A tunable transition metal dichalcogenide entangled photon-pair source.

Entangled photon-pair sources are at the core of quantum applications like quantum key distribution, sensing, and imaging. Operation in space-limited and adverse environments such as in satellite-based and mobile communication requires robust entanglement sources with minimal size and weight requirements. Here, we meet this challenge by realizing a cubic micrometer scale entangled photon-pair source in a 3R-stacked transition metal dichalcogenide crystal. Its crystal symmetry enables the generation of polarization-entangled Bell states without additional components and provides tunability by simple control of the pump polarization. Remarkably, generation rate and state tuning are decoupled, leading to equal generation efficiency and no loss of entanglement. Combining transition metal dichalcogenides with monolithic cavities and integrated photonic circuitry or using quasi-phasematching opens the gate towards ultrasmall and scalable quantum devices.

Generation of infrared photon pairs by spontaneous four-wave mixing in a CS2-filled microstructured optical fiber.

We experimentally demonstrate frequency non-degenerate photon-pair generation via spontaneous four-wave mixing from a novel CS2-filled microstructured optical fiber. CS2 has high nonlinearity, narrow Raman lines, a broad transmission spectrum, and also has a large index contrast with the microstructured silica fiber. We can achieve phase matching over a large spectral range by tuning the pump wavelength, allowing the generation of idler photons in the infrared region, which is suitable for applications in quantum spectroscopy. Moreover, we demonstrate a coincidence-to-accidental ratio of larger than 90 and a pair generation efficiency of about [Formula: see text] per pump pulse, which shows the viability of this fiber-based platform as a photon-pair source for quantum technology applications.

Spectral tailoring of photon pairs from microstructured suspended-core optical fibers with liquid-filled nanochannels.

We theoretically study the generation of photon pairs via spontaneous four-wave mixing (SFWM) in a liquid-filled microstructured suspended-core optical fiber. We show that it is possible to control the wavelength, group velocity, and bandwidths of the two-photon states. Our proposed fiber structure shows a large number of degrees of freedom to engineer the two-photon state. Here, we focus on the factorable state, which shows no spectral correlation in the two-photon components of the state, and allows the heralding of a single-photon pure state without the need for spectral post-filtering.

Subdiffraction Quantum Imaging with Undetected Photons.

We propose a nonlinear imaging scheme with undetected photons that overcomes the diffraction limit by transferring near-field information at one wavelength to far-field information of a correlated photon with a different wavelength generated through spontaneous photon-pair generation. At the same time, this scheme allows for retrieval of high-contrast images with zero background, making it a highly sensitive scheme for imaging of small objects at challenging spectral ranges with subdiffraction resolutions.

Activity Coefficients and Solubilities of NaCl in Water-Methanol Solutions from Molecular Dynamics Simulations.

We obtain activity coefficients and solubilities of NaCl in water-methanol solutions at 298.15 K and 1 bar from molecular dynamics (MD) simulations with the Joung-Cheatham, SPC/E, and OPLS-AA force fields for NaCl, water, and methanol, respectively. The Lorentz-Berthelot combining rules were adopted for the unlike-pair interactions of Na+, Cl-, and the oxygen site in SPC/E water, and geometric combining rules were utilized for the remainder of the cross interactions. We found that the selection of appropriate combining rules is important in obtaining physically realistic solubilities. The solvent compositions studied range from pure water to pure methanol. Several salt concentrations were investigated at each solvent composition, from the lowest concentrations permitted by the system size used up to the experimental solubilities. We first calculated individual ion activity coefficients (IIACs) for Na+ and Cl- from the free energy change due to the gradual insertion of a single cation or anion into the solution, accompanied by a neutralizing background. We obtained the salt solubilities by comparing the chemical potentials in solution with solid NaCl chemical potentials calculated previously using the Einstein crystal method. Mean ionic activity coefficients obtained from the IIACs are in reasonable agreement with experimental data, with deviations increasing for solutions of higher methanol content. Predictions for the salt solubility are in surprisingly good agreement with experimental data, despite well-known challenges in the simultaneous calculation of activity coefficients and solubilities with classical MD simulations. The present study demonstrates that good predictions for these two important phase equilibrium properties can be obtained for mixed-solvent electrolyte solutions using existing nonpolarizable models and further suggests that the previously proposed single ion insertion technique can be extended to complex mixed-solvent solutions as well.

