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Conservation Laws and Electromagnetic Interactions
Title
Conservation Laws and Electromagnetic Interactions.
Creator
Kajorndejnukul, Veerachart, Dogariu, Aristide, Abouraddy, Ayman, Kik, Pieter, Rahman, Talat, University of Central Florida
Abstract / Description
Aside from energy, light carries linear and angular momenta that can be transferred to matter. The interaction between light and matter is governed by conservation laws that can manifest themselves as mechanical effects acting on both matter and light waves. This interaction permits remote, precise, and noninvasive manipulation and sensing at microscopic levels. In this dissertation, we demonstrated for the first time a complete set of opto-mechanical effects that are based on nonconservative...
Show moreAside from energy, light carries linear and angular momenta that can be transferred to matter. The interaction between light and matter is governed by conservation laws that can manifest themselves as mechanical effects acting on both matter and light waves. This interaction permits remote, precise, and noninvasive manipulation and sensing at microscopic levels. In this dissertation, we demonstrated for the first time a complete set of opto-mechanical effects that are based on nonconservative forces and act at the interface between dielectric media. Without structuring the light field, forward action is provided by the conventional radiation pressure while a backward movement can be achieved through the natural enhancement of linear momentum. If the symmetry of scattered field is broken, a side motion can also be induced due to the transformation between spin and orbital angular momenta. In experiments, these opto-mechanical effects can be significantly amplified by the long-range hydrodynamic interactions that provide an efficient recycling of energy. These unusual opto-mechanical effects open new possibilities for efficient manipulation of colloidal microparticles without having to rely on intricate structuring or shaping of light beams. Optically-controlled transport of matter is sought after in diverse applications in biology, colloidal physics, chemistry, condensed matter and others.Another consequence of light-matter interaction is the modification of the optical field itself, which can manifest, for instance, as detectable shifts of the centroids of optical beams during reflection and refraction. The spin-Hall effect of light (SHEL) is one type of such beam shifts that is due to the spin-orbit transformation governed by the conservation of angular momentum. We have shown that this effect can be amplified by the structural anisotropy of random nanocomposite materials.
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Date Issued
2015
Identifier
CFE0005961, ucf:50818
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0005961
Optically Induced Forces in Scanning Probe Microscopy
Title
Optically Induced Forces in Scanning Probe Microscopy.
Creator
Kohlgraf-Owens, Dana, Dogariu, Aristide, Christodoulides, Demetrios, Kik, Pieter, DeWilde, Yannick, University of Central Florida
Abstract / Description
The focus of this dissertation is the study of measuring light not by energy transfer as is done with a standard photodetector such as a photographic film or charged coupled device, but rather by the forces which the light exerts on matter. In this manner we are able to replace or complement standard photodetector-based light detection techniques. One key attribute of force detection is that it permits the measurement of light over a very large range of frequencies including those which are...
Show moreThe focus of this dissertation is the study of measuring light not by energy transfer as is done with a standard photodetector such as a photographic film or charged coupled device, but rather by the forces which the light exerts on matter. In this manner we are able to replace or complement standard photodetector-based light detection techniques. One key attribute of force detection is that it permits the measurement of light over a very large range of frequencies including those which are difficult to access with standard photodetectors, such as the far IR and THz. The dissertation addresses the specific phenomena associated with optically induced force (OIF) detection in the near-field where light can be detected with high spatial resolution close to material interfaces. This is accomplished using a scanning probe microscope (SPM), which has the advantage of already having a sensitive force detector integrated into the system. The two microscopies we focus on here are atomic force microscopy (AFM) and near-field scanning optical microscopy (NSOM). By detecting surface-induced forces or force gradients applied to a very small size probe ( diameter), AFM measures the force acting on the probe as a function of the tip-sample separation or extracts topography information. Typical NSOM utilizes either a small aperture ( diameter) to collect and/or radiate light in a small volume or a small scatterer ( diameter) in order to scatter light in a very small volume. This light is then measured with an avalanche photodiode or a photomultiplier tube.These two modalities may be combined in order to simultaneously map the local intensity distribution and topography of a sample of interest. A critical assumption made when performing such a measurement is that the distance regulation, which is based on surface induced forces, and the intensity distribution are independent. In other words, it is assumed that the presence of optical fields does not influence the AFM operation. However, it is well known that light exerts forces on the matter with which it interacts. This light-induced force may affect the atomic force microscope tip-sample distance regulation mechanism or, by modifying the tip, it may also indirectly influence the distance between the probe and the surface. This dissertation will present evidence that the effect of optically induced forces is strong enough to be observed when performing typical NSOM measurements. This effect is first studied on common experimental situations to show where and how these forces manifest themselves. Afterward, several new measurement approaches are demonstrated, which take advantage of this additional information to either complement or replace standard NSOM detection. For example, the force acting on the probe can be detected while simultaneously extracting the tip-sample separation, a measurement characteristic which is typically difficult to obtain. Moreover, the standard field collection with an aperture NSOM and the measurement of optically induced forces can be operated simultaneously. Thus, complementary information about the field intensity and its gradient can be, for the first time, collected with a single probe. Finally, a new scanning probe modality, multi-frequency NSOM (MF-NSOM), will be demonstrated. In this approach, the tuning fork is driven electrically at one frequency to perform a standard tip-sample distance regulation to follow the sample topography and optically driven at another frequency to measure the optically induced force. This novel technique provides a viable alternative to standard NSOM scanning and should be of particular interest in the long wavelength regime, e.g. far IR and THz.
