Publications by authors named "Dario Gerace"

27 Publications

  • Page 1 of 1

Generation of a heterozygous GAPDH-Luciferase human ESC line (HVRDe008-A-1) for in vivo monitoring of stem cells and their differentiated progeny.

Stem Cell Res 2021 05 27;53:102371. Epub 2021 Apr 27.

Department of Stem Cell and Regenerative Biology, Harvard University, Howard Hughes Medical Institute, Harvard Stem Cell Institute, Boston MA, USA. Electronic address:

Human stem cell-derived beta (SC-β) cells are a candidate for cell replacement therapy for type 1 diabetes. Whilst refinements to the differentiation protocol have resulted in the production of SC-β cells that resemble adult beta cells, the unsolved challenge to protect transplanted SC-β cells from the host immune system remains. To monitor the survival of SC-β cells in vivo, we knocked-in the Firefly luciferase gene into the GAPDH locus of the HUES8 human embryonic stem cell (hESC) line, such that differentiated islet cells constitutively express luciferase.
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http://dx.doi.org/10.1016/j.scr.2021.102371DOI Listing
May 2021

Dynamical Localization Simulated on Actual Quantum Hardware.

Entropy (Basel) 2021 May 23;23(6). Epub 2021 May 23.

Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell'Insubria, Via Valleggio 11, 22100 Como, Italy.

Quantum computers are invaluable tools to explore the properties of complex quantum systems. We show that dynamical localization of the quantum sawtooth map, a highly sensitive quantum coherent phenomenon, can be simulated on actual, small-scale quantum processors. Our results demonstrate that quantum computing of dynamical localization may become a convenient tool for evaluating advances in quantum hardware performances.
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http://dx.doi.org/10.3390/e23060654DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8224768PMC
May 2021

Machine learning workflows identify a microRNA signature of insulin transcription in human tissues.

iScience 2021 Apr 31;24(4):102379. Epub 2021 Mar 31.

Centre for Transplant and Renal Research, Westmead Institute for Medical Research, University of Sydney, 176 Hawkesbury Road, Westmead, NSW 2145, Australia.

Dicer knockout mouse models demonstrated a key role for microRNAs in pancreatic β-cell function. Studies to identify specific microRNA(s) associated with human (pro-)endocrine gene expression are needed. We profiled microRNAs and key pancreatic genes in 353 human tissue samples. Machine learning workflows identified microRNAs associated with (pro-)insulin transcripts in a discovery set of islets (n = 30) and insulin-negative tissues (n = 62). This microRNA signature was validated in remaining 261 tissues that include nine islet samples from individuals with type 2 diabetes. Top eight microRNAs (miR-183-5p, -375-3p, 216b-5p, 183-3p, -7-5p, -217-5p, -7-2-3p, and -429-3p) were confirmed to be associated with and predictive of (pro-)insulin transcript levels. Use of doxycycline-inducible microRNA-overexpressing human pancreatic duct cell lines confirmed the regulatory roles of these microRNAs in (pro-)endocrine gene expression. Knockdown of these microRNAs in human islet cells reduced (pro-)insulin transcript abundance. Our data provide specific microRNAs to further study microRNA-mRNA interactions in regulating insulin transcription.
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http://dx.doi.org/10.1016/j.isci.2021.102379DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8082091PMC
April 2021

Doubly-Resonant Photonic Crystal Cavities for Efficient Second-Harmonic Generation in III-V Semiconductors.

Nanomaterials (Basel) 2021 Feb 28;11(3). Epub 2021 Feb 28.

Dipartimento di Fisica, Università di Pavia, via Bassi 6, 27100 Pavia, Italy.

