Random Transformations Of Optical Fields And Applications

The interaction of optical waves with material systems often results in complex, seemingly random fields. Because the fluctuations of such fields are typically difficult to analyze, they are regarded as noise to be suppressed. Nevertheless, in many cases the fluctuations of the field result from a linear and deterministic, albeit complicated, interaction between the optical field and the scattering system. As a result, linear systems theory (LST) can be used to frame the scattering problem and highlight situations in which useful information can be extracted from the fluctuations of the scattered field. Three fundamental problems can be posed in LST regardless of the nature of the system: one direct and two inverse problems. The direct problem attempts to predict the response of a known system to a known input. The problem may be simple enough to admit analytical solutions as in the case of homogeneous materials, phase and amplitude screens, and weakly scattering materials; or the problem may require the use of numerical techniques. This dissertation will focus on the two inverse problems, namely the determination of either the excitation field or the scattering system. Traditionally, the excitation determination problem has relied on designing optical systems that respond to the property of interest in a simple, easily quantified way. For example, gratings can be used to map wavelength onto direction of propagation while waveplates and polarizers can map polarization properties onto intensity. The primary difficulty with directly applying the concepts of LST to scattering systems iv is that, while the outputs are still combinations of the inputs, they are not ``simple\u27\u27 combinations such as Fourier transforms or spatially dispersed spectral components of the input spectrum. Instead, the scattered field can be thought of as a massive sampling and mixing of the excitation field. This dissertation will show that such complicated sampling functions can be characterized and that the corresponding scattering medium can then be used as an optical device such as a lens, polarimeter, or spectrometer. The second inverse problem, system determination, is often more difficult because the problem itself may be ill-posed. For scattering systems that are dominated by low-order scattering, the statistical properties of the scattered light may serve as a fingerprint for material discrimination; however, in many situations, the statistical properties of the output do not depend on the material properties. Rather than analyzing the scattered field from one realization of the random interaction, several measurement techniques have been developed that attempt to extract information about the material system from modifications of the scattered field in response to changes in either the excitation or the intrinsic dynamics of the medium itself. One such technique is dynamic light scattering. This dissertation includes an extension to this method that allows for a polarimetric measurement of the scattered light using a reference beam with controllable polarization. Another system determination problem relates to imaging the reflectivity of a target that is being randomly illuminated. It will be demonstrated that an approach based on the correlation between the integrated scattered intensity and the corresponding illumination intensity distribution can prove superior to standard imaging microscop

Finding the field transfer matrix of scattering media

When illuminated by temporally coherent light, multiply scattering media produce speckle patterns that in many situations are unpolarized on spatial averaging. As a result, the underlying field statistics are assumed to be Gaussian and information about them can be extracted from intensity-intensity correlations. However, such an approach cannot be applied to any scattering medium where the interaction leads to partially developed speckle patterns. We present a general procedure to directly measure the field transfer matrix of a linear medium without regard to the scattering regime. Experimental results demonstrate the ability of our procedure to correctly measure field transfer matrices and use them to recover the polarization state of incident illumination

Optically Induced Forces In Scanning Probe Microscopy

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 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 nearfield scanning optical microscopy (NSOM). By detecting surface-induced forces or force gradients applied to a very small size probe (~ 20 nm 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 (~ 50 150 nm diameter) to collect and/or radiate light in a small volume or a small scatterer (~ 20 nm 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 iv 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

Structural control of nonlinear optical absorption and refraction in dense metal nanoparticle arrays

The linear and nonlinear optical properties of a composite containing interacting spherical silver nanoparticles embedded in a dielectric host are studied as a function of interparticle separation using three dimensional frequency domain simulations. It is shown that for a fixed amount of metal, the effective third-order nonlinear susceptibility of the composite chi((3))(omega) can be significantly enhanced with respect to the linear optical properties, due to a combination of resonant surface plasmon excitation and local field redistribution. It is shown that this geometry-dependent susceptibility enhancement can lead to an improved figure of merit for nonlinear absorption. Enhancement factors for the nonlinear susceptibility of the composite are calculated, and the complex nature of the enhancement factors is discussed

Using a multimode fiber as a high resolution, low loss spectrometer

We propose and demonstrate that a conventional multimode fiber can function as a high resolution, low loss spectrometer. The proposed spectrometer consists only of the fiber and a camera that images the speckle pattern generated by interference among the fiber modes. While this speckle pattern is detrimental to many applications, it encodes information about the spectral content of the input signal which can be recovered using calibration data. We achieve a spectral resolution of 0.15 nm over 25 nm bandwidth using 1 meter long fiber, and 0.03 nm resolution over 5 nm bandwidth with a 5 meter fiber. The insertion loss is less than 10%, and the signal to noise ratio in the reconstructed spectra is over 1000.Comment: 3 pages, 3 figure

Design and testing of a frequency selective surface (FSS) based wide-band multiple antenna system

Since the first radio link was built by Hertz in 1886, antennas have become a critical technology which allows people to stay connected and informed. Several advances have been made in the field of antenna theory and technology in the past hundred twenty years. Among them is the characterization of frequency selective surfaces (FSS), which are periodic arrays of passive elements or slots that act as a band stop or a band pass filters respectively to propagating electromagnetic waves. The purpose of this project was to construct an antenna which is transmissive outside of the band of operation. For example, the antenna designed in this project operates in a band of 1-2 GHz. The goal of this project is to be able to place an antenna operating at 4-8 GHz behind this antenna and have it be able to “look though” the first antenna as if it wasn’t there. This will allow the user to stack antennas one behind the other and thus increase the density of antennas in a given area. This is advantageous in applications where the available real estate upon which to place antennas is limited, such as on ships and submarines. This antenna has two main components - an array of radiating elements and a reflector. The radiating array will be transmissive at 4-8 GHz as long as it does not radiate energy at this frequency and does not significantly scatter energy. These constraints are easily met by creating an array of wire elements. Reflectors, on the other hand, are commonly composed of a solid metal plate, which will reflect energy at any frequency. However, this project uses an element FSS for a reflector. As a result this reflector will only reflect energy in the stop band. Sufficiently outside of this band, it will be transmissive. While an entire antenna was designed for sake of completeness, the focus of this project was the design and testing of the FSS reflector. There were two main components to this project. The first was to use computational codes to design the antenna. Specifically, the antenna was designed using a Method of Moments (MoM) code, which calculates gain patterns for finite antennas. These results were then compared to a periodic moment method code, which calculates the ideal result for an infinite structure. This design process was completed in several steps. First the FSS array was designed to be reflective in the L band (1-2 GHz) and transmissive outside of this band. Following this the radiating array was designed to realize sufficiently flat L band bandwidth. The FSS reflector and radiating array were then combined together and the gain and transmissivity were then calculated for the entire antenna. Finally a prototype of the FSS reflector was built and tested. Time constraints prevented the construction of the entire antenna. The results of these tests are in very good agreement with each other. MoM tests show the FSS is within 1 dB of perfect reflectivity over the entire L band range. The prototype was within 2 dB of perfect reflectivity over the same range. This deviation is explained by unavoidable human error in the construction of the FSS. The periodic moment method code is also computed similar results. The bandwidth wasn’t quite as large in the PMM test, but this is expected and is explained by the fact that edge diffraction on finite structures increases the bandwidth. The transmissivity of this FSS is within 2 dB of perfect transmissivity in the C band (4-8 GHz.) Finally the gain of the radiating array has a 2 dB variation over L band, and the gain of the entire antenna has a 3 dB variation over L band