Focusing surface plasmons with a plasmonic lens

We report the focusing of surface plasmon polaritons by circular and elliptical structures milled into optically thick metallic films or plasmonic lenses. Both theoretical and experimental data for the electromagnetic nearfield is presented. The nearfield is mapped experimentally using nearfield scanning optical microscopy and plasmonic lithography. We find that the intensity at the focal points of the plasmonic lenses increases with size.     

Extraordinary optical transmission brightens near-field fiber probe

Near-field scanning optical microscopy (NSOM) offers high optical resolution beyond the diffraction limit for various applications in imaging, sensing, and lithography; however, for many applications the very low brightness of NSOM aperture probes is a major constraint. Here, we report a novel NSOM aperture probe that gives a 100× higher throughput and 40× increased damage threshold than conventional near-field aperture probes. These brighter probes facilitate near-field imaging of single molecules with apertures as small as 45 nm in diameter. We achieve this improvement by nanostructuring the probe and by employing a novel variant of extraordinary optical transmission, relying solely on a single aperture and a coupled waveguide. Comprehensive electromagnetic simulations show good agreement with the measured transmission spectra. Due to their significantly increased throughput and damage threshold, these resonant configuration probes provide an important step forward for near-field applications.     

Fluorescence lifetime imaging with near-field scanning optical microscopy

Near-field scanning optical microscopy (NSOM) is a high-resolution scanning probe technique capable of obtaining simultaneous optical and topographic images with spatial resolution of tens of nanometers. We have integrated time-correlated single-photon counting and NSOM to obtain images of fluorescence lifetimes with high spatial resolution. The technique can be used to measure either full fluorescence lifetime decays at individual spots with a spatial resolution of <100 nm or NSOM fluorescence images using fluorescence lifetime as a contrast mechanism. For imaging, a pulsed Ti:sapphire laser was used for sample excitation and fluorescent photons were time correlated and sorted into two time delay bins. The intensity in these bins can be used to estimate the fluorescence lifetime at each pixel in the image. The technique is demonstrated on thin films of poly(9,9'-dioctylfluorene) (PDOF). The fluorescence of PDOF is the results of both inter- and intrapolymer emitting species that can be easily distinguished in the time domain. Fluorescence lifetime imaging with near-field scanning optical microscopy demonstrates how photochemical degradation of the polymer leads to a quenching of short-delay intrachain emission and an increase in the long-delay photons associated with interpolymer emitting species. The images also show how intra- and interpolymer species are uniformly distributed in the films.     

Design of metal-cladded near-field fiber probes with a dispersive body-of-revolution finite-difference time-domain method

A dispersive body-of-revolution finite-difference time-domain method is developed to simulate metal-cladded near-field scanning optical microscope (NSOM) probes. Two types of NSOM probe (aperture and plasmon NSOM probes) are analyzed and designed with this fast method. The influence of the metal-cladding thickness and the excitation mode on the performance of the NSOM probes is studied. We introduce a new scheme of illumination-mode NSOM by employing the plasmon NSOM probe with the TM01 mode excitation. Such a NSOM probe is designed, and we demonstrate its advantages over the conventional aperture NSOM probe by scanning across a metallic object.     

Plasmonic nanofabrication by long-range excitation transfer via DNA nanowire

Driven by the demand for ongoing integration and increased complexity of today's microelectronic circuits, smaller and smaller structures need to be fabricated with a high throughput. In contrast to serial nanofabrication techniques, based, e.g., on electron beam or scanning probe methods, optical methods allow a parallel approach and thus a high throughput. However, they rarely reach the desired resolution. One example is plasmon lithography, which is limited by the utilized plasmonic metal structures. Here we show a new approach extending plasmonic lithography with the potential for a highly parallel nanofabrication with a higher level of complexity based on nanoantenna effects combined with molecular nanowires. Thereby femtosecond laser pulse light is converted by Ag nanoparticles into a high plasmonic excitation guided along attached DNA structures. An underlying poly(methyl methacrylate) (PMMA) layer acting as an electron-sensitive resist is so structured along the former DNA position. This apparently DNA-guided effect leads to nanometer grooves reaching even micrometers away from the excited nanoparticle, representing a novel effect of long-range excitation transfer along DNA nanowires.     

