Local Optical Properties Research Articles

Optofluidic needle probe integrating targeted delivery of fluid with optical coherence tomography imaging.

We present an optofluidic optical coherence tomography (OCT) needle probe capable of modifying the local optical properties of tissue to improve needle-probe imaging performance. The side-viewing probe comprises an all-fiber-optic design encased in a hypodermic needle (outer diameter 720μm) and integrates a coaxial fluid-filled channel, terminated by an outlet adjacent to the imaging window, allowing focal injection of fluid to a target tissue. This is the first fully integrated OCT needle probe design to incorporate fluid injection into the imaging mechanism. The utility of this probe is demonstrated in air-filled sheep lungs, where injection of small quantities of saline is shown, by local refractive index matching, to greatly improve image penetration through multiple layers of alveoli. 3D OCT images are correlated against histology, showing improvement in the capability to image lung structures such as bronchioles and blood vessels.

Strain-induced direct band gap shrinkage in local Ge-on-insulator structures fabricated by lateral liquid-phase epitaxy

Direct band gap shrinkage in Ge-on-insulator (GOI) structures fabricated by lateral liquid-phase epitaxy (LLPE) was investigated by means of micro(μ)-photoluminescence and μ-Raman spectroscopy. The LLPE method, based on the rapid thermal process, was found to be an effective and feasible way to produce highly strained local GOI structures. We observed a significant redshift of direct gap emission amounting to 45 meV from the tensile-strained GOI layer. Strain analysis and temperature dependent PL spectra indicated that a direct band gap shrinkage was mainly due to a tensile strain of about 0.4% induced by rapid crystallization from the Ge melting point during the LLPE process. The local optical properties along the LLPE-grown GOI wire were examined and discussed on the basis of μ-PL mapping images.

Non‐Uniform Helix Unwinding of Cholesteric Liquid Crystals in Cells with Interdigitated Electrodes

A microspectrophotometer was used to elucidate the local optical properties of cholesteric liquid crystals (CLCs) in cells with interdigitated electrodes as a function of applied voltage. The spectra collected from a spatially selective and micron-sized sampling area allow for new insights into the spectral properties of CLCs in the gaps between patterned electrodes. The microscopic electro-optic response is shown to be highly dependent on the cell thickness and the electrode periodicity. Specifically, the helix unwinding of the CLC superstructure does not always occur uniformly in the sample, as a result of field gradients through the cell thickness: for cells with relatively narrow gaps and electrodes, the redshift occurs initially only in the CLC layers closest to the substrate with the electrodes, leading to broad reflection spectra and different reflection colors depending on which side of the cell is illuminated. Theoretical estimates of the expected shift in the reflection band gap based on the critical field for a given CLC material and the spatial variation of electric field in the cell are found to be in good agreement with the complex behavior observed experimentally. In contrast, in thin cells with wider gaps, the pitch increase affects the whole CLC layer uniformly, because the electric field gradient is small.

Dislocations in bilayer graphene

Dislocations represent one of the most fascinating and fundamental concepts in materials science. Most importantly, dislocations are the main carriers of plastic deformation in crystalline materials. Furthermore, they can strongly affect the local electronic and optical properties of semiconductors and ionic crystals. In materials with small dimensions, they experience extensive image forces, which attract them to the surface to release strain energy. However, in layered crystals such as graphite, dislocation movement is mainly restricted to the basal plane. Thus, the dislocations cannot escape, enabling their confinement in crystals as thin as only two monolayers. To explore the nature of dislocations under such extreme boundary conditions, the material of choice is bilayer graphene, the thinnest possible quasi-two-dimensional crystal in which such linear defects can be confined. Homogeneous and robust graphene membranes derived from high-quality epitaxial graphene on silicon carbide provide an ideal platform for their investigation. Here we report the direct observation of basal-plane dislocations in freestanding bilayer graphene using transmission electron microscopy and their detailed investigation by diffraction contrast analysis and atomistic simulations. Our investigation reveals two striking size effects. First, the absence of stacking-fault energy, a unique property of bilayer graphene, leads to a characteristic dislocation pattern that corresponds to an alternating AB B[Symbol: see text]AC change of the stacking order. Second, our experiments in combination with atomistic simulations reveal a pronounced buckling of the bilayer graphene membrane that results directly from accommodation of strain. In fact, the buckling changes the strain state of the bilayer graphene and is of key importance for its electronic properties. Our findings will contribute to the understanding of dislocations and of their role in the structural, mechanical and electronic properties of bilayer and few-layer graphene.

