Electro-Optical Properties of Different H_xLi_1-xNbO₃Phases in Proton-Exchanged LiNbO₃Waveguides

Subsurface disorder and electro-optical properties of proton-exchanged LiNbO3 waveguides produced by different techniques

“…Our data give the direct evidence of high structural disorder, as the strong broadening of the Raman lines (see Figure 2) shows clearly an increase of the phonon damping (related to crystalline disorder [14]) in the PE and APE waveguides with any H x Li 1-x NbO 3 phase, even α-phase with low x. The proton exchange degree, x, has been determined using its well known relations with the exchange-induced refractive index change ∆n (e) , determined from dark m-lines spectroscopy data and from Raman data on phonon frequencies [8,14,18]. The region of Raman spectra ranging from 550 cm −1 to 900 cm −1 have been studied in details (Figure 2 and Table 1), as important information on the structural disorder can be obtained from Raman data of this range [8].…”

“…The proton exchange degree, x, has been determined using its well known relations with the exchange-induced refractive index change ∆n (e) , determined from dark m-lines spectroscopy data and from Raman data on phonon frequencies [8,14,18]. The region of Raman spectra ranging from 550 cm −1 to 900 cm −1 have been studied in details (Figure 2 and Table 1), as important information on the structural disorder can be obtained from Raman data of this range [8]. In particular the A 1 (LO)-phonon mode was established to be a good marker of structural disorder [8,20].…”

“…The fabrication parameters were chosen in order to obtain a waveguide composed quasi exclusively of a homogeneous α-phase layer. To determine the phase composition of the exchanged layer, we used the structural phase diagram of H x Li 1-x NbO 3 solid solution [8,14]. The APE waveguides were fabricated with standard APE technique (APE1 sample was annealed in dry air at 360 • C) [2] and improved APE technique (APE2 sample was annealed in wet air at 360 • C) [5,8].…”

“…Subsurface disorder was studied with micro-Raman and IRreflection spectroscopy technique, as broadening of Raman phonon bands and decrease of reflectivity are well-established clear indications of a marked structural disorder in LiNbO 3 [8,15]. Raman spectroscopy was performed on planar and channel waveguides with the aid of a Jobin-Yvon LabRam HR800 spectrometer operating at excitation wavelength of 632.8 nm.…”

“…It has been established [5] that the loss of water during annealing is a major contributor to the formation of crystalline disorder and, therefore, annealing under a high pressure of water vapor prevents such a disorder to appear. However, using this method only a partial suppression of the subsurface disorder and the related degradation of waveguide properties were obtained [8]. In this paper we show that, using the soft proton exchange (SPE) process [9]− [11] known as allowing the fabrication of low loss and highly efficient nonlinear components, we obtain waveguides in the α-phase [12,13] and free from any water loss [6,8,14], and therefore, we avoid completely these undesirable effects.…”