Designing subwavelength-structured light sources

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. 133-142).The laser has long been established as the best possible optical source for fundamental studies and applications requiring high field intensity, single mode operation, a high degree of coherence, a narrow linewidth and short pulses. There are many applications that require lasers of varying frequencies, powers, and far-field properties. In science, the laser is a useful source in spectroscopy and microscopy, for investigating nonlinear optics phenomena and nuclear fusion. More commonly, we find them in barcode readers, laser pointers, and printers. They are also widely used for military, medical, and industrial applications. This thesis is focused on achieving new understanding of the principles and phenomena involved in the interaction of light with a variety of material systems, which will in turn guide the designs of compact lasers with feedback structures having features at the subwavelength-scale. The thesis begins by describing the interaction of light with an arbitrary complex material system, and implementing them into the electromagnetic model using two different theoretical techniques suitable for analyzing microstructured lasers: exact finite-difference time-domain calculations and a semi-analytic coupled-mode formalism. These methods are first applied to analyze lasing action in the photonic crystal (PhC) slabs. This class of lasers, commonly referred to as the photonic crystal surface-emitting lasers (PCSELs), can be integrated on-chip and is essentially the two-dimensional (2D) versions of the second-order distributed feedback lasers, where the higher quality factor lasing mode (dark Fano resonance) is selected through the symmetry mismatch to the free-space modes. The PCSELs have not only achieved the highest surface-emitting single-mode power but also the ability to control the shapes, polarizations and directions of their far-fields. However, as in all laser cavities, the lasing areas of PCSELs are limited by two fundamental constraints; a large area tends to promote both multi-mode and multi-area lasing. We propose to overcome both constraints to achieve single-mode PCSELs of larger areas, and thus higher output powers, by tuning the regular lasing bandedges of quadratic dispersions in typical PCSELs to form a single accidental Dirac cone of linear dispersion at the Brillouin zone center. Moreover, an additional frequency-locking phenomenon at the accidental point, with potentially high density of states, is analyzed. We demonstrate and distinguish experimentally the existence of the dark Fano resonances in a macroscopic 2D silicon nitride PhC slab consisting of a square array of holes. We characterize the passive PhC slab in terms of its resonant frequencies and radiation behaviors using temporal coupled-mode theory and symmetry considerations. We also realize lasing at a dark Fano resonance with diluted solutions of R6G molecules as the gain medium. Next, we turn our attention to the organic dye lasers whose high tunability in the visible wavelengths has attracted interests for many years due to their low-cost processing, flexible choice of substrates, and large emission cross sections that can cover the spectral region from ultraviolet to the near infrared. We investigate the laser dynamics in systems of sub-wavelength photonic structures consisting of organic dye molecules, including their photobleaching effects. Our analysis considers both the chemical properties of the dyes and optical properties of the cavities. We also systematically studied the feasibility of lasing under continuous-wave excitations in optically pumped monolithic organic dye lasers. This study suggests routes to realize an organic laser that can potentially lase with a threshold of only a few W/cm² Lastly, we investigate far-infrared (FIR) (~ 0.2 - 2 THz) laser emission from optically-pumped rotationally excited molecular gases confined in a metallic cavity. Terahertz radiation has already been used in packaging inspection for quality control, chemical composition analysis, and security screening. Submillimeter spatial resolution imaging and incredibly specific molecular recognition are other compelling uses for terahertz radiation. To apply terahertz radiation beyond laboratory or close range ( 100 mW) and efficient sources are required to see through highly attenuating obscurants (including the atmosphere). The fundamental limitations in the performances of FIR molecular gas lasers reside in the molecular gas physics due to the so-called vibrational bottleneck. We seek to overcome the resulting challenges through novel optical designs of the feedback structures. To undertake this task, we generalize previous works to allow for a realistic description of the spatio-temporal dynamics characterizing the molecular collisional and diffusion processes. This work expands the current understanding of lasing action in FIR gas lasers and, thus, could contribute to the development of a new class of terahertz sources able to operate efficiently at room temperature. The advent of quantum cascade lasers to replace CO₂ pump lasers may combine to produce truly compact submillimeter-wave laser sources in the near future.by Song-Liang Chua.Ph.D

Larger-area single-mode photonic crystal surface-emitting lasers enabled by an accidental Dirac point

By altering the lattice geometry of the photonic crystal (PhC) surface-emitting lasers (PCSELs), we tune the regular lasing band edges of quadratic dispersions to form a single accidental Dirac point of linear dispersion at the Brillouin zone center. This not only increases the mode spacing by orders of magnitude but also eliminates the distributed in-plane feedback to enable single-mode PCSELs of substantially larger area and thus substantially higher output power. The advantages of using accidental Dirac cones are systematically evaluated through two-dimensional in-plane calculations and confirmed by three-dimensional simulations of PhC slab devices.National Science Foundation (U.S.) (MRSEC Program, award no. DMR-0819762)United States. Army Research Office. Institute for Soldier Nanotechnologies (Contract W911NF-13-D-0001)United States. Dept. of Energy (MIT S3TEC Energy Research Frontier Center, Grant No. DE-SC0001299)Spain. Ministerio de Ciencia e Innovación (Ramon-y-Cajal program, Grant No. RyC-2009-05489

