Rajib Chakraborty

Bio: Rajib Chakraborty is an academic researcher from University of Calcutta. The author has contributed to research in topics: Lithium niobate & Coating. The author has an hindex of 10, co-authored 62 publications receiving 288 citations. Previous affiliations of Rajib Chakraborty include Indian Institute of Technology Kharagpur.

Analysis of photonic spot profile converter and bridge structure on SOI platform for horizontal and vertical integration

Abstract: Horizontal spot size converter required for horizontal light coupling and vertical bridge structure required for vertical integration are designed on high index contrast SOI platform in order to form more compact integrated photonic circuits. Both the structures are based on the concept of multimode interference. The spot size converter can be realized by successive integration of multimode interference structures with reducing dimension on horizontal plane, whereas the optical bridge structure consists of a number of vertical multimode interference structure connected by single mode sections. The spot size converter can be modified to a spot profile converter when the final single mode waveguide is replaced by a slot waveguide. Analysis have shown that by using three multimode sections in a spot size converter, an Gaussian input having spot diameter of 2.51 μm can be converted to a spot diameter of 0.25 μm. If the output single mode section is replaced by a slot waveguide, this input profile can be converted to a flat top profile of width 50 nm. Similarly, vertical displacement of 8μm is possible by using a combination of two multimode sections and three single mode sections in the vertical bridge structure. The analyses of these two structures are carried out for both TE and TM modes at 1550 nm wavelength using the semi analytical matrix method which is simple and fast in computation time and memory. This work shows that the matrix method is equally applicable for analysis of horizontally as well as vertically integrated photonic circuit.

Semianalytical approach to study the loss characteristics of a symmetrical multilayered plasmonic waveguide.

Abstract: A metal waveguide of finite width and thickness surrounded by a dielectric provides increased propagation length of the surface plasmon polariton and is therefore called long-range surface plasmon polaritons (LR-SPP). In this work, a new structure is proposed by modifying the refractive index of the dielectric surrounding the metal waveguide, leading to further improvement of the propagation length. It is shown that, when the single dielectric surrounding is replaced by a multilayered surrounding where each layer has different thicknesses and refractive index, the propagation loss gets reduced, leading to increased propagation length of the LR-SPP. The propagation loss is calculated semianalytically from the FWHM of the Lorentzian peak obtained in the plot of excitation efficiency of such waveguide for different values of the propagation constant. Before doing this calculation, the 2D variation of refractive index is first converted into a 1D effective refractive index. All the steps of analysis are discussed in detail, and, wherever necessary, the calculated results are matched with similar results of other researchers.

Superlensing property of 2-D glass photonic crystal

Abstract: A two dimensional square lattice photonic crystal (PhC) formed by placing glass matrix in air is proposed for subwavelength imaging around 840 nm. The superlensing behavior at relatively lower wavelength compared to other reported PhC superlens is obtained by this configuration. Other advantages of using glass is that they have lower optical absorption at this wavelength and is relatively cheap. By placing the proposed PhC arrangement between the object and the objective of a conventional optical microscope, superlensing effect can be realized. Moreover, any change in radius of glass rod during fabrication process can result in the shift of superlensing wavelength.

Investigation of IR pyrometer-captured thermal signatures and their role on microstructural evolution and properties of Inconel 625 tracks in DED-based additive manufacturing

Abstract: In Laser Directed Energy Deposition (DED-L) process, based on the particular thermal cycle followed during the laser processing, a wide range of microstructural and dimensional outcomes are envisaged in the end product on account of the interaction of the material or the alloy with the laser. Various parameters can influence the thermal cycle and a direct correlation between the thermal characteristics with the mechanical/metallurgical properties would be helpful to control the process in a better way. Keeping in view of this, the present study is aimed at analyzing thermal signals received from infrared pyrometer and understanding the correlation between thermal signatures, phase prediction data using CALPHAD simulation, microstructure and deposition characteristics. The thermal signatures captured from infrared pyrometer revealed close interrelationship between the input process parameters and the molten pool characteristics, which have a huge influence on the solidification rate vis-à-vis microstructural properties. Total life of melt-pool showed an increase in magnitude from 137 ms to 264 ms with decrease in cooling rates from 3000 °C/s to 1750 °C/s. The grain size increased from 3.48 μm to 7.74 μm with increase in solidification shelf life from 32 ms to 110 ms. The higher solidification shelf life and lower cooling rates also resulted in the larger and the more prominent metal carbide precipitate and intermetallic formations as evident from XRD analysis . Microhardness of the deposits also increased with decrease in solidification shelf life and increase in cooling rates of molten pool. The dimensions of laser deposited tracks were also found to increase with increment in total molten pool lifetime both at increasing laser power and decreasing laser scan peed conditions. • Inconel 625 tracks were deposited at constant power, scan speed and fluence conditions by DED-L. • Shift in solidification shelf life indicated dilution of Fe from SS 304 L substrate. • Increment of solidification shelf life at constant fluence was detected. • Solidification shelf life and cooling rate influenced microstructure and microhardness. • 244 % increase in shelf life led to 122 % increase in grain size at constant scan speed. • Increase in track cross-sectional dimension at constant fluence condition was observed.

Reduction of Degradation of Solar Cells Using One Dimensional Photonic Crystal Based UV Filters

Abstract: Efficiency degradation of solar cell over time is most common incident occurs in practice. Active material loses its stable carrier generation rate results in degradation in conversion efficiency after a long interaction with UV in both inorganic as well as organic solar cells. In this paper, we proposed an UV filter based on one dimensional periodic thin film structure. The effectiveness of the designed UV filter is also analyzed for most simple planer silicon thin film solar cell structure.

Photonic crystals

Abstract: 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.

Integrated optical components utilizing long-range surface plasmon polaritons

Abstract: 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.

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

Abstract: 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.

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

Abstract: 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.

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

Abstract: 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.