The Department of Physics held an open house on April 8, 2016. Some 45 prospective students of all levels attended talks, ate pizza, visited labs, and spoke with faculty and current students. It was a great success!

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Physics Department students Kevin O’Mara (left) and James Heller (right) are participating in a newly inaugurated program, the Summer Undergraduate Research Program. This program, sponsored by the Office of Research, financially supports undergraduates while they carry out research with SDSU faculty. Only a third of the proposals submitted were funded. O’Mara is working with Dr. Johnson while Heller works with Dr. Anderson (in background).

]]>**Title:** “Multiple Vortex Beam Interference Using a Programmable Spatial Light Modulator”

**Time:** 9:30 am, Friday, May 8th, 2015

**Place:** P-250

**Abstract:**

The interference of light, a form of electromagnetic radiation, has been studied in depth for many years. Early experiments were performed with incoherent sources but the field has taken on new focus with the advent of the laser, which allows us to work with monochromatic coherent sources. While the effects of linear interference are now commonplace study in the university setting, we wish to take this idea to a more complex regime where the electric fields and in turn the interference patterns produced by them are a function of azimuthal angle.

In this work we first lay the foundation for point source interference before expanding the concept to multiple source interference. We then use this idea as an analog for beams of light with azimuthally varying phase distributions known as vortex beams.

These vortex beams can be combined with positive and negative topological charge to create azimuthal interference patterns. The combination of two beams with opposite charges creates an azimuthal two-beam interference pattern where the intensity varies sinusoidally with azimuthal angle. We create a series of these vortex interference patterns with varying pairs of charge and show their respective outputs. We combine several of these beams to create patterns where the interference becomes more sharply defined in the azimuthal direction for some points and where destructive interference eliminates or reduces some of the other points. The process for combining these beams is complicated by the fact that the radii of the different vortex beams are dependent on their respective topological charge as well as the Fourier lens used to focus them to an observation plane.

We outline an experimental system centered on the spatial light modulator that will be used to create these vortex beams. We first generate a series of patterns where the interference patterns of different charge are focused with different focal lengths such that their radii agree. To encode these patterns on to one spatial light modulator, we outline an algorithm that provides a way to virtually propagate different focal lengths to the same plane and superimpose them virtually. We are then able to show the effects of multiple vortex beam interference for varied combinations of charge and focal length. Experimental results agree with theory.

Committee Chair: Dr. Jeffrey Davis

Committee Members: Dr. Calvin Johnson and Dr. Ricardo Carretero

]]>**Title:** “Vector Field Generator for a Direct Mapping of the First Order Poincaré Sphere”

**Time:** 9:00 am, Wednesday, May 6, 2015

**Place:** P-250

**Abstract:**

This thesis presents an optical system able to generate all polarization states on the zero order Poincaré sphere. An important characteristic of the zero order sphere is its spatial uniformity. This means that the polarization of the beam is uniform. This characterization can be proven using polarizers. Any change in the polarization of the beam will be consistent for all points in the beam. This is not necessarily true for all types of polarization states. There are new polarization states that are spatially variant in which the polarization is no longer uniform. The assumption that the polarization at one point in the beam is the same at all points is no longer valid, which allows for strange behavior. These are defined as higher order polarization states which have their own Poincaré spheres that are similar to the zero order sphere but are spatially variant. These states are the focus of this thesis. Maxwell’s equations are shown and the solution for light is derived. From this, Jones vectors are used to describe the polarization and how they relate to the Poincaré sphere. Jones matrices are applied to the incoming polarization state to reflect the changes a waveplate causes to the system, and how to create a rotator to rotate the axis of polarization. These matrices describe an optical system consisting of a variable waveplate and a rotator created from 2 quarter waveplates and an additional variable waveplate that able to change the latitude and longitude of a polarization state on the Poincaré sphere. The system is able to achieve any coordinate on the surface of the sphere. The system is applied to the zero order Poincaré sphere and the positive and negative first order Poincaré sphere. The experimental results are presented.