High-Speed Parallel Plasmonic Direct-Writing Nanolithography Using Metasurface-Based Plasmonic Lens

Simple and efficient nanofabrication technology with low cost and high flexibility is indispensable for fundamental nanoscale research and prototyping. Lithography in the near field using the surface plasmon polariton (i.e., plasmonic lithography) provides a promising solution. The system with high stiffness passive nanogap control strategy on a high-speed rotating substrate is one of the most attractive high-throughput methods. However, a smaller and steadier plasmonic nanogap, new scheme of plasmonic lens, and parallel processing should be explored to achieve a new generation high resolution and reliable efficient nanofabrication. Herein, a parallel plasmonic direct-writing nanolithography system is established in which a novel plasmonic flying head is systematically designed to achieve around 15 nm minimum flying-height with high parallelism at the rotating speed of 8–18 m·s⁻¹. A multi-stage metasurface-based polarization insensitive plasmonic lens is proposed to couple more power and realize a more confined spot compared with conventional plasmonic lenses. Parallel lithography of the nanostructures with the smallest (around 26 nm) linewidth is obtained with the prototyping system. The proposed system holds great potential for high-freedom nanofabrication with low cost, such as planar optical elements and nano-electromechanical systems.     

Experimental study of plasmonic structures with variant periods for sub-wavelength focusing: analyses of characterization errors

Characterization issues of plasmonic structures are highlighted and investigated in detail in this paper. Combining with the plasmonic structures functioning for sub-wavelength focusing, optical characterization was carried out using a near-field scanning optical microscope (NSOM) system. Characterization errors that originated from both the nanofabrication using a focused ion beam (FIB) direct milling technique and misalignment of the NSOM system were analyzed in comparison to the theoretical computational results. Our experimental results demonstrated that the focusing function of the structures is in agreement with that of the designed structure. However, the measured beam spot size is larger than the designed value due to the direct measurement error originating from the NSOM and the indirect error from the FIB fabrication process.     

Nanometer scale polarimetry studies using a near-field scanning optical microscope

We describe a new technique that incorporates polarization modulation into near-field scanning optical microscopy (NSOM) for nanometer scale polarimetry studies. By using this technique, we can quantitatively measure the optical anisotropy of materials with both the high sensitivity of dynamic polarimetry and the high spatial resolution of NSOM. The magnitude and relative orientation of linear birefringence or linear dichroism are obtained simultaneously. To demonstrate the sensitivity and resolution of the microscope, we map out stress-induced birefringence associated with submicrometer defects at the fusion boundaries of SrTiO3 bicrystals. Features as small as 150 nm were imaged with a retardance sensitivity of approximately 3 x 10(-3) rad.     

Electromagnetic field enhancement and spectrum shaping through plasmonically integrated optical vortices

We introduce a new design approach for surface-enhanced Raman spectroscopy (SERS) substrates that is based on molding the optical powerflow through a sequence of coupled nanoscale optical vortices "pinned" to rationally designed plasmonic nanostructures, referred to as Vortex Nanogear Transmissions (VNTs). We fabricated VNTs composed of Au nanodiscs by electron beam lithography on quartz substrates and characterized their near- and far-field responses through combination of computational electromagnetism, and elastic and inelastic scattering spectroscopy. Pronounced dips in the far-field scattering spectra of VNTs provide experimental evidence for an efficient light trapping and circulation within the nanostructures. Furthermore, we demonstrate that VNT integration into periodic arrays of Au nanoparticles facilitates the generation of high E-field enhancements in the VNTs at multiple defined wavelengths. We show that spectrum shaping in nested VNT structures is achieved through an electromagnetic feed-mechanism driven by the coherent multiple scattering in the plasmonic arrays and that this process can be rationally controlled by tuning the array period. The ability to generate high E-field enhancements at predefined locations and frequencies makes nested VNTs interesting substrates for challenging SERS applications.     

Optimizing the near field around silver tips

Owing to their promise of obtaining optical as well as topographic information in nanometer scale, apertureless near-field scanning optical microscopy (NSOM) and apertureless near-field scanning optical spectroscopy have drawn much attention recently. However, NSOM is still not a mature technique. A proper understanding of and the ability to tune the near field around the tip end is critically important in NSOM instrumentation and in NSOM image interpretation. On the basis of reflection geometry, we systematically studied the effects of a number of parameters pertinent in the application of apertureless NSOM, e.g., polarization, incident angle, wavelength of the incident laser, tip material, and tip length, by using the generalized field propagator technique. Our results show that all the above parameters have a significant influence on near-field enhancement and that care must be taken in the design of the experiment in order to maximize the near field. In addition to apertureless NSOM and spectroscopy, apertureless near-field lithography can benefit from these simulation results.     

