Index Matching Oil Research Articles

SPP excitation and coupling mechanism based onmicro/nano fibers.

A hot trend in the development of optoelectronic devices is how to use the principle of surface plasmon resonance to enhance the performance of integrated photonics devices and achieve miniaturization. This paper proposes an accompanying waveguide coupling structure of micro/nano fibers, which consists of two parallel-placed micro/nano fibers (MNFs) coated with a silver film in the waist region and infused with a refractive index matching oil. In the overlapping region, there exists a segment of surface plasmon polaritons (SPPs) coupling area. The excitation and coupling characteristics of SPPs are studied through numerical simulation. Optimal coupling enhancement configuration is obtained by studying variables such as spacing distance, coupling length, and metal film thickness. A comparison is made with the SPP intensity of a single MNF, showing a 220% increase in electric field intensity, demonstrating its excellent coupling effect. By using this coupling structure, exploration of SPPs excitation and coupling mechanisms is enhanced, and structures resembling interferometric devices can be designed, providing new insights for high-performance miniaturized devices.

Impact of substrates and quantum effects on exciton line shapes of 2D semiconductors at room temperature

Abstract Exciton resonances in monolayer transition-metal dichalcogenides (TMDs) provide exceptionally strong light–matter interaction at room temperature. Their spectral line shape is critical in the design of a myriad of optoelectronic devices, ranging from solar cells to quantum information processing. However, disorder resulting from static inhomogeneities and dynamical fluctuations can significantly impact the line shape. Many recent works experimentally evaluate the optical properties of TMD monolayers placed on a substrate and the line shape is typically linked directly to the material’s quality. Here, we highlight that the interference of the substrate and TMD reflections can strongly influence the line shape. We further show how basic, room-temperature reflection measurements allow investigation of the quantum mechanical exciton dynamics by systematically controlling the substrate reflection with index-matching oils. By removing the substrate contribution with properly chosen oil, we can extract the excitonic decay rates including the quantum mechanical dephasing rate. The results provide valuable guidance for the engineering of exciton line shapes in layered nanophotonic systems.

Ultra-High-Throughput Absorbance-Activated Droplet Sorting for Enzyme Screening at Kilohertz Frequencies.

Droplet microfluidics is a valuable method to "beat the odds" in high throughput screening campaigns such as directed evolution, where valuable hits are infrequent and large library sizes are required. Absorbance-based sorting expands the range of enzyme families that can be subjected to droplet screening by expanding possible assays beyond fluorescence detection. However, absorbance-activated droplet sorting (AADS) is currently ∼10-fold slower than typical fluorescence-activated droplet sorting (FADS), meaning that, in comparison, a larger portion of sequence space is inaccessible due to throughput constraints. Here we improve AADS to reach kHz sorting speeds in an order of magnitude increase over previous designs, with close-to-ideal sorting accuracy. This is achieved by a combination of (i) the use of refractive index matching oil that improves signal quality by removal of side scattering (increasing the sensitivity of absorbance measurements); (ii) a sorting algorithm capable of sorting at this increased frequency with an Arduino Due; and (iii) a chip design that transmits product detection better into sorting decisions without false positives, namely a single-layered inlet to space droplets further apart and injections of "bias oil" providing a fluidic barrier preventing droplets from entering the incorrect sorting channel. The updated ultra-high-throughput absorbance-activated droplet sorter increases the effective sensitivity of absorbance measurements through better signal quality at a speed that matches the more established fluorescence-activated sorting devices.

