Method for measuring the near-field signal

A method for measuring the near field signal of a sample in a scattering-type near field microscope comprising a probe and a sample, wherein said method comprises the steps of:

• a) measuring the scattering signal S of the probe as a function of a distance-determining parameter P, whereby the functional dependence H(P) of the probe-sample distance H on P is known, by measuring the scattering signal S(P) for at least two values of the distance-determining parameter P corresponding to two probe-sample distances H in the range H0 to H1, wherein H1 is larger than H0, and the scattering signal S substantially consists of background signal for all distances H>H0,
• b) determining the fit coefficients of a background fit function B(P), wherein the background fit function is equation (I):
• B(P)=c0+c1*(exp(i*k*H(P))+c2*exp(−i*k*H(P)))2 (I),
• wherein B(P) is the scattering signal S(P) as measured in step a), c0, c1 and c2 are fit coefficients, k is a fixed parameter or a fit coefficient, or wherein the background fit function B(P) is an at least first order approximation of equation (I) in terms of P,
• c) measuring the scattering signal SN of the probe for at least one value of the distance-determining parameter PN corresponding to a probe-sample distance HN smaller than H0, and
• d) determining the near field signal N for at least one value of the distance-determining parameter PN by subtracting from the scattering signal SN measured at step c) the background signal BN which is calculated by extrapolating the background fit function B(P) to the probe-sample distance HN using the fit coefficients determined in step b).