Small Changes In Refractive Index Research Articles

Extremely High Sensitive Plasmonic Refractive Index Sensors Based on Metallic Grating

This work shows that a grating-based surface plasmon (SP) resonance sensing system can exhibit extremely high sensitivity to detect a small change of refractive index in an analyte. The corresponding sensitivity can be much higher than that of the prism-based systems. Both analytical calculation and rigorous coupled-wave analysis are used to study the angular sensitivity of the system. It is found that the system’s sensitivity can be over 600° per unit index change if (1) first-order diffracted wave is chosen to excite SP mode, (2) large SP resonant angles are used in the operation, and (3) grating filling factor is selected to be varied between 0.3 and 0.7. Furthermore, the sensing system has the best performance for detecting low-index analyte with a small change of refractive index.

Evaluation of polymer coatings for ammonia vapor sensing with surface plasmon resonance spectroscopy

Seven polymeric membranes were evaluated for their potential to improve the sensitivity and impart chemical selectivity to surface plasmon resonance (SPR)-based sensors. The membranes tested encompass a variety of deposition methods, providing an insight of the contact requirements between polymers and the plasmon supporting metal. Among the membranes evaluated, preliminary results utilizing polyelectrolyte multilayer membranes displayed reliable detection of vapor-phase ammonia at ∼40 ppm levels with no visible interference from hydrogen sulfide and methane while only minor effects were observed in the presence of up to 24% relative humidity. In addition to probing biomolecular interactions, yielding information relating to binding kinetics, modifying plasmon supporting surfaces with bioreceptors enables access to biosensing applications. Gas-phase sensing with SPR has largely remained unexplored primarily due to the small changes in refractive index from low molecular weight molecules. Coating SPR sensors with tailored polymers have been discussed as a viable approach to amplifying refractive index changes related to low molecular weight analytes. Gas-phase ammonia was chosen as a test candidate for the coatings evaluated due to its widespread use and health monitoring interest at low-ppm levels. The extent of scientific contributions devoted to monitoring of ammonia further provide a baseline for comparison of the proposed SPR platform with different sensing approaches.

A Comparison of Single-Cell Elasticity of Osteogenic Cells Measured with the Optical Stretcher and Holographic Microscope

While it has been known for some time that bone mass is remodeled in response to mechanical stress, the identity of the primary mechanosensor has yet to be clearly established. To determine if cellular elasticity may play a role in the cell's ability to detect a pressure variation, we have used the optical stretcher to measure the elasticity of individual osteogenic cells. To determine cell elasticity from measurements of cellular deformation, the optical pressure on the cell surface is computed using a ray optics model which assumes a value for the index of refraction of the cell. Previously we have estimated this value from measurements of other eukaryotic cells, but the optical pressure varies significantly with small changes in refractive index. Therefore, digital holographic microscopy is used to improve estimates of this critical parameter. We consider the overall impact that a spatial variation in the index of refraction can have on the determination of the optical stress, and compare single-cell elasticity measurements of red blood cells, 2T3 osteoblast and MLO-Y4 osteocyte cells. ∗ This work was supported by P20 RR016475 from the INBRE Program of the National Center for Research Resources.

Improvement of photopolymer materials for holographic data storage

Photopolymer materials are practical materials for use as holographic recording media due to the fact that they are inexpensive, self-processing materials with the ability to record low loss, highly diffraction efficient volume holographic gratings. In general these materials absorb light of an appropriate wavelength, causing photo-polymerization of the local monomer, inducing a change in the material’s refractive index. These small changes in refractive index enable the storage of large quantities of data using holographic techniques. In an attempt to further develop the data storage capacity and quality of the information stored, i.e., resolution, in such materials, a deeper understanding of the photochemical mechanisms present during the formation of holographic gratings has become ever more crucial. From this understanding the response of an acrylamide/polyvinylalcohol based photopolymer to high spatial frequency information is improved through the addition of a chain transfer agent, i.e., sodium formate, HCOONa.

