The heart of the device is a two-dimensional photonic crystal, a periodic lattice of air holes etched into a thin silicon slab that manipulates the flow of light in much the same way that a semiconductor crystal manipulates electrons. Because the refractive index alternates periodically between silicon and air, the structure exhibits a photonic band gap, a range of wavelengths that cannot propagate through the crystal. By introducing defects into this lattice, engineers can trap light in tiny resonant cavities whose transmission spectrum contains extremely sharp peaks. When something changes the optical properties of the surrounding material, such as mechanical stress altering the refractive index of silicon, those peaks shift in a measurable and repeatable way, turning a purely optical effect into a sensitive physical measurement.

Earlier work by the same community showed that a single hexagonal ring resonator in a photonic crystal suffers from relatively low sensitivity and a modest quality factor, the figure of merit that describes how long photons circulate inside a resonator before leaking away. A high quality factor translates directly into narrow resonance linewidths, and narrow linewidths make it possible to detect very small wavelength shifts. The new design therefore couples two hexagonal ring resonators in series, an arrangement that dramatically improves optical confinement and extends the effective photon lifetime within the structure. The result is a resonance that is both sharper and more responsive to external perturbations than anything a single ring can deliver.

The physical architecture of the sensor is a study in miniaturisation. A silicon substrate 450 micrometres thick and 14 by 14 micrometres in lateral size supports a three-micrometre silicon dioxide layer, on top of which sits the 220-nanometre-thick photonic crystal slab. The active sensing region measures 12 by 12 micrometres and contains an array of 40 by 34 air holes arranged in a hexagonal lattice, with a hole radius of 136 nanometres and a lattice constant of 420 nanometres. The researchers chose the hexagonal lattice deliberately, citing studies showing that it delivers wider photonic band gaps and better performance metrics than triangular or square alternatives. The ratio of hole radius to lattice constant falls between 0.3 and 0.4, a range chosen to keep the design realistic for existing fabrication processes.

Beneath the photonic crystal lies a cavity that leaves the diaphragm free to deform downward when pressure is applied. The team computed the photonic band structure of the defect-free lattice using the plane wave expansion method, finding a band gap spanning wavelengths from 1.3506 to 1.9425 micrometres, comfortably encompassing the 1550-nanometre telecommunications band where the sensor is designed to operate. When the input port is excited with a broadband source, a prominent resonance peak appears at 1.56674 micrometres with a full width at half maximum of just 0.03 nanometres, corresponding to a quality factor of 52,224. Field distribution maps at the resonant and off-resonant conditions confirm that light is efficiently coupled into the double-ring structure only on resonance, the behaviour required for reliable wavelength-shift readout.

The opto-mechanical side of the sensor was analysed with the finite element method, while the optical behaviour was simulated with the finite-difference time-domain technique, a combination that captures both the structural deformation under load and the resulting electromagnetic response. When pressure is applied to the diaphragm as a boundary load, the membrane deflects downward and surface stress rises. Stress changes the refractive index of silicon through the photoelastic effect, governed by a stress coefficient of 1.56 times ten to the minus eleven, and this refractive index change shifts the resonance wavelength. The stress distribution across the diaphragm is not uniform: it oscillates between high and low values as the edge alternates between air holes and solid silicon, reaching maxima at the midpoints of the diaphragm edges and a minimum at the centre.

The numerical results, tabulated across the full 0 to 12 megapascal operating range, show a beautifully linear response. At one megapascal the resonance sits at 1.57842 micrometres; by twelve megapascal it has moved to 1.7104 micrometres. Intriguingly, the quality factor does not degrade as pressure increases but actually climbs, from 52,224 at zero load to 57,013 at the top of the range, because the resonance wavelength itself increases while the linewidth remains fixed at 0.03 nanometres. The researchers also calculated the burst pressure of the diaphragm using the fracture stress of silicon, arriving at a maximum safe operating pressure of approximately twelve megapascals, which conveniently defines the sensor’s full measurement span.

How does this compare with the existing literature? The authors assembled a comparison table of photonic crystal pressure sensors that makes the advance strikingly clear. A two-dimensional photonic crystal Mach-Zehnder interferometer reported in 2018 managed a sensitivity of just 0.02237 nanometres per megapascal, while a 2023 ring resonator design reached only 0.003. Even the strongest earlier entries from the same research group, a photonic crystal microring resonator diaphragm sensor with a sensitivity of 1.37 nanometres per megapascal and a quality factor of 26,180, are surpassed by nearly an order of magnitude in sensitivity and doubled in quality factor. The new device also achieves a minimum detectable pressure of 0.00253 megapascal, roughly seventeen times finer than the earlier diaphragm sensor’s 0.0437 megapascal.

The significance of the work extends beyond the raw numbers. Because the entire sensor is built on a silicon-on-insulator platform using standard photonic crystal geometries, it is inherently compatible with the same semiconductor manufacturing infrastructure that produces computer chips, opening a plausible route to mass production of optical pressure sensors that are immune to electromagnetic interference and capable of operating in harsh environments. The researchers suggest the design could serve applications ranging from industrial process monitoring to biomedical instrumentation, wherever small pressures must be measured with high precision on a chip-scale footprint.

There remain, of course, the usual caveats that separate simulation from product. The reported performance comes from finite element and finite-difference time-domain modelling rather than from a fabricated and tested device, and real-world fabrication tolerances, surface roughness and coupling losses will inevitably erode some of the simulated quality factor. Nevertheless, the design parameters were deliberately constrained to fabrication-friendly values, and the underlying physics of serially coupled resonators is well established experimentally. If the laboratory results survive contact with a cleanroom, serially coupled hexagonal photonic crystal resonators may well become a standard building block for the next generation of chip-scale optical pressure sensors, proving once again that in photonics, as in so much of engineering, the cleverest tricks often come from coupling simple things together in the right way.

Subject of Research: Photonic crystal ring resonator pressure sensor design and simulation

Article Title: Highly sensitive and high Q-factor photonic crystals serially coupled hexagonal resonator for pressure sensing

Article References: Basavaprasad, Kolli, V. R., P.R., Y., K.B., S. K., Ramesh, M., Manivannan, G. S., & Talabattula, S. (2026). Highly sensitive and high Q-factor photonic crystals serially coupled hexagonal resonator for pressure sensing. Results in Optics, 25, Article 101167. https://doi.org/10.1016/j.rio.2026.101167

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101167

Keywords: photonic crystals, pressure sensor, hexagonal ring resonator, silicon-on-insulator, quality factor, FDTD, finite element method, optical sensing, photonic band gap, microelectromechanical systems, wavelength shift, silicon photonics