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Laser cavity-solitons can appear in systems comprised of a nonlinear microcavity nested within an amplifying fiber loop. These states are robust and self-emergent and constitute an attractive class of solitons that are highly suitable for microcomb generation. Here, we present a detailed study of the free-running stability properties of the carrier frequency and repetition rate of single solitons, which are the most suitable states for developing robust ultrafast and high repetition rate comb sources. We achieve free-running fractional stability on both optical carrier and repetition rate (i.e., 48.9 GHz) frequencies on the order of 1 0 − 9 for a 1 s gate time. The repetition rate results compare well with the performance of state-of-the-art (externally driven) microcomb sources, and the carrier frequency stability is in the range of performance typical of modern free-running fiber lasers. Finally, we show that these quantities can be controlled by modulating the laser pump current and the cavity length, providing a path for active locking and long-term stabilization. [ABSTRACT FROM AUTHOR]

Extremely short, high-energy pulses are essential in modern ultrafast science. In a seminal paper in 19961, Nisoli and co-workers demonstrated the first intense pulse compression employing a gas-filled hollow-core fibre. Despite the huge body of scientific work on this technology stemming from ultrafast and attosecond research, here we identify an unexplored few-cycle visible-light generation mechanism, which relies on the nonlinear mixing of hollow-core fibre modes. Using a commercially available ytterbium laser, we generate 4.6 fs, 20 μJ pulses centred at around 600 nm (~2 cycles, ~4 GW peak power), ~40 times shorter than the input 175 fs, 1 mJ pulses at 1,035 nm. Our approach thus directly projects few-hundred-femtosecond-long infrared pulses into the single-cycle regime at visible frequencies, without the need for additional post-compression. As a powerful application of our findings, we present a compact, multicolour pump–probe set-up with a temporal resolution of a few optical cycles. Direct generation of few-cycle high-energy visible pulses is demonstrated via the nonlinear mixing of hollow-core fibre modes. Compression of near-infrared laser pulses by a factor of 40 with no additional dispersion compensation delivers 4.6 fs, 20 μJ pulses (~2 cycles, ~4 GW peak power) centred at around 600 nm. [ABSTRACT FROM AUTHOR]

The accurate determination of the photothermal response of nanomaterials represents an essential aspect in many fields, such as nanomedicine. Specifically, photothermal cancer therapies rely on the precise knowledge of the light-to-heat transfer properties of plasmonic nanoparticles to achieve the desired temperature-induced effects in biological tissues. In this work, we present a novel method for the quantification of the photothermal effect exhibited by nanoparticles in aqueous dispersions. By combining the spatial and temporal thermal dynamics acquired at terahertz frequencies, the photothermal conversion efficiency associated with the geometry of the plasmonic nanoparticles can be retrieved in a noncontact and noninvasive manner. The proposed technique can be extended to the characterization of all those nanomaterials which feature a temperature-dependent variation of the refractive index in the terahertz regime. [ABSTRACT FROM AUTHOR]

The influence of surface chemistry in plasma etching of multicomponent oxides was investigated through measurements of the ion energy dependence of the etch yield. Using pulsed-laser-deposited CaxBa(1-x)Nb2O6 (CBN) and SrTiO3 thin films as examples, it was found that the etching energy threshold shifts toward values larger or smaller than the sputtering threshold depending on whether or not ion-assisted chemical etching is the dominant etching pathway and whether surface chemistry is enhancing or inhibiting desorption of the film atoms. In the case of CBN films etched in an inductively coupled Cl2 plasma, it is found that the chlorine uptake is inhibiting the etching reaction, with the desorption of nonvolatile NbCl2 and BaCl2 compounds being the rate-limiting step. [ABSTRACT FROM AUTHOR]

