Sub-diffraction Limit Research Articles

Non-melting super-resolution near-field apertures in Sb–Te alloys

The super-resolution near-field structure permits the formation of short-lived subdiffraction limit apertures that channel electromagnetic energy at the aperture boundary. This effect is commonly observed when a laser is focused onto a thin film of Sb based material. The aperture formation within Sb, Sb2Te3, Sb2Te, and SbTe is investigated by time resolved optical pump-probe techniques and found to occur without melting. Ab initio modeling has shown a threshold-like change in the optical properties below the melting temperature which leads to the formation of a near-field aperture. This threshold is shown to be a consequence of thermally induced misalignment of p-type bonding.

Beam pen lithography

Lithography techniques are currently being developed to fabricate nanoscale components for integrated circuits, medical diagnostics and optoelectronics. In conventional far-field optical lithography, lateral feature resolution is diffraction-limited. Approaches that overcome the diffraction limit have been developed, but these are difficult to implement or they preclude arbitrary pattern formation. Techniques based on near-field scanning optical microscopy can overcome the diffraction limit, but they suffer from inherently low throughput and restricted scan areas. Highly parallel two-dimensional, silicon-based, near-field scanning optical microscopy aperture arrays have been fabricated, but aligning a non-deformable aperture array to a large-area substrate with near-field proximity remains challenging. However, recent advances in lithographies based on scanning probe microscopy have made use of transparent two-dimensional arrays of pyramid-shaped elastomeric tips (or 'pens') for large-area, high-throughput patterning of ink molecules. Here, we report a massively parallel scanning probe microscopy-based approach that can generate arbitrary patterns by passing 400-nm light through nanoscopic apertures at each tip in the array. The technique, termed beam pen lithography, can toggle between near- and far-field distances, allowing both sub-diffraction limit (100 nm) and larger features to be generated.

Experimental Investigation of Focusing of Gold Planar Plasmonic Lenses

To experimentally demonstrate the subwavelength focusing of depth-tuned or non-depth-tuned plasmonic lenses, we first designed this type of lens using diffraction-coupling-angle based method, then fabricated the structure in gold thin film with focused ion beam, and finally characterized its focusing behavior using near-field scanning optical microscope. It is found that this type of lens has a resolution limit on the focal plane due to the field represented by angular spectrum having a cut-off frequency, while at the near field the lens has sub-diffraction limit focusing capability due to the existence of high-angular-frequency components in the field.

Leaky wave radiation from planar anisotropic metamaterial slabs

We describe an optical leaky waveguide, which is achieved with uniaxially anisotropic metamaterials and can support both forward and backward leaky waves. The radiation behavior is analyzed with a transverse resonance approach, and supported by finite-element simulation results. The remarkable backward leaky nature can be exploited to develop a subdiffraction limit imaging system.

The generation of nano-patterns on a pure silicon wafer in air and argon with sub-diffraction limit nanosecond laser pulses

Intense nanosecond laser pulses were delivered to sub-diffraction limit spots on pure Si wafers with a near field scanning optical microscope (NSOM) for possible nanoscale direct patterning. Compared with the previous results obtained with a femtosecond laser, no evidence of nanoscale laser ablation on Si targets was observed even under the maximum allowable laser energy (i.e. before the damage of NSOM probes). The formation of nano-patterns on pure Si wafers with nanosecond laser pulses, which were in the form of protrusions, can be explained as resulting from the melting and recrystallization along with the oxidation of pure Si targets during and after the laser pulses. Note that these nano-protrusions were primarily composed of SiO2 and nano-crystallized Si (nc-Si).

Fluorescence advantages with microscopic spatiotemporal control.

