Session V48: Focus Session: Advanced Optical Probes of Soft Matter - Forces, Imaging, Excitons

Brownian vortex circulation due to spin orbit conversion in a circularly polarized optical tweezer

Strong focusing of circularly polarized beams converts spin angular momentum into orbital angular momentum. We describe this process in terms of a generalized vector potential, involving the amplitude, phase, and polarization of the light. This gives a more general understanding of this force in terms of experimentally accessible parameters. In addition, this formalism provides a framework for understanding other polarization induced forces, which arise from the curl of the spin angular momentum density. Experimentally we demonstrate deterministic polarization-induced circulation with trapped clusters of 1$\mu$m polystyrene spheres, and Brownian vortex circulation for a single sphere trapped in elliptically polarized optical tweezers.   

Pulling Particles Backward Using a Forward Propagating Beam

Can the scattering force of a forward propagating beam pull a particle backward? A photon carries a momentum of $\hbar k$, so one may expect light will push against any object standing in its path. However, light can indeed ``attract'' in some cases. For example, if light is focused to a spot, small particles will be attracted towards it due to the gradient force. But it is probably more appropriate to say that the gradient force ``grabs'' rather than ``pulls'', as the particle will remain stable in the trap after being drawn to the focus. Here, we discuss another possibility---a backward scattering force which is always opposite to the propagation direction of the beam so that the beam keeps on pulling an object towards the source without an equilibrium point. In the absence of intensity gradient, using a light beam to pull a particle backwards is counter intuitive. The underlining physics is the maximization of forward scattering via interference of the radiation multipoles. We show explicitly that the necessary condition to realize a pulling force is the simultaneous excitation of multipoles in the particle and if the projection of the total photon momentum along the propagation direction is small, attractive optical force is possible.   

Lateral optical binding forces between two colloidal Mie particles

Micro particles in an intense optical field can self-organize into an array with well defined structure. This phenomenon, first reported by Burns et al., as optical binding, was believed to be caused by the optical gradient force. In spite of many attempts to calculate the binding forces and the colloidal structures, there has been a lack of experiments directly measuring the forces between the particles. Positioning two micron-sized polystyrene particles, each held by a tightly focused laser beam from a single coherence laser source, we found the lateral optical binding force oscillates with the inter particle separation, as well as the relative phase between the beams due to retardation. By independently changing the polarization directions at each optical trap, we examined the periodicity and magnitude of the forces. Our research indicates the forces under such conditions require a model beyond dipole approximation. In addition, an accurate calculation based on Mie theory with consideration of high focusing compare well with our experimental findings.   

Super-Resolution Imaging Using Randomly Diffusing Probes

Recent advances in super-resolution microscopy allow imaging of biological tissues labeled with fluorescent dyes with unprecedented resolution. These techniques often rely on the fact that single emitters can be localized with nm accuracy. When multiple emitters reside within a diffraction-limited spot they are serially photo-switched to ensure that they emit one at the time. This approach has not been applied to other imaging modalities, for example imaging local electromagnetic field enhancement, mainly because photo-switching would be infeasible. Here we present a super-resolution imaging technique which circumvents the requirement for serial photoswitching by using the random motion of single dye molecules to scan the surface in a stochastic manner. This technique allowed us to image electromagnetic field enhancement of a single spot formed on thin metallic film with 1.2nmn accuracy and gain insight into the mechanism for generating field enhancement.   

Three-dimensional thermal noise images of single biopolymer filaments allows to determine their orientation and quantify their mechanical properties

Intracellular biopolymer networks perform many essential functions for living cells. Most of these networks show a highly nonlinear mechanical response that is well-studied on the macroscopic scale. While much work has been done to connect the macroscopic responses of networks to specific network properties, such as filament persistence length, cross-linking geometry and pore size, there is a lack of experimental techniques that can simultaneously determine the structure and the mechanical properties of a network in situ on the single filament level. Thermal Noise Imaging is a scanning probe technique that utilizes the confined thermal motion of an optically trapped particle as a three-dimensional, noninvasive scanner for soft, biological material. It achieves nanometer precision in probe position detection at MHz bandwidth. Thermal noise imaging visualizes single biopolymer filaments as nanoscale tunnels and allows for the quantification of their mechanical properties from their transversal fluctuations. The experiments presented here pave the way for investigating force distributions inside biopolymer networks on the single filament level, as well as establish thermal noise imaging as a quantitative tool for studying biological material on the nanometer scale.   

Microscopy method for the characterization of structural color on a single wing scale of Morpho butterfly

The structural color and the iridescence of Morpho rhetenor were investigated using an optical microscope and a digital camera. Incoherent white light source was used for both spatial and spectral analysis. The scattering pattern from the micrometer sized single scale in the back focal plane of the objective lens was observed with Bertrand lens equipped in the optical microscope. We precisely controlled incident angle of the light using common components typically embedded in most optical microscope; aligning aperture stop at the center or off-center. Wide range of the angular scattering pattern from a single scale was measured and the iridescence of Morpho rhetenor was measured quantitatively. The single scale of Morpho rhetenor diffusively reflected the normally illuminated light, while blue band was more effectively reflected than green and red band. We retrieved the raw intensity data generated at the imaging sensor of the digital camera and quantitatively analyzed the spatial distribution of the scattered light. The reflectivity measured by the digital camera was comparable to the result from microspectrometer reported earlier.   

