Near-field Aperture Research Articles

Silicon Nitride Membrane Dynamic Masking Allows Improved Shapes of Near-Field Optical Apertures Fabricated by FIB

Silicon Nitride Membrane Dynamic Masking Allows Improved Shapes of Near-Field Optical Apertures Fabricated by FIB

An Aperture Probe of Near-Field Optical Microscope: Near-Field Tool to Modulate the Laser Diode Emissions

In situ emission profiles of AlGaInP multiple quantum well laser diodes (LD's) observed by a near-field scanning optical microscope aperture probe shows a single broad elliptic profile similar to a far-field observation, whereas the local emission spectrum was different depending where the aperture probe was placed within the emission profile. This observation suggests the existence of a near-field coupling between the aperture and the LD cavity, which modulates the lasing wavelength among the longitudinal multimodes determined by the LD cavity length. [DOI: 10.1380/ejssnt.2009.757]

Improved focused ion beam fabrication of near-field apertures using a silicon nitride membrane

We report an improved fabrication method for C-shaped near-field apertures resonant in the near-IR regime. The apertures are created in a metal layer on a silicon nitride membrane using a focused ion beam and a through membrane milling technique that avoids two problems with fabricating very small apertures: gallium contamination and edge rounding. Finite-difference time-domain simulations predict a 63x more intense near field with a 2.2x smaller spot versus conventionally milled apertures. We verify the position of the simulated resonance peaks with experimental far-field transmission measurements where we also find an increase of 8.8x in intensity. Our method has applications to many other plasmonic devices including bow-tie and fractal apertures, periodic arrays, and gratings.

Design, Fabrication, and Characterization of Near-Field Apertures for 1 Tbit/in$^{2}$ Areal Density

Today, conventional magnetic recording schemes are coming to an end because of the superparamagnetic limit. Heat-assisted magnetic recording (HAMR) may ultimately extend data densities beyond 1 TB/in2. HAMR systems utilize the phenomenon during which the magnetic properties of the recording media could be locally modified via heating (optionally, by an optical source in the near field) to temperature in the vicinity of the Curie value of the media material. As a result, heat induced by the optical source can temporarily reduce the magnetic coercivity of high anisotropy material to a level attainable by the magnetic writing head, thus making it feasible to record on relatively small ultra-high anisotropy (and thermally stable) grains, consequently enhancing the areal density dramatically. The key challenge is to develop a near-field transducer capable of delivering over 50 nW into a spot diameter of 30 nm. Traditional fiber schemes are barely capable of 0.1 nW. To resolve the issue, a laser diode could be placed with the emitting edge only a few nanometers away from the recording media. The light can propagate through a nanoaperture on the surface of an aluminum-coated emitting edge. This paper will present an experimental study of recording characteristics of various near-field transducers fabricated via focused ion beam (FIB). To count the number of photons emitted in the near field, a scanning near-field optical microscopy system has been implemented. The experiments indicate that the FIB-fabricated transducers could deliver power of over a few microwatt into a 30-nm spot (Fig. 7).

Reduction of Truncation Errors in Planar Near-Field Aperture Antenna Measurements Using the Gerchberg-Papoulis Algorithm

A simple and effective procedure for the reduction of truncation errors in planar near-field measurements of aperture antennas is presented. The procedure relies on the consideration that, due to the scan plane truncation, the calculated plane wave spectrum of the field radiated by the antenna is reliable only within a certain portion of the visible region. Accordingly, the truncation error is reduced by extrapolating the remaining portion of the visible region by the Gerchberg-Papoulis iterative algorithm, exploiting a condition of spatial concentration of the fields on the antenna aperture plane. The proposed procedure is simple and computationally efficient; it does not require any modification of the measurement procedure and it allows for the usual probe correction. Far-field patterns reconstructed from both simulated and measured truncated near-field data demonstrate its effectiveness and stability against measurement inaccuracies.

Fabrication of optical tips from photonic crystal fibers

We present a procedure for fabricating optical tips from photonic crystal fibers which feature a solid core surrounded by a cladding with a hexagonal, multilayer arrangement of air channels running along the length of the fiber. Such optical tips may have unique advantages for the production of near-field optical aperture probes (i.e., metal-coated optical tips with a subwavelength aperture at the tip apex). With both cladding and core made of pure silica, these fibers are fluorescence-free; they support only a single mode over a broad wavelength range (covering the visible and near-infrared spectrum), which makes them useful for multicolor experiments; and they exhibit zero group velocity dispersion at visible wavelengths, which opens up the possibility of femtosecond applications in the near field. Our tip fabrication procedure leads to a sharp, protruding, central tip formed exclusively from the fiber core amidst a regular arrangement of smaller tips from the inner, microstructured region of the cladding. A mechanism for tip formation is proposed based on optical observations at various stages, which explains the self-centering nature of the process.

