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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.

Ghost imaging, based on single-pixel detection and multiple pattern illumination, is a crucial investigative tool in difficult-to-access wavelength regions. In the terahertz domain, where high-resolution imagers are mostly unavailable, ghost imaging is an optimal approach to embed the temporal dimension, creating a “hyperspectral” imager. In this framework, high resolution is mostly out of reach. Hence, it is particularly critical to developing practical approaches for microscopy. Here we experimentally demonstrate time-resolved nonlinear ghost imaging, a technique based on near-field, optical-to-terahertz nonlinear conversion and detection of illumination patterns. We show how space–time coupling affects near-field time-domain imaging, and we develop a complete methodology that overcomes fundamental systematic reconstruction issues. Our theoretical-experimental platform enables high-fidelity subwavelength imaging and carries relaxed constraints on the nonlinear generation crystal thickness. Our work establishes a rigorous framework to reconstruct hyperspectral images of complex samples inaccessible through standard fixed-time methods.

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.

Modern optical systems increasingly rely on complex physical processes. Advanced light sources, such as supercontinuum (SC) [1], are highly sought for imaging and metrology, and are based on nonlinear dynamics where the output properties must often finely match target performance characteristics. However, in these systems, the availability of control parameters and the means to adjust them in a versatile manner are usually limited. Moreover, finding the ideal parameters for a specific application can become inherently complex. Here, we use an actively-controlled photonic chip to prepare and manipulate patterns of femtosecond optical pulses seeding supercontinuum generation [1]. Taking advantage of machine learning concepts [2], we exploit this access to an enhanced and tunable parameter space and experimentally demonstrate the customization of nonlinear interactions responsible for tailoring supercontinuum properties [3].

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.

Micro-resonator-based frequency combs, or micro-combs, have gained considerable interest in recent years due to their many potential applications such as high-speed communication systems, spectroscopy and ultrafast optical clocks [1]. Most micro-combs systems are based on laser pumped optical parametric oscillation and are typically non-self-starting, requiring a well-defined warm-up strategy involving smart control [2]. An alternative approach to micro-combs is represented by the Filter-Driven Four-Wave Mixing (FD-FWM) laser [3–5], based on a nonlinear micro-resonator nested in a main amplifying fibre cavity. Although this system has demonstrated self-starting regimes, stable operation typically imposes a strict relation between the minimum free-spectral range (FSR) of the main-cavity and the Q-factor of the micro-resonator. The use of longer main-cavity fibre lengths (highly desirable for several positive features, such as a larger gain) results in unrecoverable unstable regimes, i.e. in super-mode instability, which arises from the existence of many oscillating main-cavity modes within each micro-resonator resonance.

One of the major challenges in photonics is the full-wave reconstruction of arbitrary field distributions. Indeed, measuring both the amplitude and the optical delay (or optical phase) entails achieving breakthroughs in a wide range of scenarios, spanning from bio-imaging to material characterisation [1]. Time-Domain Spectroscopy (TDS) is routinely employed to perform field-sensitive measurements at terahertz (THz) frequencies. THz has been widely employed in developing advanced spectroscopy applications, thanks to its non-ionizing nature and to the fact that several materials possess a distinctive THz signature. All these features allow to precisely determine the composition of complex samples with critical implications in several fields, such as in-vivo biological imaging, medical diagnosis, security scanners and manufacturing control [2–3]. Despite the large body of research, however, the limited availability of high-resolution, full-wave imaging devices (i.e. field-sensitive THz cameras) poses a critical technological limit in this promising field.

Nonlinear optics in sub-wavelength structures plays a crucial role in the development of integrated photonics platforms and devices [1]. Surfaces, nanostructures and two-dimensional media are inherently free from longitudinal phase matching constraints and constitute an attractive framework to engineer complex forms of lightmatter interactions [2–3]. Most recent efforts in this area have focused on the generation of frequencies in the optical and infrared regions, e.g., second or third harmonic generation. An open question, however, is whether the broadband frequency conversion enabled by nanoscale nonlinear media could lead to the generation of long-wavelength radiation in a process known as Optical Rectification (OR). To date, OR of ultrafast optical pulses in nonlinear media has been subject of intense research, since it provides a robust and versatile approach to generate and manipulate broadband Terahertz (THz) fields. Using non-centrosymmetric crystals such as ZnTe and LiNbO 3 , large bandwidth THz pulses limited mainly by longitudinal phase-matching constraints, could be achieved [4]. Highly nonlinear Quasi-2D structures represent an intriguing alternative to this popular approach.

