Wavelength-Sensitive Superconducting Single-Photon Detectors on Thin Film Lithium Niobate Waveguides.

Lithium niobate, because of its nonlinear and electro-optical properties, is one of the materials of choice for photonic applications. The development of nanostructuring capabilities of thin film lithium niobate (TFLN) permits fabrication of small footprint, low-loss optical circuits. With the recent implementation of on-chip single-photon detectors, this architecture is among the most promising for realizing on-chip quantum optics experiments. In this Letter, we report on the implementation of superconducting nanowire single-photon detectors (SNSPDs) based on NbTiN on 300 nm thick TFLN ridge nano-waveguides. We demonstrate a waveguide-integrated wavelength meter based on the photon energy dependence of the superconducting detectors. The device operates at the telecom C- and L-bands and has a footprint smaller than 300 × 180 µm2 and critical currents between ∼12 and ∼14 µA, which ensures operation with minimum heat dissipation. Our results hold promise for future densely packed on-chip wavelength-multiplexed quantum communication systems.

Coincident Fluorescence-Burst Analysis of the Loading Yields of Exosome-Mimetic Nanovesicles with Fluorescently-Labeled Cargo Molecules.

The possible targeting functionality and low immunogenicity of exosomes and exosome-like nanovesicles make them promising as drug-delivery carriers. To tap into this potential, accurate non-destructive methods to load them and characterize their contents are of utmost importance. However, the small size, polydispersity, and aggregation of nanovesicles in solution make quantitative characterizations of their loading particularly challenging. Here, an ad-hoc methodology is developed based on burst analysis of dual-color confocal fluorescence microscopy experiments, suited for quantitative characterizations of exosome-like nanovesicles and of their fluorescently-labeled loading. It is applied to study exosome-mimetic nanovesicles derived from animal extracellular-vesicles and human red blood cell detergent-resistant membranes, loaded with fluorescently-tagged dUTP cargo molecules. For both classes of nanovesicles, successful loading is proved and by dual-color coincident fluorescence burst analysis, size statistics and loading yields are retrieved and quantified. The procedure affords single-vesicle characterizations well-suited for the investigation of a variety of cargo molecules and biological nanovesicle combinations besides the proof-of-principle demonstrations of this study. The results highlight a powerful characterization tool essential for optimizing the loading process and for advanced engineering of biomimetic nanovesicles for therapeutic drug delivery.

Charged Exciton Kinetics in Monolayer MoSe2 near Ferroelectric Domain Walls in Periodically Poled LiNbO3.

Monolayer semiconducting transition metal dichalcogenides are a strongly emergent platform for exploring quantum phenomena in condensed matter, building novel optoelectronic devices with enhanced functionalities. Because of their atomic thickness, their excitonic optical response is highly sensitive to their dielectric environment. In this work, we explore the optical properties of monolayer thick MoSe2 straddling domain wall boundaries in periodically poled LiNbO3. Spatially resolved photoluminescence experiments reveal spatial sorting of charge and photogenerated neutral and charged excitons across the boundary. Our results reveal evidence for extremely large in-plane electric fields of ≃4000 kV/cm at the domain wall whose effect is manifested in exciton dissociation and routing of free charges and trions toward oppositely poled domains and a nonintuitive spatial intensity dependence. By modeling our result using drift-diffusion and continuity equations, we obtain excellent qualitative agreement with our observations and have explained the observed spatial luminescence modulation using realistic material parameters.

Edge-enhanced optical parametric generation in periodically poled LiNbO3.

We demonstrate enhanced optical parametric gains occurring at the edge of periodically poled LiNbO3 (PPLN) regions. Experiments performed in MgO-doped PPLN samples, pumped at 532 nm with parametric signal outputs around 800 nm and 1550 nm, exhibit good agreement with numerical simulations of the nonlinear wave dynamics in the system, based on the assumption of an average refractive index increase Δn = 5.3×10-5 in the PPLN region. Excitation in proximity to the PPLN edge with a pump power of 8.1 mW results in a 3.6-fold output power increase with respect to parametric generation inside the PPLN area.

Modal birefringence-free lithium niobate waveguides.

We investigate polarization-insensitive waveguide designs afforded by the interplay of material and waveguide birefringence in LiNbO3-on-insulator photonic wires. Fundamental mode birefringence-free operation in the 0.8-1.8 µm spectral range is predicted for a suitable choice of waveguide widths in the 375-600 nm range. Optimized buried waveguide designs yield broadband (1350-1625 nm) index matching between TE00 and TM00 modes. Furthermore, simultaneous phase- and group-velocity matching at infrared wavelengths appears feasible for pulse durations as short as 100 fs.

Focus issue introduction: Advanced Solid-State Lasers (ASSL) 2015.