We present a clever design concept of using femtosecond laser pulses in microscopy by selective excitation or de-excitation of one fluorophore over the other overlapping one. Using either a simple pair of femtosecond pulses with variable delay or using a train of laser pulses at 20-50 Giga-Hertz excitation, we show controlled fluorescence excitation or suppression of one of the fluorophores with respect to the other through wave-packet interference, an effect that prevails even after the fluorophore coherence timescale. Such an approach can be used both under the single-photon excitation as well as in the multi-photon excitation conditions resulting in effective higher spatial resolution. Such high spatial resolution advantage with broadband-pulsed excitation is of immense benefit to multi-photon microscopy and can also be an effective detection scheme for trapped nanoparticles with near-infrared light. Such sub-diffraction limit trapping of nanoparticles is challenging and a two-photon fluorescence diagnostics allows a direct observation of a single nanoparticle in a femtosecond high-repetition rate laser trap, which promises new directions to spectroscopy at the single molecule level in solution. The gigantic peak power of femtosecond laser pulses at high repetition rate, even at low average powers, provide huge instantaneous gradient force that most effectively result in a stable optical trap for spatial control at sub-diffraction limit. Such studies have also enabled us to explore simultaneous control of internal and external degrees of freedom that require coupling of various control parameters to result in spatiotemporal control, which promises to be a versatile tool for the microscopic world.

Subwavelength plasmonic waveguides based on ZnO nanowires and nanotubes: A theoretical study of thermo-optical properties

We show theoretically that plasmonic waveguide structures in ZnO nanowires and nanotubes working at optical frequencies can achieve photonic waveguiding in a subdiffraction limit. The output intensity distribution, propagation length, and thermo-optical properties with different waveguide configurations are investigated. Our results show that these waveguides have the potential to develop either high performance thermally controlled nanoscale plasmonic devices or thermally insensitive waveguides by optimizing waveguide configurations.

DNA fluorocode: A single molecule, optical map of DNA with nanometre resolution

We present a new method for single-molecule optical DNA mapping using an exceptionally dense, yet sequence-specific coverage of DNA with a fluorescent probe. The method employs a DNA methyltransferase enzyme to direct the DNA labelling, followed by molecular combing of the DNA onto a polymer-coated surface and subsequent sub-diffraction limit localization of the fluorophores. The result is a ‘DNA fluorocode’; a simple description of the DNA sequence, with a maximum achievable resolution of less than 20 bases, which can be read and analyzed like a barcode. We demonstrate the generation of a fluorocode for genomic DNA from the lambda bacteriophage using a DNA methyltransferase, M.HhaI, to direct fluorescent labels to four-base sequences reading 5′-GCGC-3′. A consensus fluorocode that allows the study of the DNA sequence at the level of an individual labelling site can be generated from a handful of molecules.

Microscopy with Nanoapertures to Reveal Membrane Organization with 1 Microsecond and 100 Nanometer Resolution

Current hypotheses for plasma membrane organization depend critically on a variety of length and time scales that are outside the resolution of conventional experimental techniques. Our novel approach utilizes nanoapertures to examine membrane organization and dynamics with near-field optical fluorescence microscopy without incorporating a scanning probe or perturbing the membrane. Conventional microscopy excitation sources and fluorescent probes are used to enable fluorescence correlation spectroscopy (FCS) on living cell membranes with super-resolution. The nanoapertures confine the excitation light to a sub-diffraction limit length scale, providing a 25-fold decrease in the illuminated area of the membrane as compared to diffraction-limited illumination.View Large Image | View Hi-Res Image | Download PowerPoint Slide

Optimal measurement constellation of the fluoroBancroft localization algorithm for position estimation in tracking confocal microscopy

The fluoroBancroft algorithm is an analytical approach that converts a collection of fluorescence intensity measurements generated by an isolated sub-diffraction limit source into an estimate with nanometer-scale precision of the source position. Based on this algorithm, we have developed a scheme for tracking single fluorescent particles in a confocal microscope. In this paper, we determine an optimal measurement constellation for the estimation algorithm. The position estimation bias and uncertainty arising from the photon counting statistics are calculated based on the assumption that the natural logarithm of a Poisson random variable with large rate can be approximated as a random variable with a Gaussian distribution. A sufficient condition for an unbiased measurement constellation and the optimal radius of a given constellation geometry with six measurements are then derived. The results are illustrated through numerical simulation.

