A photonic-crystal nanocavity is a defect in a periodic dielectric lattice that traps light of one narrow
wavelength in a tiny mode volume. Part of the trapped field leaks slightly outside the solid as an
evanescent field. A molecule binding inside that field nudges the local refractive index and shifts the
resonance by a fixed, tiny amount — regardless of Q. What changes with Q is the resonance
linewidth (FWHM = λ₀ / Q): a low-Q cavity has a broad peak that swallows the shift; a high-Q
cavity has a needle-thin peak that resolves the very same shift clearly. That is why single-molecule
biosensing needs an engineered high-Q dielectric cavity, not just a narrow structure.
- λ₀ ≈ 1550 nm (telecom-band silicon photonic-crystal cavity)
- FWHM = λ₀ / Q
- Per-molecule shift is fixed (≈ 5 pm) — only detectability changes with Q