Systematic Study of Amorphous ABC Heterostructures at the Atomic Scale as a Nonlinear Optical Metamaterial

TL;DR

The paper demonstrates that amorphous ABC oxide heterostructures (SiO$_2$/TiO$_2$/Al$_2$O$_3$) grown by plasma-enhanced ALD can exhibit strong, bulk-like second-order nonlinear optical responses arising from interfacial nonlinearity, despite all constituents being centrosymmetric in bulk. By systematically varying the ABC period thickness $t_{\text{ABC}}$ while keeping total thickness fixed, they achieve a maximum effective bulk $\,\chi^{(2)}$ of $\chi_{zzz}=2.0\pm0.2$ pm/V at $t_{\text{ABC}}=1.5$ nm, with the nonlinear signal enhancing with higher interface density and deteriorating when layers intermingle around sub-nm scales. The work combines XRR, STEM, ellipsometry, and a Hermans-based SHG analysis that accounts for temporal walk-off to quantify the interface-driven nonlinear response, and demonstrates polarity flipping between ABC and CBA stacking, confirming the interfacial origin of the effect. These CMOS-compatible amorphous nanolaminates offer a scalable route to form-birefringent nonlinear metamaterials with potential applications in nanophotonic waveguides, metasurfaces, and near-UV devices.

Abstract

The systematic exploration of ABC type heterostructures reveals that nanoscale morphological modification markedly improves nonlinear optical properties to maximize the artificial bulk second-order susceptibility. These amorphous birefringent heterostructures are fabricated through cyclic plasma-enhanced atomic layer deposition of three oxides, effectively breaking centrosymmetry. We observe a dependence of optical nonlinearity on the thickness variation of three constituent materials: SiO$_2$ (A), TiO$_2$ (B), and Al$_2$O$_3$ (C), ranging from tens of nanometers to the atomic scale, and these materials exhibit second-order susceptibility at their interfaces. Our findings reveal that the enhancement of nonlinear optical properties is strongly correlated with a high density of layers and superior interface quality, where the interface second-order nonlinearity transitions to bulk-like second-harmonic generation. An effective bulk second-order susceptibility of $χ_{zzz}\nobreakspace{}=\nobreakspace{}2.0\nobreakspace{}\pm\nobreakspace{}0.2$ pm/V is achieved, comparable to typical values for conventional monocrystalline nonlinear materials.

Figures (13)

• Figure 1: The visualization of the ABC type heterostructures with an optical axis perpendicular to the layers. Changing the density of layers influences nonlinear optical properties.
• Figure 2: Graphical schema of the optical setup used for second harmonic measurements of the ABC type heterostructures. From the left: femtosecond laser, HW1: half-wave plate, P1: polarizer, HW2: half-wave plate, L1: plano-convex lens, LP: long pass filter, tilting stage with the sample, SP: short pass filter, L2: biconvex lens, P2: Rochon prism analyzer, BP: bandpass filter, camera.
• Figure 3: (a) Visualizing the angle-dependent measurement for the analysis of second harmonic generation from ABC heterostructures with the angle of incidence $\vartheta$ and the axis orientation. (b) Showing effects present in the experiment. A stronger SH signal comes from the ABC layers, and it interferes constructively and destructively with the weaker SH signal from the back side of the substrate. The temporal walk-off effect also influences the interference by making the local minima shallower.
• Figure 4: ADF STEM images showing the SiO$_2$/TiO$_2$/Al$_2$O$_3$ nanolaminates on the silicon wafer substrate of: (left) sample S3 with the ABC period thickness $t_{\textup{ABC}}$ of 1.5nm; TiO$_{2}$ layers appear darker; (right) sample S2 with $t_{\textup{ABC}}$ of 0.7nm. The layers are no longer visible. The dark area on the top of both nanolaminates is the Pt/C protection layer used for TEM specimen preparation.
• Figure 5: Results of the X-ray reflectivity measurements proving the existence of the layers and verifying the period thickness $t_{\text{ABC}}$. Sample ID from the top: S13, S11, S6, S3, S2.