Fast, background-free, 3D super-resolution optical fluctuation imaging (SOFI)

Super-resolution optical microscopy is a rapidly evolving area of fluorescence microscopy with a tremendous potential for impacting many fields of science. Several super-resolution methods have been developed over the last decade, all capable of overcoming the fundamental diffraction limit of light. We present here an approach for obtaining subdiffraction limit optical resolution in all three dimensions. This method relies on higher-order statistical analysis of temporal fluctuations (caused by fluorescence blinking/intermittency) recorded in a sequence of images (movie). We demonstrate a 5-fold improvement in spatial resolution by using a conventional wide-field microscope. This resolution enhancement is achieved in iterative discrete steps, which in turn allows the evaluation of images at different resolution levels. Even at the lowest level of resolution enhancement, our method features significant background reduction and thus contrast enhancement and is demonstrated on quantum dot-labeled microtubules of fibroblast cells.

Defining a Superlens Operating Regime for Imaging Fluorescent Molecules

It has been shown that thin metal-based films can at certain frequencies act as planar near-field lenses for certain polarization components. A desirable property of such “lenses” is that they can also enhance and focus some large transverse spatial frequency components which contain sub-diffraction limit details. Over the last decade there has been much work in optimizing designs to reduce effects (such as material losses and surface roughness) that are detrimental to image reconstruction. One design that can reduce some of these undesirable effects, and which has received a fair amount of attention recently, is the stacked metal-dielectric superlens. Here we theoretically explore the imaging ability of such a design for the specific purpose of imaging a fluorescent dye (the common bio-marker GFP) in the vicinity of the superlens surface. Our calculations take into consideration the interaction (damping) of an oscillating electric dipole with the metallic layers in the superlens. We also assume a Gaussian frequency distribution spectrum for the dipole. We treat the metallic-alloy and dielectric-alloy layers separately using an appropriate effective medium theory. The transmission properties are evaluated via Transfer matrix (-matrix) calculations that were performed in the MatLab and MathCad environments. Our study shows that it is in principle possible to image fluorescent molecules using a simple bilayer planar superlens. We find that optimal parameters for such a superlens occur when the peak dipole emission-frequency is slightly offset from the Surface Plasmon resonance frequency of the metal-dielectric interfaces. The best resolution is obtained when the fluorescent molecules are not too close ( nm) or too far ( nm) from the superlens surface. The realization and application of a superlens with the specified design is possible using current nanofabrication techniques. When combined with e.g. a sub-wavelength grating structure (such as in the far-field superlens design previously proposed [1]) or a fast near-field scanning probe, it could provide a means for fast fluorescent imaging with sub-diffraction limit resolution.

Analysis of replication factories in human cells by super-resolution light microscopy

BackgroundDNA replication in human cells is performed in discrete sub-nuclear locations known as replication foci or factories. These factories form in the nucleus during S phase and are sites of DNA synthesis and high local concentrations of enzymes required for chromatin replication. Why these structures are required, and how they are organised internally has yet to be identified. It has been difficult to analyse the structure of these factories as they are small in size and thus below the resolution limit of the standard confocal microscope. We have used stimulated emission depletion (STED) microscopy, which improves on the resolving power of the confocal microscope, to probe the structure of these factories at sub-diffraction limit resolution.ResultsUsing immunofluorescent imaging of PCNA (proliferating cell nuclear antigen) and RPA (replication protein A) we show that factories are smaller in size (approximately 150 nm diameter), and greater in number (up to 1400 in an early S- phase nucleus), than is determined by confocal imaging. The replication inhibitor hydroxyurea caused an approximately 40% reduction in number and a 30% increase in diameter of replication factories, changes that were not clearly identified by standard confocal imaging.ConclusionsThese measurements for replication factory size now approach the dimensions suggested by electron microscopy. This agreement between these two methods, that use very different sample preparation and imaging conditions, suggests that we have arrived at a true measurement for the size of these structures. The number of individual factories present in a single nucleus that we measure using this system is greater than has been previously reported. This analysis therefore suggests that each replication factory contains fewer active replication forks than previously envisaged.

