Analysis of trapped microscopic objects using fluorescence and Raman spectroscopy is gaining considerable interest. We report on the development of single fiber ultrafast optical tweezers and its use in simultaneous two-photon fluorescence (TPF) excitation of trapped fluorescent microscopic objects. Using this method, trapping depth of a few centimeters was achieved inside a colloidal sample with TPF from the trapped particle being visible to the naked eye. Owing to the propagation distance of the Bessel-like beam emerging from the axicon-fiber tip, a relatively longer streak of fluorescence was observed along the microsphere length. The cone angle of the axicon was engineered so as to provide better trapping stability and high axial confinement of TPF. Trapping of the floating objects led to stable fluorescence emission intensity over a long period of time, suitable for spectroscopic measurements. Furthermore, the stability of the fiber optic trapping was confirmed by holding and maneuvering the fiber by hand so as to move the trapped fluorescent particle in three dimensions. Apart from miniaturization capability into lab-on-a-chip microfluidic devices, the proposed noninvasive microaxicon tipped optical fiber can be used in multifunctional mode for in-depth trapping, rotation, sorting, and ablation, as well as for two-photon fluorescence excitation of a motile sample.

Polypropylene, poly(ethylene terephthalate), ethylene chloro tetrafluoroethylene, ethylene tetrafluoroethylene, and epoxy vinyl ester resin (Derakane 470-300) were evaluated in aqueous HCl containing chlorine gas at high temperature as a corrosion media. Fourier transform infrared, X-ray diffraction, energy-dispersive X-ray, and dynamic mechanical analyzer are used for identification of nature of chemical reactions on polymer chain. Puncture resistance and hardness tests were done to evaluate the mechanical strength after the exposure. The scanning electron microscopy image was taken to check the morphological change of polymer surface. Chlorination and oxidation reactions were observed to be responsible for the stability behavior of polymer. A mechanism proposed for both chlorination and oxidation on polymer.

Reorientation of adhering cell(s) with respect to other cell(s) has not been yet possible, thus limiting study of controlled interaction between cells. Here, we report cell detachment upon irradiation with a focused near-infrared laser beam, and reorientation of adherent cells. The detached cell was transported along the axial direction by scattering force and trapped at a higher plane inside the media using the same laser beam by a gravito-optical trap. The trapped cell could then be repositioned by movement of the sample stage and reoriented by rotation of the astigmatic trapping beam. The height at which the cell was stably held was found to depend on the laser beam power. Viability of the detached and manipulated cell was found not to be compromised as confirmed by propidium iodide fluorescence exclusion assay. The reoriented cell was allowed to reattach to the substrate at a controlled distance and orientation with respect to other cells. Further, the cell was found to retain its shape even after multiple detachments and manipulation using the laser beam. This technique opens up new avenues for noncontact modification of cellular orientations that will enable study of intercellular interactions and design of engineered tissue.

Atomic Force Microscope (AFM) imaging, due to the scanning method of recording, requires significant recording time for examination of wide sample area. In contrast, digital holographic microscopy (DHM), owing to the wide-field method, allows recording of the hologram in very fast rate which could be numerically analyzed to reveal surface of the sample with axial resolution at the nanometer scale. However, DHM yields quantitative phase properties of the sample, and therefore sensitive to changes in refractive index along with physical thickness. Therefore, to accurately determine the refractive index map, it is imperative to estimate the physical thickness map of the sample. This was achieved by AFM imaging. Further, since the transverse resolution of DHM is limited by diffraction limit, co-registration of AFM image provided higher transverse resolution at nanometer scale. The interference of the AFM probe was observed to be minimal during simultaneous AFM and DHM recording due to the transparent nature and bent configuration of the optical fiber based AFM cantilever. Integration of DHM and AFM led to realization of a powerful platform for nanoscale imaging. The integrated AFM-DHM system was built on an inverted fluorescence microscope to enable fluorescence imaging of the sample. The integrated system was employed to analyze fluorescent polystyrene microspheres, two-photon polymerized microstructures and red blood cells.

No abstract

Two photon polymerization (TPP) has enabled three-dimensional microfabrication with sub-diffraction limited spatial resolution. However, depth at which TPP could be achieved, has been limited due to the high numerical aperture microscope objective, used to focus the ultrafast laser beam. Here, we report fiber-optic two photon polymerization (FTP) for in-depth fabrication of microstructures from a photopolymerizable resin. A cleaved single mode optical fiber coupled with tunable femtosecond laser could achieve TPP, forming extended waveguide on the fiber itself. The length of the FTP tip was found to depend on the laser power and exposure duration. Microfabricated fiber tip using FTP was employed to deliver continuous wave laser beam on to polystyrene microspheres in order to transport and manipulate selected particles by scattering force and 2D trapping. Such microstructures formed by TPP on tip of the fiber will also enable puncture and micro-surgery of cellular structures. With use of a cleaved fiber or axicon tip, FTP structures were fabricated on curved surfaces at large depth. The required Power for FTP and the polymerization rate was faster while using an axicon tip optical fiber. This enabled fabrication of complex octopus-like microstructures.

Irradiating carbon nanoparticles (CNPs) with near-infrared laser beam leads to generation of heat, therefore it has potential to be used in many applications including the destruction of cancer cells. Though pulsed laser beams have been used earlier to transform shapes of metallic and semiconductor nanoparticles, changing shape of CNPs required intense electron beam irradiation. In this paper, we report significant size reduction of CNPs under continuous-wave (cw) near-infrared (NIR) laser beam micro-irradiation which was attributed to melting and vaporization or fragmentation of the carbon nanoparticles. Further, we show that the spherical shape of the CNPs can be transformed into ellipsoidal, by exposure to cw NIR laser microbeam irradiation for a few seconds. In-situ measurements using atomic force microscopy (AFM) reveal the shape and size changes of the CNPs upon laser micro-irradiation. Most importantly, cw NIR laser microbeam irradiation led to ultra-structural phase transformation of CNPs as detected via Raman spectroscopic imaging. While the graphitic CNPs could be changed to diamond-like carbon (DLC), no phase change in DLC nanoparticles was observed. These transformations did not require presence of any special chemical (catalyst, functionalization) or physical (pressure, temperature) arrangement. In-situ control of CNP-size, shape and ultra-structural properties opens new possibilities in multiple nanotechnology adventures.

No abstract