Gases¶

Gas materials are built-in names (He, Ne, Ar, Kr, Xe, N2, O2, CO2, H2, Air) or a material with a gas block at any pressure and temperature:

"materials": {
  "argon2bar": { "gas": { "species": "Ar", "pressureBar": 2.0, "temperatureK": 293.15 } },
  "n2cell":    { "gas": { "species": "N2", "pressureBar": 1.5 }, "rotationalRaman": { "dephasingTimeS": 60e-12 } },
  "air":       { "gas": { "species": "Air", "pressureBar": 1.01325 } }
}

	index	electronic n2 (1 bar, 273 K)	rotational Raman	ionisation potential
He, Ne, Ar, Kr, Xe	Börzsönyi 2008 fits	3.5e-25 ... 8.0e-23 m²/W	none (atoms)	24.59 ... 12.13 eV
N2	Börzsönyi 2008	8.0e-24	B = 1.990 cm⁻¹, Δα = 0.70 ų, even:odd = 2:1	15.58 eV
O2	Zhang 2008	1.03e-23	B = 1.438 cm⁻¹, Δα = 1.14 ų, odd J only	12.07 eV
CO2	Bideau-Mehu 1973	1.4e-23 (±50 %)	B = 0.390 cm⁻¹, Δα = 2.10 ų, even J only	13.78 eV
H2	Peck & Huang 1977	6.9e-24 (±30 %)	B = 59.3 cm⁻¹, Δα = 0.30 ų, ortho:para = 3:1	15.43 eV
Air	Ciddor 1996	8.5e-24	N2 + O2 by mole fraction	N2 and O2 channels

The index and the electronic n2 scale with density p T_ref / (p_ref T). Values are literature values good to 10-30 %; override any of them in the material (n2, index, rotationalRaman).

Rotational Raman (molecular orientation)¶

Linear molecules align in the field: an index change that builds up over ~100 fs, persists, and revives at the rotational period 1/(2Bc) (8.4 ps for N2). The engine computes it from the rigid-rotor density matrix (first order, thermal populations with nuclear-spin weights, centrifugal distortion, dephasing time T2 ∝ 1/p):

Δ⟨cos²θ⟩(t) = ∫ χ(t - t') F(t') dt', χ(τ) = (4/15ħ) Σ_J (J+1)(J+2)/(2J+3) (p_J - p_{J+2}) sin(ω_J τ) e^{-τ/T2},

with F = Δα I / (2 n₀ c ε₀). Light polarised along the field sees Δn_∥ = N Δα Δ⟨cos²θ⟩ / (2 n₀ ε₀), perpendicular light -1/2 of that. For arbitrary polarisation the response is a tensor,

Δn_ij = R ⊛ [ (3/2) Re(A_i A_j*) - (1/2) δ_ij |A|² ],

so it acts on the polarisation itself (nonlinear ellipse rotation, XPM between x and y) and circular light sees 1/4 of the linear response. Its time integral is the adiabatic orientational n2 (for N2 at 1 atm ≈ 2.7·10⁻²³ m²/W, three times the electronic part): long pulses (≳ 1 ps) see n2_electronic + n2_rotational, femtosecond pulses mostly the electronic part plus a delayed, red-shifting response. ultrafast materials show N2 prints both.

The response is on by default for N2, O2, CO2, H2 and air whenever physics.raman is true (switch it off with the material's "rotationalRaman": false or an element's "raman": false). It works in 1-D, on spatial grids (scalar and vector) and in carrier-resolved mode (it follows the cycle-averaged intensity).

Air in the gaps¶

A gap is air by default, and "Air" is the Ciddor index at 20 C, 101 325 Pa, dry, 450 ppm CO2. The system's environment.air (temperature, pressure, relative humidity 0..1, CO2) only supplied the air used for data quoted relative to air (OpticStudio exports). To make it the air of every gap (and of every other use of the name "Air"), switch it on:

"environment": { "air": { "temperatureC": 24, "pressurePa": 100500, "humidity": 0.45, "applyToGaps": true } }

It is off by default, so existing results do not change; a note in the result says when it is on. A gap can override it: {"id": "g", "type": "gap", "lengthM": 2.0, "air": {"humidity": 0.2}} (the gap's keys overlay the environment's). air may also name another index model with "kind": the hitran plugin adds the absorption lines and the dispersion of H2O, CO2 and O2 (a model that provides both an index and an absorption is used for both). Python: build.air(...), build.environment(air=...), build.gap(..., air=...).

Ionisation¶

Gases can ionise: see Ionisation and plasma.

refractiveindex.info pages¶

Any gas page of the refractiveindex.info database works as an index (formula 6 pages are scaled to your pressure and temperature from the page's SPECS): see Materials.