A-axis oriented Zn_0.72Mg_0.28O epitaxial thin films with large second-order nonlinear susceptibility

Abstract

Large nonlinear susceptibility that originates from strong electronic polarization enables ultrafast nonlinear optical devices. This work discovers that Mg incorporation has important contribution for enhancing second-order nonlinear susceptibility (χ(2)) and laser-induced surface-damage threshold of wurtzite ZnO epitaxial thin films deposited by radio-frequency magnetron sputtering method. Second-harmonic generation measurements derive that as-deposited Zn0.72Mg0.28O shows a gain of 48%, 77% and 33% in χ33, χ31 and χ15 with respect to as-deposited ZnO. Specially, the annealed Zn0.72Mg0.28O has a χ33 value of -57.0 ± 1.8 pm/V, which gets comparable to that of LiNbO3 crystals. Triple-axis X-ray diffraction measurements conclude that the Mg incorporation should increase the χ(2) under optical frequency electric field by strengthening electronic polarization rather than increasing the residual strain in the film. Furthermore, the annealed Zn0.72Mg0.28O exhibits an increase of 48% in laser-induced surface-damage threshold relative to [11-20] ZnO crystals. These findings open the way of the Zn0.72Mg0.28O thin films to ultrafast nonlinear optical devices.

Large Enhancements in Optical and Piezoelectric Properties in Ferroelectric Zn1‐xMgxO Thin Films through Engineering Electronic and Ionic Anharmonicities

“…Nonetheless, each of these phenomena, namely the $$d_{ijk}^{{\rm{Piezo}}}$$, [ 22–24 ] the $$d_{ijk}^{{\rm{SHG}}}$$, [ 25,26 ] the bandgap, [ 27–29 ] and in addition, the electro‐optic tensor, [ 30 ] have been separately reported to be enhanced with Mg substitution in ZnO. This work revisits and confirms the first three of these experimental measurements in a single set of films, though the magnitudes of enhancement reported here significantly differ from the literature for $$d_{ijk}^{{\rm{SHG}}}$$.…”

Strong nonlinear-polarization in ZnMgO epitaxial thin-films with Li incorporation

“…Furthermore, the |χ 33 | value of Zn 0.67 Mg 0.33 O:Li reaches 36.1 pm V −1 , which is about 2.9 times larger than that of Zn 0.67 Mg 0.33 O and increases by 37.8% relative to that of ZnO. The |χ 33 | value of ZnO is smaller than previously reported value [14], which is measured using a E p of 12.5 GW cm −2 . The utilization of much larger E p (81.2 GW cm −2 ) can damage the sample to some extent.…”

“…It can be observed that E opt is apparently boosted with increasing Mg incorporation. The χ 33 of these thin-films can be evaluated with a good approximation by using the equation in the literature [14], since their thickness (200 nm) is much less than their coherence length (2.1-4.8 µm). The derived |χ 33 | values of some samples are exhibited in table 1.…”

“…The thickness of the thin-films is approximately 200 nm. The detailed fabrication methods can be found elsewhere [14]. The chemical composition in thin-films were analyzed by utilizing x-ray photoelectron spectroscopy (XPS) with Al Kα radiation.…”

“…It receives increasing attention to study Mg incorporated ZnO (Zn 1−x Mg x O) thin-films for χ (2) process, because Zn 1−x Mg x O has wide bandgap of up to 6.2 eV [11], low lightpropagation loss of less than 0.5 dB cm −1 [12], and moderate coercive-field of about 1.5 MV cm −1 for polarization reversal [13]. It was discovered that moderate Mg incorporation (x = 0.28) significantly increased the χ (2) of c-axis preferentially oriented ZnO polycrystalline thin-films [12] and aaxis oriented ZnO epitaxial thin-films [14]. Then, Yuan et al reported that Mg incorporation (x = 0.28) into a-axis oriented ZnO obviously enhanced the effective pockels electrooptic coefficient (r c ) [15].…”