Measured Intensity Distribution Research Articles

Spectrally resolved confocal microscopy for laser mode imaging and beam characteristic investigations

In this letter, confocal optical microscopy is used to investigate the intensity patterns of an infrared vertical cavity surface emitting laser in three dimensions with high spectral resolution. The measurements were performed between the near field (Fresnel) regime and the far field (Fraunhofer) regime. The calculated intensity patterns were found to be in good agreement with the measured intensity distribution. The calculations together with the measurements can be used to determine the phase relation between different positions inside the gain medium.

Experimental and Numerical Determination of Thermal Radiative Properties of ZnO Particulate Media

The radiative characteristics of dependently scattering packed-beds of ZnO particles, applied in the design of high-temperature solar thermochemical reactors, were investigated experimentally. ZnO samples of varying thickness were exposed to a continuous beam of near monochromatic thermal radiation in the 0.5–1 μm wavelength range. The overall transmitted fraction measured as a function of sample thickness s obeys an exponential trend exp(−As), with the fit parameter A ranging from 4000±100 m−1 at 555 nm to 2100±100 m−1 at 1 μm. In the forward directions, the measured intensity distribution is approximately isotropic, whereas in the backward directions it is well approximated by a Henyey–Greenstein equation with asymmetry factors g≈−0.4 at 555 nm and g≈−0.1 at 1 μm. A Monte Carlo ray-tracing model of the experimental setup is employed to extract the extinction coefficient and the scattering albedo for the case of a nongray absorbing-scattering medium.

Dynamic polarization states and birefringence distributions measurements in spatial elliptical polariscope using Fourier analysis method

A new method to measure the light polarization state and the birefringent media parameters is proposed. We have used the setup described previously, consisting of two pairs of the linear Wollaston and circular compensators which form a set of two spatially modulated elliptical compensators. We have modified this setup introducing some carrier frequencies in all compensators and assuming that the second linear one would introduce the frequency which is a multiplicity of the basis frequency of the first linear compensator. Both of these modifications allow calculating all polarization parameters of polarized light or birefringent medium from only one measured intensity distribution of the light outcoming the described setup. They allow measuring not only the parameters of homogeneous beams/mediums but also x,y-distributions of all desired parameters, like azimuth and ellipticity angles of the light or first medium eigenvector and the phase difference introduced by this medium. The proposed calculation method comprises of Fourier analysis of obtained intensity distribution with some manipulation of coordinate system and filtration of obtained data. This method is claimed to be simple and fast enough to be treated as a real-time method.

Spinoptics: Spin-Based Plasmonics in Nanostructures

T interaction between light and metallic subwavelength structures results in various anomalous effects, such as extraordinary optical transmission1 and beaming.2 These effects have been elegantly explained by a mechanism that involves the coupling of light to collective surface-confined electronic oscillations known as surface plasmon-polaritons (SPPs). Extensive research has been carried out in the field of electromagnetic surface waves, which has rich technological potential. Apparently, the handedness of the light’s polarization (spin of photons) may provide an additional degree of freedom in nanoscale photonics. Here, we demonstrate the spin-dependent behavior of SPPs, which results from a geometric Berry phase that we found experimentally while exploring the interaction of light with metallic anisotropic and inhomogeneous nanoscale structures.3,4 The proposed anisotropic inhomogeneous plasmonic structure was produced on top of a thin metal film evaporated onto a glass plate. The element consisted of a spiral Bragg grating with a central defect surrounded by a coupling grating, as shown in part (a) of the figure. The structure was illuminated by circularly polarized light (R=right-handed, L=left handed). The intensity in the plasmonic cavity was measured by a near-field scanning optical microscope (NSOM). The measured intensity distribution exhibits a strong dependence on the incident spin. An annular ring structure with a dark spot in the center for R illumination and with a bright spot for L illumination indicates coupling to different spiral plasmonic modes (plasmonic vortices). The origin of the spin-dependent change in the near-field intensity distributions is in the geometric phase of the excited plasmonic mode. In the most general case, when a wave carrying an arbitrary intrinsic angular momentum (spin) changes its direction of propagation and polarization state, the geometric phase is given by a simple expression stemming from the Coriolis effect,3 which appears due to rotation of the reference frame represented by the local direction of the grating grooves. Accordingly, a spiral geometric phase with spin-dependent helicity arises in a circular grating.4 In the spiral structure, such as that shown in (a), an additional dynamic phase arises as a result of a space-variant path difference. The overall phase in the spiral cavity is the sum of the geometric and dynamic phases, which is manifested by different spiral modes that are obtained in the cavity for different polarizations. One of the possible technological implementations of the plasmonic geometric phase could be a spin-dependent plasmonic focusing lens. The proposed structure is presented in (b). We illumi(a) The scanning electron microscope (SEM) image of the spiral nanostructure and the scheme of the optical setup. Intensity distribution in the cavity measured by an NSOM for R and L illumination. The blue dashed line represents the spiral phasefront obtained in the cavity. (b) Spin-dependent plasmonic lens based on a plasmonic spin Hall effect. The intensity distributions measured by an NSOM for R and L illumination and the corresponding transverse cross-sections in the focal plane of the lens (measured: R=blue squares, L=red circles; calculated: R=solid blue line, L=dashed red line). The SEM picture of the element is depicted in the inset.

