The Study of Nanophotonic Switching Mechanisms in Photonic and Metallic Heterostructures

In this thesis, nanophotonic switching mechanisms and light-matter interactions are explored in photonic and metallic heterostructures and nanocomposites. These heterostructures are made using various combinations of photonic crystals (PCs), quantum dots (QDs), and graphene or metal nanoparticles (MNPs). PC heterostructures are formed by combining different PCs so that photons in a specific energy range can propagate in certain regions along one direction and cannot propagate in others. This band structure engineering is used to form photonic quantum wells (PQWs) that have discrete energy states along one dimension. By simulating the photon transmission along the direction of confinement, resonant photon tunnelling is shown to occur at discrete energies. Double PQW (DPQW) heterostructures are also considered, where it is found that resonant states appear in split pairs due to coupling between PQWs. Nonlinear DPQW heterostructures are also investigated, whereby two regions in the structure are made of Kerr-nonlinear PCs. Here it is shown that the application of an external pump laser field can be used to optically switch the resonant frequencies of bound states in the DPQW. Energy transfer in a heterostructure made by embedding a QD-graphene nanodisk nanocomposite in a Kerr-nonlinear PC has been studied. Here it is shown that energy transfer occurs between the QD and graphene due to a dipole-dipole interaction. Energy transfer occurs for two distinct frequencies of an external probe laser field, and can be switched by changing the separation between the QD and graphene or by applying a pump laser field to the PC. An alternative QD-graphene nanocomposite was investigated, where the local field created by plasmons in graphene is used to manipulate two-photon absorption in the QD. An external gate voltage is applied to graphene to modify the plasmon resonance frequency and therefore the frequency at which the local field enhancement has its maximum value. It is demonstrated that two-photon absorption in this nanocomposite can be switched on or off by modifying the gate voltage. Finally, nonlinear second harmonic (SH) generation and two-photon photoluminescence (TPPL) has been studied experimentally and theoretically in QD-MNP hybrid systems. It is found that a secondary laser field resonant with the plasmons in the MNP can be used to enhance SH generation in the QDs

Simulations of Dust in Interacting Galaxies

A new Monte-Carlo radiative-transfer code, Sunrise, is used to study the effects of dust in N-body/hydrodynamic simulations of interacting galaxies. Dust has a profound effect on the appearance of the simulated galaxies. At peak luminosities, about 90% of the bolometric luminosity is absorbed, and the dust obscuration scales with luminosity in such a way that the brightness at UV/visual wavelengths remains roughly constant. A general relationship between the fraction of energy absorbed and the ratio of bolometric luminosity to baryonic mass is found. Comparing to observations, the simulations are found to follow a relation similar to the observed IRX-Beta relation found by Meurer et al (1999) when similar luminosity objects are considered. The highest-luminosity simulated galaxies depart from this relation and occupy the region where local (U)LIRGs are found. This agreement is contingent on the presence of Milky-Way-like dust, while SMC-like dust results in far too red a UV continuum slope to match observations. The simulations are used to study the performance of star-formation indicators in the presence of dust. The far-infrared luminosity is found to be reliable. In contrast, the H-alpha and far-UV luminosity suffer severely from dust attenuation, and dust corrections can only partially remedy the situation.Comment: 4 pages, 5 figures, to appear in the proceedings of the conference "The Spectral Energy Distribution of Gas-Rich Galaxies", eds. C.C. Popescu & R.J. Tuffs (Heidelberg, October 2004

Extraordinary nonlinear plasmonics in graphene nanoislands

Nonlinear optical processes rely on the intrinsically weak interactions between photons enabled by their coupling with matter. Unfortunately, many applications in nonlinear optics are severely hindered by the small response of conventional materials. Metallic nanostructures partially alleviate this situation, as the large light enhancement associated with their localized plasmons amplifies their nonlinear response to record high levels. Graphene hosts long-lived, electrically tunable plasmons that also interact strongly with light. Here we show that the nonlinear polarizabilities of graphene nanoislands can be electrically tuned to surpass by several orders of magnitude those of metal nanoparticles of similar size. This extraordinary behavior extends over the visible and near-infrared for islands consisting of hundreds of carbon atoms doped with moderate carrier densities. Our quantum-mechanical simulations of the plasmon-enhanced optical response of nanographene reveal this material as an ideal platform for the development of electrically tunable nonlinear optical nanodevices.Comment: 16 pages, 12 figures, 54 reference

