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Photonic crystal (PC) cavities, which possess the advantages of compactness, flexible design, and suitability for integration in a lab-on-a-chip system, are able to distinguish slight variations in refractive index with only a small amount of analyte. Combined with the newly proposed optofluidic technology, PC-cavity devices stimulate an emerging class of miniaturized and label-free biochemical sensors. In this review, an overview of optofluidic PC cavities based biochemical sensors is presented. First, the basic properties of the PC, as well as the sensing principle of the PC cavity, are discussed. Second, the applications of the sensors in detecting gas, liquid, and biomolecule concentrations are reviewed, with a focus on their structures, sensing principles, sensing properties, advantages, and disadvantages. Finally, the current challenges and future development directions of optofluidic PC-cavity-based biochemical sensors are discussed. [ABSTRACT FROM AUTHOR]

This review covers photonic crystal cavities (PCCs) and their applications in optical sensors, with a particular focus on the structures of different PCCs. For each kind of optical sensor, the specific measurement principle, structure of PCC, and the corresponding sensing properties are all presented in detail. The summary of the reported works and the corresponding results demonstrate that it is possible to realize miniature and high-sensitive optical sensors due to the ultra-compact size, excellent resonant properties, and flexibility in structural design of PCCs. Finally, the key problems and new directions of PCCs for sensing applications are discussed. [ABSTRACT FROM AUTHOR]

A method for simultaneous measurement of magnetic field and temperature with high sensitivity and high precision was realized according to magnetic fluid (MF)-infiltrated photonic crystal cavity, where two different types of MF were, respectively, infiltrated into certain air holes adjacent to a waveguide to form two cascaded cavities. As the refractive index (RI) of MF is dependent on external magnetic field and temperature, the two independent resonant dips of cascaded cavities that can be simultaneously monitored at the output spectrum of the waveguide would all shift with the change of external magnetic field or temperature. Using finite-difference time-domain method, the RI sensitivity of the proposed cavity was firstly analyzed and optimized, and then the linear relationships between the shifts of two resonant wavelengths and external magnetic field/temperature were calculated. Finally, combined with the dual-wavelength matrix method, the magnetic field detection limit could reach to 1.333\times 10^\mathbf -4 T with the uncertainty of \pm 0.22 \times 10^\mathbf -4 T (coverage factor k = 2) and detection range from 0 to 0.06 T. Simultaneously, the temperature detection limit could reach to 0.301 K with the uncertainty of ±0.051 K (k = 2) and detection range from 250 to 340 K. [ABSTRACT FROM PUBLISHER]

A miniaturized, high-sensitive , and high-precision gas sensor for the measurement of methane concentration was realized in a cryptophane E infiltrated photonic crystal (PC) microcavity, by combing selective adsorption property of cryptophane E to methane and excellent resonant property of PC microcavity. The concentration variation of methane would change the refractive index (RI) of cryptophane E that infiltrated in defected holes of PC microcavity, and then induce a shift in resonant wavelength , allowing precision measurement of methane concentration. Simulation results of PC microcavity showed that the RI sensitivity of 363.8 nm/RIU, quality factor of 12,923, and transmission of 0.9122 could be obtained by increasing the radii of defected holes, which does not require sophisticate geometrical adjustment and high-precision requirement for fabrication. However, its resonant property is comparable to other subtle-tuned PC microcavities . Besides, the designed methane sensor is only in response to methane, which can prevent the interrupt of other gases and enhance the measurement accuracy. Benefiting from the high RI sensitivity and high quality factor of the microcavity, a theoretical detection limit of 697.35 ppm for methane sensing can be achieved. [ABSTRACT FROM AUTHOR]

Abstract: A novel harmonic detection theory and method for multi-component gas sensing based on photonic crystal waveguide (PCW) slow light is proposed. The PCW is used as gas chamber, and harmonic detection method was adopted for signal processing. This system could real-time and remote monitoring multi-component gases simultaneously with sensitivities increased by 10,278, 8650 and 6282 times respectively compared with system PCW not used. The proposed theory and method possesses powerful practicability and favorable application prospects. It could be also applied to other fluid concentration detection system, thus providing a new idea for expanding applications of slow light in sensing fields. [Copyright &y& Elsevier]

Broadly tunable and ultra-wideband slow light was theoretically demonstrated in a line defected photonic crystal waveguide (PCW) by infiltrating fluid with suitable refractive index into the first row of air holes adjacent to the waveguide. Simulation results showed that the working wavelength of the flat slow light in the optimized optofluidic PCW could be tuned from 1542.5 nm to 1569 nm with nearly constant group index of 80. Namely, broadly tunable slow light could be realized with the ultra-wide bandwidth over 26.5 nm and the group index up to 80. These achievements could not only open the possibility for achieving many excellent optical devices based on slow light in PCW, but also highlight the versatility, flexibility, and tunability offered by optofluidic infiltration schemes. [ABSTRACT FROM PUBLISHER]

Abstract: A high sensitive and compact refractive index sensor based on slotted photonic crystal waveguide (S-PhCW) is demonstrated. This design is worked on a Mach–Zehnder interferometer (MZI) configuration with S-PhCW as the measuring arm, which can be used to detect any changes in refractive index that correspond to different concentration of the measuring liquid. Combining the slow light enhancement in photonic crystal waveguide (PhCW) with the advantage of excellent optical confinement in slot waveguide, the sensitivity of this simple scheme can reach to 2.3×109 nm/RIU with the active region of only 1mm long. [Copyright &y& Elsevier]