Machine studies for the development of storage cells at the ANKE facility of COSY

We present a measurement of the transverse intensity distributions of the COSY proton beam at the target interaction point at ANKE at the injection energy of 45 MeV, and after acceleration at 2.65 GeV. At 2.65 GeV, the machine acceptance was determined as well. From the intensity distributions the beam size is determined, and together with the measured machine acceptance, the dimensions of a storage cell for the double-polarized experiments with the polarized internal gas target at the ANKE spectrometer are specified. An optimum storage cell for the ANKE experiments should have dimensions of 15 mm vertical×20 mm horizontal×390 mm longitudinal, whereby a luminosity of about 2.5×10 29 cm −2 s −1 with beams of 10 10 particles stored in COSY could be reached.

Millimeter-wave radiation from a Teflon dielectric probe and its imaging application

The beam profile of a millimeter wave radiated from the tip of a Teflon dielectric probe was characterized experimentally by using a three-dimensional scanning dielectric probe and numerically by using the finite difference time domain (FDTD) method. The measured intensity distribution and polarization of the millimeter wave radiated from the tip of the probe was in good agreement with those of the FDTD simulation. A reflection type of a millimeter- wave imaging system using this dielectric probe was constructed. The resolution of the imaging system was as small as 1 mm, which was slightly smaller than a half wavelength, 1.6 mm, of the radiation wave. Translucent measurement of a commercially manufactured IC card which consists of an IC chip and a leaf-shaped antenna coil was demonstrated. Not only the internal two-dimensional structures but also the vertical information of the card could be provided.

Array-based goniospectroradiometer for measurement of spectral radiant intensity and spectral total flux of light sources

We present a description of a new goniospectroradiometric measurement system developed at the National Physical Laboratory (NPL). The instrument incorporates a modified array spectrometer and a series of rotary stages to allow measurement of the spectral radiant intensity distribution of a variety of different types of light source from 350 to 830 nm. Associated source properties such as chromaticity and correlated color temperature distributions and total spectral flux are then calculated from the radiant intensity data. A preliminary comparison with NPL's integrating sphere-based luminous flux scale shows agreement to within 0.4%, well within the combined measurement uncertainty. Measurements of the luminous intensity and color temperature distributions and the spectral total flux of a tungsten filament flux standard, a white LED cluster and a compact fluorescent source made using the goniospectroradiometer, are also presented.

Algorithm for the field-phase distribution reconstruction in Hartmann sensor measurements

A new method for the wavefront reconstruction is described which is based on measurements of the beam intensity distributions in two cross sections and of wavefront tilts by a Hartmann sensor in one of these cross sections and makes it possible to find the field phase distribution taking into account possible phase jumps. The iteration algorithm for finding this distribution is developed and its mathematical description based on the calculation of the field in the Fresnel approximation is presented. The algorithm is studied numerically. Relations are obtained to estimate the optimal parameters of a measurement system, in particular, the distance between the cross sections and the number of subapertures of the Hartmann sensor depending on the mode composition of laser radiation.

