Photonic nanojets typically require a spherical object onto which a laser beam is illuminated. The presence of the spherical object creates a near-field nanojet where light is emitted in the vicinity of the sphere. We explore the nanojet emanating from various diameters of the spheres and find that two different regimes can be present. In the first regime, the sphere is small when the transverse dimension of the nanojet is smaller than the diffraction limit while the axial dimension is diffraction limited, as has been recently shown. In addition, we find another regime where the sphere diameter is much larger than wavelength, when the axial dimension of the nanojet is now smaller than the diffraction limit keeping the transverse dimension the same as diffraction limit. This yields very high numerical apertures. We also show that we can optically confine particles with a axial corner frequency which is a factor of three larger than regular optical trapping with 100x objective due to the subdiffractive axial nanojet.
Atomic Force Microscopes (AFM) with 10 nm tip is employed to estimate work of adhesion at nano-scale. The AFM tip is pressed against the surface with forces around a few nano-Newtons and retracted back until it breaks from the surface. Thus estimating the work of adhesion due to this technique can be termed as “hard probing” of the surface. Whereas, we propose another configuration in which a spherical particle is trapped near the surface using a linearly polarized light and the particle attaches to the surface by work of adhesion. Here, by moving the surface in tangential direction, the particle is forced into a rolling motion. This motion can be used to estimate work of adhesion and this technique can be called “soft probing”. We used the soft probing configuration to estimate rolling work of adhesion of a birefringent 3 μm particle on a glass surface. Further, we have studied the effects of PolydimethylSiloxane (PDMS) which is a hydrophobic surface. This technique is used to probe the rolling work of adhesion of 500 nm nanodiamond bearing Nitrogen-vacancy centers which are birefringent due to the stress in the crystal. These nanodiamonds have a contact diameter as small as 50 nm because of their relatively high Young’s modulus. The rolling work of adhesion estimated using our soft probing configuration is about 1 mJ/m2, while using the AFM tips to estimate work of adhesion at nanoscale yields about 50 mJ/m2.
The cell membrane has fluctuations due to thermal and athermal sources. That causes the membrane to flicker. Conventionally, only the normal (perpendicular to the membrane) fluctuations are studied and then used to ascertain the membrane properties like the bending rigidity. It is here that we introduce a different concept, namely the slope fluctuations of the cell membrane which can be modelled as a gradient of the normal fluctuations. This can be studied using a new technique where a birefringent particle placed on the membrane turns in the out of plane sense, called the pitch sense. We introduce the pitch detection technique in optical tweezers relying upon asymmetric scattering from a birefringent particle under crossed polarizers. We then go on to use this pitch detection technique to ascertain the power spectral density of membrane slope fluctuations and find it to be (frequency)−1 while the normal fluctuations yields (frequency)−5/3. We also explore a different regime where the cell is applied with the drug Latrunculin-B which inhibits actin polymerization and find the effect on membrane fluctuations. We find that even as the normal fluctuations now become (frequency)−4/3, the slope fluctuations spectrum still remains (frequency)−1, with exactly the same coefficient as the case when the drug was not applied. Thus, this presents a convenient opportunity to study the membrane parameters like bending rigidity as a function of time after applying the drug. This would be the first time the membrane bending rigidity could be studied as a function of time upon the application of Lat-B without reverting to AFM.
Up-converting particles (UCP) absorb wavelengths in IR region and emit light in visible region by multiphoton absorption process. When optically trapped with 975 nm laser, these particles show active Hot Brownian Motion (HBM) due to the temperature difference created across the particle by the trapping laser. This is akin to an active particle optically confined in a tweezers with properly oriented motion. However, the activity vanishes when trapped with 1064 nm laser. We carefully maneuver the activity dependence of UCPs on laser wavelength to build a Stirling engine. A Stirling cycle consists of an isothermal expansion followed by isochoric cooling, isothermal compression and isochoric heating. Here, activity of the UCP in an optical trap is analogous to effective temperature which is controlled by the 975 nm laser. Whereas, the confinement of the trapped particle is similar to volume which can be altered by changing the trap stiffness of the 1064 nm laser trap. In this work, We first trap a UCP simultaneously with 1064 nm laser and 975 nm laser. Gradually decreasing 1064 nm laser power keeping 975 nm laser power constant decreases the trap stiffness resulting in less confinement of the UCP while keeping the activity constant. This process is considered as isothermal expansion. There can also be another process where 975 nm is increased and 1064 nm laser power is reduced leaving the total intensity constant. That would amount to isochoric process. We explore all these processes towards the Stirling cycle.