Near-field optical apertured tip and modified structures for local field enhancement

We present experimental measurements and simulation of the spatial distribution of near-field light at the aperture of a Si micromachined near-field scanning optical microscopy (NSOM) probe. A miniature aperture at the apex of a SiO(2) tip on a Si cantilever was fabricated with the low temperature oxidation and selective etching technique. An optical transmission efficiency (optical throughput) of the fabricated probe was determined to be approximately 10(-2) when the aperture size was approximately 100 nm, which is several orders of magnitude higher than that for conventional optical fibers. A three-dimensional finite difference time domain (FDTD) simulation shows that the near-field light is well confined within the aperture area with a throughput of 1% for a 100-nm aperture, which is in good agreement with the measurement. The spatial distribution of the near-field light at an aperture of 300-nm diameter shows a full width at half-maximum of 250 nm with a sharp peak that is nearly 60 nm wide. The 2.4% throughput for a 300-nm aperture was estimated based on the measured spatial distribution of the near-field light that is almost the same as the experimental result. We also present the initial results of the fabrication of high throughput coaxial and surface plasmon enhancement NSOM probes.     

Compact method for optical induction of proximal probe heating and elongation

A tapered, metal-coated, optical fiber probe will elongate when heated by light input through a fiber. The induced motion can be used for data storage or nanostructuring of a surface. The elongation produced by this alignment-free system is measured with force feedback in a near-field scanning optical microscope (NSOM). The input light intensity controls the elongation magnitude, which ranges from a few nanometers to more than 100 nm. A 0.5-mW input energy yields approximately 20 nm of probe elongation. The elongation quantified here can create artifacts in any experiment using pulsed laser light with a NSOM or an atomic force microscope.     

Near-Field Photothermal Heating with a Plasmonic Nanofocusing Probe

Noble metal nanostructures support plasmon resonances—collective oscillation of charge carriers at optical frequencies—and serve as effective tools to create bright light sources at the nanoscale. These sources are useful in broad application areas including, super-resolution imaging and spectroscopy, nanolithography, and near-field optomechanical transducers. The feasibility of these applications relies on efficient conversion of free-space propagating light to plasmons. Recently, we demonstrated a hybrid nanofocusing scheme for efficient coupling of light to plasmons at the apex of a scanning probe. In the approach, free-space light is coupled to propagating surface plasmon polaritons (SPPs) on the tapered shaft of the scanning probe. The SPPs propagate adiabatically towards the probe tip where they are coupled to localized plasmons (LSPs). The nanofocusing scheme was explored in a near-field scanning optical microscope for super-resolution imaging, near-field transduction of nanomechanical vibrations, and local detection of ultrasound. Owing to the strong concentration of light at the probe, significant heating of the tip and a sample positioned in the optical near-field is expected. This paper investigates the local heating produced by the plasmonic nanofocusing probe under steady-state conditions using the tip-enhanced Raman scattering approach. In addition, a finite element model is explored to study the coupling of free propagating light to LSPs, and to estimate the temperature rise expected in a halfspace heated by absorption of the LSPs. This study has implications for exploring the plasmonic nanofocusing probe in heat-assisted nanofabrication and fundamental studies of nanoscale heat transport in materials.     

Development of high-throughput, polarization-maintaining, near-field probes

A novel, chemical-etching technique produces very high throughput, polarization-maintaining probes for near-field, scanning, optical microscopy (NSOM). The process includes coating the tips with aluminum and forming the apertures with a focused ion beam (FIB). The elliptical core fibers used resulted in elliptical apertures for the probes. The throughput of the probes depends on the incident polarization. For polarization parallel to the minor axis, the tip presents an insertion loss of only 20 dB for aperture widths of 55 nm. Probes have a typical polarization extinction of 100 to 1 in the far field. These tips produced NSOM images of gold dots on a GaAs substrate in reflection mode.     

Plasmonic Luneburg and Eaton lenses

Plasmonics takes advantage of the properties of surface plasmon polaritons, which are localized or propagating quasiparticles in which photons are coupled to the quasi-free electrons in metals. In particular, plasmonic devices can confine light in regions with dimensions that are smaller than the wavelength of the photons in free space, and this makes it possible to match the different length scales associated with photonics and electronics in a single nanoscale device. Broad applications of plasmonics that have been demonstrated to date include biological sensing, sub-diffraction-limit imaging, focusing and lithography and nano-optical circuitry. Plasmonics-based optical elements such as waveguides, lenses, beamsplitters and reflectors have been implemented by structuring metal surfaces or placing dielectric structures on metals to manipulate the two-dimensional surface plasmon waves. However, the abrupt discontinuities in the material properties or geometries of these elements lead to increased scattering of surface plasmon polaritons, which significantly reduces the efficiency of these components. Transformation optics provides an alternative approach to controlling the propagation of light by spatially varying the optical properties of a material. Here, motivated by this approach, we use grey-scale lithography to adiabatically tailor the topology of a dielectric layer adjacent to a metal surface to demonstrate a plasmonic Luneburg lens that can focus surface plasmon polaritons. We also make a plasmonic Eaton lens that can bend surface plasmon polaritons. Because the optical properties are changed gradually rather than abruptly in these lenses, losses due to scattering can be significantly reduced in comparison with previously reported plasmonic elements.     

