Provided are a highly transmissive optical thin film having an improved
structure, in which, optical reflection (due to a difference in the
refractive index between a semiconductor material and the air, when light
is extracted from a semiconductor light emitting device into the air) may
be suppressed, an optical output loss may be reduced and light
transmittance efficiency may be maximized or increased, a semiconductor
light emitting device having the same, and methods of fabricating the
same. The optical thin film may include a first material layer having a
first refractive index, a second material layer formed on the first
material layer and having a second refractive index that is smaller than
the first refractive index, and a graded-refractive index layer
interposed or inserted between the first material layer and the second
material layer and having a multi-layer structure in which refractive
index distribution gradually decreases in the range between the first
refractive index and the second refractive index as the refractive index
distribution progresses from the first material layer toward the second
material layer.