High sensitivity visible light refractive index sensor based on high order mode Si3N4 photonic crystal nanobeam cavity

Hybridization of surface plasmons and photonic crystal resonators for high-sensitivity and high-resolution sensing applications

In this paper, an optical refractive index (RI) sensor based on a hybrid plasmonic-photonic crystal (P-PhC) is designed. In the sensor's structure, some metallic rods are embedded in a rod-type photonic crystal (PhC) structure. Numerical simulations are performed based on the finite-difference time-domain (FDTD) method. The obtained results illustrate that the localized surface plasmons (LSP) induced by metallic rods can be excited in a PhC lattice to generate a hybrid P-PhC mode. According to the results, the hybrid mode provides unique opportunities. Using metallic rods in the coupling regions between waveguides and the resonant cavity significantly increases the interaction of the optical field and analyte inside the cavity. The simulation results reveal that high sensitivity of 1672 nm/RIU and an excellent figure of merit (FoM) of 2388 RIU-1 are obtained for the proposed hybrid P-PhC sensor. These values are highest compared to the purely plasmonic and or purely PhC sensors reported in the literature. The proposed sensor could simultaneously enhance sensitivity and FoM values. Therefore, the proposed hybrid P-PhC RI sensor is a more fascinating candidate for high-sensitivity and high-resolution sensing applications at optic communication wavelengths.

Topological photonic crystal biosensor with valley edge modes based on a silicon-on-insulator slab

In the development of integrated sensing, how to reduce losses and improve robustness has always been one of the key problems to be solved. The topological photonic crystal structure based on the quantum Hall effect has gradually attracted the attention of researchers due to its unique immune defect performance and anti-scattering performance. Here, we have successfully applied the valley photonic crystal structures to topologically manipulate the light within the band gap of 252 THz-317 THz in a silicon-on-insulator platform. We experimentally demonstrated that satisfactory transmission performance can be obtained using the valley-dependent topological edge states below light cone, even if there are structure defects such as lattice missing and lattice mistake near the interface between two kinds VPCs. Based on the features of topological protection, a triangular cavity consisting of three 10×a-length sides is proposed, and the Q factor value reaches 1.83×10⁵ with little influence from defects. Finally, based on drying etching technology, a biosensor with cavity-coupled waveguide structure was prepared, and the RI sensitivity was 1228 nm/RIU.

Color-changing refractive index sensor based on Fano-resonant filtering of optical modes in a porous dielectric Fabry-Pérot microcavity

Refractometry is a ubiquitous technique for process control and substance identification in the chemical and biomedical fields. Herein, we present an all-dielectric, wafer-scalable, and compact Fabry-Pérot microcavity (FPMC) device for refractive index (RI) sensing. The FPMC consists of a highly porous SiO₂ microcavity capped with a thin, quasi-periodically patterned TiO₂ hole array partial reflector that enables rapid, nanoliter-scale analyte transport to and from the sensor. Liquid (alcohols) or condensed-vapor (water from human breath) infiltration resulted in spectral redshifts up to 100 nm, highly apparent visible color change, rapid recovery (< 20 s), and RI sensitivity of up to 680 nm/RIU. The sensor can also be used in spectral or single-wavelength detection modes. Effective-medium and finite-difference time-domain optical simulations identified that Fano-resonant scattering modes induced by the quasi-periodic TiO₂ outcoupling layer effectively filter higher-order Fabry-Pérot cavity modes and thereby confer an easily identifiable red-to-green color transition during analyte infiltration.

Straight Long-Range Surface Plasmon Polariton Waveguide Sensor Operating at l0 = 850 nm

A bulk refractive index sensor based on a straight long-range surface plasmon polariton (LRSPP) waveguide is theoretically designed. The waveguide sensor consists of an Au stripe that is embedded in ultraviolet sensitive polymer SU-8. The geometric parameters are optimized by finite difference eigenmode method at the optical wavelength of 850 nm. The sensitivity of 196 dB/RIU/mm can be obtained with a 1.5 μm wide, 25 nm thick Au stripe waveguide. Straight LRSPP waveguides are fabricated by a double layer lift-off process. Its optical transmission is characterized to experimentally prove the feasibility of the proposed design. This sensor has potential for the realization of a portable, low-cost refractometer.

Photonic Crystal Nanobeam Cavities for Nanoscale Optical Sensing: A Review

The ability to detect nanoscale objects is particular crucial for a wide range of applications, such as environmental protection, early-stage disease diagnosis and drug discovery. Photonic crystal nanobeam cavity (PCNC) sensors have attracted great attention due to high-quality factors and small-mode volumes (Q/V) and good on-chip integrability with optical waveguides/circuits. In this review, we focus on nanoscale optical sensing based on PCNC sensors, including ultrahigh figure of merit (FOM) sensing, single nanoparticle trapping, label-free molecule detection and an integrated sensor array for multiplexed sensing. We believe that the PCNC sensors featuring ultracompact footprint, high monolithic integration capability, fast response and ultrahigh sensitivity sensing ability, etc., will provide a promising platform for further developing lab-on-a-chip devices for biosensing and other functionalities.

