Aperture Diffraction Research Articles

Space-based hyperspectral technologies for the thermal infrared

Various approaches now exist for obtaining spectral imagery over a broad range of infrared wavelengths. One approach involves the use of a single grating element in two grating orders with dualband focal plane array (FPA) technology. This approach offers greater efficiency over the mid-wave infrared and long-wave infrared and eliminates the need for separate FPAs, dispersing elements, and optical beamsplitters. Another approach achieves similar results by exploiting an FPA which has a broad wavelength response with an innovative grating which has useable efficiency that extends beyond the single octave limits of traditional gratings. Significant advantages result, in either case, for space-based hyperspectral imagers, for which a reduction in cryo-cooled mass translates into prodigious savings in overall payload mass, cryo-cooling requirements, and waste heat removal. By contrast, longer term approaches might realize infrared “hyperspectral pixels” in two-dimensional imaging FPAs. In this case, each pixel would detect different wavelengths of radiation, at different depths, and the resulting “spectral photocurrents” would be transported to read-out circuitry through a vertical grid of electrical contacts. Although not yet realized in practice, the conceptual basis for accomplishing this, with the widely available HgCdTe detector material, has been described. With regard to employment, space-based thermal hyperspectral imaging is characterized by coarser ground resolution as a result of aperture diameter limitations and diffraction considerations at the longer infrared wavelengths. The resulting subpixel detections, based on spectral signature, are often complementary with higher resolution, shorter wavelength, panchromatic imagery.

Phase-shift anomaly caused by subwavelength-scale metal slit or aperture diffraction

Terahertz time-domain spectroscopy probes anomalous phase-shift caused by wave diffraction from a subwavelength-scale metal slit or aperture. Carrier frequency phase measurements in the far-field region reveals that nearly 30° phase advance is induced from a subwavelength slit diffraction and that 180° phase-advance from a subwavelength aperture. These results indicate that the conventional 90° phase advance of diffracted waves in the far-field region, known as the Gouy phase shift, is not valid for subwavelength diffraction phenomena. The physical origin of these phase-shift anomalies is attributed to induced electric currents or magnetic dipole radiation, and theoretical analyses based on these factors are in good agreement with the experimental results.

Microfocusing of the FERMI@Elettra FEL beam with a K–B active optics system: Spot size predictions by application of the WISE code

FERMI@Elettra, the first seeded EUV-SXR free electron laser (FEL) facility located at Elettra Sincrotrone Trieste has been conceived to provide very short (10–100fs) pulses with ultrahigh peak brightness and wavelengths from 100nm to 4nm. A section fully dedicated to the photon transport and analysis diagnostics, named PADReS, has already been installed and commissioned. Three of the beamlines, EIS-TIMEX, DiProI and LDM, installed after the PADReS section, are in advanced commissioning state and will accept the first users in December 2012. These beam lines employ active X-ray optics in order to focus the FEL beam as well as to perform a controlled beam-shaping at focus.Starting from mirror surface metrology characterization, it is difficult to predict the focal spot shape applying only methods based on geometrical optics such as the ray tracing. Within the geometrical optics approach one cannot take into account the diffraction effect from the optics edges, i.e. the aperture diffraction, and the impact of different surface spatial wavelengths to the spot size degradation. Both these effects are strongly dependent on the photon beam energy and mirror incident angles.We employed a method based on physical optics, which applies the Huygens–Fresnel principle to reflection (on which the WISE code is based). In this work we report the results of the first measurements of the focal spot in the DiProI beamline end-station and compare them to the predictions computed with Shadow code and WISE code, starting from the mirror surface profile characterization.

Solução do problema da difração na abertura circular

O propósito do presente artigo consiste em apresentar uma formulação alternativa para o problema de encontrar a intensidade das franjas de difração numa fenda simples e estendê-la para a difração na abertura circular. Em seguida, estudamos algumas propriedades das funções de Bessel com o objetivo de compreender a solução da equação para o caso circular.

광경로차가 있는 광학기기의 변조전달함수에 관한 연구

광학기기가 대상물체를 영상으로 완벽하게 재현할 수는 없으므로 이러한 광학기기의 특성을 평가하는 방법으로서 변조전달함수(MTF)가 보편적으로 사용된다. 그런데 한편으로 렌즈의 수차가 적어지면 광학시스템 구경의 회절과 수차 사이의 반응이 매우 복잡해지므로 기존의 MTF의 방법으로는 광학기기를 평가하기가 어렵다. 이 경우에는 렌즈의 수치구경, 탈초점, 광경로차 및 위상차를 고려한 MTF를 구해야 한다. 본 논문에서는 이러한 방법으로서 가장 보편적으로 알려진 베셀함수를 이용하여 이의 타당성을 검증하고 실례를 통하여 이를 보여주고자 하였다. The modulation transfer function is generally used as a method to estimate the characteristics of this kind of optical system because it cannot reappear or implement image of the object perfect. By the way, it is difficult to assess the optical system with existing MTF method since the reaction between diffraction and aberration of aperture is very complicate if the aberration is very small. The MTF has to be calculated considering the numerical aperture, defocusing, OPD and phase difference. We validate its feasibility using bessel function as an estimating method, and then realize it through illustration in this paper.