Determination of optical properties by interstitial white light spectroscopy using a custom fiber optic probe

We demonstrate interstitial recovery of absorption and scattering coefficients using a custom optical probe and a Monte Carlo (MC)-based recovery algorithm. The probe consists of six side-firing spectroscopy fibers contained in a 1.1-mm outer diameter cladding, with each fiber having a different axial and angular position on the probe. Broadband white light is delivered by one of the fibers and is detected steady-state by the remaining fibers. These spatially and spectrally resolved data are analyzed using a MC-based fitting algorithm in order to extract the local optical properties. The technique was verified in tissue-simulating phantoms consisting of Intralipid-20% as a scatterer and either manganese meso-tetra (4-sulfanatophenyl) porphine or intact human erythrocytes as an absorber. Absorption coefficients were recovered with a mean error of 9% and scattering coefficients were recovered with a mean error of 19%, whereas the hemoglobin oxygen saturation was recovered with a mean error of 12%. These results demonstrate the feasibility of optical property recovery for situations in which surface-contact spectroscopy is not a possibility, and where only a single probe can be inserted into the tissue.

Effect of Al3+ Co-doping on the Dopant Local Structure, Optical Properties, and Exciton Dynamics in Cu+-Doped ZnSe Nanocrystals

The dopant local structure and optical properties of Cu-doped ZnSe (ZnSe:Cu) and Cu and Al co-doped ZnSe (ZnSe:Cu,Al) nanocrystals (NCs) were studied with an emphasis on understanding the impact of introducing Al as a co-dopant. Quantum-confined NCs with zinc blende crystal structure and particle size of 6 ± 0.6 Å were synthesized using a wet chemical route. The local structure of the Cu dopant, studied by extended X-ray absorption fine structure, indicated that Cu in ZnSe:Cu NCs occupies a site that is neither substitutional nor interstitial and is adjacent to a Se vacancy. Additionally, we estimated that approximately 25 ± 8% of Cu was located on the surface of the NC. Al(3+) co-doping aids in Cu doping by accounting for the charge imbalance originated by Cu(+) doping and consequently reduces surface Cu doping. The Cu ions remain distorted from the center of the tetrahedron to one of the triangular faces. The lifetime of the dopant-related photoluminescence was found to increase from 550 ± 60 to 700 ± 60 ns after Al co-doping. DFT calculations were used to obtain the density of states of a model system to help explain the optical properties and dynamics processes observed. This study demonstrates that co-doping using different cations with complementary oxidation states is an effective method to enhance optical properties of doped semiconductor NCs of interest for various photonics applications.

Shifting Focus in Optical Image-Guided Cancer Therapy

Cancer patients could benefit from a surgical procedure that helps the surgeon to determine adequate tumor resection margins. Systemic injection of tumor-specific fluorescence agents with subsequent intraoperative optical imaging can guide the surgeon in this process. However, tumor heterogeneity hampers tumor-specific targeting. In addition, determination of adequate resection margins can be very challenging due to invasive tumor strands that are difficult to resolve and because of the confounding effect of variations in tissue optical properties in the surgical margin. We provide an overview of the "classic approach" of imaging tumor-specific targets or tumor-associated pathophysiological processes, and explain the limitations of these targeting strategies. It is proposed that problems of tumor heterogeneity can theoretically be circumvented by shifting focus of tumor targeting towards the follicle-stimulating hormone receptor (FSHR). Furthermore, we discuss why objective determination of resection margins is required to improve resection of the invasive strands, a goal that may be achieved by targeting the FSHR. When invasive strands would nevertheless extend beyond such a standardized resection margin, we suggest that adjuvant photodynamic therapy would be a very suitable therapeutic regimen. Finally, we describe how point optical spectroscopy can be used to scrutinize suspect tissue that is difficult to differentiate from normal tissue by measuring the local tissue optical properties to recover a local intrinsic fluorescence measurement.