Rectangular single-mode polymer optical fiber for femtosecond laser inscription of FBGs

In this study, a novel rectangular polymer single-mode optical fiber for femtosecond (fs) laser-inscribed fiber Bragg gratings (FBGs) is proposed and demonstrated. The cylindrical geometry of the widely used circular fiber elongates the fs laser beam along the fiber axis, resulting in reduced laser intensity and requiring index-matching oil immersion during FBG inscription. However, the flat geometry and negligible surface roughness of the featured fiber significantly diminish this lensing distortion and eliminate the need for oil immersion, thereby resulting in optimal focusing of the laser beam, permitting direct and efficient inscription of FBGs within the optical fiber. The core and cladding of the rectangular fiber were fabricated using two different grades of ZEONEX, a cyclo olefin polymer, which have slightly different refractive indices. The similar glass transition temperature for core and cladding simplifies the fiber drawing process, and a rectangular single-mode optical fiber with dimensions of 213 μm × 160 μm and core diameter of 9.4 μm was fabricated using an in-house fiber drawing facility. A second harmonic (520 nm) fs laser beam was used to successfully inscribe a 2-mm-long FBG in the rectangular fiber within a few seconds with a point-by-point technique. The inscription of a single FBG leads to the excitation of higher order FBG peaks at 866.8 and 1511.3 nm, corresponding to widely used wavelength bands in fiber optic sensing. The strain and temperature sensitivities of the FBG were measured to be 7.31 nm/ % ε ( 0.731 pm / με ) and 10 pm/°C, and 12.95 nm / % ε ( 1.29 pm / με ) and 15 pm/°C at 866.8 nm and 1511.3 nm, respectively.

Silicon-Nitride-Assisted Edge Coupler Interfacing With High Numerical Aperture Fiber

Reducing the mode size mismatch between high-index-contrast silicon waveguide and single-mode fiber takes an important role in photonic integration. A low-loss and broadband silicon-nitride-assisted edge coupler for the coupling of light between silicon wire waveguides and high numerical aperture fiber is proposed and optimized for TE polarization. The mode size converter is composed of silicon inverse taper waveguides in the core and two silicon nitride interlayers spaced by silica in cladding layers. The edge coupler is characterized by cleaved high numerical aperture fiber (mode field diameter is about $6.5~\mu \text{m}$ ). With the help of refractive index matching oil ( $n =1.46$ ), the coupling loss for TE polarized light input is 0.35 dB at 1570 nm. The 1-dB bandwidth is 95 nm. The 1-dB alignment tolerances are higher than ± 2.5 and $\pm ~1~\mu \text{m}$ in the horizontal and vertical directions, respectively, and higher than $25~\mu \text{m}$ in the axial direction.

Nonlinear refractive index of electric field aligned gold nanorods suspended in index matching oil measured with a Hartmann-Shack wavefront aberrometer.

The capability to dynamically control the nonlinear refractive index of plasmonic suspensions may enable innovative nonlinear sensing and signaling nanotechnologies. Here, we experimentally determine the effective nonlinear refractive index for gold nanorods suspended in an index matching oil aligned using electric fields, demonstrating an approach to modulate the nonlinear optical properties of the suspension. The nonlinear optical experiments were carried out using a Hartmann-Shack wavefront aberrometer in a collimated beam configuration with a high repetition rate femtosecond laser. The suspensions were probed at 800 nm, overlapping with the long-axis absorption peak of the nanorods. We find that the effective nonlinear refractive index of the gold nanorods suspension depends linearly on the orientational order parameter, S, which can be understood by a thermally induced nonlinear response. We also show the magnitude of the nonlinear response can be varied by ∼ 60%.

See-through multi-projection three-dimensional display using transparent anisotropic diffuser.

We propose a see-through multi-projection three-dimensional (3D) display using a transparent anisotropic diffuser. By immersing a metal-coated anisotropic diffuser into index matching oil which has the same refractive index of anisotropic diffuser, a transparent anisotropic diffuser is implemented. The reflectance of the transparent anisotropic diffuser is analyzed with the transfer matrix. Two multi-projection methods are proposed based on reflection type integral imaging and multi-view method. Especially, the reflection type multi-view-based system is realized with a curved anisotropic diffuser. High resolution see-through 3D display can be realized with the proposed methods. They can be used in various applications with the two multi-projection methods. In order to show the augmented reality features, real objects and virtual 3D images are presented at the same time in the experimental setup.