Optical trapping of janus particles has turned out to be complicated due to lack of control on the direction of orientation. Here we use an alternative strategy, where we optically trap a NaYF4:Yb,Er upconverting nanoparticle on the pump wavelength at 975 nm and show that there is much greater visible emission in the backscatter direction than the forward scattered direction, leading to greater heating in the backscatter region of the nanoparticle. This then generates a temperature gradient across the nanoparticle to push it in the axial direction. The Mean Square Displacement (MSD) bears signature of the Hot Brownian Motion (HBM) when trapped at 975 nm, which becomes regular diffusive when the trapping wavelength is changed to non-pump wavelength 1064 nm. The effective velocity of the particle while trapped in the tweezers can be directly estimated. Thus, this is the first time that an active janus-like particle has been optically trapped in tweezers.
3D Pitch rotational motion has been generated in spherical particles using holographic optical tweezers by maneuvering the laser spots to control the rotational motion. However, since the spherical particles, required to minimise complications due to the drag forces, are perfectly isotropic, a controllable torque cannot be applied with it. It is here that we trap birefringent particles in two tweezers beams and then change the depth of one of the beam foci controllably to generate a proper pitch rotational torque wrench. We also detect the rotation with our newly developed pitch rotational motion detection technique which could not be done conventionally on isotropic spherical particles.
Single beam thermo-optical tweezers has been used for rotating particles continuously by 360o in pitch sense. The particle is rotated by placing it in a sample chamber with a thin layer of gold coated coverslip in an optical tweezers setup. The combined effects of convection current generated from heating of water and the optical trap rotate the particle in pitch sense near the hot spot present at the gold layer. This method works effectively until a water vapor bubble is formed due to continuous heating of the gold layer. We have tried to delay the onset of bubble formation by using viscoelastic medium instead of water. Viscoelastic medium is prepared by mixing polyacrylamide in water. Pitch rotation of hexagonal shaped particle has been observed for media having different concentrations of polyacrylamide. The frequency of rotation is measured at various concentrations. A suitable mathematical function is determined empirically which can describe the change in frequency while varying the concentration.
It is well known that thermo-optical tweezers leads to deposition of continuous patterns on substrates mediated by Marangoni convection currents around micro-bubbles. While performing the deposition, we also find that there is an accompanying emission. On first look, one would expect this to be thermal broadband emission. However, the spectrum of emission seems to start from green and extend all the way to near infra red. Such peak wavelengths would correspond to 3000 K or even higher. Generation of such high temperatures at the local hot spot would melt the glass substrate. However, such melting facets have never been seen. Thus we speculate that the emission is actually two photon fluorescence from the incident light on the deposited pattern. Soft Oxometallate material is known to exhibit photoluminescence in the green-red region of spectrum. This kind of emission is also observed in carbon nanotubes when incident with a focused 1064 nm light, the origin of which appears to be similar multiphoton fluorescence processes.
We show a new instability in sessile water droplets when a particle is trapped close to the edge interface of air and water by optical tweezers when the light beam heats up the glass substrate and generate thermophoretic forces that direct the particle outward from the tweezers trap. There is competition between the optical trapping and the thermophoretic forces which direct the particle away from the trap to generate this instability.
We study the normal fluctuations of an MCF-7 cell membrane and calibrate the optical trap to detect pitch motion to get information about the rocking motion of a birefringent particle. We could show both theoretically and experimentally that the Z power spectrum has a power-law behavior of (frequency)−5/3, and We find that the power spectrum of slope fluctuations is proportional to (frequency)−1. We could extract parameters like bending rigidity directly from the power spectrum fitting parameters in 5 Hz to 1 kHz range. Our method was powerful enough to identify pitch rotation for a spherical birefringent particle to a high resolution using optical tweezers.
Rare-earth doped upconverting nanoparticles like NaYF4:Yb,Er absorb infra-red light and emit visible radiation. We use light at the absorption resonance, namely 975 nm, to optically trap the nanoparticles and subsequently study the effect on the trapping parameters like the trap stiffness. We trap hexagonal shaped upconverting nanoparticles and find that these align with their side along the tweezers beam. We also place a polarizers to study the backscatter emission features and find that the emission spectra vary depending upon the orientation of the polarizer with respect to the side axis of the particle. We use this to find the rotational Brownian motion along the yaw sense. We also use the fluorescent emission to ascertain the axial Brownian motion of the particle. This can be used to ascertain the motion of the particle while trapped on resonance in tweezers without actually accessing the infra-red trapping light.
