Fig. 3. Comparison of PMMA subsurface layers and thin PMMA surface layers.

a, c Peak heights ${\phi }_n^{\max }$ and b, d spectral peak positions ${\omega }_n^{\max }$ extracted from experimental (large dots) and calculated (lines) nano-FTIR phase spectra, as a function of PMMA depth d₂ (left column) and PMMA thickness t₁ (right column), for different demodulation orders n = 3 (red) and n = 4 (blue). The solid and dashed lines are calculated with and without the far-field factor (1 + r)² in Eq. (1), respectively. The gray horizontal dotted line indicates the peak position ω_ref experimentally obtained on thick (bulk) PMMA samples. Inset: definition of ${\phi }_n^{\max }$ and ${\omega }_n^{\max }$. Each experimental data point shows mean values and standard deviation (error bars) obtained from ten averaged (5×) spectra (16 averaged spectra for d₂ ≥ 85 nm).