Photonic crystal sensors use photonic crystals: nanostructures composed of periodic arrangements of dielectric materials that interact with light depending on their particular structure, reflecting lights of specific wavelengths at specific angles. Any change in the periodicity or refractive index of the structure can give rise to a change in the reflected color, or the color perceived by the observer or a spectrometer. [1] That simple principle makes them useful colorimetric intuitive sensors for different applications including, but not limited to, environmental analysis, temperature sensing, magnetic sensing, biosensing, diagnostics, food quality control, security, and mechanical sensing.[ citation needed ] Many animals in nature such as fish or beetles employ responsive photonic crystals for camouflage, signaling or to bait their prey. [2] The variety of materials utilizable in such structures ranging from inorganic, organic as well as plasmonic metal nanoparticles makes these structures highly customizable and versatile. In the case of inorganic materials, variation of the refractive index is the most commonly exploited effect in sensing, while periodicity change is more commonly exhibited in polymer-based sensors. Besides their small size, current developments in manufacturing technologies have made them easy and cheap to fabricate on a larger scale, making them mass-producible and practical.
As properly designed photonic crystals exhibit high sensitivity, selectivity, stability, and their electricity-free operation if needed, they have become highly researched portable biological sensors. Developments in analysis, device miniaturization, fluidic design and integration have catapulted the development of integrated photonic crystal sensors in what is known as lab-on-a-chip devices of high sensitivity, low limit of detection, faster response time and low cost. [3] A large range of analytes of biological interest such as proteins, DNA, [4] cancer cells, [5] glucose [6] and antibodies can be detected with this kind of sensors, providing fast, cheap and accurate diagnostic and health-monitoring tools that can detect concentrations as low as 15 nM.[ citation needed ] Certain chemical or biological target molecules can be integrated within the structure to provide specificity. [7]
As chemical analytes have their own specific refractive indices, they can fill porous photonic structures, altering their effective index and consequently their color in a finger-print like manner. On the other hand, they can alter the volume of polymer-based structures, resulting in a change in the periodicity leading to a similar end effect. In ion-containing hydrogels, their selective swelling results in their specificity. Applications in gaseous and aqueous environment have been studied to detect concentrations of chemical species, solvents, vapors, [8] ions, [9] pH [10] and humidity. The specificity and sensitivity can be controlled by the appropriate choice of materials and their interaction with the analytes, that can achieve even label-free sensors. [11] The concentration of chemical species in vapor or liquid phases as well as in more complex mixtures can be determined with high confidence. [12] [13]
Different mechanical signals such as pressure, strain, torsion and bending can be detected with photonic crystal sensors. Commonly, they are based on the deformation-induced change in the lattice constants in flexible materials such as elastomeric composites or colloidal crystals, causing a mechano-chromic effect as they stretch or contract. [14]
Synthetic opals are three dimensional photonic crystals usually made of self-assembled nanospheres of diameters on the order of hundreds of nanometers, where the high refractive index material is that of the spheres and the low-index material is air or another filler. On the other hand, inverse opals are structures where the interstitial space between the spheres is filled with another material and the spheres are consequently removed, providing a larger free volume for faster diffusion of chemical species. [15]
Photonic crystal fibers are a special types of optical fibers that has contain air holes distributed in specific patterns around a solid or hollow core. Due to their high sensitivity, inherent flexibility, and small diameters, they can be used in a variety of situations requiring high robustness and portability. Compared to traditional optical fibers, they are highly birefringent with tailorable dispersion, limited loss and endless single-mode propagation for a long range of wavelengths and have a very fast sensing response. [16]
One-dimensional slabs with two dimensional order cause by selective removal of material, creating a pattern of holes or grooves in an otherwise homogeneous material is a popular photonic crystal structure used in sensing. [17]
Fabry-Pérot mirrors are planar photonic crystal where the periodicity is maintained only in the z-dimension. [18] sputtered porous inorganic sensors, spin-coated polymer sensors and self-assembled block-copolymers are a few of the commonly used planar 1D structures. [19] [20]
In electromagnetics, an evanescent field, or evanescent wave, is an oscillating electric and/or magnetic field that does not propagate as an electromagnetic wave but whose energy is spatially concentrated in the vicinity of the source. Even when there is a propagating electromagnetic wave produced, one can still identify as an evanescent field the component of the electric or magnetic field that cannot be attributed to the propagating wave observed at a distance of many wavelengths.
