Quasi-BIC Nanowire Cavity Design

• The paper demonstrates high-Q confinement by engineering quasi-bound states through modal interference that balances bandwidth and field confinement.
• It utilizes a non-Hermitian Hamiltonian and FDTD simulations to analyze modal coupling and predict resonant performance in nanowire cavities.
• The study offers design guidelines for quantum photonics, spintronics, and hybrid systems, enabling efficient single-photon sources and robust spin filtering.

A nanowire cavity based on quasi-bound states in the continuum (quasi-BICs) is a photonic, electronic, or hybrid structure that achieves high-quality-factor (high-Q) confinement by engineering the interference and coupling of resonant modes such that leakage into the continuum is strongly suppressed, but not fully eliminated as in a true BIC. This approach enables highly efficient, compact, and tunable cavities at the nanoscale, supporting applications in quantum networks, spintronics, and optoelectronics. Distinct manifestations exist across optical, electronic, and hybrid quantum systems.

1. Fundamental Concepts: Quasi-BICs and Nanowire Cavities

A quasi-bound state in the continuum arises when a localized state, situated energetically within the extended states (“the continuum”) of a nanowire or nanostructure, retains a long lifetime due to destructive interference or symmetry-induced decoupling from the continuum. In practice, the resonance acquires a high but finite Q due to imperfect isolation:

• Quasi-BIC formation: Typically emerges via strong (often non-Hermitian) coupling of two or more leaky modes with specific symmetry properties, leading to an avoided crossing in their eigenfrequencies and the partial cancellation of far-field radiation channels.
• Cavity principle: In nanowires, the approach exploits the modal structure of the waveguide (Mie, Fabry-Pérot, or more complex) to realize broadband, high-field-confinement resonances with directional emission and operational tunability (Gangopadhyay et al., 7 Jan 2026, Huang et al., 2019).

The quasi-BIC regime, as opposed to the “true” BIC (with Q → ∞), is technologically advantageous because it balances bandwidth, field confinement, and out-coupling—essential for single-photon sources and active device integration.

2. Modal Coupling Mechanism and Eigenproblem Analysis

The central physical mechanism involves the hybridization of two near-degenerate resonant modes (labeled, e.g., EH₁₁ and HE₁₁ in hexagonal InP nanowires (Gangopadhyay et al., 7 Jan 2026), or TE(m,l) and TE(m±2,l∓2) in rectangular nanowires (Huang et al., 2019)). This is captured mathematically by a non-Hermitian $2\times2$ Hamiltonian:

$$H(\Delta) = \begin{pmatrix} E_1(\Delta) & V \ W & E_2(\Delta) \end{pmatrix}$$

with hybrid eigenvalues

$$E_{\pm}(\Delta) = E_0 + \frac{\Delta}{2} - i \gamma_0 \pm \frac{1}{2}\sqrt{\Delta^2 + 4VW}$$

The avoided crossing occurs when the real parts of $E_1$ and $E_2$ coincide, and, for appropriately tuned parameters, the imaginary part (related to radiative loss) of one eigenvalue approaches zero, yielding a high-Q quasi-BIC. The symmetry of the coupled modes (such as parity with respect to relevant structural planes) is critical for achieving the interference condition that suppresses far-field emission (Gangopadhyay et al., 7 Jan 2026, Huang et al., 2019).

In momentum and multipole space, the radiation quenching corresponds to vanishing amplitude in the dominant radiation channel(s) due to destructive interference:

$a_{m_0} + b_{m_0} \to 0$

for dominant multipole indices $m_0$.

3. Implementation: Cavity Geometry, Materials, and Performance

Optical Quasi-BIC Nanowire Cavity

• Structure: Hexagonal wurtzite InP core (n ≈ 3.44), diameter 420 nm, height 1,375 nm, on a 12 nm SiO₂ buffer above a gold mirror. The cavity supports guided modes with in-plane polarization symmetry suitable for strong coupling (Gangopadhyay et al., 7 Jan 2026).
• Quantum emitter: An in-plane dipole (representing a quantum dot) located 30 nm below the top-facet antinode.
• Simulation and extraction: Finite-difference time-domain (FDTD) modeling with uniform 10 nm mesh, PML boundaries; Purcell factor and out-coupling directly read from field monitors.

