A method for inspecting an article containing a target material is provided. A sampling position of the article is illuminated by a beam of laser light having a wavelength λ. Illumination focus to a focal point at the sampling position of the article to produce molecular excitation of the target material by simultaneous absorption of n incident photons of the beam of laser light, wherein n is equal to or greater than two. An output light exited from the article is analyzed by a detector, wherein the output light is of a wavelength range between 0.8λ and 1.2λ.