Trapping and controlled rotation of low-refractive-index particles using dual line optical tweezers

We show that dual line optical tweezers provides a convenient and dynamically reconfigurable approach for trapping and transport of low refractive index microscopic particles. By varying the spacing between the two line tweezers, particles of varying sizes could be trapped. Further, simultaneous rotation of the dual line tweezers could be used for controlled rotation of the trapped low-index particles. The transverse trapping force and the efficiency of the trap measured along the direction perpendicular to the line tweezers are in very good agreement with the theoretically estimated value. PACS 07.60.-j; 87.80.Cc; 87.80.Fe     

Three-dimensional orientation of microscopic objects using combined elliptical and point optical tweezers

We report the development of a simple approach for controlled three-dimensional orientation of microscopic objects. The approach makes use of the fact that whereas in an elliptical trap, an object lacking spherical symmetry orients with its long axis along the major axis of the elliptical tweezers, in point tweezers it orients with its long axis along the trapping beam. Therefore, a combined use of point and elliptical tweezers and a control over the relative power of the two trapping beams can be used for three-dimensional orientation of the object. The use of this approach for three-dimensional orientation of biological objects varying in size from 2 to 20 μm is discussed. PACS 07.60.-j; 87.80.Cc     

Three-dimensional dynamic optical manipulation by combining a diffractive optical element and a spatial light modulator

Since the introduction of computer-controlled spatial light modulators (SLMs), holographic optical tweezers have become an important tool for dynamic parallel optical manipulation. In this paper we clarify the usefulness of a new configuration for optical trapping that creates light patterns using the combination of a diffractive optical element (DOE) and an SLM. This configuration not only enables the use of the higher part of the SLM’s diffraction efficiency curve, because a simple hologram can be chosen for the SLM, but also achieves three-dimensional dynamic optical manipulation over a large spatial range. By switching blaze-like holograms displayed on the SLM, we demonstrated simultaneous transportation of three 6-μm-diameter polystyrene beads over a range of 90 μm in the vertical direction and 37.5 μm in the horizontal direction. Compared with the same manipulation executed using only the SLM, the range of this method is extended four-fold in the vertical direction and three-fold in the horizontal direction.     

Probing of pair interaction of magnetic microparticles with optical tweezers

Magnetic interaction of paramagnetic Brownian submicron-sized particles is studied by optical tweezers technique. Correlation analysis allows one to extract magnetic interaction of two particles 0.4 μm in size, which are optically trapped at the distance of 3 μm one from each other and placed in a static magnetic field of 30 Oe, from the background of their Brownian motion. The magnetic interaction force is estimated to be of approximately 100 fN. Two configurations of the mutual orientation of the magnetic field vector and the line connecting two centers of optical traps are used in the experiment. For field vector orientation parallel/perpendicular to this line, the magnetic interaction is detected by the cross-correlation function increase/decrease in comparison with the absence of magnetic field on the time scales of 1 ms.     

Fast position measurements with scanning line optical tweezers

Scanning line optical tweezers are a powerful tool for the study of colloidal or biomolecular systems in the low-force regime. We present a fast, high-resolution particle position measurement scheme that extends the capabilities of these instruments into the realm of dynamic measurements. The technique is based on synchronous detection of forward-scattered laser light during a line scan. We demonstrate a position resolution of better than 50 nm for bandwidths of as much as 40 kHz for pairs of microspheres trapped in a flat line potential at center-to-center separations of 1.7-6 microm.     

Pearl-chain waveguides written at megahertz repetition rate

Waveguides in the form of connected pearls, written with sub-30 fs pulses centered at 800 nm at a repetition rate of 10 MHz, show high changes of the refractive index and, as a consequence, simple in-coupling. The value of the refractive index modification of these waveguides is as high as 10^-2, optical losses are 6 dB/cm (at 1.55 μm) and the mode field diameter is 8 μm at 670 nm. PACS 42.62.-b; 87.80.Mj; 42.82.Et     

微米级锥形光纤的近场光镊

分别从理论和实验上分析了光纤表面倏逝场强度的分布(z=10 nm, 100 nm, 500 nm,1 000 nm),研究了微米级光纤光镊对微球的操纵。实验中把直径为125 m的普通单模光纤拉制成锥腰直径为2 m的锥形光纤。当光纤通光时,在光纤锥区倏逝场的作用下,直径3 m的聚苯乙烯微球保持平衡状态,并且光纤附近的微球被吸引到光纤表面,以5.3 m /s的速度沿着光束的传播方向运动。这个实验不仅实现了对微球的成功捕获,而且验证了光纤光镊的力学作用。光纤光镊对微球的无接触、无损伤操纵,将在生物传感领域有潜在的应用。     

Light-driven oscillations of entangled nematic colloidal chains

Laser tweezers have been used to drive the oscillations of a chain of entangled colloidal particles in the nematic liquid crystal 5CB. The amplitude and phase of light-driven oscillations have been determined for the motion of individual colloidal particles. The collective motion of 4.8μm silica particles is highly damped for a driving frequency above 0.5Hz. The results were compared to an effective bead-spring model, where the motion of elastically coupled particles is hindered by viscous damping and hydrodynamic coupling. Qualitative agreement between theory and experiment was obtained.     