Optimization and Characterization of Femtosecond Laser Inscribed In-Fiber Microchannels for Liquid Sensing

In-fiber microchannels were fabricated directly in standard single mode fiber using the femtosecond laser inscribe and etch technique. This method of creating in-fiber microchannels offers great versatility, since it allows complex three-dimensional structures to be inscribed and then preferentially etched with hydrofluoric acid. In addition, inscription does not require a photosensitive fiber; the modification is induced through nonlinear processes triggered by an ultrashort laser pulse. Four in-fiber microchannel designs were experimentally investigated using this technique—microhole, microslot channel along the core, microslot channel perpendicular to the core and helical channel around the core. Each device design was evaluated through monitoring the optical spectral change while inserting a range of index matching oils into each microchannel; an R.I. sensitivity up to 1.55 dB/RIU was achieved in these initial tests. Furthermore, an all femtosecond laser inscribed Fabry–Perot-based refractometer with an R.I. sensitivity of 2.75 nm/RIU was also demonstrated. The Fabry–Perot refractometer was formed by positioning a microchannel between two femtosecond laser inscribed point-by-point fiber Bragg gratings.

Ball Lens Based Common Path Fiber Optic Interferometer Sensor

A common path interferometer has been made with a ball lens packaged in a metal cylinder, dielectric mirror, and lucent type connector (LC) fiber patch cord. By simply placing metal cylinder packaged mirror and a ball lens directly in front of a fiber patch cord, a compact and potentially disposable sensing probe for optical bio-sensor could be implemented. To achieve a sufficiently long working distance, the proper ball lens diameter and the distance between the ball lens and the fiber patch cord were investigated. A working distance of up to 1.6 mm was achieved. With common path sensing probe, measurements of the index matching oil and glucose solutions were implemented and used to demonstrate the fesibility as the liquid sensing probe.

Real-time Doppler-assisted tomography of microstructured fibers by side-scattering.

We introduce the concept of Doppler-assisted tomography (DAT) and show that it can be applied successfully to non-invasive imaging of the internal microstructure of a photonic crystal fiber. The fiber is spun at ~10 Hz around its axis and laterally illuminated with a laser beam. Monitoring the time-dependent Doppler shift of the light scattered by the hollow channels permits the azimuthal angle and radial position of individual channels to be measured. An inverse Radon transform is used to construct an image of the microstructure from the frequency-modulated scattered signal. We also show that DAT can image sub-wavelength features and monitor the structure along a tapered fiber, which is not possible using other techniques without cutting up the taper into several short pieces or filling it with index-matching oil. The non-destructive nature of DAT means that it could potentially be applied to image the fiber microstructure as it emerges from the drawing tower, or indeed to carry out tomography on any transparent microstructured cylindrical object.

Modeling of Au-Nanowire Waveguide for Plasmonic Sensing in Liquids

We theoretically demonstrate a plasmonic nanosensor, using Au-nanowire waveguide to measure the refractive-index changes in aqueous solutions. Based on finite element method simulations, waveguiding properties of Au nanowires for plasmonic sensing in liquids are investigated, with Au nanowire diameter down to 10 nm. A plasmonic nanowire Mach–Zehnder interferometer is proposed to measure the phase shift introduced by the index changes of surroundings. We find that, for a typical Au nanowire with 100-nm diameter, the calculated sensitivity is as high as 5.5π/(μm·RIU), and the sensitivity can be increased by reducing the nanowire diameter. Besides, for reference, we have also investigated Au nanowire plasmonic sensing in other liquids including ethylene glycol and index-matching oil. The nanowire plasmonic sensing scheme proposed here represents a high-sensitivity nanosensor with ultra-small footprint, and may open new opportunities for miniaturized sensing platform based on highly confined 1-D waveguiding plasmons.

