At the core of an ideal single-photon detector is an active material that absorbs and converts every incident photon to a discriminable signal. A large active material favours efficient absorption, ...but often at the expense of conversion efficiency, noise, speed and timing accuracy. In this work, short (8.5 μm long) and narrow (8 × 35 nm(2)) U-shaped NbTiN nanowires atop silicon-on-insulator waveguides are embedded in asymmetric nanobeam cavities that render them as near-perfect absorbers despite their small volume. At 2.05 K, when biased at 0.9 of the critical current, the resulting superconducting single-photon detectors achieve a near-unity on-chip quantum efficiency for ∼1,545 nm photons, an intrinsic dark count rate <0.1 Hz, a reset time of ∼7 ns, and a timing jitter of ∼55 ps full-width at half-maximum. Such ultracompact, high-performance detectors are essential for progress in integrated quantum optics.
We describe a powerful and intuitive theoretical technique for modeling light-matter interactions in classical and quantum nanoplasmonics. Our approach uses a quasinormal mode (QNM) expansion of the ...photon Green function within a metal nanoresonator of arbitrary shape, together with a Dyson equation, to derive an expression for the spontaneous decay rate and far field propagator from dipole oscillators outside resonators. For a single QNM, at field positions outside the quasi-static coupling regime, we give a closed form solution for the Purcell factor and generalized effective mode volume. We augment this with an analytic expression for the divergent local density of optical states very near the metal surface, which allows us to derive a simple and highly accurate expression for the electric field outside the metal resonator at distances from a few nanometers to infinity. This intuitive formalism provides an enormous simplification over full numerical calculations and fixes several pending problems in QNM theory.
Formulas are presented that provide clear physical insight into the phenomenon of extrinsic optical scattering loss in photonic crystal waveguides due to random fabrication imperfections such as ...surface roughness and disorder. Using a photon Green-function-tensor formalism, we derive explicit expressions for the backscattered and total transmission losses. Detailed calculations for planar photonic crystals yield extrinsic loss values in overall agreement with experimental measurements, including the full dispersion characteristics. We also report that loss in photonic crystal waveguides scales inversely with group velocity, at least, thereby raising serious questions about future low-loss applications based on operating frequencies that approach the photonic band edge.
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Silicon microcavity-based optical trapping of Au nanoparticles with diameters as small as ≈24 nm is achieved using optical powers <1 mW. By comparing measured and modeled histograms of transmission ...time series data obtained when a particle is trapped in the cavity, it is shown that the influence of backaction on the transmitted light dynamics alone can be used to determine the size of trapped particles with nanometer precision.
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High coincidence-to-accidental ratio (CAR) is crucial for photon-pair sources (PPSs) integrated with pump reject filters (PRFs) in silicon, but CAR values currently reported for integrated PPS/PRF ...chips still fall short of those achieved using stand-alone sources with external PRFs. Here we report measured and modelled CAR values for a micro-ring resonator PPS integrated with a PRF consisting of a three-stage, cascaded (via their through ports), contra-directional coupler (CDC) that compare favorably even with some stand-alone sources. CDC-based PRFs provide the benefits of compact area and wide reject bands without a need for tuning, in comparison to prior-art implementations.
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•Equilibrium shape and surface facets of PbS QDs vary considerably by their size.•Size-dependent composition model predicts increasing the Pb:S ratio by size reduction.•Surface-ligand ...coordination model studied type and ratio of ligands on QDs surfaces.•Methods for measuring the theoretical and experimental ligand weight percentages.•Mechanism of air exposure oxidation of different surface facets of QDs.
This paper reports theoretical and experimental studies that explain how the size-dependent shape and elemental composition of PbS quantum dots (QDs) affect the arrangement and density of adsorbed ligands by QDs surfaces, and their related susceptibility to oxidation. By considering the differences of Pb:S ratios of the particles, we predict a size-dependent Pb:S ratio that is consistent with our measurements, and those reported in the literature. We further connect the shape and facet Pb:S composition to surface ligand densities, and relate this to the particle-size dependence of oxidation when exposed to air. While the surfaces of smaller particles (d < 2.7 nm) are entirely covered by Pb terminated (1 1 1) facets, bigger QDs (2.7 < d < 7.5 nm) are truncated at their (1 0 0) facets to form cuboctahedron-shape particles. The (1 1 1) facets representing the largest portion of both smaller and bigger particle’s surfaces can be well-passivated by anionic oleate/hydroxide ligands. Investigation on the surface chemistry of PbS QDs by the means of FT-IR, NMR, XPS and combustion elemental analyses, as well as steric stability considerations suggested that hydroxide to oleate atomic ratio in (1 1 1) facet of PbS QDs is about 1:5. On the other side, (1 0 0) surface facets have a checkerboard arrangement of lead and sulfur atoms and are coordinated by oleic acid ligands. Low density and weak binding energy of such ligands make the (1 0 0) facets vulnerable to oxidation in an air atmosphere. XPS investigations indicate that the oxide includes both PbSO3 and PbSO4 components on the surface of PbS QDs.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
We present a new fab-in-the-loop reinforcement learning algorithm for the design of nano-photonic components that accounts for the imperfections present in nanofabrication processes. As a ...demonstration of the potential of this technique, we apply it to the design of photonic crystal grating couplers fabricated on an air clad 220 nm silicon on insulator single etch platform. This fab-in-the-loop algorithm improves the insertion loss from 8.8 to 3.24 dB. The widest bandwidth designs produced using our fab-in-the-loop algorithm can cover a 150 nm bandwidth with less than 10.2 dB of loss at their lowest point.
