Mohammad H. Maleki

Tomographic reconstruction from optical scattered intensities

A generalized tomographic reconstruction procedure is described for determining the complex-valuedindex-of-refraction distribution of a semitransparent, three-dimensional inhomogeneous object from observations of the far-field intensity patterns generated by the object in a sequence of scattering experiments. The inversion procedure is based on the wave equation governing the scattered optical field and fully accounts for the diffraction and propagation effects associated with the interaction of the incident wave with the object and the subsequent free-space propagation of the scattered wave to the wave zone (far field). The reconstruction of the object’s index-of-refraction distribution is performed digitally directly from the far-field intensity of the scattered wave and does not require direct measurement or retrieval of the phase of the scattered field. An optical scattering experiment is reported in which the cross-sectional profile of the index-of-refraction distribution of an optical fiber is reconstructed from the measured intensity of the diffraction pattern of the fiber by using the described inversion procedure.

Noniterative reconstruction of complex-valued objects from two intensity measurements

We present results obtained from the application of a novel phase-retrieval algorithm for recovering a complex-valued object from a set of two intensity measurements. The algorithm requires two intensity measurements at different distances from a weak scatterer, where the total transmitted field is composed of the coherent sum of an incident plane wave and the scattered wave. The algorithm is noniterative and does not have the convergence problems associated with iterative algorithms. The new technique shows great promise for inverse-scattering applications, such as optical diffraction tomography and in-line holography of complex-valued objects, with the aim of eliminating the twin-image problem. Results are presented from a computer simulation of a simple object and from experimental data obtained from a microlens array. Our results obtained using the new algorithm on experimental data compare well with those obtained with a modified form of the Gerchberg-Saxton algorithm, at a significantly reduced computational cost.

Monte Carlo simulation of the subsurface growth mode during pulsed laser deposition

The pulsed laser deposition (PLD) growth of aluminum (Al) metal in the presence of noble background gases is studied using a Monte Carlo model constructed on the basis of srim 2010 calculations. It is shown that Al ions are deposited with a high kinetic energy of about 100 eV. These high kinetic incident energies lead to the implantation of ions into the existing film and resputtering from its surface. The consequent film growth is in the subsurface or subplantation growth mode, in which the material does not grow on top of the already deposited film. It is proved that by considering the role of resputtering, which is neglected in other theoretical models of PLD, experimentally observed deviations from the stoichiometric growth of multicomponent materials and dips in thickness profiles of elemental materials at the film center can be explained. The calculated implantation depths are also consistent with the reported measured ones. Taking into account the role of sputtering yield in calculating the deposit...

Microscopic description of the thermalization process during pulsed laser deposition of aluminium in the presence of argon background gas

The presence of background gases is typical in most pulsed laser deposition (PLD) applications and there is a need for methods which do not suffer from the oversimplified assumption of collisionless transfer of the target atoms onto the substrate in analytical descriptions. In this work, we give a microscopic description of a thermalization process by following the history of each ablated particle by Monte Carlo calculations. TRIM code (SRIM2010), which is capable of quantum mechanical treatment of ion?atom collisions, has been used in our simulations. Various kinetic parameters of ablated Al ions during target-to-substrate transfer have been calculated to demonstrate the efficient role of background gas atoms in thermalization of ions. Moreover, the growing parameters of interest in PLD have been calculated to achieve the optimal deposition conditions in the presence of a background gas. A base pressure of 1 ? 10?1?Torr and 2?3?cm of target-to-substrate distance have been found to be the optimal conditions in PLD of Al in Ar gas environment. Our model can be used to obtain the first estimates of nonreactive PLD parameters, such as the background gas pressure and the target-to-substrate distance for the growth of even more complex materials in the presence of different background gases.

New ducting model for analyzing the Gaussian beam propagation in nonlinear Kerr media and its application to spatial self-phase modulations

Knowing the Gaussian beam parameters, such as its radius of curvature and spot size during propagation in nonlinear Kerr media, is of paramount importance in describing the observable far-field diffraction ring patterns as well as in design and stability analysis of Kerr-lens mode-locked resonators. Specifically, the sign of the beam radius of curvature after exiting these media has been proposed to be of assistance in recognizing their optical nonlinearity sign through determining the type of diffraction ring pattern in the far field. In order to be able to trace the evolution of the beam parameters in the Gaussian beam formalism, we have used the common aberration-free theory. We have shown that the nonlinear propagation problem of a fundamental Gaussian beam in a Kerr medium with an intensity-dependent index of refraction can be handled by assuming a ducting index profile along the propagation direction. Knowing the familiar ABCD matrix of a duct, the evolution of the mentioned beam parameters can be traced during propagation using the ABCD law in Gaussian beam theory. We have validated our ducting model by comparing its results with the outcomes of one widely used and accepted model which has been known to yield consistent results when electronic optical nonlinearity prevails. We have shown that when thermal optical nonlinearity is dominant, as in diffraction ring observation experiments, our ducting model yields sensible results and should be used. Our model predicts that when the sign of the thermal nonlinearity and the beam radius of curvature on the entrance plane of the medium are positive, the sign of the beam radius of curvature on the exit plane may have either sign, depending on the medium thickness used in the experiment. Hence, two types of diffraction ring pattern may be obtained using the same medium with two different thicknesses and this may cast doubt on the validity of the methods proposing the detection of the optical nonlinearity signs by observing these patterns. We have proposed a simple procedure for experimentally obtaining the two different types of diffraction pattern from the same medium.