Activity coefficients of aqueous electrolytes from implicit-water molecular dynamics simulations.

We obtain activity coefficients in NaCl and KCl solutions from implicit-water molecular dynamics simulations, at 298.15 K and 1 bar, using two distinct approaches. In the first approach, we consider ions in a continuum with constant relative permittivity (ɛr) equal to that of pure water; in the other approach, we take into account the concentration-dependence of ɛr, as obtained from explicit-water simulations. Individual ion activity coefficients (IIACs) are calculated using gradual insertion of single ions with uniform neutralizing backgrounds to ensure electroneutrality. Mean ionic activity coefficients (MIACs) obtained from the corresponding IIACs in simulations with constant ɛr show reasonable agreement with experimental data for both salts. Surprisingly, large systematic negative deviations are observed for both IIACs and MIACs in simulations with concentration-dependent ɛr. Our results suggest that the absence of hydration structure in implicit-water simulations cannot be compensated by correcting for the concentration-dependence of the relative permittivity ɛr. Moreover, even in simulations with constant ɛr for which the calculated MIACs are reasonable, the relative positioning of IIACs of anions and cations is incorrect for NaCl. We conclude that there are severe inherent limitations associated with implicit-water simulations in providing accurate activities of aqueous electrolytes, a finding with direct relevance to the development of electrolyte theories and to the use and interpretation of implicit-solvent simulations.

Individual Ion Activity Coefficients in Aqueous Electrolytes from Explicit-Water Molecular Dynamics Simulations.

We compute individual ion activity coefficients (IIACs) in aqueous NaCl, KCl, NaF, and KF solutions from explicit-water molecular dynamics simulations. Free energy changes are obtained from insertion of single ions-accompanied by uniform neutralizing backgrounds-into solution by gradually turning on first Lennard-Jones interactions, followed by Coulombic interactions using Ewald electrostatics. Simulations are performed at multiple system sizes, and all results are extrapolated to the thermodynamic limit, thus eliminating any possible artifacts from the neutralizing backgrounds. Because of controversies associated with measurements of IIACs from electrochemical cells with ion-selective electrodes, the reported experimental data are not widely accepted; thus there remains a knowledge gap with respect to the contributions of individual ions to solution nonidealities. Our results are in good qualitative agreement with these reported measurements, though significantly larger in magnitude. In particular, the relative positioning for the activity coefficients of anions and cations matches the experimental ordering for all four systems. This work establishes a robust thermodynamic framework, without a need to invoke extra hypotheses, that sheds light on the behavior of individual ions and their contributions to nonidealities of aqueous electrolyte solutions.

Scalable Functionalization of Optical Fibers Using Atomically Thin Semiconductors.

Atomically thin transition metal dichalcogenides are highly promising for integrated optoelectronic and photonic systems due to their exciton-driven linear and nonlinear interactions with light. Integrating them into optical fibers yields novel opportunities in optical communication, remote sensing, and all-fiber optoelectronics. However, the scalable and reproducible deposition of high-quality monolayers on optical fibers is a challenge. Here, the chemical vapor deposition of monolayer MoS2 and WS2 crystals on the core of microstructured exposed-core optical fibers and their interaction with the fibers' guided modes are reported. Two distinct application possibilities of 2D-functionalized waveguides to exemplify their potential are demonstrated. First, the excitonic 2D material photoluminescence is simultaneously excited and collected with the fiber modes, opening a novel route to remote sensing. Then it is shown that third-harmonic generation is modified by the highly localized nonlinear polarization of the monolayers, yielding a new avenue to tailor nonlinear optical processes in fibers. It is anticipated that the results may lead to significant advances in optical-fiber-based technologies.

A fully automated dual-tip scanning near-field optical microscope for localized optical excitation and detection in the visible and near-infrared.

Near-field optical microscopes with two independent tips for simultaneous excitation and detection can be essential tools for studying localized optical phenomena on the subwavelength scale. Here, we report on the implementation of a fully automated and robust dual-tip scanning near-field optical microscope (SNOM), in which the excitation tip is stationary, while the detection tip automatically scans the surrounding area. To monitor and control the distance between the two probes, mechanical interactions due to shear forces are used. We experimentally investigate suitable scan parameters and find that the automated dual-tip SNOM can operate stably for a wide range of parameters. To demonstrate the potential of the automated dual-tip SNOM, we characterize the propagation of surface plasmon polaritons on a gold film for visible and near-infrared wavelengths. The good agreement of the measurements with numerical simulations verifies the capability of the dual-tip SNOM for the near-field characterization of localized optical phenomena.

