Abstract
Optical index of refraction n is studied by spectroscopic ellipsometry in epitaxial nanofilms of NaNbO3 with thickness ∼10 nm grown on different single-crystal substrates. The index n in the transparency spectral range (n ≈ 2.1 – 2.2) exhibits a strong sensitivity to atmospheric-pressure gas ambience. The index n in air exceeds that in an oxygen ambience by δn ≈ 0.05 – 0.2. The thermo-optical behaviour n(T) indicates ferroelectric state in the nanofilms. The ambience-sensitive optical refraction is discussed in terms of fundamental connection between refraction and ferroelectric polarization in perovskites, screening of depolarizing field on surfaces of the nanofilms, and thermodynamically stable surface reconstructions of NaNbO3.
Highlights
Perovskite-structure ABO3 ferroelectric crystals have long been employed in optical and photonic devices owing to a number of remarkable properties including wide bandgaps (>3 eV), large electro-optical and nonlinear optical coefficients, high static dielectric constants, and the possibility of sustaining and switching a spontaneous polarization [1,2,3]
We show that due to fundamental connection between polarization and optical properties of perovskite ferroelectrics, the sensitivity of ferroelectric polarization to surface state in ferroelectric nanofilms can lead to ambiencesensitive optical refraction
We report on atmospheric-pressure ambience-sensitive index of refraction in electrode-less epitaxial nanofilms of NaNbO3
Summary
Perovskite-structure ABO3 ferroelectric crystals have long been employed in optical and photonic devices owing to a number of remarkable properties including wide bandgaps (>3 eV), large electro-optical and nonlinear optical coefficients, high static dielectric constants, and the possibility of sustaining and switching a spontaneous polarization [1,2,3]. The polarization in epitaxial ferroelectric films can differ from that in bulk samples leading to new optical properties of the films compared to crystals. Either electronic charge in metallic electrodes or extra ions, or point defects at the electrode-less surfaces can provide screening of the depolarizing field and stabilize the ferroelectric phase in films with thickness of a few nanometres only, or nanofilms. We show that due to fundamental connection between polarization and optical properties of perovskite ferroelectrics, the sensitivity of ferroelectric polarization to surface state in ferroelectric nanofilms can lead to ambiencesensitive optical refraction. We report on atmospheric-pressure ambience-sensitive index of refraction in electrode-less epitaxial nanofilms of NaNbO3. The index n stabilized in air significantly exceeds the index in an oxygen ambience This observation is discussed in terms of thermodynamically stable surface reconstructions of NaNbO3 resulting in different screening of the depolarizing field. The dielectric functions and the optical properties of the substrates were determined using separate independent measurements
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