Integration of glass microspheres and planar waveguides for microsphere lasers

Microsphere resonators with sizes in the micrometer range are reported to support very high Q’s of more than 109 for a fused silica microsphere. This high Q value represents many promising characteristics such as low cavity loss, long cavity life time, and narrow band width. With their remarkable characteristics, microsphere resonators can be used in various applications such as the narrow band filter, add-drop multiplexer, microlasers, and etc. In this work, the integration of microspheres with planar waveguides is the main focus. High quality neodymium-doped BK7 microspheres are fabricated and characterised to observe laser oscillation at 1.06 µm from the 4F3/2 - 4I11/2 transition. Theoretical calculation of the microsphere mode and field and the characteristic equation which describes the relation of the sphere mode numbers l, m, n to the wavelength, are obtained. The theoretical expressions of the microsphere WGM can be described with the sphere mode numbers based on spherical Bessel and Hankel functions. Coupling and quality factors of the waveguide-coupled system are obtained by integrating the overlapped fields of the microsphere and waveguide, and the study of Q factors as a function of sphere/waveguide separation are elaborated. Theoretical models to calculate the total loss and gain of the microsphere lasers and the Q which is required for lasing action, are developed. The expressions for power threshold in the microsphere and in the input waveguide are derived, and the threshold pump power as a function of sphere/waveguide separation is obtained. Experimental work to observe the WGM propagation of a passive microsphere has been carried out, and results obtained at wavelengths in the 800 nm and 1550 nm regions, and modal assignment including the evaluation of Q for each system are carried out. The experiments to observe the fluorescence and the lasing oscillation of the neodymium-doped BK7 microspheres are demonstrated. The measurement of the fluorescence lifetime of a neodymium-doped BK7 microsphere, is obtained. A laser oscillation is demonstrated with the free-space pump excitation, with the threshold pump power of 8 mW at lambda = 808nm

Computational modelling of placental amino acid transfer as an integrated system

AbstractPlacental amino acid transfer is essential for fetal development and its impairment is associated with poor fetal growth. Amino acid transfer is mediated by a broad array of specific plasma membrane transporters with overlapping substrate specificity. However, it is not fully understood how these different transporters work together to mediate net flux across the placenta. Therefore the aim of this study was to develop a new computational model to describe how human placental amino acid transfer functions as an integrated system. Amino acid transfer from mother to fetus requires transport across the two plasma membranes of the placental syncytiotrophoblast, each of which contains a distinct complement of transporter proteins. A compartmental modelling approach was combined with a carrier based modelling framework to represent the kinetics of the individual accumulative, exchange and facilitative classes of transporters on each plasma membrane. The model successfully captured the principal features of transplacental transfer. Modelling results clearly demonstrate how modulating transporter activity and conditions such as phenylketonuria, can increase the transfer of certain groups of amino acids, but that this comes at the cost of decreasing the transfer of others, which has implications for developing clinical treatment options in the placenta and other transporting epithelia

Integration of computational modeling with membrane transport studies reveals new insights into amino acid exchange transport mechanisms

Uptake of system L amino acid substrates into isolated placental plasma membrane vesicles in the absence of opposing side amino acid (zero-trans uptake) is incompatible with the concept of obligatory exchange, where influx of amino acid is coupled to efflux. We therefore hypothesized that system L amino acid exchange transporters are not fully obligatory and/or that amino acids are initially present inside the vesicles. To address this, we combined computational modeling with vesicle transport assays and transporter localization studies to investigate the mechanism(s) mediating [14C]L-serine (a system L substrate) transport into human placental microvillous plasma membrane (MVM) vesicles. The carrier model provided a quantitative framework to test the 2 hypotheses that L-serine transport occurs by either obligate exchange or nonobligate exchange coupled with facilitated transport (mixed transport model). The computational model could only account for experimental [14C]L-serine uptake data when the transporter was not exclusively in exchange mode, best described by the mixed transport model. MVM vesicle isolates contained endogenous amino acids allowing for potential contribution to zero-trans uptake. Both L-type amino acid transporter (LAT)1 and LAT2 subtypes of system L were distributed to MVM, with L-serine transport attributed to LAT2. These findings suggest that exchange transporters do not function exclusively as obligate exchangers.—Widdows, K. L., Panitchob, N., Crocker, I. P., Please, C. P., Hanson, M. A., Sibley, C. P., Johnstone, E. D., Sengers, B. G., Lewis, R. M., Glazier, J. D. Integration of computational modeling with membrane transport studies reveals new insights into amino acid exchange transport mechanisms

