Sean Kirkpatrick

University of Georgia | UGA · Department of Physics and Astronomy

Sightings of unidentified flying objects (UFOs) or unidentified anomalous phenomena (UAP) have been reported throughout history. Given the potential security and safety risks they pose, as well as scientific curiosity, there is increasing interest in understanding what these sighting reports represent. We approach this problem as an important one o...

During the absorption of a laser pulse of moderate length, the leading edge can experience excited state absorption out of the first singlet state. The measurement of this excited state absorption spectrum can only be accurately probed using short pulse pump-probe techniques. Specifically, we examine the excited state absorption of AF-380 in THF us...

Optical limiting requires the development of materials exhibiting strong nonlinear absorption. Multiphoton absorption in materials can take the form of instantaneous two-photon absorption or time-integrating excited state absorption. We examine both types of mechanisms for optical limiting. Several experimental methods are used to study these pheno...

Progress in femtosecond laser technology has spawned a rapidly emerging area of investigation covering diverse research and development sectors such as novel laser source development, complex understandings of propagation physics, easily accessible higher order nonlinear physics, advanced material processing, or manipulation of biomolecules and nan...

Organic materials exhibiting strong two-photon absorption cross-sections and subsequent up-converted fluorescence have been targeted for use in a variety of applications including optical data storage, nondestructive imaging, frequency up-converted lasing, and microfabrication. In order for these materials to be useful in practical application they...

In order to achieve the high amount of index of refraction mismatch necessary for the fabrication of a photonic bandgap device, a highly ordered hybrid material of organic and inorganic compounds must be developed. Through the identification of peptides from the diatom Cylindrotheca fusiformis, simple silica nanospheres can now be synthesized from...

Organic materials exhibiting strong two-photon absorption cross-sections and subsequent up-converted fluorescence have been targeted for use in a variety of applications including optical data storage, nondestructive imaging, frequency up-converted lasing, and microfabrication. The nonlinear photophysical properties of a two-photon absorbing chromo...

We report photophysical measurements and application of an effective three-photon absorption model that characterize the two-photon and excited state absorption in organic D-pi-A chromophores. The key parameter is an effective three-photon absorption coefficient that depends on the intrinsic molecular two-photon absorption cross section and excited...

The synthesis of composite materials that exhibit inhomogeneities on the sub-micron or nanometer scale using chemical and physical deposition processes is a challenge. The use of a biomimetic approach may compliment existing methods in creating materials that exhibit properties that would not be otherwise achieved. We describe herein our efforts to...

Porous silicon (poSi) has been the focus of many studies recently due to its unique nonlinear optical properties. The nonlinear properties of poSi wafers were studied as a function of intensity using a 790nm, 75 femtosecond laser system. The effects of pulse stretching were measured as a function of beam intensity and sample porosity. Cross correla...

The formation of a Graded Index of Refraction (GRIN) lens is possible by using two-photon induced polymerization. Preliminary GRIN profiles are presented with a short discussion on application to deformable lenses. Summary Using the technique of single beam pulse formatting, a graded index of refraction (GRIN) lens can be fabricated using two photo...

In this paper we describe improvements and changes to a 3He/ 4He dilution refrigerator (DR) which is precooled by a Gifford-McMahon (GM) refrigerator instead of a helium bath cryostat. The general layout of this "dry" DR has been described in detail before. First we report about the installation of two more step heat exchangers into the dilution un...

The return signal frequency of an eye-safe ladar system is upconverted from the infrared to the visible through sum-frequency generation by incorporation of periodically poled LiNbO3 into the receiver. A quantitative analysis of the angular acceptance and the quantum efficiency is then presented for a single macroscopic receiver optic and a multiap...

Two-photon holographic photopolymerization was used to form switchable Bragg gratings composed of layers of phase-separated liquid-crystal (LC) domains interspersed with cured, crosslinked polymer. These holographic polymer-dispersed liquid crystals form a periodic structure which diffracts red light due to nanostructured planes ∼250nm in spacing....

We investigate the space-bandwidth product of a ladar system incorporating an upconversion receiver. After illuminating a target with an eye-safe beam, we direct the return into a piece of periodically poled LiNbO3 where it is upconverted into the visible spectrum and detected with a CCD camera. The theoretical and experimental transfer functions a...

Green fluorescent protein (GFP) has been of interest to researchers recently due to its use in nonlinear microscopy and biomedical applications. Through the use of nonlinear characterization, GFP has been previously shown to exhibit saturable nonlinear absorption. This nonlinear saturation suggests the possibility of a two-photon induced population...

Red fluorescent molecule (RFM) has recently been found to be useful as a reporter gene in the control of gene expression, and is of interest to researchers for its uses in biomedical and microfabrication applications. Ultrafast nonlinear spectroscopy, linear fluorescence, absorption and excitation spectroscopy techniques are employed to investigate...

Green fluorescent protein (GFP) extracted from the bioluminescent jellyfish Aequorea Victoria[1] and its mutants are novel nanoscale systems, which have been shown to exhibit desirable linear and nonlinear optical properties[2]. In this paper, a combination of both linear and nonlinear optical spectroscopic techniques was used to investigate dynami...

