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A complex problem in terahertz photonics is the need for thin emitters [1] . Traditional free-space terahertz (THz) emission crystals rely on relatively thick crystals (on the orders of millimetres), as the terahertz emission scales with the length of the crystal. Thick crystals, however, come with a number of drawbacks. One such drawback is that the emitted THz bandwidth is limited by phase-matching conditions [2] . This amounts to thicker crystals giving a lower bandwidth, making them unsuitable for broadband spectroscopy. In near-field imaging, the image resolution depends critically on how close one can approach the sample [3] : in this regime, thick crystals can quickly become unpractical as they effectively limit the achievable resolution.

Optical-Pump Terahertz (THz) Near-Field Microscopy (OP-TNFM) is a novel technique for trans- carrier hyperspectral imaging, capable of extracting recombination dynamics and the spatially distributed complex refractive index that results from thermalised carriers in semiconducting materials.

In an open nonlinear system, emergent complex structures are generally determined by a set of global parameters. Cavity-solitons are an example of such structures and, in their temporal form, have seeded the last decade of major developments in microresonator-based optical frequency combs. These optical pulses typically require complex procedures and external feedback to be induced and maintained. Recent progress has seen the microresonators directly embedded into a laser cavity [1] , [2] where it is possible to simply initiate cavity-soliton formation by increasing the laser-diode driving current [3] . Hybrid laser-microresonator schemes have been considered since 2012 [4] , and extremely efficient laser cavity-solitons have been experimentally and theoretically demonstrated in a laser microcomb [5] .

International audience; We summarize our results on the generation of temporal laser cavity-solitons in a system comprising an optical micro-cavity nested in a fiber laser. We will discuss their features, region of existence, potential and challenges ahead. Dissipative solitons are self-confined pulses which appear in driven and lossy systems when the phase dispersion is balanced by the nonlinear phase-shift. Ultrashort pulses generated by passive mode-locking lasers are a very important example of temporal dissipative solitons in optics. Temporal cavity-solitons [1,2,3] also belong to this class of pulses and have been instrumental in the development of optical frequency combs in nonlinear micro-cavities, or "micro-combs" [1]. Temporal cavity-solitons have been largely studied in a 'driven' configuration, where an external pumping source is resonantly coupled in the nonlinear micro-resonator to sustain and excite the solitary pulses. More recently, we demonstrated that it is possible to generate localised pulses in a configuration where the micro-cavity is inserted in a fiber laser loop, as described in Figure 1. In particular, we reported the observation of laser cavity-solitons [4], which have previously attracted large attention especially in spatial configurations, such as in semiconductor lasers [5]. By merging their properties with the physics of both micro-resonators and multi-mode systems, this scheme represents a fundamentally new paradigm for the generation, stabilisation and control of solitary optical pulses in micro-cavities. Fig. 1. Scheme of the nested, travelling-wave cavities configuration: a Kerr micro-resonator (green loop) is nested in an amplifying fibre-loop (black). The amplifying cavity comprises a gain fibre (erbium-ytterbium doped amplifier, EYDFA), a tuneable delay line with optical couplers (OC), polarising beam splitter (PBS), polarisation control and a band-pass filter (BPF). The output of the laser is monitored with an optical spectrum analyser (OSA), a second-harmonic non-collinear autocorrelator and an oscilloscope to measure the radio-frequency noise of the system. © 2 0 2 0 T h e Au t h o r (s)

We demonstrate a Terahertz Ghost-Imaging technique combining nonlinear pattern generation and single-pixel Time-Domain-Spectroscopy (TDS) detection. Our technique allows reconstructing the morphology and spectral features of subwavelength samples.

We report the characterization of a balanced ultrafast photodetector. For this purpose, we use a recently developed time-domain laser-based vector network analyzer (VNA) to determine the common-mode rejection ratio (CMRR) of the device under test. This includes the frequency-domain response above the single-mode frequency of the coaxial connector. Although the balanced photodetector has a nominal bandwidth of 43 GHz, it generates voltage pulses with frequency components up to 180 GHz. We obtain a CMRR of better than 30 dB up to 70 GHz and better than 20 dB up to 110 GHz. The laser-based measurements are compared with the measurements using a digital sampling oscilloscope and with the frequency-domain measurements using a conventional VNA. We obtain good agreement between the three techniques with the laser-based method providing the largest measurement bandwidth, although it also constitutes the most complicated characterization setup.