Heterogeneous integration of InGaAsP microdisk laser on a silicon platform using optofluidic assembly

Heterogeneous integration of InGaAsP microdisk lasers on a silicon platform is demonstrated experimentally using an optofluidic assembly technique. The 200-nm-thick, 5- and 10-μm-diameter microdisk lasers are fabricated on InP and then released from the substrates. They are reassembled on a silicon platform using lateral-field optoelectronic tweezers (LOET). The assembled laser with 5-μm diameter exhibits a threshold pump power of 340 μW at room temperature under pulse condition. The heterogeneously-integrated InGaAsP-on-Si microdisk laser could provide the much needed optical source for CMOS-based silicon photonics. The small footprint and low power consumption make them attractive for optical interconnect applications. The optofluidic assembly technique enables efficient use of the III–V epitaxial materials in silicon photonic integrated circuits.     

Rotation rate of a three-wing rotor illuminated by upward-directed focused beam in optical tweezers

The optical torque and the trapping position (focal point) in optical tweezers are analyzed for upward-directed focused laser illumination using a ray optics model, considering that laser light is incident at not only the lower surface but also the side surface of a 3-wing rotor. The viscous drag force due to the pressure and the shearing stress on all surfaces of the rotor is evaluated using computational fluid dynamics. The rotation rate is simulated in water by balancing the optical torque with the drag force, resulting in 500 rpm for an SU-8 rotor with 20 μm diameter at a laser power of 200 mW. The trapping position is estimated to be 7.6 μm in the rotor with an upward-directed laser at 200 mW via an objective lens having a numerical aperture of 1.4. Both the rotation rate and the trapping position agree well with the values obtained in the experiment.     

Microfluidic sorting with blinking optical traps

It is shown that by appropriately choosing the periodicity of a blinking optical trap only larger sized colloidal spheres can be selectively trapped out of a mixed population. This happens because smaller sized, more agile, spheres escape out of the trap volume during the off period of the trap beam. Therefore, by scanning an array of blinking traps over a mixed sample, bigger spheres can be forced to move with the traps and eventually could be taken to the output side. Experimental demonstration of sorting between 1 μm and 2 μm diameter silica spheres is presented.     

Quantification of glucose levels in flowing blood using M-mode swept source optical coherence tomography

The increase of glucose levels in blood changes the viscosity of flowing fluids and shape of the erythrocytes. Both of these can affect the details of light scattering as can be quantified via decorrelation times measured by optical coherence tomography (OCT). The relative contributions of these competing effects have been studied by examining the motion dynamics of deformable asymmetrical (red blood cells, RBCs with ∼7 μm diameter and ∼2 μm thickness) and non deformable symmetrical (polystyrene microspheres, PSM with 1.4 μm diameter) flowing scattering particles. The fluid flow under the action of gravity was modulated by changing the glucose concentrations. Quantitative analysis of the OCT’s M-mode autocorrelation functions enabled the derivations of the translational diffusion coefficients. These systematic studies are aimed at eventual tissue imaging scenarios with speckle-variance OCT to obtain local glucose concentrations maps.     

Optical tweezers for confocal microscopy

In confocal laser scanning microscopes (CLSMs), lasers can be used for image formation as well as tools for the manipulation of microscopic objects. In the latter case, in addition to the imaging lasers, the light of an extra laser has to be focused into the object plane of the CLSM, for example as optical tweezers. Imaging as well as trapping by optical tweezers can be done using the same objective lens. In this case, z-sectioning for 3D imaging shifts the optical tweezers with the focal plane of the objective along the optical axis, so that a trapped object remains positioned in the focal plane. Consequently, 3D imaging of trapped objects is impossible without further measures. We present an experimental set-up keeping the axial trapping position of the optical tweezers at its intended position whilst the focal plane can be axially shifted over a distance of about 15 μm. It is based on fast-moving correctional optics synchronized with the objective movement. First examples of application are the 3D imaging of chloroplasts of Elodea densa (Canadian waterweed) in a vigorous cytoplasmic streaming and the displacement of zymogen granules in pancreatic cancer cells (AR42 J). Received: 24 March 2000 / Revised version: 23 June 2000 / Published online: 11 October 2000     

Picosecond optical switching of a 1.5 μm active birefringent optical fiber loop filter using 1.3 μm control optical pulses and a 1.3 μm semiconductor optical amplifier

Less than 100ps, polarization-independent switching operation of an active birefringent optical fiber loop filter using 1.3 μm control optical pulses as well as a 1.3 μm semiconductor optical amplifier (SOA) has been demonstrated. In the proposed SOA-based active birefringent filter operating at 1.55 μm wavelength, 1.3 μm SOA is employed to control the polarization-mode dispersion in the loop part. By injecting 1.3 μm ps gain-switched optical control pulses into the SOA, 1.5 μm input signals can be switched from the transmission port to the reflection port with less than 100 ps rise time.     

