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

The work, carried out by Abhishek Kumar Pandey, Deepak Jain, and Catherine Baskiotis, exploits spontaneous four-wave mixing, a nonlinear optical process in which two pump photons at the same frequency are annihilated to create a correlated pair of photons at two very different frequencies. Energy conservation forces the sum of the two new frequencies to equal twice the pump frequency, while momentum conservation, known as the phase-matching condition, dictates which frequency combinations can actually occur. In a waveguide, the phase-matching condition can be engineered by shaping the geometry of the silicon core, allowing the generated signal and idler photons to be tuned with remarkable precision across a wide spectral range.

What makes the new designs remarkable is the sheer distance between the two photons. The flagship design, targeted at atmospheric quantum key distribution, produces a signal photon at 3.905 micrometres, deep inside a mid-infrared atmospheric transparency window, while its idler partner lands at 1.541 micrometres in the telecom C-band. That separation of roughly 2,364 nanometres is dramatically larger than the record of about 1,125 nanometres theoretically reported in earlier all-solid silicon waveguide studies. Crucially, the design achieves this in the true quantum regime, where at most one photon pair is produced per pump pulse, rather than relying on the high pump powers used in classical demonstrations.

This distinction matters. Previous experiments with air-clad silicon waveguides demonstrated large signal-idler separations of about 2,058 nanometres, but only by using pump peak powers around 20 watts to achieve phase matching. In the quantum regime, pump power must be kept low to suppress multi-pair emission, and with those earlier designs the phase-matched separation collapsed to 1,780 nanometres, placing the mid-infrared photon outside any atmospheric transmission window. The new all-solid designs sidestep this problem by building the phase matching into the waveguide geometry itself, so that it holds even at the milliwatt pump levels required for single-pair emission.

The choice of an all-solid, fully oxide-clad structure is itself strategic. Air-clad waveguides suffer from significant scattering losses caused by sidewall roughness, because the optical mode interacts strongly with the etched surfaces. By surrounding the silicon core with fused silica cladding and a buried oxide layer, the proposed waveguides reduce the modal overlap with etched sidewalls, mitigating fabrication-induced scattering. The team’s calculations show that even at the longest signal wavelength, where silica is nominally opaque, the majority of the optical power remains confined to the silicon core, keeping total material attenuation over the 2-centimetre waveguide to acceptable levels.

To give their predictions quantitative teeth, the researchers adopted a model for the probability of photon-pair generation per pulse and validated it against published experimental data from photon-pair generation in photonic crystal fibre. By modelling the fibre as a simple step-index structure and feeding in the experimental parameters, they computed photon-pair rates that closely matched the measured values across a range of pump powers, outperforming the original theoretical treatment of those experiments. Because the model takes as inputs only the modal properties, the nonlinear index, and the operating conditions, the authors argue that the validation is generic and applies equally to the silicon waveguide geometries, whose modal fields they computed independently using full-vectorial finite element simulations.

Each of the three proposed waveguides is tailored to a specific application. Two designs generate signal photons at 3.265 and 3.461 micrometres, which coincide with absorption bands of methane and nitrogen dioxide respectively, the fingerprint region where these gases reveal their presence. Paired with telecom-band idlers, these sources could power schemes for sensing with undetected light, in which the mid-infrared photon probes the gas while only the telecom photon is ever detected, allowing the entire measurement to be performed with cheap, mature silicon-compatible detectors. The third design, optimised for atmospheric quantum key distribution, places the signal photon at 3.905 micrometres, where atmospheric transmission is dramatically better than at the 1.55-micrometre wavelength used in current daylight quantum communication experiments.

The shift to the mid-infrared could be transformative for free-space quantum communication. At 3 to 4 micrometres, the atmosphere exhibits reduced Rayleigh scattering, lower solar background noise, and greater resistance to weather-related variations, all of which translate into more reliable quantum links and an improved ability to operate in daylight. Shrinking the photon-pair source onto a chip also addresses a critical constraint for satellite payloads, where every gram of mass and cubic centimetre of volume counts. Silicon-on-insulator fabrication is cheap, scalable, and compatible with existing CMOS foundries, making the prospect of flight-qualified quantum sources considerably more realistic than with bulk nonlinear crystals.

The team also addressed the practical obstacles that have plagued mid-infrared quantum optics in silicon. Spontaneous Raman scattering, which can contaminate photon-pair spectra, is predicted to be negligible because the generated photons lie more than 330 Raman linewidths away from the silicon Raman gain peak, placing the Raman gain at the signal frequencies at the level of one part in a million. Two-photon absorption, the bane of silicon nonlinear optics at telecom wavelengths, falls off sharply for pump wavelengths above 1.9 micrometres, and the chosen pump wavelengths between 2.1 and 2.21 micrometres sit in a sweet spot combining strong Kerr nonlinearity with suppressed carrier generation. Numerical tolerance studies further showed that small fabrication imperfections of plus or minus 10 nanometres in the core dimensions cause only slight wavelength drifts that can be compensated by tuning the pump laser.

The predicted operating points are within reach of existing technology. Each design targets a probability of photon-pair generation per pulse of about 0.05, a value considered a practical compromise between source brightness and multi-pair suppression, and achieves this with peak pump powers between 9.2 and 32.2 milliwatts from 5-picosecond pulses. Such pump powers are already delivered by mature thulium and holmium-doped solid-state lasers around 2 micrometres, while integrated 2-micrometre laser sources continue to advance toward the required output levels. With mid-infrared single-photon detectors, including superconducting nanowire devices and frequency-upconversion schemes, progressing rapidly, the authors argue that their chip-scale sources could soon move from simulation to experiment, opening a path toward quantum communication and sensing that spans from the fingerprint region of the molecules to the heart of the telecom infrastructure.

Subject of Research: Silicon waveguide design for quantum-correlated photon-pair generation bridging mid-infrared and telecom bands via spontaneous four-wave mixing

Article Title: Quantitative study of silicon waveguides for the generation of quantum correlated photon pairs bridging mid-infrared and telecom bands

Article References: Pandey, A. K., Jain, D., & Baskiotis, C. (2026). Quantitative study of silicon waveguides for the generation of quantum correlated photon pairs bridging mid-infrared and telecom bands. Results in Optics, 25, Article 101158. https://doi.org/10.1016/j.rio.2026.101158

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101158

Keywords: silicon photonics, quantum optics, photon pairs, four-wave mixing, mid-infrared, telecom C-band, quantum key distribution, gas sensing, silicon-on-insulator, phase matching, Raman scattering, integrated photonics