Driving electronic dynamics with external fields

[Input: recipes/electronicdynamics/driving/]

In this section we will describe how to drive the Q band of chlorophyll a using both a continuous and pulsed laser. The first step is to find the transition dipole moment direction of the Q band. This is done using the calc_timeprop_maxpoldir script either available after make install of DFTB+, of located in the tools/misc directory under the dftbplus source tree. The invocation of the script is as follows:

calc_timeprop_maxpoldir -d 20.0 -w 636.0

which produces the output:

PolarizationDirection = -0.08808129 0.99564018 -0.03069709

these are the three Cartesian components of the transition dipole moment vector. This vector points in the direction in which a driving laser, if it is in resonance with the excitation, will have maximum absorption. This vector is the eigenvector of the polarizability tensor at that energy that has the largest eigenvalue. It is equivalent to a principal axis of inertia in rigid body rotation.

Using the ElectronDynamics block that follows:

ElectronDynamics = {
  Steps = 60000
  TimeStep [au] = 0.2
  Perturbation = Laser {
    PolarizationDirection = -0.08808129 0.99564018 -0.03069709
    LaserEnergy [eV] = 1.94944
  FieldStrength [V/A] = 0.0001

We resonantly excite the Q band along the direction of its maximum polarizability. The obtained magnitude of the dipole moment as a function of time is shown in the following figure:

Dipole moment as a function of time.

An initial transient, the dipole moment maxima and minima grow in absolute value as a linear function of time after, confirming that the applied field is in resonance with the excitation and within the linear response regime. The slope of this growth is related to the transition dipole of the excitation.

Off-resonant excitation

[Input: recipes/electronicdynamics/driving-oot/]

An off resonance excitation at 1.9 eV produces the following result for the dipole moment:

Dipole moment as a function of time.

showing characteristic beats, the frequency of which are related to the amount of detuning. The amplitude of the dipole moment change caused by the illumination is also much smaller that when the laser is in tune with the excitation.