Barium strontium titanate (Ba0.5Sr0.5TiO3 or BST) films were grown on SiO2/Si substrate by means of a reactive pulsed laser deposition technique. The microstructural and surface morphology of BST films were characterized by x-ray diffraction, scanning electron microscopy, and atomic force microscopy. Films deposited at 400 °C were found to be amorphous and present wide cracks. The crystallization of the films was induced by annealing at 600 °C. The BST films deposited at 550 °C present a cubic perovskite structure with a dense and smooth surface. The optical constants (n and k as a function of wavelength) of the films were obtained using variable angle spectroscopic ellipsometry in the UV-vis-NIR regions. Microfabricated BST/SiO2/Si ridged waveguides were patterned using UV photolithography and high-density plasma etching. The propagation and loss characteristics at the telecommunication wavelength of 1.55 μm were investigated using top-view scattering and Fabry-Pérot resonance methods. For specific ridge widths, we obtained single-mode propagation with relatively low losses (∼1 dB/cm), thereby demonstrating the strong potential of BST films for guided-wave components for advanced optical integrated systems. [ABSTRACT FROM AUTHOR]

We present a novel class of CMOS-compatible devices aimed to perform the solid-state-biased coherent detection of ultrashort terahertz pulses, i.e., featuring a gap-free bandwidth at least two decades-wide. Such a structure relies on a 1-µm-wide slit aperture located between two parallel aluminum pads, embedded in a 1-µm-thick layer of silicon nitride, and deposited on a quartz substrate. We show that this device can detect ultra-broadband terahertz pulses by employing unprecedented low optical probe energies of only a few tens of nanojoules. This is due to the more than one order of magnitude higher nonlinear coefficient of silicon nitride with respect to silica, the nonlinear material employed in the previous generations. In addition, due to the reduced distance between the aluminum pads, very high static electric fields can be generated within the slit by applying extremely low external bias voltages (in the order of few tens of volts), which strongly enhance the dynamic range of the detected THz waveforms. These results pave the way to the integration of solid-state ultra-broadband detection in compact and miniaturized terahertz systems fed by high repetition-rate laser oscillators and low-noise, low-voltage generators. [ABSTRACT FROM AUTHOR]

We report on terahertz (THz) generation via optical rectification in a room temperature gallium arsenide (GaAs) crystal pumped at a wavelength of 1.8 lm using a modified tilted-pulse-front scheme, leading to a 0.05% energy conversion efficiency. The spectral content of the measured THz pulses, ranging from 0.1 to 3 THz, confirmed a good broadband phase matching between the pump and the THz pulses over several millimeters (>20mm) in a semi-insulating h110i cut bulk GaAs crystal. Our findings also suggest that the pump-to-THz conversion efficiency can be further increased by using a pump source with a narrower bandwidth. [ABSTRACT FROM AUTHOR]

We report on the excitation of sharp Fano-like resonances in lattices of metamolecules composed of two differing types of metaatoms. The proposed structures exhibit modes originating from the individual metaatoms as well as a very sharp mode from the collective excitation of the metamolecule lattice as a whole. Next-generation thin film sensors (e.g., for bio/chemical hazard detectors) could especially benefit from such artificial materials. Having multiple modes at different spectral positions enables the characterization of dispersive materials, while the high Q-factors of the eigenmodes lead to a very high sensitivity. [ABSTRACT FROM AUTHOR]

We investigate the nonlinear wave-mixing in gases between intense, short optical pulses and long-wavelength fields (mid infrared and terahertz). We show numerically that the beating between the sum- and difference-frequency generation components can be isolated in the spectrogram of the interaction, and can be used to sample the electric field oscillations of the long-wavelength pulses. This, in turn, could be employed as a possible characterization method that provides information on the real electric field amplitude. Our numerical model is supported by spectrally resolved measurements of the four-wave mixing signals obtained from the interaction of intense, single-cycle terahertz fields (λ > 15 μm) and optical pulses (λ ≃ 800 nm, 50 fs duration) in air. [ABSTRACT FROM AUTHOR]

