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- Title
- STUDY OF THE EXCITED-STATE ABSORPTION PROPERTIES OF POLYMETHINE MOLECULES.
- Creator
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Lepkowicz, Richard Stanley, Van Stryland, Eric W., University of Central Florida
- Abstract / Description
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This dissertation investigates excited-state nonlinearities in a series of polymethine dyes for the application of nanosecond optical limiting. Optical limiters are devices that for low intensity light exhibit a high linear transmittance, but for high intensity light strongly attenuate the incident radiation. These devices would serve to protect optical sensors from intense laser radiation by clamping the maximum energy allowed through an optical system below the damage threshold of the...
Show moreThis dissertation investigates excited-state nonlinearities in a series of polymethine dyes for the application of nanosecond optical limiting. Optical limiters are devices that for low intensity light exhibit a high linear transmittance, but for high intensity light strongly attenuate the incident radiation. These devices would serve to protect optical sensors from intense laser radiation by clamping the maximum energy allowed through an optical system below the damage threshold of the sensor. The search is ongoing for optical materials that are both broadband and have high damage thresholds to be effective materials for limiting applications. Polymethine dyes are promising compounds due to a strong and broad excited-state absorption (ESA) band in the visible region. However, the effectiveness of polymethine molecules as applied to optical limiting is hindered by a saturation of the ESA process at high fluences. Experiments and theoretical modeling are performed to determine the root causes of this saturation effect in both the picosecond and nanosecond time regime. The polymethine molecules studied have chromophore lengths from di- to pentacarbocyanine (2 to 5 -CH=CH- groups) with various bridge structures. This allows us to develop relationships between the molecular parameters of the polymethine molecules and overall nonlinear absorption performance. The experiments conducted included femtosecond white light continuum pump-probe experiments to measure ESA spectra, picosecond two-color polarization-resolved pump-probe to measure excited-state dynamics and the orientation of transition dipole moments, and picosecond and nanosecond optical limiting and z-scans. From these experiments we are able to develop energy level models that describe the nonlinear absorption processes in polymethines from the picosecond to nanosecond time regime. This work, along with the quantum chemical modeling performed at the Institute of Physics and National Academy of Sciences of Ukraine, has resulted in the creation of dyes that have improved photochemical stability with larger nonlinearities. These are useful not only for optical limiting but also for a wide variety of nonlinear optical applications.
Show less - Date Issued
- 2004
- Identifier
- CFE0000002, ucf:46115
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0000002
- Title
- EXPERIMENTAL TECHNIQUES FOR NONLINEAR MATERIAL CHARACTERIZATION: A NONLINEAR SPECTROMETER USING A WHITE-LIGHT CONTINUUM Z-SCAN.
- Creator
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Balu, Mihaela, Van Stryland, Eric, University of Central Florida
- Abstract / Description
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The main goal of this dissertation is to introduce and demonstrate a new method for the rapid determination of the nonlinear absorption spectra and the dispersion of the nonlinear refraction of optical materials in the visible and near IR spectral regions. However, conventional methods like, white-light continuum pump-probe and Z-scan techniques were used to measure the peak 2PA cross-sections for a number of commercially available photoinitiators. In the new method mentioned above, a high...
Show moreThe main goal of this dissertation is to introduce and demonstrate a new method for the rapid determination of the nonlinear absorption spectra and the dispersion of the nonlinear refraction of optical materials in the visible and near IR spectral regions. However, conventional methods like, white-light continuum pump-probe and Z-scan techniques were used to measure the peak 2PA cross-sections for a number of commercially available photoinitiators. In the new method mentioned above, a high energy, broadband femtosecond white-light continuum is used to replace the single wavelength source conventionally used in a Z-scan experiment. In a Z-scan experiment, the transmittance of a focused beam through a sample is monitored as the sample travels through the focus, in the Z direction, along the focused beam. Providing the sample exhibits nonlinear absorption and/or refraction, the detector monitors a change in transmittance and/or a change in the beam divergence (if the energy is partially collected through an aperture in front of the detector). Replacing the single wavelength source with a white-light continuum allows for a much faster way of measuring nonlinear absorption/refraction spectra. This could eliminate the need for using other tunable sources (e.g. Optical Parameter Generators/Amplifiers) for nonlinear measurements. These sources made nonlinear spectroscopy using Z-scan experiments a time consuming task. This new source/method allows for rapid and simultaneous measurement of the nonlinear absorption spectrum and the dispersion of the nonlinear refraction. We have confirmed the functionality of the continuum as a source for nonlinear optical characterization of materials by using it to perform Z-scans on the well characterized semiconductors ZnSe and ZnS and on solutions of organic dyes.
