Kishan Dholakia

Sub-millimeter helical fiber created by Bessel vortex beam illumination

We demonstrate a self-written sub-millimeter (>300 μm) helical fiber in a photo-cure resin by irradiation of non-diffractive 1st-order Bessel beam with an orbital angular momentum. The twisted direction of the helical fiber can be controlled by only reversing the sign of the topological charge of Bessel beam.

Levitated optomechanics of silica microparticles in vacuum placed in 2D and 3D optical potentials possessing orbital angular momentum (Conference Presentation)

We demonstrate the transfer of orbital angular momentum (OAM) to optically levitated microparticles in vacuum. We create two-dimensional (2D) and three-dimensional (3D) optical potentials possessing OAM. In the former case the microparticle is placed within a Laguerre-Gaussian (LG) beam and orbits the annular beam profile with increasing angular velocity as the air drag coefficient is reduced. Our results reveal that there is a fundamental limit to the OAM that may be transferred to a trapped particle, dependent upon the beam parameters and inertial forces present. Whilst a LG beam scales in size with azimuthal index, recently we have created a “perfect vortex” beam whose radial intensity profile and radius are both independent of topological charge. As the Fourier transform of a “perfect vortex” yields a Bessel beam, imaging a “perfect vortex”, with its subsequent propagation thus realises a complex three-dimensional optical field. In this scenario we load individual silica microparticles into this field where the optical gradient and scattering forces interplay with the inertial and gravitational forces acting on the trapped particle. As a result the trapped microparticle exhibits a complex three-dimensional motion that includes a periodic orbital motion between the Bessel and the “perfect vortex” beam. We are able to determine the three dimensional optical potential in situ by tracking the particle. This first demonstration of trapping microparticles within a complex 3D optical potential in vacuum opens up new possibilities for fundamental studies of many-body dynamics, mesoscopic entanglement, and optical binding.

Electrostrictive in-situ nanoparticle detection with coherent Rayleigh-Brillouin scattering (Conference Presentation)

We report on the development and application of coherent Rayleigh-Brillouin scattering for the in situ detection of large molecules and nanoparticles. This four wave mixing diagnostic technique relies on the creation of an electrostrictive optical lattice in a medium due to the interaction between polarized particles and the intense electric field gradient created by the optical interference of two intense pulsed laser beams. Though this interaction, we can detect the temperature, pressure, relative density, polarizability and speed of sound of a gas and gas mixture. This diagnostic was already successfully demonstrated in atomic and molecular gaseous environments, where the different gas polarizabilities and pressures were successfully measured. We are currently conducting in situ measurements with large molecules and nanoparticles produced in an arc discharge, the results of which will be presented in this meeting.

Angular momentum exchange between light and small particles (Conference Presentation)

We present a few simple examples to illustrate certain fundamental properties of the EM field. Using elementary physical concepts, we explain the nature of interactions that involve exchanges of energy, linear momentum, and angular momentum between EM fields and material media. First, the radiation force experienced by a small, polarizable particle which has a predetermined dielectric susceptibility will be examined. The dielectric susceptibility of small spherical particles will be related to their refractive index (with proper accounting for the effects of radiation resistance). We describe the relation between the energy and orbital angular momentum of a cylindrical harmonic EM wave trapped inside a hollow cylindrical cavity, and explore the relations among the energy, linear momentum, and angular momentum picked up by a small particle under illumination by a cylindrical harmonic EM wave. In light of this analysis, it becomes clear why a small particle spins around its own axis when illuminated by a light beam that carries spin angular momentum, whereas the same particle tends to orbit around an axis of vorticity when exposed to a beam (such as a vector cylindrical harmonic) that possesses orbital angular momentum.

Front Matter: Volume 10347

This PDF file contains the front matter associated with SPIE Proceedings Volume 10347, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Evanescent single-molecule biosensing with quantum limited precision (Conference Presentation)

Techniques to observe and track single unlabelled biomolecules are crucial for many areas of nanobiotechnology; offering the possibility for lab-on-a-chip medical diagnostics operating at their ultimate detection limits, and to shed light on important nanoscale biological processes such as binding reactions, conformational changes, and motor molecule dynamics. Impressive progress has been made over the past few years to extend the sensitivity of such techniques, primarily via the evanescent field enhancement provided by optical microcavities [1, 2] or plasmonic resonators [3]. However, such approaches expose the biological system to greatly increased optical intensity levels, which can severely impact biological function, growth, structure and viability [4]. Here, we introduce an evanescent biosensing platform that operates at the fundamental precision limit introduced by quantisation of light. This allows a five order-of magnitude reduction in optical intensity whilst maintaining state-of-the-art sensitivity and enabling quantum noise limited tracking of single biomolecules as small as 3.5 nm.

