Second-harmonic generation of 2D-hBN

Step 0: Theoretical framework

In this tutorial, we compute the Second-harmonic generation (SHG) from the dynamics of the Bloch-states. The equation-of-motion of the Bloch-states is explained in the tutorial on Linear response from Bloch-states dynamics. Independently of the 'experiment' we simulate, the part of the integration of motion stays the same. This means that any experiment, including non-linear optics, can be performed at the level of the theory listed for the computation of the dielectric function:

• independent (quasi)particle
• time-dependent Hartree (RPA level)
• time-dependent DFT (and DPFT)
• time-dependent Hartree+Screened exchange (BSE level)

What changes when we want to calculate the SHG (or higher harmonics) is:

• the time-dependence of the input electric field and
• the post-processing of the signal

Time dependence of the electric Field: we choose a sinusoidal electric field, thus a monochromatic laser field. This allows one to expand the polarization in the form [math]\displaystyle{ \bf{P}(t) = \sum_{n=-\infty}^{+\infty} \bf{p}_n e^{-i\omega_n t} }[/math] where the coefficient [math]\displaystyle{ \bf{p}_1,...,\bf{p}_n }[/math] are related to [math]\displaystyle{ \chi^{(1)},...,\chi^{(n)} }[/math] through a coefficient depending on a power of the strength of the field (with the power depending on the order of the response).

At difference with a delta-like perturbation, a real-time simulation gives the response at the laser-field only. Then, to obtain the spectrum for the desired range of frequency, we have to perform so many simulations as the frequencies in the desired range.

Post-processing of the signal: The switch-on of the electric field corresponds to a weak delta-like kick. Thus, even if it may not be noticeable, the polarization results from both the sinusoidal field and the delta-like kick. The latter excites all eigenfrequencies of the system. Though weak, the resulting signal is comparable with the second-harmonic signal. To eliminate the signal from the eigenfrequencies, we apply a dephasing ([math]\displaystyle{ \gamma_{deph} }[/math]). To have a signal useful to sample the second-harmonic signal, we need to wait a time [math]\displaystyle{ \bar t }[/math] much larger than the dephasing-time [math]\displaystyle{ 1/\gamma_{deph} }[/math]. Note that we cannot choose a too short dephasing time (to shorten the simulation time), as this would result in a too large broadening of the spectrum. After [math]\displaystyle{ \bar t }[/math], we sample $2N+1$ times in a period and use discrete Fourier transform to extract [math]\displaystyle{ {p}_n }[/math] for [math]\displaystyle{ n = 0,...,N }[/math], where n=2 gives the SHG. This is schematically illustrated in figure:

The scheme from Ref. summarised the workflow for computing the SHG (or higher-harmonics) in a energy range [math]\displaystyle{ [\Omega_1,\Omega_2] }[/math].

Step 1: Prerequisites

In this example, we will consider a single layer of hexagonal boron nitride (hBN). Before running real time simulations to compute the SHG in yambo you need to download input files and Yambo databases for this tutorial here: hBN-2D-RT.tar.gz. Unzip the tarball and change directory to YAMBO and run the setup (yambo_nl -nompi). Execute the command

ypp_nl -fixsym -F 00_removesym.in  

to create the input, to remove the symmetries which are not compatible with the field:

fixsyms                          # [R] Remove symmetries not consistent with an external perturbation
% Efield1
 1.000000 | 1.000000 | 0.000000 |        # First external Electric Field
%
% Efield2
 0.000000 | 0.000000 | 0.000000 |        # Additional external Electric Field
%
BField= 0.000000           T     # [MAG] Magnetic field modulus
Bpsi= 0.000000             deg   # [MAG] Magnetic field psi angle [degree]
Btheta= 0.000000           deg   # [MAG] Magnetic field theta angle [degree]
#RmAllSymm                     # Remove all symmetries
RmTimeRev                     # Remove Time Reversal
#RmSpaceInv                    # Remove Spatial Inversion
#KeepKGrid                     # Do not expand the k-grid

where we chose the field along the xy direction and we remove time-reversal symmetry.

Then run the pre-processing job:

ypp_nl -F 00_removesym.in J 00_removesym

This creates the directory FixSymm. Change directory to FixSymm. This contains the SAVE directory with reduced symmetries, compatible with the direction of the field.

Step 2: Independent-particle approximation

Use the command:

 yambo_nl -u -F Inputs_shg/01_td_ip.in

to generate the input (NOTE THIS IS A PLACEHOLDER):

nloptics                      # [R NL] Non-linear optics
NL_Threads= 1                # [OPENMP/NL] Number of threads for nl-optics
% NLBands
  3 |  6 |                   # [NL] Bands
%
NLstep=   0.0100       fs    # [NL] Real Time step length
NLtime=55.000000      fs    # [NL] Simulation Time
NLverbosity= "high"           # [NL] Verbosity level (low | high)
NLintegrator= "INVINT"       # [NL] Integrator ("EULEREXP/RK4/RK2EXP/HEUN/INVINT/CRANKNIC")
NLCorrelation= "IPA"         # [NL] Correlation ("IPA/HARTREE/TDDFT/LRC/JGM/SEX/HF")
NLLrcAlpha= 0.000000         # [NL] Long Range Correction
% NLEnRange
 0.200000 | 8.000000 | eV    # [NL] Energy range 
%
NLEnSteps= 1                 # [NL] Energy steps
NLDamping= 0.000000    eV    # [NL] Damping
% ExtF_Dir
 0.000000 | 1.000000  | 0.000000 |        # [NL ExtF] Versor
%
ExtF_kind=  "DELTA"          # [NL ExtF] Kind(SIN|SOFTSIN|RES|ANTIRES|GAUSS|DELTA|QSSIN)

ADD A DESCRIPTION OF THE MAIN PARAMETERS, LINKING THEM WITH THE THEORETICAL PART

Run the simulation:

 yambo_nl -F Inputs_shg/01_td_ip.in -J TD-IP_nl -C TD-IP_nl

TO DO: The run now is performing a real-time simulation per frequency, and therefore it takes approximately X times more time than the linear response simulation (NB check as well length of the simulation and amend)

TO DO: Add a plot of the polarization. Note that it is sinusoidal etc. and the nonlinear part is not visible since every order is as a rule of a thumb 6-7 orders of magnitude smaller than the previous.

Output post-processing: the dielectric function

Once we obtained the polarization, we Fourier-transform the polarization to obtain the dielectric function.

Use the command:

 ypp_nl -u -F Inputs_shg/ypp_shg.in

to generate the input file (THIS IS A PLACEHOLDER):

nonlinear                    # [R] NonLinear Optics Post-Processing
Xorder= 1                    # Max order of the response functions
% TimeRange
-1.000000 |-1.000000 | fs    # Time-window where processing is done
%
ETStpsRt= 1001                # Total Energy steps
% EnRngeRt
  0.00000 | 10.00000 | eV    # Energy range
%
DampMode= "LORENTZIAN"             # Damping type ( NONE | LORENTZIAN | GAUSSIAN )
DampFactor=  0.10000   eV    # Damping parameter

TO-DO: describe the main keys and parameters.

To run the post-processing, use the command:

ypp_nl -F Inputs_shg/ypp_shg.in -J TD-IP_nl -C TD-IP_nl 

TO-DO describe which files are produced

TO-DO: Plot the SHG and comments