Detection Of Refractive Index Variations Research Articles

MIM plasmonic sensors based on single-side ring cavity with one stub and their applications

A plasmonic sensor is proposed, comprising a metal–insulator–metal (MIM) straight waveguide and a ring cavity with one stub (RCS). Using the finite element method, its transport properties are simulated and systematically analyzed. By optimizing the structure parameters, the sensor obtains the maximum sensitivity (S) of 2010 nm/RIU and the maximum figure of merit (FOM) of 49219.04 RIU−1. It demonstrates a sensing resolution (SR) of 4.98 × 10−7 RIU in the detection of refractive index variation. Based on the optimized parameters, temperature sensing is investigated utilizing Polydimethylsiloxane (PDMS) as the temperature-sensitive medium, and the temperature sensitivity is found to be −0.90 nm/°C. In addition, multiple independently tunable resonances are achieved by adding a ring cavity (RC) above the straight waveguide. This derived structure enables the simultaneous detection of electrolyte samples (Na+ and K+) in blood with bio-sensing sensitivities reaching 0.1833 nm·dL/mg and 0.2 nm·dL/mg. These results have directive significance for the development of multifunctional and ultra-compact plasmonic sensor.

Gold-coated split laser-induced periodic surface structures as refractometric sensors

The generation of surface plasmon resonances (SPR) in laser-induced periodic surface structures (LIPSS) allows their application in the field of optical sensing, such as the detection of refractive index variations in gases and liquids. We have fabricated gold-coated LIPSS nanostructures on stainless steel substrates by using femtosecond laser nano-ablation. This technique is a low-cost and high-throughput fabrication method applicable to fast and large-scale manufacturing. The depth profile of the fabricated LIPSS shows a central dip at the top of each ripple that split the geometry. The actual topography is modeled and included in a computational electromagnetism package to obtain the expected optical response under the experimental conditions. The measured and simulated spectral reflectances are compared, and the differences are explained by the departure of the fabricated LIPSS from the ideal topography. The experiments and simulations showed excellent agreement for the main spectral characteristics, like the Fano-like lineshapes of the spectral reflectance. This fitting provides the values used to determine the refractometric performance of the fabricated device, that shows a sensitivity of 518 nm/RIU and a figure of merit of 32 RIU−1 for an aqueous analyte. Our experimental results show that the fabricated devices are competitive in terms of cost and simplicity when compared to existing devices with similar performance.

Plasmon-induced transparency sensor for detection of minuscule refractive index changes in ultra-low index materials

Detection of low-index materials such as aerogels and also detection of refractive index variations in these materials is still a challenging task. Here, a high figure of merit (FOM) sensor based on plasmon-induced transparency (PIT) is proposed for the detection of aerogel refractive index changes. In the proposed PIT sensor, the transparency window in an opaque region arises from the coupling between surface plasmon polariton (SPP) mode and planar waveguide mode. By comprising sub-wavelength grating (SWG) in the planar waveguide region, the maximum of the electric field of waveguide occurs in a low index media. This facilitates detection of the aerogels when they are used as the low index material (sensing material). Application of the subwavelength grating waveguide also improves the sensitivity of the sensor by a factor of six compared to a conventional structure with a homogenous waveguide. The proposed structure has a quality factor of Q ≥ 1800, and a reflection of 86%, and can detect the refractive index changes as low as Δn = 0.002 (around n = 1.0). The lineshape, Q-factor, and resonant wavelength of the transparency spectrum can be controlled by tailoring the structural parameters. Our work also has potential application in switching, filtering, and spectral shaping.

A highly sensitive Bezier polygonal hollow core photonic crystal fiber biosensor based on surface plasmon resonance

Photonic crystal fiber- based plasmonic resonance sensor for detection of refractive index variations of liquids in the range of 1.36–1.39 is proposed. The five layer air hole fiber structure, with the first inner air hole layer forming the air core using Bezier polygon, confines light in the core by the principle of photonic band gaps. Plasmon waves induced on the surface of embedded metallic gold nanowires due to the light propagating in the air core are influenced by the liquid filled in the core. The study on properties of plasmonic excitation is carried out using finite element method. Numerical investigations reveal that the sensor exhibits a high sensitivity value of 34,000 nm RIU−1 and confinement loss factor as high as 73 dB/cm is achieved. The sensor is more suitable in biosensing applications for detecting variations in refractive indices of different biochemical and chemical analytes.

Spectral Optical Readout of Rectangular-Miniature Hollow Glass Tubing for Refractive Index Sensing.

