120 MeV swift Au9+ion induced phase transition in ZrO2: monoclinic to tetragonal and cubic to tetragonal structure.

This study reports the effect of 120 MeV swift Au9+ion irradiation on the structures of monoclinic, tetragonal and cubic ZrO2, probed through x-ray diffraction (XRD) and Raman spectroscopy. Three phases of ZrO2were prepared using the solution combustion method. The tetragonal and cubic phases of ZrO2were stabilized at room temperature by adding 6% and 10% of yttrium ions, respectively. Both the XRD and Raman results confirm the partial phase transition from monoclinic to tetragonal, which was approximately 74%. Tetragonal ZrO2is stable under 120 MeV Au9+ion irradiation. Interestingly, a phase transition from cubic to tetragonal ZrO2was observed under 120 MeV Au9+ion irradiation. The roles of transient temperature, defects and strain in the lattice induced by swift heavy ions are discussed. This study reveals the structural stability of different phases of ZrO2under swift heavy ion irradiation and should be helpful in choosing potential hosts for various applications such as inert fuel matrix inside the core of nuclear reactors, oxygen sensors and accelerators, and radiation shielding.

Role of Ni substitution on structural, magnetic and electronic properties of epitaxial CoCr2O4 spinel thin films.

Cubic spinel CoCr2O4 has recently attained attention due to its multiferroic properties. However, the Co site substitution effect on the structural and magnetic properties has rarely been studied in thin film form. In this work, the structural and magnetic properties of Co1-x Ni x Cr2O4 (x= 0, 0.5) epitaxial thin films deposited on MgAl2O4 (100) and MgO (100) substrates to manipulate the nature of strain in the films using pulsed laser deposition (PLD) technique are presented. The epitaxial nature of the films was manifested through x-ray diffraction (XRD), reciprocal space mapping (RSM) and Rutherford backscattering spectrometry (RBS) measurements. Raman measurements revealed a disappearance of characteristic A 1 g and F 2 g modes of the CoCr2O4 with increase in the Ni content. Atomic force microscopy (AFM) and field emission scanning electron microscopy (FE-SEM) studies show a modification of the surface morphology upon Ni substitution. Magnetic measurements disclose that the ferrimagnetic Curie temperature (T C) of the CoCr2O4 in thin film grown on MgAl2O4 (100) and MgO (100) substrates were found to be 100.6 ± 0.5 K and 93.8 ± 0.2 K, respectively. With Ni substitution the T C values were found to be enhanced to 104.5 ± 0.4 K for MgAl2O4 (100) and 108.5 ± 0.6 K for MgO (100) substrates. X-ray photoelectron spectroscopy (XPS) suggests Cr3+ oxidation states in the films, while Co ions are present in a mixed Co2+/Co3+ oxidation state. The substitution of Ni at Co site significantly modifies the line shape of the core level as well as the valence band. Ni ions are also found to be in a mixed 2+/3+ oxidation state. O 1s core level display asymmetry related to possible defects like oxygen vacancies in the films.

Zernike expansion of pupil filters: optimization of the signal concentration factor.

Amplitude pupil filters for optimizing the signal concentration factor for a point spread function of given transverse and/or axial widths are derived. The pupil is expanded in a basis of Zernike polynomials. It is shown that the pupil that maximizes the signal concentration factor for a given transverse gain has a quadratically varying amplitude profile, as was shown in a previous paper, while the pupil that maximizes the signal concentration factor for a given axial gain has a quartic amplitude profile.

Optimization of pupil filters for maximal signal concentration factor.

A new optimization for a continuously varying amplitude pupil filter that maximizes the signal concentration factor for a given transverse gain is derived. The filter has a simple parabolic amplitude transmittance, and is an example of a Sonine filter. The connection between different definitions of gain factor is discussed.

Focusing of vortex beams: Lommel treatment.

Focusing of vortex beams by a lens with circular aperture in the paraxial scalar Debye regime is analyzed. The amplitude in the focal region can be expressed naturally in terms of higher order Lommel functions of two variables. Using recurrence relationships, these can then be expressed in terms of low-order Lommel functions. The phase variation in the focal region is investigated, showing some interesting behavior of the Gouy phase anomaly.

Rayleigh-Sommerfeld diffraction formula in k space.

An angular spectrum representation in three dimensions is used to develop three-dimensional Fourier forms of the first and second Rayleigh-Sommerfeld diffraction formulae and the Kirchhoff diffraction formula. For forward-propagating waves, these reduce to three-dimensional Fourier representations for diffraction in the forward half-space.

