Dinosaur Photonic Crystal Cavity Interfaces for Color Center Coupling to Triangular Nanostructures

TL;DR

The paper introduces Dinosaur photonic crystal cavities with corrugated triangular cross-sections and a tapered cavity–waveguide interface to enable efficient spin–photon coupling for embedded color centers. Using finite-element simulations and Bayesian optimization, the authors demonstrate high intrinsic quality factors ($Q$ on the order of 10^5), small mode volumes ($V < 0.4(\,\lambda_c/n)^3$), and near-unity cavity–waveguide coupling efficiencies ($\beta_{WG}$ up to 0.931), while enabling tunability across color centers in 4H-SiC and diamond. The design achieves strong light–matter interaction via controlled Purcell enhancement, with detailed adaptation for k-V_Si centers and diamond NV/SnV centers, and shows potential for scalable quantum photonic integration with fewer fabrication steps than prior Sawfish cavities. These results position Dinosaur cavities as promising building blocks for quantum repeater nodes in distributed quantum networks, with avenues for experimental realization and optomechanical extensions.

Abstract

Waveguide-coupled photonic crystal cavities with a triangular cross section fabricated by angled etching are suitable to interface embedded color centers with flying photonic qubits in quantum information applications. Moreover, their fabrication requires fewer processing steps compared to nanostructures produced by quasi-isotropic undercutting. As an alternative to established hole-based photonic crystal cavities, we introduce corrugated triangular 'Dinosaur' photonic crystal cavities, and develop a tapered, quasi loss-free cavity-waveguide interface to adiabatically interconvert Bloch and waveguide modes. We optimize the cavity-waveguide interface to minimize photon losses and demonstrate that its adjustment allows precise tuning of the light-matter interaction.

Figures (5)

• Figure 1: Concept of a Dinosaur spin--photon interface. A spin-active solid-state color center (star with dark blue arrow) is embedded in the center of a tapered 'Dinosaur' photonic crystal cavity. On its left, the cavity possesses a strong mirror (section I). On its right, there is a weak mirror (section II) that transitions to a cavity-attached waveguide (section III) to adiabatically transfer photons (red arrow) from the cavity to the waveguide and vice versa.
• Figure 2: Dinosaur unit cell. (a) A Dinosaur unit cell of length $a_i$ features a triangular cross section, determined by the sidewall angle $\delta$, and a $\cos^4$ corrugation profile, which is defined by an amplitude $A_0$ and offset from the yz symmetry plane by the gap width $g$. It resembles the back plates of a Stegosaurus (inset). (b) A periodic sequence of identical unit cells yields an optical band diagram that supports TE-like (dark blue lines) and TM-like (light blue lines) Bloch bands for wave vectors $k_\mathrm{z}$ along the stacking direction that exceed the light line $\nu = c|\vec{k}|$ (gray line). $c$ is the speed of light. Solid lines refer to a unit cell length $a_0=315.9\,$nm, and dashed lines to a length $a_3=346.1\,$nm. A TE-like optical bandgap (cyan-shaded area for $a_0$, dashed area for $a_3$) separates the two lowermost TE-like Bloch bands. The k-$\mathrm{V}_\mathrm{Si}$ ZPL emission frequency is indicated by a red solid line. Insets visualize the electric field intensities of both the fundamental TE-like (lower inset) and TM-like (upper inset) Bloch modes at $k_\mathrm{z}=\pi/a_0$ (blue diamonds).
• Figure 3: Symmetric Dinosaur cavity. (a) Starting with a stacked sequence of Dinosaur unit cells with a length $a_3$, a PhC cavity with an additional xy symmetry plane at $z=0$ is introduced by decreasing the length of the innermost three unit cells monotonically to $a_0$. The unit cell with length $a_0$ ends at $z=0$, where the confined cavity mode's electric field intensity is maximal, as visualized in an xz cross-sectional plane through the cavity. Along the y-direction, the plane is located at $y=0$, i.e., at a depth below the cavity surface matching one-third of the unit cells' triangular cross section height. (b) The quality factor $Q$ (red triangles) and the mode volume $V$ (blue triangles) of the confined cavity mode depends on the the number of stacked unit cells $N$, counting from the cavity's xy symmetry plane to one of its ends. Here, $\lambda_\mathrm{c}=917.0\,$nm denotes the cavity's resonance wavelength after convergence ($N\geq 24$), matching its resonance frequency $\nu_\mathrm{c}=326.9\,$THz, and $n$ the refractive index of 4H-SiC (refer to main text). Panel (b) is adapted from Bopp2025 and originally published under CC BY 4.0.
• Figure 4: Tapered, asymmetric waveguide-coupled Dinosaur cavity. (a) The taper profile $x(z)$ of the tapered section interfacing the cavity with a waveguide of width $d_\mathrm{WG}$ is modulated by a third-order polynomial. Its peak heights $X_i$ and gap widths $g_i$ are determined by Eq. \ref{['eq:taper_A']} and Eq. \ref{['eq:taper_P']}, respectively. The profile is symmetric with respect to $x=0$. (b) The waveguide coupling efficiency $\beta_\mathrm{WG}$ depends on the number of unit cells $N_\mathrm{w}$ constituting the cavity's weak mirror and on the number of unit cells $M$ forming the tapered section within the weak mirror. (c) Likewise, the Purcell factor $F_\mathrm{P}$ depends on the weak mirror parameters $N_\mathrm{w}$ and $M$. In (b) and (c), the cavity's strong mirror possesses $N_\mathrm{s}=30$ unit cells. The encircled cavity configuration exhibits the highest observed waveguide coupling efficiency of $93.1\,\%$, corresponding to a Purcell factor of $105.9$.
• Figure 5: Cavity parameter adjustment for diamond color centers. (a) The resonance frequency $\nu_\mathrm{c}$ of Dinosaur PhC cavities can be tuned by varying the gap width $g$ or the corrugation amplitude $A_0$ to accommodate the ZPL of the negatively-charged diamond NV (cyan crosses) or SnV (blue diamonds) centers. (b) Such tuning likewise affects the quality factor $Q$. The figure is adapted from Bopp2025 and originally published under CC BY 4.0.