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Garth J Simpson

Publications and source records attributed to Garth J Simpson.

13 recordsLinked to original sources
PubMed2006

Mechanism of the chiral SHG activity of bacteriorhodopsin films.

The nonlinear optical activity of bacteriorhodopsin (bR) Langmuir-Blodgett (LB) films were investigated computationally and experimentally. The second harmonic generation optical rotary dispersion (SHG-ORD) response was calculated directly from the known structure and orientation of the PSB retinal chromophore within bR with no adjustable parameters. The predicted results agree remarkably well in sign, magnitude, and trend with the experimental SHG-ORD measurements. The calculations indicated negligible chirality with the tensor for the PSB retinal chromophore, but significant chiral-specific activity for the thin film through a relatively simple orientational mechanism.

Bacteriorhodopsins↗
PubMed2006

Nanoscale dielectrophoretic spectroscopy of individual immobilized mammalian blood cells.

Dielectrophoretic force microscopy (DEPFM) and spectroscopy have been performed on individual intact surface-immobilized mammalian red blood cells. Dielectrophoretic force spectra were obtained in situ in approximately 125 ms and could be acquired over a region comparable in dimension to the effective diameter of a scanning probe microscopy tip. Good agreement was observed between the measured dielectrophoretic spectra and predictions using a single-shell cell model. In addition to allowing for highly localized dielectric characterization, DEPFM provided a simple means for noncontact imaging of mammalian blood cells under aqueous conditions. These studies demonstrate the feasibility of using DEPFM to monitor localized changes in membrane capacitance in real time with high spatial resolution on immobilized cells, complementing previous studies of mobile whole cells and cell suspensions.

Animals↗
PubMed2005

Reduction of tip-sample contact using dielectrophoretic force scanning probe microscopy.

Dielectrophoretic force microscopy is shown to allow for facile noncontact imaging of systems in aqueous media. Electrokinetic tip-sample forces were predicted from topography measurements of an interface and compared with experimental images. Correlation function and power spectral density analyses indicated that image feedback was maintained without mechanical contact using moderate potentials (e.g., approximately 18 nm off the surface for a 7-Vpp, 100-kHz waveform). The applied dielectrophoretic force and the corresponding increase in effective tip radius were predictably adjusted by changing the peak potential.

Mathematics↗
PubMed2005

Electronic and vibrational second-order nonlinear optical properties of protein secondary structural motifs.

A perturbation theory approach was developed for predicting the vibrational and electronic second-order nonlinear optical (NLO) polarizabilities of materials and macromolecules comprised of many coupled chromophores, with an emphasis on common protein secondary structural motifs. The polarization-dependent NLO properties of electronic and vibrational transitions in assemblies of amide chromophores comprising the polypeptide backbones of proteins were found to be accurately recovered in quantum chemical calculations by treating the coupling between adjacent oscillators perturbatively. A novel diagrammatic approach was developed to provide an intuitive visual means of interpreting the results of the perturbation theory calculations. Using this approach, the chiral and achiral polarization-dependent electronic SHG, isotropic SFG, and vibrational SFG nonlinear optical activities of protein structures were predicted and interpreted within the context of simple orientational models.

Algorithms↗
PubMed2005

Self-consistent approach for simplifying the molecular interpretation of nonlinear optical and multiphoton phenomena.

A method is developed for simplifying molecular interpretations of nonlinear optical phenomena. General sum-over-states expressions derived from perturbation theory can be written identically and self-consistently as simple products of lower-order effects. Electric dipole-allowed expressions for the nonlinear polarizability reduce to straightforward formulas directly connected to intuitive molecular properties without sacrificing mathematical rigor. This approach is sufficiently general to allow its application in treating electronic, vibrational, and vibronic interactions for both parametric (passive) processes (e.g., wave-mixing spectroscopies, sum- and difference-frequency generation, harmonic generation, etc.) and nonparametric (active) processes (e.g., hyper-Raman spectroscopy, multiphoton absorption, etc.). Explicit examples for sum-frequency generation and for four-wave mixing provide a convenient context for interpreting higher order nonlinear optical processes.

Journal Article↗
PubMed2005

Dielectrophoretic force microscopy of aqueous interfaces.

A novel scanning probe microscopy technique has allowed dielectrophoretic force imaging with nanoscale spatial resolution. Dielectrophoresis (DEP) traditionally describes the mobility of polarizable particles in inhomogeneous alternating current (ac) electric fields. Integrating DEP with atomic force microscopy allows for noncontact imaging with the image contrast related to the local electric polarizability. By tuning the ac frequency, dielectric spectroscopy can be performed at solid/liquid interfaces with high spatial resolution. In studies of cells, the frequency-dependent dielectrophoretic force is sensitive to biologically relevant electrical properties, including local membrane capacitance and ion mobility. Consequently, dielectrophoretic force microscopy is well suited for in vitro noncontact scanning probe microscopy of biological systems.

Journal Article↗
PubMed2005

Direct determination of effective interfacial optical constants by nonlinear optical null ellipsometry of chiral films.