Backlight units based on light extraction from a curved optical fiber

A backlight unit is constructed by laying out a plastic optical fiber (POF) in a curved trench fabricated in a light-guide plate. First, the light leaks out of the POF at curved sections and enters the plate. Next, the light is extracted from the plate by some microstructures fabricated on the surfaces of the plate. Coupled to a laser diode, its optical power can be efficiently and uniformly delivered over a large area via the POF. In this experiment, we fabricated a 10 cm×10 cm×3 mm prototype unit with off-the-shelf components. It becomes see-through when the space around the POF is filled with index-matching oil. One can build an arbitrary-shaped planar light source by tiling multiple cells and connecting them by a POF. The light inside the POF is depleted as it propagates downstream. This can be compensated by decreasing the radii of curvature. Microstructures on the light-guide plate can distort the passage of ambient light. For a see-through unit, we can distribute them sparsely and/or use absorbers. A see-through backlight unit is a relatively unexplored device, and it might pave the way for new applications.

Radiation pattern of plasmonic nano-antennas in a homogeneous medium.

Radiation patterns from plasmonic nano-antennas formed on a glass substrate were investigated using index-matching oils. It was confirmed that the pattern from single nano-antennas for various cases of index-mismatching between the substrate and the oil is explained well by the patterns of infinitesimal electric dipoles. We found that for an angular resolution of 2°, the index mismatch must be smaller than 0.001 to realize isotropic radiation. By using the appropriate condition, the radiation patterns of nano Yagi-Uda antennas in a quasi-homogeneous medium were obtained experimentally.

Large Core Diameter POF/PCF Optical Switch Using Shape Memory Alloy (SMA) Coil Actuator for Local Area Network (LAN) System

The polymer optical fiber (POF) is mostly suitable for house held optical switches because of its large core diameter. A high performance optical switch building process is described. In order to achieve high performance switching and low data loss, we examine different technique while building different parts. Furthermore, we show the comparison of surface roughness of POF/PCF using different processing methodology to minimize the optical insertion loss. This research developed a fiber fixing Jig for Mechanical polishing method, and we discovered that, the Mechanical polishing method able to provide 1/3 (0.08 dB) optical insertion loss than the Mechanical cutting method. Another challenge was to build the main structure of the switch, and how to Seal inside’s matching oil properly. As long as, our target is to build a low-cost switch, we selected an acrylic substrate for the body. Switching speed is less than 0.5 second, and the insertion loss of the fabricated switch is 0.4 to 0.5 dB. The insertion loss is 0.06 to 0.09 dB when using index-matching oil. 

Optimizing POF/PCF based optical switch for indoor LAN

For indoor local area network (LAN) the Polymer optical fiber (POF) is mostly appropriate, because of its large core diameter and flexible material. A 1×2 optical switch for indoor LAN using POF and a shape memory alloy (SMA) coil actuator with magnetic latches was successfully fabricated and tested. To achieve switching by the movement of a POF, large displacement is necessary because the core diameter is large (e.g., 0.486mm). A SMA coil actuator is used for large displacement and a magnetic latching system is used for fixing the position of the shifted POF. The insertion loss is 0.40 to 0.50dB and crosstalk is more than 50dB without index-matching oil. Switching speed is less than 1s at a driving current of 80mA. A cycling test was performed 1.4 million times. Polymer clad fiber optical (PCF) switch also fabricated and tasted.

Experimental demonstration of surface plasmon resonance enhancement of the tapered optical fiber coated with Au/Ti thin film

Dependence of taper angle of the surface plasmon resonance (SPR)-based fiber optic sensor on the SPR absorption and sensitivity was investigated. The SPR wavelength was found to increase from 700nm to 1200nm with the increase of the refractive index of the surrounding oils from 1.404 to 1.434. The SPR sensitivity increased with the increase of metal-coated taper angle (Ω) from 0.07° to 1.86° by changing the down/up tapering length or waist diameter. The absorption intensity of the SPR spectrum was found to decrease and the FWHM was broadened with the increase of refractive index of the index matching oils (n=1.404–1.434) due to the leakage of the power fraction of guided rays into the surrounding medium and the spatial spreading and scattering of the conduction electrons, respectively.