Dust events and their influence on aerosol optical properties over Jaipur in Northwestern India

In this study, we systematically document the link between dust episodes and local scale regional aerosol optical properties over Jaipur located in the vicinity of Thar Desert in the northwestern state of Rajasthan. The seasonal variation of AOT(500 nm) (aerosol optical thickness) shows high values (0.51 ± 0.18) during pre-monsoon (dust dominant) season while low values (0.36 ± 0.14) are exhibited during winter. The Ångström wavelength exponent has been found to exhibit low value (<0.25) indicating relative dominance of coarse-mode particles during pre-monsoon season. The AOT increased from 0.36 (Aprilmean) to 0.575 (May-June(mean)). Consequently, volume concentration range increases from April through May-June followed by a sharp decline in July during the first active phase of the monsoon. Significantly high dust storms were observed over Jaipur as indicated by high values of single scattering albedo (SSA(440 nm) = 0.89, SSA(675 nm) = 0.95, SSA870 nm = 0.97, SSA(1,020 nm) = 0.976) than the previously reported values over IGP region sites. The larger SSA values (more scattering aerosol), especially at longer wavelengths, is due to the abundant dust loading, and is attributed to the measurement site's proximity to the Thar Desert. The mean and standard deviation in SSA and asymmetry parameter during pre-monsoon season over Jaipur is 0.938 ± 0.023 and 0.712 ± 0.017 at 675 nm wavelength, respectively. Back-trajectory air mass simulations suggest Thar Desert in northwestern India as the primary source of high aerosols dust loading over Jaipur region as well as contribution by long-range transport from the Arabian Peninsula and Middle East gulf regions, during pre-monsoon season.

Review on theory and applications of wavefront recording plane framework in generation and processing of digital holograms

The wavefront recording plane (WRP), subsequently generalized to be known as the virtual diffraction plane (VDP), is a recent concept that has been successfully deployed in fast generation and processing of digital holograms. In brief, the WRP and its extension, the VDP, is a hypothetical plane that is located between the hologram and the object scene, and which is at close proximity to the latter. As such, the fringe patterns on the hypothetical plane are carrying the holistic information of the hologram, as well as the local optical properties of the object scene. This important property enables a hologram to be processed with classical image processing techniques that are normally unsuitable for handling holographic information. In this paper we shall review a number of works, that have been developed based on the framework of the WRP and the VDP.

Mapping the local dielectric response at the nanoscale by means of plasmonic force spectroscopy

At the present, the local optical properties of nanostructured materials are difficult to be measured by available instrumentation. We investigated the capability of plasmonic force spectroscopy of measuring the optical response at the nanoscale. The proposed technique is based on force measurements performed by combining Atomic Force Microscopy, or optical tweezers, and adiabatic compression of surface plasmon polaritons. We show that the optical forces, caused by the plasmonic field, depend on the local response of the substrates and, in principle, allow probing both the real and the imaginary part of the local permittivity with a spatial resolution of few nanometers.

Single Atom Microscopy

We show that aberration-corrected scanning transmission electron microscopy operating at low accelerating voltages is able to analyze, simultaneously and with single atom resolution and sensitivity, the local atomic configuration, chemical identities, and optical response at point defect sites in monolayer graphene. Sequential fast-scan annular dark-field (ADF) imaging provides direct visualization of point defect diffusion within the graphene lattice, with all atoms clearly resolved and identified via quantitative image analysis. Summing multiple ADF frames of stationary defects produce images with minimized statistical noise and reduced distortions of atomic positions. Electron energy-loss spectrum imaging of single atoms allows the delocalization of inelastic scattering to be quantified, and full quantum mechanical calculations are able to describe the delocalization effect with good accuracy. These capabilities open new opportunities to probe the defect structure, defect dynamics, and local optical properties in 2D materials with single atom sensitivity.