A three dimensional rigid spherical microscopic object can rotate in either the pitch, yaw or roll fashion. Among these, yaw motion has been conventionally studied using the intensity of the scattered light from birefringent microspheres through crossed polarizers. So far, however, there is no way to study the pitch rotational motion in spherical microspheres. Here we suggest a new method towards the study of such pitch motion in birefringent microspheres under crossed polarizers by measuring the 2-fold asymmetry in the scattered signal using video microscopy. We show a simple example of pitch rotation determination using video microscopy for a microsphere attached with a kinesin molecule while moving along a microtubule. It can also be extended to optical tweezers.
We have developed a technique of generating a micro-bubble inside an optical trap by using a material (Mo-based Soft Oxometalate (SOM) compound) that absorbs at the trapping laser wavelength. A high concentration aqueous dispersion of the SOMs is taken in a sample chamber, and the trapping laser is focused on SOMs adsorbed on one of the surfaces of the chamber, so as to create a hot spot due to which a microbubble is nucleated. Due to the temperature gradient on the bubble, a surface tension gradient results, which leads to Marangoni type flows around the bubble. The resultant Marangoni ow around the bubble causes self-assembly of material at its base, which undergoes a phase transition into a crystalline state when the laser spot is translated causing the bubble to follow due to convective effects.1 We have used this technique to pattern materials ranging from dyes to carbon nano-tubes to conducting polymers which co-assemble in a mixture with the SOMs. The method is rather universal and has been used to develop catalytic chips2 and solution processed printable electronics. The flow generated by the bubbles can be studied by mapping the trajectories of probe particles in the vicinity of the bubble. We show interesting self-assembly of the particles on the bubble surface, as well as manipulation of trajectories of the particles by multiple bubbles. The bubble can also be used to capture, transport, and release particles in a perfectly controlled manner.
Asymmetric particles, such as biological cells, often experience torque under optical tweezers. The cause is believed to be either birefringence or unbalanced scattering forces. The estimate of torque relies on the accurate measurement of rotational motion. Here we present a new technique to quantify the asymmetry of trapped particles relying upon the cross coupling between rotational and translational Brownian motion. We observe that RBC does indeed show cross coupling indicating asymmetry of the shape. Further we also show by polarimetry that the retardance of the RBC is not sufficient to make it rotate since the scattering torque is much higher.
We have developed a new technique of optical micro-patterning using micro bubbles based on light induced self-
assembly of materials using thermo-optic tweezers. Presently, we use a liquid matter- Soft Oxo-Metalate (SOMs)
that have high absorption near the wavelength of the tweezers laser at 1064 nm. An aqueous dispersion of the
sample solution is taken in a glass sample chamber and introduced into the translation stage of our tweezers
set-up (inverted microscope) where a highly focused laser beam is aimed at SOM particles adsorbed on the top
surface of the sample chamber. The high absorptivity of SOMs ensures the creation of a local `hot-spot' which
leads to the nucleation of a micro-bubble in this region. Thus, a large local surface-tension gradient is introduced
in the vicinity of the micro-bubble due to the temperature gradient produced at the two ends of the bubble, which
leads to a Marangoni type convective
ow around the bubble. This
ow causes material to be self-assembled
at the base of the bubble. As the translation stage is moved, the `hot-spot' moves simultaneously, and due
to the resulting
ow dynamics, the microbubble is also translated thus causing continuous accumulation of the
SOMs around it. Simultaneously, due to the sudden thermal shock generated when the `hot-spot' is moved away
from the self-assembled SOMs, they undergo a phase transition from soft (liquid) to hard (crystalline) state,
resulting in the formation of a stable permanent pattern of choice on the glass substrate. This technique can
have diverse applications with materials other than SOMs including carbon nano tubes, organic dyes, catalysts
and conducting polymers, etc, being co-deposited from aqueous dispersions of the particular material with the
SOMs. The patterns thus formed have been used for various applications including the development of catalytic
micro-chips, and solution processed printable micro-circuits.