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Date Issued
2013
Identifier
CFE0004705, ucf:49829
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0004705
Particle Manipulation Via Optical Forces and Engineering Soft-Matter Systems With Tunable Nonlinearities.
Title
Particle Manipulation Via Optical Forces and Engineering Soft-Matter Systems With Tunable Nonlinearities.
Creator
Fardad, Shima, Christodoulides, Demetrios, Hagan, David, Amezcua Correa, Rodrigo, Likamwa, Patrick, Chen, Zhigang, University of Central Florida
Abstract / Description
One of the most intriguing properties of light-matter interaction is the ability of an electromagnetic field to exert mechanical forces on polarizable objects. This phenomenon is a direct consequence of the fact that light carries momentum, which in turn can be transferred to matter. Mediated by scattering, this interaction usually manifests itself as a (")pushing force(") in the direction of beam propagation. However, it is possible to judiciously engineer these optical forces, either by...
Show moreOne of the most intriguing properties of light-matter interaction is the ability of an electromagnetic field to exert mechanical forces on polarizable objects. This phenomenon is a direct consequence of the fact that light carries momentum, which in turn can be transferred to matter. Mediated by scattering, this interaction usually manifests itself as a (")pushing force(") in the direction of beam propagation. However, it is possible to judiciously engineer these optical forces, either by tailoring particle polarizability, and/or by structuring the incident light field. As a simple example, a tightly focused laser beam demonstrates strong gradient forces, which may attract and even trap particles with positive polarizability in the focal volume. The opposite occurs in the regime of negative polarizability, where particles are expelled from the regions of highest intensity. Based on this fundamental principle, one can actively shape the beam using spatial light modulators to manipulate individual objects as well as ensembles of particles suspended in a liquid. In the latter case, a modulation of the local particle concentration is associated with changes of the effective refractive index. The result is an artificial nonlinear medium, whose Kerr-type response can be readily tuned by the parameters of its constituent particles.In the course of this work, we introduce a new class of synthetic colloidal suspensions exhibiting negative polarizabilities, and observe for the first time robust propagation and enhanced transmission of self-trapped light over long distances. Such light penetration in strongly scattering environments is enabled by the interplay between optical forces and self-activated transparency effects. We explore various approaches to the design of negative-polarizability arrangements, including purely dielectric as well as metallic and hybrid nanoparticles. In particular, we find that plasmonic resonances allow for extremely high and spectrally tunable polarizabilities, leading to unique nonlinear light-matter interactions. Here, for the first time we were able to observe plasmonic resonant solitons over more than 25 diffraction lengths, in colloidal nanosuspensions.
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Date Issued
2014
Identifier
CFE0005610, ucf:50239
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0005610
NEAR-FIELD OPTICAL INTERACTIONS AND APPLICATIONS
Title
NEAR-FIELD OPTICAL INTERACTIONS AND APPLICATIONS.
Creator
Haefner, David, Dogariu, Aristide, University of Central Florida
Abstract / Description
The propagation symmetry of electromagnetic fields is affected by encounters with material systems. The effects of such interactions, for example, modifications of intensity, phase, polarization, angular spectrum, frequency, etc. can be used to obtain information about the material system. However, the propagation of electromagnetic waves imposes a fundamental limit to the length scales over which the material properties can be observed. In the realm of near-field optics, this limitation is...