A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of natural crystals gives rise to X-ray diffraction and that the atomic lattices of semiconductors affect their conductivity of electrons. Photonic crystals occur in nature in the form of structural coloration and animal reflectors, and, as artificially produced, promise to be useful in a range of applications.
Nanosensors are nanoscale devices that measure physical quantities and convert these to signals that can be detected and analyzed. There are several ways proposed today to make nanosensors; these include top-down lithography, bottom-up assembly, and molecular self-assembly. There are different types of nanosensors in the market and in development for various applications, most notably in defense, environmental, and healthcare industries. These sensors share the same basic workflow: a selective binding of an analyte, signal generation from the interaction of the nanosensor with the bio-element, and processing of the signal into useful metrics.
Polydiacetylenes (PDAs) are a family of conducting polymers closely related to polyacetylene. They are created by the 1,4 topochemical polymerization of diacetylenes. They have multiple applications from the development of organic films to immobilization of other molecules.
An optical ring resonator is a set of waveguides in which at least one is a closed loop coupled to some sort of light input and output. The concepts behind optical ring resonators are the same as those behind whispering galleries except that they use light and obey the properties behind constructive interference and total internal reflection. When light of the resonant wavelength is passed through the loop from the input waveguide, the light builds up in intensity over multiple round-trips owing to constructive interference and is output to the output bus waveguide which serves as a detector waveguide. Because only a select few wavelengths will be at resonance within the loop, the optical ring resonator functions as a filter. Additionally, as implied earlier, two or more ring waveguides can be coupled to each other to form an add/drop optical filter.
Photonic-crystal fiber (PCF) is a class of optical fiber based on the properties of photonic crystals. It was first explored in 1996 at University of Bath, UK. Because of its ability to confine light in hollow cores or with confinement characteristics not possible in conventional optical fiber, PCF is now finding applications in fiber-optic communications, fiber lasers, nonlinear devices, high-power transmission, highly sensitive gas sensors, and other areas. More specific categories of PCF include photonic-bandgap fiber, holey fiber, hole-assisted fiber, and Bragg fiber. Photonic crystal fibers may be considered a subgroup of a more general class of microstructured optical fibers, where light is guided by structural modifications, and not only by refractive index differences.
Surface plasmon resonance (SPR) is a phenomenon that occurs where electrons in a thin metal sheet become excited by light that is directed to the sheet with a particular angle of incidence, and then travel parallel to the sheet. Assuming a constant light source wavelength and that the metal sheet is thin, the angle of incidence that triggers SPR is related to the refractive index of the material and even a small change in the refractive index will cause SPR to not be observed. This makes SPR a possible technique for detecting particular substances (analytes) and SPR biosensors have been developed to detect various important biomarkers.
A dielectric mirror, also known as a Bragg mirror, is a type of mirror composed of multiple thin layers of dielectric material, typically deposited on a substrate of glass or some other optical material. By careful choice of the type and thickness of the dielectric layers, one can design an optical coating with specified reflectivity at different wavelengths of light. Dielectric mirrors are also used to produce ultra-high reflectivity mirrors: values of 99.999% or better over a narrow range of wavelengths can be produced using special techniques. Alternatively, they can be made to reflect a broad spectrum of light, such as the entire visible range or the spectrum of the Ti-sapphire laser. Mirrors of this type are very common in optics experiments, due to improved techniques that allow inexpensive manufacture of high-quality mirrors. Examples of their applications include laser cavity end mirrors, hot and cold mirrors, thin-film beamsplitters, high damage threshold mirrors, and the coatings on modern mirrorshades and some binoculars roof prism systems.
