KEYWORDS: Near field optics, Near field, Silicon, Electron microscopy, Optical imaging, Quantum nanophotonics, Quantum fields, Light-matter interactions, Imaging spectroscopy, Resonators
Experimental examination of light-matter interactions at the nanoscale is challenging since the corresponding electromagnetic near-field is often confined within volumes below the resolution of conventional optical microscopy. Here, we demonstrate that photoemission electron microscopy (PEEM) can image near-field optical fields in nanophotonic structures. We present concurrent spectroscopy and imaging of the near-field distribution of resonance modes supported by broken-symmetry silicon metasurfaces. Additionally, we can examine collective modes and deduce that coupling between eight resonators establishes the collective excitations. Altogether, the high-spatial resolution of this hyperspectral imaging approach is valuable for the metrology of collective, non-local optical resonances in nanophotonic systems.
High-harmonic generation (HHG) has been used to generate extreme ultra-violet (EUV) light sources to probe fast electron dynamics in the attosecond time scale. While traditionally observed in rare-gas atoms, HHG has also recently been reported in solids, with reduced threshold pump field and the additional advantage of producing stable EUV waveforms in a compact setup. Unfortunately, above-band-gap absorption restricts the HHG process to a very thin layer of the solid-state material (typically tens of nanometers in thickness), significantly limiting the generation efficiency. Here, we use a material operating in its epsilon-near-zero (ENZ) region, where the real part of its permittivity vanishes, to greatly boost the efficiency of the HHG process at the microscopic level. In experiments, we report high-harmonic emission up to the 9th order directly from a low-loss, solid-state ENZ medium: indium-doped cadmium oxide, with an excitation intensity at the GW cm-2 level. Furthermore, the observed HHG signal exhibits a pronounced spectral red-shift as well as linewidth broadening, resulting from the photo-induced electron heating and the consequent time-dependent resonant frequency of the ENZ film. Our results provide a novel nanophotonic platform for strong field physics, reveal new degrees of freedom for spectral and temporal control of HHG, and open up possibilities of compact solid-state attosecond light sources
Three-dimensional (3D) metafilms composed of periodic arrays containing single and multiple micrometer-scale vertical split ring resonators per unit cell were fabricated using membrane projection lithography. In contrast to planar and stacked planar structures such as cut wire pairs and fishnet structures, these 3D metafilms have a thickness t ~λd/4, allowing for classical thin film effects in the long wavelength limit. The infrared specular far-field scattering response was measured for metafilms containing one and two resonators per unit cell, and compared to numerical simulations. Excellent agreement in the frequency region below the onset of diffractive scattering was obtained. The metafilms demonstrate strong bi-anisotropic polarization dependence. Further, we show that for 3D metafilms, just as in solids, complex unit cells with multiple atoms (inclusions) per unit cell possess a richer set of excitation mechanisms. The highlight of these new coupling mechanisms is the excitation of the 3D analog to the 2D cut-wire-pair magnetic response.
Metasurfaces have been investigated for various applications ranging from beam steering, focusing, to polarization conversion. Along with passive metasurfaces, significant efforts are also being made to design metasurfaces with tunable optical response. Among various approaches, voltage tuning is of particular interest because it creates the possibility of integration with electronics. In this work, we demonstrate voltage tuning of reflectance from a complementary metasurface strongly coupled to an epsilon-near-zero (ENZ) mode in an ultrathin semiconductor layer. Our approach involves electrically controlling the carrier concentration of the ENZ layer to modulate the polaritonic coupling between the dipole resonances of the metasurface and the ENZ mode for modulating the reflectance of the metasurface. The hybrid structure we fabricate is similar to MOSCAP configuration where the complementary metasurface offers a continuous gold top layer for biasing and positive/negative bias to the metasurface leads to accumulation/depletion of carriers in the ENZ layer beneath it. We optimized our structure by using InGaAs as the ENZ material because of its high mobility and low effective mass. This allowed us to reduce the doping requirement and thereby reduce the ionized impurity scattering as well as the reverse bias required to deplete the ENZ layer. For low leakage and efficient modulation of carrier density, we used Hafnia as the gate dielectric. We further added a reflecting backplane below the ENZ layer to enhance the interaction and by applying bias, we achieved spectral shifts of 500 nm and amplitude modulation of 11% of one of the polariton branches at 14 µm.