Nanohybrid materials based on nanoparticles of the intrinsically microporous polymer PIM-1 and graphene oxide (GO) are prepared from aqueous dispersions with a re-precipitation method, resulting in the surface of the GO sheets being decorated with nanoparticles of PIM-1. The significant blueshift in fluorescence signals for the GO/PIM-1 nanohybrids indicates modification of the optoelectronic properties of the PIM-1 in the presence of the GO due to their strong interactions. The stiffening in the Raman G peak of GO (by nearly 6 cm^{-1}) further indicates p-doping of the GO in the presence of PIM. Kelvin probe force microscopy (KPFM) and electrochemical reduction measurements of the nanohybrids provide direct evidence for charge transfer between the PIM-1 nanoparticles and the GO nanosheets. These observations will be of importance for future applications of GO-PIM-1 nanohybrids as substrates and promoters in catalysis and sensing.

Plasmonic metasurfaces provide a compact platform to engineer the wave-front of optical beams by tuning the material and its morphology, hence enabling advanced functionalities in ultra-thin photonic systems [1,2]. In standard metasurfaces, however, the optical response is usually static and fixed by design. An appealing possibility to achieve ultrafast dynamical tuning is given by optically-induced plasmonic systems, where the metallic response of narrow-bandgap semiconductors is driven by high-fluence illumination. Under these conditions, the surface of the semiconductor can be overflown with photo-carriers inducing a transient metallic state [3]. An intriguing question is whether the transient metallization could be employed to dynamically engineer the optical response and to control light-matter interactions on the surface.

We report on 'slow' pulsing dynamics in a silica resonator-based laser system: by nesting a high-Q rod-resonator inside an amplifying fiber cavity, we demonstrate that trains of microsecond pulses can be generated with repetition rates in the hundreds of kilohertz. We show that such pulses are produced with a period equivalent to several hundreds of laser cavity roundtrips via the interaction between the gain dynamics in the fiber cavity and the thermo-optical effects in the high-Q resonator. Experiments reveal that the pulsing properties can be controlled by adjusting the amplifying fiber cavity parameters. Our results, confirmed by numerical simulations, provide useful insights on the dynamical onset of complex self-organization phenomena in resonator-based laser systems where thermo-optical effects play an active role. In addition, we show how the thermal state of the resonator can be probed and even modified by an external, counter-propagating optical field, thus hinting towards novel approaches for all-optical control and sensing applications. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

We consider the passive modulation of the surface field at a junction between semiconductors and 2D-materials. We experimentally demonstrate that this boosts the optical-to-terahertz conversion efficiency beyond that of benchmark surface emitters.

Plasmonic metamaterials enable access to extremely nonlinear regimes with remarkable full-field control. We theoretically and experimentally demonstrate a novel form of photo-induced semiconducting Time-Dependent metamaterial at THz frequencies.

We demonstrate the first fully solid-state technique for the coherent detection of ultra-broadband THz pulses (0.1-10 THz), relying on the electric-field-induced second-harmonic generation attained in integrated CMOS-compatible devices.

We experimentally investigated a novel form of photo-induced plasmonic response, in nanostructured silicon, at THz frequencies which can be employed to precisely control the full-wave properties, i.e. amplitude and phase, of the generated THz pulse.

The interest in surface terahertz emitters lies in their extremely thin active region, typically hundreds of atomic layers, and the agile surface scalability. The ultimate limit in the achievable emission is determined by the saturation of the several different mechanisms concurring to the THz frequency conversion. Although there is a very prolific debate about the contribution of each process, surface optical rectification has been highlighted as the dominant process at high excitation, but the effective limits in the conversion are largely unknown.\ud \ud The current state of the art suggests that in field-induced optical rectification a maximum limit of the emission may exist and it is ruled by the photocarrier induced neutralisation of the medium's surface field. This would represent the most important impediment to the application of surface optical rectification in high-energy THz emitters.\ud \ud We experimentally unveil novel physical insights in the THz conversion at high excitation energies mediated by the ultrafast surface optical rectification process. The main finding is that the expected total saturation of the Terahertz emission vs pump energy does not actually occur. At high energy, the surface field region contracts towards the surface. We argue that this mechanism weakens the main saturation process, re-establishing a clearly observable quadratic dependence between the emitted THz energy and the excitation. This is relevant in enabling access to intense generation at high fluences.

