There are two closely related coherent optical phenomena for IT (Induced Transparency), which turn an opaque (absorbing) medium into a transparent (allowing propagating) one:
It is difficult to know which one is most basic, fundamental and a possible generalization for the other. The two effects, although related, also bear several strong differences.
Electromagnetic Induced Transparency (EIT) involves interferences of two external coherent driving field of a 3LS, whereby the two field cancel by destructive interference the excitation of the excited state. Terminology goes that one beam is the pump, the other is the probe (going through) but this is a bit misleading as to the nature of the effect. This involves instead of "coherent population trapping" [which was itself discovered by Alzetta et al.[1] and formalized into a general principle by Kocharovskaya[2].
Self-Induced Transparency (SIT) does not require two fields but takes care of the reshaping of the dielectric medium all by itself, furthermore typically composed of two-level resonantly interacting quantum systems, through strong-coupling of the transition by the strong pulse that subsequently travels as a soliton. This was discovered by McCall and Hahn[3] and first observed by Boller et al. (co-authored by A. Imamoglu who subsequently worked a lot laser without inversion, which interestingly led him to polaritons lasers).
and also: