Thank you for your kind help.
Actually, I have tried the strength parameter sigma as small as 1e-20. But the
field rises to infinity rapidly. sigma can be used to tune the gain, can't it?
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在 2020-07-26 04:47:29，"Alexander Cerjan" <alexcer...@gmail.com> 写道：
This is likely the physically expected behavior. If your gain is coupling to a
mode of your system whose loss rate is less than that of the rate of stimulated
emission, your system will begin to lase, i.e. the field will begin to grow
exponentially. In real, physical systems, this is then compensated by gain
saturation, which prohibits the fields from growing exponentially forever.
However, as the Lorentzian susceptibility model does not contain this physics,
nothing prohibits the fields from continuing their exponential growth.
On Sat, Jul 25, 2020 at 2:02 PM 裴延波 <peiya...@163.com> wrote:
I am trying to use dispersive complex epsilon to describe gain in my
calculation. The parameters for the epsilon is defined as follows.
freq_32 = 2 # emission frequency (units of 2\pi c/a)
gamma_32 = 0.306 # FWHM emission linewidth in sec^-1 (units of 2\pi c/a)
sigma_32 = 1e-4
susceptibilities = [mp.LorentzianSusceptibility(frequency=freq_32,
gamma=-gamma_32, sigma=sigma_32)]
geometry = [mp.Block(center=mp.Vector3(z=0),
size=mp.Vector3(mp.inf,mp.inf,dcell),
material=mp.Medium(epsilon=2.25,E_susceptibilities=susceptibilities))]
Here gamma is negative indicating the material has gain, just as that in the
tutorial. However, when running the field increase to infinity even though
sigma is very very small. I try to set gamma positive and sigma negative. In
this case there is a result which looks normal. I don't know why and what is
the problem in my code. Can anyone explain this. Thanks in advance.
The following is my full python code.
import meep as mp
import math
resolution = 100
dimensions = 3
ns = 1.0
nlead = ns
dlead = 2.0
npad = ns
dpad = 2.0
dpml = 2.0
Ncell = 20
dcell = 96
sz = dcell + dlead + dpad + 2*dpml
cell_size = mp.Vector3(0,0,sz)
pml_layers = [mp.PML(dpml)]
freq_32 = 2 # emission frequency (units of 2\pi c/a)
gamma_32 = 0.306 # FWHM emission linewidth in sec^-1 (units of 2\pi c/a)
df1 = gamma_32/2/math.pi
sigma_32 = 1e-4 # dipole coupling strength (hbar = 1)
default_material = mp.Medium(index=ns)
sources = [mp.Source(mp.GaussianSource(freq_32, fwidth=df1), component=mp.Ex,
center=mp.Vector3(-dcell/2-dpad/2))]
sim = mp.Simulation(cell_size=cell_size,
sources=sources,
resolution=resolution,
boundary_layers=pml_layers,
dimensions = dimensions,
default_material=default_material)
nfreq = 50 #number of frequencies at which to compute flux
pt = mp.Vector3(0,0,dcell/2+dpad/2)
flux_detection_point = mp.FluxRegion(center=pt)
incidence = sim.add_flux(freq_32,df1,nfreq,flux_detection_point)
sim.run(until_after_sources=mp.stop_when_fields_decayed(50,mp.Ex,pt,1e-3))
incident_flux = mp.get_fluxes(incidence)
sim.reset_meep()
susceptibilities = [mp.LorentzianSusceptibility(frequency=freq_32,
gamma=-gamma_32, sigma=sigma_32)]
geometry = [mp.Block(center=mp.Vector3(z=0),
size=mp.Vector3(mp.inf,mp.inf,dcell),
material=mp.Medium(epsilon=2.25,E_susceptibilities=susceptibilities))]
geometry.append(mp.Block(center=mp.Vector3(z=-sz/2+(dpml+dlead)/2),
size=mp.Vector3(mp.inf,mp.inf,dpml+dlead),
material=mp.Medium(index=nlead)))
geometry.append(mp.Block(center=mp.Vector3(z=sz/2-(dpml+dpad)/2),
size=mp.Vector3(mp.inf,mp.inf,dpml+dpad),
material=mp.Medium(index=npad)))
sim = mp.Simulation(cell_size=cell_size,
sources=sources,
resolution=resolution,
boundary_layers=pml_layers,
geometry=geometry,
dimensions = dimensions,
default_material=default_material)
transmission = sim.add_flux(freq_32,df1,nfreq,flux_detection_point)
sim.run(until_after_sources=mp.stop_when_fields_decayed(50,mp.Ex,pt,1e-3))
transmitted_flux = mp.get_fluxes(transmission)
flux_freqs = mp.get_flux_freqs(transmission)
data1 = open("lasing.dat",'w')
for ii in range(0,nfreq):
data1.write("%f %f %f %f\n"
%(flux_freqs[ii],incident_flux[ii],transmitted_flux[ii],transmitted_flux[ii]/incident_flux[ii]))
data1.close()
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