Refractive Index Sensing Using Quasi One-Dimensional Nanoslit Arrays

This letter describes the optical properties of quasi one-dimensional (1D) Au nanoslit arrays on a microscale pitch. The transmission spectra exhibited multiple minima that were well characterized by 1D surface plasmon polariton Bloch wave modes. We found that all higher order modes showed a linear response to small changes of refractive index (RI) with sensitivities up to 560 nm per RI unit, which is comparable to that of two-dimensional nanohole arrays. By calibrating the RI response of the nanoslit arrays, we could use the multiple modes to determine the RI of unknown, nonabsorbing solutions.

ENHANCED REFRACTOMETRIC OPTICAL SENSING BY USING ONE-DIMENSIONAL TERNARY PHOTONIC CRYSTALS

One-dimensional ternary photonic crystals are suggested as refractometric sensing elements, for sensing very small refractive index changes of a medium. These one-dimensional ternary photonic crystals based refractometric sensing elements are not only remarkably smaller, but are also more sensitive than one-dimensional binary photonic crystal based sensing element recently suggested by researchers.

Laser-induced phase change in Langmuir films observed using nanosecond pump-probe Brewster angle microscopy

A technique is described to observe transient events in thin interfacial films and monolayers. p-polarized light has minimum reflectivity at the Brewster angle. When an interface is viewed with light that is both incident and reflected at the Brewster angle the resulting image is dark. However, small refractive index changes can increase the reflectivity producing a high-contrast image of an altered interface with a dark background level. Using this phenomenon, with imaging optics, photo-induced phase change in Langmuir films was monitored. Two synchronized 5-ns pulsed lasers were used in the pump–probe configuration to induce changes at an air–liquid interface and to monitor the resulting morphology changes at selected time delays after photo-excitation. The photo-responsive layers were made from photochromic spiropyrans, having long aliphatic chain substituents. When irradiated with UV light the closed form of the molecule converts to a more planar ring open merocyanine form. This results in the layers changing their morphology in order to accommodate the new molecular form.

Femtomol SPR detection of DNA–PNA hybridization with the assistance of DNA-guided polyaniline deposition

The inability of surface plasmon resonance (SPR) spectroscopy to detect extremely small refractive index changes has hindered its applications in ultrasensitive DNA analysis. In this study we report a signal amplification strategy that uses DNA-templated polyaniline deposition, suitable for DNA hybridization analysis with charge neutral peptide nucleic acid (PNA) being probes. Under acidic conditions, protonated aniline monomers are adsorbed on DNA backbones through electrostatic interaction. The microenvironment provided by the DNA facilitates oxidative aniline polymerization initialized by H 2O 2 in the presence of horseradish peroxide. Under optimal conditions, the detection limit is lowered from 5 nM for conventional SPR detection to 0.1 pM. The significant sensitivity improvement is attributed to the in-situ polymer chain growth along DNA strands, which introduces drastic refractive index increases. This signal amplification approach does not involve secondary hybridization processes. The detection sensitivity obtained is much better than that of gold nanoparticle-based amplification involving a secondary hybridization process and labeled DNA detection probes.