Coupling of metasurfaces to intersubband transitions (ISTs) in semiconductor quantum wells (QWs) has been extensively studied for various applications ranging from generating giant nonlinear optical response to designing tunable metasurfaces for applications such as ultrafast spatial optical modulators and voltage tunable filters. In this work, we experimentally demonstrate a fundamentally new approach of actively controlling the coupling of ISTs in QWs to a metasurface for voltage tuning its optical response. Unlike previous approaches, we use voltage-controlled quantum tunneling to control the carrier concentration in the QWs for turning on/off the ISTs. We design a multi-quantum well structure consisting of four undoped InGaAs wells with AlInAs barriers grown on top of a highly doped InGaAs layer that acts as an electron reservoir. The heterostructure is optimized such that the first IST in all the wells is at 11µm. A complementary gold metasurface with dipole resonances at 11µm is fabricated on top of the QW structure. We designed the heterostructure such that by applying a bias of 1V, the energy bands of all the QWs get aligned simultaneously, leading to the occupation of the ground state of all the QWs via quantum tunneling of the electrons from the electron reservoir. The ISTs which were turned off due to negligible electron density gets turned on at 1V, and this leads to coupling between the ISTs and the dipoles resonances of the metasurface. The voltage induced coupling leads to reflectance modulation which we confirmed experimentally by rapid scan double modulation FTIR measurements.
Membrane projection lithography (MPL) has been demonstrated as a fabrication method for creation of layers of 3D unit cell metamaterials. Here we report an extensive modeling study of the electromagnetic behavior of split ring resonator (SRR) based metamaterial layers using rigorous coupled wave analysis, with particular attention to the MPL fabrication related aspects.
Micro-optical 5mm lenses in 50mm sub-arrays illuminate arrays of photovoltaic cells
with 49X concentration. Fine tracking over ±10° FOV in sub-array allows coarse tracking by
meter-sized solar panels. Plastic prototype demonstrated for 400nm<λ<1600nm.
Straightforward extension of canonical microwave metamaterial structures to optical and IR frequency dimensions is
complicated by both the size scale of the resulting structures, requiring cutting edge lithography to achieve the requisite
line-widths, as well as limitations on assembly/construction into final geometry. We present a scalable fabrication
approach capable of generating metamaterial structures such as split ring resonators and split wire pairs on a micron/sub-micron
size scale on concave surfaces with a radius of curvature ~ SRR diameter. This talk outlines the fabrication
method and modeling/theory based interpretation of the implications of curved metamaterial resonators.
Temporal image correlation provides a powerful fluorescence technique for measuring several biologically relevant
parameters of molecules in living cells. These parameters include, but are not limited to local concentrations, diffusion
dynamics, and aggregation states of biomolecules. However, the complex cellular environment presents several
limitations, precluding high quantitative accuracy and constraining biological implementation. In order to address these
issues, high speed spectral imaging was employed to compare the results of image correlation from spectrally unmixed
and virtually implemented fluorescence emission filters. Of particular interest in this study is the impact of cellular
autofluorescence, which is ubiquitous in fluorescence imaging of cells and tissues. Using traditional instrumentation,
corrections for autofluorescence are commonly estimated as a static offset collected from a separate control specimen.
While this may be sufficient in highly homogenous regions of interest, the low analyte concentrations requisite to
fluctuation-based methods result in the potential for unbounded error resulting from spectral cross-talk between local
autofluorescence inhomogeneities and the fluorescence signal of interest. Thus we demonstrate the importance of
accurate autofluorescence characterization and discuss potential corrections using a case study focusing on fluorescence
confocal spectral imaging of immune cells before and after stimulation with lipopolysaccheride (LPS). In these
experiments, binding of LPS to the membrane receptor, YFP-TLR4, is observed to result in initiation of the immune
response characterized by altered receptor diffusion dynamics and apparent heterogeneous aggregation states. In addition
to characterizing errors resulting from autofluorescence spectral bleed-through, we present data leading to a deeper
understanding of the molecular dynamics of the immune response and suggest hypotheses for future work utilizing
hyperspectrally enabled multi-label fluorescence studies on this system of high biological import.
KEYWORDS: Reflectivity, Temperature metrology, Sensors, FT-IR spectroscopy, High dynamic range imaging, Mirrors, Reflectometry, Signal detection, Aluminum, Data modeling
We have developed a system to measure the directional thermal emission from a surface, and in turn, calculate its
emissivity. This approach avoids inaccuracies sometimes encountered with the traditional method for calculating
emissivity, which relies upon subtracting the measured total reflectivity and total transmissivity from unity. Typical total
reflectivity measurements suffer from an inability to detect backscattered light, and may not be accurate for high angles
of incidence.
