Sunita Ahlawat

Optical orientation and rotation of trapped red blood cells with Laguerre-Gaussian mode

We report the use of Laguerre-Gaussian (LG) modes for controlled orientation and rotation of optically trapped red blood cells (RBCs). For LG modes with increasing topological charge the resulting increase in size of the intensity annulas led to trapping of the cells at larger tilt angle with respect to the beam axis and thus provided additional control on the stable orientation of the cells under trap. Further, the RBCs could also be driven as micro-rotors by a transfer of orbital angular momentum from the LG trapping beam having large topological charge or by rotating the profile of LG mode having fractional topological charge.

Hemoglobin degradation in human erythrocytes with long-duration near-infrared laser exposure in Raman optical tweezers

Near-infrared laser (785-nm)-excited Raman spectra from a red blood cell, optically trapped using the same laser beam, show significant changes as a function of trapping duration even at trapping power level of a few milliwatts. These changes in the Raman spectra and the bright-field images of the trapped cell, which show a gradual accumulation of the cell mass at the trap focus, suggest photoinduced aggregation of intracellular heme. The possible role of photoinduced protein denaturation and hemichrome formation in the observed aggregation of heme is discussed.

Three dimensional optical twisters-driven helically stacked multi-layered microrotors

We demonstrate tunable helically stacked multi-layered microrotors realized in vortex-embedded three dimensional (3D) optical twister patterns. Intensity-tunable annular irradiance profiles with higher order vortex are generated as well as simultaneously unfolded by phase-engineered multiple plane wave interference. In the individually tunable 3D helical bright arms of these unfolded vortex structures, 2 μm silica beads are optically trapped as spiraling multilayered handles of multi-armed microrotors. Further, multiple rows of such microrotors are parallelly actuated with controllable sense of rotation. We also present our observation on helical 3D stacking of micro-particles in these longitudinally gyrating multi-armed rotor traps.

Studies on erythrocytes in malaria infected blood sample with Raman optical tweezers

Raman spectroscopy was performed on optically trapped red blood cells (RBCs) from blood samples of healthy volunteers (h-RBCs) and from patients suffering from P. vivax infection (m-RBCs). A significant fraction of m-RBCs produced Raman spectra with altered characteristics relative to h-RBCs. The observed spectral changes suggest a reduced oxygen-affinity or right shifting of the oxygen-dissociation curve for the intracellular hemoglobin in a significant fraction of m-RBCs with respect to its normal functional state.

Trapping of micron-sized objects at a liquid–air interface

The limited working distance of the high numerical aperture microscope objectives used in conventional optical tweezers makes it difficult to trap objects at the liquid-air interface. Since with a weakly focused optical beam the gradient forces are not sufficient to overcome the axial scattering force, we investigated the possibility of the use of surface tension forces generated when the object is pushed against the liquid-air interface by a weakly focused optical beam to balance the axial scattering force. In contrast to the expected trapping of objects at the focal point of the trap beam the objects were observed to get trapped in an annular region about the trap beam. The experimental results and their analysis reveal that, apart from optical and surface tension forces, the laser-induced heating of the interface and the resulting thermocapillary effect are responsible for the observed trapping of objects.

Raman spectroscopic investigations on optical trap induced deoxygenation of red blood cells

Raman spectroscopic investigations on the oxygenation status of optically trapped red blood cells show that the cellular site in the trap beam is more deoxygenated compared to the rest of the cell, and the level of deoxygenation increases with an increase in the trap beam power. These observations and the changes in the Raman spectrum of hemoglobin solution as a function of the trapping beam power suggest that observed deoxygenation may be due to photodissociation of oxygen from hemoglobin at increased trapping power.

Optical sorting in holographic trap arrays by tuning the inter-trap separation

Particle motion through a holographic trap array has been investigated theoretically and experimentally, and it is shown that a change in inter-trap separation can be used to selectively control the motion of particles of different sizes. By an appropriate choice of inter-trap separation in a holographically generated two-dimensional trap array, optical potential channels can be created in orthogonal directions such that, from a suspension having a mixture of two different particle sizes, the particles can be sorted in the two orthogonal channels. The use of the approach to sort 3 and 5 μm silica spherical particles in the two orthogonal channels, from a mixed suspension of these, has also been demonstrated.

Microfluidic sorting with a moving array of optical traps

Optical sorting was demonstrated by selective trapping of a set of microspheres (having specific size or composition) from a flowing mixture and guiding these in the desired direction by a moving array of optical traps. The approach exploits the fact that whereas the fluid drag force varies linearly with particle size, the optical gradient force has a more complex dependence on the particle size and also on its optical properties. Therefore, the ratio of these two forces is unique for different types of flowing particles. Selective trapping of a particular type of particles can thus be achieved by ensuring that the ratio between fluid drag and optical gradient force on these is below unity whereas for others it exceeds unity. Thereafter, the trapped particles can be sorted using a motion of the trapping sites towards the output. Because in this method the trapping force seen by the selected fraction of particles can be suitably higher than the fluid drag force, the particles can be captured and sorted from a fast fluid flow (about 150  μm/s). Therefore, even when using a dilute particle suspension, where the colloidal trafficking issues are naturally minimized, due to high flow rate a good throughput (about 30  particles/s) can be obtained. Experiments were performed to demonstrate sorting between silica spheres of different sizes (2, 3, and 5 μm) and between 3 μm size silica and polystyrene spheres.

Optical trapping near a colloidal cluster formed by a weakly focused laser beam

We report on the results of our studies on the stable trapping of colloidal particles by a cluster of microspheres formed using a weakly focused trapping laser beam. Since the observed trapping action was believed to arise due to interference of the coherently scattered optical fields from this multiparticle cluster, experiments were carried out on a one-dimensional chain of polystyrene microspheres, which is simpler to model. A chain of three microspheres, which was generated using an elliptical trap beam, was observed to trap a microsphere at a distance of about ~3 µm on its symmetry axis. This is in reasonable agreement with the trapping sites predicted based on the calculated intensity pattern of the coherently scattered optical fields from this multiparticle cluster.

Long-distance axial trapping with Laguerre–Gaussian beams

We show that the axial spread of the focal volume of a tightly focused beam propagating through a glass-water interface is much reduced for Laguerre-Gaussian (LG) modes as compared to the TEM(00) mode. Therefore, use of the LG beam helps in achieving a significant improvement of the axial trapping range in optical tweezers. We demonstrate the use of LG modes to manipulate biological cells from the bottom layer of the medium to the top surface layer. Exposure of the cells to a higher oxygen concentration at the surface layer is used for estimation of the intramembrane oxygen diffusion rate.