A review of the physical properties and range of practical applications of layered (quasy two-dimensional (2-D)) GaSe-type (gallium selenide) semiconductors is given in the present article with emphasizes on GaSe. First data on the NLO properties of ε-GaSe (hereinafter GaSe) has been reported in 1972 by the Azerbaijanian (Institute of Physics Azerbaijan National Academy of Sciences, Baku, Azerbaijan) and Russian (Lebedev Institute of Physics USSR Academy of Sciences, Moscow, Russia) teams. GaSe has been recognized by the world scientific community as a crystal with an outstanding NLO properties which may be characterize shortly as follows: high optical birefringence Δn ~ 0.3 at 700 nm; wide optical transparency range of 0.65 nm - 18 μm with low absorption coefficient in this range ( α ≤0.3 cm-1); one of the highest second-order NLO coefficient (d22 ≈ 86 ± 17 pm/V); high power optical damage threshold; possibility to perform frequency conversion under phase-matching conditions in the near- to mid-IR and THz- range of spectra, etc. Occurrence of different polytypes (ε, γ, β, δ), in the bulk grown crystal highly increases the interest to study the physical properties of GaSe and leads to formation of a domain structure. The latter is discussed also in the present article as studied by confocal microscopy 2D imaging experiments.
Some results on the optical and NLO properties of nanoparticles of this class of materials are considered also. Present article, does not pretend to be one reflecting all existing papers on above mentioned subjects.