A method for analyzing a substance, comprising the following steps: - arranging a surface of said substance to be analyzed on a surface of an optical medium, so that at least a portion of the surface of the optical medium is in contact with the substance surface; - emitting an excitation light beam with an excitation wavelength through the region of the surface of the optical medium in contact with the substance surface, onto the substance surface, to enable an absorption of excitation light in the substance inducing a temperature increase, which leads to a local change in the refraction number or refraction index of the optical medium in contact with said substance; - emitting a measurement light beam through the optical medium onto the region of the surface of the optical medium which is in direct contact with the substance surface, such that the measurement light beam is reflected at an interface of the optical medium and the substance surface; characterized in that the light path of the measurement light beam is arranged such that it is emitted through the optical medium to said region of the surface, and that the locations where the measurement light beam and the excitation light beam impinge onto the interface of the optical medium and the surface of the substance overlap, and that the method comprises the following steps: - detecting a deflection of the reflected measurement light beam, which is caused by said local change in the refraction number or refraction index, as dependent on the wavelength of the excitation light beam; and - analyzing the substance on the basis of the detected deflection of the measurement light beam as dependent on the wavelength of the excitation light beam, wherein the extent of the deflection correlates with the absorption of the excitation light beam in said substance. - arranging a surface of said substance to be analyzed on a surface of an optical medium, so that at least a portion of the surface of the optical medium is in contact with the substance surface; - emitting an excitation light beam with an excitation wavelength through the region of the surface of the optical medium in contact with the substance surface, onto the substance surface, to enable an absorption of excitation light in the substance inducing a temperature increase, which leads to a local change in the refraction number or refraction index of the optical medium in contact with said substance; - emitting a measurement light beam through the optical medium onto the region of the surface of the optical medium which is in direct contact with the substance surface, such that the measurement light beam is reflected at an interface of the optical medium and the substance surface; - detecting a deflection of the reflected measurement light beam, which is caused by said local change in the refraction number or refraction index, as dependent on the wavelength of the excitation light beam; and - analyzing the substance on the basis of the detected deflection of the measurement light beam as dependent on the wavelength of the excitation light beam, wherein the extent of the deflection correlates with the absorption of the excitation light beam in said substance.