Plasmon polaritons in the near infrared on fluorine doped tin oxide films

Here we investigate plasmon polaritons in fluorine doped tin oxide (FTO) films. By fitting reflectance and transmittance measurements as a function of wavelength λ ∊ [1.0µm,2.5µm] we derive a Drude dispersion relation of the free electrons in the transparent conducting oxide films. Then we compute the dispersion curves for the bulk and surface modes together with a reflectance map over an extended wavelength region (λ ⇑ 10µm). Although the surface polariton dispersion for a single FTO/air interface when neglecting damping should appear clearly in the plots in the considered region (since it is supposedly far and isolated from other resonances), a complex behaviour can arise. This is due to different characteristic parameters, such as the presence of a finite extinction coefficient, causing an enlargement and backbending of the feature, and the low film thickness, via coupling between the modes from both the glass/FTO and FTO/air interfaces. Taking into account these effects, computations reveal a general behaviour for thin and absorbing conducting films. They predict a thickness dependent transition region between the bulk polariton and the surface plasmon branches as previously reported for indium tin oxide. Finally, attenuated total reflection measurements vs the incidence angle are performed over single wavelengths lines R(θ) (λ=0.633,0.830,1.300,1.550µm) and over a two dimensional domain R(θ,λ) in the near infrared region λ ∊ [1.45µm,1.59µm]. Both of these functions exhibit a feature which is attributed to a bulk polariton and not to a surface plasmon polariton on the basis of comparison with spectrophotometer measurements and modeling. The predicted range for the emergence of a surface plasmon polariton is found to be above λ≥2.1µm, while the optimal film thickness for its observation is estimated to be around 200nm.