Dispersion engineered AlGaAs-on-insulator nanophotonics by distributed feedback

TL;DR

The paper tackles on-chip dispersion engineering for nanophotonics by integrating Fabry-Pérot resonators with distributed Bragg reflectors in an AlGaAs-on-insulator platform. It introduces a shape-constrained inverse design that tunes a corrugation profile $\Gamma(x)$ to establish a prescribed dispersion, leveraging a fast 1D coupled-wave model to guide optimization. Experimentally, PhC nanobeam cavities demonstrate high intrinsic quality factors ($Q_i \gtrsim 10^5$) and dispersion profiles that agree with theory, with pure PhC designs yielding particularly accurate results while FP–PhC designs reveal boundary-induced deviations linked to group-velocity mismatch $\Delta v_g$ at interfaces. The work provides a general, efficient dispersion-engineering framework for high-index-contrast nanophotonics with potential for broadband operation and integration of nonlinear functionalities.

Abstract

Technological advances in the fabrication of nanophotonic circuits have driven the scientific community to increasingly focus on the precise tailoring of their key optical properties, over a broadband spectral domain. In this context, the modulation of the local refractive index can be exploited to customize an effective reflectivity by the use of distributed Bragg mirrors, enabling the on-chip integration of Fabry-Pérot resonators. The resulting cavity length is strongly wavelength-dependent, offering practical solutions to the growing demand of dispersion engineering. Owing to their typically high core-to-cladding refractive index contrast and exceptional nonlinear properties, III-V semiconductor-based platforms represent promising candidates for the fabrication of Bragg reflectors. In this work, we propose an AlGaAs-on-insulator linear resonator based on distributed Bragg mirrors. We discuss the first experimental demonstration of a systematic, shape-constrained inverse design technique which tailors a prescribed dispersion profile, showing a strong agreement between simulations and measurements. In perspective, the proposed approach offers an efficient and general response to the challenge of dispersion engineering in integrated optical circuits.

Figures (6)

• Figure 1: (a) Schematic of the device. (b) PhC unit cell. (c) Photonic band diagram for specified PhC waveguide geometries. Spatial confinement of a PhC cavity (d) and of a FP-PhC (e). The resulting eignemodes have HG shaped or hybrid profiles, respectively.
• Figure 2: (a) Profile of the quantity $\Gamma$ along the PhC cavity. (b) Cavity spectrum and confining potential well with the corresponding (c) intrinsic quality factor of the modes. (d) 2D and (e) 1D spatial optical confinement of an high-order HG mode, and (f,g) corresponding 2D and 1D Fourier transforms.
• Figure 3: (a) SEM pictures of the fabricated device. (b) Transmission. (c) Resonance fitting. (d) Loaded quality factors statistics. (e) Integrated dispersion statistics ($D_{\mathrm{int}}$).
• Figure 4: Spatial confinement of a flat dispersion FP-PhC cavity with a homogeneous central section of $L_0=0\ \mu$m (a), $L_1=40\ \mu$m (c), $L_2=150\ \mu$m (e), respectively. To improve readability, only one out of every two solutions is reported in panel (e). In (b), (d) and (f) we report the corresponding sinusoidal corrugation amplitude $\Gamma(x)$. (g) Minimization of the cost function for the three different optimizations $L_{0,1,2}$.
• Figure 5: Comparison between the designed and experimental $D_{\mathrm{int}}$ for the resonators (a) $L_0$, (b) $L_1$ and (c) $L_2$. Examples of optical transmission for (d) $L_0$ and (e) $L_2$.