The editors introduce the focus issue on "Advanced Solid-State Lasers (ASSL) 2015", which is based on the topics presented at a congress of the same name held in Berlin, Germany, from October 4 to October 9, 2015. This focus issue, jointly prepared by Optics Express and Optical Materials Express, includes 23 contributed papers (17 for Optics Express and 6 for Optical Materials Express) selected from the voluntary submissions from attendees who presented at the congress and have extended their work into complete research articles. We hope this focus issue offers a good snapshot of a variety of topical discussions held at the congress and will contribute to the further expansion of the associated research areas.

Focus issue introduction: Advanced solid-state lasers (ASSL) 2014.

The editors introduce the focus issue on "Advanced Solid-State Lasers (ASSL) 2014," which is based on the topics presented at a congress of the same name held in Shanghai, China, from October 27 to November 1, 2014. This focus issue, jointly prepared by Optics Express and Optical Materials Express, includes 28 contributed papers (21 for Optics Express and 7 for Optical Materials Express) selected from the voluntary submissions by attendees who presented at the congress and have extended their work into complete research articles. We hope this focus issue offers a useful snapshot of the variety of topical discussions held at the congress and will contribute to the further expansion of the associated research areas.

Focus issue introduction: Advanced Solid-State Lasers (ASSL) 2013.

The editors introduce the focus issue on "Advanced Solid-State Lasers (ASSL) 2013," which is based on the topics presented at a congress of the same name held in Paris, France, from October 27 to November 1, 2013. This focus issue, jointly prepared by Optics Express and Optical Materials Express, includes 21 contributed papers (18 for Optics Express and 3 for Optical Materials Express) selected from the voluntary submissions from attendees who presented at the congress and have extended their work into complete research articles. We hope this focus issue offers a good snapshot of a variety of topical discussions held at the congress and will contribute to the further expansion of the associated research areas.

Broadband parametric processes in χ(2) nonlinear photonic crystals.

We develop a general model, based on a (2+1)D unidirectional pulse propagation equation, for describing broadband noncollinear parametric interactions in 2D quadratic lattices. We apply it to the analysis of twin-beam optical parametric generation in hexagonally poled LiTaO3, gaining further insights into experimental observations.

Antibody-Loading of Biological Nanocarrier Vesicles Derived from Red-Blood-Cell Membranes.

Antibodies, disruptive potent therapeutic agents against pharmacological targets, face a barrier in crossing immune systems and cellular membranes. To overcome these, various strategies have been explored including shuttling via liposomes or biocamouflaged nanoparticles. Here, we demonstrate the feasibility of loading antibodies into exosome-mimetic nanovesicles derived from human red-blood-cell membranes, which can act as nanocarriers for intracellular delivery. Goat-antichicken antibodies are loaded into erythrocyte-derived nanovesicles, and their loading yields are characterized and compared with smaller dUTP-cargo molecules. Applying dual-color coincident fluorescence burst analyses, the loading yield of nanocarriers is rigorously profiled at the single-vesicle level, overcoming challenges due to size-heterogeneity and demonstrating a maximum antibody-loading yield of 38-41% at the optimal vesicle radius of 52 nm. The achieved average loading yields, amounting to 14% across the entire nanovesicle population, with more than two antibodies per loaded vesicle, are fully comparable to those obtained for the much smaller dUTP molecules loaded in the nanovesicles after additional exosome-spin-column purification. The results suggest a promising new avenue for therapeutic delivery of antibodies, potentially encompassing also intracellular targets and suitable for large-scale pharmacological applications, which relies on the exosome-mimetic properties, biocompatibility, and low-immunogenicity of bioengineered nanocarriers synthesized from human erythrocyte membranes.

Purely nonlinear disorder-induced localizations and their parametric amplification.

We investigate spatial localization in a quadratic nonlinear medium in the presence of randomness. By means of numerical simulations and theoretical analyses we show that, in the down conversion regime, the transverse random modulation of the nonlinear susceptibility generates localizations of the fundamental wave that grow exponentially in propagation. The localization length is optically controlled by the pump intensity that determines the amplification rate. The results also apply to cubic nonlinearities.

Multistep quadratic cascading in broadband optical parametric generation.

We theoretically and experimentally investigate multistep parametric processes in broadband optical parametric generators (OPGs) based on periodically poled 1 mol. % MgO-doped stoichiometric LiTaO3. We demonstrate that parametric collateral processes may deplete or enhance spectral portions of the OPG output, depending on pump pulse duration.

Ultra-broadband optical parametric generation in periodically poled stoichiometric LiTaO3.