Abstract: A high sensitive multi-component gas sensing system was proposed, in which the slotted photonic crystal waveguide (SPCW) was used as the gas cell and the fiber Fabry–Perot (FFP) was adopted as the wavelength filter. By choosing the refractive index that infiltrated in the first and second rows of air holes adjacent to the slot respectively, the working wavelength of SPCW could be tuned to matches with one absorption line of the measured gas, thus, the system could detect different kinds of gases selectively. Meanwhile, a very high group index of 150 with flat bandwidth could be achieved and the electric field enhancement factor would reach to 12. Combined with the wavelength modulation techniques (WMS) for signal processing, the minimum detection limit of 1.56ppm was achieved. [Copyright &y& Elsevier]

Abstract: A miniature and sensitivity gas sensor based on slotted photonic crystal waveguide (SPCW) was presented, in which the correlation spectroscopy method was applied for signal processing and the SPCW was used for the gas cell. To enhance the system sensitivity, the structure of the SPCW was optimized to possess extremely high group index and very large light confinement in the slot. Simulation results indicated that the measuring resolution of acetylene gas of 1ppm was achieved with good temperature stability. Additionally, the active region was only 1mm long, which could be of great advantage in the use of the limited space. [Copyright &y& Elsevier]

Abstract: This paper proposed a high sensitivity gas sensing system based on slow light in photonic crystal waveguide (PCW). The PCW was used as the gas chamber, and the harmonic detection method was adopted for signal processing. Combined the Maxwell equation with perturbation theory, the effective gas absorption factor was deduced which was greatly enhanced due to slow light in PCW. For the measurement of CO gas, a PCW was designed as the gas chamber and the detection sensitivity could be increased by 14.45 times compared with the traditional system that without PCW as the chamber. Thus the measuring detection limit of 21.5ppm is achieved with active region is only 1mm long. The proposed method provided a potential for expending the applications of slow light in the sensor fields, having powerful practicability and favorable application prospects. [Copyright &y& Elsevier]

A photonic crystal (PC) cavity with high quality factor (Q), wide measurement range, and high refractive index (RI) sensitivity is proposed and its possibility in tetrahydrofuran (THF) vapor sensing is theoretically demonstrated. The resonance characteristics of the PC cavity are simulated and optimized by using finite difference time domain (FDTD) method. Cholesteric liquid crystal (CLC) is selected as an example to infiltrate into local air holes of the PC cavity. Since the RI of the CLC and consequently the resonance wavelength of the PC cavity would change with the variation of the THF vapor concentration, this proposed PC cavity can be used for THF vapor sensing. By optimizing structural parameters of several air holes, the simulation results show that the RI sensitivity can be up to 194 nm RIU−1, along with a high Q of 2 × 105 and a detection limit (DL) as low as 4 × 10−5 RIU. For the THF sensing, the theoretical sensitivity and detection limit of THF vapor concentration can be 0.128 nm · L mmol−1 and 0.06 mmol L−1, respectively. Besides, it was theoretically verified that the PC cavity sensor has the ability to maintain RI sensitivity over a wide refractive index range of 1.33–1.53, enabling it to measure more other parameters. [ABSTRACT FROM AUTHOR]

A high-sensitive gas sensing method is proposed, in which miniaturized photonic crystal (PC) cavity is utilized as the sensing element, and cryptophane E molecular is infiltrated in defected holes of PC cavity as the sensitive material to methane gas. Particularly, slow light in side-coupled PC waveguide is optimized to enhance refractive index sensitivity of the PC cavity and fiber loop ring-down technology is used to demodulate output resonant spectrum of the sensing system. Simulation results demonstrate that a refractive index sensitivity of 450 nm/RIU and a quality factor of 1105 can be obtained in the slow light engineered PC cavity. For measurement of methane concentration, the theoretical sensitivity can be up to 4.21 μs/% and the minimum detectable concentration can be as accurate as 2.37 ppm. [ABSTRACT FROM AUTHOR]

Abstract: This work proposed a methodology based on the liquid infiltration of slotted photonic crystal waveguide (SPCW). By choosing the refractive index that infiltrated in the first and second rows of air holes adjacent to the slot, respectively, SPCW was optimized to possess wideband slow light with large group index and low dispersion. The properties of SPCW were numerically simulated by plane wave expansion (PWE) method and finite-difference time-domain (FDTD) method. Simulation results showed that the designed SPCW could control the group index for the same SPCW with the nearly constant group index of 50, 68, 81, 150, and 200 over 7.5nm, 5.5nm, 3.1nm, 1.65nm, and 1.15nm. In addition, we demonstrated that this post-fabrication liquid infiltrated technology has the potential for realizing reconfigurable and tunable SPCW, in which the flexible wavelength range of SPCW can also be controlled by different liquid infiltration. [Copyright &y& Elsevier]

• A new C-type photonic crystal fiber array was first proposed for multi-channel sensing. • Surface Plasmon Resonance was chosen as the sensing principle to get high sensitivity. • A gradual changing sensitivity of 13091 nm/RIU ~ 45322 nm/RIU was achieved. A new C-type photonic crystal fiber array was first proposed for multi-channel sensing. This design utilized the high birefringence characteristic of C-type structure and the excellent of photonic crystal array. Surface Plasmon Resonance (SPR) was chosen as the sensing principle to get high sensitivity. Through a detailed optimization process, the planar array got an optimal structure model, whose inner layer pores and defect are the main sensing channels. If some analytes with different refractive indices were entered into these channels respectively, resonance -matching conditions would change, which would superimpose on the original birefringence effect and output multiple characteristic signals. Finally, in a multi-phase infiltration condition, the device showed 5 characteristic output peaks that corresponded to different analytes. A gradual changing sensitivity of 13,091–45,322 nm/RIU was achieved in a wide refractive index range (1.33–1.44). Such a new sensing device could be regarded as a multi-channel micro-laboratory, which had a great potential for simultaneous sensing of multiple component parameters. [ABSTRACT FROM AUTHOR]