Nonlinear Anisotropy in Phase-Tuned Wide-Gap Halides

TL;DR

The paper addresses how crystallographic phase affects nonlinear optical responses in noncentrosymmetric halide semiconductors. It employs polarization-resolved second-harmonic generation and two-photon photoluminescence on phase-pure gamma-AgI (zincblende, 111) and beta-AgI (wurtzite, 101) thin films grown by physical vapor deposition. The main findings show a sixfold SHG pattern and isotropic 2PPL in gamma-AgI, and a twofold SHG pattern with anisotropic 2PPL in beta-AgI, consistent with symmetry-informed tensor analyses of chi^(2) and chi^(3). The results demonstrate phase control as a route to tailor nonlinear light–matter interactions in halide semiconductors, with potential implications for nanoscale optoelectronics and quantum photonics.

Abstract

Silver iodide (AgI) thin films offer a compelling platform for studying nonlinear optical phenomena due to their intrinsic noncentrosymmetric lattice and direct band gap. Here, we investigate the nonlinear optical properties of AgI thin films grown by physical vapor deposition that selectively produce zincblende (\zbAgI) and wurtzite (\wzAgI) phases. Using a combination of polarization-resolved second harmonic generation (SHG) and two-photon photoluminescence (2PPL) spectroscopy, we identify clear phase- and morphology-dependent anisotropic nonlinear responses. Triangular \zbAgI $(111)$ flakes exhibit sixfold SHG symmetry and isotropic 2PPL emission, while rod-shaped \wzAgI $(101)$ samples display twofold-symmetric patterns in both SHG and 2PPL, which are explained by polarization analysis using second- and third- order nonlinear susceptibilities. These findings establish AgI as a promising halide semiconductor platform for phase-selective nonlinear optics and quantum photonic applications.

Figures (6)

• Figure 1: Electron microscopy and spectroscopy characterization of two AgI morphologies prepared as FIB lift-out cross-section lamellae. Triangular platelet: (a) SEM plan-view image of triangular platelets, with a dashed yellow line indicating the cut location for cross-sectioning. (b) HAADF-STEM cross-section reveals a uniform AgI film of approximately 500 nm thickness on a Si substrate. (c) Bright-field optical image of the same sample confirms the characteristic triangular morphology. (d) STEM–EDS mapping shows a stoichiometric and compositionally uniform distribution of Ag (magenta) and I (green); O (blue), Si (red), and Pt (yellow) originate from the substrate and protective cap, respectively. (e) High-resolution STEM image along a zone axis reveals atomic lattice fringes consistent with the $\gamma$-AgI (111) stacking sequence with a zone axis [110]. Rod-like platelet: (f) Optical micrograph of an elongated nanorod grown at higher temperature. (g) SEM plan-view of a nanorod, with the yellow dashed line indicating the cross-section cut for TEM. (h) ADF-STEM cross-section shows a compact, rectangular shape with dense internal microstructure. (i) STEM–EDS elemental mapping confirms the rod is composed of Ag and I, with a stoichiometric distribution.
• Figure 2: Wavelength-resolved nonlinear emission from an individual AgI triangular flake. Normalized photoluminescence spectra were collected while tuning the excitation wavelength from 784 nm to 798 nm. Each trace exhibits two distinct features: (i) a narrow band at 388–398 nm attributed to second-harmonic generation (SHG), which shifts according to $\lambda/2$ of the fundamental excitation; and (ii) a broader band centered at 412–420 nm assigned to excitonic emission, whose peak position remains nearly constant while its intensity varies with the pump wavelength. The inset shows a representative triangular crystal from the same substrate used for the measurement. All intensities are plotted in arbitrary units (A.U.).
• Figure 3: Polarization-resolved nonlinear-optical response of two AgI micro-crystals. (a) Optical micrograph of a triangular flake; (b) SHG polar plot from flake (a), exhibiting six-lobed pattern that follows a $\cos^2[3(\theta-\phi_0)]$ dependence; (c) 2PPL response of the triangle. (d) Optical micrograph of a nanorod; (e) SHG polar plot recorded from the nanorod showing twofold symmetry aligning with simulation values for ${d_{1}>d_{2}}$; (f) corresponding 2PPL revealing pronounced anisotropy, contrasting the isotropic response from (a) – maxima are aligned with the long axis of the rod.
• Figure 4: Relation between Cartesian and crystallographic coordinate systems, and definition of beam orientation angles. (a) Alignment of Cartesian and crystallographic axes in a cubic unit cell. (b) Alignment of Cartesian and crystallographic axes in a hexagonal unit cell. (c) Definition of the angles $\theta$, $\phi$, and $\delta$ describing the orientation of the incident laser beam (wave vector $\mathbf{k}$) and its optical electric field $\mathbf{E}$.
• Figure S1: Characterization of AgI nanorods ($\beta$-AgI). (a--c) SEM images of AgI rods grown at $470\,^\circ\mathrm{C}$. (d) HRSTEM image of the rod cross-section acquired along the $[21\bar{2}]$ zone axis; lattice planes indexed to $(101)$ are indicated. (e) X-ray diffraction (XRD) confirming the nanorod growth direction as $(101)$ with a peak at $2\theta = 25.3^\circ$ consistent with the TEM analysis.