Near-field imaging of plasmonic nanopatch antennas with integrated semiconductor quantum dots

Plasmonic nanopatch antennas that incorporate dielectric gaps hundreds of picometers to several nanometers thick have drawn increasing attention over the past decade because they confine electromagnetic fields to grossly sub-diffraction-limited volumes. Substantial control over the optical properties of excitons and color centers confined within these plasmonic cavities has already been demonstrated with far-field optical spectroscopies, but near-field optical spectroscopies are essential for an improved understanding of the plasmon–emitter interaction at the nanoscale. Here, we characterize the intensity and phase-resolved plasmonic response of isolated nanopatch antennas by cathodoluminescence microscopy. Furthermore, we explore the distinction between optical and electron beam spectroscopies of coupled plasmon–exciton heterostructures to identify constraints and opportunities for future nanoscale characterization and control of hybrid nanophotonic structures. While we observe substantial Purcell enhancement in time-resolved photoluminescence spectroscopies, negligible Purcell enhancement is observed in cathodoluminescence spectroscopies of hybrid nanophotonic structures. The substantial differences in measured Purcell enhancement for electron beam and laser excitation can be understood as a result of the different selection rules for these complementary experiments. These results provide a fundamentally new understanding of near-field plasmon–exciton interactions in nanopatch antennas, which is essential for myriad emerging quantum photonic devices

Mapping the Pathways of Photo-induced Ion Migration in Organic-inorganic Hybrid Halide Perovskites

Organic-inorganic hybrid perovskites (OIHPs) exhibiting exceptional photovoltaic and optoelectronic properties are of fundamental and practical interest, owing to their tunability and low manufacturing cost. For practical applications, however, challenges such as material instability and the photocurrent hysteresis occurring in perovskite solar cells under light exposure need to be understood and addressed. While extensive investigations have suggested that ion migration is a plausible origin of these detrimental effects, detailed understanding of the ion migration pathways remains elusive. Here, we report the characterization of photo-induced ion migration in OIHPs using \textit{in situ} laser illumination inside a scanning electron microscope, coupled with secondary electron imaging, energy-dispersive X-ray spectroscopy and cathodoluminescence with varying primary electron energies. Using methylammonium lead iodide (MAPbI$_3$), formamidinium lead iodide (FAPbI$_3$) and hybrid formamidinium-methylammonium lead iodide as model systems, we observed photo-induced long-range migration of halide ions over hundreds of micrometers and elucidated the transport pathways of various ions both near the surface and inside the bulk of the OIHPs, including a surprising finding of the vertical migration of lead ions. Our study provides insights into ion migration processes in OIHPs that can aid OIHP material design and processing in future applications

Genomic run-on evaluates transcription rates for all yeast genes and identifies gene regulatory mechanisms

Most studies of eukaryotic gene regulation have been done looking at mature mRNA levels. Nevertheless, the steady-state mRNA level is the result of two opposing factors: transcription rate (TR) and mRNA degradation. Both can be important points to regulate gene expression. Here we show a new method that combines the use of nylon macroarrays and in vivo radioactive labeling of nascent RNA to quantify TRs, mRNA levels, and mRNA stabilities for all the S. cerevisiae genes. We found that during the shift from glucose to galactose, most genes undergo drastic changes in TR and mRNA stability. However, changes in mRNA levels are less pronounced. Some genes, such as those encoding mitochondrial proteins, are coordinately regulated in mRNA stability behaving as decay regulons. These results indicate that, although TR is the main determinant of mRNA abundance in yeast, modulation of mRNA stability is a key factor for gene regulation

PIK3CA dependence and sensitivity to therapeutic targeting in urothelial carcinoma

Background Many urothelial carcinomas (UC) contain activating PIK3CA mutations. In telomerase-immortalized normal urothelial cells (TERT-NHUC), ectopic expression of mutant PIK3CA induces PI3K pathway activation, cell proliferation and cell migration. However, it is not clear whether advanced UC tumors are PIK3CA-dependent and whether PI3K pathway inhibition is a good therapeutic option in such cases. Methods We used retrovirus-mediated delivery of shRNA to knock down mutant PIK3CA in UC cell lines and assessed effects on pathway activation, cell proliferation, migration and tumorigenicity. The effect of the class I PI3K inhibitor GDC-0941 was assessed in a panel of UC cell lines with a range of known molecular alterations in the PI3K pathway. Results Specific knockdown of PIK3CA inhibited proliferation, migration, anchorage-independent growth and in vivo tumor growth of cells with PIK3CA mutations. Sensitivity to GDC-0941 was dependent on hotspot PIK3CA mutation status. Cells with rare PIK3CA mutations and co-occurring TSC1 or PTEN mutations were less sensitive. Furthermore, downstream PI3K pathway alterations in TSC1 or PTEN or co-occurring AKT1 and RAS gene mutations were associated with GDC-0941 resistance. Conclusions Mutant PIK3CA is a potent oncogenic driver in many UC cell lines and may represent a valuable therapeutic target in advanced bladder cancer

Pre-Micro RNA Signatures Delineate Stages of Endothelial Cell Transformation in Kaposi Sarcoma

MicroRNAs (miRNA) have emerged as key regulators of cell lineage differentiation and cancer. We used precursor miRNA profiling by a novel real-time QPCR method (i) to define progressive stages of endothelial cell transformation cumulating in Kaposi sarcoma (KS) and (ii) to identify specific miRNAs that serve as biomarkers for tumor progression. We were able to compare primary patient biopsies to well-established culture and mouse tumor models. Loss of mir-221 and gain of mir-15 expression demarked the transition from merely immortalized to fully tumorigenic endothelial cells. Mir-140 and Kaposi sarcoma–associated herpesvirus viral miRNAs increased linearly with the degree of transformation. Mir-24 emerged as a biomarker specific for KS