Second-order nonlinear effects, such as second-harmonic generation, can be strongly enhanced in nanofabricated photonic materials when both fundamental and harmonic frequencies are spatially and temporally confined. Practically designing low-volume and doubly-resonant nanoresonators in conventional semiconductor compounds is challenging owing to their intrinsic refractive index dispersion. In this work we review a recently developed strategy to design doubly-resonant nanocavities with low mode volume and large quality factor via localized defects in a photonic crystal structure. We built on this approach by applying an evolutionary optimization algorithm in connection with Maxwell equations solvers; the proposed design recipe can be applied to any material platform. We explicitly calculated the second-harmonic generation efficiency for doubly-resonant photonic crystal cavity designs in typical III-V semiconductor materials, such as GaN and AlGaAs, while targeting a fundamental harmonic at telecom wavelengths and fully accounting for the tensor nature of the respective nonlinear susceptibilities. These results may stimulate the realization of small footprint photonic nanostructures in leading semiconductor material platforms to achieve unprecedented nonlinear efficiencies.
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http://dx.doi.org/10.3390/nano11030605DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7997186PMC
February 2021

Charging a quantum battery via nonequilibrium heat current.

Phys Rev E 2020 Dec;102(6-1):062133

Instituto de Física, Universidade Federal do Rio de Janeiro, CP68528, Rio de Janeiro, Rio de Janeiro 21941-972, Brazil.

When a quantum system is subject to a thermal gradient it may sustain a steady nonequilibrium heat current by entering into a so-called nonequilibrium steady state (NESS). Here we show that NESS constitute a thermodynamic resource that can be exploited to charge a quantum battery. This adds to the list of recently reported sources available at the nanoscale, such as coherence, entanglement, and quantum measurements. We elucidate this concept by showing analytic and numerical studies of a two-qubit quantum battery that is alternatively charged by an incoherent heat flow and discharged by application of a properly chosen unitary gate. The presence of a NESS for the charging step guarantees steady operation with positive power output. Decreasing the duration of the charging step results in a time-periodic steady state accompanied by increased efficiency and output power. The device is amenable to implementation with different nanotechnology platforms.
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http://dx.doi.org/10.1103/PhysRevE.102.062133DOI Listing
December 2020

Metasurface Integrated Monolayer Exciton Polariton.

Nano Lett 2020 Jul 17;20(7):5292-5300. Epub 2020 Jun 17.

Electrical and Computer Engineering, University of Washington, Seattle, Washington 98189, United States.

Monolayer transition-metal dichalcogenides (TMDs) are the first truly two-dimensional (2D) semiconductor, providing an excellent platform to investigate light-matter interaction in the 2D limit. The inherently strong excitonic response in monolayer TMDs can be further enhanced by exploiting the temporal confinement of light in nanophotonic structures. Here, we demonstrate a 2D exciton-polariton system by strongly coupling atomically thin tungsten diselenide (WSe) monolayer to a silicon nitride (SiN) metasurface. Via energy-momentum spectroscopy of the WSe-metasurface system, we observed the characteristic anticrossing of the polariton dispersion both in the reflection and photoluminescence spectrum. A Rabi splitting of 18 meV was observed which matched well with our numerical simulation. Moreover, we showed that the Rabi splitting, the polariton dispersion, and the far-field emission pattern could be tailored with subwavelength-scale engineering of the optical meta-atoms. Our platform thus opens the door for the future development of novel, exotic exciton-polariton devices by advanced meta-optical engineering.
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http://dx.doi.org/10.1021/acs.nanolett.0c01624DOI Listing
July 2020

Tailoring Dispersion of Room-Temperature Exciton-Polaritons with Perovskite-Based Subwavelength Metasurfaces.

Nano Lett 2020 Mar 24;20(3):2113-2119. Epub 2020 Feb 24.

Université de Lyon, Institut des Nanotechnologies de Lyon, INL-UMR5270, CNRS, Ecole Centrale de Lyon, 36 avenue Guy de Collongue, F-69134 Ecully, France.

