magnons in a ferromagnetic grating where GHz coherent phonons act as the quasiharmonic excitation and the broadband impact arises from pulsed optical excitation followed by spin dynamics in the ferromagnetic nanostructure

Giant photoelasticity of polaritons for detection of coherent phonons in a superlattice with quantum sensitivity

The functionality of phonon-based quantum devices largely depends on the efficiency of interaction of phonons with other excitations. For phonon frequencies above 20 GHz, generation and detection of the phonon quanta can be monitored through photons. The photon-phonon interaction can be enormously strengthened by involving an intermediate resonant quasiparticle, e.g. an exciton, with which a photon forms a polariton. In this work, we discover a giant photoelasticity of exciton-polaritons in a short-period superlattice and exploit it for detecting propagating acoustic phonons. We demonstrate that 42 GHz coherent phonons can be detected with extremely high sensitivity in the time domain Brillouin oscillations by probing with photons in the spectral vicinity of the polariton resonance.Comment: 6 pages, 3 figures, Supplemental Material

Enhanced Photon–Phonon Interaction in WSe2 Acoustic Nanocavities

Acoustic nanocavities (ANCs) with resonance frequencies much above 1 GHz are prospective to be exploited in sensors and quantum operating devices. Nowadays, acoustic nanocavities fabricated from van der Waals (vdW) nanolayers allow them to exhibit resonance frequencies of the breathing acoustic mode up to f ∼ 1 THz and quality factors up to Q ∼ 103. For such high acoustic frequencies, electrical methods fail, and optical techniques are used for the generation and detection of coherent phonons. Here, we study experimentally acoustic nanocavities fabricated from WSe2 layers with thicknesses from 8 up to 130 nm deposited onto silica colloidal crystals. The substrate provides a strong mechanical support for the layers while keeping their acoustic properties the same as in membranes. We concentrate on experimental and theoretical studies of the amplitude of the optically measured acoustic signal from the breathing mode, which is the most important characteristic for acousto-optical devices. We probe the acoustic signal optically with a single wavelength in the vicinity of the exciton resonance and measure the relative changes in the reflectivity induced by coherent phonons up to 3 × 10–4 for f ∼ 100 GHz. We reveal the enhancement of photon–phonon interaction for a wide range of acoustic frequencies and show high sensitivity of the signal amplitude to the photoelastic constants governed by the deformation potential and dielectric function for photon energies near the exciton resonance. We also reveal a resonance in the photoelastic response (we call it photoelastic resonance) in the nanolayers with thickness close to the Bragg condition. The estimates show the capability of acoustic nanocavities with an exciton resonance for operations with high-frequency single phonons at an elevated temperature