Performance enhancements to absorbance-modulation optical lithography. I. Plasmonic reflector layers

The ability to improve the transmission and intensity profiles in absorbance-modulation optical lithography [J. Opt. Soc. Am. A 23, 2290-2294 (2006) and Phys. Rev. Lett. 98, 043905 (2007)] through the introduction of a plasmonic metal layer is investigated. In this part of the work, a plasmonic reflector layer (PRL) is placed beneath the photoresist layer. Improvement is expected due to surface plasmons being induced on the plasmonic layer and supporting the transmission of the image deeper into the imaging layer. The introduction of the plasmonic reflector improves the depth of focus markedly, with the image confinement extended up to 60 nm but with a penalty of up to a 50% increase in the minimum full width at half-maximum of the intensity profile. The presented work demonstrates that a PRL can be a valuable tool for near-field lithography.     

Efficient numerical representation of the optical field for the propagation of partially coherent radiation with a specified spatial and temporal coherence function

We propose a method to narrow the gap between the rigorous methods for the propagation of partially coherent light, which require excessive computational capacity, and the numerical methods used in practical engineering applications, where it is not clear how to handle spatial and temporal coherence in a statistically correct manner. As is the case for the latter methods, the numerical method described can deal with fields with a large spatial and temporal extent, which is necessary in practical applications such as laser fusion or optical lithography. However, the method also takes a few steps toward a more rigorous, yet efficient, representation of the optical field, which depends on detailed specified coherence properties of the radiation. The described method uses a set of independent monochromatic fields at different oscillation frequencies. The frequencies are chosen such that the statistical properties of the integrated intensity closely resemble those from a full-time trace treatment. Finally, we demonstrate the capabilities and limitations of the method with a few numerical examples of the propagation of a large field with a specified spatial and temporal coherence.     

Printing colour at the optical diffraction limit

The highest possible resolution for printed colour images is determined by the diffraction limit of visible light. To achieve this limit, individual colour elements (or pixels) with a pitch of 250?nm are required, translating into printed images at a resolution of ～100,000 dots per inch (d.p.i.). However, methods for dispensing multiple colourants or fabricating structural colour through plasmonic structures have insufficient resolution and limited scalability. Here, we present a non-colourant method that achieves bright-field colour prints with resolutions up to the optical diffraction limit. Colour information is encoded in the dimensional parameters of metal nanostructures, so that tuning their plasmon resonance determines the colours of the individual pixels. Our colour-mapping strategy produces images with both sharp colour changes and fine tonal variations, is amenable to large-volume colour printing via nanoimprint lithography, and could be useful in making microimages for security, steganography, nanoscale optical filters and high-density spectrally encoded optical data storage.     

Plasmonic Amplifiers: Engineering Giant Light Enhancements by Tuning Resonances in Multiscale Plasmonic Nanostructures

The unique ability of plasmonic nanostructures to guide, enhance, and manipulate subwavelength light offers multiple novel applications in chemical and biological sensing, imaging, and photonic microcircuitry. Here the reproducible, giant light amplification in multiscale plasmonic structures is demonstrated. These structures combine strongly coupled components of different dimensions and topologies that resonate at the same optical frequency. A light amplifier is constructed using a silver mirror carrying light‐enhancing surface plasmons, dielectric gratings forming distributed Bragg cavities on top of the mirror, and gold nanoparticle arrays self‐assembled into the grating grooves. By tuning the resonances of the individual components to the same frequency, multiple enhancement of the light intensity in the nanometer gaps between the particles is achieved. Using a monolayer of benzenethiol molecules on this structure, an average SERS enhancement factor <EF> ～10⁸ is obtained, and the maximum enhancement in the interparticle hot‐spots is ～3 × 10¹⁰, in good agreement with FDTD calculations. The high enhancement factor, large density of well‐ordered hot‐spots, and good fidelity of the SERS signal make this design a promising platform for quantitative SERS sensing, optical detection, efficient solid state lighting, advanced photovoltaics, and other emerging photonic applications.     

Differential near-field scanning optical microscopy

We theoretically and experimentally illustrate a new apertured near-field scanning optical microscopy (NSOM) technique, termed differential NSOM (DNSOM). It involves scanning a relatively large (e.g., 0.3-2 mum wide) rectangular aperture (or a detector) in the near-field of an object and recording detected power as a function of the scanning position. The image reconstruction is achieved by taking a two-dimensional derivative of the recorded power map. Unlike conventional apertured NSOM, the size of the rectangular aperture/detector does not determine the resolution in DNSOM; instead, the resolution is practically determined by the sharpness of the corners of the rectangular aperture/detector. Principles of DNSOM can also be extended to other aperture/detector geometries such as triangles and parallelograms.