Exciton-polaritons represent a promising platform for studying quantum fluids of light and realizing prospective all-optical devices. Here we report on the experimental demonstration of exciton-polaritons at room temperature in resonant metasurfaces made from a sub-wavelength two-dimensional lattice of perovskite pillars. The strong coupling regime is revealed by both angular-resolved reflectivity and photoluminescence measurements, showing anticrossing between photonic modes and the exciton resonance with a Rabi splitting in the 200 meV range. Moreover, by tailoring the photonic Bloch mode to which perovskite excitons are coupled, polaritonic dispersions are engineered exhibiting linear, parabolic, and multivalley dispersions. All of our results are perfectly reproduced by both numerical simulations based on a rigorous coupled wave analysis and an elementary model based on a quantum theory of radiation-matter interaction. Our results suggest a new approach to study exciton-polaritons and pave the way toward large-scale and low-cost integrated polaritonic devices operating at room temperature.
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http://dx.doi.org/10.1021/acs.nanolett.0c00125DOI Listing
March 2020

Optimal efficiency of the Q-cycle mechanism around physiological temperatures from an open quantum systems approach.

Sci Rep 2019 11 13;9(1):16657. Epub 2019 Nov 13.

Department of Physics, University of Pavia, I-27100, Pavia, Italy.

The Q-cycle mechanism entering the electron and proton transport chain in oxygenic photosynthesis is an example of how biological processes can be efficiently investigated with elementary microscopic models. Here we address the problem of energy transport across the cellular membrane from an open quantum system theoretical perspective. We model the cytochrome [Formula: see text] protein complex under cyclic electron flow conditions starting from a simplified kinetic model, which is hereby revisited in terms of a Markovian quantum master equation formulation and spin-boson Hamiltonian treatment. We apply this model to theoretically demonstrate an optimal thermodynamic efficiency of the Q-cycle around ambient and physiologically relevant temperature conditions. Furthermore, we determine the quantum yield of this complex biochemical process after setting the electrochemical potentials to values well established in the literature. The present work suggests that the theory of quantum open systems can successfully push forward our theoretical understanding of complex biological systems working close to the quantum/classical boundary.
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http://dx.doi.org/10.1038/s41598-019-52842-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6853958PMC
November 2019

Optimal efficiency of the Q-cycle mechanism around physiological temperatures from an open quantum systems approach.

Sci Rep 2019 11 13;9(1):16657. Epub 2019 Nov 13.

Department of Physics, University of Pavia, I-27100, Pavia, Italy.

The Q-cycle mechanism entering the electron and proton transport chain in oxygenic photosynthesis is an example of how biological processes can be efficiently investigated with elementary microscopic models. Here we address the problem of energy transport across the cellular membrane from an open quantum system theoretical perspective. We model the cytochrome [Formula: see text] protein complex under cyclic electron flow conditions starting from a simplified kinetic model, which is hereby revisited in terms of a Markovian quantum master equation formulation and spin-boson Hamiltonian treatment. We apply this model to theoretically demonstrate an optimal thermodynamic efficiency of the Q-cycle around ambient and physiologically relevant temperature conditions. Furthermore, we determine the quantum yield of this complex biochemical process after setting the electrochemical potentials to values well established in the literature. The present work suggests that the theory of quantum open systems can successfully push forward our theoretical understanding of complex biological systems working close to the quantum/classical boundary.
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http://dx.doi.org/10.1038/s41598-019-52842-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6853958PMC
November 2019

High-Efficiency Lentiviral Gene Modification of Primary Murine Bone-Marrow Mesenchymal Stem Cells.

Methods Mol Biol 2019 ;2029:197-214

The School of Life Sciences and Centre for Health Technologies, University of Technology Sydney, Sydney, NSW, Australia.

Lentiviral vectors are the method of choice for stable gene modification of a variety of cell types. However, the efficiency with which they transduce target cells varies significantly, in particular their typically poor capacity to transduce primary stem cells. Here we describe the isolation and enrichment of murine bone-marrow mesenchymal stem cells (MSCs) via fluorescence-activated cell sorting (FACS); the cloning, production, and concentration of high-titer second generation lentiviral vectors via combined tangential flow filtration (TFF) and ultracentrifugation; and the subsequent high-efficiency gene modification of MSCs into insulin-producing cells via overexpression of the furin-cleavable human insulin (INS-FUR) gene.
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http://dx.doi.org/10.1007/978-1-4939-9631-5_16DOI Listing
April 2020

Expansion of Murine MSC Impairs Transcription Factor-Induced Differentiation into Pancreatic -Cells.