High Fidelity Detection of Defects in Polymer Films using Surface-Modified Nanoparticles

Defects are ubiquitous to materials and material surfaces. As we push the thresholds of length scale producing defect free materials, surfaces and interfaces, it becomes increasingly difficult to detect their presence over multiple length scales. The ideal example is the semiconductor industry where the driving force is higher performance and lower cost devices with material interfaces. In this regard, Chemical mechanical planarization, CMP, emerged as the premier method for achieving ultra flat surfaces below the 0.35 micron technology node, enabling many of the advanced electronic devices currently in production. The interactions between numerous process settings and output metrics are difficult to predict and often lead to defects. Failure analysis (FA) is critical and the current methods involve advanced imaging methods such as SEM. We design a cost effective method to detect physico-chemical defects at multiple length scales through Polymer-Nanoparticles (NPs) interactions in relatively shorter period of time (time of imaging is reduced 50 fold). This method can be used in conjunction with the traditional imaging methods to pin point location of these defects, which can be further analyzed.   

A Study of the Polarizability of Single-Walled Carbon Nanotubes in an Optical Field

Whereas the behavior of single-walled carbon nanotubes (SWCNT) in an electric field has been extensively studied, the polarizability of SWCNTs at optical frequencies remains unclear due to the difficulty in direct detection. It was demonstrated by utilizing Raman spectroscopy as a characterization means, optical tweezers could selectively aggregate SWCNTs. While it was commonly believed that the trapping effect due to the large optical field gradient caused the strong response of tubes to the laser beam, we expect the aligning effect due to the optical polarization also has a considerable contribution. To quantify these two possible effects experienced by an ensemble of individual DNA-SWCNTs of different chiralities, and address the issue of tube-tube interaction, We design an experiment by applying optical tweezers with variable polarization states and insepcting resonance Raman excitation for sensitive detection. Specifically, we measure the radial breathing mode signal of SWCNTs as a function of laser power and the direction of polarization for different tube types and concentrations. The research may lead to a more complete understanding of sorting phenomenon of individual SWCNTs in an optical field at microscopic level.   

Dynamics of Aerial Tower Formation in Bacillus subtilis Biofilms

Biofilms are highly-organized colonies of bacteria that form on surfaces. These colonies form sophisticated structures which make them robust and difficult to remove from environments such as catheters, where they pose serious infection problems. Previous work has shown that sub-mm sized aerial towers form on the surface of Bacillus subtilis colony biofilms. Spore-formation is located preferentially at the tops of these towers, known as fruiting bodies, which aid in the dispersal and propagation of the colony to new sites. The formation of towers is strongly affected by the quorum-sensing molecule surfactin and the cannibalism pathway of the bacteria. In the present work, we use confocal fluorescence microscopy to study the development of individual fruiting bodies, allowing us to visualize the time-dependent spatial distribution of matrix-forming and sporulating bacteria within the towers. With this information, we investigate the physical mechanisms, such as surface tension and polymer concentration gradients, that drive the formation of these structures.   

The Orientation of Luminescent Excitons in Layered Organic Nanomaterials

A fundamental understanding of optoelectronics in organic semiconductors is complicated by the diversity of excitons which can exist within a single material system. Measurements that distinguish between different exciton types are crucial for a complete understanding of organic materials. By fitting experimental curves of angle-, polarization-, and energy-dependent PL to analytical Purcell calculations we quantify the relative dipole moments for in-plane and out-of-plane oriented excitons in organic and inorganic layered nanomaterials. In mono- and bi-layers of Molybdenum Disulfide (MoS2) and Graphene Oxide the luminescence arises only from in-plane oriented excitons. In the perylene derivative PTCDA, however, we show that PL arises from both in-plane and out-of-plane excitons. We observe a difference in emission frequency between the dipole orientations which indicates the existence of two distinct exciton species: an in-plane oriented Frenkel exciton and an out-of-plane oriented Charge Transfer exciton. Based on these results we devise and implement a method for isolating luminescence from either exciton species. We observe different temporal dynamics for the two distinct excitons, highlighting the power of this technique for fundamental studies of organic materials.   

Accessing exciton transport in light-harvesting structures with plasmonic nanotip

Natural light-harvesting complexes, such as that of plant cells or photosynthetic bacteria, are considered as possible prototypes for artificially designed solar cell materials. In these structures the energy of light absorbed by a peripheral antenna is transmitted very efficiently in a form of excitons to a reaction center. Usually, information about the exciton transport is obtained from time-resolved nonlinear optical experiments where the frequencies of a pump and a probe fields select particular electronic transitions in the light-harvesting complex. We explore a complimentary setup utilizing a plasmonic nanotip as a local sub-wavelength probe of excitation dynamics. As specific examples we consider an LHII complex involved in the light-harvesting process of purple bacteria and a Fenna-Matthews-Olson pigment-protein complex of green-sulphur bacteria.