Session E2: Poster Session II

Macroscopic Cavity Optomechanics

We propose and develop systems for demonstrating macroscopic cavity optomechanical phenomena, a field that is heavily focused on microscopic mechanical oscillators and resonant cavities. Our group seeks to observe the same optomechanical phenomena on the cm-scale in order to push the limits of observing quantum-mechanics. Specifically, our group has developed high-$Q$ superconducting radio-frequency cavities with quality factors up to 1.7 x 10$^{9}$. The highest $Q$ we achieved to date when integrated with a mechanical oscillator is 5 x 10$^{7}$. In addition, we are developing a capacitive electrode system to enhance the characterization and measurements of the optomechanical system. We aim to use this system to explore physics such as squeezing, parametric amplification, and the dynamical Casimir effect.   

Frequency-tunable Conically-shaped Stub Cavities for Microwave Optomechanics

We report on our progress in constructing conical coaxial quarter-wave cavities having frequency tunability and optomechanical coupling. 3D optomechanical cavities have recently been exploited for macroscopic quantum mechanical experiments. We will discuss the response of quarter-wave cavities under two different frequency tuning conditions. One where a dielectric rod is inserted and a second where the conical central conductor is extended. We will also present results for the inclusion of an aluminum plane which simulates an optomechanical element. Based on our room-temperature results, we find that quarter-wave cavities of this design are attractive candidates for use as high quality factor superconducting radio-frequency systems for ultra-sensitive metrology and sensing.   

Optoelectronic Microwave Oscillator

The optoelectronic microwave oscillator (OEO) is a device that takes light from a continuous wave pump laser as an input and converts it into a high repetition rate optical pulse stream and an ultra-stable microwave signal. We report on our initial results with an OEO operating at 10 GHz incorporating a coaxial stub cavity. Light from a 1550 nm laser is sent through a variable fiber delay (between 10 m and 10 km) before being detected. The electrical signal is then amplified and spectrally filtered before being fed back into an electro-absorption modulator to modulate the laser. This generates a series of side bands on the optical beam around the center wavelength, spaced at 10 GHz intervals. The OEO can be used to generate microwave signals, or be used as the basis for an optical clock recovery setup. Instead of the traditional electronic band pass filter, this setup uses a fabricated aluminum stub cavity as a band pass filter. We envision incorporating the OEO in a 10-GHz superconducting radio frequency system for use in microwave optomechanics experiments. Using different materials and stub geometries, we can alter the quality factor of the cavity and study the influence it has on optoelectronic devices such as the OEO.   

Resonance Threshold and Bistability in a Parametrically-Driven Pendulum

We examine the steady-state energy of a weakly-damped pendulum driven into parametric resonance. Motion control and data acquisition of the pendulum are accomplished using a micro-controller, which we use to record the passage time of the pendulum through its equilibrium position and obtain the maximum speed per oscillation as a function of time. As examples of the interesting physics which the experiment reveals, we present contour plots depicting the energy of the system as functions of driven frequency and modulation depth, as well as the bistability of the system for various modulation depths. We observe the transition to steady state oscillation and compare the experimental oscillation threshold with theoretical expectations. The equations of motion can be derived from first principles, and the cost-friendly and easily constructed apparatus makes this robust experiment a method for undergraduate students to thoroughly understand damped harmonic motion and parametric amplification in an upper-division mechanics class environment.   

Writing as a Mediator for Critical Thinking and Conceptual Change: A Targeted Activity to Help Students Uncover Their Misconceptions in an Introductory Physics Class.

This poster reports the essay analysis of a targeted writing assignment, designed to examine how well physics students developed a more complex conceptual framework regarding microscopic models of friction and to the extent the writing task mediated critical thinking to achieve that framework. The assignment directed students to write an essay which examined the results of \textit{other student's} misconceptions about microscopic friction, by reading research articles that reported classroom dynamics involving these students and their misconceptions. This study linguistically examines the submitted essays for sentence structure and use of lexicon, to evaluate learning as modeled by conceptual change through negotiation of conceptual conflict and misconception. The results suggest that the writing assignment helped a majority of students to begin to restructure their ideas of microscopic friction as distinctly different from macroscopic models of friction.   

