The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

New optical waveguide technology for integrated optics, based on propagation of long-range surface plasmon polaritons (LR-SPPs) along metal stripes embedded in dielectric, is presented. Guiding and routing of electromagnetic radiation along nanometer-thin and micrometer-wide gold stripes embedded in polymer via excitation of LR-SPPs is investigated in the wavelength range of 1250-1650 nm. LR-SPP guiding properties, such as the propagation loss and mode-field diameter, are investigated for different stripe widths and thicknesses. A propagation loss of /spl sim/6 dB/cm, a coupling loss of /spl sim/0.5 dB (per facet), and a bend loss of /spl sim/5 dB for a bend radius of 15 mm are evaluated for 15-nm-thick and 8-/spl mu/m-wide stripes at the wavelength of 1550 nm. LR-SPP-based 3-dB power Y-splitters, multimode interference waveguides, and directional couplers are demonstrated and investigated. At 1570 nm, coupling lengths of 1.9 and 0.8 mm are found for directional couplers with, respectively, 4- and 0-/spl mu/m-separated waveguides formed by 15-nm-thick and 8-/spl mu/m-wide gold stripes. LR-SPP-based waveguides and waveguide components are modeled using the effective-refractive-index method, and good agreement with experimental results is obtained.

/pdf/integrated-optical-components-utilizing-long-range-surface-ej45tsgl8i.pdf

Integrated optical components utilizing long-range surface plasmon polaritons

High-performance bioceramics are required for preventing failure and prolonging the life-time of bone grafting scaffolds and osseous implants. The proper identification and development of materials with extended functionalities addressing socio-economic needs and health problems constitute important and critical steps at the heart of clinical research. Recent findings in the realm of ion-substituted hydroxyapatite (HA) could pave the road towards significant developments in biomedicine, with an emphasis on a new generation of orthopaedic and dentistry applications, since such bioceramics are able to mimic the structural, compositional and mechanical properties of the bone mineral phase. In fact, the fascinating ability of the HA crystalline lattice to allow for the substitution of calcium ions with a plethora of cationic species has been widely explored in the recent period, with consequent modifications of its physical and chemical features, as well as its functional mechanical and in vitro and in vivo biological performance. A comprehensive inventory of the progresses achieved so far is both opportune and of paramount importance, in order to not only gather and summarize information, but to also allow fellow researchers to compare with ease and filter the best solutions for the cation substitution of HA-based materials and enable the development of multi-functional biomedical designs. The review surveys preparation and synthesis methods, pinpoints all the explored cation dopants, and discloses the full application range of substituted HA. Special attention is dedicated to the antimicrobial efficiency spectrum and cytotoxic trade-off concentration values for various cell lines, highlighting new prophylactic routes for the prevention of implant failure. Importantly, the current in vitro biological tests (widely employed to unveil the biological performance of HA-based materials), and their ability to mimic the in vivo biological interactions, are also critically assessed. Future perspectives are discussed, and a series of recommendations are underlined.

https://www.mdpi.com/1996-1944/11/11/2081/pdf

Cationic Substitutions in Hydroxyapatite: Current Status of the Derived Biofunctional Effects and Their In Vitro Interrogation Methods.

Vibration sensing is critical to monitor and ultimately preserve the health state of engineering systems. These systems with a large structure are typically working in some harsh environments including strong magnetic fields. However, traditional electrical sensors are difficult to accurately measure the vibration under harsh environments. Besides these instinct advantages of normal fiber optic sensors (FOS) sensors such as compact size, passive sensing, resistance to electromagnetic interference, etc., fiber Bragg grating (FBG) sensors have a capability of distributed sensing based on wavelength demodulation and resistance to light intensity fluctuation and unwanted fiber bending losses. Such merits lead them to be a hot topic in FOS field and excellent candidates for vibration sensing. Three types of FBG-based vibration sensors have been classified based on the difference of vibration-strain coupling way to FBG in this survey, which are pasted FBG-based, axial property of FBG-based and transverse property of FBG-based, respectively. FBG-based vibration sensors' principles and designs have been introduced and discussed. Recent advances in the applications of FBG-based vibration sensors have been investigated. The limitations and prospects of the FBG-based vibration sensing technologies have been analyzed and discussed.

Recent Advances and Tendency in Fiber Bragg Grating-Based Vibration Sensor: A Review

To facilitate patient healing in injuries and bone fractures, metallic implants have been in use for a long time. As metallic biomaterials have offered desirable mechanical strength higher than the stiffness of human bone, they have maintained their place. However, in many case studies, it has been observed that these metallic biomaterials undergo a series of corrosion reactions in human body fluid. The products of these reactions are released metallic ions, which are toxic in high dosages. On the other hand, as these metallic implants have different material structures and compositions than that of human bone, the process of healing takes a longer time and bone/implant interface forms slower. To resolve this issue, researchers have proposed depositing coatings, such as hydroxyapatite (HA), polycaprolactone (PCL), metallic oxides (e.g., TiO2, Al2O3), etc., on implant substrates in order to enhance bone/implant interaction while covering the substrate from corrosion. Due to many useful HA characteristics, the outcome of various studies has proved that after coating with HA, the implants enjoy enhanced corrosion resistance and less metallic ion release while the bone ingrowth has been increased. As a result, a significant reduction in patient healing time with less loss of mechanical strength of implants has been achieved. Some of the most reliable coating processes for biomaterials, to date, capable of depositing HA on implant substrate are known as sol-gel, high-velocity oxy-fuel-based deposition, plasma spraying, and electrochemical coatings. In this article, all these coating methods are categorized and investigated, and a comparative study of these techniques is presented.

