Reduced blood deformability is clinically linked to several diseases. It is important to develop sensitive tools to measure the loss of blood deformability. The evanescent field of an optical waveguide can trap and propel red blood cells along the waveguide. Here we propose to use the evanescent field from a narrow optical waveguide to trap and deform red blood cells. We demonstrate that the intensity gradient of the evanescent field at the edge of narrow waveguides (1-3 μm) can be used to squeeze a blood cell. The RBCs are squeezed to a size comparable to the waveguide width. When the laser is switched on the cell is attracted towards the waveguide and is held in place. Subsequently, the part of the cell not on the waveguide is pulled in across the waveguide. The result is a cell (7-8 μm in diameter) squeezed down to a significantly smaller width (typically 3 μm). The cell regains its original shape when laser is switched-off.
An integrated optical sensor is developed for measuring gas concentration for subsea and atmospheric applications. The optical sensor is based on a waveguide Mach-Zehnder interferometer (MZI). In MZI, the light is spilt into a sensing and a reference arm and after a certain distance the branches are recombined. The sensing branch is covered with a sensitive layer that has high affinity towards a specified gas. The presence of the gas gives a change in the refractive index of the sensing arm, which is translated into a change in the output signal. With a prior calibration, the change in the output signal is correlated to the gas concentration. The waveguide should be single-mode and it is desirable to have high intensity in
the evanescent field. By using a high refractive index material and a thin waveguide core, the intensity of the evanescent field can be enhanced. Simulations are performed to obtain waveguide parameters with low losses and high sensitivity. The maximum sensitivity at wavelength 785 nm was obtained for a waveguide of core thickness 150 nm, rib height 5 nm and width 1 m for TM polarization. The first measurements of phase sensitivity of 12456π rad/RIU was obtain by the Hydrogen Chlorine (HCl) measurement. This is comparable to the phase sensitivity of 14268π rad/RIU obtained by the simulation.μ
Waveguide Mach-Zehnder interferometers (MZI) have been successfully used for a number of sensing applications due to their high sensitivity. As the MZI-sensors have a built-in reference, they are ideally not dependent on temperature variations. However, due to their high sensitivity, a temperature difference or an imbalance between the two arms, can give an unwanted temperature dependence of the output signal. Here, we present an experimental study of the temperature sensitivity of waveguide MZI. Three designs, balanced MZI, unbalanced MZI and tapered MZI are studied. In order to investigate the temperature sensitivity of the interferometer, we measure the phase shift at the output of the interferometer as function of temperature. It is found that the balanced MZI is very stable with temperature. For an unbalanced MZI having a 1 mm length difference between the two arms, a small phase shift is observed. The phase shift was less than one period for a 10°C change. The phase shift can be precisely determined for the tapered MZI. Initial measurements of the temperature sensitivity for a balanced, tapered MZI, gave output variations of some 150° for the phase, for a temperature change of 10°C. This corresponds to a temperature difference of 2.6 mK within the oil covering the two arms and demonstrates how sensitive the device is to temperature differences between the sensing and the reference arms.
We propose to optically trap nanoparticles utilizing a single nanostructured glass-fiber tip. 3D translation of optically trapped nanoparticles - nano tweezers - presents vast application possibilities and has not yet been shown. The input end of the fibre probe is a standard fibre, providing easy coupling to a light source. The output end is tapered down and covered with gold, with a nanoaperture fabricated on the tip. The nanoaperture provides the strong field gradient necessary for trapping of nanoparticles. We discuss probe geometries supported by numerical simulations. The fabrication procedure for the fibre probe, using a focused ion beam, is described. A set-up for the experiments has been made and preliminary trapping results are presented.
The evanescent field from an optical waveguide is used for near-field trapping and transporting of fluorescent microspheres. Out-of-focus fluorescence imaging is used to track the trapped particle in 3-D with nanometer precision (<100 nm). A prior calibration is done to determine the relationship between the z-coordinate and the radius of the outermost diffraction ring in the image of the sphere. This gives precise information about how much the particle moves up and down during propulsion along the waveguide. Results are presented for trapping and tracking a 1 μm fluorescent particle on a strip waveguide.
The evanescent field from a waveguide can be used to trap and propel a particle. An optical waveguide loop with an
intentional gap at the center is used for planar transport and stable trapping of particles. The waveguide acts as a
conveyor belt to trap and deliver spheres towards the gap. At the gap, the counter-diverging light fields hold the sphere
at a fixed position. Numerical simulation based on the finite element method was performed in three dimensions using a
computer cluster. The field distribution and optical forces for rib and strip waveguide designs are compared and
discussed. The optical force on a single particle was computed for various positions of the particle in the gap. Simulation
predicted stable trapping of particles in the gap. Depending on the gap separation (2-50 μm) a single or multiple spheres
and red blood cells were trapped at the gap. Waveguides were made of tantalum pentaoxide material. The waveguides
are only 180 nm thick and thus could be integrated with other functions on the chip.
