Optical Processes in Microparticles and Nanostructures

The following sections are included:

• CONTENTS

• FOREWORD

• PREFACE

• Front Cover

• Back Cover

• Fundamental Physical Constants

Earlier pioneering work of Richard Chang set the foundation for the development of secondharmonic generation and sum-frequency generation as surface-specific spectroscopic tools that have created many new opportunities for research in surface science.

After some special points from the long history of surface-enhanced Raman scattering (SERS) and a discussion of silver (Ag) films on nano-spheres and their relation to the “chemical effect” the focus of this chapter is on SERS and infrared reflection absorption spectroscopy (IRRAS) of carbon dioxide (CO₂) on cold-deposited copper films. The SERS spectra of CO₂ on copper films deposited at 40 K display neutral species at SERS active sites with bands not observed by Raman spectroscopy of CO₂ gas, but identical to the loss bands of gaseous CO₂ in electron energy loss spectroscopy. The absence of one component of the Fermi doublet of CO₂ in SERS proves that the local electromagnetic field enhancement at SERS active sites cannot deliver signals above the noise level. The activated anionic CO₂⁻ is observed by transient electron transfer from the anionic molecule to the copper metal at a subgroup of SERS active sites, which are annealed below 200 K. The IRRAS spectra show only the expected infrared (IR) active modes of neutral CO₂ representing the “majority species” of adsorbed CO₂.

Since its first use in Richard Chang's laboratory in 1982 in a comparative study with vibrational coherent anti-Stokes Raman scattering (VCARS) in a flame, pure rotational coherent anti-Stokes Raman scattering (RCARS) has gained tremendous importance for gas temperature and relative species concentration measurements in combustion diagnostics. The field of application covers basic studies on diagnostics development and on flame research as well as its use in technical combustion systems, e.g., for the determination of the gas-phase temperature in the vaporizing spray of a gasoline direct injection (GDI) injector or for the simultaneous measurement of gas temperature and exhaust-gas-recirculation rate (EGR rate) in a homogeneous charge compression ignition (HCCI) engine. An overview is given on the fundamentals of the technique and on its most important technical applications.

Laser imaging techniques have become productive tools for studying combustion. A variety of laser light scattering mechanisms can be used to measure temperature, species concentrations, and velocities in flames. In addition to giving valuable insight through visualization of the flow, imaging experiments can provide quantitative data that can be used to guide and verify combustion models. Several different approaches will be considered, ranging from a set of multiparameter imaging experiments in a turbulent nonpremixed flame designed to measure reaction rate and scalar dissipation, to measurements of soot in laminar flames. The first set of experiments required simultaneous imaging of four different quantities, using four lasers, while the second was carried out using a consumer digital camera. Data from both types of measurements are valuable for quantitative comparisons with model predictions.

Laser light scattering measurements from single levitated solid particles, or microdroplets, have been used to study a wide variety of physical, chemical, and optical properties and processes. A brief overview of the electrodynamic levitation is provided, and numerous applications of light scattering are reviewed. The interpretation of elastic scattering data based on Mie theory has made it possible to measure precisely the size and refractive index of microspheres and to apply such measurements to analyze droplet evaporation and condensation processes for pure components and multicomponent droplets. In addition, the Rayleigh limit of charge on a droplet has been explored using elastic scattering to determine the size and charge corresponding to droplet explosions, when a droplet reaches the Rayleigh limit. Phase function measurements (intensity versus scattering angle) and data on morphology-dependent resonances (MDRs) provide alternative methods to determine the size and refractive index of droplets and microspheres, and the characteristics of coated spheres. The absorption of electromagnetic radiation leads to particle heating, and for volatile droplets this increases the evaporation rate. A review of theory and applications of microsphere heating is provided. Inelastic scattering (Raman and fluorescence scattering) has been used to measure the chemical composition of microparticles and to study gas/particle and gas/droplet chemical reactions such as the uptake of carbon dioxide by reaction with sorbent particles. Inelastic scattering has also been used to follow polymerization of monomer microparticles. It is shown that the temperature of a microparticle can be determined by measuring the ratio of the anti-Stokes to Stokes scattering intensities. Finally, the application of inelastic scattering to the detection of biological particles such as pollen and bacteria is explored.