Committee Chair: Dr. Jeffrey Davis

Committee Members: Dr. Fridolin Weber and Dr. Ricardo Carretero

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**William Spinella** (Numerical Astrophysics):

**Advisor:** Dr. Fridolin Weber

**Abstract:**

Neutrino-pair bremsstrahlung due to interactions between electrons and the crystalline lattice in the quark-hadron mixed phase of high mass neutron stars has been previously studied by Na et al. 2012. We extend this study by first replacing the MIT bag model with the nonlocal three-flavor Nambu-Jona-Lasinio model to describe the quark matter phase. We then include rod and slab rare-phase geometries in addition to spherical blobs. Finally we compare contributions due to Bragg diffraction and electron-phonon scattering. We find that the neutrino emissivity due to electron-lattice interactions in the mixed phase may be substantial at low temperature and quark fraction.

**Martin Kandes** (Bose-Einstien Condensates)

**Advisor:** Dr. Ricardo Carretero

**Abstract:**

We present the implementation of a method-of-lines approach for numerically approximating solutions of the time-dependent Gross-Pitaevksii equation in non-uniformly rotating reference frames. Implemented in parallel using a hybrid MPI + OpenMP framework, which will allow for scalable, high-resolution numerical simulations, we utilize an explicit, generalized 4th-order Runge-Kutta time-integration scheme with 2nd- and 4th-order central differences to approximate the spatial derivatives in the equation. The principal objective of this project is to model the effect(s) of inertial forces on quantized vortices within weakly-interacting dilute atomic gas Bose-Einstein condensates in the mean-field limit of the Gross-Pitaevskii equation. Here, we discuss our work-to-date and preliminary results.

**Omair Zubairi** (Numerical Astrophysics)

**Advisor:** Dr. Fridolin Weber

**Abstract:**

Conventional models of compact objects such as neutron stars assume they are perfect spheres. However, due to high magnetic fields, certain classes of neutron stars such as magnetars and neutron stars containing color-superconducting quark matter cores are expected to be deformed (non-spherical). In this work, we seek to examine the stellar structure of such objects in the framework of general relativity. We derive the stellar structures equations of non-spherical neutron stars and calculate stellar properties such as masses, radii, along with pressure and density profiles and investigate any changes from standard spherical models.

**Micah Schuster** (Nuclear Physics)

**Advisor:** Dr. Calvin Johnson

**Abstract:**

A goal of nuclear theory is to make quantitative predictions of low-energy nuclear observables starting from accurate microscopicinternucleon forces. Modern effective interaction theory, applying unitary transformations to soften the nuclear Hamiltonian and hence accelerate the convergence of *ab initio* calculations as a function of the model space size, is a major element of such an effort. The consistent simultaneous transformation of external operators, however, has been overlooked in applications of the theory, particularly for non-scalar transitions. We study the evolution of the electric dipole operator in the framework of the similarity-renormalization group method and apply the renormalized matrix elements to the calculation of the ^{4}He total photo absorption cross section and electric dipole polarizability. All observables are calculated within the *ab initio* no-core shell model. We find that, although seemingly small, the effects of induced operators on the photo absorption cross section are comparable in magnitude to the correction produced by including the three-nucleon force and cannot be neglected.

**Alexis Romero** (Numerical Astrophysics)

**Advisor:** Dr. Fridolin Weber

**Abstract:**

Non-rotating neutron stars are generally treated in theoretical studies as perfect spheres. Such a treatment, however, may not be correct if strong magnetic fields are present and/or the pressure of the matter in the cores of neutron stars is non-isotropic, leading to neutron stars which are deformed. In this work, we investigate the impact of deformation on the gravitational redshift of neutron stars in the framework of general relativity. Using a parameterized metric to model non-spherical mass distributions, we derive an expression for the gravitational redshift in terms of the mass, radius, and deformity of a neutron star. Numerical solutions for the redshifts of sequences of deformed neutron stars are presented and observational implications are pointed out.