A comprehensive experimental study of whispering gallery modes in a cylindrical microresonator excited by a tilted fiber taper

Whispering gallery modes (WGMs) excitation in a cylindrical microresonator formed by a section of silica optical fiber has been studied. Evanescent light coupling into the microresonator is realized using a tapered optical fiber, fabricated by a microheater brushing technique. Several types of silica fibers with different diameters are studied as microresonators, and the influence of the resonator's diameter on the excitation of WGMs is investigated. The excitation of WGMs in a cylindrical fiber resonator were studied with changes to the tilt angle between the microcylinder and the fiber taper in the range of angles from a perpendicular position (0°) to large tilt angles (24°). The evolution of the fiber taper transmission spectrum with the change of the tilt angle results in changes in the intensity, broadening of and a blue shift in the WGM resonance spectra. Overall losses in the taper transmission spectrum decrease with the increase of the taper tilt angle from its perpendicular position, followed by a complete disappearance of the WGM resonances at large tilt angles greater than 20°

Computational modelling of amino acid exchange and facilitated transport in placental membrane vesicles

AbstractPlacental amino acid transport is required for fetal development and impaired transport has been associated with poor fetal growth. It is well known that placental amino acid transport is mediated by a broad array of specific membrane transporters with overlapping substrate specificity. However, it is not fully understood how these transporters function, both individually and as an integrated system. We propose that mathematical modelling could help in further elucidating the underlying mechanisms of how these transporters mediate placental amino acid transport.The aim of this work is to model the sodium independent transport of serine, which has been assumed to follow an obligatory exchange mechanism. However, previous amino acid uptake experiments in human placental microvillous plasma membrane vesicles have persistently produced results that are seemingly incompatible with such a mechanism; i.e. transport has been observed under zero-trans conditions, in the absence of internal substrates inside the vesicles to drive exchange. This observation raises two alternative hypotheses; (i) either exchange is not fully obligatory, or (ii) exchange is indeed obligatory, but an unforeseen initial concentration of amino acid substrate is present within the vesicle which could drive exchange.To investigate these possibilities, a mathematical model for tracer uptake was developed based on carrier mediated transport, which can represent either facilitated diffusion or obligatory exchange (also referred to as uniport and antiport mechanisms, respectively). In vitro measurements of serine uptake by placental microvillous membrane vesicles were carried out and the model applied to interpret the results based on the measured apparent Michaelis–Menten parameters Km and Vmax. In addition, based on model predictions, a new time series experiment was implemented to distinguish the hypothesised transporter mechanisms. Analysis of the results indicated the presence of a facilitated transport component, while based on the model no evidence for substantial levels of endogenous amino acids within the vesicle was found

Integrated microsphere planar lightwave circuits

Multicomponent glass microspheres self-assembled on optical waveguides combine tailored optical properties with strong light/material interaction potentially leading to compact low-power photonic devices. Progress and prospects for microsphere/waveguide integration will be described

DNA methylation of amino acid transporter genes in the human placenta

IntroductionPlacental transfer of amino acids via amino acid transporters is essential for fetal growth. Little is known about the epigenetic regulation of amino acid transporters in placenta. This study investigates the DNA methylation status of amino acid transporters and their expression across gestation in human placenta.MethodsBeWo cells were treated with 5-aza-2′-deoxycytidine to inhibit methylation and assess the effects on amino acid transporter gene expression. The DNA methylation levels of amino acid transporter genes in human placenta were determined across gestation using DNA methylation array data. Placental amino acid transporter gene expression across gestation was also analysed using data from publically available Gene Expression Omnibus data sets. The expression levels of these transporters at term were established using RNA sequencing data.ResultsInhibition of DNA methylation in BeWo cells demonstrated that expression of specific amino acid transporters can be inversely associated with DNA methylation. Amino acid transporters expressed in term placenta generally showed low levels of promoter DNA methylation. Transporters with little or no expression in term placenta tended to be more highly methylated at gene promoter regions. The transporter genes SLC1A2, SLC1A3, SLC1A4, SLC7A5, SLC7A11 and SLC7A10 had significant changes in enhancer DNA methylation across gestation, as well as gene expression changes across gestation.ConclusionThis study implicates DNA methylation in the regulation of amino acid transporter gene expression. However, in human placenta, DNA methylation of these genes remains low across gestation and does not always play an obvious role in regulating gene expression, despite clear evidence for differential expression as gestation proceeds