Ultrasonic nondestructive evaluation of thin viscoelastic plates : the inverse problem

Vita.The acoustic response of a think, linear-viscoelastic plate is completely described by four of its properties: thickness, wavespeed, density and attenuation. The qualifer "thin" is used to describe a situation when the duration of the incident pulse (or the length of the incident pulse) exceeds the round-trip time (or the twice the specimen thickness), i.e any two successive echoes, due to the incident pulse, from the front and the back surfaces of the plate are inseparable. In this dissertation, a fequency-domain technique for the ultrasonic nondestructive evaluation of thin plates is presented. The technique utilizes plane-longitudinal waves normally incident upon a viscoelastic plate immersed in an elastic fluid (water). The through-transmission transfer function of a plate is examined in detail for a variety of engineering materials. The theory and the experiment are compared for engineering materials over a wide range of frequencies. The comparison is found to be excellent except at certain frequencies; the discrepancy is attrbuted to (conjectured) excitation of the spurious leaky-Lamb waves. A systmeatic analysis of the sensitivity of the transfer function to each of the acoustical properties is presented. The inverse problem, defined as the determination of the acoustical properties from the comparison of the measured and predicted transfer functions, is solved for the following cases: (a) measurment of any one of the four acoustical properties, given the remaining three; the inversion algorithm utilizes the well-known secant method. (b) simultaneous measurements of two, three or all of the four acoustical properties; the inversion algorithm utilizes the Gauss method in conjunction with the Incremental search method. The technique is used to measure the acoustical properties of two classes of materials: thin elastic plates made up aluminum, stainless steel, glass, tungsten, and leaded-brass; and thin visocelastic plates made up of Plexiglas, graphite/epoxy composite and bulk aerospace adhesive. The errors are found to be frequency dependent and become progressively worse as frequency decreases. Finally, a new technique is developed for the simultaneous measurment of thickness and wavespeed in thin adhesive layers bonding a pair fo thick adherends. This technique is automated and computer-controlled and can be easily adapted for in-situ applications. Results are presented for aluminum-bonded joints and the properties of the cured bond are compared with that of the bulk adhesive

Ultrafast Energy Dynamics of Two Dimensional Semiconducting and Topological Materials

Invention of the transistor in the 1950s marked a revolution in the history of scientific advancement. Today, our lives are propelled by the tremendous power of electronics which is within everyone’s reach. This has been possible due to the thorough understanding of the transistor and other related electronic components. Silicon, which is abundantly available in nature, has been the material of choice for fabricating most of the electronic components we use today. It is a very robust and well performing semiconductor. However, we are reaching a stage where silicon electronics can no longer be improved, and the scientific community is keenly looking for alternatives to achieve continued progress. The search for faster computing and better electronic devices has led to the opening up of several promising routes, which are being actively researched. These include spintronics, valleytronics, topological computing, quantum computing and exotic two-dimensional (2D) material based systems, to name a few. Each field has seen a tremendous amount of effort in the past decade. Broadly, this thesis explores some of the fundamental aspects of 2D semiconducting and topological materials, via an optical spectroscopy approach. The technique of choice is femtosecond ultrafast pump and probe laser spectroscopy, which has proven to be a powerful tool for investigating fundamental microscopic phenomena. First, we look at emerging 2D materials, black phosphorus and tellurium, which have useful properties such as anisotropic electronic and optical response, good carrier mobility and direct bandgap. Black phosphorus and tellurium are Van der Waals materials, similar to graphene and hence can be cleaved using scotch tape or in liquid phase to obtain thin samples. In this work, a 65 nm black phosphorus flake was obtained and its relaxation following optical excitation was probed from near-infrared (1500 nm) to mid-infrared (4500 nm) wavelengths. The bandgap of the material was clearly resolved and relaxation times in the sub 100 ps range was observed. Wavelength dependent electron-phonon coupling and recombination was observed, which points to the importance of choosing the correct probing energies in ultrafast experiments. An upper estimate of the carrier mobility was obtained from the extracted electron-phonon scattering time. Moreover, the spectral evolution of reflectance was used to obtain hot carrier relaxation times by modeling with a Fermi-Dirac distribution. Lastly, surface oxidation effects were proposed to explain anomalous features in the transient data. Tellurium flakes with thicknesses ranging from 12 nm to 160 nm were obtained by solution growth and a strong dependence of the recombination times on thickness was observed. Thin flakes show a fast decay on the order of 20 ps, whereas thicker flakes have a decay in the 100s of ps range. The recombination mechanism in thin flakes was attributed to fast carrier capture by midgap defect states arising from surface defects and that in the thick flakes to radiative recombination. Recombination coefficients were extracted using a diffusion-recombination model. A surface and bulk scattering model was used to qualitatively explain the observed thickness dependent field effect mobilities in literature. Next, we look at another emerging material, Bi2Te2Se (BTS221). This material is a Van der Waals topological insulator and can be cleaved using scotch tape. Similar to black phosphorus and tellurium, BTS221 is also a direct band gap semiconductor, with a comparable bandgap. However, BTS221 possesses an exotic property due to its topological nature. It has Dirac like energy states on the surface, crossing the bandgap