# Pump and Probe

**This tutorial works only with Yambo version > 5.5, that will be released soon.**

In this tutorial we will show you how to setup two external fields in yambo_nl, to perform pump and probe simulation.

The input simulation parameters of these tutorial require large computational power,
we advice you to run them in parallel, or to use simplified parameters for example less bands, k-points and so on.
This tutorial supposes that you are already familiar with real-time simulation with Yambo,
if it is not the case please check these tutorials: Tutorials#Non_linear_response

We start from the h-BN monolayer with an in place lattice constant a=2.5 Å and a box large 30 a.u. in the z-direction.
Standard DFT input for hBN monolayer for ABINIT or QuantumEspresso can be found here Tutorials.

We used a 24x24x1 k-points grid and 100 bands in the non-self consistent calculation.
In order to have faster calculation we advice you to decrease the k-points grid to 12x12x1,
results will be very close to the converged ones.

In our example we choose direction [0,1,0] for the pump and the probe. We generate the ypp.in to remove symmetries with the command ypp -y and we modify it as:

fixsyms # [R] Remove symmetries not consistent with an external perturbation % Efield1 0.000000 | 1.000000 | 0.000000 | # First external Electric Field % % Efield2 0.000000 | 1.000000 | 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

then we go in the FixSymm directory and run again the setup. Now we calculate collisions,
since we want to include excitonic effects in the calculations.

Using the command: yambo_nl -v h+sex+cvonly -e -X s -r we get the input file:

collisions # [R] Collisions em1s # [R][Xs] Statically Screened Interaction dipoles # [R] Oscillator strenghts (or dipoles) DIP_Threads=0 # [OPENMP/X] Number of threads for dipoles X_Threads=0 # [OPENMP/X] Number of threads for response functions RT_Threads=0 # [OPENMP/RT] Number of threads for real-time RandQpts= 30000000 # [RIM] Number of random q-points in the BZ RandGvec= 1 RL # [RIM] Coulomb interaction RS components CUTGeo= " box Z" # [CUT] Coulomb Cutoff geometry: box/cylinder/sphere/ws/slab X/Y/Z/XY.. % CUTBox 0.00000 | 0.00000 | 29.50000 | # [CUT] [au] Box sides % Chimod= "HARTREE" # [X] IP/Hartree/ALDA/LRC/PF/BSfxc % BndsRnXs 1 | 100 | # [Xs] Polarization function bands % NGsBlkXs= 10000 mHa # [Xs] Response block size % LongDrXs 0.000000 | 1.000000 | 0.000000 | # [Xs] [cc] Electric Field % XTermKind= "BG" # [X] X terminator ("none","BG" Bruneval-Gonze) % COLLBands 3 | 6 | # [COLL] Bands for the collisions % HXC_Potential= "SEX+HARTREE+CVONLY" # [SC] SC HXC Potential HARRLvcs= 10000 mHa # [HA] Hartree RL components EXXRLvcs= 10000 mHa # [XX] Exchange RL components CORRLvcs= 10000 mHa # [GW] Correlation RL components

Notice that in the previous input file we added the flag CVONLY that force Yambo to calculate collisions integrals only between valence and conductions bands. This approximation is usually safe and speed up calculation, but it could fail in particular non-linear phenomena.

Now that we calculated the COLLISIONS we will perform three different real-time calculations: 1) the probe field alone; 2) the pump field alone; 3) the pump and probe configuration.

We can generate the input file for a generic Pump and Probe calculations with the command: yambo_nl -u p -V qp -F yambo.in_probe:

nloptics # [R] Non-linear spectroscopy NLogCPUs=0 # [PARALLEL] Live-timing CPU`s (0 for all) PAR_def_mode= "balanced" # [PARALLEL] Default distribution mode ("balanced"/"memory"/"workload") DIP_CPU= "" # [PARALLEL] CPUs for each role DIP_ROLEs= "" # [PARALLEL] CPUs roles (k,c,v) DIP_Threads=0 # [OPENMP/X] Number of threads for dipoles NL_Threads=0 # [OPENMP/NL] Number of threads for nl-optics % NLBands 3 | 6 | # [NL] Bands range % NLverbosity= "high" # [NL] Verbosity level (low | high) NLstep= 5.00000 as # [NL] Time step length NLtime= 80.000000 fs # [NL] Simulation Time NLintegrator= "CRANKNIC" # [NL] Integrator ("EULEREXP/RK2/RK4/RK2EXP/HEUN/INVINT/CRANKNIC") NLCorrelation= "SEX" # [NL] Correlation ("IPA/HARTREE/TDDFT/LRC/LRW/JGM/SEX") NLLrcAlpha= 0.000000 # [NL] Long Range Correction NLDamping= 0.000000 eV # [NL] Damping (or dephasing) #EvalCurrent # [NL] Evaluate the current HARRLvcs= 913 RL # [HA] Hartree RL components EXXRLvcs= 913 RL # [XX] Exchange RL components % GfnQP_E 3.594325 | 1.000000 | 1.000000 | # [EXTQP G] E parameters (c/v) eV|adim|adim % % Field1_Freq 0.100000 | 0.100000 | eV # [RT Field1] Frequency % Field1_Int= 1000.00 kWLm2 # [RT Field1] Intensity Field1_Width= 0.000000 fs # [RT Field1] Width Field1_kind= "DELTA" # [RT Field1] Kind(SIN|RES|ANTIRES|GAUSS|DELTA|QSSIN) Field1_pol= "linear" # [RT Field1] Pol(linear|circular) % Field1_Dir 0.000000 | 1.000000 | 0.000000 | # [RT Field1] Versor % % Field1_Dir_circ 0.000000 | 1.000000 | 0.000000 | # [RT Field1] Versor_circ % Field1_Tstart= 5.00000 fs # [RT Field1] Initial Time

