Optical tweezers enable non-destructive, contact-free manipulation of ultrasound contrast agent (UCA) microbubbles, which are used in medical imaging for enhancing the echogenicity of the blood pool and to quantify organ perfusion. The understanding of the fundamental dynamics of ultrasound-driven contrast agent microbubbles is a first step for exploiting their acoustical properties and to develop new diagnostic and therapeutic applications. In this respect, optical tweezers can be used to study UCA microbubbles under controlled and repeatable conditions, by positioning them away from interfaces and from neighboring bubbles. In addition, a high-speed imaging system is required to record the dynamics of UCA microbubbles in ultrasound, as their oscillations occur on the nanoseconds timescale.
In this work, we demonstrate the use of an optical tweezers system combined with a high-speed camera capable of 128-frame recordings at up to 25 million frames per second (Mfps), for the study of individual UCA microbubble dynamics as a function of the distance from solid interfaces.
We discuss a new technique to generate force gradient in arrays of optical traps. The arrays can be configured in two or three dimensions by means of phase diffractive optical elements displayed on a spatial light modulator. The design of the diffractive optical elements is based on the approach of spherical wave propagation and superposition, which enables to individually control the strength of each optical trap. Computer simulation and experimental results are discussed for two and three dimensional arrays of traps. An example with silica micro-beads trapped with different forces in two different planes is presented to demonstrate the validity of our approach.
We present and discuss a new experimental setup to perform small angle X-ray scattering and diffraction (SAXSD) of localized liposome colloidal microparticles. A home-built inverted infrared laser tweezers microscope is used to trap, manipulate and aggregate micron-scale liposome particles at single locations inside a 100 microns glass capillary. The micro-focused X-ray and the laser beams are aligned to intersect each other perpendiculary, allowing to associate the X-ray diffraction signal to the micron-sized region of interest inside the capillary. Throughout the laser tweezer setup, using diffractive optical elements implemented on a spatial light modulator, we are able to manipulate small aggregates of colloidal particles (liposomes) and fix them in the optical path of the X-ray beam.
We present and discuss first scattering and diffraction experiments on phospholipid liposomes, at the ID13 microfocus beamline of the European Synchrotron Radiation Facility (ESRF). The results demonstrate that we can push the limit of measurable cluster size close to a single liposome.
We present techniques to generate multiple vortex with different topological charge by means of diffractive optical
elements. Analytical formulae to describe the Fresnel and Fraunhofer diffraction of the Gaussian beam by a helical
axicon (HA) are introduced. The relations are presented as a series of the hypergeometric functions. By setting the
axicon parameter equal to zero, the solution for the HA changes to that for the spiral phase plate (SPP). The
performance of the aforesaid optical elements is tested both through computer simulation and by experiments using a
spatial light modulator, in view of optical miroparticle manipulation.
We present a unified view regarding the use of diffractive optical elements (DOEs) for microscopy applications a wide
range of electromagnetic spectrum. The unified treatment is realized through the design and fabrication of DOE through
which wave front beam shaping is obtained. In particular we show applications ranging from micromanipulation using
optical tweezers to X-ray differential interference contrast (DIC) microscopy. We report some details on the design and
physical implementation of diffractive elements that beside focusing perform also other optical functions: beam
splitting, beam intensity and phase redistribution or mode conversion. Laser beam splitting is used for multiple trapping
and independent manipulation of spherical micro beads and for direct trapping and manipulation of biological cells with
non-spherical shapes. Another application is the Gauss to Laguerre-Gaussian mode conversion, which allows to trap
and transfer orbital angular momentum of light to micro particles with high refractive index and to trap and manipulate
low index particles. These experiments are performed in an inverted optical microscope coupled with an infrared laser
beam and a spatial light modulator for DOEs implementation. High resolution optics, fabricated by means of e-beam
lithography, are demonstrated to control the intensity and the phase of the sheared beams in X-ray DIC microscopy.
DIC experiments with phase objects reveal a dramatic increase in image contrast compared to bright-field X-ray
We present an experimental system based on the use of a spatial light modulator which enables to perform simultaneously 3D optical manipulation and optical sectioning. This has been achieved by modifying the wave front of the trapping beam with properly diffractive optical elements displayed on a computer controlled spatial light modulator. We demonstrated the capability of the system in two experimental schemes, in a first one we performed a 3D optical scanning of 6 trapped beads by displacing the beads through a fixed imaging plane. In a second one we scan the imaging plane and simultaneously compensate for the movement of the objective in order to keep the trapping plane at a fixed position.
