We introduce all-dielectric millimeter-wave silicon photonic crystal cavities as chip-scale perturbative sensors in small quantum-material samples. By avoiding superconducting elements, the all-silicon geometry is naturally compatible with cryogenic temperatures and strong magnetic fields, opening a route to measurements in regimes where superconducting microwave cavities lose performance. We characterize a bare cavity over the 4.3–294 K range, with the fundamental mode reaching a total quality factor exceeding ${10}^5$ at 4.3 K in the W-band. As a room-temperature proof of concept, we position a hexagonal boron nitride (hBN)-capped multilayer-graphene (Si/MLG/hBN) heterostructure at an electric-field antinode and measure the perturbative response. From the resulting internal-linewidth shift, we extract a volume-weighted in-plane conductivity ${\sigma }_{\text{sample}}\approx 1\times {10}^5\mathrm{S}/\mathrm{cm}$. The platform extends naturally to multimode spectroscopy via the cavity’s higher-order modes and to terahertz frequencies by scaling the geometry.

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• Electrical conductivity
• Photonic crystals
• Radio frequency techniques
• Graphene
• Heterostructures
• Cavity resonators
• Perturbative methods
• Spectroscopy