The nonlinear energy dynamics and charge carrier generation of a model donor-acceptor two-photon dye, AF-380, is investigated using ultrafast nonlinear spectroscopy. This material has been the focus of several investigations due to its large purported two-photon absorption cross-section, the discrepancies between long and short pulse measurements,...

Coiled-coil proteins are assemblies of two to four alpha-helices that pack together in a parallel or anti-parallel fashion. Coiled-coil structures can confer a variety of functional capabilities, which include enabling proteins, such as myosin, to function in the contractile apparatus of muscle and non-muscle cells. The TlpA protein encoded by the...

The diversity of biological sensing and biocatalysis is astounding. A considerable effort has been directed at not only understanding the mechanism of these biological processes, but also how this activity can be maintained or duplicated in an artificial environment. We will present work on the formation of functional optical devices that convert b...

Diatoms are of interest to the materials research community because of their ability to create highly complex and intricate silica structures under physiological conditions: what these single-cell organisms accomplish so elegantly in nature requires extreme laboratory conditions to duplicate-this is true for even the simplest of structures. Followi...

Two-photon absorption is a nonlinear phenomenon that is observed in materials irradiated with the combined energy of two photons, which matches the transition energy between the ground state and the excited state. Molecules exhibiting strong two-photon absorption have great potential in a wide range of applications such as three-dimensional fluores...

Green fluorescent protein (GFP) has been shown to be an efficient two-photon absorber. It has been studied extensively and is used in nonlinear microscopy and biomedical applications. In this work, we investigate the two-photon absorption cross section of GFP using standard ultrafast Z-scan measurements, nonlinear fluorescence measurements, and sem...

The fabrication of both passive and active devices on the micron and sub-micron scale have previously been reported using the ultrafast Holographic Two-Photon Induced Polymerization (H-TPIP) technique. The fabrication of various types of structures, ranging from photonic devices to biosensors, has been demonstrated in a variety of materials systems...

Electrically Switchable Gratings Formed Using Ultrafast Holographic Two-Photon-Induced Photopolymerization Periodic gratings were formed using ultrafast holography to initiate two-photon-induced photopolymerization of a reactive monomer mixed with liquid crystal. The resulting structure shown in the cross-sectional SEM micrograph (scale bar = 3 mum...

Non-cryogenic, high sensitivity infrared detection is one of the fundamental criteria for future sensor technology. To this end, we propose mimicking the natural infrared detection structures found in certain snake pit tissue. We hypothesize that the pit organ behaves like a photonic bandgap in that a regular arrangement of sub-micron micropits rem...

Previously, we have reported the first demonstration of holographic two-photon induced photopolymerization (H-TPIP) in the construction of transmission holograms. This technique relies on the coupling of a two-photon absorbing chromophore and a photocurable optical resin. Several different systems have been successfully explored; all involving vari...

Two-photon chromophores developed by the Air Force Research Laboratory were used to internally generate visible light at 475 nm which, in turn, activates a photoinitiator enabling polymerization to occur. The advantage of the technique is two folds. First, a greater depth of penetration can be achieved in organic materials with the use of infrared...

Non-cryogenic, high sensitivity infrared detection is one of the fundamental criteria for future sensor technology. To this end, we propose mimicking the natural infrared detection structures found in certain snake pit tissue. We hypothesize that the pit organ behaves like a photonic bandgap in that a regular arrangement of sub-micron micropits rem...

Phosphorescence studies of reverse saturable absorption materials (RSA) are discussed. Absorption and emission measurements of an RSA material, specifically Zinc 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraphenylporphyrin (ZnOBP), are performed in a proprietary host polymer. Spectra are analyzed to estimate the relative energy level of the first...

We investigate the effects of the relaxation of wave vector conservation due to broken translational symmetry on the anharmonic decay process. The relaxation of momentum conservation in the vicinity of a defect site is considered and its implications in various experimental methods of investigation are discussed.

The spectroscopy and nonlinear absorption of bis(diphenylamino) diphenyl polyenes have been studied in octane and dichloromethane solvents. The amines exhibit high fluorescence quantum yield and two photon excited emission. Two photon absorption cross section, (sigma) 2, was measured by Z-scan experiments. Strong two photon absorption is indicated...

The excited state absorption of lead phthalocyanine is investigated using ultrafast transient white light absorption spectroscopy. Singlet state absorption features are identified on the basis of time. Increases in optical density up to 1 in the near infrared region of 900 nm to 1.1 micrometers are observed by pumping at 800 nm. In addition, severa...

To understand the photophysics of nonlinear absorbers, we have investigated the photophysics of a series of di(2- thienyl-3,3',4,4'-butyl)polyenes. Spectroscopic measurements, including UV/Vis, fluorescence, fluorescence lifetimes, fluorescence quantum yields, triplet state lifetime, solvent effects and two-photon absorption coefficient were obtain...

The molecular third-order optical nonlinearity γR (second hyperpolarizability or nonlinear refractive index) was measured for a series of sulfur-rich molecules:  sulfur (S8), carbon−sulfur compounds, and metal thiolate clusters. Z-scan techniques (pulse width 27 ps, wavelength 527 nm) were used to measure these properties in solution by comparing t...