Two-Dimensional FDTD Analysis of the Readout Characteristics of an Optical Near Field Disk

Various kinds of optical near-field apertures have been proposed for higher throughput and smaller spot size. However, few studies have mentioned the readout characteristics of the recorded marks on an optical disk illuminated by a near field optical light. In this paper, we have investigated the scattering light by a two-dimensional recorded mark on a phase change disk with Finite-Difference Time-Domain (FDTD) simulations. Instead of using Recursive Convolution (RC) and Piecewise Linear Recursive Convolution (PLRC) scheme, we integrated the motion equation of free electron into conventional FDTD method to solve the electromagnetic field in the metallic materials. The validity of the proposed method is illustrated by comparing its results with those from the analytic exact solution. We analyzed the distributions of optical near-field around a two-dimensional metallic nano-aperture, and then calculated the far-field scattering pattern from a two-dimensional recorded mark on a phase change disk while it was illuminated by an optical near-field though a nanoaperture. The sum signal by a condenser lens was calculated from far-field pattern, and its relationships with the width of recorded mark and the thickness of each layer in the phase change disk were illustrated. The cross-talk between the recorded marks was also discussed.

Minimization of the impact of a broad bandwidth high-gain nonlinear preamplifier to the amplified spontaneous emission pedestal of the Vulcan petawatt laser facility

To generate petawatt pulses using the Vulcan Nd:glass laser requires a broad bandwidth high-gain preamplifier. The preamplifier used is an optical parametric amplifier that provides a total gain of 10(8) in three amplification stages. We report on a detailed investigation of the effect of the Vulcan optical parametric chirped pulse amplification (OPCPA) preamplifier on contrast caused by the amplified spontaneous emission (ASE) pedestal that extends up to 2 ns before the arrival of the main pulse. The contrast after compression is improved to 4x10(8) of the intensity of the main pulse using near-field apertures between the stages of the OPCPA preamplifier. Further reduction of the level of the ASE pedestal can be achieved at the cost of a reduction in amplified bandwidth by solely phosphate glass amplification after initial preamplification rather than a mixed glass amplification scheme.

Near-field, broadband optical spectroscopy of metamaterials

Near-field, broadband optical spectroscopy of metamaterials is reported. A compact, easy to assemble supercontinuum light source, coupled with a near-field scanning optical microscope and spectrometer provide the means to locally probe the spectral response of a nanorod metamaterial sample. Spectral maps of near-field transmittance are obtained for the spectral range from 500 to 950nm. In comparison to far-field measurement, the average of the near-field mappings shows a significant increase in transmittance. The uniqueness of the spectral maps and the increased transmittance indicates a strong nanoantenna effect between the sample and the plasmonic near-field aperture.

Stand-alone device for the electrolytic fabrication of scanning near-field optical microscopy aperture probes

Near-field optical applications require the fast, stable, and reproducible fabrication of scanning near-field optical microscopy (SNOM) aperture probes in the submicrometer range. We have developed a stand-alone device for the electrolytic etching of nanoapertures with an integrated current and optical transmission monitoring and control. Probes with an aperture ranging from 50to100nm were reproducibly fabricated with great reliability. With these probes, high resolution SNOM images of 100nm test patterns and single dye molecules (Rhodamine 6G in poly(vinyl alcohol)) are measured and presented. Not requiring a SNOM setup, the stand-alone device is not only inexpensive and compact, but also insensitive to external disturbances.

Fractal extensions of near-field aperture shapes for enhanced transmission and resolution

Families of fractals are investigated as near-field aperture shapes. They are shown to have multiple transmission resonances associated with their multiple length scales. The higher iterations exhibit enhanced transmission, and spatial resolution exceeding the first order. Near-field enhancements of greater than 400 times the incident intensity and resolutions of better than ?/20 have been shown with apertures modeled after third iteration prefractals. Enhancements as large as 1011 have been shown, when compared with conventional square apertures that produce the same spot size. The effects of the complex permittivity values of the metal film are also addressed.

Complex Padé approximants for bidirectional and nonparaxial beam propagation method

The nonparaxial and bidirectional beam propagation method suitable for modelling near-field and high numerical aperture (NA) optical storage systems is suggested for 2D geometry and TE polarization of incident light beam. The complex Pade approximants are introduced for correct approximation of evanescent field in the near-field optics. Pole–zero shifting and branch-cut rotation methods of building complex Pade approximants are studied and compared.

New dimension in nano-imaging: breaking through the diffraction limit with scanning near-field optical microscopy

In recent years scanning near-field optical microscopy (SNOM) has developed into a powerful surface analytical technique for observing specimens with lateral resolution equal to or even better than 100 nm. A large number of applications, from material science to biology, have been reported. In this paper, two different kinds of near-field optical microscopy, aperture and scattering-type SNOM, are reviewed together with recent studies in surface analysis and biology. Here, near-field optical techniques are discussed in comparison with related methods, such as scanning probe and standard optical microscopy, with respect to their specific advantages and fields of application.