Nonlinear optical null ellipsometry (NONE) measurements of chiral interfaces allowed direct experimental measurement of the linear interfacial optical constants in surface second harmonic generation (SHG) measurements. Since phase information is retained in NONE measurements, the real and imaginary components of the interfacial refractive index (n and k, respectively) were uniquely obtained from the measured chiral chi((2)) tensor elements of a fluorescein-labeled bovine serum albumin film. The sensitivity of the calculated chi((2)) tensor elements on the assumed values of the interfacial optical constants allowed measurements of n and k to four significant figures with no additional adjustable parameters and independent of molecular symmetry. The optical constants measured by SHG agreed within a relative error of 0.8% with values predicted independently using a simple effective medium approximation, also with no adjustable parameters. Additionally, those same optical constants produced relationships between the achiral chi((2)) tensor elements in excellent agreement with predictions for systems exhibiting weak orientational order. This study suggests that the far-field intensity and polarization state of the nonlinear optical beam may be largely independent of the near-field optical constants within the interfacial layer in the limit of a film thickness much less than the wavelength of light.

Journal Article↗
PubMed2004

Molecular origins of the remarkable chiral sensitivity of second-order nonlinear optics.

Recent observations of remarkably large chiroptical effects in second-harmonic generation (SHG) and sum-frequency generation (SFG) measurements suggest exciting possibilities for the development of new chiral-specific spectroscopies and novel chiral materials for nonlinear optics. Several fundamental studies designed to elucidate the molecular and macromolecular origins of the chiral responses are reviewed to provide a framework for development of this emerging field. In general, the chiral activity in SHG and SFG has the potential to arise from complex interactions between hosts of different competing effects. Fortunately, relatively simple electric dipole-allowed mechanisms routinely dominate the nonlinear optical chiral activities of most practical systemsexpressions can often be generated to link the. This substantial reduction in complexity allows for the development of simple models connecting the macroscopic nonlinear optical response to intuitive molecular and supramolecular properties.

Journal Article↗
PubMed2004

Ellipsometric approach for the real-time detection of label-free protein adsorption by second harmonic generation.

Second harmonic generation (SHG) was performed using a novel ellipsometric detection approach to selectively probe the real-time surface binding kinetics of an unlabeled protein. The coherence of nonlinear optical processes introduces new possibilities for exploiting polarization that are unavailable with incoherent methods, such as absorbance and fluorescence. Adsorption of bovine serum albumin (BSA) at silica/aqueous solution interfaces resulted in changes in the polarization state of the frequency-doubled light through weak, dynamic interactions with a coadsorbed nonlinear optical probe molecule (rhodamine 6G). Using a remarkably simple instrumental approach, signals arising exclusively from surface interactions with BSA were spatially isolated and selectively detected with high signal-to-noise. The relative intensities acquired during the kinetics experiments using both circularly and linearly polarized incident beams were in excellent agreement with the responses predicted from SHG ellipsometry polarization measurements. Analysis of the polarization-dependent SHG generated during BSA adsorption at glass/aqueous solution interfaces provided direct evidence for slow conformational changes within the protein layer after adsorption, consistent with protein denaturation. This polarization selection approach is sufficiently general to be easily extended to virtually all coherent nonlinear optical processes and a variety of different surface interactions and architectures.

Adsorption↗
PubMed2003

Experimental confirmation of the importance of orientation in the anomalous chiral sensitivity of second harmonic generation.

Macromolecular interactions were demonstrated to yield large chiroptical effects in second harmonic generation measurements of ultrathin surface films. Second harmonic generation (SHG) has recently shown to be several orders of magnitude more sensitive to chirality in oriented systems than common linear methods, including absorbance circular dichroism (CD) and optical rotary dispersion (ORD). Numerous mechanisms have been developed to explain this anomalous sensitivity, with a general emphasis on understanding the molecular origins of the chromophore chirality. In this work, orientational effects alone are shown to be the dominant factor for generating large SHG chiral dichroic ratios in many surface systems. Three distinct uniaxial surface films of SHG-active achiral chromophores oriented at chiral templated surfaces were observed to yield chiral dichroic ratios as great as 40% in magnitude.

Journal Article↗
PubMed2002

Coupled electrorotation of polymer microspheres for microfluidic sensing and mixing.

We show that coupled electrorotation (CER) of microscopic particles using microfabricated electrodes can be used for localized sensing and mixing. The effective use of microelectromechanical systems and micro total analysis systems requires many types of control. These include the abilityto (1) manipulate objects within microchannels by noncontact means, (2) mix fluids, and (3) sense local chemical parameters. Coupled electrorotation, in which the interactions between induced electric dipoles of adjacent particles lead to particle rotation, addresses aspects of all three challenges simultaneously. CER is a simple means of controlling the rotation of dielectric objects using homogeneous external radio frequency electric fields. CER is sensitive to several chemical and physical parameters such as the solution conductivity, pH, and viscosity. As a step toward integrating CER devices into microfluidic systems, a simple chip was designed to induce local mixing and to detect local changes in salt concentration, pH, and viscosity.

Journal Article↗
PubMed2002

Coupled electrorotation: two proximate microspheres spin in registry with an AC electric field.

We report a novel approach to micro- and nanoparticle rotation, uniting the fine translational control afforded by optical trapping with the flexibility and simplicity of dipole-field-induced coupled electrorotation (CER). Fluorescence imaging using a microparticle photopatterning technique was combined with optical trapping to quantify both the senses and speeds of rotation for individual pairs of particles. Laser tweezers allowed controlled positioning of a pair of particles within a dipole field while simultaneously providing an axis about which the particles rotated. The particle-particle interactions inherent in CER offer several distinct advantages compared with electrorotation in multipole fields. Results from several investigations highlight the utility of this approach, including quantification of rotation in spheres as small as 750 nm in diameter, observation of rotation rates as high as 1800 rpm, fabrication of coupled electrorotational "antigears", trapping and rotation of sphere dimers, and exploitation of the registry of sphere rotation to probe the dielectric properties of immobile objects.

Electrochemistry↗