Mode-size converter with high coupling efficiency and broad bandwidth

An ultralow coupling loss and broad bandwidth fiber-to-waveguide mode-size converter is demonstrated for nano-scale waveguides on SOI platform using CMOS technology in this paper. The mode-size converter consists of a cantilevered PECVD SiO(2) waveguide and a-Si nano-tapers by removing the adjacent SiO(2) layer and underlying substrate Si. The a-Si waveguide is located at the center of the cantilevered SiO(2) waveguide. We characterized the cantilevered mode-size converter using cleaved optical single mode fiber with 10.5 µm mode field diameter. With refractive index (1.375) matching oil, the measured coupling efficiencies between the cleaved optical fiber and this converter are higher than 80% per facet and 70% per facet for TE and TM modes at 1600 nm, respectively. The polarization dependent loss and the coupling loss variation of this converter are less than 1.0 dB at the wavelength range of 1520~1640 nm. The 1-dB bandwidths for both TE and TM modes are more than 120 nm. The alignment tolerances for TE and TM modes are ± 2.8 µm and ± 2.1 µm at 1-dB excess loss in horizontal direction and vertical direction, respectively.

Dynamically adjustable three-dimensional gray masks operated by electrostatic force modulation for the fabrication of microlens arrays in microchannels

We propose a new approach to fabricating micro-optical elements, in particular, microlenses and microlens arrays using an adjustable gray-scale mask and a microfluidic conveyor module. A liquid with acrylate oligomer is photopolymerized while it flows through this module, thus forming the micro-optical components. The adjustable gray-scale mask contains a layer filled with a UV-absorbing liquid and transparent elastomer structures in the shape of microlenses. By applying a high voltage, the shape of these microlenses can be altered, enabling one to change the intensity transmission distribution of the gray-scale mask and, as a consequence, the surface profile of the corresponding microlens structures in the microfluidic conveyor module below. The produced microlens structures are washed out of the conveyor module without adhesion on the walls of the PDMS microchannel surfaces due to the existence of an oxygen-aided inhibition layer. The dependence of the produced microlens morphology on the index-matching oil, the concentration of UV-absorbing liquids, and the electrostatic force modulation of the adjustable gray-scale mask are characterized. The adjustable gray-scale mask combined with a suitable flow/stop sequence enables one to fabricate microlenses and microlens arrays in a continuous and high-throughput mode.

Surface plasmon excited infrared-to-visible upconversion in Er 3+-doped transparent glass ceramics

The surface plasmon (SP) excited infrared-to-visible upconversion luminescence has been studied in the oxyfluoride glass ceramics containing Er 3+-doped fluoride nano-crystals. Upconversion luminescence was observed at glass ceramics. Quantum yield of upconversion luminescence increased with increasing heat-treatment temperature. The transparent glass ceramics heat-treated at 700 °C was covered with a 50 nm gold film, and then attached to an SF10 prism with index-matching oil to make an attenuated total reflection (ATR) illumination. The intense upconversion luminescence bands at 540 and 660 nm were observed at the SP resonance angle by using p-polarized laser beam. The apparent dependences of the upconversion efficiency on laser polarization and on incident angle demonstrated the SP excited upconversion of Er 3+.

Two-dimensional flow towards a guarded downhole sampling probe: An experimental study

The guarded-probe concept for downhole fluid sampling is analogous to that of focused electrodes for borehole electrical measurements. The guard part of the probe surrounds the sample probe and draws fluid from the near-wellbore region of the rock formation where pore fluid is contaminated with drilling fluid filtrate (invasion fluid). Fluid entering the central sample probe comes from points deeper in the formation, where contamination is lower. We report experiments performed to study the guard probe concept in two dimensions. For these experiments, glass beads contained in a thin cell formed the porous medium. The original, uncontaminated pore fluid was an optical index-matching oil. The invasion fluid was the same oil with a dissolved dye. A camera recorded the displacement of the dyed fluid by the clear fluid and the dye concentration was measured in the flow lines through which fluid was withdrawn. The experiments verify that in 2 dimensions a guarded probe produces clean samples of original pore fluid faster than would a simple probe. The experiments were successfully modeled, without adjustable parameters, by means of a finite-difference numerical reservoir simulator and by computations based on an approximate velocity field given by a complex variable solution of the Laplace equation.