Bloch surface eigenstates within the radiation continuum

From detailed numerical calculations, we demonstrate that in simple photonic crystal structures, a discrete number of Bloch surface-localized eigenstates can exist inside the continuum of free-space modes. Coupling to the free space causes the surface modes to leak, but the forward and back-reflected leakage may interfere destructively to create a perfectly bound surface state with zero leakage. We perform analytical temporal coupled-mode theory analysis to show the generality of such phenomenon and its robustness from variations of system parameters. Periodicity, time-reversal invariance, two-fold rotational symmetry and a perfectly reflecting boundary are necessary for these unique states.United States. Dept. of Energy. Office of Science (Solid-State Solar-Thermal Energy Conversion Center Grant DE-SC0001299)National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Award DMR-0819762)Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (Contract W911NF-07-D0004

Synergistic Microbial Consortium for Bioenergy Generation from Complex Natural Energy Sources

Microbial species have evolved diverse mechanisms for utilization of complex carbon sources. Proper combination of targeted species can affect bioenergy production from natural waste products. Here, we established a stable microbial consortium with Escherichia coli and Shewanella oneidensis in microbial fuel cells (MFCs) to produce bioenergy from an abundant natural energy source, in the form of the sarcocarp harvested from coconuts. This component is mostly discarded as waste. However, through its usage as a feedstock for MFCs to produce useful energy in this study, the sarcocarp can be utilized meaningfully. The monospecies S. oneidensis system was able to generate bioenergy in a short experimental time frame while the monospecies E. coli system generated significantly less bioenergy. A combination of E. coli and S. oneidensis in the ratio of 1 : 9 (v : v) significantly enhanced the experimental time frame and magnitude of bioenergy generation. The synergistic effect is suggested to arise from E. coli and S. oneidensis utilizing different nutrients as electron donors and effect of flavins secreted by S. oneidensis. Confocal images confirmed the presence of biofilms and point towards their importance in generating bioenergy in MFCs

C-di-GMP regulates <i>Pseudomonas aeruginosa</i> stress response to tellurite during both planktonic and biofilm modes of growth

Stress response plays an important role on microbial adaptation under hostile environmental conditions. It is generally unclear how the signaling transduction pathway mediates a stress response in planktonic and biofilm modes of microbial communities simultaneously. Here, we showed that metalloid tellurite (TeO(3)(2–)) exposure induced the intracellular content of the secondary messenger cyclic di-GMP (c-di-GMP) of Pseudomonas aeruginosa. Two diguanylate cyclases (DGCs), SadC and SiaD, were responsible for the increased intracellular content of c-di-GMP. Enhanced c-di-GMP levels by TeO(3)(2–) further increased P. aeruginosa biofilm formation and resistance to TeO(3)(2–). P. aeruginosa ΔsadCΔsiaD and PAO1/p(lac)-yhjH mutants with low intracellular c-di-GMP content were more sensitive to TeO(3)(2–) exposure and had low relative fitness compared to the wild-type PAO1 planktonic and biofilm cultures exposed to TeO(3)(2–). Our study provided evidence that c-di-GMP level can play an important role in mediating stress response in microbial communities during both planktonic and biofilm modes of growth

Spawning rings of exceptional points out of Dirac cones

The Dirac cone underlies many unique electronic properties of graphene and topological insulators, and its band structure--two conical bands touching at a single point--has also been realized for photons in waveguide arrays, atoms in optical lattices, and through accidental degeneracy. Deformations of the Dirac cone often reveal intriguing properties; an example is the quantum Hall effect, where a constant magnetic field breaks the Dirac cone into isolated Landau levels. A seemingly unrelated phenomenon is the exceptional point, also known as the parity-time symmetry breaking point, where two resonances coincide in both their positions and widths. Exceptional points lead to counter-intuitive phenomena such as loss-induced transparency, unidirectional transmission or reflection, and lasers with reversed pump dependence or single-mode operation. These two fields of research are in fact connected: here we discover the ability of a Dirac cone to evolve into a ring of exceptional points, which we call an "exceptional ring." We experimentally demonstrate this concept in a photonic crystal slab. Angle-resolved reflection measurements of the photonic crystal slab reveal that the peaks of reflectivity follow the conical band structure of a Dirac cone from accidental degeneracy, whereas the complex eigenvalues of the system are deformed into a two-dimensional flat band enclosed by an exceptional ring. This deformation arises from the dissimilar radiation rates of dipole and quadrupole resonances, which play a role analogous to the loss and gain in parity-time symmetric systems. Our results indicate that the radiation that exists in any open system can fundamentally alter its physical properties in ways previously expected only in the presence of material loss and gain

