Optical element and spectroscopic device using the same

An optical element comprising a multilayer structure containing a periodic structural portion as at least one region constituted by repetition of a predetermined period, wherein:

• an end surface of said multilayer structure non-parallel to layer surfaces of said multilayer structure is used as a light input surface whereas one or each of opposite surfaces of said multilayer structure parallel to said layer surfaces is used as a light output surface, or one of opposite surfaces of said multilayer structure parallel to said layer surfaces is used as a light input surface whereas an end surface of said multilayer structure non-parallel to said layer surfaces is used as a light output surface;
• and
• said multilayer structure satisfies the conditions (1) and (2):
• 0<ks·λ0/(2π·ns)<1 (1)
• which is the condition to be satisfied in a direction that is parallel to said layer surfaces of said multilayer structure and that does not have the periodic structural portion,
• wherein the periodic structural portion of said multilayer structure is regarded as a one-dimensional photonic crystal, and
• wherein λ0 is a wavelength in vacuum of used light, ks is a magnitude of a wave number vector in a coupling band (which is not the lowest) of said photonic crystal in accordance with said wavelength λ0, and ns is a refractive index of medium taken at said wavelength λ0 and brought into contact with a surface of said multilayer structure which serves as said light output surface or said light input surface;
• and
• 0.2≦λ0/(a·nM2)≦0.4 (2)
• in which a is a length of one period of said periodic structural portion of said multilayer structure in accordance with said wavelength λ0, and nM is an average refractive index in one period of said periodic structural portion in accordance with said wavelength λ0,
• wherein a number of repeated periods in said periodic structural portion of said multilayer structure is not smaller than 10.