Our design allows us to vary the measurement angle (θ) from near-normal to ~80°, and can accommodate samples as
small as 7 mm on a side by controlling the sample interrogation area. The sample mount is open-backed to eliminate
shine-through, can be heated up to 200 °C, and is kept under vacuum to avoid oxidizing the sample. A cold shield
reduces the background noise and stray signals reflected off the sample. We describe the strengths, weaknesses, trade-offs,
and limitations of our system design, data analysis methods, the measurement process, and present the results of our
validation of this Variable-Angle Directional Emissometer.
Cellular autofluorescence, though ubiquitous when imaging cells and tissues, is often assumed to be small in comparison
to the signal of interest. Uniform estimates of autofluorescence intensity obtained from separate control specimens are
commonly employed to correct for autofluorescence. While these may be sufficient for high signal-to-background
applications, improvements in detector and probe technologies and introduction of spectral imaging microscopes have
increased the sensitivity of fluorescence imaging methods, exposing the possibility of effectively probing the low signal-to-background regime. With spectral imaging, reliable monitoring of signals near or even below the noise levels of the
microscope is possible if autofluorescence and background signals can be accurately compensated for. We demonstrate
the importance of accurate autofluorescence determination and utility of spectral imaging and multivariate analysis
methods using a case study focusing on fluorescence confocal spectral imaging of host-pathogen interactions. In this
application fluorescent proteins are produced when bacteria invade host cells. Unfortunately the analyte signal is
spectrally overlapped and typically weaker than the cellular autofluorescence. In addition to discussing the advantages
of spectral imaging for following pathogen invasion, we present the spectral properties of mouse macrophage
autofluorescence. The imaging and analysis methods developed are widely applicable to cell and tissue imaging.
Confocal fluorescence imaging of biological systems is an important method by which researchers can investigate
molecular processes occurring in live cells. We have developed a new 3D hyperspectral confocal fluorescence
microscope that can further enhance the usefulness of fluorescence microscopy in studying biological systems. The new
microscope can increase the information content obtained from the image since, at each voxel, the microscope records
512 wavelengths from the emission spectrum (490 to 800 nm) while providing optical sectioning of samples with
diffraction-limited spatial resolution. When coupled with multivariate curve resolution (MCR) analyses, the microscope
can resolve multiple spatially and spectrally overlapped emission components, thereby greatly increasing the number of
fluorescent labels, relative to most commercial microscopes, that can be monitored simultaneously. The MCR algorithm
allows the "discovery" of all emitting sources and estimation of their relative concentrations without cross talk, including
those emission sources that might not have been expected in the imaged cells. In this work, we have used the new
microscope to obtain time-resolved hyperspectral images of cellular processes. We have quantitatively monitored the
translocation of the GFP-labeled RelA protein (without interference from autofluorescence) into and out of the nucleus
of live HeLa cells in response to continuous stimulation by the cytokine, TNFα. These studies have been extended to
imaging live mouse macrophage cells with YFP-labeled RelA and GFP-labeled IRF3 protein. Hyperspectral imaging
coupled with MCR analysis makes possible, for the first time, quantitative analysis of GFP, YFP, and autofluorescence
without concern for cross-talk between emission sources. The significant power and quantitative capabilities of the new
hyperspectral imaging system are further demonstrated with the imaging of a simple fluorescence dye (SYTO 13)
traditionally used to stain the nucleus of live cells. We will demonstrate the microscope system's ability to actually
discover and quantify the presence of two separate SYTO 13 fluorescent species shifted in wavelength by only a few
nm. These two emission components exhibit very different spatial distributions in macrophage cells (i.e., nucleus vs.
cytoplasm + nucleus). Two highly overlapped autofluorescence components in addition to the two SYTO 13
components were also observed, and the spatial distributions of the two autofluorescence components were
quantitatively mapped throughout the cells in three dimensions.
The development of colloidal quantum dots (QDs) for biological imaging has brought a new level of sensitivity to live
cell imaging. Single particle tracking (SPT) techniques in particular benefit from the superior photostability, high
extinction coefficient and distinct emission spectra of QDs. Here we describe the use of QDs for SPT to study the
dynamics of membrane proteins in living cells. We work with the RBL-2H3 mast cell model that signals through the
high affinity IgE receptor, Fc&Vegr;RI. Using wide field or Total Internal Reflection Fluorescence (TIRF) microscopy we
have achieved simultaneous imaging of two spectrally distinct QDs with frame rates of up to 750 frames/s and
localization accuracy of ~10 nm. We also describe the imaging and analysis of QDs using a novel hyperspectral
microscope and multivariate curve resolution analysis for multi-color QD tracking. The same QD-tag used for SPT is
used to localize proteins at <10 nm resolution by electron microscopy (EM) on fixed membrane sheets.