We present an analytical model describing the full electromagnetic propagation in a THz time-domain spectroscopy (THz-TDS) system, from the THz pulses via Optical Rectification to the detection via Electro Optic-Sampling. While several investigations deal singularly with the many elements that constitute a THz-TDS, in our work we pay particular attention to the modelling of the time-frequency behaviour of all the stages which compose the experimental set-up. Therefore, our model considers the following main aspects: (i) pump beam focusing into the generation crystal; (ii) phase-matching inside both the generation and detection crystals; (iii) chromatic dispersion and absorption inside the crystals; (iv) Fabry-Perot effect; (v) diffraction outside, i.e. along the propagation, (vi) focalization and overlapping between THz and probe beams, (vii) electro-optic sampling. In order to validate our model, we report on the comparison between the simulations and the experimental data obtained from the same set-up, showing their good agreement. [ABSTRACT FROM AUTHOR]

In this paper, we investigate semiconductor nanowaveguides (i.e. ridge waveguides with core-widths narrower than 1 μm) intended to act as novel optical light sources through nonlinear wavelength/frequency conversion. In particular, numerical calculations have been performed in order to design suitable photonic devices (fabricated in the AlGaAs/GaAs platform) capable of high efficiency second harmonic generation. Particular interest has been dedicated to the effective conversion of optical signals from 1520-1600 nm (the third telecom window) down to 760-800 nm. We demonstrate that the output wavelength (resulting from modal phase-matching) can be dynamically tuned by proper adjustment of the temperature and/or geometrical parameters of the waveguides. In addition, by changing the waveguide width it is also possible to modify the device dispersion characteristics, giving the possibility to work in the region of anomalous dispersion and thus allowing for the generation of temporal solitons. [ABSTRACT FROM AUTHOR]

In this paper, we investigate semiconductor nanowaveguides (i.e. ridge waveguides with core-widths narrower than 1 μm) intended to act as novel optical light sources through nonlinear wavelength/frequency conversion. In particular, numerical calculations have been performed in order to design suitable photonic devices (fabricated in the AlGaAs/GaAs platform) capable of high efficiency second harmonic generation. Particular interest has been dedicated to the effective conversion of optical signals from 1520-1600 nm (the third telecom window) down to 760-800 nm. We demonstrate that the output wavelength (resulting from modal phase-matching) can be dynamically tuned by proper adjustment of the temperature and/or geometrical parameters of the waveguides. In addition, by changing the waveguide width it is also possible to modify the device dispersion characteristics, giving the possibility to work in the region of anomalous dispersion and thus allowing for the generation of temporal solitons. [ABSTRACT FROM AUTHOR]

We investigate the generation of free-space terahertz (THz) radiation from photoconductive antennas using single crystal and polycrystalline ZnSe substrates. The photoconductive antennas have been excited above (400 nm) and below (800 nm) the bandgap. We investigate the dependence of the THz electric field radiated from biased ZnSe emitters on the applied bias field and on the incident optical fluence for bias fields as high as 26 kV/cm and for optical fluences of 0.005–3 mJ \cm^-2. The saturation fluence is observed to be significantly different both above and below bandgap excitation. These results show that, in comparison with GaAs substrates, ZnSe has strong potential as a high-power THz emitter. [ABSTRACT FROM PUBLISHER]

Integrated multiple-wavelength laser sources, critical for important applications such as high-precision broadband sensing and spectroscopy, molecular fingerprinting, optical clocks and attosecond physics, have recently been demonstrated in silica and single-crystal microtoroid resonators using parametric gain. However, for applications in telecommunications and optical interconnects, analogous devices compatible with a fully integrated platform do not yet exist. Here, we report a fully integrated, CMOS-compatible, multiple-wavelength source. We achieve optical ‘hyper-parametric’ oscillation in a high-index silica-glass microring resonator with a differential slope efficiency above threshold of 7.4% for a single oscillating mode, a continuous-wave threshold power as low as 54 mW, and a controllable range of frequency spacing from 200 GHz to more than 6 THz. The low loss, design flexibility and CMOS compatibility of this device will enable the creation of multiple-wavelength sources for telecommunications, computing, sensing, metrology and other areas. [ABSTRACT FROM AUTHOR]

