Sudipta Sarkar Pal

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.

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.

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.

Influence of confinement on single-electron charging in a network of nanoparticles

We investigated the single-electron tunneling (SET) behavior in a network of ligand stabilized Au nanoparticles (NPs) that are self-organized on an Au(111) surface by means of low-temperature scanning tunneling microscopy and spectroscopy. We demonstrate that for a proper combination of ligand chain length and NP radius the ligand shell is able to isolate a particle from the neighboring ones. This results in SET spectra with a clear Coulomb blockade and a regular staircase, similar to SET spectra obtained for isolated particles. A fraction of the investigated particles exhibits additional fine structure on top of the Coulomb charging peaks in the tunneling conductance spectra. The origin of the fine structure can be related to quantum size effects due to the very small NP size rather than to inter-particle capacitive coupling. Our findings indicate the possibility of using an individual particle in the self-organized network as the central Coulomb island in a double-barrier tunnel junction configuration, ...

Magnetron Sputtering Coated Optical Fiber Probe Designs for Surface Plasmon Resonance Sensor

In this paper, we have reported the design of the surface plasmon resonance probes prepared from partially etched polymer optical probe and specially tapered single-mode fiber tip probe which are gold coated in magnetron-sputtering unit. The coating parameters and conditions are discussed in details to achieve sensitive surface plasmon resonance probes. The sensor probes work in transmission modes. The gold-coated etched in-line polymer fiber probe demonstrates high sensitivity, ~2459 nm/RIU and working range includes visible to infrared wavelength. The fiber tip probe with very small interaction area demonstrates sensitivity, ~166 nm/RIU. The in-line fiber probe could be used for large volume sample and the pointed fiber tip could be used for small volume samples.

Image processing in frequency domain for common path optical coherence tomography (CP-OCT)

CP-OCT is a single arm OCT technique which offer significant advantages over dual arm OCT. These techniques are used for presenting a cross sectional view of 3D structure and inner layers of the structure especially turbid in nature. Image processing is a major challenge in OCT instrumentation. This paper focuses on the imaging process on acquired power spectrum of the collected data from a novel optical fiber tip as a CP-OCT probe. Image enhancement techniques applied to these data sets improve the visualization of the image. The work is done by using image processing and signal processing toolbox of MATLAB vR2010. The purpose of this paper is only to show improvement in the visualization of the cross sectional image of a sample by applying mentioned techniques for advantageous CPOCT environment. Arbitrarily, cello tape is chosen as a sample as it contains more number of layers so that it will be easy to distinguish the visualization quality in the outcome images.

Study of FBG sensors for cryo temperature monitoring

Experimental characterization of a fiber Bragg grating sensor probe has been carried out for cryo-temperature monitoring. The temperature sensitivity was found to be 16.7 pm/°C at room temperature and 90 pm/°C pm/°C near liquid nitrogen temperature.

Optical fiber tip: Nanotweezer, nanoantenna and plasmonic hotspot

We have introduced a technique to grow novel optical fiber tip (OFT) which can be widely used in nano-photonics research. It takes help of capillary action and surface tension meniscus of hydrofluoric acid (HF) around the fiber during etching process to grow ultrafine OFT. The optical fiber used has different core material, which is etched at higher rate in HF compared to its cladding. The optical properties of the probe makes the tip very attractive as nano-optical tweezer, optical nanoantenna, probe for surface enhanced and tip enhanced Raman scattering (SERS and TERS) etc. The nano structure is also very effective for higher harmonic generation. To validate our claim we have explored few applications of the OFT, such as Bessel beam generation, second harmonic generation (SHG), optical tweezer and optical nanoantenna generating plasmonic hotspot.