Array-based optical nanolithography using optically trapped microlenses

Current demands on optical nanolithography require the ability to rapidly and cost-effectively write arbitrary patterns over large areas with sub-diffraction limit feature sizes. The challenge in accomplishing this with arrays of near-field probes is maintaining equal separations between the substrate and each probe, even over non-planar substrates. Here we demonstrate array-based laser nanolithography where each probe is a microsphere capable of fabricating 100 nm structures using 355 nm light when self-positioned near a surface by Bessel beam optical trapping. We achieve both a feature size uniformity and relative positioning accuracy better than 15 nm, which agrees well with our model. Further improvements are possible using higher power and/or narrower Bessel beam optical traps.

Subdiffraction Limited, Remote Excitation of Surface Enhanced Raman Scattering

We demonstrate that focused laser excitation at the end of silver nanowires of 50-150 nm diameter excites SERS hot-spots at points of nanoparticle adsorption many micrometers along the wire due to the plasmon waveguide effect. The total SERS intensity detected at the hot-spots following wire-end excitation correlates with the known wavelength, polarization, and distance dependences of surface plasmon polariton (SPP) propagation in nanowires. The SERS spectra obtained at the hot-spots following wire-end excitation show very little background compared to when excitation occurs directly at the hot-spot, suggesting that a much smaller SERS excitation volume is achieved by remote, waveguide excitation. The ability to transfer SERS excitation over several micrometers, through a structure with a subdiffraction limit diameter, is discussed with respect to potential high-resolution SERS imaging applications.

Multicolor Three-dimensional Whole-cell Imaging With Nanometer Scale Resolution

The ability to directly visualize nanoscopic cellular structures and their spatial relationship in all three dimensions will greatly enhance our understanding of molecular processes in cells. In this work, we have developed multicolor three-dimensional (3D) stochastic optical reconstruction microscopy (STORM) as a tool to probe molecular structures and their interactions on sub-diffraction length scales. STORM achieves sub-diffraction limit image resolution by using photoswitchable fluorescent probes to separate the spatially overlapping images of individual probes in time. Only a small subset of probes was activated at any given time, allowing us to resolve individual activated probes and determine their positions with high precision. A super-resolution image was then constructed by plotting the measured probe positions accumulated over time. With this we have generated 3D whole cell images, several micrometers thick, with a spatial resolution of 20 - 30 nm and 60 - 70 nm in the lateral and axial dimensions, respectively. Using this approach, we imaged the entire mitochondrial network in mammalian cells and studied the spatial relationship between mitochondria and the microtubule cytoskeleton. The 3D STORM images clearly resolved the hollow mitochondria outer membrane structures obscured in conventional fluorescence images. Distinct mitochondrial morphologies were observed, ranging from thin elongated tubes to globular compartments. Interestingly, while globular mitochondria are relatively dispersed in size, from 200 nm to 1500 nm, the tubular structures are more uniform in diameter, taking a narrow distribution around 200 nm. The images also displayed several distinct interaction modes between mitochondria and microtubules. Notably, elongated mitochondria were observed to “inchworm” along microtubules with discrete attachment sites, while such an interaction mode was completely unresolvable with conventional fluorescence. Super-resolution optical microscopy techniques such as STORM promise to significantly expand the understanding of biological structures and their interactions on a molecular level.