A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by creating a periodic variation in the refractive index of the fiber core, which generates a wavelength-specific dielectric mirror. Hence a fiber Bragg grating can be used as an inline optical fiber to block certain wavelengths, can be used for sensing applications, or it can be used as wavelength-specific reflector.
A distributed Bragg reflector (DBR) is a reflector used in waveguides, such as optical fibers. It is a structure formed from multiple layers of alternating materials with different refractive index, or by periodic variation of some characteristic of a dielectric waveguide, resulting in periodic variation in the effective refractive index in the guide. Each layer boundary causes a partial reflection and refraction of an optical wave. For waves whose vacuum wavelength is close to four times the optical thickness of the layers, the interaction between these beams generates constructive interference, and the layers act as a high-quality reflector. The range of wavelengths that are reflected is called the photonic stopband. Within this range of wavelengths, light is "forbidden" to propagate in the structure.
A slot-waveguide is an optical waveguide that guides strongly confined light in a subwavelength-scale low refractive index region by total internal reflection.
A colloidal crystal is an ordered array of colloidal particles and fine grained materials analogous to a standard crystal whose repeating subunits are atoms or molecules. A natural example of this phenomenon can be found in the gem opal, where spheres of silica assume a close-packed locally periodic structure under moderate compression. Bulk properties of a colloidal crystal depend on composition, particle size, packing arrangement, and degree of regularity. Applications include photonics, materials processing, and the study of self-assembly and phase transitions.
Multiphoton lithography of polymer templates has been known for years by the photonic crystal community. Similar to standard photolithography techniques, structuring is accomplished by illuminating negative-tone or positive-tone photoresists via light of a well-defined wavelength. The fundamental difference is, however, the avoidance of reticles. Instead, two-photon absorption is utilized to induce a dramatic change in the solubility of the resist for appropriate developers.
PhoSFOS is a research and technology development project co-funded by the European Commission.
A holographic sensor is a device that comprises a hologram embedded in a smart material that detects certain molecules or metabolites. This detection is usually a chemical interaction that is transduced as a change in one of the properties of the holographic reflection, either refractive index or spacing between the holographic fringes. The specificity of the sensor can be controlled by adding molecules in the polymer film that selectively interacts with the molecules of interest.
Optofluidics is a research and technology area that combines the advantages of fluidics and optics. Applications of the technology include displays, biosensors, lab-on-chip devices, lenses, and molecular imaging tools and energy.
Polyvinylcarbazole (PVK) is a temperature-resistant thermoplastic polymer produced by radical polymerization from the monomer N-vinylcarbazole. It is a photoconductive polymer and thus the basis for photorefractive polymers and organic light-emitting diodes.
Photonic molecules are a form of matter in which photons bind together to form "molecules". They were first predicted in 2007. Photonic molecules are formed when individual (massless) photons "interact with each other so strongly that they act as though they have mass". In an alternative definition, photons confined to two or more coupled optical cavities also reproduce the physics of interacting atomic energy levels, and have been termed as photonic molecules.
Ester H. Segal is an Israeli nanotechnology researcher and professor in the Department of Biotechnology and Food Engineering at the Technion - Israel Institute of Technology, where she heads the Laboratory for Multifunctional Nanomaterials. She is also affiliated with the Russell Berrie Nanotechnology Institute at the Technion - Israel Institute of Technology. Segal is a specialist in porous silicon nanomaterials, as well as nanocomposite materials for active packaging technologies to extend the shelf life of food.
A chemical sensor array is a sensor architecture with multiple sensor components that create a pattern for analyte detection from the additive responses of individual sensor components. There exist several types of chemical sensor arrays including electronic, optical, acoustic wave, and potentiometric devices. These chemical sensor arrays can employ multiple sensor types that are cross-reactive or tuned to sense specific analytes.