Show moreThe propagation symmetry of electromagnetic fields is affected by encounters with material systems. The effects of such interactions, for example, modifications of intensity, phase, polarization, angular spectrum, frequency, etc. can be used to obtain information about the material system. However, the propagation of electromagnetic waves imposes a fundamental limit to the length scales over which the material properties can be observed. In the realm of near-field optics, this limitation is overcome only through a secondary interaction that couples the high-spatial-frequency (but non-propagating) field components to propagating waves that can be detected. The available information depends intrinsically on this secondary interaction, which constitutes the topic of this study. Quantitative measurements of material properties can be performed only by controlling the subtle characteristics of these processes. This dissertation discusses situations where the effects of near-field interactions can be (i) neglected in certain passive testing techniques, (ii) exploited for active probing of static or dynamic systems, or (iii) statistically isolated when considering optically inhomogeneous materials. This dissertation presents novel theoretical developments, experimental measurements, and numerical results that elucidate the vectorial aspects of the interaction between light and nano-structured material for use in sensing applications.
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Date Issued
2010
Identifier
CFE0003095, ucf:48318
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0003095
Analysis of Behaviors in Crowd Videos
Title
Analysis of Behaviors in Crowd Videos.
Creator
Mehran, Ramin, Shah, Mubarak, Sukthankar, Gita, Behal, Aman, Tappen, Marshall, Moore, Brian, University of Central Florida
Abstract / Description
In this dissertation, we address the problem of discovery and representation of group activity of humans and objects in a variety of scenarios, commonly encountered in vision applications. The overarching goal is to devise a discriminative representation of human motion in social settings, which captures a wide variety of human activities observable in video sequences. Such motion emerges from the collective behavior of individuals and their interactions and is a significant source of...
Show moreIn this dissertation, we address the problem of discovery and representation of group activity of humans and objects in a variety of scenarios, commonly encountered in vision applications. The overarching goal is to devise a discriminative representation of human motion in social settings, which captures a wide variety of human activities observable in video sequences. Such motion emerges from the collective behavior of individuals and their interactions and is a significant source of information typically employed for applications such as event detection, behavior recognition, and activity recognition. We present new representations of human group motion for static cameras, and propose algorithms for their application to variety of problems.We first propose a method to model and learn the scene activity of a crowd using Social Force Model for the first time in the computer vision community. We present a method to densely estimate the interaction forces between people in a crowd, observed by a static camera. Latent Dirichlet Allocation (LDA) is used to learn the model of the normal activities over extended periods of time. Randomly selected spatio-temporal volumes of interaction forces are used to learn the model of normal behavior of the scene. The model encodes the latent topics of social interaction forces in the scene for normal behaviors. We classify a short video sequence of $n$ frames as normal or abnormal by using the learnt model. Once a sequence of frames is classified as an abnormal, the regions of anomalies in the abnormal frames are localized using the magnitude of interaction forces.The representation and estimation framework proposed above, however, has a few limitations. This algorithm proposes to use a global estimation of the interaction forces within the crowd. It, therefore, is incapable of identifying different groups of objects based on motion or behavior in the scene. Although the algorithm is capable of learning the normal behavior and detects the abnormality, but it is incapable of capturing the dynamics of different behaviors.To overcome these limitations, we then propose a method based on the Lagrangian framework for fluid dynamics, by introducing a streakline representation of flow. Streaklines are traced in a fluid flow by injecting color material, such as smoke or dye, which is transported with the flow and used for visualization. In the context of computer vision, streaklines may be used in a similar way to transport information about a scene, and they are obtained by repeatedly initializing a fixed grid of particles at each frame, then moving both current and past particles using optical flow. Streaklines are the locus of points that connect particles which originated from the same initial position.This approach is advantageous over the previous representations in two aspects: first, its rich representation captures the dynamics of the crowd and changes in space and time in the scene where the optical flow representation is not enough, and second, this model is capable of discovering groups of similar behavior within a crowd scene by performing motion segmentation. We propose a method to distinguish different group behaviors such as divergent/convergent motion and lanes using this framework. Finally, we introduce flow potentials as a discriminative feature to recognize crowd behaviors in a scene. Results of extensive experiments are presented for multiple real life crowd sequences involving pedestrian and vehicular traffic.The proposed method exploits optical flow as the low level feature and performs integration and clustering to obtain coherent group motion patterns. However, we observe that in crowd video sequences, as well as a variety of other vision applications, the co-occurrence and inter-relation of motion patterns are the main characteristics of group behaviors. In other words, the group behavior of objects is a mixture of individual actions or behaviors in specific geometrical layout and temporal order.We, therefore, propose a new representation for group behaviors of humans using the inter-relation of motion patterns in a scene. The representation is based on bag of visual phrases of spatio-temporal visual words. We present a method to match the high-order spatial layout of visual words that preserve the geometry of the visual words under similarity transformations. To perform the experiments we collected a dataset of group choreography performances from the YouTube website. The dataset currently contains four categories of group dances.
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Date Issued
2011
Identifier
CFE0004482, ucf:49317
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0004482