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**Title**: ” A Comparison of Demons Deformable Image Registration Algorithms and their Applicability to Monitoring Longitudinal Changes in 3D MRI Femorotibial Cartilage Images in Subjects with Osteoarthritis: Data from the OsteoArthritis Initiative”

**Time**: 9:30-11 am, Thursday, October 30, 2014

**Place**: P-250

**Abstract**:

Osteoarthritis (OA) is a slowly progressing disease characterized clinically by pain, deformity, enlargement of the joints, and limitation of motion. OA causes, among other changes, loss in cartilage volume that increases as the disease progresses. OA is a complex disease and objective documentation of disease progression or response to treatment is challenging. Approximately 27 million adults age 25 and older have clinically diagnosed OA; however, cartilage loss with disease progression is small and localized to sub-regions of the cartilage. Detection of these changes is challenging and manual methods are tedious and error prone.

Magnetic resonance imaging (MRI) is a non-invasive modality that provides high-resolution, 3-dimensional images with high contrast between cartilage and the surrounding anatomy. Highly accurate measures of cartilage volume, and thickness (global and local) can be extracted from morphological MR images. The focus of this research is on development of accurate tools to quantify the small and localized changes in cartilage morphology to facilitate comparisons between patient cohorts with varying degrees of OA as well as to track longitudinal changes (normal progression and response to treatment). The application area is the femoral cartilage but the methodology can be readily extended to the patellar and tibial cartilage.

We explored a fast, readily implementable algorithm called the ‘Demons Algorithm’. We implemented and compared the registration accuracy of four variants of the algorithm on cartilage image volumes. The registration algorithms were also evaluated for the accuracy of the average Jacobians. Evaluation was performed on 36 subjects using the baseline and later time point images acquired after 12 months. The symmetric evolved demons algorithm provided the best in registration accuracy evaluated using quantitative metrics of mean squared error and voxel overlap. The average Jacobian of the cartilage was compared to the ratio of volume change for validation. The symmetric simple demons and symmetric evolved demons performed equally well in terms of the Jacobians. The techniques developed here will be used, in future studies, to explore differences in cohorts segregated by disease severity and correlation of local changes to clinical variables.

**Committee Members**:

Dr. Usha Sinha (Chair)

Dr. Mauro Tambasco

Dr. Faramarz Valafar

]]>Undergraduate physics student Grace Cordes and Electrical Engineering student Daniel White head off to the 98th annual Frontiers in Optics/Laser Science conference. This year the conference is being held in Tuscon, AZ from October 19th to the 23rd. Both students work the physics department newest faculty, Dr. Lyuba Kuznetsova in the areas of Optics. Both students are participating in the conference by presenting their latest research at the poster session of this conference. See below for their abstracts.

**Grace Cordes**

**Title:** “Noninvasive Nanoparticles Detection Using Silicon Microdisks”

**Abstract:**

We demonstrate a noninvasive ultra-sensitive biodetection platform using a 2μm diameter silicon microcavity by measuring the resultant resonance frequency shift in the microcavity when a nanoparticle (e.g., viruses) is in contact with the microdisk. Results show detection and sizing of particles (e.g., HIV virus) down to a single nanoparticle.

**Daniel White**

**Title:** “Fabrication and Characterization of SiO2/Al Nanolayered Metamaterial”

**Abstract:**

We have fabricated and characterized a SiO2/AL nanolayered metamaterial using sputtering technique. The Al layer thickness was adjusted based on effective medium approximation to achieve hyperbolic dispersion in visible spectral range. Sputtering parameters (RF and DC powers, chamber pressure) were varied to find optimal conditions for low loss metamaterial.

]]>**Title: **” A Rapid CTDI Check Using In-Air Measurements and a Method to Measure the Spatially-Varying HVL in CT Scanners Using a Real-Time Dose Probe”

**Time: **12:30-1:30 pm, Thursday, September 11, 2014

**Place: **P-245

**Abstract:**

The simplest and most standardized metric for reporting the estimated dose from a series of computed tomography (CT) scans is the CT dose index (CTDI). Although this quality control (QC) procedure is required on an annual basis by the FDA, performing such measurements more frequently (e.g. monthly or even weekly) would present the opportunity to expose an unexpected drift in CT scanner output from its commissioned settings and allow for an earlier correction to the problem. This would ensure scans are performed with optimal image quality and avoid the possibility of exposing patients to ancillary ionizing radiation. However, correctly aligning CTDI phantoms can be a laborious, time-intensive process. The first part of this study is a proposed method to construct a CTDI ratio look-up to derive values of CTDIw using only in-air measurements. This could serve as a quick monthly or weekly CTDIw QC check with a minimum of three simple in-air measurements without devoting a large amount of time toward CTDI phantom setup.

