A method to measure the refractive index of a sample, the method including: • providing a plasmonic sensor capable to allow, upon illumination with electromagnetic radiation, a surface plasmon resonance condition, the plasmonic sensor including a sensing surface in contact with the sample to be measured; • providing an optical resonator having a output channel, the plasmonic sensor being integrated in the optical resonator as a reflecting surface of the same; • providing a first input field of electromagnetic radiation as a primary carrier having a first frequency and having a TE and/or a TM polarized component with respect to the sensing surface; • providing a second input field of electromagnetic radiation as a secondary carrier having a second frequency different from the first frequency and defined as: second frequency = first frequency + Δ v where Δv is the frequency difference between the first frequency and the second frequency and having a TE and/or a TM polarized component, the first and second input field being part of the same beam and travelling along the same optical path; • positioning a source of the first and second input fields (4,4') and the plasmonic sensor in such a fashion that the first and second input fields (4,4') meet a surface of the plasmonic sensor at an angle of incidence enabling a surface plasmon resonance phenomenon; the method being characterized by • impinging simultaneously with the first and second input field (4, 4') the plasmonic sensor integrated in the optical resonator so as to excite two orthogonally-polarized modes of the optical resonator to produce a first and second intra-cavity field in the optical resonator corresponding to the TE or TM component of the first input field and to the TM or TE component of the second input field; • tuning the frequency of the first field and/or the value of Δv till the value of the first frequency and the value of the second frequency are such that the first intra-cavity field corresponding to the TE field or TM field component of the first input field is resonating in the optical resonator and the second frequency at which the second intra-cavity field corresponding to the TM field or TE field component of the second input field is resonant in the optical resonator simultaneously, obtaining a first resonating frequency v 1 and a second resonating frequency v 2; • detecting a resonator output power corresponding to the first and second intra-cavity fields resonating in the optical resonator; • determining a difference between the first resonating frequency v 1 and the second resonating frequency v 2 by determining a beat of the reflected first and second input fields (4,4') resonant in the resonant optical cavity; • calculating the refractive index of the sample from the difference between the first and second frequency. • providing a plasmonic sensor capable to allow, upon illumination with electromagnetic radiation, a surface plasmon resonance condition, the plasmonic sensor including a sensing surface in contact with the sample to be measured; • providing an optical resonator having a output channel, the plasmonic sensor being integrated in the optical resonator as a reflecting surface of the same; • providing a first input field of electromagnetic radiation as a primary carrier having a first frequency and having a TE and/or a TM polarized component with respect to the sensing surface; • providing a second input field of electromagnetic radiation as a secondary carrier having a second frequency different from the first frequency and defined as: second frequency = first frequency + Δ v where Δv is the frequency difference between the first frequency and the second frequency and having a TE and/or a TM polarized component, the first and second input field being part of the same beam and travelling along the same optical path; • positioning a source of the first and second input fields (4,4') and the plasmonic sensor in such a fashion that the first and second input fields (4,4') meet a surface of the plasmonic sensor at an angle of incidence enabling a surface plasmon resonance phenomenon; • impinging simultaneously with the first and second input field (4, 4') the plasmonic sensor integrated in the optical resonator so as to excite two orthogonally-polarized modes of the optical resonator to produce a first and second intra-cavity field in the optical resonator corresponding to the TE or TM component of the first input field and to the TM or TE component of the second input field; • tuning the frequency of the first field and/or the value of Δv till the value of the first frequency and the value of the second frequency are such that the first intra-cavity field corresponding to the TE field or TM field component of the first input field is resonating in the optical resonator and the second frequency at which the second intra-cavity field corresponding to the TM field or TE field component of the second input field is resonant in the optical resonator simultaneously, obtaining a first resonating frequency v 1 and a second resonating frequency v 2; • detecting a resonator output power corresponding to the first and second intra-cavity fields resonating in the optical resonator; • determining a difference between the first resonating frequency v 1 and the second resonating frequency v 2 by determining a beat of the reflected first and second input fields (4,4') resonant in the resonant optical cavity; • calculating the refractive index of the sample from the difference between the first and second frequency.