Show less - Date Issued
- 2006
- Identifier
- CFE0001241, ucf:46934
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0001241
- Title
- MOLECULAR STRUCTURE NONLINEAR OPTICAL PROPERTY RELATIONSHIPS FOR A SERIES OF POLYMETHINE AND SQUARAINE MOLECULES.
- Creator
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Fu, Jie, Van Stryland, Eric, University of Central Florida
- Abstract / Description
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This dissertation reports on the investigation of the relationships between molecular structure and two-photon absorption (2PA) properties for a series of polymethine and squaraine molecules. Current and emerging applications exploiting the quadratic dependence upon laser intensity, such as two-photon fluorescence imaging, three-dimensional microfabrication, optical data storage and optical limiting, have motivated researchers to find novel materials exhibiting strong 2PA. Organic materials...
Show moreThis dissertation reports on the investigation of the relationships between molecular structure and two-photon absorption (2PA) properties for a series of polymethine and squaraine molecules. Current and emerging applications exploiting the quadratic dependence upon laser intensity, such as two-photon fluorescence imaging, three-dimensional microfabrication, optical data storage and optical limiting, have motivated researchers to find novel materials exhibiting strong 2PA. Organic materials are promising candidates because their linear and nonlinear optical properties can be optimized for applications by changing their structures through molecular engineering. Polymethine and squaraine dyes are particularly interesting because they are fluorescent and showing large 2PA. We used three independent nonlinear spectroscopic techniques (Z-scan, two-photon fluorescence and white-light continuum pump-probe spectroscopy) to obtain the 2PA spectra revealing 2PA bands, and we confirm the experimental data by comparing the results from the different methods mentioned. By systematically altering the structure of polyemthines and squaraines, we studied the effects of molecular symmetry, strength of donor terminal groups, conjugation length of the chromophore chain, polarity of solvents, and the effects of placing bridge molecules inside the chromophore chain on the 2PA properties. We also compared polymethine, squaraine, croconium and tetraon dyes with the same terminal groups to study the effects of the different additions inserted within the chromophore chain on their optical properties. Near IR absorbing squaraine dyes were experimentally observed to show extremely large 2PA cross sections ( 30000GM). A simplified three-level model was used to fit the measured 2PA spectra and detailed quantum chemical calculations revealed the reasons for the squaraine to exhibit strong 2PA. In addition, two-photon excitation fluorescence anisotropy spectra were measured through multiple 2PA transitions. A theoretical model based on four-levels with two intermediate states was derived and used for analysis of the experimental data.
Show less - Date Issued
- 2006
- Identifier
- CFE0001350, ucf:46967
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0001350
- Title
- CHARACTERIZATION OF THE NONLINEAR REFRACTIVE INDEX OF CARBONDISULFIDE OVER AN EXTENDED SPECTRAL AND TEMPORAL RANGE.
- Creator
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Seidel, Marcus, Van Stryland, Eric, University of Central Florida
- Abstract / Description
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The intensity dependent refractive index change of a medium is frequently described in terms of the product n2xI where n2 is the nonlinear refractive index and I the light intensity. The nonlinear refractive index is often treated as constant which is a reasonable assumption if the light interacts only with bound electrons. In the case of carbon disulfide (CS2) however, nuclear motions contribute to n2. These motions occur on the sub picosecond time scale and thus become especially relevant...
Show moreThe intensity dependent refractive index change of a medium is frequently described in terms of the product n2xI where n2 is the nonlinear refractive index and I the light intensity. The nonlinear refractive index is often treated as constant which is a reasonable assumption if the light interacts only with bound electrons. In the case of carbon disulfide (CS2) however, nuclear motions contribute to n2. These motions occur on the sub picosecond time scale and thus become especially relevant for ultrashort laser pulses. The neat liquid CS2 is studied because it exhibits a large nonlinear refractive index in comparison to other liquids. Therefore, it is employed in optical switching, optical limiting, and beam filamentation applications. This thesis presents effective n2 values for Gaussian shaped linearly polarized pulses with central wavelengths of 700nm. A theoretical model describing the time evolution of the material response is applied to distinguish between the instantaneous electronic, the ultrafast nuclear and the slow nuclear origins of the nonlinear refractive index. Moreover, the tensor nature of the material response function is studied by means of circularly polarized light. The relative magnitudes of bound electronic and nuclear contributions to n2 are experimentally determined. Eventually, the dispersion of the instantaneous electronic response is measured in the spectral range between 411nm and 1064nm.
