E-symmetry optical phonons at the Γ point of LiNbO3 and LiTaO3 were experimentally resolved in spectra measured by infrared and Raman spectroscopy. For this purpose, congruent and nearly stoichiometric crystals of LiNbO3 and LiTaO3, and mixed LiNb1−xTaxO3, crystals were studied. The results show that some of the E modes have weak intensities in Raman or infrared spectra. Thus the complete assignment of E-symmetry modes has been achieved by comparing Raman and infrared data. In addition, this assignment has been confirmed by Raman measurements at low temperatures.

In 1928, Zachariasen discovered the LiNbO 3 phase. [1] The first single crystals were grown using the flux method by Matthias Over the past five decades, LiNbO 3 and LiTaO 3 single crystals and thin films have been studied intensively for their exceptional acoustic, electro-optical, and pyroelectric and ferroelectric properties. Today, LiNbO 3 single crystals in electro-optics are equivalent to silicon in electronics, and about 70% of radio-frequency (RF) filters, based on surface acoustic waves, are fabricated on these single crystals. These materials in the form of thin films are needed urgently for the development of the next-generation of high-frequency and/or wide-band RF filters or tuneable frequency filters adapted to the fifth generation of infrastructures/networks/communications. The integration of LiNbO 3 films in guided nanophotonic devices will allow higher operational frequencies, wider bandwidth, and miniaturized optical devices in line with improved electronic conversion. Here, the challenges and the achievements in the epitaxial growth of LiTaO 3 and LiNbO 3 thin films and their integration with silicon technology and to acoustic and guided nanophotonic devices are discussed in detail. The systematic representation and classification of all epitaxial relationships reported in the literature have been carried out in order to help the prediction of the epitaxial orientations in the new heterostructures. Future prospects of potential applications and the expected performances of thin film devices are overviewed, as well.Figure 2. Schematic representation of the layer transfer process by the ion slicing technique consisting of a) ion implantation, b) wafer bonding, c) laser irradiation or heating in order to obtain d) a single crystalline layer on the host (Si) substrate. Reproduced with permission. [53]Figure 3. a,b) Electron microscopy images and c) schematic diagram of a microresonator based on LiNbO 3 film on Si and fabricated by the layer transfer technique. Reproduced with permission. [53]The first reports on the growth of c-axis-oriented LN films by liquid phase epitaxy on an LT substrate and by RF sputtering Adv. Mater. Interfaces 2017, 4, 1600998 www.advmatinterfaces.de www.advancedsciencenews.com Figure 6. Frequency response of a) HBAR and b) FBAR based on thinned X-cut LiNbO 3 layer on Si. Schematic representations of a) HBAR and b) FBAR structures are given in the insets. Reproduced with permission. [78]