Near-field Subwavelength Imaging Research Articles

Rigorous Analysis and Systematical Design of Double-Layer Metal Superlens for Improved Subwavelength Imaging Mediated by Surface Plasmon Polaritons.

A double-layer metal superlens was rigorously analyzed and systematically designed to improve subwavelength imaging ability. It was revealed that transmission properties of the imaging system could be accurately interpreted by the five-layer waveguide mode theory—each amplification peak among the spatial frequency range of evanescent waves was associated with a corresponding surface plasmon polariton (SPP) mode of an insulator-metal-insulator-metal-insulator (IMIMI) structure. On the basis of such physical insight, evanescent waves of higher spatial frequency were effectively amplified via increasing propagation constants of symmetrically coupled short-range SPP (s-SRSPP) and antisymmetrically coupled short-range SPP (a-SRSPP), and evanescent waves of lower spatial frequency were appropriately diminished by approaching to cut off symmetrically coupled long-range SPP (s-LRSPP). A flat and broad optical transfer function of the imaging system was then achieved, and improved subwavelength imaging performance was validated by imaging an ideal thin object of two slits with a 20-nm width distanced by a 20-nm spacer, under 193-nm illumination. The resolution limit of the designed imaging system with double-layer superlens was further demonstrated to be at least ~λ/16 for an isolated two-slit object model. This work provided sound theoretical analysis and a systematic design approach of double-layer metal superlens for near-field subwavelength imaging, such as fluorescent micro/nanoscopy or plasmonic nanolithography.

Near-infrared subwavelength imaging using Al:ZnO-based near-field superlens

Near-field subwavelength imaging has been realized at visible and mid-infrared frequencies, but it has not been achieved at near-infrared (NIR). In this work, the transparent conducting oxides (TCO)-based near-field superlenses working at NIR frequencies were proposed. As one of easily available TCO materials, Al-doped ZnO (AZO) was considered in both stratified ZnO-AZO-ZnO and single-layered AZO structures, which had the subwavelength resolution of better than λ/25 at a wavelength of 2.57μm andλ/20 at 2.01μm, respectively. These findings reveal that the TCO can find the applications not only in liquid-crystal displays, photovoltaic devices, and electromagnetic interface shielding, but also in superresolution and subsurface imaging at NIR frequencies.

Resonantly loaded apertures for sub‐diffraction near‐field imaging and surface probing

It is demonstrated that electromagnetic (EM) transmission through a subwavelength or non-resonant aperture in a conductive screen can be significantly enhanced by loading it with folded metallic strips exhibiting resonant properties. When illuminated by an EM plane wave, these loaded apertures enable very tight, subwavelength, collimation of the EM power in the near-field zone. The authors propose quasi-planar resonant insertion geometries that should allow, for the first time, simultaneous subwavelength near field focusing of both electric and magnetic fields. The proposed technique for resonance transmission enhancement and near-field confinement forms a basis for a new class of microwave near-field imaging probe with subwavelength resolution capable of operating over a wide range of imaging distances (0.01–0.25λ). Measurement results demonstrate the possibility of high-contrast (more than 3 dB in amplitude and 40° in phase) near-field subwavelength imaging of active sources as well as the characterisation of planar subwavelength non-resonant dielectric targets located in free space and at the interface of moderately lossy layered media.

Subwavelength Imaging Using Conducting Sheets

Here we demonstrate that, paradoxically, subwavelength imaging can be produced by purely resistive means. Space acts like a low pass filter for highly evanescent field components, and if a sheet or thin layer of imperfectly conducting material is placed adjacent to a source, such that the layer overcomes the larger impedance of the spatial low pass filter, no relative attenuation of evanescent components is experienced at the location of the sheet. This results in near-field subwavelength imaging, which also holds for reactive sheets. The conducting layer enables us to trade definition for amplitude. Impedance sheets are commonplace in radio frequencies or microwaves, hence the phenomenon identified here is widespread, and can be extended into the IR or optical region, as well as to other areas of physics where wave motion exists.

Object distance-independent near-field subwavelength imaging of metal waveguide arrays

A prism-shape microlens constructed with two-dimensional nanoscale metal waveguide arrays (MWGAs) is introduced for near-field subwavelength imaging. Finite-difference time-domain numerical simulations reveal that the microlens shows interesting object distance-independent subwavelength imaging as the object distance ranges from 100 nm to 1000 nm and the concave angle of the prism-shape lens is less than 150°. The possibility of using such microlens for array microimaging is also demonstrated. The microlens may provide potential applications in nanophotonics, lithography, and superresolution imaging.

Simulation of photodetection using finite-difference time-domain method with application to near-field subwavelength imaging based on nanoscale semiconductor photodetector array

Simulation of detecting photoelectrons using multi-level multi-electron (MLME) finite-difference time-domain (FDTD) method with an application to near-field subwavelength imaging based on semiconductor nanophotodetector (NPD) array is reported. The photocurrents from the photodiode pixels are obtained to explore the resolution of this novel NPD device for subwavelength imaging. One limiting factor of the NPD device is the optical power coupling between adjacent detector pixels. We investigate such power coupling in the presence of absorbing media as well as the spatial distributions of the electric field and photoelectron density using the MLME FDTD simulation. Our results show that the detection resolution is about one tenth of the operating wavelength, which is comparable to that of a near-field scanning optical microscope based on metal clad tapered fiber.

Near-field enhancement and imaging in double planar polariton-resonant structures

It is shown that a system of two coupled planar material sheets possessing surface mode (polariton) resonances can be used for the purpose of evanescent field restoration and, thus, for sub-wavelength near-field imaging. The sheets are placed in free space so that they are parallel and separated by a certain distance. Due to interaction of the resonating surface modes (polaritons) of the sheets an exponential growth in the amplitude of an evanescent plane wave in the system can be achieved. This effect was predicted earlier for backward-wave (double-negative or Veselago) slab lenses. The alternative system considered here is proved to be realizable at microwaves by grids or arrays of resonant particles. The necessary electromagnetic properties of the resonating grids and the particles are investigated and established. Theoretical results are supported by microwave experiments that demonstrate amplification of evanescent modes.

Broadband near-field interference spectroscopy of metal nanoparticles using a femtosecond white-light continuum.

We demonstrate a new nanoscale spectroscopic technique that combines subwavelength near-field imaging with broadband interference spectroscopy. We apply this technique to study phase spectra of surface plasmons in individual gold nanoparticles and nanoparticle dimers. Collective plasmon oscillations in selected nanostructures are excited by a femtosecond white-light continuum transmitted through a subwavelength aperture. The interference spectra detected in the far field result from the coherent superposition of the aperture field and the secondary field re-emitted by the nanostructure. The analysis of these spectra allows us to accurately measure the positions and damping constants of single-nanostructure plasmon resonances.