Show less - Date Issued
- 2011
- Identifier
- CFE0004052, ucf:49141
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004052
- Title
- EXPERIMENTAL AND THEORETICAL APPROACHES TO CHARACTERIZATION OF ELECTRONIC NONLINEARITIES IN DIRECT-GAP SEMICONDUCTORS.
- Creator
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Cirloganu, Claudiu, Van Stryland, Eric, University of Central Florida
- Abstract / Description
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The general goal of this dissertation is to provide a comprehensive description of the limitations of established theories on bound electronic nonlinearities in direct-gap semiconductors by performing various experiments on wide and narrow bandgap semiconductors along with developing theoretical models. Nondegenerate two-photon absorption (2PA) is studied in several semiconductors showing orders of magnitude enhancement over the degenerate counterpart. In addition, three-photon absorption ...
Show moreThe general goal of this dissertation is to provide a comprehensive description of the limitations of established theories on bound electronic nonlinearities in direct-gap semiconductors by performing various experiments on wide and narrow bandgap semiconductors along with developing theoretical models. Nondegenerate two-photon absorption (2PA) is studied in several semiconductors showing orders of magnitude enhancement over the degenerate counterpart. In addition, three-photon absorption (3PA) is studied in ZnSe and other semiconductors and a new theory using a Kane 4-band model is developed which fits new data well. Finally, the narrow gap semiconductor InSb is studied with regard to multiphoton absorption, free-carrier nonlinearities and decay mechanisms. The non-degenerate two-photon absorption was investigated in several direct-gap semiconductors with picosecond and femtosecond pulses. Large enhancements in 2PA were demonstrated when employing highly non-degenerate photon pairs and the results were shown to be consistent to a simple 2-parabolic band theory based on a ÃÂ"dressedÃÂ" state approach. The nonlinear refractive index induced in such configurations was also calculated and possible implications of such extreme behavior are discussed. A large number of measurements of 3PA were taken at multiple wavelengths and in several semiconductors. The subsequent analysis has shown that simple 2-band model calculations (based on either perturbative or tunneling approaches) do not adequately describe the experimental trends. A more comprehensive model, based on KaneÃÂ's 4-band theory was developed and we calculate three-photon spectra for zincblende structures within the perturbative framework. We have confirmed the results of our calculations performing a series of Z-scans in semiconductors ZnSe and ZnS, yielding complete experimental three-photon spectra. A systematic approach based on using a large variety of pulse durations was needed to quantify the wealth of nonlinear optical processes in InSb, accessible in the mid-infrared range. Femtosecond pulses provided a lower limit to measurements of the instantaneous effects (absorptive and refractive), while picosecond pulses allowed further characterization of the free-carrier effects, including population dynamics in the high density regime (Auger effects). The model developed permitted us to verify the temperature dependence of free-carrier absorption recently predicted, and to successfully model optical limiting data with longer, nanosecond pulses.
Show less - Date Issued
- 2010
- Identifier
- CFE0003401, ucf:48417
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0003401
- Title
- NONLINEAR ABSORPTION AND FREE CARRIER RECOMBINATION IN DIRECT GAP SEMICONDUCTORS.
- Creator
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Olszak, Peter, Van Stryland, Eric, University of Central Florida
- Abstract / Description
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Nonlinear absorption of Indium Antimonide (InSb) has been studied for many years, yet due to the complexity of absorption mechanisms and experimental difficulties in the infrared, this is still a subject of research. Although measurements have been made in the past, a consistent model that worked for both picosecond and nanosecond pulse widths had not been demonstrated. In this project, temperature dependent two-photon (2PA) and free carrier absorption (FCA) spectra of InSb are measured using...