Highly Sensitive Color Tunablility by Scalable Nanomorphology of a Dielectric Layer in Liquid-Permeable Metal-Insulator-Metal Structure

A liquid-permeable concept in a metal-insulator-metal (MIM) structure is proposed to achieve highly sensitive color-tuning property through the change of the effective refractive index of the dielectric insulator layer. A semicontinuous top metal film with nanoapertures, adopted as a transreflective layer for MIM resonator, allows to tailor the nanomorphology of a dielectric layer through selective etching of the underneath insulator layer, resulting in nanopillars and hollow voids in the insulator layer. By allowing outer mediums to enter into the hollow voids of the dielectric layer, such liquid-permeable MIM architecture enables to achieve the wavelength shift as large as 323.5 nm/RIU in the visible range, which is the largest wavelength shift reported so far. Our liquid-permeable approaches indeed provide dramatic color tunablility, a real-time sensing scheme, long-term durability, and reproducibility in a simple and scalable manner.

Applications of Photonic Crystal Nanobeam Cavities for Sensing

In recent years, there has been growing interest in optical sensors based on microcavities due to their advantages of size reduction and enhanced sensing capability. In this paper, we aim to give a comprehensive review of the field of photonic crystal nanobeam cavity-based sensors. The sensing principles and development of applications, such as refractive index sensing, nanoparticle sensing, optomechanical sensing, and temperature sensing, are summarized and highlighted. From the studies reported, it is demonstrated that photonic crystal nanobeam cavities, which provide excellent light confinement capability, ultra-small size, flexible on-chip design, and easy integration, offer promising platforms for a range of sensing applications.

High-Q, low-index-contrast photonic crystal nanofiber cavity for high sensitivity refractive index sensing

We present the design of simultaneous high-quality (Q)-factor and high-sensitivity (S) photonic crystal nanofiber cavities (PCNFCs) made of single silica nanofiber that have a low-index contrast (ratio=1.45). By using the three-dimensional finite-difference time-domain method, two different resonant modes, dielectric mode (DM) and air mode (AM), are designed and optimized to achieve an ultrahigh figure of merit (FOM), respectively. Numerical simulations are performed to study the Q-factors and sensitivities of the proposed PCNFCs. It shows that for both DM- and AM-based PCNFCs, respectively, the Q-factors and sensitivities of Q∼1.1×10⁷, S=563.6  nm/RIU and Q∼2.1×10⁵, S=736.8  nm/RIU can be estimated, resulting in FOMs as high as 4.31×10⁶ and 1.13×10⁵, respectively. To the best of our knowledge, this is the first silica nanofiber cavity geometry that simultaneously features high Q and high S for both DM and AM in PCNFCs. Compared with the state of the art of nanofiber-based cavities, the cavity Q-factor to mode volume (V) ratio (Q/V) in this work has been improved more than two orders of magnitude. The demonstration of a high Q/V cavity in low-index-contrast nanofibers can open up versatile applications using a broad range of functional and flexible fibers. Moreover, due to the extended evanescent field and small mode volumes, the proposed PCNFCs are ideal platforms for remote ultra-sensitive refractive-index-based gas sensing without the need for complicated coupling systems.

Highly-Sensitive Refractive Index Sensing by Near-infrared Metatronic Nanocircuits

In this work, we present a highly-sensitive refractive index sensor based on metatronic nanocircuits operating at near-infrared spectral range. The structure is designed based on simple nanorod geometry and fabricated by nanopatterning of transparent conducting oxides. The functionality of these polarization dependent metatronic nanocircuits is enhanced by applying tunable response. This feature is investigated by depositing NH₂ (Amine) groups via plasma polymerization technique on top of indium-tin-oxide nanorods. The dielectric constant of Amine groups is a function of their thickness, which can be controlled by the RF power and the time duration of the applied plasma polymerization process. The resonance wavelengths of nanocircuits shift to higher wavelength, as the dielectric constant of the deposited material increases. An excellent agreement between the design and experimental results are obtained. Our metatronic based nanosensor offers a high-sensitive performance of 1587 nm/RIU with a satisfactory figure of merit for this class of sensors.

Post-fabrication trimming of photonic crystal nanobeam cavities by electron beam irradiation

We demonstrate that resonant wavelength of photonic crystal (PhC) nanobeam cavities can be individually post-fabrication tuned by electron beam irradiation. By measuring the transmission spectrum of the cavities before and after trimming, it is shown that resonant wavelength shifts are proportional to the scanning time and the acceleration voltage. Furthermore, larger resonant wavelength shifts can be achieved by scanning the region where the electric field is highly localized. The measurement results show that the resonant wavelength difference can be reduced from 5.5 nm (before trimming) to 0.4 nm (after trimming), while the quality factor of the cavities can be maintained.