A system for characterising a material is provided. The system includes an optical sensor including an optical waveguide, the optical waveguide having first and second ends and being characterised by having a numerical aperture greater than or equal to 0.2, and a microresonator including an optically active material, the microresonator being positioned in an optical near field of an end face of the first end of the optical waveguide such that the optically active material is excitable by light. The system further includes a light source for exciting the optically active material of the microresonator so as to generate whispering gallery modes (WGMs) in the microresonator and a light collector for collecting an intensity of light that is associated with the WGMs excited in the microresonator.