Stem Cells Int 2019 10;2019:1395301. Epub 2019 Mar 10.

The School of Life Sciences and Centre for Health Technologies, Faculty of Science, University of Technology Sydney, Sydney, Australia.

Combinatorial gene and cell therapy as a means of generating surrogate -cells has been investigated for the treatment of type 1 diabetes (T1D) for a number of years with varying success. One of the limitations of current cell therapies for T1D is the inability to generate sufficient quantities of functional transplantable insulin-producing cells. Due to their impressive immunomodulatory properties, in addition to their ease of expansion and genetic modification , mesenchymal stem cells (MSCs) are an attractive alternative source of adult stem cells for regenerative medicine. To overcome the aforementioned limitation of current therapies, we assessed the utility of expanded bone marrow-derived murine MSCs for their persistence in immune-competent and immune-deficient animal models and their ability to differentiate into surrogate -cells. CD45/Ly6 murine MSCs were isolated from the bone marrow of nonobese diabetic (NOD) mice and nucleofected to express the bioluminescent protein, . The persistence of a subcutaneous (s.c.) transplant of -expressing MSCs was assessed in immune-competent (NOD) ( = 4) and immune-deficient (NOD/) ( = 4) animal models of diabetes. -expressing MSCs persisted for 2 and 12 weeks, respectively, in NOD and NOD/ mice. expanded MSCs were transduced with the HMD lentiviral vector (MOI = 10) to express furin-cleavable human insulin () and murine and . This was followed by the characterization of pancreatic transdifferentiation via reverse transcriptase polymerase chain reaction (RT-PCR) and static and glucose-stimulated insulin secretion (GSIS). -expressing MSCs were assessed for their ability to reverse diabetes after transplantation into streptozotocin- (STZ-) diabetic NOD/ mice ( = 5). Transduced MSCs did not undergo pancreatic transdifferentiation, as determined by RT-PCR analyses, lacked glucose responsiveness, and upon transplantation did not reverse diabetes. The data suggest that expanded MSCs lose their multipotent differentiation potential and may be more useful as gene therapy targets prior to expansion.
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http://dx.doi.org/10.1155/2019/1395301DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6431458PMC
March 2019

Quantum nonlinearities at the single-particle level.

Nat Mater 2019 03;18(3):200-201

CNR NANOTEC, Institute of Nanotechnology, Lecce, Italy.

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http://dx.doi.org/10.1038/s41563-019-0298-3DOI Listing
March 2019

Optimizing band-edge slow light in silicon-on-insulator waveguide gratings.

Opt Express 2018 Apr;26(7):8470-8478

A systematic analysis of photonic bands and group index in silicon grating waveguides is performed, in order to optimize band-edge slow-light behavior in integrated structures with low losses. A combination of numerical methods and perturbation theory is adopted. It is shown that a substantial increase of slow light bandwidth is achieved when decreasing the internal width of the waveguide and the silicon thickness in the cladding region. It is also observed that a reduction of the internal width does not undermine the performance of an adiabatic taper.
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http://dx.doi.org/10.1364/OE.26.008470DOI Listing
April 2018

Partial pancreatic transdifferentiation of primary human hepatocytes in the livers of a humanised mouse model.

J Gene Med 2018 05 16;20(5):e3017. Epub 2018 Apr 16.

School of Life Sciences, University of Technology Sydney, Sydney, Australia.