Direct calculation of a three-dimensional diffracted field.

We present an approach to calculating the complex amplitude of a three-dimensional (3D) diffracted light field in the paraxial approximation based on a 3D Fourier transform. Starting from the Huygens-Fresnel principle, the method is first developed for the computation of the light distribution around the focus of an apertured spherical wave. The method, with modification, is then extended to treat the 3D diffraction of an aperture with an arbitrary transmittance function.

Design considerations for refractive solid immersion lens: application to subsurface integrated circuit fault localization using laser induced techniques.

With fast scaling and advancement of integrated circuit (IC) technology, circuitries have become smaller and denser. New materials and more sophisticated designs have evolved. These changes reduced the effectiveness of conventional laser induced fault localization techniques. Since IC fault localization is the most critical step in failure analysis, there are strong motivations to improve both spatial resolution and sensitivity of such systems to meet the new challenges from advanced technology. Refractive solid immersion lens (RSIL) is well known to enhance the laser spot size which directly affects resolution and sensitivity in back side fault localizations. In practice, it is difficult to operate RSIL at the ideal configurations to obtain the smallest spot resolution. It is necessary to understand the resolution performance at the other design focal planes. Besides resolution, there are also other factors that affect sensitivity in a RSIL enhanced system. This paper identifies and characterizes key RSIL design parameters to optimize RSIL performance on laser induced techniques. We report that the most efficient conditions are achieved close to aplanatic RSIL design to within 20-25 microm (for a 1 mm diameter lens), and the backing objective should be the minimum numerical aperture required for optimum resolution performance. The size of the mechanical clear aperture opening should be large enough (>80%) to exploit the advantage of aplanatic RSIL. RSIL is developed on a laser scanning optical microscope in this work, and a resolution of 0.3 microm (for a wavelength of 1340 nm) was achieved over a range of operating conditions. A quantitative resolution of 0.25 microm is achieved and a pitch structure of 0.4 microm is easily resolvable. Close to 15 times enhancement in laser induced signal is obtained.

Superresolution along extended depth of focus with binary-phase filters for the Gaussian beam.

In the paraxial Debye regime, simple and power-efficient pupil filters are designed to break the diffraction limit along a large depth of focus (DOF) for the Gaussian beam. Dependences of the superresolution factor, DOF gain, Strehl ratio, sidelobe strength, and axial intensity nonuniformity on the Gaussian profile in the pupil plane are characterized using the numerical method. Optimal filter designs are proposed for either high-resolution or ultra-large-DOF applications followed by experimental verifications.

Molecular contrast of EGFR expression using gold nanoparticles as a reflectance-based imaging probe.

Advanced reflectance-based optical techniques for in vivo imaging often suffer from low contrast between neoplastic and normal tissue and are unable to image early biomolecular changes associated with carcinogenesis, thus limiting their clinical value. In this study, we exploit the resonance light scattering property of gold nanoparticles at their surface plasmon resonance to develop them as potential molecular contrast probes for imaging biomolecular changes during carcinogenesis under reflectance-mode imaging techniques. Gold nanoparticles were synthesized and conjugated to anti-epidermal growth factor receptor (EGFR). Their localization on the EGFR of nasopharyngeal carcinoma CNE2 cells and normal human lung fibroblast (NHLF) cells were imaged and compared under confocal microscopy in vitro. We have shown that the localization of gold bioconjugates on EGFR increases the reflectance properties of CNE2 cells and the regions of increased reflectance correspond to regions of high EGFR expression in the cells. The optical properties of normal fibroblast cells are not greatly affected. These gold bioconjugates are thus able to map the expression of relevant biomarkers and elicit an optical contrast for cancer cells over normal cells under confocal reflectance microscopy. Our study demonstrates the potential of gold nanoparticles to target and probe cancer cells and illuminates them for cancer detection under reflectance-based imaging systems based on biomolecular changes.

Double-reflection polygon mirror for high-speed optical coherence microscopy.

We report on a high-speed, high-efficiency, high-duty-cycle, path-length-maintaining and linear beam scanner suitable for en face scanning optical coherence microscopy. Fast transverse beam scanning is achieved by use of a double-reflection polygon mirror (DRPM) rotating at a constant speed. With a motor speed of 18,000 rpm and a scanner diameter of 50 mm, the DRPM provides a line rate up to 3 kHz, +/-1.8 degrees scanning range, and 90% duty cycle. A much higher scanning speed and much larger scanning range can be readily achieved by increasing the scanner diameter.

Binary-phase spatial filter for real-time swept-source optical coherence microscopy.