Progressive Addition Lens Measurement by Point Diffraction Interferometry

Design a device for accurate measurements of local optical properties of progressive addition lenses (PALs). A point diffraction interferometer has been adapted to measure local prescriptions of PALs. The most basic configuration of the interferometer for the measurement of PALs showed in this work presents high dynamic range and accuracy as well as the possibility of choosing the number and position of measurement points. Measurements are taken within a region of interest within a radius of about 0.4 to 1.5 mm. Different PAL designs are measured by the method proposed here and compared with results by a last generation commercial lens mapper. With the point diffraction interferometer we also compared several PAL designs in order to analyze their properties in the progression zone. The device is compact, robust, and fairly accurate, and the operational principle is very simple. By direct measurements it provides the local dioptric power, i.e., the second order wavefront properties, of the lens for selected regions of interest. The position and area can be chosen by the user. The only mobile part of the setup allows for the selection of the measurement points without any additional prismatic correction or movement of the PAL.

Scanning Tunneling Microscopy and Spectroscopy of Supported Nanostructures

Recent advances in low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) have provided new opportunities for the investigation of the local geometric, electronic, magnetic, and optical properties of nanostructures. This review focuses on the presentation and discussion of single molecules, supramolecular assemblies, and other nanostructures; all research results obtained in our laboratory. The emphasis is directed to the observation of new effects, where the properties of matter at the nanoscale differ from those at the mesoscopic or macroscopic scale: small is different. This fact is illustrated for the conservation of chirality in a hierarchical supramolecular assembly of organic molecules and for local light emission from supported molecules. The latter indicates a possible route towards an optical spectroscopic analysis on the scale of single molecules.

Optical characterization of a InGaN/GaN microcavity with epitaxial AlInN/GaN bottom DBR

ABSTRACTResonant coupling of an optical mode confined within a microcavity and an emitter is the basic prerequisite for the observation of Bose-Einstein condensation phenomena and the development of novel optical devices based on cavity polaritons.We demonstrate highly spatially resolved 2” wafer characterization of the reflectivity and emission properties of a nitride based multi quantum well semi microcavity (i.e. structure without top Bragg reflector) to verify resonant regions. Photoluminescence and reflectivity spectra recorded at the same positions on the wafer exhibit a strong spatial dependence of the multi quantum well emission and the center wavelength of the stop band of the bottom Bragg reflector across the sample. Resonance, i.e., matching of the emission and the center wavelength of the stop band, is found in a region 8 mm off the center of the wafer.The thickness profile across the AlInN/GaN Bragg reflector and multi quantum well layers was obtained by x-ray mappings over the full wafer. A perfect correlation between the local optical properties and the x-ray thickness distribution is found. Additional transmission electron microscopy investigations indicate a complete crack free structure and smooth interfaces between the layers within the Bragg reflector making the structure appropriate for strong coupling applications.

Surface Polarity-Dependent Cathodoluminescence in Hydrothermally Grown ZnO Hexagonal Rods

Vertically oriented ZnO hexagonal rod arrays were produced by site-specific hydrothermal growth. These ZnO nanostructures possess well-defined surfaces composed of Zn-terminated (0001) planes (for the top surface) and nonpolar {1010} planes (for the rectangular side faces), thereby providing an opportunity to investigate the correlation between surface structures and local optical properties. The cathodoluminescence (CL) spectra of various sized ZnO rods revealed that the ratio of the deep-level emission (DLE) to the near-band-edge emission (NBE) peak intensities increased continuously with increasing diameter, which is contradictory to the general trend that the DLE increases by enhancing the surface-to-volume ratio. From the CL spectral mapping, significant NBE quenching was observed at the Zn-terminated (0001) surface, whereas the DLE was bright for every surface. Based on these observations, the anomalous behavior observed in our ZnO rods can be attributed to the surface polarity-dependent NBE charac...

Development of a noncontact diffuse optical spectroscopy probe for measuring tissue optical properties

Optical reflectance probes are often used as tools to obtain optical spectra from superficial tissues and subsequently determine optical and physiological properties associated with early stage cancer. These probes, when placed directly on the tissue, are known to cause significant pressure-dependent changes in local optical properties. To address this, we fit the probe with an optical device that images the illumination and collection fibers onto the tissue surface, eliminating the influence of contact probe pressure on the sampling area. The noncontact probe addition addresses new optical conditions that may affect its performance such as tissue surface contour, and specular reflections by implementing an autofocusing mechanism and cross polarization. Extracted optical properties of tissue simulating phantoms yield errors of 3.46% in reduced scattering and 8.62% in absorbance. Autofocusing has extended the depth of field from 4 mm to throughout the 12 mm range of autofocus travel, while cross polarization has removed the incidence angle dependent specular reflection component from the collected signal.