Influence of particle size and concentration on the second-harmonic signal generated at colloidal surfaces

Second harmonic generation (SHG) spectroscopy is a recently developed technique for the investigation of surface properties of particles. To apply the method to technical colloidal systems, the dependences of several experimental parameters on the signal have to be studied. In this work the influence of particle concentration on the SHG signal from the surfaces of colloids (polystyrene beads in a size range of 0.1 μm to 2.9 μm) is investigated. A simple model, based on Lambert–Beer’s law, to describe the measured dependences is derived. The model agrees with the experimental observations for particles smaller 1.1 μm and with a small modification also for larger particles. Based on the new model an analytical equation for determining the optimum concentration, where highest signals in colloidal SHG spectroscopy measurements are obtained, is derived. PACS 42.25.Fx; 42.65.-k; 82.70.Dd     

Optical forces and torques in nonuniform beams of light

The spin angular momentum in an elliptically polarized beam of light plays several noteworthy roles in optical traps. It contributes to the linear momentum density in a nonuniform beam, and thus to the radiation pressure exerted on illuminated objects. It can be converted into orbital angular momentum, and thus can exert torques even on optically isotropic objects. Its curl, moreover, contributes to both forces and torques without spin-to-orbit conversion. We demonstrate these effects experimentally by tracking colloidal spheres diffusing in elliptically polarized optical tweezers. Clusters of spheres circulate deterministically about the beam's axis. A single sphere, by contrast, undergoes stochastic Brownian vortex circulation that maps out the optical force field.     

Advances toward MR microscopy of single biological cells

Here we report further progress toward the goal of achieving proton magnetic resonance imaging microscopy with resolution approaching a few micrometers in all three dimensions. We obtain proton images of a phantom sample — a microcapillary containing water and 39 μm diameter polymer microspheres — with a resolution of a few micrometers (perhaps about 5 μm) in all three spatial dimensions.     

A vision chip with column-level amplification of optical data signals for indoor optical wireless local area networks

We present a vision chip architecture with column-level photo-amplification of optical data signals for optical wireless local area networks (LANs) to reduce the pixel area. Based on the architecture, we have fabricated a prototype vision chip in a standard 0.8 μm bipolar complementary metal-oxide-semiconductor (BiCMOS) technology. The device offers position detection of other optical transceivers in the LAN and 4-ch concurrent high-speed optical data acquisition. A data rate of 400 Mbps was demonstrated. The pixel size was 125 μm square, which can be shrunk to smaller than around 60 μm square in 0.35 μm or more advanced BiCMOS or CMOS technologies.     

Ring beam shaping optics fabricated with ultra-precision cutting for YAG laser processing

In this study, a method for generating ring intensity distribution at a refraction-type lens with an aspheric element was proposed, and the beam shaping optical element was finished using only ultra-precision cutting. The shape of the optical element and its irradiance pattern were determined from numerical calculation based on its geometrical and physical optics. An ultra-precision lathe was employed to fabricate beam shaping optical elements, and acrylic resin was used as the material. The transmittance of an optical element (a rotationally symmetrical body) with an aspheric surface fabricated using a single-crystal diamond tool was over 98%, and its surface roughness was 9.6 nm Ra. The method enabled the formation of a circular melting zone on a piece of stainless steel with a thickness of 300 μm through pulse YAG laser (λ 1:06 μm) processing such that the average radius was 610 μm and the width was 100–200 μm. Circular processing using a ring beam shaping optical element can be realized by single-pulse beam irradiation without beam scanning.     

Raman scattering enhancement by dielectric spheres

Raman scattering experiments were performed on Si(60 nm)/metal/substrate structures with and without silica microspheres (with a diameter between 0.5 and 5 µm) on top. Raman scattering from the thin Si layer exhibits enhancements (~20) due to the dielectric spheres, where the enhancement factors depend on the diameter of the spheres. The interaction between light and dielectric spheres has been simulated by finite difference time domain calculations (FDTD), wherein particularly the electric energy density (ED) distribution in the thin Si layer was of concern. For microspheres with a diameter less than ~3 µm, the transverse ED distribution (perpendicular to the incident light direction) within the Si layer is characterised by a single peak centered on the optical axis. For larger diameters, a multimodal transverse ED distribution develops where the maximum is not centered on the optical axis. Using an ad‐hoc approach for surface enhanced Raman scattering in combination with the FDTD calculations, the experimental Raman observations are well accounted for. Copyright © 2013 John Wiley & Sons, Ltd.