We report a novel scheme to optimize the focusing condition for real-time, swept-source optical coherence microscopy. The axial and lateral behaviors of four-zone binary-phase spatial filters are presented numerically. A nearly constant axial intensity distribution along an extended depth of focus of 1.5 mm and a lateral resolution of 5 microm are experimentally verified. The A-line scan rate is up to 16 kHz, yielding a frame rate of 25 Hz and 640 lines per image.

Focusing of pseudoradial polarized beams.

The focusing of a light beam with radial polarization has substantial advantages as the irradiance distribution in the focal plane is symmetric and there is maximum absorption at the focus. Using half wave plates cut into four quadrants with each quadrant having a linear polarization directed outwards gives a total field that approximates radial polarization, called pseudoradial polarization. The irradiance distributions in the focal region for different polarizations and beam profiles are compared. The irradiance is calculated by the numerical integration of the two-dimensional Rayleigh-Sommerfeld diffraction integral of the first kind using the 2DSC method for both circular and annular apertures.

Linear phase imaging using differential interference contrast microscopy.

We propose an extension to Nomarski differential interference contrast microscopy that enables isotropic linear phase imaging. The method combines phase shifting, two directions of shear and Fourier-space integration using a modified spiral phase transform. We simulated the method using a phantom object with spatially varying amplitude and phase. Simulated results show good agreement between the final phase image and the object phase, and demonstrate resistance to imaging noise.

Hyperresolving phase-only filters with an optically addressable liquid crystal spatial light modulator.

Hyperresolving (sometimes called 'superresolving' or 'ultraresolving') phase-only filters can be generated using an optically addressable liquid crystal spatial light modulator. This approach avoids the problems of low efficiency, and coupling between amplitude and phase modulation, that arise when using conventional liquid crystal modulators. When addressed by a programmed light intensity distribution, it allows filters to be changed rapidly to modify the response of a system or permit the investigation of different filter designs. In this paper we present experimental hyperresolved images obtained using an optically addressable parallel-aligned nematic LCD with two zone Toraldo type phase-only filters. The images are compared with theoretical predictions.

Wigner function for nonparaxial wave fields.

The generalized optical transfer function and the spectral correlation function are investigated for nonparaxial two-dimensional wave fields. The angle-impact marginal of the four-dimensional Wigner function is derived directly. For focused wave fields of semiangle greater than 90 degrees, the spectral correlation function exhibits overlapping and interference. For focused wave fields for which the semiangle is known to be less than 180 degrees, the magnitude and phase can be recovered directly from knowledge of the intensity in the focal region.

Bessel pulse beams and focus wave modes.

Free-space propagation of ultrashort pulses is investigated. Space-time couplings are reduced for a particular form of beams that is termed a pulse beam, or a type 3 pulsed beam. General conditions for the formation of pulse beams in the paraxial approximation are presented. The free-space propagation of spatially localized ultrashort laser pulses is investigated. This treatment is based on a particular pulsed form of the well-known Bessel beam, which is termed a Bessel pulse beam. The connections with focus wave modes and X waves are discussed.

High-aperture beams.

Beams of a high angle of convergence and divergence are called high-aperture beams. Different ways of defining high-aperture generalizations to paraxial beams are reviewed for both scalar beams and electromagnetic beams. The different approaches are divided into three types. The particular examples of Gaussian beams and Bessel beams are discussed. For Gaussian beams, beams that exhibit a Gaussian variation in the waist necessarily include evanescent components, which rules out their use in describing propagation over all space. Generalizations of the definitions of beam width and the beam-propagation factor M2 for high-aperture beams are described. The similarities among the three types of high-aperture beams and the different models of ultrashort-pulsed beams are discussed.

Optimization of second-harmonic generation microscopy.

We describe the principles and characteristics of second-harmonic generation imaging (SHGI) and explore various methods for optimization of the technique. Second-harmonic imaging is optimized for ultrashort laser pulses, high numerical aperture microscope objectives, a highly sensitive non-descanned large area detector, pseudo-phase-matching, and specimens with large second-order non linearity or which exhibit surface plasmon enhanced phenomena. We also compare and contrast the techniques of SHGI and two-photon excited fluorescence imaging.

Measurement of thin coatings in the confocal microscope.

The use of the confocal microscope for measurement of the thickness of thin transparent coatings, such as the varnish layer on compact discs, is described. The relationship between true and apparent thickness varies in a non-linear fashion, but intensity profiles show a good correspondence with calculated profiles. This provides the basis of a nomogram for prediction of coating thickness.