def test_multilevel_atom(self):
resolution = 20
ncav = 1.5
Lcav = 1
dpad = 1
dpml = 1
sz = Lcav + dpad + dpml
cell_size = mp.Vector3(z=sz)
dimensions = 1
pml_layers = [mp.PML(dpml, side=mp.High)]
omega_a = 40
freq_21 = omega_a / (2 * math.pi)
gamma_perp = 4
gamma_21 = (2 * gamma_perp) / (2 * math.pi)
theta = 1
sigma_21 = 2 * theta * theta * omega_a
rate_21 = 0.005
N0 = 28
Rp = 0.0051
t1 = mp.Transition(1,
2,
pumping_rate=Rp,
frequency=freq_21,
gamma=gamma_21,
sigma_diag=mp.Vector3(sigma_21, sigma_21, sigma_21))
t2 = mp.Transition(2, 1, transition_rate=rate_21)
ml_atom = mp.MultilevelAtom(sigma=1,
transitions=[t1, t2],
initial_populations=[N0])
two_level = mp.Medium(index=ncav, E_susceptibilities=[ml_atom])
geometry = [
mp.Block(center=mp.Vector3(z=(-0.5 * sz) + (0.5 * Lcav)),
size=mp.Vector3(mp.inf, mp.inf, Lcav),
material=two_level)
]
sim = mp.Simulation(cell_size=cell_size,
resolution=resolution,
boundary_layers=pml_layers,
geometry=geometry,
dimensions=dimensions)
def field_func(p):
return 1 if p.z == (-0.5 * sz) + (0.5 * Lcav) else 0
def check_field(sim):
fp = sim.get_field_point(
mp.Ex, mp.Vector3(z=(-0.5 * sz) + Lcav + (0.5 * dpad))).real
self.assertAlmostEqual(fp, 1.8040684243391956)
sim.init_sim()
sim.fields.initialize_field(mp.Ex, field_func)
sim.run(mp.at_end(check_field), until=7000)
# Finally, note the lack of 4*pi in the above conversion that is written in many published SALT papers.
# This 4*pi comes from using Gaussian units, in which the displacement field, D = E + 4*pi*P, whereas
# in SI units, D = eps0*E + P, which is what MEEP uses.
# Gain medium pump and decay rates are specified in units of c/a.
rate_21 = 0.005 # non-radiative rate (units of c/a)
N0 = 37 # initial population density of ground state
Rp = 0.0051 # pumping rate of ground to excited state
# so for example, these parameters have D_0 (SALT) = 0.0693.
# Make the actual medium in MEEP:
transitions = [
mp.Transition(1,
2,
pumping_rate=Rp,
frequency=freq_21,
gamma=gamma_21,
sigma_diag=mp.Vector3(sigma_21, 0, 0)),
mp.Transition(2, 1, transition_rate=rate_21)
]
ml_atom = mp.MultilevelAtom(sigma=1,
transitions=transitions,
initial_populations=[N0])
two_level = mp.Medium(index=ncav, E_susceptibilities=[ml_atom])
# Specify the cavity geometry:
geometry = [
mp.Block(center=mp.Vector3(z=-0.5 * sz + 0.5 * Lcav),
size=mp.Vector3(mp.inf, mp.inf, Lcav),
material=two_level)
]