The spin orbit interaction (SOI) of light leading to the evolution of trajectory dependent geometric phase and associated spin Hall shift (SHS) in circularly polarized light has led to several fascinating manifestations in scattering, tight focusing, and imaging processes. However, most of these observations are at the sub-wavelength level, with somewhat limited applications of a general nature. We investigate the SOI in an optical trap for a linearly polarized trapping beam where the both the trajectory dependent geometric phase as well as the SHS are magnified significantly due to a stratified medium. The stratified medium is created using an index mismatched cover slip that modifies the radial intensity distribution near the focal plane of the trap due to diffraction effects. The modified intensity distribution causes trapping of polystyrene beads in ring-like patterns, while the tight focusing in the stratified medium also leads to a large spin redirection geometric phase that creates intensity side lobes in the azimuthal direction near the focal plane. Single trapped asymmetric particles can be trapped in the side lobes and translated along the ring by changing the polarization angle of the input beam. A 3D analysis of polarization reveals the generation of polarization vortices as well as spatially separated regions of opposite circular polarizations near the focal plane leading to controlled rotation of trapped particles, again by a linearly polarized input beam. The study can have several interesting consequences in the manipulation of mesoscopic particles in an optical trap.
Artificial micro-swimmers are fast emerging as models to mimic and thereby understand the movement patterns of microorganisms and biological cells which self-propel themselves by generating fields or gradients that cause fluid flow around their surface by phoretic surface effects, such as thermophoresis or electrophoresis. In this paper, we demonstrate that radiation pressure can lead to spontaneous revolution of a micron-sized asymmetric particle inside an annular potential formed due to geometrical aberrations of a Gaussian beam focused into a stratified medium using a high numerical aperture microscopic objective. The rate of revolution can be controlled from a few Hz to tens of Hz by changing the intensity of the trapping light which can be achieved either by modifying the laser power or the annular trap diameter. Theoretical simulations performed using Finite-difference time- domain method in Lumerical verify the experimental observations. The electric field distribution confirms that the particle revolution is the effect of asymmetrical scattering by the particle in the annular potential that gives rise to a tangential force. A proper Maxwell stress-tensor analysis of the problem demonstrates this uniform tangential force acting on the particle inside the ring. The model also shows that particles could be custom designed in order to spontaneously revolve in such annular trapping potentials. Thus, such systems could be used in place of LG beams to apply torque on DNA strands in order to study protein-DNA interaction, or to study the hydrodynamic synchronization among multiple rotating objects.
Asymmetric particles, such as biological cells, often experience torque under optical tweezers due to birefringence or unbalanced scattering forces, which makes precise determination of the torque crucial for calibration and control of the particles. The estimate of torque relies on the accurate measurement of rotational motion, which has been achieved by various techniques such as measuring the intensity fluctuations of the forward scattered light, or the polarization component orthogonal to the trapping light polarization in plasmonic nanoparticles and vaterite crystals. Here we present a simple yet high sensitive technique to measure rotation of such an asymmetric trapped particle by detecting the light backscattered onto a quadrant photodiode, and subtracting the signals along the two diagonals of the quadrants. This automatically suppresses the common mode translational signal obtained by taking the difference signal of the adjacent quadrants, while amplifying the rotational signal. Using this technique, we obtain a S/N of 200 for angular displacement of a trapped micro-rod by 5 degrees, which implies a sensitivity of 50 mdeg with S/N of 2. The technique is thus independent of birefringence and polarization properties of the asymmetric particle and depends only on the scattering cross-section.
Back-focal plane interferometry is typically used to determine displacements of a trapped bead which lead to
trapping force measurements in optical tweezers. In most cases, intensity shifts of the back-scattered interference
pattern due to displacements of the bead are measured by a position sensitive detector placed in the microscope
back-focal plane. However, in intensity-based measurements, the axial displacement resolution is typically worse
than the lateral resolution since for axial displacements, the inherent resolution of the position detector cannot
be used. In this paper, we demonstrate that measurement of the phase of the back-scattered light yields high
axial displacement resolution, and can also be used for lateral displacement measurement. In our experiments,
we separate out the back-scattered light from the trapped bead and reflected light from the top surface of
the sample chamber by a confocal arrangement consisting of a spatial filter used in combination with two
apertures. We proceed to beat the two separated components in a Mach-Zehnder interferometer where we
employ balanced detection to improve our fringe contrast, and thus the sensitivity of the phase measurement.
For lateral displacement sensing, we match experimental results to within 10% with a theoretical simulation
determining the shift of the overall phase contour of the back-scattered light due to a given lateral displacement
by using plane wave decomposition in conjunction with Mie scattering theory. Our technique is also able to track
the Brownian motion of trapped beads from the phase jitter so that, similar to intensity-based measurements,
we can use it to determine the spring constant of the trap, and thus the trapping force. The sensitivity of our
technique is limited by path drifts of the external interferometer which we have currently stabilized by locking
it to a frequency stabilized diode laser to obtain displacement measurement resolution ~200 pm.
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