Near-field Applications Research Articles

Evanescent Bessel beams

Bessel beams of the evanescent kind are presented and analyzed. They rapidly decay with propagation but retain their original transversal shape. A physical method of generation is proposed in the form of propagation of an ordinary (nonevanescent) Bessel beam across an interface between two different dielectric media. Transmission and reflection coefficients are calculated for this type of beam. The analysis is vectorial and is fully consistent with Maxwell’s equations. Apodized beams of Gauss–Bessel and Circ–Bessel types are propagated by numerical simulation and are shown also to retain a narrow central lobe. Beams of these types have evident advantages in near-field applications, microscopy, and high-density data storage with subwavelength resolution.

Calibration of electric coaxial near-field probes and applications

A new calibration technique for application to near-field probes has been developed. For this, a simple electric coaxial near-field probe for application in the 0.05-20-GHz band has been developed, theoretically analyzed, and calibrated using a known field. By using the finite-difference time-domain (FDTD) method, this field probe is theoretically analyzed to determine its most sensitive probe segment. Taking the amplitude of the normal electric field at this segment as a known field, the probe is calibrated by defining a performance factor (PF), which is the ratio of the known field amplitude to the probe signal amplitude. Comparing the calculated results with measured results, the agreement is good. Additionally, the measurements are experimentally characterized with respect to the influence of the distance between the probe and the device-under-test (DUT), the influence of the input signal on the probe, and the spatial resolution. Two measurement examples are demonstrated, which investigate the mode and field distribution within passive coplanar microwave components.

The inverse scanning tunneling near-field microscope (ISTOM) or tunnel scanning near-field optical microscope (TSNOM) 3D simulations and application to nano-sources

We propose an application of the ISTOM to characterize nano-sources used in Scanning Near-Field Microscopies. The model takes into account the coupling between the nano-source and the hemispherical lens of the ISTOM set-up. By changing the angle of detection, experimental data are related to the Fourier spectrum of the source. We show “images” calculated with two different distances between tip end and lens surface.

Combined microwave heating and surface cooling of the cornea

We investigated a nonsurgical means of reshaping the cornea to correct hyperopia, keratoconus, or myopia. The object was to heat the central stroma of the cornea to the shrinkage temperature of collagen, 55-58 degrees C. The heating device was an open-ended, coaxial, near-field applicator driven at 2450 MHz; it incorporates cooling of the cornea surface by flow of saline. We investigated the system theoretically by computing the 2-D, axisymmetric temperature distribution with the finite element method. We investigated the system experimentally by heating excised steer corneas. Histology showed the system could shrink the stroma to a depth of 0.6 mm while sparing the epithelium in 75% of cases; the diameter of shrinkage was 1.3 mm. Theory predicted a significantly deeper and narrower region of shrinkage than was observed.