Microparticles, particularly in the form of spheres and cylinders with radii larger than the wavelength of light, as well as coated gas bubbles, are at the center of various fields of study that include linear and nonlinear optics, combustion diagnostics, fuel dynamics, colloid chemistry, atmospheric science, telecommunications, and pulmonary medicine. The spectroscopy of single microparticles is feasible nowadays due to the development of various optical and electromagnetic trapping techniques. While data derived from elastic scattering, such as the angular distribution of the scattered radiation or the radiation pressure acting on spherical resonators, e.g., microdroplets, provides mainly information about the morphology of the particle, inelastic light scattering, e.g., Raman spectroscopy, yields additional information concerning the chemical composition of the material under investigation. Trapping techniques allow to obtain Raman spectra of single particles, whose sizes are of the order of or larger than the wavelength of the exciting light. However, in scattering systems with well-defined geometries, e.g., cylindrical, spherical, or spheroidal cavities, the use of Raman spectroscopy as a diagnostic probe becomes complicated due to morphologydependent resonances (MDRs) of the cavity. Such cavity resonances may give rise to sharp peaks in a Raman spectrum that are not present in bulk Raman spectra. These peaks result from resonanceinduced enhancements to the Raman scattering. The physical nature of these resonances can be described for dielectric particles by means of the well-known Lorenz-Mie theory. These MDRs can be used together with Raman data for a comprehensive study of the physical properties as well as the time dependence of chemical reactions. Here, we present a short review of our own work on combined inelastic/elastic (Raman/Mie) light scattering studies and their applications to several microchemical reactions as well as on elastic light scattering on a femtosecond timescale. A few representative examples have been chosen to demonstrate the power of such light scattering studies of microparticles trapped by optical or electrodynamical forces.

We revisit Prof. Chang's early experiments on morphology-dependent resonances (MDRs) in this chapter. We observed MDRs in the lasing spectra of dye-coated polystyrene microspheres and the fluorescence spectra in quantum dot-coated microspheres. An Ar⁺ laser is used to excite lasing/fluorescence from the dye molecules and the quantum dots while a near-IR laser tweezer beam is used to trap individual microspheres and control their positions with respect to the Ar⁺ beam. Photobleaching prevented us from observing the transition from spontaneous to stimulated emission in quantum dot-coated microspheres.

We investigate the scattering threshold and cavity-enhanced gain in nonlinear spheres with second- or third- order permittivity. Pairs of pump-driven and signal modes are considered, satisfying morphology-dependent resonance conditions. The thresholds and gain coefficients of amplified and stimulated Raman scattering, parametric downconversion and analogous parametric processes in microspheres are derived and evaluated under typical conditions. Applications may include the measurement of chemical impurity concentrations or the creation of low-threshold optical parametric amplifiers using microspheres.

The development of techniques for bioaerosol detection and characterization has flourished during the last decade. A brief summary of the advancements of Prof. Richard K. Chang and his research group at Yale University, together with collaborators at the US Army Research Laboratory (ARL), is given here. We focus on the development of the Single-Particle Fluorescence Spectrometer (SPFS). The SPFS is capable of real-time, in-situ monitoring, classification, sorting, and collection of bioaerosols. The SPFS rapidly samples single aerosol particles having sizes in the 1- to 10-μmdiameter range, illuminates them one-by-one with a pulsed UV laser, disperses the fluorescence generated, measures the fluorescence spectra and elastic scattering, and uses these measurements to classify the particles. Bioaerosol particles can be sorted one-by-one, according to their fluorescence spectra, using an aerodynamic puffer, and collected to yield an enriched aerosol sample defined by the particles' fluorescence “signature.” A system to further identify specific particle types in the enriched sample has also been investigated. The intent is to provide a system capable of monitoring harmful aerosols in the highly variable and complex atmospheric environment. Here we describe our investigations of ultraviolet-laser-induced fluorescence (UV-LIF) of aerosols, the evolution of the SPFS technology, and the application of the SPFS to characterization of atmospheric aerosols at Adelphi, MD, New Haven, CT, and Las Cruces, NM, USA.