A new method to demonstrate frustrated total internal reflection in the visible band

We describe a new method to demonstrate frustrated total internal reflection in the visible band using the 100nm thick air film near the center of Newton’s rings. Experimental measurements of the light intensity distribution validate the theoretical predictions.

Stochastic Scattering Polarimetry

We introduce the general concept of stochastic scattering polarimetry and demonstrate that the anisotropic polarizability of a scattering object can be obtained by analyzing the statistical moments of polarimetrically measured intensity distributions. This general procedure is valid even in situations where the state of polarization of the incident field is not known. The efficiency of recovering different scattering polarizabilities is demonstrated numerically for several particular cases pertaining to both far- and near-field optics.

Localized Modes in a Finite-Size Open Disordered Microwave Cavity

We present measurements of the spatial intensity distribution of localized modes in a two-dimensional open microwave cavity randomly filled with cylindrical dielectric scatterers. We show that each of these modes displays a range of localization lengths, and we successfully relate the largest value to the measured leakage rate at the boundary. These results constitute unambiguous signatures of the existence of strongly localized electromagnetic modes in two-dimensional open random media.

Fiber Optic Sensors For Detection of Toxic and Biological Threats.

Protection of public and military personnel from chemical and biological warfareagents is an urgent and growing national security need. Along with this idea, we havedeveloped a novel class of fiber optic chemical sensors, for detection of toxic and biologicalmaterials. The design of these fiber optic sensors is based on a cladding modificationapproach. The original passive cladding of the fiber, in a small section, was removed and thefiber core was coated with a chemical sensitive material. Any change in the opticalproperties of the modified cladding material, due to the presence of a specific chemicalvapor, changes the transmission properties of the fiber and result in modal powerredistribution in multimode fibers. Both total intensity and modal power distribution (MPD)measurements were used to detect the output power change through the sensing fibers. TheMPD technique measures the power changes in the far field pattern, i.e. spatial intensitymodulation in two dimensions. Conducting polymers, such as polyaniline and polypyrrole,have been reported to undergo a reversible change in conductivity upon exposure tochemical vapors. It is found that the conductivity change is accompanied by optical propertychange in the material. Therefore, polyaniline and polypyrrole were selected as the modifiedcladding material for the detection of hydrochloride (HCl), ammonia (NH₃), hydrazine(H₄N₂), and dimethyl-methl-phosphonate (DMMP) {a nerve agent, sarin stimulant},respectively. Several sensors were prepared and successfully tested. The results showeddramatic improvement in the sensor sensitivity, when the MPD method was applied. In thispaper, an overview on the developed class of fiber optic sensors is presented and supportedwith successful achieved results.

Light scattering from ultracold atoms in optical lattices as an optical probe of quantum statistics

We study off-resonant collective light scattering from ultracold atoms trapped in an optical lattice. Scattering from different atomic quantum states creates different quantum states of the scattered light, which can be distinguished by measurements of the spatial intensity distribution, quadrature variances, photon statistics, or spectral measurements. In particular, angle-resolved intensity measurements reflect global statistics of atoms (total number of radiating atoms) as well as local statistical quantities (single-site statistics even without optical access to a single site) and pair correlations between different sites. As a striking example we consider scattering from transversally illuminated atoms into an optical cavity mode. For the Mott-insulator state, similar to classical diffraction, the number of photons scattered into a cavity is zero due to destructive interference, while for the superfluid state it is nonzero and proportional to the number of atoms. Moreover, we demonstrate that light scattering into a standing-wave cavity has a nontrivial angle dependence, including the appearance of narrow features at angles, where classical diffraction predicts zero. The measurement procedure corresponds to the quantum nondemolition measurement of various atomic variables by observing light.

Direct Observation of Localized Modes in an Open Disordered Microwave Cavity

We present measurements of the spatial intensity distribution of localized modes in a two-dimensional open microwave cavity randomly filled with cylindrical dielectric scatterers. The robustness of these modes with respect to a modification of the boundaries of the scattering region is successfully established. From a comparison with numerically calculated modes, we can extract precise values of the localization length which are in agreement with the measured spectral widths. These results constitute unambiguous signatures of the existence of strongly localized electromagnetic modes in two-dimensional open random media.