In this contribution, we will review our recent activities in the development of double nested cavity lasers. Several stable operating regimes can be achieved over a wide range of conditions.

The interplay between ultrafast and slow nonlinearities, like Kerr and thermo-optical nonlinearities, is well-known for generating bistability, self-pulsing and chaotic regimes in optical microcavities. These effects have been observed in different kinds of devices, such as doped silica glass optical microcavities with high refractive index [1], and numerical models have been investigated to provide a description of such dynamics, e.g. in semiconductor nanocavities [2-3]. Specifically, self-pulsing can be explored to generate all-optical stochastic sources, and it has been recently demonstrated with opto-mechanical nonlinearities [4].

We demonstrate a robust control over the repetition rate of a micro-comb laser by using the Filter-Driven Four Wave Mixing scheme. Results show that the coherence can be kept during the whole process.

We experimentally investigate the enhancement and phase change of the THz emission from photo-excited randomly structured black silicon substrates mediated by an induced metallic state of the nanostructured surface.

We numerically and experimentally demonstrate the generation of Type II micro-combs in a Filter-Driven four wave mixing laser, showing that the generated Type II micro-combs are fully coherent.

In this paper, we demonstrate a novel dual-pump approach to generate robust optical frequency comb with varying free spectral range (FSR) spacing in a CMOS-compatible high-Q micro-ring resonator (MRR). The frequency spacing of the comb can be tuned by an integer number FSR of the MRR freely in our dual-pump scheme. The dual pumps are self-oscillated in the laser cavity loop and their wavelengths can be tuned flexibly by programming the tunable filter embedded in the cavity. By tuning the pump wavelength, broadband OFC with the bandwidth of >180 nm and the frequency-spacing varying from 6 to 46-fold FSRs is realized at a low pump power. This approach could find potential and practical applications in many areas, such as optical metrology, optical communication and signal processing systems, for its excellent flexibility and robustness.

This review summarizes the results obtained in 2011 toward the development of an "integrated optical oscilloscope," encompassing research on new technologies for the measurement of ultrafast optical pulses in both amplitude and phase through the use of integrated devices, some of which are compatible with electronic circuit technology (CMOS).

We demonstrate temporal measurements of sub-picosecond optical pulses via time-to-frequency conversion in a 45-cm-long CMOS compatible high-index glass spiral waveguide. The measurements are based on efficient four-wave mixing in the C-band, using around 1 W of peak pump power. We achieve a resolution of 400 fs over a time window of 100 ps, representing a time-bandwidth product >; 250.

We report on light-induced optical tuning of colloidal photonic crystals doped with gold nanoparticles (Au-nps). By resonantly exciting the Au-np surface plasmon absorption with picosecond pulses at 0.53 micron in a standard pump-probe setup, we observed permanent changes in the stop band resonance around 1.7 micron, with blue wavelength shifts as large as 30 nm and associated to a nanoparticle reshaping. Fine tuning was achieved by controlling either the pulse energy or the irradiation time.

We demonstrate two-dimensional spatial solitons excited by near-infrared picosecond pulses in Kerr-like heavy metal oxide glasses with a nonlinearity one order of magnitude larger than in fused silica. Solitons were obtained at 820 nm owing to the presence of multiphoton absorption, which prevented catastrophic collapse. © 2007 Optical Society of America., This work was funded by an “Integrated Action” between Italy and Spain (IT1890/HI2006-0095) and by the Spanish Ministry of Education and Science (MAT2005-06508-C02-01 and TEC2005-00074/MIC).

We summarize here our recent results on high-repetition-rate laser mode-locking via microring resonators. The recent realization of multiple wavelength oscillators based on high Q-factor resonators has profoundly impacted optical frequency combs for metrology and precision measurements and other areas. The ability to mode-lock these devices is important for achieving high-precision optical clocks in a monolithic format, offering enormous benefits in performance, cost, size, and power consumption for optical waveform synthesis, high-capacity telecommunications, optical analog-to-digital conversion, and many other applications. Here, we demonstrated a mode-locked laser based on a monolithic high Q resonator. This device produces stable, self-starting transform-limited optical pulses at 200.3 GHz, with a sub-100 kHz linewidth and extremely narrow amplitude noise bandwidth of

We demonstrated coherent Terahertz characterization based on the Terahertz Field Induced Second Harmonic effect in few microns thick Silica (SiO2) samples that can be easily operated with as little as a few hundred Volts. Our sample is an infinitely long 30 μm slit written in gold. The underlying idea of our work is that of exploiting the large breakdown voltage and the high nonlinearity of glass to achieve a large signal to noise ratio detection with a very weak optical probe at moderate bias fields, thus drastically reducing the electrodes gap. Our results pave the way to a novel approach towards broadband THz detection.