Background: Gene therapy is one treatment that may ultimately cure type 1 diabetes. We have previously shown that the introduction of furin-cleavable human insulin (INS-FUR) to the livers in several animal models of diabetes resulted in the reversal of diabetes and partial pancreatic transdifferentiation of liver cells. The present study investigated whether streptozotocin-diabetes could be reversed in FRG mice in which chimeric mouse-human livers can readily be established and, in addition, whether pancreatic transdifferentiation occurred in the engrafted human hepatocytes.

Methods: Engraftment of human hepatocytes was confirmed by measuring human albumin levels. Following delivery of the empty vector or the INS-FUR vector to diabetic FRG mice, mice were monitored for weight and blood glucose levels. Intraperitoneal glucose tolerance tests (IPGTTs) were performed. Expression levels of pancreatic hormones and transcription factors were determined by a reverse transcriptase-polymerase chain reaction (RT-PCR) and immunohistochemistry.

Results: Diabetes was reversed for a period of 60 days (experimental endpoint) after transduction with INS-FUR. IPGTTs of the insulin-transduced animals were not significantly different from nondiabetic animals. Immunofluorescence microscopy revealed the expression of human albumin and insulin in transduced liver samples. Quantitative RT-PCR showed expression of human and mouse endocrine hormones and β-cell transcription factors, indicating partial pancreatic transdifferentiation of mouse and human hepatocytes. Nonfasting human C-peptide levels were significantly higher than mouse levels, suggesting that transdifferentiated human hepatocytes made a significant contribution to the reversal of diabetes.

Conclusions: These data show that human hepatocytes can be induced to undergo partial pancreatic transdifferentiation in vivo, indicating that the technology holds promise for the treatment of type 1 diabetes.
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http://dx.doi.org/10.1002/jgm.3017DOI Listing
May 2018

CRISPR-targeted genome editing of mesenchymal stem cell-derived therapies for type 1 diabetes: a path to clinical success?

Stem Cell Res Ther 2017 03 9;8(1):62. Epub 2017 Mar 9.

The School of Life Sciences, Chronic Disease Solutions Team and the Centre for Health Technologies, University of Technology Sydney, PO Box 123, Broadway, NSW, 2007, Australia.

Due to their ease of isolation, differentiation capabilities, and immunomodulatory properties, the therapeutic potential of mesenchymal stem cells (MSCs) has been assessed in numerous pre-clinical and clinical settings. Currently, whole pancreas or islet transplantation is the only cure for people with type 1 diabetes (T1D) and, due to the autoimmune nature of the disease, MSCs have been utilised either natively or transdifferentiated into insulin-producing cells (IPCs) as an alternative treatment. However, the initial success in pre-clinical animal models has not translated into successful clinical outcomes. Thus, this review will summarise the current state of MSC-derived therapies for the treatment of T1D in both the pre-clinical and clinical setting, in particular their use as an immunomodulatory therapy and targets for the generation of IPCs via gene modification. In this review, we highlight the limitations of current clinical trials of MSCs for the treatment of T1D, and suggest the novel clustered regularly interspaced short palindromic repeat (CRISPR) gene-editing technology and improved clinical trial design as strategies to translate pre-clinical success to the clinical setting.
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http://dx.doi.org/10.1186/s13287-017-0511-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5345178PMC
March 2017

Digital quantum simulators in a scalable architecture of hybrid spin-photon qubits.

Sci Rep 2015 Nov 13;5:16036. Epub 2015 Nov 13.

Dipartimento di Fisica e Scienze della Terra, Università di Parma, I-43124 Parma, Italy.

Resolving quantum many-body problems represents one of the greatest challenges in physics and physical chemistry, due to the prohibitively large computational resources that would be required by using classical computers. A solution has been foreseen by directly simulating the time evolution through sequences of quantum gates applied to arrays of qubits, i.e. by implementing a digital quantum simulator. Superconducting circuits and resonators are emerging as an extremely promising platform for quantum computation architectures, but a digital quantum simulator proposal that is straightforwardly scalable, universal, and realizable with state-of-the-art technology is presently lacking. Here we propose a viable scheme to implement a universal quantum simulator with hybrid spin-photon qubits in an array of superconducting resonators, which is intrinsically scalable and allows for local control. As representative examples we consider the transverse-field Ising model, a spin-1 Hamiltonian, and the two-dimensional Hubbard model and we numerically simulate the scheme by including the main sources of decoherence.
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http://dx.doi.org/10.1038/srep16036DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4643341PMC
November 2015

Optimising apodized grating couplers in a pure SOI platform to -0.5 dB coupling efficiency.