Methods of computational physics in the problem of mathematical interpretation of laser investigations

It is shown that in laser experiments performed by using an 'imperfect' setup when instrumental distortions are considerable, sufficiently accurate results can be obtained by the modern methods of computational physics. It is found for the first time that a new instrumental function — the 'cap' function — a 'sister' of a Gaussian curve proved to be demanded namely in laser experiments. A new mathematical model of a measurement path and carefully performed computational experiment show that a light beam transmitted through a mesoporous film has actually a narrower intensity distribution than the detected beam, and the amplitude of the real intensity distribution is twice as large as that for measured intensity distributions.

Modelling of diffraction from fibre texture gradients in thin polycrystalline films

Crystallographic textures in thin polycrystalline films typically exhibit a rotational symmetry,i.e.they occur as a fibre texture with the texture pole being orientated in the direction of the substrate normal. As a further characteristic of thin-film textures, it was often observed that the degree of preferred orientation increases with increasing thickness. It is shown in this work how a fibre texture gradient may be modelled in kinematical X-ray diffraction and which effects it has on the intensity mapping of theIHKLreflection, when theHKLpole is the fibre axis. A general expression forIHKLis derived for a depth-dependent fibre texture that is based on the finite Laplace transform of the texture distribution. The concept is outlined for the cosnψ function to model the tilt-angle dependence of intensity, with the parameterndenoting the degree of texture. It is found that the measured intensity distribution sensitively depends on the ratio of texture gradient over X-ray attenuation coefficient. For particular cases, it is found that the maximum intensity may occur for non-zero tilt angles and thus arise at a different tilt angle from the pole of the fibre texture.

Measurement of the laser beam intensity distribution using optical tomography

Optical tomography is proposed as a method for studying the intensity distribution in the laser beam cross section. The tomographic projection of the beam cross section is obtained with the help of an optical fiber possessing a high radiation resistance, which makes it possible to employ this technique for studying high-power laser radiation. The results of tomographic reconstruction of the bean cross section from experimental projections are compared with the results of reconstruction from model projections.

Scaling behaviour of pair correlation functions for randomly faulted hexagonal close-packed structures

Monte Carlo simulations and analytical results are used to demonstrate that the hexagonal close-packed (h.c.p.) pair correlation functions for different values of randomly distributed growth and deformation faults can be collapsed into master curves when plotted against a spatial variable, scaled with respect to a characteristic length (L). The functional dependences of L on different types of faults are found to be non-universal. A simple method for the determination of L from the measured intensity distributions is also outlined.

Mapping inelastic intensities in diffraction patterns of magnetic samples using the energy spectrum imaging technique

We present the quantitative measurement of inelastic intensity distributions in diffraction patterns with the aim of studying magnetic materials. The relevant theory based on the mixed dynamic form factor (MDFF) is outlined. Experimentally, the challenge is to obtain sufficient signal for core losses of 3d magnetic materials (in the 700–900 eV energy-loss range). We compare two experimental settings in diffraction mode, i.e. the parallel diffraction and the large-angle convergent-beam electron diffraction configurations, and demonstrate the interest of using a spherical aberration corrector. We show how the energy spectrum imaging (ESI) technique can be used to map the inelastic signal in a data cube of scattering angle and energy loss. The magnetic chiral dichroic signal is measured for a magnetite sample and compared with theory.

Vacancy rearrangement processes in multiply ionized atoms

We demonstrate that in order to interpret the x-ray satellite structure of Pd Lα1,2(L3M4,5) transitions excited by fast O ions, which was measured using a high-resolution von Hamos crystal spectrometer, the vacancy rearrangement processes, taking place prior to the x-ray emission, have to be taken into account. The measured spectra were compared with the predictions of the multi-con.guration Dirac-Fock (MCDF) calculations using the fluorescence and Coster-Kronig yields which were modiffed due to a reduced number of electrons available for relaxation processes and the effect of closing the Coster-Kronig transitions. We demonstrate that the vacancy rearrangement processes can be described in terms of the rearrangement factor, which can be calculated by solving the system of rate equations modelling the flow of vacancies in the multiply ionized atom. By using this factor, the ionization probability at the moment of collision can be extracted from the measured intensity distribution of x-ray satellites. The present results support the independent electron picture of multiple ionization and indicate the importance of use of Dirac-Hartree-Fock wave functions to calculate the ionization probabilities.