New raster-scanned CO2 laser heater for pulsed laser deposition applications: design and modeling for homogenous substrate heating

We report on designing a new raster-scanned CO2 laser heater for homogenous heating of the disk-shaped substrates during pulsed laser depositioning of materials. This new design aims at concentrating the laser energy near the substrate peripheral edge, which mostly tends to remain cooler than the inner parts during the heating process. A comprehensive heat diffusion model has been developed to predict the temperature and its homogeneity on the substrate depositioning face. Conduction and radiation heat transfer in three dimensions with temperature dependant material properties and a moving heat source are taken into account in this transient model. The model is validated by a simple stationary Gaussian laser heat source whose results are in good agreement with our measured values. Using this model, optimum conditions for the growth of garnets are calculated. Short heat-up times, some within minutes, are corroborated by calculations and measurements. An experimental procedure is designed to test the possibility of optical damage occurring in the substrate during this short temperature heat-up time.

Phase retrieval in inverse scattering

This paper discusses application of phase retrieval in inverse scattering, using optical diffraction tomography as an example. We consider two algorithms for recovering the phase of a scattered field from intensity measurements. The first algorithm is iterative and uses one intensity measurement and support information of the object to retrieve the phase. The second algorithm is non-iterative and uses two intensity measurements at different distances from the object to recover the phase by solving a pair of coupled algebraic equations. The second algorithm requires that the object by weakly scattering, a condition also required by the usual assumption of the Born or the Rytov approximations in diffraction tomography. The paper includes computer simulations of the two phase retrieval algorithms and compares the results to reconstruction obtained from full simulated data and a direct intensity-only reconstruction algorithm.

Design and modeling of a new CO2 laser heater for thin film deposition applications

In this paper, we report on designing a new raster-scanned CO2 laser heater for homogeneous heating of the disk-shaped substrates. This new design aims at concentrating the laser energy near the substrate peripheral edge, which mostly tends to remain cooler than the inner parts during the heating process. A comprehensive heat diffusion model has been developed to predict the temperature and its homogeneity on the substrate depositioning face. This new kind of laser heater can favorably be used in preparation of nanostructured thin films where the shape and size of the embedded nanoparticles depend on the maintained temperature during the depositioning time interval. This heater can also be used for CO2 laser conditioning of the prepared thin films to enhance their damage threshold for high power laser applications.

Experimental investigation of phase retrieval for inverse scattering

We present results obtained from the application of a novel phase retrieval algorithm on experimental scattering data from a microlens array, with the aim of reconstructing the phase profile across the array. The algorithm, originally developed for electron microscopy, requires two intensity measurements at different distances from a weak scatter, where the total transmitted field is composed of the coherent sum of an incident plane wave and the scattered wave. The algorithm is noniterative and does not have the convergence problems associated with iterative algorithms. Our results obtained using the new algorithm on experimental data are comparable to those obtained with the Gerchberg-Saxton algorithm, and the computational cost is much less. The new technique shows great promise for inverse scattering applications, such as optical diffraction tomography and rough surface profiling, where the scattering can be assumed to be weak.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Modeling adapted to manufacturing of holographic super Gaussian gratings

Abstract. A few holographic metallic (gold and silver) gratings with maximum diffraction efficiency (DE) in the 700 to 1100 nm wavelength range were designed. These designed diffraction gratings can be attractive for stretching and compressing pulses in chirped pulse amplification (CPA) technology. They were optimized for transverse magnetic polarized light in the first order (littrow mount). KPP1206 photoresist was used to model the final pattern on photoresist thin film. To obtain super Gaussian profiles, approximately identical exposure energy density intervals were used for optimizing grating profile. The optimized gold and silver gratings were compared in terms of DE, bandwidth and incidence angle range. Dependence of DE on the depth and duty cycle of super Gaussian gold and silver gratings showed that the silver grating had higher DE than the gold grating in 1064 nm. Fabricated silver grating based on modeling results had good agreement with experimental results.