Opt Express 2015 Jun;23(12):16289-304

We present a theoretical optimisation of 1D apodized grating couplers in a "pure" Silicon-On-Insulator (SOI) architecture, i.e. without any bottom reflector element, by means of a general mutative method. We perform a comprehensive 2D Finite Difference Time Domain study of chirped and apodized grating couplers in 220 nm SOI, and demonstrate that the global maximum coupling efficiency in that platform is capped to 65% (-1.9 dB). Moving to designs with thicker Si-layers, we identify a new record design in 340 nm SOI, with a simulated coupling efficiency of 89% (-0.5 dB). Going to thicker Si layers does not further improve the efficiency, implying that -0.5 dB may be a global maximum for a grating coupler in SOI without a bottom-reflector. Even after allowing for 193 nm UV-lithographic fabrication constraints, the 340 nm design still offers -0.7 dB efficiency. These new apodized designs are the first pure SOI couplers compatible with deep-UV lithography to offer better than -1 dB insertion losses. With only very minor changes to existing deposition and lithography recipes, they are compatible with the multi-project wafer runs already offered by Si-Photonics foundries.
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http://dx.doi.org/10.1364/OE.23.016289DOI Listing
June 2015

An all-silicon single-photon source by unconventional photon blockade.

Sci Rep 2015 Jun 10;5:11223. Epub 2015 Jun 10.

Institute of Theoretical Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.

The lack of suitable quantum emitters in silicon and silicon-based materials has prevented the realization of room temperature, compact, stable, and integrated sources of single photons in a scalable on-chip architecture, so far. Current approaches rely on exploiting the enhanced optical nonlinearity of silicon through light confinement or slow-light propagation, and are based on parametric processes that typically require substantial input energy and spatial footprint to reach a reasonable output yield. Here we propose an alternative all-silicon device that employs a different paradigm, namely the interplay between quantum interference and the third-order intrinsic nonlinearity in a system of two coupled optical cavities. This unconventional photon blockade allows to produce antibunched radiation at extremely low input powers. We demonstrate a reliable protocol to operate this mechanism under pulsed optical excitation, as required for device applications, thus implementing a true single-photon source. We finally propose a state-of-art implementation in a standard silicon-based photonic crystal integrated circuit that outperforms existing parametric devices either in input power or footprint area.
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http://dx.doi.org/10.1038/srep11223DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4462143PMC
June 2015

Optimizing polarization-diversity couplers for Si-photonics: reaching the -1dB coupling efficiency threshold.

Opt Express 2014 Jun;22(12):14769-81

Polarization-diversity couplers are low-cost industrially-scalable passive devices that can couple light of unknown polarization from a telecom fiber-mode to a pair of TE-polarized wave-guided modes in the Silicon-on-Insulator platform. These couplers offer significantly more relaxed alignment tolerances than edge-coupling schemes, which is advantageous for commercial fiber-packaging of Si-photonic circuits. However, until now, polarization-diversity couplers have not offered sufficient coupling efficiency to motivate serious commercial consideration. Using 3D finite difference time domain calculations for device optimization, we identify Silicon-on-Insulator polarization-diversity couplers with 1,550 nm coupling efficiencies of -0.95 dB and -1.9 dB, for designs with and without bottom-reflector elements, respectively. These designs offer a significant improvement over state-of-the-art performance, and effectively bridge the "performance gap" between polarization-diversity couplers and 1D-grating couplers. Our best polarization-diversity coupler design goes beyond the -1dB efficiency limit that is typically accepted as the minimum needed for industrial adoption of coupler devices in the telecoms market.
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http://dx.doi.org/10.1364/OE.22.014769DOI Listing
June 2014

Room temperature Bloch surface wave polaritons.

