Interactions between trapped microspheres have been studied in two geometries so far: (i) using line optical tweezers and (ii) in traps using two counter propagating laser beams. In both trap geometries, the stable inter bead separations have been attributed to optical binding. One could also trap two such beads in a single beam Gaussian laser trap. While there are reports that address this configuration through theoretical or simulation based treatments, there has so far been no detailed experimental work that measures the interactions.
In this work, we have recorded simultaneously the fluctuation spectra of two beads trapped along the laser propagation direction in a single Gaussian beam trap by measuring the back scattered signal from the trapping and a tracking laser beam that are counter propagating . The backscattering from the trapping laser monitors the bead encountered earlier in the propagation path. The counter propagating tracking laser, on the other hand, is used to monitor the fluctuations of the second bead. Detection is by using quadrant photo detectors placed at either end. The autocorrelation functions of both beads reveal marked departures from that obtained when there is only one bead in the trap. Moreover, the fall-off profiles of the autocorrelation indicates the presence of more than one relaxation time. This indicates a method of detecting the presence of a second bead in a trap without directly carrying out measurements on it. Further, a careful analysis of the relaxation times could also reveal the nature of interactions between the beads.
Optical Tweezers are capable of trapping individual particles of sizes that range from micrometers to sub
micrometers. One can compute the trap strength experienced by a particle by analyzing the fluctuations in the
position of the trapped particle with time. It is reported that the trap strength of a dielectric bead increases linearly
with increase in the power of the trapping laser. The situation with metallic particles, however, is strongly dependent
on the particle size. Available literature shows that metallic Rayleigh particles experience enhanced trap strengths
when compared to dielectric particles of similar sizes due to a larger polarizability. On the contrary, micrometer
sized metallic particles are poor candidates for trapping due to high reflectivity. We report here that commercially
available micrometer sized metal oxide core - dielectric shell (core – shell) beads are trapped in a single beam optical
tweezer in a manner similar to dielectric beads. However as the laser power is increased these core – shell beads are
trapped with a reduced corner frequency, which represents a lowered trap strength, in contrast to the situation with
ordinary dielectric beads. We attribute this anomaly to an increase in the temperature of the medium in the vicinity
of the core – shell bead due to an enhanced dissipation of the laser power as heat. We have computed autocorrelation
functions for both types of beads at various trapping laser powers and observe that the variation in the relaxation
times with laser power for core - shell beads is opposite in trend to that of ordinary dielectric beads. This supports
our claim of an enhanced medium temperature about the trapped core – shell bead. Since an increase in temperature
should lead to a change in the local viscosity of the medium, we have estimated the ratio of viscosity to temperature
for core – shell and dielectric beads of the same size. We observe that while for ordinary dielectric beads this ratio
remains a constant with increasing laser power, there is a decrease for core – shell beads. We plan to extend this
work towards studying the hydrodynamic correlations between a pair of trapped beads where one of the beads acts as
a heat source.
The phase of a negative axicon is combined with that of a Fresnel zone lens (FZL) to obtain an element labelled as
conical FZL, which can generate a focused ring pattern at the focal plane of the FZL. The phase integration is achieved
by modifying the location and width of zones of FZL in accordance with the phase variation of the negative axicon. The
element was designed for a high power laser with a wavelength of 1064 nm, focal length and diameter of conical FZL of
30 mm and 8 mm respectively and for a ring diameter of 50 μm. The element was fabricated using photolithography. The
pattern was transferred from the resist layer to the borosilicate glass plates by dry etching to achieve an etch depth of
1064 nm. The etch depth measured using confocal microscope was 1034 nm at the central part and 930 nm for the
outermost part of the device with a maximum error of 12.5% at the outermost part and 3% at the central part. The
element was used in an optical trapping experiment. The ring pattern generated by the conical FZL was reimaged into the
trapping plane using a tightly focusing microscopic objective. Polystyrene beads with diameters of 3 μm were
suspended in deionized distilled water at the trapping plane. The element was found to trap multiple particles in to the
A normal human red blood cell (RBC) when trapped with a linearly polarized laser, reorients about the electric polarization direction and then remains rotationally bound to this direction. This behavior is expected for a birefringent object. We have measured the birefringence of distortion-free RBCs in an isotonic medium using a polarizing microscope. The birefringence is confined to the cell’s dimple region and the slow axis is along a diameter. We report an average retardation of 3.5±1.5 nm for linearly polarized green light (λ=546 nm). We also estimate a retardation of 1.87±0.09 nm from the optomechanical response of the RBC in an optical trap. We reason that the birefringence is a property of the cell membrane and propose a simple model attributing the origin of birefringence to the phospholipid molecules in the lipid bilayer and the variation to the membrane curvature. We observe that RBCs reconstituted in shape subsequent to crenation show diminished birefringence along with a sluggish optomechanical response in a trap. As the arrangement of phospholipid molecules in the cell membrane is disrupted on crenation, this lends credence to our conjecture on the origin of birefringence. Dependence of the birefringence on membrane contours is further illustrated through studies on chicken RBCs.
A microscopic object finds an equilibrium orientation under a laser tweezer such that a maximum of its volume lies in the region of highest electric field. Furthermore, birefringent microscopic objects show no rotational diffusion after reorienting under a linearly polarized optical trap and also are seen to follow the plane of polarization when the latter is changed using a half wave plate. We observe that a healthy human Red Blood Cell (RBC) reproduces these observations in an optical tweezer, which confirms it to be birefringent. Polarization microscopy based measurements reveal that the birefringence is confined to the cell’s dimple region and the mean value of retardation for polarized green light (λ = 546nm) is 9 ± 1.5nm. We provide a simple geometrical model that attributes the birefringence to the nature of arrangement of the phospholipid molecules of the bilayer. This predicts the observed variation in the measured birefringence, from the dimple to the rim of the cell which we further show, can serve to demarcate the extent of the dimple region. This points to the value of birefringence measurements in revealing cell membrane contours. . We extend this technique to understand the birefringence of a chicken RBC, an oblate shaped cell, wherein the slow axis is identified to be coincident with the long axis of the cell. Further, we observe the birefringence to be confined to the edges of the cell. Experiments to probe the optomechanical response of the chicken RBC are in progress.
We report here on studies of reorientation of human red blood cells (RBCs) in an optical trap. We have measured the time required, t re , for the plane of the RBC entering the optical trap to undergo a 90-deg rotation to acquire an edge on orientation with respect to the beam direction. This has been studied as a function of laser power, P , at the trap center. The variation of t re with increasing P shows an initial sharp decrease followed by a much smaller rate of further decrease. We find that this experimentally measured variation is not in complete agreement with the variation predicted by a theoretical model where the RBC is treated as a perfectly rigid circular disk-like body. We argue that this deviation arises due to deformation of the RBC. We further reason that this feature is dominated by the elastic behavior of the RBC membrane. We compare the studies carried out on normal RBCs with RBCs where varying conditions of membrane stiffness are expected. We propose that the value of energy used for maximum deformation possible during a reorientation process is an indicator of the membrane elasticity of the system under study.