Hyperspectral imaging provides complex image data with spectral information from many fluorescent species contained within the sample such as the fluorescent labels and cellular or pigment autofluorescence. To maximize the utility of this spectral imaging technique it is necessary to couple hyperspectral imaging with sophisticated multivariate analysis methods to extract meaningful relationships from the overlapped spectra. Many commonly employed multivariate analysis techniques require the identity of the emission spectra of each component to be known or pure component pixels within the image, a condition rarely met in biological samples. Multivariate curve resolution (MCR) has proven extremely useful for analyzing hyperspectral and multispectral images of biological specimens because it can operate with little or no a priori information about the emitting species, making it appropriate for interrogating samples containing autofluorescence and unanticipated contaminating fluorescence. To demonstrate the unique ability of our hyperspectral imaging system coupled with MCR analysis techniques we will analyze hyperspectral images of four-color in-situ hybridized rat brain tissue containing 455 spectral pixels from 550 - 850 nm. Even though there were only four colors imparted onto the tissue in this case, analysis revealed seven fluorescent species, including contributions from cellular autofluorescence and the tissue mounting media. Spectral image analysis will be presented along with a detailed discussion of the origin of the fluorescence and specific illustrations of the adverse effects of ignoring these additional fluorescent species in a traditional microscopy experiment and a hyperspectral imaging system.
The use of modern microstreolithography (MSL) technology gives optics developers the freedom to integrate mounting and positioning structures directly into an optical mask structure. We have created Hadamard spectrometer masks with increments of 150 μm and less using the Sony SCS-6000 microstereolithography apparatus (MSLA). Due to laser over cure and other parameters, adjustments were made iteration by iteration until appropriate mask tolerances were met. A mounting structure was integrated with the mask for testing and application. At the computer aided design (CAD) model level, the mounting geometry can be adjusted to fit any specific mounting apparatus. By using the MSLA, features as small as 75 μm and larger than 300 mm can be created in the same build. Additionally, the conceptual design of an entire positioning system constructed using layer additive MSLA microfabrication is underway. This positioning system may be built as an integrated assembly, encapsulating necessary components. Optical characterization results are presented.
We describe the development of a MEMS-based correlation radiometer for remote detection of chemical species. The radiometer utilizes a new type of MEMS programmable diffraction grating called the Polychromator. The Polychromator contains an array of 1024 electrostatically actuated reflective beams that are 10 microns wide by 1 cm long, and have a vertical travel of approximately 2 - 4 microns. The Polychromator grating is used to replace the reference cell of conventional correlation radiometry. Appropriate programming of the deflection profile of the grating array enables the production of any spectral transfer function desired for the correlation measurement. Advantages of this approach to correlation radiometry include the ability to detect multiple chemical species with a compact
instrument, the ability to optimize the reference spectra to eliminate chemical interferences, and the ability to produce
reference spectra for hazardous and transient species.
Multivariate curve resolution (MCR) using constrained alternating least squares algorithms represents a powerful analysis capability for a quantitative analysis of hyperspectral image data. We will demonstrate the application of MCR using data from a new hyperspectral fluorescence imaging microarray scanner for monitoring gene expression in cells from thousands of genes on the array. The new scanner collects the entire fluorescence spectrum from each pixel of the scanned microarray. Application of MCR with nonnegativity and equality constraints reveals several sources of undesired fluorescence that emit in the same wavelength range as the reporter fluorphores. MCR analysis of the hyperspectral images confirms that one of the sources of fluorescence is due to contaminant fluorescence under the printed DNA spots that is spot localized. Thus, traditional background subtraction methods used with data collected from the current commercial microarray scanners will lead to errors in determining the relative expression of low-expressed genes. With the new scanner and MCR analysis, we generate relative concentration maps of the background, impurity, and fluroescent labels over the entire image. Since the concentration maps of the fluorescent labels are relativly uaffected by the presence of background and impurity emissions, the accuracy and useful dynamic range of the gene expression data are both greatly improved over those obtained by commercial microarray scanners.
There are many applications for micro-optics. Perhaps the most exciting use of micro-optics is in fiber optical communication systems, getting the light signals in and out of the fibers. Other possible uses include data processing, chemical sensing, and spectroscopic applications. In this talk we will describe (mu) ChemLab, the Polychromator, a new spectroscopic gas identification device, a fiberoptic status monitor, and a deep X-ray lithographic technique for fabricating micro-optical systems.