Performance metrics from (Gangopadhyay et al., 7 Jan 2026):

Parameter	Value (Optimal Quasi-BIC)	Notes
Resonant wavelength	$\lambda_0 \approx 900$ nm	Centered for InP/quantum dot emission
Bandwidth ($\Delta \lambda$)	$\approx 4$ nm ($\sim$2.8 THz)	Supports biexciton/exciton covering
Purcell Factor ($F_p$)	$\sim$17	Lifetime reduction to $\sim$60 ps
Quality Factor ($Q$)	$\sim$225 (loaded)	Intrinsic $Q$ could be higher
Mode Volume ($V$)	(0.3–0.5)$(\lambda/n)^3$	High field confinement
Gaussian overlap ($\eta$)	88%	Near-diffraction-limited emission
Extraction efficiency ($\eta_\mathrm{ext}$, NA=0.8)	74%	Efficient photon out-coupling

Electronic Quasi-BIC Cavity in Nanowire Networks

For electronic transport, as in hashtag-type InSb nanowire networks, BICs and quasi-BICs appear as sharp Fano resonances or antiresonances in conductance (Martínez et al., 2024). The decoupling condition is enforced by geometry and flux quantization. Design rules specify wire dimensions ($L=50$ nm, diameter $\sim$100 nm), dead-end chain placement, and fine magnetic field ($B\sim0.1$ T) and Fermi level tuning to reach the destructive interference point.

4. Experimental Demonstrations and Tuning Strategies

Optical Regime

Experimental realization in Si nanowires has demonstrated Q-factors up to $Q\approx380$ for TE(3,5) modes in wires with optimized aspect ratios ($R\approx0.868$) (Huang et al., 2019). High-Q modes manifest as sharp dips in scattering spectra, with lineshapes well-described by Fano resonance models.

Electronic Regime

In InSb nanowire loops, adjusting the magnetic flux through the loop area achieves periodic BIC formation, evidenced by pinned conductance zeros or peaks at predictable field values (Martínez et al., 2024). Rashba spin–orbit coupling can broaden or obscure these resonances, but also enables gate-controlled spin filtering at BIC-induced antiresonances.

Hybrid Quantum Regime

Hybrid architectures, such as proximitized nanowires capacitively coupled to microwave cavities, exploit quasi-bound Majorana or Andreev states for robust, parity-sensitive cavity response. Selectively coupling the cavity to quasi-bound lobes enables both diagnostics (via microwave absorption visibility) and active initialization of quantum parity (Prem et al., 16 Sep 2025).

5. Applications in Quantum Photonics, Electronics, and Spintronics

• Entangled photon sources: The quasi-BIC nanowire cavity supports broadband, high-extraction, Purcell-enhanced emission compatible with simultaneous biexciton and exciton enhancement, critical for quantum repeaters and network nodes. The moderate Q and bandwidth ensure indistinguishable single photons while retaining practical collection rates (Gangopadhyay et al., 7 Jan 2026).
• Spintronics: InSb nanowire networks with quasi-BICs can function as electrically- and magnetically-tuned spin filters, with nearly complete polarization at BIC antiresonances (Martínez et al., 2024).
• Quantum information: Hybrid systems use the spatial nonlocality of quasi-Majorana bound states for noninvasive state discrimination and robust, cavity-driven quantum state initialization (Prem et al., 16 Sep 2025).

6. Design Guidelines and Trade-offs

Specific prescriptions for cavity realization include:

Design Lever	Effect on Quasi-BIC	Remarks
Aspect ratio (R)	Tunes avoided crossing, Q	Fine-tuning around $R_c$ critical for high-Q
Material index (n)	Raises Q, reduces leakage	High-n (e.g., n=4 Si) enables $Q\sim 2\times10^4$
Geometry	Sets modal interference	Square, hexagonal, hashtag, dead-end chains control channel decoupling
Mirror placement	Shapes field, emission	Gold mirror creates standing-wave antinodes, enhances directionality
Rashba SO coupling	Detrimental for Q, can enable spin logic	Strong SO coupling broadens quasi-BIC resonance
Magnetic field	Controls BIC position, Q	Flux quantization critical for electronic BICs

Trade-offs exist between Q-factor, bandwidth, extraction efficiency, and tolerance to fabrication variations. For quantum photonics, operation in the quasi-BIC regime balances spectral coverage (biexciton/exciton) and purity (no significant birefringence), facilitating high-fidelity entanglement under realistic conditions (Gangopadhyay et al., 7 Jan 2026).

7. Prospects, Limitations, and Technological Relevance

Nanowire cavities based on quasi-bound states in the continuum realize robust, strongly confined, tunable resonances with high efficiency, directionality, and broad applicability across quantum network platforms, electronic transport, and spin logic. Technological implementation benefits from scalable bottom-up growth, straightforward tuning mechanisms (geometry, field, gate voltage), and compatibility with on-chip integration.

Limitations include sensitivity to fabrication-induced symmetry breaking (limiting Q in practice), trade-off between Q and bandwidth, and in electronic systems, the detrimental effects of strong spin–orbit coupling on BIC isolation. Nevertheless, the mode engineering principles extend broadly to various nanowire materials, network architectures, and hybrid quantum devices, providing a general route to high-performance nanoscale cavities (Gangopadhyay et al., 7 Jan 2026, Huang et al., 2019, Martínez et al., 2024, Prem et al., 16 Sep 2025, Giovenale et al., 2022).