Label-free optical biosensor on a single chip

The healthcare and pharmaceutical sectors continuously demand more powerful analytical and diagnostic tools for the identification of disease, the development of new medicines, and better diagnostic tests. High-capability optical biosensing systems are emerging as a way to achieve these goals. Based on the combination of optical materials and photonic components, optical sensing offers many advantages. For instance, the technology is non-invasive, immune to electromagnetic interference, and the risk of electrical shocks or explosions is nonexistent. A key enabler for these systems is the development of costeffective miniaturization technology, which permits the efficient integration of mechanical, fluidic, and photonic components. In typical biosensors, detection of specific pathogens or proteins begins by immobilizing appropriate bioreceptors on the sensing areas of a chip. When analytes are introduced into those areas, only the target biomolecules will be bound to their corresponding biomolecular receptors. This binding process is usually monitored with commercial analytical techniques that require transduction labeling elements, such as fluorescent dyes or radioactive isotopes, to generate a physically readable signal from a recognition event. However, labeling chemistry is expensive and time-consuming. In contrast, recent progress in biosensing systems, in which biomolecules are unlabeled or unmodified (label-free biosensing), is showing promising results. The sensing transduction signals in optical label-free biosensing platforms are based on small changes in refractive index caused by the immobilization or binding reaction of biomolecules. Planar photonic biosensors have measured biomolecular binding interactions using well-known photonic structures, including surface plasmon resonance, 4 interferometers, resonators, 10 gratings, 12 and photonic crystals. 14 These photonic sensing biochips are attractive because they integrate multiple compact devices on a single chip. But, although they use low cost mature semiconductor technology, planar photonic biochips normally need complex optical coupling systems, such as inverted taper and grating couplers, which make the devices less cost effective. Our vision is to create a highly sensitive, label-free optical biosensing system that is fully compatible with semiconductor technology and can be fabricated and interrogated at wafer or disposable chip level. Our holistic approach combines biophotonic resonant or interferometric sensing cells and advanced micron and sub-micron spot size high-sensitivity optical interrogation technologies. This approach leads to a new generation of high-value biochips that are scalable at the wafer level without the need for complex coupling systems, thus offering an inexpensive solution for integration and packaging. Our optical sensing system uses a tightly focused beam (see Figure 1), which allows in situ measurement of micron and submicron size geometries. In our current work, we have already demonstrated the capability to measure extremely small volumes of analytes (on the order of 0.1fL per hole). 16 This system makes routine screening more cost effective and, because each wafer can integrate hundreds of sensing structures, it is also suitable for performing hundreds of measurements on one or multiple samples with high throughput and on-wafer monitoring. To determine the interactions with receptor biomolecules with more reliability and sensitivity, the system will be tremendously sensitive to refractive index variations: on the order of 10 or even 10 refractive index units (RIUs). This means that direct detection of picomolar or even lower concentrations is feasible. Both observation of the reflectivity profile and magnification of the biophotonic resonant or interferometric sensing cells will facilitate this sensitivity. Label-free optical biosensors are expected to grow most in importance in the healthcare, biomedical, and biopharmaceutical sectors, since they can provide new analytical tools with reduced size as well as facilitate large-scale screening of a very wide range of samples for many different parameters. Our optical biosensing system, which uses mixed technologies, such as op-

Sensitive Label‐Free Biomolecular Detection Using Thin Silicon Waveguides

We review our work developing optical waveguide‐based evanescent field sensors for the label‐free, specific detection of biological molecules. Using high‐index‐contrast silicon photonic wire waveguides of submicrometer dimension, we demonstrate ultracompact and highly sensitive molecular sensors compatible with commercial spotting apparatus and microfluidic‐based analyte delivery systems. We show that silicon photonic wire waveguides support optical modes with strong evanescent field at the waveguide surface, leading to strong interaction with surface bound molecules for sensitive response. Furthermore, we present new sensor geometries benefiting from the very small bend radii achievable with these high‐index‐contrast waveguides to extend the sensing path length, while maintaining compact size. We experimentally demonstrate the sensor performance by monitoring the adsorption of protein molecules on the waveguide surface and by tracking small refractive index changes of bulk solutions.

Binary Number Sequence Multilayer Structure Based Refractometric Optical Sensing Element

New mod. 4 up count binary number sequence multilayer structures are suggested as refractometric-sensing elements, for sensing very small refractive index changes of a medium. Theoretical results show that these mod. 4 up count binary number sequence multilayer structures exhibit isolated narrow transmission peaks in their forbidden wavelength regions and a slight change in refractive index of material layers induce large peak shifts in the transmission spectra. These new up count binary number sequence multilayer based refractometric sensing elements are not only remarkably smaller, but are also more sensitive than photonic crystal based sensing element recently suggested by researchers.

