Abstract
Cutting-edge acousto-optic devices require optically transparent thin films, which possess a high index of refraction and large elasto-optic coefficients. For the wide near-infrared to ultraviolet spectral region, the mainstream technology employs lithium niobate crystals, which interferes with the vital demands of global sustainability. Here, we demonstrate unprecedented elasto-optic properties in thin films of sustainable and environmentally friendly strontium titanate [SrTiO3 (STO)]. Compared to cubic STO, a nearly twofold increase in elasto-optic coefficients is achieved in epitaxial tetragonal STO films, which concurrently exhibit excellent transparency and a high index of refraction at wavelengths from 400 to 1700 nm. The room-temperature non-polar state is evidenced by the thermo-optical behavior of the films. The obtained enhancement is related to the tetragonal antiferrodistortive phase of STO. It is suggested that such films can form a platform for future sustainable acousto-optic materials.
Highlights
In bulk unstressed STO crystals, the transition from the cubic paraelectric (PE) phase to the tetragonal AFD phase occurs on cooling at ∼104 K
In contrast to the low-temperature AFD phase in crystals, the high-temperature AFD phase can be stabilized in epitaxial films, grown on compressive substrates.[19–21]
We note that LSAT substrates enable an in-plane lattice parameter of 3.868 Å, which is close to that of ∼3.840 Å for STO on Si substrates
Summary
In bulk unstressed STO crystals, the transition from the cubic paraelectric (PE) phase to the tetragonal AFD phase occurs on cooling at ∼104 K. The dielectric functions (ε∗ = ε1 + iε2) and optical constants (index of refraction n, extinction coefficient k, and absorption coefficient α) were determined as a function of wavelength (or photon energy) in the STO films and crystal
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