In recent years, there are reported many examples in which a photonic crystal is applied to a semiconductor laser. Patent Literature 1 discloses a surface emitting laser in which a two-dimensional photonic crystal (two-dimensional diffraction grating) is formed in a vicinity of an active layer containing light emission material. This is one type of a distributed feedback surface emitting laser. This two-dimensional photonic crystal has cylindrical air holes or the like disposed periodically in a semiconductor layer, and has a two-dimensionally periodic refractive index profile. This periodic refractive index profile enables light generated in the active layer to resonate and form a standing wave for laser oscillation.
In the above-mentioned Patent Literature 1, one of a square lattice and a triangular lattice is adopted for the lattice structure of the two-dimensional photonic crystal. Since these two lattice structures have the same length of the primitive translation vectors in plane (namely lattice constant), diffraction is equivalent in some directions in plane. Therefore, diffraction light rays in different directions are combined to each other so as to generate coherent laser oscillation with uniform phase in a two-dimensional manner. This light generated by laser oscillation is extracted in the direction perpendicular to the plane by first-order diffraction by the photonic crystal. According to the principle described above, the semiconductor laser described in Patent Literature 1 works as a surface emitting laser that emits coherent light in a two-dimensional manner.
In addition, there is developed a two-dimensional photonic crystal surface emitting laser using a lattice structure having different lengths of two primitive translation vectors in plane without limiting to the square lattice or the triangular lattice. For instance, Patent Literature 2 proposes a two-dimensional photonic crystal surface emitting laser using a rectangular lattice.
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