Although CTDI serves as a useful metric for quantifying CT radiation output to compare different scanning protocols, it does not contain enough information to accurately evaluate dose to a specific patient or organ of interest. For this reason, Monte Carlo (MC) simulations are used to compute patient-specific dose for research purposes. Accurate calculations largely rely on a researcher’s ability to correctly model the x-ray source fluence and spectra, which are largely defined by an internal bow tie (BT) filter. The second part of this project explores the feasibility of employing an aluminum cylinder half value layer (HVL) measurement technique in conjunction with a real-time dose probe to completely assess the HVL along the BT filter axis in a CT scanner with a minimum of three scans. The data from this method could be used to calculate the angle-dependent fluence and energy spectrum along the BT filter axis, which could be used as beam model input for MC dose computations. This technique is significant not only for its rapid approach, but also that each scan can be completed using routine scan protocols rather than service or localization protocols – eliminating the possible reliance on a service engineer.

**Committee Members:**

Dr. Mauro Tambasco (Chair)

Dr. Usha Sinha

Dr. Satish Sharma

]]>**Title:** “The Rheology and Dynamics of the Jammed State.”

**Time:** 10-11:30 am, Friday, August 8, 2014

**Place:** P-250

**Abstract:**

The jammed state occurs when the shearing wall of a polymer system is stuck in place and restricted to small scale oscillations. This state has characteristics unique to itself as well as those that resonate throughout the system. This thesis aims to explore the dynamics and rheology of the jammed state through the use of a mixed molecular dynamics computational model. Computational methods are employed to explore the jammed state and uncover the underlying behaviour. Phenomena that are explored include the correlation between the wall and the chain stretch entropy, the self organized criticality of the wall and the layering of the system in both the Y and Z direction.

**Committee Members:**

Dr. Arlette Baljon (Chair)

Dr. Matt Anderson

Dr. Peter Blomgren

]]>**Title:** “Dosimetric Comparison of Pinnacle3, EclipseTM 11.0, and iPlan 4.1 Algorithms with Heterogeneous Phantoms”

**Time:** 2-3 pm, Wednesday, August 6, 2014

**Place:** P-245

**Abstract: **

Our goal is to compare the dosimetric accuracy of the EclipseTM 11.0 Anisotropic Analytical Algorithm (AAA) and Acuros® XB (AXB), Pinnacle3 9.2 Collapsed Cone Convolution Superposition, and the iPlan 4.1 Monte Carlo (MC) and Pencil Beam (PB) algorithms using measurement as the gold standard. Measurements were taken for 6, 10, and 18 megavoltage (MV) beams in heterogeneous block phantoms and an E2E®stereotactic body radiation therapy* (*SBRT) Lung Phantom. The measurements were taken using a pinpoint chamber and EDGE diode. Data from the planning systems were computed for each scenario, and compared to our benchmark measurements using percent differences.The best results between data from the algorithms and our measurements occur after the buildup region in solid water for scenarios 1 and 2, and at the treatment isocenter in scenario 3. The cumulative results from scenarios 1 and 2, using a 3% difference from benchmark as our passing criteria, indicate that AXB performs the best overall. All algorithms except PB were within 5% difference from our benchmark measurements at the isocenter. Differences between our measurements and algorithm data are much greater for the off-axis points. In summary there is an obvious lack of accuracy for doses to critical structures outside any primary beam. Further studies will need to be done to understand the risk of dose to OAR’s outside the primary fields. The data for the anthropomorphic phantom indicates that all the algorithms accurately calculate dose to targets within lung, excluding PB. Finally, our first two scenarios display the possible power behind AXB, overall outperforming iPlan MC code.

**Committee Members:**

Dr. Mauro Tambasco (Chair)

Dr. Usha Sinha

Dr. Faramarz Valafar

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