Single-core and Dual-core Fiber Optical Parametric Oscillators

We report on two designs for fiber-optic parametric oscillators (FOPOs): a single-core system which exhibits interesting polarization behavior, and a dual-core design which we expect will provide wavelength flexibility and exponential gain. In a single-core FOPO, polarization-based output coupling leads to an increase of output power by 20$\% $-30$\% $ compared with a non-polarization-based output coupling. We present a geometrical visualization of the polarization through the Poincare sphere and compare with experimental results. The coupling coefficient between cores in a dual-core FOPO provides additional phase-matching flexibility. Our proposed dual-core fiber has a coupling coefficient of 0.42 $m^{-1}$ and nonlinear coefficient of 1.6 $W^{-1}{km}^{-1}$ in a weakly guided mode which should deliver exponential gain dependence on the pump power under a variety of conditions. We anticipate that this system will be a unique platform for quantum optics where signal and idler photons are generated indistinguishably in both cores of the fiber.   

Guiding High-Energy, Pulsed-Laser Light through Hollow-Core Optical Fibers

We report on studies of guiding high-energy, pulsed-laser light through hollow-core optical fibers. The 1064 nm, 9 ns, laser pulses generated from a pulsed Nd:YAG laser (operating at 10 Hz and capable of pulse energies up to 450 mJ) are coupled into a 750-$\mu$m-core-diameter fiber. In particular, we characterize the guiding properties of both AgI-coated and uncoated fibers in terms of their efficiency and induced polarization effects as functions of input coupling and bend radius.   

Progress Towards Construction of A Magneto-Optical Trap to Study the Spinor Dynamics of a Bose Thermal Gas

We present our progress towards creating ultracold gases to study the spinor dynamics of a Bose thermal gas. We have designed, constructed, and tested low-cost, external-cavity diode lasers (ECDLs) to operate below the rubidium D2 transition at 780 nm for use in an undergraduate-only research laboratory. The ECDL operates in a Littrow configuration with a mode-hop free range of 1-2 GHz and a linewidth $<$ 6 MHz. The trap and repump lasers will be frequency-stabilized using a sub-Doppler Dichroic Atomic Vapor Laser Lock (DAVLL). The locking electronics are discussed, as well as the vacuum chamber design.   

Numerical Analyses of the Landau-Zener Transition with a Finite Sweeping Time

We present detailed numerical analyses of the Landau-Zener Transition in a quantum two-level system with finite sweeping time. Our goal is to obtain accurate description of the Landau-Zener Transition, as well as an analyzation at or near the gap $(t=0)$. We study the transition probability for the case of a symmetric crossing, where $t_i < 0$, $t_f > 0$ and $|t_i| = t_f$, and the crossing occuring at $t=0$, as well as the half crossing, which requires two simulations with the time-spans being from $-200 \leq t \leq 0$ and $0 \leq t \leq 200$. Furthermore, we numerically simulated the Landau-Zener Transition for an adiabatic process, and a non-adiabatic process for both the symmetric and half crossing. Lastly, we will compare our numerical results with the analytical results presented by N.V. Vitanov and B.M. Garraway. The Landau-Zener Transition exhibits very little or no transition probability for both the excited and ground state in an adiabatic process. Yet, for a system in a non-adiabatic process, the Landau-Zener Transition probability increases for the excited state as we increase the velocity at which our two-level quantum system changes. As a result, the transition probability for the excited state approaches 1 for the symmetric crossing, and 1/2 for half crossing.   