We report on broadband gain in an optical parametric generator based on periodically poled 1 mol% magnesium-doped stoichiometric LiTaO3 (PPMg:SLT). More than an octave-spanning parametric gain, stretching from near to mid-infrared, is generated by pumping the crystals close to the point where, at parametric degeneracy, the waves experience zero group-velocity dispersion. Using a picosecond Ti:sapphire source, we measured the broadest parametric gain bandwidths, 180 THz at 10 dB, in PPMg:SLT gratings with a period of 25 µm pumped at 860 nm.

Analysis of acceptable spectral windows of quadratic cascaded nonlinear processes in a periodically poled lithium niobate waveguide.

We report a systematic and comparative study of the acceptance bandwidths of two cascaded quadratic nonlinear processes in periodically poled lithium niobate waveguides, namely cascaded second-harmonic generation and difference-frequency generation (cSHG/DFG) and cascaded sum-frequency generation and difference-frequency generation (cSFG/DFG). We first theoretically and experimentally study the acceptance bandwidths of both the individual second-harmonic generation (SHG) and sum-frequency generation (SFG) processes in the continuous wave (CW) and pulsed-pump regimes. Our results show that the SHG bandwidth is approximately half that of the SFG process in the CW regime, whereas the SHG acceptance bandwidth can approach the CW SFG bandwidth limit when pulsed-pump is used. As a consequence we conclude that the tuning bandwidths of both cascaded processes should be similar in the pulsed pump regime once the pump pulse bandwidths approach that of SFG (i.e. the cSHG/DFG bandwidth is not limited by the CW SHG bandwidth). We confirm that this is the case experimentally.

Phase-regenerative wavelength conversion in periodically poled lithium niobate waveguides.

We propose and experimentally demonstrate phase-regenerative wavelength conversion in periodically poled lithium niobate waveguides, using either: a single-stage implementation based on a simultaneous combination of two cascaded second-order nonlinear effects in a single periodically poled lithium niobate waveguide, or a two-stage implementation where two separate devices are used in sequence to give rise to the same nonlinear effects. The phase regeneration properties of the proposed wavelength conversion schemes are also investigated.

OTDM to WDM format conversion based on quadratic cascading in a periodically poled lithium niobate waveguide.

We propose and demonstrate error-free conversion of a 40 Gbit/s optical time division multiplexed signal to 4 x 10 Gbit/s wavelength division multiplexed channels based on cascaded second harmonic and difference frequency generation in a periodically poled lithium niobate waveguide. The technique relies on the generation of spectrally (and temporally) flat linearly chirped pulses which are then optically switched with short data pulses in the nonlinear waveguide. Error-free operation was obtained for all channels with a power penalty below 2dB.

Elimination of the chirp of optical pulses through cascaded nonlinearities in periodically poled lithium niobate waveguides.

We propose and demonstrate a novel method for the elimination of arbitrary frequency chirp from short optical pulses. The technique is based on the combination of two cascaded second-order nonlinearities in two individual periodically poled lithium niobate waveguides. The proposed scheme operates independently of the spectral phase characteristics of the input pulse, producing a near-transform-limited output.

Aperiodically poled silica fibers for bandwidth control of quasi-phase-matched second-harmonic generation.

Precise control of the bandwidth of quasi-phase-matched second-harmonic generation in silica fibers is realized through chirped-period poling. The bandwidth is expanded by a factor of 33 over a uniform-period poled fiber of the same interaction length.

Phase sensitive amplification based on quadratic cascading in a periodically poled lithium niobate waveguide.

We propose and demonstrate phase-sensitive amplification based on cascaded second harmonic generation and difference frequency generation within a periodically poled lithium niobate waveguide. Excellent agreement between our numerical simulations and proof-of-principle experiments using a 3-cm waveguide device operating at wavelengths around 1550 nm is obtained. Our experiments confirm the validity and practicality of the approach and illustrate the broad gain bandwidths achievable. Additional simulation results show that the maximum gain/attenuation factor increases quadratically with input pump power, reaching a value of +/- 19.0 dB at input pump powers of 33 dBm for a 3 cm-long waveguide. Increased gains/reduced powers for a fixed gain could be achieved using longer crystals.

Golden Ratio Gain Enhancement in Coherently Coupled Parametric Processes.

Nonlinear optical processes are an essential tool in modern optics, with a broad spectrum of applications, including signal processing, frequency conversion, spectroscopy and quantum optics. Ordinary parametric devices nevertheless still suffer from relatively low gains and wide spectral emission. Here we demonstrate a unique configuration for phase-matching multiple nonlinear processes in a monolithic 2D nonlinear photonic crystal, resulting in the coherent parametric emission of four signal and idler modes, featuring an exponential gain enhancement equal to the Golden Ratio. The results indicate a new route towards compact high-brightness and coherent sources for multi-photon generation, manipulation and entanglement, overcoming limitations of conventional parametric devices.