Quantum dot nanophotonics - from waveguiding to integration

Due to its unique optoelectronic properties, the quantum dot (QD) has become a promising material for realizing photonic components and devices with high quantum efficiencies. Quantum dots in colloidal form can have their surfaces modified with various molecules, which enables new fabrication process utilizing molecular self-assembly and can result in new QD photonic device structures in nano-scale. In this review paper, we describe QD waveguides for sub-diffraction-limit waveguiding, nano-scale QD photodetectors for sensing with high spatial resolution and sensitivity, as well as integration of these two nanophotonic components. The paper will provide an overview on the operating principles, fabrications and characterizations of the devices. The QD waveguide achieved a transmission loss of 3 dB/4 micron, which is lower than the experimental results from other sub-diffraction limit waveguides that have been reported. It also demonstrated a comparable waveguiding effect through a waveguide with a sharp bend. The QD photodetector showed a sensitivity of 60 pW over a device with a nano-gap of 25 nm for detection. The compatibility between the fabrication processes for these two components with colloidal QDs allows integration of them through self-assembly fabrications.

Nanolithography using high transmission nanoscale ridge aperture probe

Nanoscale ridge apertures provide a highly confined radiation spot with a high transmission efficiency when used in the near field approach. The radiation confinement and enhancement is due to the electric–magnetic field concentrated in the gap between the ridges. This paper reports the experimental demonstration of radiation enhancement using such antenna apertures and lithography of nanometer size structures. The process utilizes a NSOM (near field scanning optical microscopy) probe with a ridge aperture at the tip, and it combines the nonlinear two photon effect from femtosecond laser irradiation to achieve sub-diffraction limit lithography resolution.

Microscale and Nanoscale Thermal Characterization Techniques

Miniaturization of electronic and optoelectronic devices and circuits and increased switching speeds have exasperated localized heating problems. Steady-state and transient characterization of temperature distribution in devices and interconnects is important for performance and reliability analysis. Novel devices based on nanowires, carbon nanotubes, and single molecules have feature sizes in 1–100 nm range, and precise temperature measurement and calibration are particularly challenging. In this paper we review various microscale and nanoscale thermal characterization techniques that could be applied to active and passive devices. Solid-state microrefrigerators on a chip can provide a uniform and localized temperature profile and they are used as a test vehicle in order to compare the resolution limits of various microscale techniques. After a brief introduction to conventional microthermocouples and thermistor sensors, various contact and contactless techniques will be reviewed. Infrared microscopy is based on thermal emission and it is a convenient technique that could be used with features tens of microns in size. Resolution limits due to low emissivity and transparency of various materials and issues related to background radiation will be discussed. Liquid crystals that change color due to phase transition have been widely used for hot spot identification in integrated circuit chips. The main problems are related to calibration and aging of the material. Micro-Raman is an optical method that can be used to measure absolute temperature. Micron spatial resolution with several degrees of temperature resolution has been achieved. Thermoreflectance technique is based on the change of the sample reflection coefficient as a function of temperature. This small change in 10−4–10−5 range per degree is typically detected using lock-in technique when the temperature of the device is cycled. Use of visible and near IR wavelength allows both top surface and through the substrate measurement. Both single point measurements using a scanning laser and imaging with charge coupled device or specialized lock-in cameras have been demonstrated. For ultrafast thermal decay measurement, pump-probe technique using nanosecond or femtosecond lasers has been demonstrated. This is typically used to measure thin film thermal diffusivity and thermal interface resistance. The spatial resolution of various optical techniques can be improved with the use of tapered fibers and near field scanning microscopy. While subdiffraction limit structures have been detected, strong attenuation of the signal reduces the temperature resolution significantly. Scanning thermal microscopy, which is based on nanoscale thermocouples at the tip of atomic force microscope, has had success in ultrahigh spatial resolution thermal mapping. Issues related to thermal resistance between the tip and the sample and parasitic heat transfer paths will be discussed.

Real‐time three‐dimensional imaging of cell division by differential interference contrast microscopy

Differential interference contrast (DIC) microscopy can provide information about subcellular components and organelles inside living cells. Applicability to date, however, has been limited to 2D imaging. Unfortunately, understanding of cellular dynamics is difficult to extract from these single optical sections. We demonstrate here that 3D differential interference contrast microscopy has sub-diffraction limit resolution both laterally and vertically, and can be used for following Madin Darby canine kidney cell division process in real time. This is made possible by optimization of the microscope optics and by incorporating computer-controlled vertical scanning of the microscope stage.