Photonic integrated circuits are a key component of future telecommunication networks, where demands for greater bandwidth, network flexibility, and low energy consumption and cost must all be met. The quest for all-optical components has naturally targeted materials with extremely large nonlinearity, including chalcogenide glasses and semiconductors, such as silicon and AlGaAs (ref. 4). However, issues such as immature fabrication technology for chalcogenide glass and high linear and nonlinear losses for semiconductors motivate the search for other materials. Here we present the first demonstration of nonlinear optics in integrated silica-based glass waveguides using continuous-wave light. We demonstrate four-wave mixing, with low (5 mW) continuous-wave pump power at λ = 1,550 nm, in high-index, doped silica glass ring resonators. The low loss, design flexibility and manufacturability of our device are important attributes for low-cost, high-performance, nonlinear all-optical photonic integrated circuits. [ABSTRACT FROM AUTHOR]

The recent theoretical predictions and experimental observations of discrete surface solitons propagating along the interface between a one- or two-dimensional continuous medium and a one- or two-dimensional waveguide array are reviewed. These discrete solitons were found in second order (periodically poled lithium niobate) and third order nonlinear media, including AlGaAs, photorefractive media and glass, respectively. [ABSTRACT FROM AUTHOR]

One-dimensional (1-D) deep-etched gratings on a specially grown AlGaAs wafer were designed and fabricated. The gratings were fabricated using state-of-the-art electron beam lithography and high-aspect-ratio reactive ion etching (RIE) in order to achieve the required narrow deep air slots with good accuracy and reproducibility. Since remarkable etch depths (up to 1.5 μm), which completely cut through the waveguide core layer, have been attained, gratings composed of only five periods (and, thus, shorter than 6 μm) have a bandgap larger than 100 nm. A defect was introduced by increasing the width of the central semiconductor tooth to create microcavities that exhibit a narrow transmission peak (less than 7 nm) around the wavelength of 1530 nm. The transmission spectra between 1460 and 1580 nm have been systematically measured, and the losses have been estimated for a set of gratings, both with and without a defect, for different periods and air slot dimensions. Numerical results obtained via a bidirectional beam propagation code allowed the evaluation of transmissivity, reflectivity, and diffraction losses. By comparing experimental results with the authors' numerical findings, a clear picture of the role of the grating's geometric parameters in determining its spectral features and diffractive losses is illustrated. [ABSTRACT FROM PUBLISHER]

The electro-optical response of calcium barium niobate ferroelectric thin films is characterized using a single beam setup in reflection geometry. Clear evidence of a Pockels response together with an r33 coefficient as high as 130 pm/V is found. This large value and the high Curie temperature of the material under study (>250 °C) can be considered as a promising feature for the fabrication of hybrid, integrated electro-optical modulators working at a high repetition rate. [ABSTRACT FROM AUTHOR]

We report the microfabrication and characterization of SrTiO3/SiO2/Si ridge waveguides. SrTiO3 films are very attractive for optical integrated systems as they present a high transparency in the visible and infrared wavelength range as well as a relatively high refractive index. In this work, SrTiO3 films were grown by means of a reactive pulsed-laser-deposition technique and patterned using UV photolithography and high-density plasma etching. Propagation and loss characteristics at the telecommunication wavelength of 1.55 μm were investigated using top-view scattering and Fabry-Perot resonance methods. For specific ridge widths, we obtained single-mode propagation with relatively low losses (∼1.5 dB/cm), thereby demonstrating the strong potential of SrTiO3 films for guided-wave components for advanced optical integrated systems. [ABSTRACT FROM AUTHOR]

In this letter, we investigate the nonlinear transmission properties of a one-dimensional micro-structured A1GaAs waveguide with a defect in the middle of a deep-etched Bragg grating. The transmitted spectrum depends on the spectral position of the incident pulse spectrum with respect to the defect mode as well as the pulse intensity. These findings are very important for all optical switching applications and can be explained by the interplay between self-phase modulation of the incident 250 fs pulses in the waveguide and the filtering properties of the defect mode. [ABSTRACT FROM AUTHOR]

Investigates the linear and nonlinear optical properties of a non-uniform waveguide array. Decrease in effective index and induction of waveguiding along the defect by reducing the width of a single waveguide; Reduction of the index difference for increasing power levels with the result that the field escapes.