Application of noncontact terahertz surface plasmon interferometry for determination of optical constants of thin tetraaminodiphenyl films

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The article presents a new noncontact plasmonic refractometry method for determination of the optical constants of weakly absorbing micron-thick dielectric layers in the THz range from measured characteristics (the refractive index and propagation length) of surface plasmon polaritons (SPPs) in a sputtered gold – test dielectric layer – air structure. The method is implemented using a plasmonic Michelson interferometer and quasi-monochromatic radiation with wavelength λ = 141 μm (2.1 THz) of the Novosibirsk free-electron laser (NovoFEL). The optical constants of films under study were determined using the actual effective optical constants of the skin layer of deposited gold, which had been determined from the SPP characteristics in an experiment with a calibrated dielectric coating of zinc sulfide of micron thickness. From the measurement results, the optical constants of tetraaminodiphenyl (TADPh) films with thickness d = 1 – 4 μm, utilized in pyroelectric THz radiation detectors, were determined for the first time at this λ . It has been found that the accuracy in the determination of the film extinction coefficient increases with the excess of the SPP energy absorbed in the film over the SPP ohmic losses in the metal, as well as at low values of the film refractive index n . The minimum optical thickness of film that can be identified by this method is nd / λ ≈ 0.01, and the minimum equivalent absorption level A ≈ 0.1%, which is an order of magnitude lower than in other known methods applied in the THz range. The revealed advantages of the THz surface plasmon refractometry demonstrate a potential for scientific research and various THz photonics applications.