Four-wave mixing in a thermal tunable 5th order cascaded CMOS compatible ring resonators is experimentally and numerically investigated. Flat bandwidth conversion of 3.1GHz is obtained for a range of parameter.

We demonstrated a 1.81 μm mode-locked figure-eight laser using an integrated 11-th order microring resonator. Mode-locked train centered at 1.81 μm with repetition rate of 9.09 MHz is achieved.

We carried out second-harmonic generation in quasi-phase-matched ? -phase lithium niobate channel waveguides realized by proton exchange and surface periodic poling. Owing to a limited ferroelectric domain depth, we could observe the interplay between second-harmonic generation and self-phase modulation due to cascading and cubic effects, resulting in a nonlinear resonance shift. Data reduction allowed us to evaluate both the quadratic nonlinearity in the near infrared as well as the depth of the uninverted domains. © 2007 Optical

We implement a simple and powerful approach to characterize the domain distribution in the bulk of quadratic ferroelectric crystals via far-field second-harmonic spectroscopy. The approach is demonstrated in a lithium tantalate sample with periodic electric field poling and random mark-to-space ratio.

We introduce and test a simple approach for the characterization of domain distribution in bulk quadratic ferroelectric crystals, specifically periodically poled Lithium Tantalate with random mark-to space ratio.

We introduce and test a simple approach for the characterization of domain distribution in bulk quadratic ferroelectric crystals, such as periodically poled Lithium Tantalate with random mark-to space ratio.

Exploiting four-wave mixing between an optical probe and a Terahertz field in a few micrometers thick Silica sample we demonstrate broadband coherent Terahertz detection by applying a bias as low as 100 V.

Second harmonic generation is investigated in lithium niobate channels realized by proton exchange and quasi-phase-matched by surface periodic-poling. The reduction in conversion efficiency at high powers is interpreted in terms of multi-photon absorption via two-color terms, yielding an estimate of the dominating three-photon process., Comment: 2 pages, 6 figures

We demonstrate ultraviolet generation in lithium tantalate channel waveguides for frequency doubling via quasi-phase-matching. The samples, proton exchanged and nanostructured by electric-field assisted surface periodic poling with domains as deep as 40 μm, yield continuous wave light at 365.4 nm with conversion efficiencies larger than 7.5% W-1 cm-2.

In this Communication, a stable self-confined propagation with ps near-infrared (NIR) pulses and over several Rayleigh lengths is demonstrated for the first time . Multiphoton absorption is shown to saturate the self-focusing response in a novel glass of the ternary system Nb2O5-O2-PbO (NPG) exhibiting a high refractive index (2-2.1) in the NIR and an energy gap of 3.52 eV. NPG belongs to the family of heavy metal oxides, widely studied for its fast and high nonlinearity in the infrared.

We report on recent results by surface periodic poling on lithium niobate and lithium tantalate. Such approach allows periodic inversion of the second order susceptibility with nanoscale features using insulating masks. We achieved a world-best 200 nm feature size, as well as good compatibility with alpha-phase proton exchanged channel waveguides in lithium niobate. Preliminary results of surface periodic poling in lithium tantalate also show similar characteristics. Surface poling is best suited for integrated optics devices in technologically-demanding configurations such as backward second harmonic generation and counter propagating optical parametric amplification

We demonstrate novel microcavity lasers in an integrated, CMOS compatible platform. This platform has promise for telecommunications and on-chip WDM optical interconnects.

We demonstrate a range of novel functions based on a high index doped silica glass CMOS compatible platform. This platform has promise for telecommunications and on-chip WDM optical interconnects for computing.

We introduce a novel algorithm for phase reconstruction X-SPIDER that significantly extends the measurement time windows and test it in an integrated CMOS SPIDER device.

We report a mode locked laser based on an integrated high-Q microring resonator with a highly monochromatic radiofrequency modulation thanks to the stable operation of two slightly shifted spectral optical comb replicas.

We report a mode locked laser based on an integrated high-Q microring resonator with a highly monochromatic radiofrequency modulation thanks to the stable operation of two slightly shifted spectral optical comb replicas.