Ridge waveguide as a near field aperture for high density data storage

The performance of the ridge waveguide as a near-field aperture in data storage systems is investigated. Finite element method (FEM) and finite-difference time-domain (FDTD) based software are used in the numerical simulations. To verify their accuracy at optical frequencies, the FEM and FDTD are first compared to analytical results. The accuracy of these techniques for modeling ridge waveguides at optical frequencies is also evaluated by comparing their results with each other for a plane wave illumination. The FEM, which is capable of modeling focused beams, is then used to simulate various geometries involving ridge waveguides. Near-field radiation from ridge waveguide transducer is expressed in terms of power density quantities. Previous studies in the literature consider the performance of the transducer in free space, rather than in the presence of a recording magnetic medium. The effect of the recording magnetic medium on the transmission efficiency and spot size is discussed using numerical simulations. The effect of various geometric parameters on the optical spot size and transmission efficiency is investigated and discussed. Based on our numerical simulations, a promising transducer design is suggested to obtain intense optical spots well below the diffraction limit. Numerical simulations suggest that a full width at half maximum spot diameter of 31nm in the recording magnetic medium can be obtained. The maximum value of the absorbed optical power density in the recording medium is about 1.67×10−4mW∕nm3 for a 100mW input power. In-track and cross-track profiles for this design are compared with Gaussian distributions.

Optical Trapping with Integrated Near-Field Apertures

In recent years, optical micromachines based on forces exerted by strongly focused beams of light have started to provide unprecedented access to nonintrusive measurement and manipulation of matter on submicron length scales. However, the sharpness of the present optical tweezers is restricted by the spatial gradients of light attainable with diffraction-limited optics. Here, we demonstrate a technique for optical trapping of single particles in a fluid, which is based on the intense near-field gradients around small apertures in a metal film. Our scheme should be able to trap smaller particles with a fraction of the laser intensity required by conventional optical tweezers. Detailed simulations of the electromagnetic fields near apertures and the resulting forces they can produce are described. We also present a proof-of-principle experiment in which the trapping of latex beads is demonstrated by following the time evolution of their fluorescence. Our scheme allows containment of particles in free soluti...

2-D near-field SAR non-destructive testing of rebars in a concrete wall

A microwave non-destructive testing technique to localize metal reinforcement bars in a concrete wall is presented. The technique is based on a two-dimensional near-field Synthetic Aperture Radar imaging algorithm in a monostatic stepped frequency arrangement. The resulting, qualitative, images of the induced sources give a clear indication on the position of the highly conductive rebars, thanks to their strongly reflective properties. The algorithm takes into account the presence of the concrete, the complex permittivity of which is assumed to be known. The technique has been succesfully tested on simulated and experimental data for a number of concrete wall samples with various reinforcement configurations.

Fabrication of a High-Throughput Cantilever-Style Aperture Tip by the Use of the Bird's-Beak Effect

A fabrication technique for a high-throughput cantilever-style aperture tip is described. Using the bird's-beak effect due to the stress caused by Si3N4 film in the field oxidation of silicon, we fabricated a near-field aperture with a diameter of about 100 nm which has a throughput of more than 1000 times that of the pyramidal pit.

Aperture shape effect on the performance of very small aperture lasers

We report on the effect of different aperture shapes on the power output of a very small aperture laser (VSAL) fabricated from commercial edge-emitting laser diodes and correlate the results to the layer structure and polarization of the optical field. A waveguide theory is used to explain the experimental observations. We show that the shape of the aperture has a significant effect on the VSAL output power. In particular shapes exploiting the asymmetry of the laser can achieve much higher throughput over square apertures while keeping the aperture area constant. This work also indicates the validity of analyzing near-field small apertures as cutoff waveguide structures.

Consideration and Control of Writing Conditions with Near-Field Aperture Solid Immersion Lens Probe

Control of writing conditions for near-field aperture-SIL (APSIL) probe is investigated with respect to polarization, axial focus position of the objective lens and beam transverse misalignment. Both FDTD simulations and edge-scan experiments are used in the investigation. The TE direction is the optimal writing polarization direction, and the probe exit plane is the optimum focus position of the illuminating lens. Spot size change and spot position shift relative to the center of the probe are observed with TE polarization when the writing beam is transversely misaligned.

Near-Field Surface Plasmon Excitation on Structured Gold Films

Surface plasmon polariton (SPP) localization control on rough metal surfaces has been achieved by varying the gap between a scanning near-field aperture probe and the surface. The qualitative explanation for this observation is based on the overlapping of spatial frequencies available for near-field excitation of the SPP, with the spectral distribution of spatial frequencies characterizing the surface roughness. From a practical point of view, this selective excitation of electromagnetic states on nanostructures provides a way to estimate the reflectivity of structures such as grooves engraved into the metal film. Qualitative aspects of transmission and scattering through parallel slit pairs acting as Fabry−Perot resonators are discussed.