On-Chip Surface-Based Detection with Nanohole Arrays

A microfluidic device with integrated surface plasmon resonance (SPR) chemical and biological sensors based on arrays of nanoholes in gold films is demonstrated. Widespread use of SPR for surface analysis in laboratories has not translated to microfluidic analytical chip platforms, in part due to challenges associated with scaling down the optics and the surface area required for common reflection mode operation. The resonant enhancement of light transmission through subwavelength apertures in a metallic film suggests the use of nanohole arrays as miniaturized SPR-based sensing elements. The device presented here takes advantage of the unique properties of nanohole arrays: surface-based sensitivity; transmission mode operation; a relatively small footprint; and repeatability. Proof-of-concept measurements performed on-chip indicated a response to small changes in refractive index at the array surfaces. A sensitivity of 333 nm per refractive index unit was demonstrated with the integrated device. The device was also applied to detect spatial microfluidic concentration gradients and to monitor a biochemical affinity process involving the biotin-streptavidin system. Results indicate the efficacy of nanohole arrays as surface plasmon-based sensing elements in a microfluidic platform, adding unique surface-sensitive diagnostic capabilities to the existing suite of microfluidic-based analytical tools.

Thermal-Optic Switch by Total Internal Reflection of Micromachined Silicon Prism

This paper reports the conceptual design and experimental demonstration of an optical switch that utilizes the thermo-optic effect (TOE) and total internal reflection (TIR) of a micromachined silicon prism. The key idea is to change the refractive index of the prism via TOE to switch the light from the transmission state to the TIR state. The structure is fabricated by microelectromechanical systems (MEMS) technology on a silicon-on-insulator wafer. It requires a temperature change of 69 K to switch from the transmission to the reflection states, which measure isolations of 15.6 and 40.1 dB, respectively. This design is advantageous over the other waveguide switches and photonic crystal devices in the aspects of the absence of beam splitting, tremendously enhanced sensitivity of switching to small change in refractive index, high compactness, and potentially fast and low power switching

Characterization of thin real-time holographic media by dithered four-wave mixing

Photosensitive optical media of the types that are used in real-time holography generally exhibit both photorefractive and photochromic response. The parameters that characterize each of these types of response can be extracted by detailed quantitative analysis of four-wave mixing experiments in the Raman–Nath regime. This principle has been tested using reactive eyeglass lenses. The techniques of Fourier optics have been applied to the analysis of diffraction from amplitude/phase gratings produced in the photochromic glass by laser illumination. Dithering of a laser-induced fringe pattern produces a combination of two-wave and four-wave mixing that can be analysed in terms of amplitudes proportional to products of Bessel functions of different orders. The incident and diffracted beams are predicted, and observed, to be modulated at the dither frequency and its harmonics. The small dither amplitudes that were employed, made possible by use of a phase-sensitive detection technique, allowed Bessel function expansions to be truncated after only a few terms for the purpose of parameter estimation. The technique has allowed the very small refractive index change associated with dispersion of the photo-induced absorption in silver halide doped photochromic glass to be measured for the first time.

Optical-fiber surface-plasmon-resonance sensor employing long-period fiber gratings in multiplexing

A new type of optical-fiber surface-plasmon-resonance (SPR) sensor based on a thin metallic film and long-period fiber gratings for measuring small changes of refractive index of analyte is presented. This sensor simply employs a long-period fiber grating with a proper period to couple a core mode (HE11) to the copropagating cladding mode that can excite a surface-plasmon wave (SPW). The mainly theoretical base used to analyze this new structure is the unconjugated form of coupled-mode equations. In this new SPR sensor, the variation of the refractive index of analyte is determined by monitoring the change of the transmitted core mode power, which is calculated by unconjugated two-mode coupled-mode equations at a fixed wavelength. The numerical results have demonstrated that this new and simple configuration may be used as a highly sensitive amplitude sensor. As far as the excitation of SPW, the model of numerical simulation, and the complexity of measurement equipment are concerned, this new structure is superior to the proposed sensor, consisting of a bent polished single-mode SPR optical fiber. Furthermore, the structure can be easily adapted for a SPR fiber optical probe if a mirror is deposited on the fiber tip.

