arXiv subjects

Thorsten Hansen

Publications and source records attributed to Thorsten Hansen.

6 recordsLinked to original sources

arXiv2019

Adiabatic Elimination and Sub-space Evolution of Open Quantum Systems

Efficient descriptions of open quantum systems can be obtained by performing an adiabatic elimination of the fast degrees of freedom and formulating effective operators for the slow degrees of freedom in reduced dimensions. Here, we perform the construction of effective operators in frequency space, and using the final value theorem or alternatively the Keldysh theorem, we provide a correction for the trace of the density matrix which takes into account the non trace-preserving character of the evolution. We illustrate our results with two different systems, ones where the eliminated fast subspace is constituted by a continuous set of states and ones with discrete states. Furthermore, we show that the two models converge for very large dissipation and at coherent population trapping points. Our results also provide an intuitive picture of the correction to the trace of the density matrix as a detailed balance equation.

quant-ph↗

arXiv2019

Two-dimensional Fano lineshapes: Excited-state absorption contributions

Fano interferences in nanostructures are influenced by dissipation effects as well as many-body interactions. Two dimensional coherent spectroscopies have just begun to be applied to these systems where the spectroscopic signatures of a discrete-continuum structure are not known. In this article we calculate the excited-state absorption contribution for different models of higher-lying excited states. We find that the characteristic asymmetry of one-dimensional spectroscopies is recovered from the many-body contributions, and that the higher-lying excited manifolds have distorted lineshapes that are not anticipated from discrete-level Hamiltonians. We show that the SE cannot have contributions from a flat continuum of states. This work completes the Ground-State Bleach and Stimulated Emission signals that were calculated previously (Phys. Rev. B. 2016, 94, 205137). The model reproduces the observations reported for molecules on surfaces probed by 2DIR.

physics.chem-ph↗

arXiv2018

Classification of Dark States in Multi-level Dissipative Systems

Dark states are eigenstates or steady-states of a system that are decoupled from the radiation. Their use, along with associated techniques such as Stimulated Raman Adiabatic Passage, has extended from atomic physics where it is an essential cooling mechanism, to more recent versions in condensed phase where it can increase the coherence times of qubits. These states are often discussed in the context of unitary evolution and found with elegant methods exploiting symmetries, or via the Bruce-Shore transformation. However, the link with dissipative systems is not always transparent, and distinctions between classes of CPT are not always clear. We present a detailed overview of the arguments to find stationary dark states in dissipative systems, and examine their dependence on the Hamiltonian parameters, their multiplicity and purity. We find a class of dark states that depends not only on the detunings of the lasers but also on their relative intensities. We illustrate the criteria with the more complex physical system of the hyperfine transitions of $^{87}$Rb and show how a knowledge of the dark state manifold can inform the preparation of pure states.

quant-ph↗

arXiv2017

The minimal model of light harvesting complex with dipole-quadrupole interaction derived from ab initio calculation

A minimal, one-parameter model of the excitonic Hamiltonian for the light harvesting complex of purple bacteria (LH2) based on ab initio calculation of the excitonic energies is proposed. The set of input parameters contains positions of atoms in 27 bacteriochlorophyll pigments only. The excitation energies, transition dipole and quadrupole moments of the bacteriochlorophyll units were calculated by advanced multiconfigurational multireference method. In the work we investigate influence of the dipole-quadrupole interaction on the particularities of the absorption spectra and circular dichroism and on the wave-functions localization. We demonstrate that although the spectra changes are small the dipole-quadrupole interaction term leads, however, to almost complete separation of the system onto two subsystems: the small B800 ring of pigments and the large B850 ring. The latter observation is seemed essential for formulation of energy transport models in light harvesting complexes.

q-bio.BM↗

arXiv2016

Coherent two-dimensional spectroscopy of a Fano model

The Fano lineshape arises from the interference of two excitation pathways to reach a continuum. Its generality has resulted in a tremendous success in explaining the lineshapes of many one-dimensional spectroscopies - absorption, emission, scattering, conductance, photofragmentation - applied to very varied systems - atoms, molecules, semiconductors and metals. Unravelling a spectroscopy into a second dimension reveals the relationship between states in addition to decongesting the spectra. Femtosecond-resolved two-dimensional electronic spectroscopy (2DES) is a four-wave mixing technique that measures the time-evolution of the populations, and coherences of excited states. It has been applied extensively to the dynamics of photosynthetic units, and more recently to materials with extended band-structures. In this letter, we solve the full time-dependent third-order response, measured in 2DES, of a Fano model and give the new system parameters that become accessible.

physics.chem-ph↗

arXiv2016

The nature of relaxation processes revealed by the action signals of phase modulated light fields

We introduce a generalized theoretical approach to study action signals induced by the absorption of two-photons from two phase modulated laser beams and subject it to experimental testing for two types of photoactive samples, solution of rhodamine 6G and GaP photodiode. In our experiment, the phases of the laser beams are modulated at the frequencies f1 and f2, respectively. The action signals, such as photoluminescence and photocurrent, which result from the absorption of two photons, are isolated at frequencies m f (f=|f1-f2|, m=0,1,2...). We demonstrate that the ratio of the amplitudes of the secondary (m=2) and the primary (m=1) signals is sensitive to the type of relaxation process taken place in the system and thus can be used for its identification. Such sensitivity originates from cumulative effects of non-equilibrated state of the system between the light pulses. When the cumulative effects are small, i.e. the relaxation time is much shorter then the laser repetition rate or the laser intensity is high enough to dominate the system behavior, the ratio achieves its reference value 1:4 (the signature of two-photon absorption). In the intermediate regimes the ratio changes rapidly with the growth of intensity from zero value in case of second order relaxation process, while it demonstrates slow monotonic decrease for linear relaxation. In the article we also determine the value of the recombination rate in a GaP photodiode by using the above approach.

physics.optics↗