A three dimensional finite element method is used to model the forces acting on red blood cells trapped on
an optical waveguide surface. The parameters are chosen to correspond to strip waveguides made of tantalum
pentoxide (Ta2O5). A wavelength of 1070 nm is used and the cells are taken to be spherical. Gradient and
scattering forces experienced by the cells are studied and found to be highly dependent on the refractive index
of the cells. Gradient forces are found to be one order of magnitude larger than scattering forces. Only the lower
part of the cells is in contact with the evanescent field of the waveguide. For low refractive indices, we find that
the lower 0.5-1 μm of the cells is sufficient to determine the optical forces. For the cell sizes considered, all forces
increase with the size.
Three dimensional finite element method is employed to determine optical trapping forces on hollow glass spheres on
an optical waveguide. The evanescent field from the waveguide interacts mostly with the shell of the hollow glass
sphere. We describe how the optical forces vary with shell thickness and particle size, and find the minimum shell
thickness allowing trapping and propulsion of hollow glass spheres. Hollow glass spheres with shell thickness less than
the minimum are repelled away from the optical waveguide. The simulation results are compared with analytical Mie
calculations and experimental data.
Design, fabrication and optimization of high refractive index (2.1 @ 1070 nm), sub-micron thickness (200 nm) Tantalum
Pentoxide waveguides is reported. Optimization of fabrication parameters reduces the propagation loss to ~ 1 dB/cm @
1070 nm for Ta2O5 waveguides. Ta2O5 waveguides were found to be stable for high power application with no significant
absorption peaks over a large range of wavelengths (600-1700 nm). Ta2O5 waveguides provide high intensity in the
evanescent field, which is useful for efficient optical propelling of micro-particles. We have employed Ta2O5 waveguide
to propel polystyrene micro-particles with 50 μm/s velocity.
The high-refractive index contrast (▵n ~0.65 as compared to silicon oxide) of Tantalum pentoxide (Ta2O5) waveguide
allows strong confinement of light in waveguides of sub-micron thickness (200 nm). This enhances the intensity in the
evanescent field, which we have employed for efficient propelling of micro-particles. The feasibility of opto-fluidics
sorting of different sized micro-particles based on their varying optical propulsion velocity is suggested. Optical
propulsion of fixed red blood cells (RBC) with velocity higher than previously obtained is also reported. The optical
propulsion velocities of RBC in isotonic solution (0.25 M sucrose) and water have been compared.
In a dielectric waveguide, the optical power is confined mostly in the core of the waveguide, where the refractive index is highest. Outside of the core the field is evanescent, i.e., the field strength decreases exponentially with the distance from the core. This evanescent field can be used to manipulate microparticles. For a particle with index of refraction higher than that of the surrounding medium (water), the optical forces due to the evanescent field act to guide the particle along the waveguide. The use of waveguides to trap particles combines the possibilities of conventional optical tweezers with the techniques employed in integrated optics, and it has the added advantage of integration of several functions on a single chip. We have experimentally observed size-dependent trapping and propulsion at velocities up to 33μm/s of polystyrene spheres, of diameters between 3 and 12μm, and in propulsion of 0.25μm diameter gold spheres at velocities up to 500μm/s. A Y-junction with a multimode input waveguide has been used to sort particles. By moving the input fibre relative to the input waveguide, the light goes into one of the two output branches. We have shown that this principle can be used to sort polystyrene microbeads. Recently we have used counter-propagating waves to move particles in both directions and also to stop a particle at a precise location. Experimental results and simulations for polystyrene microbeads, yeast cells and gold particles are presented.
Optical microsphere resonators, with their exceptionally low optical losses and high Q-factors, are attracting a lot of interest in integrated optics and related fields. Not being accessible by free-space beams, whispering gallery modes (WGM) of a microsphere resonator require near-field coupler devices. Efficient evanescent coupling has been demonstrated previously by using thin tapered fibres, fibre half-block couplers, angle-polished fibres and bulk prisms. In this work, we demonstrate WGM excitation in microspheres, from 8 to 15 μm in diameter, by using an integrated optics channel waveguide. Light from a tunable laser was coupled into a single mode K+ ion-exchanged channel waveguide formed in BK7 glass substrate. Dry borosilicate glass microspheres were dispersed on the substrate surface. Polystyrene microspheres were suspended in electrolyte water solution and confined in a closed cell on top of the waveguide. The light was coupled to the particles sitting on the waveguide surface. The scattered light was observed through the microscope. As the laser wavelength was tuned, the observed images were recorded with a CCD camera. WGM excitation was observed through the increased scattered light intensity at certain wavelengths. In the case of glass microspheres and a Ti:Sapphire tunable laser, the obtained resonance quality (Q-) factors were about 400. The resonances observed in polystyrene microspheres using a tunable diode laser had lower Q-factors and were deteriorating with decreasing particle size.