Potentiation of Anticancer Drugs: Effects of Pentoxifylline on Neoplastic Cells

The drug efflux activity of P-glycoprotein (P-gp, a product of the mdr1 gene, ABCB1 member of ABC transporter family) represents a mechanism by which tumor cells escape death induced by chemotherapeutics. In this study, we investigated the mechanisms involved in the effects of pentoxifylline (PTX) on P-gp-mediated multidrug resistance (MDR) in mouse leukemia L1210/VCR cells. Parental sensitive mouse leukemia cells L1210, and multidrug-resistant cells, L1210/VCR, which are characterized by the overexpression of P-gp, were used as experimental models. The cells were exposed to 100 μmol/L PTX in the presence or absence of 1.2 μmol/L vincristine (VCR). Western blot analysis indicated a downregulation of P-gp protein expression when multidrug-resistant L1210/VCR cells were exposed to PTX. The effects of PTX on the sensitization of L1210/VCR cells to VCR correlate with the stimulation of apoptosis detected by Annexin V/propidium iodide apoptosis necrosis kit and proteolytic activation of both caspase-3 and caspase-9 monitored by Western blot analysis. Higher release of matrix metalloproteinases (MMPs), especially MMP-2, which could be attenuated by PTX, was found in L1210/VCR than in L1210 cells by gelatin zymography in electrophoretic gel. Exposure of resistant cells to PTX increased the content of phosphorylated Akt kinase. In contrast, the presence of VCR eliminated the effects of PTX on Akt kinase phosphorylation. Taken together, we conclude that PTX induces the sensitization of multidrug-resistant cells to VCR via downregulation of P-gp, stimulation of apoptosis and reduction of MMPs released from drug-resistant L1210/VCR cells. These facts bring new insights into the mechanisms of PTX action on cancer cells

Calculated energy performance certificate of a duplex house and energy renovation analysis

V diplomski nalogi je predstavljena izdelava računske energetske izkaznice s poudarkom na priporočilih za energetsko sanacijo dvostanovanjske stavbe, locirane v Medvodah. Stavba je bila zgrajena leta 1985. V tem obdobju se je začela uporabljati toplotna izolacija, saj so se že pojavljala vprašanja o energetski učinkovitosti stavb. Energetska izkaznica stavb je potrebna pri prodaji in najemu stavbe, če je obdobje najema daljše od enega leta. Obvezna je tudi pri novogradnji kot del projekta izvedenih del. V uvodnem delu sem predstavil veljavno zakonodajo, ki ureja področje učinkovite rabe energije v stavbah. Potem sem s pomočjo programa TEDI in TOST izdelal računsko energetsko izkaznico za obravnavani objekt v obstoječem stanju, nato pa sem se osredotočil na učinkovitost posameznega ukrepa energetske sanacije. Pri vsakem ukrepu sem komentiral dobljene rezultate in se odločil o smiselnosti njegove izvedbe v odvisnosti od tehnične zahtevnosti in posledičnega prihranka energije. Pri obravnavani stavbi se največ energije, potrebne za ogrevanje z menjavo transparentnega dela in mehanskim prezračevanjem, prihrani z vračanjem toplote odpadnega zraka, in sicer blizu 50 %. Z izborom vseh ukrepov, za katere menim, da so smiselni, pa se prihrani preko 80 % potrebne letne toplote za ogrevanje stavbe. Tudi koeficient specifičnih transmisijskih izgub se zmanjša za 60 %. Poveča pa se potreba po hlajenju, kar je tipična stranska škoda zelo dobro toplotno izoliranih stavb. V zaključnem delu so predstavljene ugotovitve, nastale tekom diplomske naloge.In the undergraduate thesis, the production of the calculated energy performance certificate for the specific building is undertaken. This is done with the emphasis on recommendations for energy renovation of the duplex residential building, which is located in Medvode, Slovenia and was built in 1985. When the building was constructed, energy efficiency was already gaining importance. Consequently many buildings constructed in this period were moderately thermally insulated. An energy performance certificate is legally required when a building is offered for purchase and when it is leased for a period longer than one year. Furthermore the certificate is mandatory for newly built properties, as a basic requirement for the project plan. The thesis initially examines the current legislation, which regulates the area of energy efficiency in buildings. Next the energy performance for the considered building will be produced. This will be done with TEDI and TOST computer software. After that, individual technical measures will be analysed. The viability of these measures will be assessed in consideration of technical complexity and estimated energy savings potential. For the considered building, the greatest energy savings can be realised with exterior joinery renovation and installation of heat recovery ventilation. The implementation of booth measures accounts for about 50% of energy savings. With the comprehensive renovation, more than 80% of the annual heat required for the building would be preserved. In this case the specific heat transfer coefficient would decrease by about 60 %. On the contrary the requirements for cooling would increase, which is the common downside of a well-insulated building