Samir K. Mondal

Optical fiber nano-tip and 3D bottle beam as non-plasmonic optical tweezers

We report a simple fiber nano-tip as non-plasmonic optical tweezer, which can manipulate submicron particles in a non-contact manner. The efficiency of an optical tweezer can be enhanced by using non-diffracting type optical beams such as Bessel beam or self-imaged Bessel beam (3D bottle beam). The present work, for the first time, realizes a non-plasmonic optical tweezer based on a miniaturized axicon like single-mode optical fiber nano-tip. The tip generates non-diffracting type 3D bottle beam by virtue of its changing wedge angle. The nano-tip is prepared from a photosensitive single-mode optical fiber by employing a novel chemical etching technique. We experimentally demonstrate trapping of ~60 nm silver particle and ~160 nm silica particle using this nano-tip optical tweezer. The nano-tweezer also succeeds to pick up the particles from aqueous solution. The proposed nano-tweezer working at smaller laser powers opens new avenues for nanomanipulation and analysis of sub-microscale specimens in the biological and physical sciences.

Optical fiber nanoprobe preparation for near-field optical microscopy by chemical etching under surface tension and capillary action

We propose a technique of chemical etching for fabrication of near perfect optical fiber nanoprobe (NNP). It uses photosensitive single mode optical fiber to etch in hydro fluoric (HF) acid solution. The difference in etching rate for cladding and photosensitive core in HF acid solution creates capillary ring along core-cladding boundary under a given condition. The capillary ring is filled with acid solution due to surface tension and capillary action. Finally it creates near perfect symmetric tip at the apex of the fiber as the height of the acid level in capillary ring decreases while width of the ring increases with continuous etching. Typical tip features are short taper length (approximately 4 microm), large cone angle (approximately 38 degrees ), and small probe tip dimension (<100 nm). A finite difference time domain (FDTD) analysis is also presented to compare near field optics of the NNP with conventional nanoprobe (CNP). The probe may be ideal for near field optical imaging and sensor applications.

Fiber Bragg grating sensor for monitoring bone decalcification

INTRODUCTION Estimation of decalcification is a vital tool to discern bone health. Different techniques are used for its quantitative measurement, e.g. DEXA, QCT & QUS. All these techniques, although noninvasive, suffer from limitations such as radiation exposure and inaccurate values. Recently, fiber optic techniques are fast emerging for medical applications owing to their various attractive features like immunity to EMI/RFI, geometric versatility, chemical inertness, etc. MATERIAL AND METHODS The effect of decalcification on strain response of a goat tibia was investigated in vitro using fiber Bragg grating (FBG) sensing technique. The bone was strained by using three-point bending technique and corresponding Bragg wavelength shifts were recorded. Two similar bone samples from the same animal were taken and one was partially decalcified. Strain response of decalcified and untreated bone was taken concurrently to monitor the effects of calcium loss and that of degradation with time. RESULTS AND CONCLUSION The strain generated for same stress increased with greater degree of decalcification and a steep increase occurred after 2g calcium loss, indicating the onset of damage. The strain response, therefore gives a direct indication of the degree of calcium present in the bone. LEVEL OF EVIDENCE Level III.

Single and Multiple Microparticle Trapping Using Non-Gaussian Beams From Optical Fiber Nanoantennas

Optical trapping of dielectric microparticles is reported using an optical tweezers based on two original chemically etched fiber nanoantenna. The nanoantenna converts Gaussian beam into nondiffracting type quasi-Bessel beam, which is used in trapping microparticles. Stable trapping in three distinct positions is observed for an antenna distance of 32.5 μm and for light powers as low as 1.3 mW. Optical trapping properties are studied by applying Boltzmann statistics to the particle position fluctuations. Harmonic trapping potentials with trap stiffness of 3.5 pN μm-1 are observed. The FDTD simulation results on the antenna optics are also included to understand the trapping mechanism.

