Alexander Cocking

Images of Gravitational and Magnetic Phenomena Derived from Two-dimensional Back-projection Doppler Tomography of Interacting Binary Stars

We have used two-dimensional back-projection Doppler tomography as a tool to examine the influence of gravitational and magnetic phenomena in interacting binaries that undergo mass transfer from a magnetically active star onto a non-magnetic main-sequence star. This multitiered study of over 1300 time-resolved spectra of 13 Algol binaries involved calculations of the predicted dynamical behavior of the gravitational flow and the dynamics at the impact site, analysis of the velocity images constructed from tomography, and the influence on the tomograms of orbital inclination, systemic velocity, orbital coverage, and shadowing. The H? tomograms revealed eight sources: chromospheric emission, a gas stream along the gravitational trajectory, a star-stream impact region, a bulge of absorption or emission around the mass-gaining star, a Keplerian accretion disk, an absorption zone associated with hotter gas, a disk-stream impact region, and a hot spot where the stream strikes the edge of a disk. We described several methods used to extract the physical properties of the emission sources directly from the velocity images, including S-wave analysis, the creation of simulated velocity tomograms from hydrodynamic simulations, and the use of synthetic spectra with tomography to sequentially extract the separate sources of emission from the velocity image. In summary, the tomography images have revealed results that cannot be explained solely by gravitational effects: chromospheric emission moving with the mass-losing star, a gas stream deflected from the gravitational trajectory, and alternating behavior between stream state and disk state. Our results demonstrate that magnetic effects cannot be ignored in these interacting binaries.

On-Chip Glass Microspherical Shell Whispering Gallery Mode Resonators

Arrays of on-chip spherical glass shells of hundreds of micrometers in diameter with ultra-smooth surfaces and sub-micrometer wall thicknesses have been fabricated and have been shown to sustain optical resonance modes with high Q-factors of greater than 50 million. The resonators exhibit temperature sensitivity of −1.8 GHz K−1 and can be configured as ultra-high sensitivity thermal sensors for a broad range of applications. By virtue of the geometry’s strong light-matter interaction, the inner surface provides an excellent on-chip sensing platform that truly opens up the possibility for reproducible, chip scale, ultra-high sensitivity microfluidic sensor arrays. As a proof of concept we demonstrate the sensitivity of the resonance frequency as water is filled inside the microspherical shell and is allowed to evaporate. By COMSOL modeling, the dependence of this interaction on glass shell thickness is elucidated and the experimentally measured sensitivities for two different shell thicknesses are explained.

Compressive coherent anti-Stokes Raman scattering holography

Coherent anti-Stokes Raman scattering (CARS) holography captures both the amplitude and the phase of the anti-Stokes field generated from a sample and can thus perform single-shot, chemically selective three-dimensional imaging. We present compressive CARS holography, a numerical technique based on the concept of compressive sensing, to improve the quality of reconstructed images by leveraging sparsity in the source distribution and reducing the out-of-focus background noise. In particular, we use the two-step iterative shrinkage threshold (TwIST) algorithm with an l1 norm regularizer to iteratively retrieve images from an off axis CARS digital hologram. It is shown that the use of compressive CARS holography enhances the CARS holographic imaging technique by reducing noise and thereby effectively emulating a higher axial resolution using only a single shot hologram.

Nonlinear characterization of two dimensional materials (Conference Presentation)

Two-dimensional materials have attracted significant interest recently for their unique optical properties compared to their bulk counterparts. Specifically, the family of transition metal dichalcogenides (TMD), such as MoS2 and WS2, have large second order nonlinear susceptibility. Extraordinary second harmonic generation and sum frequency generation have been observed. Here we investigate the second order nonlinearity of 2D materials, including TMD layered materials with dopants and defects. Experimental results and preliminary theoretical analysis will be discussed.

Surface enhanced Raman scattering in whispering gallery mode microresonators

In this talk I will discuss surface enhanced Raman scattering in silica microsphere resonators based on whispering gallery mode resonance. Recently silica microspheres have attracted attention as a novel substrate for surface enhanced Raman scattering. Whispering gallery mode resonance has been identified as a major enhancement mechanism, along with other effects such as photonic nanojets. In most of the previous experiments, however, free space pumping of the microsphere has been used, which has low efficiency in coupling to the whispering gallery modes. In our approach, we use a tapered fiber coupler for a highly efficient coupling to the whispering gallery modes. Coupling to the microresonator is monitored using a tunable laser. We observe both pump enhancement and Purcell enhancement in the microresonator. Since the linewidth of the whispering gallery modes is much smaller than that of the Raman peaks, sharp peaks corresponding to the whispering gallery modes are overlaid on top of the Raman spectrum of the bulk material. To demonstrate the system’s potential for Raman analysis, I will present the whispering gallery mode surface enhanced Raman spectrum of rhodamine 6G thin film coated on a microsphere resonator.

Raman sensing in optical microresonantors(Conference Presentation)

Ultrahigh-quality whispering gallery mode optical microresonators have been studied for their use as highly sensitive sensors. In this talk, we discuss the use of microsphere microresonators in Raman spectroscopy for interrogating particles adhered to the surface of the resonator. An external cavity diode laser is tuned to a resonant high-Q mode and the circulating optical field experiences a large buildup, resulting in enhanced Raman scattering. Here we present studies of Raman scattering spectroscopy of single particles. Raman sensing with different Qs is discussed.

Development of an opto-electronic fiber device with multiple nano-probes.

We present the fabrication and characterization of an opto-electronic fiber device which can allow for both electromechanical functionality and optical waveguiding capability. The air holes of a photonic crystal fiber are selectively sealed and then pumped with molten metal under pressure. The metal filled holes act as electrodes to which individual carbon nanotubes (CNT) are attached precisely by a laser-welding technique or a focused ion beam assisted pick-and-bond technique. The optical modal profile and the group velocity dispersion of the fabricated device are studied both numerically and experimentally. We also present preliminary experimental proof showing the feasibility of electric actuation of a pair of nanotubes by applying up to 40 V potential difference between the filled electrodes. Furthermore, numerical simulations are carried out which agree with the experimentally observed displacement of the CNT upon electric actuation. The unique aspect of our device is that it provides optical waveguiding and electromechanical nano-probing capability in a single package. Such combined functionality can potentially enable simultaneous electrical and optical manipulation and interrogation at the nanoscale.

Raman sensing in microresonators

High-quality whispering-gallery-mode optical resonators have garnered interest in particle sensing for a variety of applications. Here, we further explore the idea of using microresonators to enhance single-particle detection and identification by monitoring the Raman scattering from a particle adhered to a silica micro-sphere. A tunable diode laser is critically coupled into a resonant mode of the micro-sphere resonator, allowing circulating power to build up within the cavity for enhanced interaction with the attached particle. Experimental results of single particle Raman scattering in microsphere resonators are presented.

Explorations into 3D Doppler Tomography of Interacting Binaries

Over the past twenty-five years, the technique of Doppler tomography has produced many 2D images of the accretion structures and other gas flows in a range of systems contain- ing compact and non-compact stars, including cataclysmic variables, polars, Algols, x-ray, and gamma-ray binaries. Recent 3D images derived from the Radioastronomical Approach (RA) have revealed prominent gas motions beyond the central plane, and display the usual character- istics found in 2D images, as well as new evidence of tilted or precessing accretion disks around the mass gainer, and magnetic loop prominences and coronal mass ejections associated with the donor star. In this work, we have compared new 3D images derived from the back projection tomography technique with those derived from the RA method. In general, back projection produces sharper and more distinctive images than the RA method, thereby permitting a more detailed study of the physical properties of the accretion sources.