https://www.mdpi.com/1996-1944/12/11/1795/pdf

Coating Techniques for Functional Enhancement of Metal Implants for Bone Replacement: A Review

While analytical techniques to study electric field distribution for an electrode-pair placed on two parallel opposite faces of a crystal (or channel) exist, but for the configuration with inclined opposite crystal faces such analysis is not straightforward. Here Schwarz-Christoffel mapping method, a special form of Conformal mapping technique, is used to study electric field distribution for parallel and non-parallel electrode-pair respectively. Being an analytical technique, the simulations are fast compared to numerical methods. A scheme on how variable polarization phase can be introduced when the electrode-pair is placed on the non-parallel faces of a specially designed trapezoidal shaped electro-optic lithium niobate crystal is also given in this work. It is also shown that by using multiple strip electrodes, tunability of the polarization phase difference can be achieved. Random optimization based on genetic algorithm has been used to optimize the device structure parameters to obtain desired phase shift.

Analysis of electric field for inclined electrodes and use of such configuration for generating tunable differential polarization phase

The translational symmetry of the periodicity in a photonic crystal can be disturbed by introducing a controlled defect in
its periodicity. The photon localization causes a pass band in the photonic bandgap. Based on this concept, we are proposing
the design of a tunable narrow band filter for multiple wavelengths used for coarse wave length division multiplexing
(CWDM) system. To achieve that, a multiple stack Fabry Perot structure with suitable stack materials and controllable
defect is considered. The proposed Fabry Perot structure consists of periodic layers of electro optic material Lithium
Niobate (nH) and Magnesium Fluoride (nL). The optical length of each such layer is their corresponding quarter wave
length width at the design wavelength λ 0 (1.55 μm). The reflection band of the quarter wavelength multilayer structure is
formed due to the periodic repetition of the (LH) N L, where L and H are the quarter wavelength width of the nL and nH
material respectively and N is the number of bilayers. A quarter wave layer L is then inserted between the groups of (LH) N 
and (HL) N to form the Fabry Perot resonator structure which can be used for narrow band transmission filter. The
simulation has been carried out in both FDTD and TMM method and compared. As the refractive index of the Lithium
Niobate can be tuned electro-optically, the filter can also be made tunable.

Design of multistack Fabry-Perot structure with defect as tunable transmission filter for CWDM using FDTD method

A waveguide based structure on silicon on insulator platform is proposed for vertical integration in photonic integrated circuits. The structure consists of two multimode interference couplers connected by a single mode (SM) section which can act as a bridge over any other underlying device. Two more SM sections acts as input and output of the first and second multimode couplers respectively. Potential application of this structure is in multilayered photonic links. It is shown that the efficiency of the structure can be improved by making some design modifications. The entire simulation is done using effective-index based matrix method. The feature size chosen are comparable to waveguides fabricated previously so as to fabricate the proposed structure easily.

Multilayered photonic integration on SOI platform using waveguide-based bridge structure

Abstract. A photonic crystal (PhC)-based structure is designed to achieve subdiffraction imaging over the entire visible wavelength range. The designed structure is a two-dimensional triangular lattice PhC formed by making circular holes in a 100-nm film of electro-optic lithium niobate (LiNbO3) grown on indium tin oxide. The analysis is done by using MEEP and MPB softwares. This shows that by applying a voltage within ±460  V, equal frequency contour can be obtained for specific values of frequencies, which corresponds to wavelength (λ) in visible range (389 to 757 nm). This indicates that the designed structure will have effective negative refraction and hence subdiffraction imaging over the entire visible wavelength (frequency) range. By placing this type of PhC structure in front of the objective, the resolution of the conventional imaging system can be increased.

Design of electro-optic photonic crystal structure for subdiffraction imaging over the entire visible wavelength range

A segmented silicon based multimode to dual port slot structure on silicon-on-insulator platform is proposed which can be used as a refractive-index sensing device. The introduction of segmentation leads to tuning the effective index of the device which results in increasing compactness of the sensing device. Although the structure supporting TM mode is more compact than TE mode, but TE mode is considered here as vertical slots in the output section enhances optical signal in the slots for TE mode only. By considering dual output, the device length is reduced further as dual self-imaging length is less compared to single self-imaging distance for symmetrical multimode section input. The surface sensitivity of the structure has a typical value of∼2249 nm/RIU. Relative sensitivity can be calculated from the ratio of field amplitudes of the arms of the dual output. Matrix method and 2D FDTD is used for the entire analysis.⁠

Rajib Chakraborty

Papers

Analysis of electric field for inclined electrodes and use of such configuration for generating tunable differential polarization phase

Design of multistack Fabry-Perot structure with defect as tunable transmission filter for CWDM using FDTD method

Multilayered photonic integration on SOI platform using waveguide-based bridge structure

Design of electro-optic photonic crystal structure for subdiffraction imaging over the entire visible wavelength range

Segmented multimode to dual port slot couplers as compact optical sensors with improved sensitivity