Density-functional theory calculations of hybrid perovskites for photovoltaics

Hybrid organometallic perovskites (CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}})$, are a promising material for solar cell applications as the power conversion efficiencies for perovskite devices have risen significantly in recent years. If we are able to make the structure stable at ambient conditions, it may provide a grid-competitive efficiency at low manufacturing cost. We have performed first-principles density-functional theory (DFT) calculations on the cubic phase of CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ using different exchange-correlation functionals to find optimized structures and noticed that the Van der Waals interaction plays a role for structural optimization. Band-structure calculations for different orientations (100 and 111) of the methylammonium ion within the unit cell indicates that the orientation does not change the overall shape of the band structure. By contrast, spin-orbit coupling significantly modifies the conduction bands, reducing the band gap. Calculated effective masses are compared with some previously published results. These preliminary calculations will help us to study light induced degradation mechanism in CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$.   

Charge noise in InAs/GaAs coupled quantum dot devices

Semiconductor quantum dots (QDs) trap individual charge carriers in discrete metastable bound states, optically addressable through interband absorption and photoluminescence. Tunnel-coupled QD pairs (CQDs) additionally host interdot exciton states whose large electric dipole moment allows for in-situ tuning of the transition energy over tens of meV with an applied electric field, conversely acting as a sensitive probe of local electric field. Measurements of the photoluminescence and absorption spectra in diode-embedded InAs/GaAs CQDs reveal interdot states with a significantly broader optical transition linewidth than the shorter-lived single-dot states, indicating spectral wandering from a noisy local charge environment, which limits the resolution obtainable with CQD-based metrology. We gain insight into the spatial distribution and dynamics of charged lattice defects and techniques to minimize fluctuation-induced broadening by analyzing the dependence of the lineshape on optical excitation conditions in different CQD diode samples, in conjunction with Monte Carlo simulations of fluctuating charge traps.   

Power-Law Statistics in Jammed Disordered Solids

Simulations have shown that in many solid materials, rearrangements within the solid obey power-law statistics. A connection has been proposed between these statistics and the ability of a system to reach a limit cycle under cyclic driving. We study experimentally a 2D jammed solid that reaches a limit cycle under cyclic driving. Our solid consists of microscopic plastic beads adsorbed at an oil-water interface and cyclically sheared by a magnetically driven needle. We track 30,000 particles over 20,000 frames to identify rearrangements. By associating particles both spatially and temporally, we can measure the size of each rearrangement event. We present power-law like statistics within our solid when it is not driven, showing promise for power-law statistics with cyclic driving.   

Using Python to Automate Electron Transport Experiments and Provide Real-Time Data Visualization

Low temperature electron transport experiments on low dimensional nanomaterials are ruled by exciting quantum effects. In order to extract meaningful information from these experiments, data needs to be collected in a reliable way. Real time visualization of the data allows us to determine efficiently the direction of the experiment. Here we present a method using the Python programming language and open source libraries to control measurement instruments such as Keithley source-meters and lock-in amplifiers while simultaneously controlling a closed-cycle cryostat with a superconducting magnet. We present additionally the implementation of a library which provides a web-based interface to interactively visualize incoming data from multiple sensors in real time. Using these tools, we have been able to fully automate data acquisition during initial tests conducted on our newly installed cryostat. These tests include the reading of temperature in separate regions inside the cryostat during cooldown and the reading and control of current supplied to the superconducting magnet through different runs to full field, 12 Tesla.   

Muon-bonding site search in MgO: possible implications for earthquake-precursor detection

A computational model is used to study and locate the muon-bonding sites in MgO. Likely, the positive muon localizes in a vacant oxygen tetrahedron. This is similar to what has been observed in other well-known oxides like Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$. Potentials are given by a Morse function (describing the muon-oxygen bonding) and Coulombic interactions between the muon and the four O[-2] ions. We plan to extend our work to a larger cluster, extending over a couple of lattice cells (and later over the whole lattice). These investigations are performed in support of Muon-Spin-Resonance ($\mu $SR) MgO {\&} MnO studies, [1] for which $\mu $SR signals have been observed. For MnO, only one signal has been observed, as expected. In MgO, an additional sharp signal has been seen, suggesting the existence of extended O[-1] states.[1] The latter can be seen as O[-1] earthquake-like precursor effects. [2] 1] C Boekema \textit{et al,} APS March meeting (2016). 2] FT Freund, Nat Hazards Earth Sys Sci \textbf{7} (2007) 1; FT Freund \textit{et al,} Phys Chem Earth \textbf{31} (2006) 389.   

