I have consulted a book by Mark Fox on Optical Properties of Solids, and it states:
" The absorption of a photon by an inter-band transition in a semiconductor or insulator creates an electron in the conduction band and hole in the valence band. The oppositely charged particles are created at the same point in space and can attract each other through their mutual Coulomb interaction... Moreover, if the conditions are satisfied, a bound electron-hole pair can be formed. This neutral bond pair is called an exciton."
My understanding is - We first need to supply E > Eg for excitons to form. Once excitons exist in a material, they facilitate the absorption of photons with E < Eg, specifically at energies corresponding to n=2, n=3 etc. If the material has not been initially supplied with photons of E > Eg, with the material containing no excitons then why do we see an additional absorption response (red lines in figure below) apart from the alpha ~ sqrt(Eg) response we normally observe? I'm confused because I saw spectra such as one below, I don't expect to see absorption below the band gap - we aren't supplying Eg for excitons to form.
I thought that the idea of a spectra is that you provide photons energy from 0 eV to final energy value greater than Eg, starting from low energy and sweep into higher energies.