The photostability of semiconductor PbSe nanocrystals (NCs) under different storage conditions and the photoluminescence of PbSe NC films on Si substrates under vacuum and under different atmospheres ...were investigated. The NCs show a small blue shift in both the emission and absorption spectra when stored in solution. This blue shift is accelerated when the particles are stored in room light compared to when the particles are stored in the dark, which indicates that photooxidation is an important process. The photooxidation process is accelerated by irradiating NC solutions with a 450 W xenon lamp. The photobleaching and appearance of a precipitate are attributed to ligand desorption followed by agglomeration. Reversible O2-induced luminescence quenching was observed on a PbSe NC film on Si substrates, which could be addressed by two mechanisms: indirect quenching by alteration of PbSe charging and neutralization dynamics or quenching through direct interaction of O2 with the exciton. To improve the photostability of PbSe NCs, PbSe/PbS core/shell NCs using (TOP)S (TOP = trioctylphosphine) and TMS2S (TMS2 = bis(trimethylsilyl)) as sulfur sources were prepared. However, PbSe/PbS core/shell NCs did not show an increased stability in solution upon irradiation with a xenon lamp when compared with PbSe core NCs. Either the PbS shell is not able to confine the charge carriers or there is incomplete shell passivation with the PbSe core, and as a result these core/shell NCs have comparable stability.
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The air exposure oxidation mechanisms of PbS quantum dot (QD) thin films and solar cells are studied in the current paper. As a novel and advantageous method, QD thin films were prepared by the ...single-step deposition of colloidal QDs treated with different ligands of butylamine (BA), mercaptopropionic acid (MPA), tetrabutylammonium iodide (TBAI), methylammonium iodide (MAI) and methylammonium lead triiodide (MAPbI3, perovskite). Photoluminescence (PL) measurements evaluated the stability of different surface treated PbS QDs during the colloidal to thin film transformation, and over the air exposure times. Blue-shift and quenching the PL spectra suggested rapid oxidation of QD thin films at the first times of air exposure. However, the oxidation rates significantly reduced for the QD thin films treated with organic MPA and all the inorganic ligands. According to the X-ray photoelectron spectroscopy (XPS) studies, thin films oxidation accompanied with the formation of PbSO3 and PbSO4 compounds on the (100) facets of PbS QDs. Although MAPbI3 treatment led to the complete passivation of QDs in the air, perovskite shelling partially oxidized to PbO and PbCO3 compounds. Furthermore, the PL enhancement phenomenon observed at the first times of air exposure for the TBAI and MAPbI3-treated QD thin films, as a result of their strong surface passivation as well as the photoenhancement and photoelectrification mechanisms. Eventually, p-n and p-i-n structured solar cells were fabricated by the single-step deposition of solution-phase treated PbS QD inks. In this case, inorganic surface treatments not only increased the power conversion efficiency (PCE) of solar cells, but also led to a high stability of fabricated devices in the air environment (lower than 1% PCE loss after 500 h of storage in the air).
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•Surface functionalization of PbS QDs with a wide range of organic and inorganic ligands.•Deposition of pre-exchanged QD thin films via advantageous single-step method.•Air oxidation stability of QDs over thin film formulation and light illumination.•Mechanisms of photooxidation and PL enhancement for the air-exposed QD thin films.•Fabrication, efficiency and air stability of single-step prepared QD solar cells.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Superconducting oscillators have been successfully used for quantum control and readout devices in conjunction with superconducting qubits. Also, squeezed states can improve the accuracy of ...measurements to subquantum, or at least subthermal, levels. Here, we show theoretically how to produce squeezed states of microwave radiation in a superconducting oscillator with tunable parameters. Its resonance frequency can be changed by controlling an rf SQUID inductively coupled to the oscillator. By repeatedly shifting the resonance frequency between any two values, it is possible to produce squeezed and subthermal states of the electromagnetic field in the (0.1-10) GHz range, even when the relative frequency change is small. We propose experimental protocols for the verification of squeezed state generation, and for their use to improve the readout fidelity when such oscillators serve as quantum transducers.
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