We report a novel geometry for OPOs in a CMOS-compatible microring resonator. It exploits non-critical lasing of the pump inherently positioned within the resonances of the microcavity, thus counteracting the effect of thermal fluctuations.

Broadband optical communication systems are rapidly becoming the key to overcome the stringent limitations imposed by standard electronic telecommunication networks. However, in order to complete the inevitable transition from electronics to photonics, several critical requirements must be addressed, including lowering energetic demands, achieving higher efficiency, increasing bandwidth and flexibility, all within a compact form factor [1–3]. In particular, it is broadly accepted that future photonic devices must be CMOS compatible in order to exploit the existing silicon fabrication technology that has been largely developed during the last 60 years [4–7]. Following this idea, there has been a tremendous growth of hybrid optoelectronic technologies that has not only responded to the need of lowering costs, but has also enabled on-chip ultra-fast signal processing. However, these hybrid solutions are an intermediate step to achieve the ambitious goal of an all-optical technology, which would bring together the intrinsic benefit of lowering the production costs and simplifying future ultrafast communication networks.

All-optical signal processing has been demonstrated extensively in Si including demultiplexing at 160Gb/s via four-wave mixing (FWM) [1] and optical regeneration [2], as well as in chalcogenide glass (ChG) waveguides[3]. The efficiency of all-optical devices can be improved by increasing the nonlinear parameter, γ = ω n 2 / c A eff (A eff is the waveguide effective area, n 2 is the Kerr nonlinearity) as well as by using resonant structures to enhance the local field intensity. Semiconductors and ChG offer excellent optical confinement and a high n 2 that has produced γ's of 200,000 W−1 km−1 for Si nanowires [2], and 93,400 W−1 km−1 in ChG nanotapers [4]. Yet silicon suffers from a poor intrinsic nonlinear figure of merit (FOM = n 2 / (β λ), where β is the two-photon absorption coefficient) due to two-photon absorption (TPA) and the resulting free carriers [5]. Here, we report a wide range of functions in a CMOS compatible high index doped silica glass platform, including an integrated multiple wavelength source [6,7], an ultra-high repetition rate modelocked laser [8], and an all-optical integrator[9]. We achieve CW oscillation in a Q=1.2 million micro-ring resonator with a slope efficiency of 7.4% (single oscillating mode, single port), a threshold power of 54mW, and frequency spacings from 200GHz to > 6THz. We also achieve modelocking with repetition rates as high as 800GHz [9]. We also report novel functions in ultra-long (45cm) spiral waveguides, including a time-lens optical oscilloscope and parametric amplifier with > 15dB of gain [10]. The success of these devices is due to very low linear loss, negligible nonlinear loss (up to 25GW/cm2), and a high γ ≅ 233W−1km−1. The low loss, design flexibility, and CMOS compatibility of this platform will enable devices for telecommunications, computing, sensing, metrology and other areas.

We present a dissipative four wave mixing tunable laser based on a integrated CMOS-compatible high-Q nonlinear ring resonator, emitting subpicosecond pulses at 200GHz-repetition rate. Quasi-sinusoidal 800GHz emission regime is also demonstrated.

We present an investigation on the nonlinear dynamics of intense pulses in an AlGaAs Bragg waveguide and we report the experimental observation of an intensity dependent blue-shift of the Bragg notch spectral line.

The recent introduction(1-3) of coherent optical communications has created a compelling need for ultrafast phase-sensitive measurement techniques operating at milliwatt peak power levels and in timescales ranging from sub-picoseconds to nanoseconds. Previous reports of ultrafast optical signal measurements(4-7) in integrated platforms(8-10) include time-lens temporal imaging(5) on a silicon chip(8,9) and waveguide-based frequency-resolved optical gating (FROG)(4,6,10). Time-lens imaging is phase-insensitive, and waveguide-based FROG methods require the integration of long tunable delay lines, which is still an unsolved challenge. Here, we report a device capable of characterizing both the amplitude and phase of ultrafast optical pulses with the aid of a synchronized incoherently related clock pulse. It is based on a novel variation of spectral phase interferometry for direct electric-field reconstruction (SPIDER)(4,7) that exploits degenerate four-wave mixing in a CMOS-compatible chip. We measure pulses with a peak power of 1 THz, and up to 100 ps pulsewidths, yielding a time-bandwidth product of >100.