Plasmon excitation by the Gaussian-like core mode of a photonic crystal waveguide

We describe resonant excitation of a plasmon by the Gaussian-like leaky mode of an effectively single mode photonic crystal (PC) waveguide. Plasmon is phase matched by design with a waveguide mode, and travels in a metallic layer on the top of a PC waveguide. We observe that small changes in the ambient refractive index just outside the metal film lead to strong variations in the losses of a waveguide core mode, making such a device a good candidate for compact sensor applications. Using low refractive index core PC waveguides makes phase matching between the plasmon and PC core mode relatively easy to enforce as modal effective index in such waveguides can be readily varied in a wide range.

Surface modes in two-dimensional photonic crystal slabs with a flat dielectric margin

We report numerical simulations of surface modes in two-dimensional high-index-contrast photonic crystal slabs with a flat dielectric margin of width on the order of the photonic crystal periodicity. Our calculations using plane wave expansion method reveal multiple surface guided modes within the photonic band gap, with some high-order modes exhibit relatively flat dispersion curves. We calculate the finite-length surface waveguide modes transmission and field patterns using two dimensional finite-difference time-domain method. We verify the surface mode dispersion curves by using spatial Fourier transform of the mode field patterns. Our study on surface modes under small ambient refractive index changes (5 x 10(-3)) shows that lower order modes exhibit larger wavelength shifts on the order of 1 nm. We also design a 4-port 3-channel bidirectional coupler using a conventional dielectric waveguide side coupled to the multimode surface waveguide.

Influence of eigenmode characteristics on optical tuning of a two-dimensional silicon photonic crystal

Ultrafast pump-probe measurements on a two-dimensional (2-D) silicon photonic crystal are used to show the influence of a pump eigenmode's spatial characteristics on the crystal's optical tuning properties. Electron-hole pairs are generated by two-photon absorption of $130\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, $1.5\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ pulses via different crystal eigenmodes and spatially modify the crystal's refractive index via Drude effects; characteristics of an eigenmode near $1.9\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ are monitored via time-resolved reflectivity. For a pump eigenmode producing an inhomogeneous carrier distribution, diffusion is responsible for an initially fast ($10\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ time scale) component of the recovery of the probe reflectivity with surface recombination accounting for a slower response ($700\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ time scale) after the carriers are nearly uniformly distributed within the silicon backbone. When carriers are initially generated homogeneously in the silicon, surface recombination alone controls the time evolution of the probed mode. A simple expression is developed for the mode-dependent two photon absorption in photonic crystals and a simple perturbation technique is used to treat modifications to photonic crystal eigenmodes by small refractive index changes.

Micrometre-scale silicon electro-optic modulator

Metal interconnections are expected to become the limiting factor for the performance of electronic systems as transistors continue to shrink in size. Replacing them by optical interconnections, at different levels ranging from rack-to-rack down to chip-to-chip and intra-chip interconnections, could provide the low power dissipation, low latencies and high bandwidths that are needed. The implementation of optical interconnections relies on the development of micro-optical devices that are integrated with the microelectronics on chips. Recent demonstrations of silicon low-loss waveguides, light emitters, amplifiers and lasers approach this goal, but a small silicon electro-optic modulator with a size small enough for chip-scale integration has not yet been demonstrated. Here we experimentally demonstrate a high-speed electro-optical modulator in compact silicon structures. The modulator is based on a resonant light-confining structure that enhances the sensitivity of light to small changes in refractive index of the silicon and also enables high-speed operation. The modulator is 12 micrometres in diameter, three orders of magnitude smaller than previously demonstrated. Electro-optic modulators are one of the most critical components in optoelectronic integration, and decreasing their size may enable novel chip architectures.

Polarization control and sensing with two-dimensional coupled photonic crystal microcavity arrays

We have experimentally studied polarization properties of the two-dimensional coupled photonic crystal microcavity arrays and observed a strong polarization dependence of the transmission and reflection of light from the structures-effects that can be employed in building miniaturized polarizing optical components. Moreover, by combining these properties with a strong sensitivity of the coupled bands on the surrounding refractive index, we have demonstrated a detection of small refractive-index changes in the environment, which is useful for construction of biochemical sensors.