Elastic scattering of light from micron-sized aerosol particles provides a wealth of information about the scatterer including shape, size, internal structure, surface roughness, composition, and orientation. The ability to understand and interpret properly the light scattering properties of irregular and nonspherical particles (including aggregates) is important to fields as diverse as global warming and food processing. Incident radiation is elastically scattered over 4π sr, however due to the complexity of the angular scattering and limitations on computing power, measurements and numerical comparisons are traditionally performed at a fixed azimuthal angle, while varying the polar angle (0° – 180°). This limited angular coverage excludes much of the information contained in the elastic scattering signal. The emergence of inexpensive detector arrays permits the measurement of twodimensional angular optical scattering (TAOS) patterns with wider angular coverage (over 2π sr). In addition, increases in computational power make it feasible to model complex particle morphologies with numerical simulations. Over the past two decades, researchers at Yale University, University of Hertfordshire, the US Army Research Laboratory (ARL), and Porton Down have developed techniques for maximizing the collection of light scattered from nonspherical particles as well as data analysis techniques capable of extracting single particle morphology from the information-rich TAOS patterns. This chapter will present their scientific achievements, discuss current research efforts, and outline the major questions remaining in the field.

Femtosecond spectroscopy opens new perspectives in bioaerosol sensing. On one side, the induced nonlinear light emission from the particles is remarkably peaked in the backward direction, which is favorable for remote detection, and on the other, quantum control schemes allow discrimination from major interference such as soot and other organic non-biological particles.

In the past decade, an unconventional laser named “random laser” has drawn much attention. The laser cavity is formed not by mirrors, but by optical scattering in a disordered gain medium. This novel laser exhibits many features distinct from a conventional laser, which lead to unique applications.

Size-dependence of efficient UV photoluminescence (PL) and absorption spectra of various sizes of zinc oxide (ZnO) quantum dots (QDs) give evidence for the quantum confinement effect. Bandgap enlargement is in agreement with the theoretical calculation based on the effective mass model for the size of ZnO QDs being comparable to the Bohr radius of bulk exciton. By using the modified spatial correlation model to fit the measured Raman spectra, we reveal that the Raman spectral shift and asymmetry for E₂(high) mode are caused by localization of optical phonons. Furthermore, we present temperature-dependent PL of different sizes of ZnO particles. The unobvious LO-phonon replicas of free exciton (FX) were observed when the ZnO particle sizes were under 12 nm in diameter. The increasing exciton energy (E_b) with the decreasing quantum dot size can be obtained from temperature-dependent PL. From the temperature-dependent change of FX emission energy, we deduce that the exciton-LO phonon coupling strength reduces as the particle size decreases. The reduced exciton Bohr radius a_B with particle size obtained from E_b and PL spectrum confirms that the exciton becomes less polar in turn reducing the Fröhlich interaction and the exciton-LO phonon interaction is reduced with decreasing size of the ZnO QDs. In addition, the nearly unchanged spectral shape in power dependent PL of ZnO quantum dots reveals stable exciton states without formation of biexcitons and exciton-exciton cattering.

Constant flux states describe the steady state response of a photonic medium with an arbitrary, possibly frequency ependent index of refraction n(x,ω) to a harmonically oscillating source. Constant flux states were initially introduced to describe steady state oscillations of complex lasers. Here, we describe their application to various phenomena in photonics and quantum optics.