Optical fabrication and trapping of superconducting nanoparticles in superfluid helium (Conference Presentation)

Superfluid helium having extremely low temperature, negligibly small viscosity, and huge thermal conductivity provides us a unique opportunity to generate a novel cryogenic space for the fabrication of nanostructures and the manipulation of their motion. Here we fabricated metallic nano- and micro-particles by laser ablation in superfluid helium and selectively trapped superconducting particles with a quadrupole magnetic field utilizing perfect diamagnetism caused by Meissner effect. We also discuss the size dependence of the superconducting transition temperatures of the trapped metallic particles by changing the temperature of liquid helium.

Optical measurements of nanoparticle vibrations for fluid mechanics on the nanoscale (Conference Presentation)

Optical measurements of acoustic oscillations in metal nanoparticles provide a sensitive probe into the mechanical properties of materials at GHz frequencies and nanometer length scales. In these experiments, an incident pump laser heats the nanoparticles, leading to their expansion and the excitation of mechanical vibrations. The vibrations produce oscillations in the plasmon resonance frequency of the nanoparticles, which are monitored by measuring the change in transmission through the sample of a second, probe laser pulse. By making these measurements on a highly monodisperse sample of bipyramidal gold nanoparticles, we were able to determine both the frequency and the decay rate of the vibrations. Measurements on nanoparticles in different solvents made it possible to determine the portion of damping and the vibrational frequency shift that are due to coupling to the surrounding liquid environment. Viscous damping could account for results at low viscosities, but significant discrepancies were observed for higher viscosities. The discrepancies were ultimately resolved by accounting for the viscoelastic nature of the surrounding liquids. For more viscous liquids, relaxation times are higher, and thus more of the vibrational energy is stored as elastic energy in the surrounding liquid. This reduces damping, and the restoring force provided by the stored energy increases the vibrational frequency, the opposite of what would occur for an ordinary Newtonian fluid. These measurements demonstrate that metal nanoparticles can serve as nanoscale rheometers, with the picosecond response times required to reveal viscoelastic effects in conventional liquids.

Optical trapping of rare earth-doped nanorods using an optical fiber tweezers approach (Conference Presentation)

NaYF4:(Er,Yb,Gd) nanorods of different size were trapped using our original optical tweezers consisting of two fiber tips facing each other. Trapping properties were found to depended drastically on the actual particle size. Small rods were efficiently trapped whereas long rods were strongly attracted by the fiber tips and their stable trapping position was situated at the apex of one single fiber tip. In the case of the long particles the trapped particle modified the fiber tip emission properties and trapping of a second nanorod at distances of some microns from the first one is observed. These experimental results will be explained by numerical simulations using the exact Maxwell Stress Tensor approach.

Force-activated substrates for high-precision, high-throughput optical trapping assays of ssDNA motor proteins(Conference Presentation)

Optical-trapping-based assays can measure individual proteins bind to and move along DNA with sub-nm resolution, and have yielded insight into a broad array of protein-DNA interactions. Unfortunately, collecting large numbers of high-resolution traces remains an ongoing challenge. Studying helicase motion along DNA exemplifies this challenge. One major difficulty is that helicase binding often requires a single stranded (ss)-double stranded (ds) DNA junction flanked by ssDNA with a minimum size and orientation. Historically, creating such DNA substrates is inefficient. More problematic is that data throughput is low in standard surface-based assays since all substrates are unwound upon introduction of ATP. The net result is ~2–4 high-resolution traces on a good day. To improve throughput, we sought to turn-on or activate a substrate for a helicase one molecule at a time and thereby sequentially study many molecules on an individual microscope slide. As a first step towards this goal, we engineered a dsDNA that contains two site-specific nicks along the same strand of the dsDNA but no ssDNA. Upon overstretching the DNA (F = 65 pN), the strand between the two nicks was mechanically dissociated. We demonstrated this with two different substrates: one yielding an internal ssDNA region of 1100 nt and the other yielding a 20-bp long hairpin flanked by 30 nt of ssDNA. Unwinding a hairpin yields a 3-fold larger signal while the 30-nt ssDNA serves as the binding site for the helicase. We expect that these force-activated substrates to significantly accelerate high-resolution optical-trapping studies of DNA helicases.