All silicon waveguide spherical microcavity coupler device

[EN] This paper was published in OPTICS EXPRESS and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://dx.doi.org/ 10.1364/OE.19.003185. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under lawA coupler based on silicon spherical microcavities coupled to silicon waveguides for telecom wavelengths is presented. The light scattered by the microcavity is detected and analyzed as a function of the wavelength. The transmittance signal through the waveguide is strongly attenuated (up to 25 dB) at wavelengths corresponding to the Mie resonances of the microcavity. The coupling between the microcavity and the waveguide is experimentally demonstrated and theoretically modeled with the help of FDTD calculations. © 2011 Optical Society of America.The authors wish to acknowledge financial support from projects FIS2009-07812; Consolider Nanolight.es 2007/0046 and Nº 1841; the Spanish Education and Science Ministry, TEC2008- 06145; the Generalitat Valenciana, project PROMETEO/2008/092 and PROMETEO/2010/043; and project Apoyo a la investigación 2009 from Universidad Politecnica de Valencia, nº reg. 4325. E. Xifré-Pérez acknowledges the financial support from the program Juan de la Cierva (Spanish Ministerio de Educación y Ciencia). J. D. Doménech acknowledges the FPI research grant BES-2009-018381. Finally we thank Prof. J. Garcia de Abajo for providing us with the MESME theoretical program we have used in the calculation of electric field intensity distribution of the Mie modes.Xifre Perez, E.; Doménech Gómez, JD.; Fenollosa Esteve, R.; Muñoz Muñoz, P.; Capmany Francoy, J.; Meseguer Rico, FJ. (2011). All silicon waveguide spherical microcavity coupler device. Optics Express. 19(4):3185-3192. https://doi.org/10.1364/OE.19.003185S31853192194Cai, M., Painter, O., Vahala, K. J., & Sercel, P. C. (2000). Fiber-coupled microsphere laser. Optics Letters, 25(19), 1430. doi:10.1364/ol.25.001430Knight, J. C., Dubreuil, N., Sandoghdar, V., Hare, J., Lefèvre-Seguin, V., Raimond, J. M., & Haroche, S. (1995). Mapping whispering-gallery modes in microspheres with a near-field probe. Optics Letters, 20(14), 1515. doi:10.1364/ol.20.001515Lefèvre-Seguin, V., & Haroche, S. (1997). Towards cavity-QED experiments with silica microspheres. Materials Science and Engineering: B, 48(1-2), 53-58. doi:10.1016/s0921-5107(97)00080-9Gorodetsky, M. L., Savchenkov, A. A., & Ilchenko, V. S. (1996). Ultimate Q of optical microsphere resonators. Optics Letters, 21(7), 453. doi:10.1364/ol.21.000453Vernooy, D. W., Ilchenko, V. S., Mabuchi, H., Streed, E. W., & Kimble, H. J. (1998). High-Q measurements of fused-silica microspheres in the near infrared. Optics Letters, 23(4), 247. doi:10.1364/ol.23.000247Vahala, K. J. (2003). Optical microcavities. Nature, 424(6950), 839-846. doi:10.1038/nature01939Serpengüzel, A., & Demir, A. (2008). Silicon microspheres for near-IR communication applications. Semiconductor Science and Technology, 23(6), 064009. doi:10.1088/0268-1242/23/6/064009Broaddus, D. H., Foster, M. A., Agha, I. H., Robinson, J. T., Lipson, M., & Gaeta, A. L. (2009). Silicon-waveguide-coupled high-Q chalcogenide microspheres. Optics Express, 17(8), 5998. doi:10.1364/oe.17.005998Yilmaz, Y. O., Demir, A., Kurt, A., & Serpenguzel, A. (2005). Optical channel dropping with a silicon microsphere. IEEE Photonics Technology Letters, 17(8), 