Simulated Annealing of Nanowire Structures using a Quantum-Based Model

We performed simulations of the equilibrium atomic structures of metallic nanowires based on a~classical energy model~which includes~a short-range (phenomenological hard-core) repulsion,~a screened Coulomb potential, and~a quantum-mechanics based potential energy that stabilizes longer wires. We simulated cylindrical sodium nanowires~with "magic" radii, on order of Angstroms, predicted to be stable by the nanoscale free electron model. The boundary conditions are chosen to simulate a wire connected to bulk contacts. Written in C,~our program is based on a Monte-Carlo simulated annealing algorithm and utilizes the double precision SIMD oriented Fast Mersenne Twister (dSFMT) pseudo-random number generator. The simulations performed found that the atoms arrange themselves in multi-shell helical structures.   

Development of a New Pressure Measuring Technique for Examining Pressure Gradients in Solids

A cryogenic pressure vessel was designed and created for the purpose of measuring pressure gradients within solid helium. Tests in the preliminary stage were performed in liquid nitrogen (77.2 K) at helium pressures up to 31 bars, using a piezo-resistive force sensor for the pressure measurement. The pressure vessel is made of OFHC copper with indium metal as a seal. Stainless steel capillary tubing provides a helium line and a wire feed-through. While the force sensor does react to the pressure change, the readings of the resistance are somewhat noisy and the setup could benefit from further noise reduction.   

Experimental measurements of pressures exerted by bimetallic shells due to thermal stress

We present measurements of the~relationship~between~volume-displacement and pressure for bimetallic shells through a broad range of temperatures. Bimetallic shells were fixed in a sealed chamber and the pressures exerted by the shells were measured mechanically as temperature was varied. We observed hysteresis behavior between the concave and convex equilibrium states. These results can be used to predict the work output from a bimetallic heat engine where the disk acts as a diaphragm. .   

Translocation dynamics of pre-packaged polymers through a pore

Many biological systems contain polymers that translocate through pores in a membrane only after being packaged by transport factors, which serve to condense the polymer by some packaging fraction as well as to provide some affinity for the inside of the pore. Here, we use a Fokker-Planck formalism to model how the properties of these transport factors interact with the pore size to affect the time of translocation, assuming the removal of these transport factors is accomplished by some other energy-consuming enzymatic action. At sufficiently high affinity, translocation time is the least when there is always exactly one transport factor inside the pore at any given time during translocation; however, at lower affinities, the increased ratcheting effect proves more important than the edge effects, and translocation time decreases as pore size increases. If we take into account that the diffusion constant of the polymer is reduced both by decreasing the packaging fraction and decreasing affinity, we are able to identify optimal and sub-optimal regimes of the parameter space, for pores of varying sizes, where small deviations from the optimal regime can increase the time of polymer translocation drastically.   

Self-Assembly of Colloidal Particles by Optical Binding

The properties of a material can be altered by assembling that material into particular microscopic patterns. The result is a metamaterial, which offers an avenue for creating exotic material properties not seen in nature. In principle, such a material could be created through self-assembly, however, it is difficult in practice to control the interactions between constituent particles. One potential method for doing this is optical binding, which is a highly tunable inter-particle force mediated by an intense optical field. We wish to develop tools, including experimental apparatus and numerical models, that can help demonstrate the self-assembly capabilities of the optical binding force.