Small displacements of a microparticle in an optical trap can be measured using back focal plane interferometry. The
position of the particle is evaluated by analyzing the fringes pattern obtained by interference between the light scattered
by the particle and unscaterred light in the back focal plane of the condenser. The fringes positions are detected with a
quadrant photo diode, allowing nanometric precision. In this paper we analyze theoretically some parameters that may
influence the measurements: laser power fluctuations, local fluid viscosity, condenser focal length, particle size.
In this work we present a numerical evaluation of the forces in an optical tweezers system, for metallic nanoparticles in the
Rayleigh regime. Initially a Gaussian beam is described in the scalar approximation, and the forces it can apply on Rayleigh
dielectric and metallic particles are computed within the point-dipole approach. The method is then extended to dielectric
and metallic Rayleigh particles in a Laguerre-Gaussian beam, i.e. a higher order beam that is increasingly used for optical
trapping experiments. We discuss the limits of the approximation for the beam intensity by comparing the numerical results
with the experimental measurements that can be found in literature.
A strongly focused laser beam through an objective microscope with high NA allows the trapping of dielectric particles
with micrometric sizes. The trapping force is proportional to the power of the laser, the relative refractive index (the ratio
between the refractive index of the particle and the refractive index of the medium surrounding it) and the trapping
geometry (shape of the laser beam, shape of the particle, transmission and reflection coefficients). Numerical models to
evaluate the trapping force can be developed for simple geometrical shapes of the trapped particle. For particles with
complicated shapes the trapping force should be measured experimentally. The goal of this paper is to evaluate a
measurement method based on the equilibrium between the drag force in a fluid with known viscosity and the transversal
trapping force. A particle with a known size is fist trapped in a cell filled with water. After stable trapping, the cell is
shifted with controlled velocities using piezoelectric actuators. If the velocity exceeds a certain threshold, the particle
escapes from the trap. This threshold allows to determine the trapping force. Experimental results obtained with high and
low index particles are presented and discussed.
Higher-order laser beams were demonstrated to enable optical manipulation of low-index-particles. In this work single-ringed Laguerre-Gaussian beams, obtained by means of phase-only diffractive optical elements, are used to perform manipulation of phospholipid-shelled gas microbubbles in water. Implementation of diffractive optical elements on a programmable spatial light modulator allows to generate also arrays of Laguerre-Gaussian traps. We show manipulation of low-index particles by properly displaying a suitable sequence of diffractive optical elements. Control over the distance between the trapped particles in real time is also demonstrated.
Deformable mirror is a popular device in adaptive optics. The characterization of this deformable mirror becomes important in a closed loop system. A shack Hartmann based wavefront sensor using a proper combination of lenslet array and a CCD camera has been used for this purpose. The typical process sequence includes, acquisition of reference and distorted image, calculation of centroide for both images, calculation of shift along X and Y axis, calculation of slope data. Finally reconstruction of the wavefront to calculate phase is performed. Integrated software based on Windows 9X platform has been developed for all above steps. This paper describes a novel technique used in this software. A multithreaded approach is used to perform above steps in parallel. Threads are introduced to acquire image from CCD camera, to calculate slope and phase data for each captured image and to display real time graphs viz. Slope matrix, 3D-phase matrix. Phase matrix has been calculated based on standard least square iterative method.
Due to the involvement of shorter time and energy consumption, rapid thermal processing (RTP) technique finds its success in Semiconductor processing. The Al-Si ohmic contact has been formed and the effect of alloying temperatures between 450 degree(s)C and 550 degree(s)C on this contact has been studied. The nature of the contact is probed well by positron lifetime spectroscopy (PLTS). The measurements are supported by I - V characterization and SEM.
Rapid Thermal Processing (RTP) technique finds its success in microelectronics due to shorter time and energy cosumption than conventional furnace processing. There is no further impurity redistribution which is of great interest for VLSIs researchers. This is mainly because of shorter time scale. Rapid heating and cooling in the process may give rise to defects. These RTP-induced defects have been first time probed by Positron Annihilation Technique(PAT). Positron annihilation methods have been proved as a powerful tool for studing defects in solids. The increasing interest in technique stems from its defects sensitive and non destructive nature. Positron lifetime measurements on samples silicon dioxide grown by furnace methods and RTA are presented and discussed. The data obtained from PAT gives information regarding nature of defects in material. 1 .
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.