Since the low index particles are repelled away from the highest intensity point, trapping them optically requires either a rotating Gaussian beam or optical vortex beams focused by a high numerical microscope objective. However, the short working distance of these microscope objectives puts a limit on the depth at which these particles can be manipulated. Here, we show that axicon like structure built on tip of a single mode optical fiber produces a focused beam that is able to trap low index particles. In fact, in addition to transverse trapping inside the dark conical region surrounded by high intensity ring, axial trapping is possible by the balance of scattering force against the buoyancy of the particles. The low-index particle system consisted of an emulsion of water droplets in acetophenone. When the fiber was kept horizontal, the low index spheres moved away along the beam and thus could be transported
by influence of the scattering force. However in the vertical position (or at an angle) of the fiber, the particles could be trapped stably both in transverse and axial directions. Chain of such particles could also be trapped and transported together by translation of the fiber. Using escape force technique, transverse trapping force and thus efficiency for particle in Mie regime was measured. Details of these measurements and theory showed that trapping of Raleigh particle is possible with such axicon-tip fibers. This ability to manipulate low-index spheres inside complex condensed environments using such traps will throw new insights in the understanding of bubble-bubble and bubble-wall interactions, thus probing the physics behind sonoluminescence and exploring new applications in biology and medicine.
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.
We have developed a holographic optical tweezers system based on diffractive optical elements (DOES) implemented
on a liquid crystal spatial light modulator (LC-SLM) able to generate fine positioned traps on the sample. Our own
algorithms and code allows to calculate phase DOES that can transform a single laser beam into an array of independent
traps, each with individually specified characteristics, arranged in arbitrary three-dimensional (3D) geometrical
configurations. Different DOEs can be dynamically projected to the SLM in order to achieve a rearrangement of the
configuration of the trapping spots. Silica or latex micro-beads are trapped in different configurations of spots to
demonstrate the fine control capability on each trap. Our setup is built on a standard video microscope coupled with a
laser source, a spatial light modulator and a three axis nano-positioning system. It allows to obtain 3D multi-trapping
and a fine calibration for the positioning of the traps.
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.
The goal of our study is to develop a setup that combines multi-trapping and manipulation with micro Raman spectroscopy of microns size particles. Multiple trapping, in 2D or 3D (two or three dimensional) configurations, is obtained in an inverted microscope scheme by shaping the trapping beam (1064 nm) with diffractive optical elements implemented on a spatial light modulator (SLM). Manipulation of multiple particles, directly trapped by the beam, can be achieved using the dynamic displaying of the SLM. Indirect trapping and manipulation of the sample can be obtained surrounding it with trapped micro beads that are manipulated by the optical tweezers. Laser light is not directly focused on the sample but is distributed on the beads and therefore the photo-induced damaging of biological samples is reduced. This technique offers also other advantages: the sample can be kept in a stable position during the spectroscopic investigation or can be moved in x-y-z to get spatial resolved information in a scanning mode measurement and the shape of a deformable sample can be changed in a controlled manner during the measurement. Sample's excitation and Raman signal collection are accomplished with a separate laser beam (514.5 nm) in a non-inverted microscope coupled with the spectrometer. Some experimental results showing multi trapping and indirect manipulation of human red blood cells 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.
The use of diffractive optical elements (DOEs) for multiple trapping of dielectric micro-spheres immersed in a fluid has been demonstrated recently. When the DOEs are implemented on a spatial light modulator (SLM), the trapped particles can be independently moved by changing the configuration of the DOE. In this paper we demonstrate phase DOEs implemented on an optical addressable SLM to move an array of trapped particles in a volume of about 20x20x6 μm. Experimental results show the usefulness of this technique for particle micromanipulation in biology.
Trapping and manipulation of microparticles using optical tweezers is usually performed within a sample cell formed by two parallel microscope cover slides. In this paper we discuss and demonstrate trapping and manipulation conditions when the cell has more complex configurations like microchannels or capillary tubes. The microchannels are fabricated on the surface of the cover slide by means of lithographic techniques. Experimental results of trapping and micromanipulation for silica microspheres and biological samples immersed in water show the usefulness of our study for microfluidics and biological applications.