A metal ion sensitive, fluorescent lipid-bilayer material (5% PSIDA/DSPC) was successfully immobilized in a silica matrix using a tetramethoxysilane (TMOS) sol-gel procedure. The sol- gel immobilization method was quantitative in the entrapment of self-assembled lipid-bilayers and yielded thin films for facile configuration to optical fiber platforms. The silica matrix was compatible with the solvent sensitive lipid bilayers and provided physical stabilization as well as biological protection. Immobilization in the silica sol-gel produced an added benefit of improving the bilayer's metal ion sensitivity by up to two orders of magnitude. This enhanced performance was attributed to a preconcentrator effect from the anionic surface of the silica matrix. Thin gels (193 micron thickness) were coupled to a bifurcated fiber optic bundle to produce a metal ion sensor probe. Response times of 10 - 15 minutes to 0.1 M CuCl2 were realized with complete regeneration of the sensor using an ethylenediaminetetraacetic acid (EDTA) solution.
As a first step toward the development of a new remote sensing technique that we call 'holographic correlation spectroscopy,' we demonstrate that diffractive optics can be used to synthesize the IR spectra of real compounds. In particular, we have designed, fabricated, and characterized a diffractive element that successfully reproduces the major features of the spectrum of gaseous HF in the region between 3600 cm-1 and 4300 cm-1. The reflection-mode diffractive optic consists of 4096 lines, each 4.5 micrometers wide, at 16 discrete depths relative to the substrate, and was fabricated on a silicon wafer using anisotropic reactive ion-beam etching in a four-mask-level process. We envision the use diffractive elements of this type to replace the cumbersome reference cells of conventional correlation spectroscopy and thereby enable a new class of compact and versatile correlation spectrometers.
We demonstrate that computer-generated diffractive optical elements can be used to synthesize the infrared spectra of real compounds. In particular, we describe a modified phase- retrieval algorithm that we have used to design diffractive elements of this type and we present experimental results for a diffractive optic which is capable of synthesizing the infrared spectrum of HF between 3600 cm-1 and 4300 cm-1. The reflection-mode diffractive optic consists of 4096 lines, each 4.5 micrometers wide, at 16 discrete depths relative to the substrate (from 0 to 1.2 micrometer), and was fabricated on a silicon wafer using anisotropic reactive ion-beam etching in a four-mask-level process. We propose the use of such elements to replace reference cells in a new type of correlation spectroscopy that we call 'holographic correlation spectroscopy.' Storage of a large number of diffractive elements, each producing a synthetic spectrum corresponding to a different target compound, in compact disk-like format, will allow a spectrometer of this type to rapidly determine the composition of unknown samples. Further, this approach can be used to perform correlation- based measurements of hazardous or transient species, for which conventional correlation spectroscopy is impractical.
We examine carrier relaxation and radiative recombination in AlGaAs-based near IR and AlGaInP-based visible fractal quantum well heterostructures. Through temperature dependent photoluminescence, we demonstrate that enhanced population of higher lying energy levels can be achieved by varying the thickness of the layers in the fractal heterostructure. This distribution of carriers results in room temperature emission over a relatively broad range of wavelengths: approximately 700-855 nm for AlGaAs structures and 575-650 nm for AlGaInP structures. Spectra are compared to theoretical calculations to evaluate the nonequilibrium nature of the carrier distributions. Time resolved photoluminescence measurements demonstrate an approximately linear relationship between the radiative decay time and the layer thickness of the structure. Correspondingly, integrated luminescence measurements at room temperature reveal a factor of four increase in the light output efficiency of the structure as the fractal layer thickness is increased from 50 angstrom to 400 angstrom. The applicability of these heterostructures to broadband LEDs is discussed.
Dielectric optical waveguides exhibit properties that are well suited to sensor applications. They have low refractive index and are transparent to a wide range of wavelengths. They can react with the surrounding environment in a variety of controllable ways. In certain sensor applications, it is advantageous to integrate the dielectric waveguide on a semiconductor substrate with active devices. In this work, we demonstrate a tamper sensor based on dielectric waveguides that connect epitaxial GaAs-GaAlAs sources and detectors. The tamper sensing function is realized by attaching particles of absorbing material with high refractive index to the surface of the waveguides. These absorbers are then attached to a lid or cover, as in an integrated circuit package or multi-chip module. The absorbers attenuate the light in the waveguides as a function of absorber interaction. The absorbers are placed randomly on the waveguides, to form a unique attenuation pattern that is registered by the relative signal levels on the photodetectors. When the lid is moved, the pattern of absorbers changes, altering the photodetector signals. This dielectric waveguide arrangement is applicable to a variety of sensor functions, and can make use of resonant coupling properties between low refractive index dielectric waveguides and high-index absorbing materials.
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