1662-1664. doi:10.1109/lpt.2005.850896Almeida, V. R., Barrios, C. A., Panepucci, R. R., & Lipson, M. (2004). All-optical control of light on a silicon chip. Nature, 431(7012), 1081-1084. doi:10.1038/nature02921Noda, S., Chutinan, A., & Imada, M. (2000). Trapping and emission of photons by a single defect in a photonic bandgap structure. Nature, 407(6804), 608-610. doi:10.1038/35036532Fenollosa, R., Meseguer, F., & Tymczenko, M. (2008). Silicon Colloids: From Microcavities to Photonic Sponges. Advanced Materials, 20(1), 95-98. doi:10.1002/adma.200701589Xifré-Pérez, E., García de Abajo, F. J., Fenollosa, R., & Meseguer, F. (2009). Photonic Binding in Silicon-Colloid Microcavities. Physical Review Letters, 103(10). doi:10.1103/physrevlett.103.103902Conwell, P. R., Barber, P. W., & Rushforth, C. K. (1984). Resonant spectra of dielectric spheres. Journal of the Optical Society of America A, 1(1), 62. doi:10.1364/josaa.1.000062García de Abajo, F. J. (1999). Multiple scattering of radiation in clusters of dielectrics. Physical Review B, 60(8), 6086-6102. doi:10.1103/physrevb.60.6086Laine, J.-P., Tapalian, C., Little, B., & Haus, H. (2001). Acceleration sensor based on high-Q optical microsphere resonator and pedestal antiresonant reflecting waveguide coupler. Sensors and Actuators A: Physical, 93(1), 1-7. doi:10.1016/s0924-4247(01)00636-7Panitchob, Y., Murugan, G. S., Zervas, M. N., Horak, P., Berneschi, S., Pelli, S., … Wilkinson, J. S. (2008). Whispering gallery mode spectra of channel waveguide coupled Microspheres. Optics Express, 16(15), 11066. doi:10.1364/oe.16.011066Taillaert, D., Van Laere, F., Ayre, M., Bogaerts, W., Van Thourhout, D., Bienstman, P., & Baets, R. (2006). Grating Couplers for Coupling between Optical Fibers and Nanophotonic Waveguides. Japanese Journal of Applied Physics, 45(8A), 6071-6077. doi:10.1143/jjap.45.6071Mukaiyama, T., Takeda, K., Miyazaki, H., Jimba, Y., & Kuwata-Gonokami, M. (1999). Tight-Binding Photonic Molecule Modes of Resonant Bispheres. Physical Review Letters, 82(23), 4623-4626. doi:10.1103/physrevlett.82.4623Smith, D. D., Chang, H., & Fuller, K. A. (2003). Whispering-gallery mode splitting in coupled microresonators. Journal of the Optical Society of America B, 20(9), 1967. doi:10.1364/josab.20.00196

An international network (PlaNet) to evaluate a human placental testing platform for chemicals safety testing in pregnancy

tThe human placenta is a critical life-support system that nourishes and protects a rapidly growing fetus; aunique organ, species specific in structure and function. We consider the pressing challenge of providingadditional advice on the safety of prescription medicines and environmental exposures in pregnancy andhow ex vivo and in vitro human placental models might be advanced to reproducible human placentaltest systems (HPTSs), refining a weight of evidence to the guidance given around compound risk assess-ment during pregnancy. The placental pharmacokinetics of xenobiotic transfer, dysregulated placentalfunction in pregnancy-related pathologies and influx/efflux transporter polymorphisms are a few caveatsthat could be addressed by HPTSs, not the specific focus of current mammalian reproductive toxicologysystems. An international consortium, “PlaNet”, will bridge academia, industry and regulators to con-sider screen ability and standardisation issues surrounding these models, with proven reproducibilityfor introduction into industrial and clinical practice