Opt Lett 2014 Apr;39(7):2068-71

Polaritons are hybrid light-matter quasi-particles that have gathered a significant attention for their capability of showing room temperature and out-of-equilibrium Bose-Einstein condensation. More recently, a novel class of ultrafast optical devices have been realized by using flows of polariton fluids, such as switches, interferometers, and logical gates. However, polariton lifetimes and propagation distances are strongly limited by photon losses and accessible in-plane momenta in normal microcavity samples. In this work, we show experimental evidence of the formation of room temperature propagating polariton states arising from the strong coupling between organic excitons and a Bloch surface wave. This result, which was only recently predicted, paves the way for the realization of polariton devices that could allow lossless propagation up to macroscopic distances.
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http://dx.doi.org/10.1364/OL.39.002068DOI Listing
April 2014

Broad parameter optimization of polarization-diversity 2D grating couplers for silicon photonics.

Opt Express 2013 Sep;21(18):21556-68

Polarization-diversity couplers, which are designed to couple the unknown polarization state of an optical fiber into the TE-polarized modes of integrated waveguides, are important for the development of practical all-optical circuits. We describe the use of a full 3D finite difference time domain (FDTD) calculation campaign to rigorously optimize the 2D photonic crystal grating that couples a single-mode telecom fiber to the silicon waveguides of a Silicon-on-Insulator (SOI) platform. With this approach we identify the unique optimum combination of etch-depth, hole-radius, and grating-pitch of the photonic crystal array for best performance at 1550 nm. The mean (polarization-averaged) coupling efficiency of 48% (-3.2dB) exceeds reported efficiencies of analogous couplers, and has only a marginal dependence on the polarization state of the input fiber (48 ± 3%). In addition, 3D-FDTD calculations are used to characterize the propagation direction, mode-profile, and polarization of light coupled from the fiber into the SOI slab. Such information is crucial for component design and goes beyond previously available results from existing approximations and simulations of 2D-grating coupler performance. Calculations of photonic mode dispersion in the grating coupler, by means of guided-mode expansion, indicate that the coupling is due to an optically active resonant guided mode in the photonic crystal array. This points towards a fast optimization scheme that enhances both the performance and the physical interpretation of 3D-FDTD simulations.
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http://dx.doi.org/10.1364/OE.21.021556DOI Listing
September 2013

Low-power continuous-wave generation of visible harmonics in silicon photonic crystal nanocavities.

Opt Express 2010 Dec;18(25):26613-24

Dipartimento di Fisica A Volta, Universit`a degli Studi di Pavia, 27100 Pavia, Italy.

We present the first demonstration of frequency conversion by simultaneous second- and third-harmonic generation in a silicon photonic crystal nanocavity using continuous-wave optical excitation. We observe a bright dual wavelength emission in the blue/green (450-525 nm) and red (675-790 nm) visible windows with pump powers as low as few microwatts in the telecom bands, with conversion efficiencies of ∼ 10 (-5) /W and ∼ 10/ W(2) for the second- and third-harmonic, respectively. Scaling behaviors as a function of pump power and cavity quality-factor are demonstrated for both second- and third order processes. Successful comparison of measured and calculated emission patterns indicates that third-harmonic is a bulk effect while second-harmonic is a surface-related effect at the sidewall holes boundaries. Our results are promising for obtaining practical low-power, continuous-wave and widely tunable multiple harmonic generation on a silicon chip.
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http://dx.doi.org/10.1364/OE.18.026613DOI Listing
December 2010

All-optical switching in silicon-on-insulator photonic wire nano-cavities.

Opt Express 2010 Jan;18(2):1450-61

Dipartimento di Fisica A. Volta and UdR CNISM, Università degli Studi di Pavia, Via Bassi 6, Pavia, Italy.