Searching for Lyman alpha emission from a possible Zel'dovich pancake

The detection of 2 x 10(exp 14) solar mass of neutral hydrogen at a redshift of 3.397 has been reported. Top-down theories of structure formation predict such a mass of hydrogen collapsing to form a protocluster of galaxies. We sought to observe this object in Lyman-alpha, which could be produced through ionization by the metagalactic ionizing radiation field or through internal ionization processes. On 29 Apr. 1992, the region of the reported HI emission for 1800 seconds with the 1.3 meter McGraw-Hill reflector at Michigan-Dartmouth-M.I.T. Observatory was observed. Because the H1 emission reported has a transverse scale of 300 sec, a 1/3.06 reducing camera and a Thomson CCD were used to obtain a field of view of about 600 sec by 840 sec. A filter 88 A wide, centered at 5354 A was used; Lyman-alpha emission at z = 3.4 is redshifted to 5347 A. In order to avoid saturating the CCD with a bright star in the field, nine 200 second exposures were taken. The combination of these images shows no obvious extended Lyman-alpha emission at a level of about 28 magnitudes per square arcsecond. The field observed also shows a distant cluster of galaxies. In order to determine if the cluster could in any way be associated with the cloud of neutral hydrogen at z = 3.4, we sought to estimate its redshift from the size and magnitude of the galaxies and of the cluster as a whole. Omega = 1 and H(sub O) = 50 km s(sup -1) Mpc(sup -1) were adopted; our redshift estimates range from z = 0.2 to z = 0.6. The cluster is clearly not associated with the HI cloud at z = 3.4

Cosmopolitan Christians: Religious Subjectivity and Political Agency in Equiano\u27s Interesting Narrative and Achebe\u27s African Trilogy

The primary texts featured in this study—the Interesting Narrative of Olaudah Equiano and two novels of Chinua Achebe’s so-called African Trilogy—each constitute responses to a sly and exploitive Christian modernity, responses which, borrowing from theories of intersubjectivity articulated by Kwame Anthony Appiah and others, might be called two cosmopolitanisms: for Equiano, a Christian cosmopolitanism, which works within available theological structures to revise Enlightenment-era notions of shared humanity; and for Achebe, a contaminated cosmopolitanism, which ironically celebrates the modern inevitability of cultural admixture. Despite their separation by time, space, and even genre, and even more than their common Igbo heritage, the two authors share a common set of discursive strategies by which they portray a resilient agency among African “converts,” whose cosmopolitan Christianities allow for and even invigorate political and cultural resistance. For the enslaved and colonized Africans who come to profess the religion of their oppressors, the final result is not utter subjection but the genesis of new, even powerfully radical subjectivities; that is, it is no longer a religion of oppression, but a new faith entirely. Ultimately, the discursive traps laid by colonial Christianity cannot restrain the new Christian cosmopolitans who emerge from these texts to meet the harrowing rhetorical demands of two pivotal, and in many ways quite similar, moments in modern history

Nonlinear Plasmonic Sensing with Nanographene

Plasmons provide excellent sensitivity to detect analyte molecules through their strong interaction with the dielectric environment. Plasmonic sensors based on noble metals are, however, limited by the spectral broadening of these excitations. Here we identify a new mechanism that reveals the presence of individual molecules through the radical changes that they produce in the plasmons of graphene nanoislands. An elementary charge or a weak permanent dipole carried by the molecule are shown to be sufficient to trigger observable modifications in the linear absorption spectra and the nonlinear response of the nanoislands. In particular, a strong second-harmonic signal, forbidden by symmetry in the unexposed graphene nanostructure, emerges due to a redistribution of conduction electrons produced by interaction with the molecule. These results pave the way toward ultrasensitive nonlinear detection of dipolar molecules and molecular radicals that is made possible by the extraordinary optoelectronic properties of graphene.Peer ReviewedPostprint (published version