For answering the growing demand of innovative micro-fluidic devices able to measure the refractive index of samples in extremely low volumes, this paper presents an overview of the performances of a micro-opto-fluidic sensing platform that employs rectangular, miniature hollow glass tubings. The operating principle is described by showing the analytical model of the tubing, obtained as superposition of different optical cavities, and the optical readout method based on spectral reflectivity detection. We have analyzed, in particular, the theoretical and experimental optical features of rectangular tubings with asymmetrical geometry, thus with channel depth larger than the thickness of the glass walls, though all of them in the range of a few tens of micrometers. The origins of the complex line-shape of the spectral response in reflection, due to the different cavities formed by the tubing flat walls and channel, have been investigated using a Fourier transform analysis. The implemented instrumental configuration, based on standard telecom fiberoptic components and a semiconductor broadband optical source emitting in the near infrared wavelength region centered at 1.55 µm, has allowed acquisition of reflectivity spectra for experimental verification of the expected theoretical behavior. We have achieved detection of refractive index variations related to the change of concentration of glucose-water solutions flowing through the tubing by monitoring the spectral shift of the optical resonances.

Extended Malus law with terahertz metallic metamaterials for sensitive detection with giant tunable quality factor

We study a polarizer-analyzer mounting for the terahertz regime with perfectly conducting metallic polarizers made of a periodic subwavelength pattern. We analytically investigate the influence on the transmission response of the multiple reflections which occur between polarizer and analyzer with a renewed Jones formalism. We demonstrate that this interaction leads to a modified transmission response: the extended Malus' Law. In addition, we show that the transmission response can be controlled by the distance between polarizer and analyzer. For particular set-ups, the mounting exhibits extremely sensitive transmission responses. This interesting feature can be employed for high precision sensing and characterization applications. We specifically propose a general design for measuring electro-optical response of materials in the terahertz domain allowing detection of refractive index variations as small as $10^{-5}$.

Optical wave microphone measurement during laser ablation of Si

Pulsed laser irradiation is used for surface treatment of a solid and ablation for particle formation in gas, liquid or supercritical phase media. When a pulsed laser is used to irradiate a solid, spatial refractive index variations (including photothermal expansion, shockwaves and particles) occur, which vary depending on the energy density of the pulsed laser. We focused on this phenomenon and applied an unique method for detection of refractive index variation using an optical wave microphone based on Fraunhofer diffraction. In this research, we analyzed the waveforms and frequencies of refractive index variations caused by pulsed laser irradiation of silicon in air and measured with an optical wave microphone.

Digital holography as a versatile optical diagnostic method for microgravity experiments

Microgravity experiments require robust, reliable and simple measurement techniques that provide the same capabilities as on-ground laboratory applications. Interferometric measurement techniques such as conventional holographic interferometry using hologram plates have several disadvantages in microgravity research - a time-consuming development process and additional experimental effort for a quantitative evaluation of interference patterns, for example. Digital holography is an advanced optical diagnostic tool for surface deformation analysis, measurement of refractive index variations and particle analysis in transparent media: Fresnel holograms are stored electronically and the reconstruction is performed by numerical methods. Due to the reconstruction process a numerical representation of the recorded wavefront can be evaluated including amplitude and phase. This paper reports on applications of digital holography to particle analysis and to the detection of refractive index variations within transparent media. The properties and advantages of this technique for microgravity experiments with respect to other techniques are discussed.

Chemical sensing by surface plasmon resonance in a multimode optical fibre

A new fibre optic sensor for the detection of refractive index variations is presented. The principle of the sensor is based on the generation of surface plasma waves at the interface between a thin metallic layer deposited on the fibre core and a surrounding dielectric medium. Specific injection of monochromatic light into the fibre allows the detection of a refractive index variation as low as for dielectric media lying in the 1.33 - 1.40 refractive index range. Modelling of sensing signal and experiments are performed. Potential applications are aqueous solution analyses and detection of gases and solvents with a specific polymeric layer.

Visualization of protein zones in preparative electrophoresis and carrier ampholyte zones on isoelectric focusing in gel slabs by two light refraction methods

Two simple arrangements for the detection of refractive index variations are described. One is based on the so-called shadow method and consists of a lamp, a parabolic mirror, and a screen. The other is based on the Toepler-schlieren method and consists of a lamp, two parabolic mirrors, an adjustable knife-edge, and a screen. These devices are used to visualize refractive index variations in a vertical polyacrylamide-gel slab used for electrophoretic separations. The variations appear on a screen as differences in illumination in a natural-size image of the gel. The electrophoresis apparatus is made of ordinary window glass and acrylic plastic. Two experiments are described: visualization of protein zones during electrophoresis and carrier ampholyte zones during isoelectric focusing. Two major applications are suggested: localization of protein zones to guide proper slicing of the gel in preparative work (for analytical purposes, more sensitive methods are necessary) and monitoring of the formation of pH gradients.