四孔径衍射屏在近场形成的光强分布及拓扑荷

四孔径衍射屏在近场形成的光强分布及拓扑荷

Diffraction of ultrashort Gaussian pulses within the framework of boundary diffraction wave theory

We study the diffraction of Gaussian pulses and beams within the framework of boundary diffraction wave theory. For the first time the boundary diffraction wave theory is applied to pulsed Gaussian beams, and it is shown that the diffracted field of a pulsed Gaussian beam on a circularly symmetric aperture can be evaluated by a single 1D integration along the diffracting aperture at every point of interest. We compare theoretical simulations to experimental measurements of ultrashort pulses diffracted off a circular aperture, an opaque disc, an annular aperture, and a system of four concentric annular apertures. Using the recently developed SEA TADPOLE measurement technique, we obtain micron spatial and femtosecond temporal resolutions in the spatio-temporal measurements of the diffracted fields.

Systematic row and zone axis STEM defect image simulations

Transmission electron microscopy (TEM) has been instrumental in advancing the field of crystalline defect analysis. Conventional TEM imaging techniques, such as bright field (BF), dark field (DF), and weak beam dark field (WBDF or g–3g) imaging, have been well-documented in the scientific literature, with simulation methods readily available for each. The present contribution highlights the use of a field-emission TEM, operated in scanning transmission electron microscopy mode, as a viable tool for defect analysis. Common techniques such as two-beam diffraction contrast and zone axis imaging are applied to defect analysis; both experimental and computational results are presented. Effects of experimental parameters such as camera length, beam divergence angle, and diffraction aperture placement are also discussed and illustrated by both experimental and computed micrographs of stacking faults.

Fresnel aperture diffraction: A phase-sensitive probe of the pairing symmetry of a superconductor

Fresnel single aperture diffraction (FSAD) is proposed as a phase-sensitive probe for pairing symmetry and Fermi surface of a superconductor. We consider electrons injected, through a small aperture, into a thin superconducting (SC) layer. It is shown that in case of SC gap symmetry $\ensuremath{\Delta}(\ensuremath{-}{k}_{x},{\mathbf{k}}_{\ensuremath{\parallel}})=\ensuremath{\Delta}({k}_{x},{\mathbf{k}}_{\ensuremath{\parallel}})$ with ${k}_{x}$ and ${\mathbf{k}}_{\ensuremath{\parallel}}$ respectively the normal and parallel components of the electron Fermi wave vector, the quasiparticle FSAD pattern developed at the image plane is zeroth-order minimum if ${k}_{x}x=n\ensuremath{\pi}$ ($n$ is an integer and $x$ is SC layer thickness). In contrast, if $\ensuremath{\Delta}(\ensuremath{-}{k}_{x},{\mathbf{k}}_{\ensuremath{\parallel}})=\ensuremath{-}\ensuremath{\Delta}({k}_{x},{\mathbf{k}}_{\ensuremath{\parallel}})$, the corresponding FSAD pattern is zeroth-order maximum. Observable consequences are discussed for iron-based superconductors of complex multiband pairings.

A non-dimensional approach to diffraction phenomena in the Fresnel approximation

The paper presents a theoretical and numerical study of the diffraction problem approached in the Fresnel approximation. The problem was formulated by a non-dimensional approach, which implies the definition of two non-dimensional quantities: F and ζ. In particular the parameter F is the well-known Fresnel number, whose value was usually used to classify diffraction regime. In analyzing the numerical approach some limitations imposed by the discretization arise; in particular these limitations constrain the level of sampling to be used for the diffracting field by requiring more and more elements when the diffraction conditions become more severe — i.e. increasing the dimension of the diffracting aperture and/or decreasing the distance of the plane where the diffracted field is observed.

General Spectral Camera Lens Simulation

AbstractWe present a camera lens simulation model capable of producing advanced photographic phenomena in a general spectral Monte Carlo image rendering system. Our approach incorporates insights from geometrical diffraction theory, from optical engineering and from glass science. We show how to efficiently simulate all five monochromatic aberrations, spherical and coma aberration, astigmatism, field curvature and distortion. We also consider chromatic aberration, lateral colour and aperture diffraction. The inclusion of Fresnel reflection generates correct lens flares and we present an optimized sampling method for path generation.

Projection of diffraction patterns for use in cold-neutral-atom trapping

Scalar diffraction theory is combined with beam-propagation techniques to investigate the projection of near-field diffraction patterns to spatial locations away from the aperture for use in optically trapping cold neutral alkali-metal atoms. Calculations show that intensity distributions with localized bright and dark spots usually found within a millimeter of the diffracting aperture can be projected to a region free from optical components such as a cloud of cold atoms within a vacuum chamber. Calculations also predict that the critical properties of the optical dipole atom traps are not only maintained for the projected intensity patterns but also can be manipulated and improved by adjustment of the optical components outside the vacuum chamber.

