Typical values of the nonlinear interaction potential for excitons are U ∼10⁻¹¹ meV-cm². Convert this value to a photon–photon χ⁽³⁾ value in units of m²/V², using the formulas of Section 4.4 to convert electric field amplitude E to photon number density, and taking a typical solid state microcavity length of 500 nm, the dielectric constant κ = 13 for a typical III–V semiconductor, and optical wavelength of 800 nm. You will first need to find the effective mass of the particles. How does your number compare to the χ⁽³⁾ values of other nonlinear optical media that you can look up?