Photonic crystal waveguides as sources of counterpropagating factorizable biphoton states.

We demonstrate numerically that photonic crystal slab waveguides can generate spectrally unentangled biphoton states, highly desired for heralding of single photons. We achieve this by modally phase matching a counterpropagating spontaneous parametric down-conversion process, in a fully integrated scheme and without the need for periodic poling. Such a configuration is an ideal integrated source of heralded single photons, as it spatially separates the photons of a pair at the source without any extra components, while allowing for generation of spectrally narrow photons on a very short length scale.

Atom-mediated spontaneous parametric down-conversion in periodic waveguides.

We propose the concept of atom-mediated spontaneous parametric down-conversion, in which photon-pair generation can take place only in the presence of a single two-level emitter, relying on the bandgap evanescent modes of a nonlinear periodic waveguide. Using a guided signal mode, an evanescent idler mode, and an atom-like emitter with the idler's transition frequency embedded in the structure, we find a heralded excitation mechanism, in which the detection of a signal photon outside the structure heralds the excitation of the embedded emitter. We use a rigorous Green's function quantization method to model this heralding mechanism in a 1D periodic waveguide and determine its robustness against losses.

Generation of Counterpropagating Path-Entangled Photon Pairs in a Single Periodic Waveguide.

We propose the use of nonlinear periodic waveguides for direct and fully integrated generation of counterpropagating photon pairs by spontaneous parametric down-conversion. Using the unique properties of Bloch modes in such periodic structures, we furthermore show that two counterpropagating phase-matching conditions can be fulfilled simultaneously, allowing for the generation of path-entangled Bell states in a single periodic waveguide. To demonstrate the feasibility of our proposal, we design a photonic crystal slab waveguide made of lithium niobate and numerically demonstrate Bell-state generation.

Resonantly Enhanced Second-Harmonic Generation Using III-V Semiconductor All-Dielectric Metasurfaces.

Nonlinear optical phenomena in nanostructured materials have been challenging our perceptions of nonlinear optical processes that have been explored since the invention of lasers. For example, the ability to control optical field confinement, enhancement, and scattering almost independently allows nonlinear frequency conversion efficiencies to be enhanced by many orders of magnitude compared to bulk materials. Also, the subwavelength length scale renders phase matching issues irrelevant. Compared with plasmonic nanostructures, dielectric resonator metamaterials show great promise for enhanced nonlinear optical processes due to their larger mode volumes. Here, we present, for the first time, resonantly enhanced second-harmonic generation (SHG) using gallium arsenide (GaAs) based dielectric metasurfaces. Using arrays of cylindrical resonators we observe SHG enhancement factors as large as 10(4) relative to unpatterned GaAs. At the magnetic dipole resonance, we measure an absolute nonlinear conversion efficiency of ∼2 × 10(-5) with ∼3.4 GW/cm(2) pump intensity. The polarization properties of the SHG reveal that both bulk and surface nonlinearities play important roles in the observed nonlinear process.

Effect of loss on slow-light-enhanced second-harmonic generation in periodic nanostructures.

We theoretically analyze the dependence of second-harmonic generation efficiency on the group index in periodic optical waveguides with loss. We investigate different possible scenarios of using slow light to enhance the efficiency of this process and show that in some cases there exists a maximally achievable efficiency reached for finite values of the group index at the point of phase-matching. Furthermore, we identify situations for which slow light, surprisingly, does not enhance the second-harmonic generation efficiency. Our results are corroborated by rigorous nonlinear simulations of second-harmonic generation in periodic nanobeam waveguides with loss.

Fabrication of nanoscale lithium niobate waveguides for second-harmonic generation.

Nanoscale waveguides are basic building blocks of integrated optical devices. Especially, waveguides made from nonlinear optical materials, such as lithium niobate, allow access to a broad range of applications using second-order nonlinear frequency conversion processes. Based on a lithium niobate on insulator substrate, millimeter-long nanoscale waveguides were fabricated with widths as small as 200 nm. The fabrication was done by means of potassium hydroxide-assisted ion-beam-enhanced etching. The waveguides were optically characterized in the near infrared wavelength range showing phase-matched second-harmonic generation.