We review results of our recent research and development efforts devoted to application of electro-optical crystalline whispering gallery mode resonators in the area of microwave photonics. In particular, we describe deployment of these resonators in functional radio frequency signal processing devices operating at frequencies laying in microwave and millimeter-wave bands. We discuss demonstrations of quadratic photonic detectors, co-herent quadratic receivers, terahertz receivers, optical pulse generators and parametric converters, as well as examples of small footprint packaging prototypes of these devices.

This chapter is intended to tell a story, in a small way, how the early studies of the optical microcavity microspheres by the author, Prof. Richard K. Chang, and Prof. John B. Fenn in the 1970's and subsequent studies of the microdisks and microrings by others in Prof. Richard K. Chang's group evolved to affect and shape the present day very large scale integrated photonics technology and its future outlook. The chapter first describes how the concept of VLSI photonics was born out in the early 2000's out of the need for high-speed and high-capacity information technology and discusses the scientific and technological issues and challenges to be worked out for its future. The chapter then reviews how the early studies on the optical microcavity microspheres in the 1970's progressed to the understanding of the optical whispering gallery modes in Prof. Richard K. Chang's group and how they later evolved to the studies of microcylinders, microdisks, and microrings, which found their applications in photonic integration and very large scale integrated photonics of today. It then discusses on the outlook of the very large scale integrated photonics in the future.

Spherical microresonators are natural building blocks of three dimensional integrated photonics. The resulting three dimensional device would be a volumetric lightwave circuit or a photonic integrated circuit. The intersphere communication channels can be evanescently coupled optical waveguides. The possible multidimensional integration of these spherical microresonators evanescently coupled with optical waveguides will usher in a new age in integrated photonics and resulting applications and systems.

This chapter exemplifies in a small way how some of Prof. Richard Kounai Chang's lasting contributions to the science and technology of optical microcavities are finding promising applications in integrated photonics. Specifically, we review the development of microspiral and double-notch-shaped microdisk resonator-based passive devices. Microspiral resonators as uni-port directional emission microcavity lasers was proposed and demonstrated by Chang's group in 2003. We review how such novel-shaped microresonator design featuring gapless coupling has been bearing fruits as a new breed of silicon photonic passive devices including channel filters and coupled-resonator optical waveguides.

In this chapter we discuss the generation of broadband terahertz radiation from nitride semiconductors by ultrafast optical pulses. We focus in particular on the generation mechanisms related to the photo-Dember effect and acceleration of photogenerated charges in built-in electric fields in-plane as well as normal to the sample surface.

Implantation of an intraocular retinal prosthesis represents one possible approach to the restoration of sight in those with minimal light perception due to photoreceptor degenerating diseases such as retinitis pigmentosa and age-related macular degeneration. In such an intraocular retinal prosthesis, a microstimulator array attached to the retina is used to electrically stimulate still-viable retinal ganglion cells that transmit retinotopic image information to the visual cortex by means of the optic nerve, thereby creating an image percept. We describe herein an intraocular camera that is designed to be implanted in the crystalline lens sac and connected to the microstimulator array. Replacement of an extraocular (head-mounted) camera with the intraocular camera restores the natural coupling of head and eye motion associated with foveation, thereby enhancing visual acquisition, navigation, and mobility tasks. This research is in no small part inspired by the unique scientific style and research methodologies that many of us have learned from Prof. Richard K. Chang of Yale University, and is included herein as an example of the extent and breadth of his impact and legacy.

This article summarizes the model for birefringent propagation in optical fibers, and describes its detail numerical implementation. Validation results against known solutions for the formulation are presented.

Physical phenomena occurring in open wave systems can be analyzed in terms of relevant quasi-normal modes (QNMs), which are metastable states characterized by complex eigenfrequencies, in a way paralleling the normal mode expansion in closed systems. In this review, we discuss the application of quasi-normal mode expansion (QNME) and the properties of QNMs for various open systems, including microdroplets, black holes and neutron stars. These systems and related wave phenomena are of physical importance in their own right, and QNME is manifestly a powerful tool to describe them.

The following sections are included:

• INDEX