Are electron tweezers possible?

Positively answering the question in the title, we demonstrate in this work single electron beam trapping and steering of 20-300nm solid Al nanoparticles generated inside opaque submicron-sized molten Al-Si eutectic alloy spheres. Imaging of solid nanoparticles and liquid alloy in real time was performed using energy filtering in an analytical transmission electron microscope (TEM). Energy-filtering TEM combined with valence electron energy-loss spectroscopy enabled us to investigate in situ nanoscale transformations of the internal structure, temperature dependence of plasmon losses, and local electronic and optical properties under melting and crystallization of individual binary alloy particles. For particles below 20nm in size, enhanced vibrations of the dynamic solid-liquid interface due to instabilities near the critical threshold were observed just before melting. The obtained results indicate that focused electron beams can act as a tool for manipulation of metal nanoparticles by transferring linear and angular mechanical momenta. Such thermally assisted electron tweezers can be utilized for touchless manipulation and processing of individual nano-objects and potentially for fabrication of assembled nanodevices with atomic level sensitivity and lateral resolution provided by modern electron optical systems. This is by three orders of magnitude better than for light microscopy utilized in conventional optical tweezers. New research directions and potential applications of trapping and tracking of nano-objects by focused electron beams are outlined.

Strong near-field optical localization on an array of gold nanodisks

By scanning near-field optical microscopy, we measured the localization of the electromagnetic field on an array of gold nanodisks illuminated in a transmission mode. We experimentally observed that the field is localized between the disks, with a pattern oriented along the incident polarization direction. We also observed that the electromagnetic field rapidly decays above the nanodisks, showing a strong vertical localization. The experimental results are in good agreement with numerical simulations performed by a finite difference time domain method. This study provides quantitative information about the local optical properties of closely-packed nanodisks that can be used for applications in biochemical sensors and nanolithography.

Plasmonic chiral contrast agents for optical coherence tomography: numerical study

Optical coherence tomography (OCT) is a widely used morphological imaging modality. Various contrast agents, which change localized optical properties, are used to extend the applicability of OCT, where intrinsic contrast is not sufficient. In this paper we propose the use of a dual-rod gold nano-structure as a polarization sensitive contrast agent. Using numerical simulation, we demonstrated that the proposed structure has tunable chiral response. Enhanced cross-section due to Plasmon resonance in gold nanoparticles, along with the chiral behavior can provide enhanced detection sensitivity. The proposed contrast agents may extend the applicability of OCT to the problems that require the molecular contrast with enhanced sensitivity.

Validation of MERIS water quality products in Murchison Bay, Lake Victoria – Preliminary results

As a step forward to validating the retrieval algorithm associated with the MERIS (Medium Resolution Imaging Spectrometer) sensor onboard the European Space Agency's ENVISAT satellite, we compare scaling factors for (442) (pigment absorption coefficient at 442 nm) versus chl-a concentration, and for (scattering coefficient for particles at 440 nm) versus TSM (total suspended matter) concentration, as well as in situ measurements of chl-a and TSM concentrations in Murchison Bay, Lake Victoria, with MERIS level 2 products. Our comparisons show that MERIS chl-a data products underestimate the in situ chl-a concentration by a factor of about 0.6, and that MERIS level 2 products overestimate the in situ TSM concentration and the yellow substance absorption coefficient by a factor of about 2. Application of a local inherent optical property (IOP) scaling factor improves the retrieval provided by MERIS level 2 products. The MERIS level 2 products also underestimate the water-leaving reflectance. These discrepancies between retrievals from MERIS data and in situ measurements are due to the current imperfect atmospheric correction algorithm for inland waters. Therefore, a need arises to develop site-specific bio-optical models and to improve existing MERIS retrieval algorithms.