Show moreNonlinear absorption of Indium Antimonide (InSb) has been studied for many years, yet due to the complexity of absorption mechanisms and experimental difficulties in the infrared, this is still a subject of research. Although measurements have been made in the past, a consistent model that worked for both picosecond and nanosecond pulse widths had not been demonstrated. In this project, temperature dependent two-photon (2PA) and free carrier absorption (FCA) spectra of InSb are measured using femtosecond, picosecond, and nanosecond IR sources. The 2PA spectrum is measured at room temperature with femtosecond pulses, and the temperature dependence of 2PA and FCA is measured at 10.6m using a nanosecond CO2 laser giving results consistent with the temperature dependent measurements at several wavelengths made with a tunable picosecond system. Measurements over this substantial range of pulse widths give results for FCA and 2PA consistent with a recent theoretical model for FCA. While the FCA cross section has been generally accepted in the past to be a constant for the temperatures and wavelengths used in this study, this model predicts that it varies significantly with temperature as well as wavelength. Additionally, the results for 2PA are consistent with the band gap scaling (Eg-3) predicted by a simple two parabolic band model. Using nanosecond pulses from a CO2 laser enables the recombination rates to be determined through nonlinear transmittance measurements. Three-photon absorption is also observed in InSb for photon energies below the 2PA band edge. Prior to this work, data on three-photon absorption (3PA) in semiconductors was scarce and most experiments were performed over narrow spectral ranges, making comparison to the available theoretical models difficult. There was also disagreement between the theoretical results generated by different models, primarily in the spectral behavior. Therefore, we studied the band gap scaling and spectra of 3PA in several semiconductors by the Z-scan technique. The 3PA coefficient is found to vary as (Eg-7), as predicted by the scaling rules of simple two parabolic band models. The spectral behavior, which is considerably more complex than for 2PA, is found to agree well with a recently published theory based on a four-band model.
Show less - Date Issued
- 2010
- Identifier
- CFE0003402, ucf:48418
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0003402
- Title
- CHEMICAL STRUCTURE - NONLINEAR OPTICAL PROPERTY RELATIONSHIPS FOR A SERIES OF TWO-PHOTON ABSORBING FLUORENE MOLECULES.
- Creator
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Hales, Joel McCajah, Van Stryland, Eric W., University of Central Florida
- Abstract / Description
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This dissertation reports on the investigation of two-photon absorption (2PA) in a series of fluorenyl molecules. Several current and emerging technologies exploit this optical nonlinearity including two-photon fluorescence imaging, three-dimensional microfabrication, site-specific photodynamic cancer therapy and biological caging studies. The two key features of this nonlinearity which make it an ideal candidate for the above applications are its quadratic dependence on the incident...
Show moreThis dissertation reports on the investigation of two-photon absorption (2PA) in a series of fluorenyl molecules. Several current and emerging technologies exploit this optical nonlinearity including two-photon fluorescence imaging, three-dimensional microfabrication, site-specific photodynamic cancer therapy and biological caging studies. The two key features of this nonlinearity which make it an ideal candidate for the above applications are its quadratic dependence on the incident irradiance and the improved penetration into absorbing media that it affords. As a consequence of the burgeoning field which exploits 2PA, it is a goal to find materials that exhibit strong two-photon absorbing capabilities. Organic materials are promising candidates for 2PA applications because their material properties can be tailored through molecular engineering thereby facilitating optimization of their nonlinear optical properties. Fluorene derivatives are particularly interesting since they possess high photochemical stability for organic molecules and are generally strongly fluorescent. By systematically altering the structural properties in a series of fluorenyl molecules, we have determined how these changes affect their two-photon absorbing capabilities. This was accomplished through characterization of both the strength and location of their 2PA spectra. In order to ensure the validity of these results, three separate nonlinear characterization techniques were employed: two-photon fluorescence spectroscopy, white-light continuum pump-probe spectroscopy, and the Z-scan technique. In addition, full linear spectroscopic characterization was performed on these molecules along with supplementary quantum chemical calculations to obtain certain molecular properties that might impact the nonlinearity. Different designs in chemical architecture allowed investigation of the effects of symmetry, solvism, donor-acceptor strengths, conjugation length, and multi-branched geometries on the two-photon absorbing properties of these molecules. In addition, the means to enhance 2PA via intermediate state resonances was investigated. To provide plausible explanations for the experimentally observed trends, a conceptually simple three level model was employed. The subsequent correlations found between chemical structure and the linear and nonlinear optical properties of these molecules provided definitive conclusions on how to properly optimize their two-photon absorbing capabilities. The resulting large nonlinearities found in these molecules have already shown promise in a variety of the aforementioned applications.
Show less - Date Issued
- 2004
- Identifier
- CFE0000005, ucf:46103
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0000005