TwoPhotonAbsorption

Model for two-photon absorption (TPA) nonlinearity which gives an intensity-dependent absorption of the form $\alpha = \alpha_0 + \beta I$. The expression for the nonlinear polarization is given below.

$P_{NL} = -\frac{c_{0}^{2} \varepsilon_{0}^{2} n_0 Re(n_0) \beta}{2 i \omega} |E|^{2} E$

Coefficient

beta: Coefficient for two-photon absorption (TPA).

Type: floating-point number

• Unit: μm/W
• Default: 0

Carrier Lifetime

tau: Carrier lifetime.

• Unit: s
• Constraint: greater than or equal to 0
• Default: 0

Free-Carrier Absorption Cross Section

sigma: Free-carrier absorption cross section.

• Unit: μm²
• Constraint: greater than or equal to 0
• Default: 0

Electron and Hole Exponents

e_e and e_h: Exponents used for electron and hole carrier densities.

• Type: floating-point number
• Unit: unitless
• Constraint: greater than or equal to 0
• Default: 1 for each

Electron and Hole Coefficients

c_e and c_h: Coefficients for electron and hole carrier effects.

• Type: floating-point number
• Unit: μm^(3 e_e) for c_e and μm^(3 e_h) for c_h
• Default: 0 for each

Linear Refractive Index and Central Frequency

NOTE

The editor does not show n0 or freq0. If they are not set, Tidy3D computes them from the simulation sources (all sources must have the same central frequency). To set them, use the Python interface.

Notes

• This frequency-domain equation is implemented in the time domain using complex-valued fields.
• Different field components do not interact nonlinearly. For example, when calculating $P_{NL,x}$, we approximate $|E|^{2}\approx|E_x|^{2}$. This approximation is valid when the $E$ field is predominantly polarized along one of the x, y, or z axes.