Notice that we introduced a scissor operator in such a way to have the main excitonic peak at 6.1 eV in agreement with experimental measurements. In this input we set only the first field that is the probe, and we use a DELTA function as field in such a way to probe all the spectra. Then we can analyze the spectra with ypp -u:

nonlinear # [R] Non-linear response analysis Xorder= 1 # Max order of the response/exc functions % TimeRange -1.000000 |-1.000000 | fs # Time-window where processing is done % ETStpsRt= 200 # Total Energy steps % EnRngeRt 0.00000 | 20.00000 | eV # Energy range % DampMode= "LORENTZIAN" # Damping type ( NONE | LORENTZIAN | GAUSSIAN ) DampFactor= 0.100000 eV # Damping parameter PumpPATH= "none" # Path of the simulation with the Pump only

and get the dielectric response. Hereafter we plot the polarization along the y-direction and the corresponding dielectric constant:

Now we start from the previous input cp yambo.in_probe yambo.in_pump and

modify it
in order perform a calculation with the Pump field only: yambo_nl -u p -V nl -F yambo.in_pump. These result will be our reference field in the final analysis:

nloptics # [R] Non-linear spectroscopy % NLBands 3 | 6 | # [NL] Bands range % NLverbosity= "high" # [NL] Verbosity level (low | high) NLstep= 5.00000 as # [NL] Time step length NLtime= 150.000000 fs # [NL] Simulation Time NLintegrator= "CRANKNIC" # [NL] Integrator ("EULEREXP/RK2/RK4/RK2EXP/HEUN/INVINT/CRANKNIC") NLCorrelation= "SEX" # [NL] Correlation ("IPA/HARTREE/TDDFT/LRC/LRW/JGM/SEX") NLLrcAlpha= 0.000000 # [NL] Long Range Correction NLDamping= 0.000000 eV # [NL] Damping (or dephasing) #EvalCurrent # [NL] Evaluate the current NoPperiodic # [DIP] Do not enforce periodicity for the Polarization HARRLvcs= 913 RL # [HA] Hartree RL components EXXRLvcs= 913 RL # [XX] Exchange RL components % GfnQP_E 3.594325 | 1.000000 | 1.000000 | # [EXTQP G] E parameters (c/v) eV|adim|adim % % Field1_Freq 0.100000 | 0.100000 | eV # [RT Field1] Frequency % Field1_Int= 1000.00 kWLm2 # [RT Field1] Intensity Field1_Width= 0.000000 fs # [RT Field1] Width Field1_kind= "DELTA" # [RT Field1] Kind(SIN|RES|ANTIRES|GAUSS|DELTA|QSSIN) Field1_pol= "linear" # [RT Field1] Pol(linear|circular) % Field1_Dir 0.000000 | 1.000000 | 0.000000 | # [RT Field1] Versor % % Field1_Dir_circ 0.000000 | 0.000000 | 0.000000 | # [RT Field1] Versor_circ % Field1_Tstart= 165.00000 fs # [RT Field1] Initial Time % Field2_Freq 6.1 | 0.00000 | eV # [RT Field2] Frequency % Field2_Int= 1.E4 kWLm2 # [RT Field2] Intensity Field2_Width= 10.000000 fs # [RT Field2] Width Field2_kind= "QSSIN" # [RT Field2] Kind(SIN|RES|ANTIRES|GAUSS|DELTA|QSSIN) Field2_pol= "linear" # [RT Field2] Pol(linear|circular) % Field2_Dir 0.000000 | 1.000000 | 0.000000 | # [RT Field2] Versor % % Field2_Dir_circ 0.000000 | 0.000000 | 0.000000 | # [RT Field2] Versor_circ % Field2_Tstart= 0.00000 fs # [RT Field2] Initial Time

We set the starting time of the Probe (Field1) to 165 fs, a time larger then the simulation run in such a way to have only the Pump(Field2) field. As Pump field we use QSSIN that is an sine-Gaussian wave, with a frequency 6.1 eV centered at the energy of the first exciton, and a width of 10 fs:

Notice that we turned on an additional flag NoPperiodic to not enforce periodicity in the polarization, see ^{[1]}. This is useful for the final Fourier analysis of the results.