We report on experimental demonstration of all-optical switching in a silicon-on-insulator photonic wire nanocavity operating at telecom wavelengths. The switching is performed with a control pulse energy as low as approximately 0.1 pJ on a cavity device that presents very high signal transmission, an ultra-high quality-factor, almost diffraction-limited modal volume and a footprint of only 5 microm(2). High-speed modulation of the cavity mode is achieved by means of optical injection of free carriers using a nanosecond pulsed laser. Experimental results are interpreted by means of finite-difference time-domain simulations. The possibility of using this device as a logic gate is also demonstrated.
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http://dx.doi.org/10.1364/OE.18.001450DOI Listing
January 2010

Low-loss guided modes in photonic crystal waveguides.

Opt Express 2005 Jun;13(13):4939-51

We study disorder-induced propagation losses of guided modes in photonic crystal slabs with line-defects. These losses are treated within a theoretical model of size disorder for the etched holes in the otherwise periodic photonic lattice. Comparisons are provided with state-of-the-art experimental data, both in membrane and Silicon-on-Insulator (SOI) structures, in which propagation losses are mainly attributed to fabrication imperfections. The dependence of the losses on the photon group velocity and the useful bandwidth for low-loss propagation are analyzed and discussed for membrane and asymmetric as well as symmetric SOI systems. New designs for further improving device performances are proposed, which employ waveguides with varying channel widths. It is shown that losses in photonic crystal waveguides could be reduced by almost an order of magnitude with respect to latest experimental results. Propagation losses lower than 0.1 dB/mm are predicted for suitably designed structures, by assuming state-of-the-art fabrication accuracy.
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http://dx.doi.org/10.1364/opex.13.004939DOI Listing
June 2005

Disorder-induced losses in photonic crystal waveguides with line defects.

Opt Lett 2004 Aug;29(16):1897-9

Istituto Nazionale per la Fisica della Materia and Dipartimento di Fisica Alessandro Volta, Università di Pavia, via Bassi 6, I-27100 Pavia, Italy.

A numerical analysis of extrinsic diffraction losses in two-dimensional photonic crystal slabs with line defects is reported. To model disorder, a Gaussian distribution of hole radii in the triangular lattice of airholes is assumed. The extrinsic losses below the light line increase quadratically with the disorder parameter, decrease slightly with increasing core thickness, and depend weakly on the hole radius. For typical values of the disorder parameter the calculated loss values of guided modes below the light line compare favorably with available experimental results.
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http://dx.doi.org/10.1364/ol.29.001897DOI Listing
August 2004

Gap maps and intrinsic diffraction losses in one-dimensional photonic crystal slabs.

Phys Rev E Stat Nonlin Soft Matter Phys 2004 May 11;69(5 Pt 2):056603. Epub 2004 May 11.

Istituto Nazionale per la Fisica della Materia and Dipartimento di Fisica Alessandro Volta, Università di Pavia, Via Bassi 6, I-27100 Pavia, Italy.

A theoretical study of photonic bands for one-dimensional (1D) lattices embedded in planar waveguides with strong refractive index contrast is presented. The approach relies on expanding the electromagnetic field on the basis of guided modes of an effective waveguide, and on treating the coupling to radiative modes by perturbation theory. Photonic mode dispersion, gap maps, and intrinsic diffraction losses of quasi guided modes are calculated for the case of self-standing membranes as well as for silicon-on-insulator structures. Photonic band gaps in a waveguide are found to depend strongly on the core thickness and on polarization, so that the gaps for transverse electric and transverse magnetic modes most often do not overlap. Radiative losses of quasiguided modes above the light line depend in a nontrivial way on structure parameters, mode index, and wave vector. The results of this study may be useful for the design of integrated 1D photonic structures with low radiative losses.
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http://dx.doi.org/10.1103/PhysRevE.69.056603DOI Listing
May 2004
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