In this paper we show a novel design of high Q‐factor silicon nitride (SiN) 1D photonic crystal (PhC) cavities side‐coupled to curved waveguides, operating with both silica and air cladding. The engineering of the etched 1D PhC cavity sidewalls angle allows for high Q‐factors over a wide range of upper cladding compositions, and the achievement of the highest calculated Q‐factor for non‐suspended asymmetric SiN PhC structures. We show the employment of these type of SiN PhC cavities in hybrid external cavity laser (HECL) configuration, with mode‐hop free single mode laser operation over a broad range of injected currents (from 25 mA to 65 mA), milliwatts of power output (up to 9 mW) and side‐mode suppression ratios in the range of 40 dB. We demonstrate the operation of these devices as compact and energy efficient optical sensors that respond to refractive index changes in the surrounding medium the measurement of sodium chloride (from 0% to 25%) and sucrose (from 0% to 25%) in aqueous solution. In HECL configuration, the RI sensor exhibits a 2 orders of magnitude improvement in detection limit compared to the passive microcavity. We also discuss the possibility for applying these devices as novel transducers for refractive index changes that are induced by analyte specific absorption of infrared radiation by the target analytes present in gas or liquid phase.

Silicon-based Micro Ring Resonators (MRR) are a powerful tool for the realization of label free optical biosensors. The sharp edge of a Fano resonance in a Silicon Nitride (Si₃N₄) platform can boost photonic sensing applications based on MRRs. In this work, we demonstrate enhanced Fano resonance features for a Si₃N₄ Micro Ring Resonator assisted by a Photonic Crystal Nanobeam (PhCN-MRR) operating in the TM-like mode at the O-band wavelengths. Our findings show that the fabricated PhCN-MRR results in increased asymmetric resonances for TM-like mode compared with TE-like mode operation in the C-band. As a result, a versatile and flexible design to realize Fano resonance with polarization dependent asymmetry in the C and O telecom bands is presented.

Laser‐based mid‐infrared (mid‐IR) photothermal spectroscopy (PTS) represents a selective, fast, and sensitive analytical technique. Recent developments in laser design permits the coverage of wider spectral regions in combination with higher power, enabling for qualitative reconstruction of broadband absorption features, typical of liquid or solid samples. In this work, we use an external cavity quantum cascade laser (EC‐QCL) that emits in pulsed mode in the region between 5.7 and 6.4 µm (1770–1560 cm ⁻¹ ), to measure the absorption spectrum of a thin film of polymethyl methacrylate (PMMA) spin‐coated on top of a silicon nitride (Si ₃ N ₄ ) micro‐ring resonator (MRR). Being the PTS signal inversely proportional to the volume of interaction, in the classical probe–pump dual beam detection scheme, we exploit a Si ₃ N ₄ transducer coated with PMMA, as a proof‐of‐principle for an on‐chip photothermal sensor. By tuning the probe laser at the inflection point of one resonance, aiming for highest sensitivity, we align the mid‐IR beam on top of the ring’s area, in a transversal configuration. To maximize the amplitude of the photoinduced thermal change, we focus the mid‐IR light on top of the ring using a Cassegrain reflector enabling for an optimal match between ring size and beam waist of the excitation source. We briefly describe the transducer design and fabrication process, present the experimental setup, and perform an analysis for optimal operational parameters. We comment on the obtained results showing that PTS allows for miniaturized robust sensors opening the path for on‐line/in‐line monitoring in several industrial processes.

Measuring refractive index (RI) changes of liquid samples is central to many sensing applications including flow injection analysis, liquid chromatography, biosensing and photothermal spectroscopy. Commercial refractive index detectors optimized for liquid chromatography suffer from a limited linear range and measurement rate, restricting their use largely to separation sciences. In contrast, microring resonators (MRR) integrated with low-volume microfluidics, offer enhanced performance by minimizing sample dilution during flow-through RI measurements and increased dynamic range. MRRs realized by modern photonic integrated circuitry (PIC) technology also have the potential to be used as transducers in more advanced sensing schemes. Here, we demonstrate a silicon nitride (Si₃N₄) MRR integrated into a low-volume microfluidic system as a compact, chip-scale RI detector capable of real-time operation under dynamic flow conditions. Two interrogation modalities were experimentally compared for flow-through liquid sensing using the same MRR for the first time: resonance wavelength scanning for wide-range refractive index detection, and fixed-wavelength probing on the resonance slope for high-speed measurements. Using glucose solutions as test samples, the device was benchmarked against a commercial RI detector, achieving a sensitivity of 113 nm/RIU and a sLOD of 2.3 × 10^–6 RIU (0.014 g/L glucose). To demonstrate the applicability of the developed RI-sensor for resolving transient RI peaks in realistic chromatographic flow conditions we also report its successful use in an isocratic separation of four sugars (sorbitol, fructose, glucose, and sucrose). These results highlight the potential of integrated Si₃N₄ MRRs as versatile, miniaturized transducers for quantitative, high-speed RI sensing in flow-based analytical systems.

Asymmetric optical resonances shaped by Fano interference can provide enhanced signal contrast and tunable lineshapes in integrated photonics. However, realizing and exploiting these effects in fabricated devices remains challenging, as nanofabrication often reduces asymmetry and slope steepness. In this work, we experimentally demonstrate a hybrid silicon nitride photonic crystal nanobeam-microring resonator (PhCN-MRR) for refractive index (RI) sensing and systematically analyze how Fano-induced line shape asymmetry impacts transduction under realistic flow conditions. Despite exhibiting only moderate asymmetry, the PhCN-MRR shows distinct optical advantages for intensity-based detection, demonstrated using glucose solutions as a model analyte. The sensor was integrated into a custom Si₃N₄ microfluidic platform and tested under both stopped-flow and dynamic flow-injection conditions over concentrations from 0.5 to 10 mg/mL. Time-resolved spectral scans (0.5–1 Hz) enabled dual transduction channels via wavelength shifts (Δλ) and fixed-wavelength intensity changes (ΔI). Compared with a conventional microring resonator (MRR) featuring symmetric Lorentzian responses, the PhCN-MRR exhibited comparable Δλ sensitivities (∼111 to 113 nm/RIU) but delivered enhanced ΔI responsivity and improved contrast at low concentrations (e.g., 2 mg/mL) due to its asymmetric resonance slopes. These results link controlled Fano asymmetry to measurable sensing gains, demonstrating how modestly asymmetric resonances can improve real-time refractometric detection and expand the design space of Si₃N₄ photonic platforms for lab-on-chip analytical applications.

In this work, we present a compact high-Q factor S 3 N 4 -based Photonic Crystal Nanobeam Cavity (PhCNC) that achieves calculated Q-factors above 10 ⁶ (experimental 10 ⁴ ) in case over a wide range of upper-cladding refractive index values (1 to 1.45), angle sidewalls (0°, 5° and 7°) and stick widths. In addition, a high sensitivity (~175nm/RIU) for liquids is demonstrated.