Molecular excitation via the simultaneous absorption of two photons can lead to improved three-dimensional control of photochemical or photophysical processes due to the quadratic dependence of the absorption probability on the incident radiation intensity. This has lead to the development of improved three-dimensional fluorescence imaging, optical...

The photophysics of a series (alpha) ,(omega) -dithienyl polyenes was studied. Both one photon excited fluorescence and fluorescence originating from two photon absorption states were investigated. There is strong evidence of two photon absorption from fluorescence studies in agreement with our earlier studies involving nonlinear absorption.

A plasma switching technique is used to generate subnanosecond, far-infrared (FIR) pulses with frequency 113cm-1. The generation of subnanosecond FIR pulses enables us to improve the time resolution of phonon spectroscopic measurements from 50 ns to 350 ps. As an application of this technique, we investigate the subnanosecond dynamics of high-frequ...

© 1998 Society of America

The presence of low-lying optical modes in LaF3 can introduce additional decay processes which cause the frequency dependence of the phonon relaxation rate to deviate from the ω5 dependence predicted by the isotropic dispersionless model. In this paper we investigate the frequency dependence of the phonon relaxation rate in LaF3:Pr3+ over a wide ra...

An improved design for optically addressed multiple-quantum-well spatial light modulators is reported. These integrated AlGaAs modulators are of high optical quality and are simple to fabricate. They exhibit excellent performance characteristics such as 10:1 reflection contrast ratio, 1.5% diffraction efficiency, 7-μm spatial resolution, and illumi...

Short pulse laser-induced fluorescence experiments on the lowest-energy electronic states (υ<10−4 cm−1) of trivalent praseodymium ions are reported. Crystals of lanthanum trichloride with a range of Pr3+ concentrations were examined. We compute cross relaxation and upconversion coefficients for the metastable energy levels using a standard rate equ...

We report on a spectroscopic analysis of ErCl3 and 1% Er³⁺:YCl3 to determine their potential as possible laser sources at 3.5 and 4.5 μm. Concentration quenching of the low lying excited states is reported to be surprisingly very weak in this system. Although some shortening of the lifetimes is measured in the fully concentrated system, they retain...

Nonequilibrium phonons created in a crystal relax by spontaneous decay due to anharmonicity of the crystal. We formulate a theory which takes into account the effects of disorder and the breaking of wave-vector conservation on the anharmonic decay of both lattice modes and localized vibrations.

The search for materials exhibiting large third order susceptibilities has been of importance to many disciplines. Semiconductor physics including nonlinear propagation in fibers, optical switching, and self focusing effects have been of recent interest, as well as the study of conjugated polymers which have application in electronics and fast opti...

Monochromatic phonons are generated in LaF_3:Pr^3+ by pumping the defect induced one phonon absorption band. The excitation source is a far-infrared superradiant cell which is pumped by a transversely excited atmospheric pressure CO2 laser. Nonequilibrium phonons are detected optically using vibronic sideband spectroscopy. Neutron scattering measur...

Using a semiconductor switching technique, we generate subnanosecond far infrared pulses and investigate the anharmonic decay rates of high frequency acoustic phonons in LaF3:Pr3 + (0.5%). We generated a nonequilibrium phonon population at 113 cm− 1 and obtained phonon transients of the decay products at 55 and 75 cm− 1.

Due to the relative simplicity of the energy levels in rm LaF_3:Pr^{3+} this system has been the source of many investigations involving vibrational dynamics. Previous studies have included energy transfer between interstitial sites in PrF_3 due to thermal phonons (Hamilton et al, 1977), and frequency dependence measurements of the anharmonic lifet...

We discuss the generation of ultrashort far infrared (FIR) pulses using external switching of the output of a FIR superradiant cell pumped by a transversely excited atmospheric CO2 laser. The external switching mechanism consists of either one or two semiconducting wafers that have been optically excited with the second harmonic of a synchronized m...

We investigate the effects of a nonequilibrium phonon population on energy transfer in PrF3. We determine the spectral and temporal dynamics associated with transfer between Pr3+ ions in interstitial and stoichiometric sites.

Defect-induced one-phonon absorption of far-infrared radiation is utilized to generate a nonequilibrium phonon population in PrF3. We observe that the nonequilibrium phonon population stimulates energy transfer between the A3 and the stoichiometric sites of the Pr3+ ion. We investigate both the spectral and temporal dynamics associated with this pr...

The optical properties of polymer gratings containing phase separated thermal-mechanical protein, TlpA, and TlpA with gold are explored. The gratings are composed of polyethelene-glycol based acrylates, which were cured into solid polymer using holographic two-photon induced polymerization, HTPIP. Gratings with no dopants, TlpA, and TlpA with gold...

Volume gratings can be written in Fe doped LiNbO3 crystals using a counter propagating two beam coupling geometry(G. Cook, C. J. Finnan, and D. C. Jones, Appl. Phys. B 68), (1999) 911. Partial thermal erasure of the grating takes place because of the heating by the incident laser beam. To study this effect in detail, it is necessary to know the loc...