Simultaneous characterization of the shape and refractive index of transparent living cells by an optical aperture

We report a method of using optical aperture diffraction to simultaneously detect the image and the refractive index of a living cell. By simulation, it is found that, when a suitable gap is introduced between the cell and the aperture, the image of the cell on the detection plane can be amplified hundreds to thousands of times, and a limit of detection of 3e-4 to 9e-5 can be reached for the refractive index of the cell. Experiments show that this method is feasible to realize, but the achievement of such a detection system is yet to be proved.

Image capture via a wedge light-guide with no margins

We report the capture of images via a wedge light-guide without the margin for fan-in needed heretofore. While this lets one look out of a slim panel as if it were a periscope, half the power is lost and resolution is degraded by aperture diffraction. Volume gratings may resolve these drawbacks at certain wavelengths and we consider how these might be extruded.

微小孔非傍轴衍射光束传输特性的研究

Based on the accurate definition of light intensity of nonparaxial scalar beams at the transverse plane,a comparative study of the beam widths,far-field divergence angles and M2 factors defined by the power in the bucket and second-order moment is made.Taking the nonparaxial diffraction of a small circular aperture as an example,the detailed numerical calculation is performed.The results show that the beam width,far-field divergence angle and M2 factor defined by the power in the bucket are less than that defined by the second-order moment.Both M2 factors tend to zero as beam waist vanishes,the propagation of beam width defined by the power in the bucket is different from the hyperbolic law.

Improved far-field optical trap for cold atoms (molecules) with phase-modulated circular aperture diffraction

We propose a scheme to trap cold atoms (or molecules) by using an improved red- or blue-detuned far-field optical trap, which is formed by an optical system composed of a binary phase plate and a circular aperture illuminated by a plane light wave. We calculate the relative intensity distribution of the far-field trap and study its dependence on the phase $\ensuremath{\varphi}$ of the binary phase plate. Our study shows that the binary phase plate can be used to improve some geometric and optical parameters of the far-field optical trap, and realize the evolution of the far-field optical trap from a red-detuned trap to a blue-detuned one. In particular, when the phase $\ensuremath{\varphi}$ of the binary phase plate is changed from 0 to $\ensuremath{-}\ensuremath{\pi}$, the maximum intensity of the far-field trap and its well depth will be enhanced by $\ensuremath{\sim}4$ times, and the corresponding trapping volume will be increased by $\ensuremath{\sim}8$ times, with respect to a trapping beam obtained from the diffraction by a circular aperture.

Differential Near-Field Scanning Optical Microscopy with THz quantum cascade laser sources

We have realized a differential Near-field Scanning Optical Microscope (NSOM) working with subwavelength resolution in the THz spectral region. The system employs a quantum cascade laser emitting at lambda approximately 105 microm as source, and the method, differently from conventional NSOM, involves diffracting apertures with size comparable to the wavelength. This concept ensures a higher signal-to-noise level at the expense of an additional computational step. In the implementation here reported lambda/10 resolution has been achieved; present limiting factors are investigated through finite difference time domain simulations.

Nonparaxial diffraction analysis of Airy and SAiry beams

We theoretically analyze Airy beams by solving the exact vectorial Helmholtz equation using boundary conditions at a diffraction aperture. As result, the diffracted beams are obtained in the whole space; thus, we demonstrate that the parabolic trajectories are larger than those previously reported, showing that the Airy beams start to form before the Fourier plane. We also demonstrate the possibility of using a new type of Airy beams (SAiry beams) with finite energy that can be generated at the focal plane of the lens due to diffraction by a circular aperture of a spherical wave modified by a cubic phase. The finite energy ensured by the principle of conservation of energy of a diffracted beam.

Fourier optics of image formation in LEEM

A Fourier optics calculation of image formation in low energy electron microscopy(LEEM) is presented. The adaptation of the existing theory for transmission electronmicroscopy to the treatment of LEEM and other forms of cathode lens electronmicroscopy is explained. The calculation incorporates imaging errors that are caused bythe objective lens (aberrations), contrast aperture (diffraction), imperfect sourcecharacteristics, and voltage and current instabilities. It is used to evaluate theappearance of image features that arise from phase objects such as surface steps andamplitude objects that produce what is alternatively called amplitude, reflectivityor diffraction contrast in LEEM. This formalism can be used after appropriatemodification to treat image formation in other emission microscopies. Implications forimage formation in the latest aberration-corrected instruments are also discussed.

Synthetic aperture diffraction tomography for three-dimensional imaging

Tomography of complex three-dimensional objects with ultrasound or microwave has been a long-standing goal since the introduction of these technologies after World War II. While current state-of-the-art systems can provide high-resolution images of cylindrical objects characterized by a two-dimensional structure, the three-dimensional case remains an open challenge owing to current limitations of sensor technology and computer power. Here, this problem is addressed by means of a synthetic aperture technique that, while using hardware technology developed for two-dimensional problems, accounts for the complexity of three-dimensional scattering and leads to high-resolution three-dimensional reconstructions. In this paper, we present the theoretical formulation of this new approach and illustrate it by means of a numerical example.