Tiantong Tang

Numerical calculation of a converging vector electromagnetic wave diffracted by an aperture using Borgnis potentials. II. Application to the study of focal shift

Focal shift of the converging spherical wavefront light diffracted by a circular aperture is numerically studied with the method of calculating the vector diffractive field by using Borgnis potentials given in Part I [J. Opt. Soc. Am. A23, 872 (2006)]. The quantitative dependence of the focal shift on the geometric parameters is discussed. The focal shift is mainly determined by the Fresnel number (N(f)) on the geometric focusing plane of the converging light, and an empirical formula between the fractional focal shift and the Fresnel number is deduced for N(f)<2. The focal shift of the same geometry is also studied on the basis of the scalar Rayleigh theory of diffraction, and its comparison with and difference from the result of our method are presented.

Vector near-field calculation of scanning near-field optical microscopy probes using Borgnis potentials as auxiliary functions

A new boundary integral equation method for solving the near field in three-dimensional vector form in scanning near-field optical microscopy (SNOM) using Borgnis potentials as auxiliary functions is presented. A boundary integral equation of the electromagnetic fields, expressed by Borgnis potentials, is derived based on Greens theorem. The harmonic expansion in rotationally symmetric SNOM probe--sample systems is studied, and the three-dimensional electromagnetic problem is partly simplified into a two-dimensional one. The boundary conditions of Borgnis potentials both on dielectric boundaries and on perfectly conducting boundaries are derived. Relevant algorithms were studied, and a computer program was written. As an example, a SNOM probe-sample system composed of a round metal-covered probe and a sample with a flat surface has been numerically studied, and the computational results are given. This new method can be used efficiently for other electromagnetic field problems with round subwavelength structures.

Numerical calculation of a converging vector electromagnetic wave diffracted by an aperture by using Borgnis potentials. I. General theory

A method is proposed, on the basis of the vector electromagnetic theory, for the numerical calculation of the diffraction of a converging electromagnetic wave by a circular aperture by using Borgnis potentials as auxiliary functions. The diffraction problem of vector electromagnetic fields is simplified greatly by solving the scalar Borgnis potentials. The diffractive field is calculated on the basis of the boundary integral equation, taking into consideration the contribution of the field variables on the diffraction screen surface, which is ignored in the Kirchhoff assumption. An example is given to show the effectiveness and suitability of this method and the distinctiveness of the diffractive fields caused by the vector characteristics of the electromagnetic fields.

Simulation of topographic images and artifacts in illumination-mode scanning-near-field optical microscopy

Scanning images in illumination-mode, scanning-near-field optical microscopy (SNOM) are numerically studied by the boundary element method based on rigorous vector electromagnetic theory. Computation results of constant-height and constant-distance images for samples with different topographic features are presented. Effects of the polarization of the input light and the optical parameters of samples on the resolution of SNOM are discussed. The artifacts in constant-distance images are also investigated. Numerical results indicate that the constant-height images for TM input light and constant-distance images for both TE and TM input light give only the local changes of the sample topography because of the loss of the low-frequency component of the topography.

Study on the polarization dependency of diffraction efficiency of perfectly conducting blazed gratings

The diffraction properties for blazing -3 order of the perfectly conducting blazed gratings in the Littrow mounting under the polarized incident beam is investigated by numerical simulation. The diffractive electromagnetic fields for S and P polarized incident lights are calculated numerically using TDFD method. The field distribution of the electromagnetic field in the near groove area is analyzed, and the polarization dependency of diffraction efficiency is studied. The spectral characteristics in 1530~1566 nm range is given. It is proved that the efficiency for S polarized incident light is higher than that of P polarized one and it is not sensitive to the blazed angle. The spectral characteristic is flat within the fiber communication spectrum range.

Integrated optical planar waveguide acousto-optic switches

A N X N integrated planar optical waveguide acousto-optical switch design is presented based on waveguide geodesic lenses and acousto-optical deflector. This kind of switches gets over the difficulties occurred in integrated switches based on strip waveguide. It can be integrated in a chip, and the channel numbers N can be up to 218. As an example, a 2 X 2 integrated planar optical waveguide acousto-optical switch has been fabricated and tested. Experiment result shows only one mode is transmitted in the Ti:LiNbO3 waveguide; the chip insertion loss is 0.8 dB; the central acoustic frequency is 150 MHz; the 3 dB bandwidth of IDT is 56 MHz; the chip acoustic frequency resolution is 6 MHz and the chip extinction ratio is larger than 20 dB.

Monte Carlo simulation for coupling between single-mode fiber and slab waveguide with spherical fiber microlenses

For the butt-joint coupling between a single mode fiber (SMF) and a single mode dielectric slab waveguide (SMDSW) with a spherical fiber microlens (SFML), because the SMF and the rectangular-section SMDSW have different symmetry, it is difficult to calculate the coupling efficiencies using the traditional analytic method. In this paper, a Monte Carlo model of the coupler is proposed. This model incorporates the geometrical surface of both the SFML and the SMDSW, the relative position of the SMF and the SMDSW, and both the aberration and diffraction effects into consideration. The propagation characteristics of the coupler are simulated. And the coupling efficiencies and misalignment loss are investigated.

Absolute measurement of photodetector quantum efficiency based on spontaneous parametric down-conversion biphoton field

A new method of absolute measurement of photodetector quantum efficiency based on spontaneous parametric down- conversion (SPDC) biphoton field is discussed. The process of SPDC is studied theoretically. The single photon detection probability and two-photon coincidence probability are derived and the measurement principle of photodetectors is explained. An experimental system has been set up. The quantum efficiency of a photon-counting photomultiplier tube was measured, and the results were compared with that obtained using conventional method.

Ray optics formation theory of geodesic lenses

The expressions of cardinal point site, principal plane and focal length of geodesic lenses are derived from ray optics. According to the obtained characteristics of focal points and principal planes, the geometry graphic imaging formation principles are presented to get the object-image-relation of geodesic lenses.

Performance of imaging photon-counting intensifiers

The configuration of imaging photon-counting intensifier (IPCI) is introduced, and the differences between traditional II generation intensifiers and the IPCI are given. Based on the single channel photon-counting technique, the measurement principle and method of the IPCIs key performances, such as dark count and photon gain are discussed theoretically. The experimental system has been developed. Measurement work of the two performances mentioned above are carried out under the condition of different MCPs operating voltage and temperature. The experimental result and its comparison with traditional II generation intensifiers are presented.