Method and system for optical detection of nano-objects in a refracting medium. A nano-object (n_oi) and the refractive medium are illuminated (A) with a periodically amplitude modulated coherent electromagnetic heating wave (HB(Ω)), to generate a specified temperature and refractive index profile in the vicinity of the nano-object, and with a coherent electromagnetic probe wave (PB) to generate an emerging probe wave (EPB(Ω)) having at least one intensity component amplitude modulated by a beat at the modulation frequency of the coherent heating wave. The intensity component amplitude modulated by a beat is detected (C) in the emerging wave (EPB(Ω)), to distinguish and represent this nano-object in the refractive medium. The invention is useful in the detection of nano-objects in an industrial, physiological or intracellular medium.