Etched multimode microfiber knot-type loop interferometer refractive index sensor

We propose a novel refractive index sensor based on multimode microfiber knot-type loop (NL) interferometer. The middle portion (~5 cm) of a 15 cm long multimode fiber is etched in 48% hydrofluoric acid to reduce its diameter to ~12 μm. A NL of diameter <1 mm is made from the etched fiber. The ends of etched fiber are spliced with single-mode fibers for launching and detecting light from the NL interferometer. The NL introduces path differences to produce interferometric spectra with free spectral range ~16 nm. The spectrum shifts as the surrounding refractive index of the loop is changed by adding chemicals. We observe the highest sensitivity of the NL interferometer ~172 nm/RIU (refractive index unit) at a refractive index value 1.370 as obtained experimentally using commonly available chemicals. The design could be used as simple, low cost, and highly sensitive biological and chemical sensor.

Ultrafine Fiber Tip Etched in Hydrophobic Polymer Coated Tube for Near-Field Scanning Plasmonic Probe

We propose a novel one-step etching technique for the preparation of an optical fiber tip using a hydrophobic polymer coated tube (HP) containing 48% hydrofluoric acid (HF) to etch photosensitive single-mode fiber (PSMF). Dipping of PSMF in an HP tube suppresses the convex meniscus (CM) of the HF solution in the tube due to formation of a concave surface tension meniscus (SM) around the PSMF. The continuous etching reduces the diameter of PSMF and shortens the SM along PSMF resulting in elevation of CM around PSMF. The elevation of the CM and faster etching rate of the PSMF core digs a capillary ring along the fiber core-cladding boundary. Capillary action fills the ring with HF, which finally recedes and creates a tip at the apex of the fiber. We have fabricated a typical tip with typical features of tip height ~ 4 μm, base diameter ~ 3 μm , cone angle ~ 25°, and aperture ~ 30 nm. For the theoretical analysis, a tip similar to the fabricated one is modeled to estimate surface plasmon (SP) enhancement, ~ 200, at the end of the tip with a silver coating.

Single fiber Bragg grating sensor with two sections of different diameters for longitudinal strain and temperature discrimination with enhanced strain sensitivity

A single fiber Bragg grating (FBG) sensor with two sections of different diameters is proposed and experimentally demonstrated for discrimination and measurement of strain and temperature. A section of single FBG is etched in hydrofluoric acid solution to reduce diameter of the fiber by factor of <1/2 to increase its strain sensitivity. Different shifts of the Bragg wavelengths of chemically etched and nonetched gratings caused by different strain sensitivities are used to discriminate and measure strain and temperature. Maximum errors of +/-13 microepsilon (microstrain) and +/-1 degrees C are reported over 1700 microepsilon and 60 degrees C measurement ranges, respectively. Depending upon the diameter of the etched fiber grating, the design can also discriminate nanostrain from temperature.

Evanescent wave assisted nanomaterial coating.

In this work we present a novel nanomaterial coating technique using evanescent wave (EW). The gradient force in the EW is used as an optical tweezer for tweezing and self-assembling nanoparticles on the source of EW. As a proof of the concept, we have used a laser coupled etched multimode optical fiber, which generates EW for the EW assisted coating. The section-wise etched multimode optical fiber is horizontally and superficially dipped into a silver/gold nanoparticles solution while the laser is switched on. The fiber is left until the solution recedes due to evaporation leaving the fiber in air. The coating time usually takes 40-50 min at room temperature. The scanning electron microscope image shows uniform and thin coating of self-assembled nanoparticles due to EW around the etched section. A coating thickness <200 nm is achieved. The technique could be useful for making surface-plasmon-resonance-based optical fiber probes and other plasmonic circuits.

Optical fiber antenna generating spiral beam shapes

A simple method is proposed here to generate vortex beam and spiral intensity patterns from a Gaussian source. It uses a special type of optical fiber antenna of aperture ∼80 nm having naturally grown surface curvature along its length. The antenna converts linearly polarized Gaussian beam into a beam with spiral intensity patterns. The experimentally obtained spiral patterns with single and double spiral arms manifest the orbital angular momentum, l = ±1, 2, carried by the output beam. Such beam can be very useful for optical tweezer, metal machining, and similar applications.

Temperature compensated transverse load sensor based on dual FBG sensor

A temperature compensation method of measuring transverse load on a beam using fiber Bragg grating (FBG) sensors is proposed. FBG sensors have been used to measure strain at two positions as a function of load on a cantilever. The theoretical analysis was also carried out under the same conditions and the experimental results were in close agreement with the theoretical predictions. The temperature effect on FBGpsilas can be cancelled out due to their close proximity and this method can be utilized for transverse load measurement in civil structures with temperature compensation.