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.

Significant progress in nonlinear and ultrafast optics has recently opened new and exciting opportunities for terahertz (THz) science and technology, which require the development of reliable THz sources, detectors, and supporting devices. In this work, we demonstrate the first 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 in a thin layer of ultraviolet fused silica. The proposed CMOS-compatible devices, which can be realized with standard microfabrication techniques, allow us to perform ultra-broadband detection with a high dynamic range by employing probe laser powers and bias voltages much lower than those used in gas-based techniques. Eventually, this may pave the way for the use of high-repetition-rate ultrafast lasers and commercially available electronics for the coherent detection of ultrashort THz pulses.

Ring opening polymerization (ROP) of N-carboxy anhydride (NCA) amino acids presents a rapid way to synthesize high molecular weight polypeptides with different amino acid compositions. The compositional and functional versatility of polypeptides make these materials an attractive choice for biomaterials. The functional performance of polypeptide materials is equally linked to their conformation which is determined by the amino acid sequence in the polymer chains. Here, the interplay between composition and conformation of synthetic polypeptides obtained by NCA polymerization was explored. Various copolypeptides from Glu(Bzl) and Ser(Bzl) were prepared to investigate how polypeptide composition affected the conformation of the resulting copolymer. Polymerization kinetics indicated that the copolymerization of Glu(Bzl) and Ser(Bzl) preferentially yielded alternating copolymers. Both the polydispersity and the conformation of the polypeptides were dependent on the Ser(Bzl) content in the polymer, demonstrating that polypeptide functionalities could be tuned directly by altering the relative amounts of amino acids in the chain. This work presents the first step toward an improved understanding and control over polypeptide conformation through modulating the amino acid composition of the material. Understanding this sequence–functionality relationship is essential to advancing the use of ROP as a technique to design smart polypeptide based materials with specific functions. © 2016 The Authors. Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 2331–2336

Modern optical systems increasingly rely on complex physical processes that require accessible control to meet target performance characteristics. In particular, advanced light sources, sought for, for example, imaging and metrology, are based on nonlinear optical dynamics whose output properties must often finely match application requirements. However, in these systems, the availability of control parameters (e.g., the optical field shape, as well as propagation medium properties) and the means to adjust them in a versatile manner are usually limited. Moreover, numerically finding the optimal parameter set for such complex dynamics is typically computationally intractable. Here, we use an actively controlled photonic chip to prepare and manipulate patterns of femtosecond optical pulses that give access to an enhanced parameter space in the framework of supercontinuum generation. Taking advantage of machine learning concepts, we exploit this tunable access and experimentally demonstrate the customization of nonlinear interactions for tailoring supercontinuum properties., Controlling complex properties of optical systems, like the output of nonlinear light sources, is increasingly important for applications. Here, Wetzel et al. use an actively-controlled photonic chip to prepare patterns of femtosecond laser pulses used for tailoring supercontinuum generation.

The FD-FWM is a proven approach to implement coherent Microcomb generation. Comb repetition rates can be finely controlled in a robust way, while maintaining the coherence during the whole process, whereas full-coherent type-I and type-II combs can be accessible. We present our recent advances towards effective control of optical combs exploiting integrated chip-based resonators.

The quest towards the integration of ultra-fast, high-precision optical clocks is reflected in the large number of high-impact papers on the topic published in the last few years. This interest has been catalysed by the impact that high-precision optical frequency combs (OFCs) have had on metrology and spectroscopy in the last decade [ [1] , [2] , [3] , [4] , [5] ]. OFCs are often referred to as optical rulers: their spectra consist of a precise sequence of discrete and equally-spaced spectral lines that represent precise marks in frequency. Their importance was recognised worldwide with the 2005 Nobel Prize being awarded to T.W. Hansch and J. Hall for their breakthrough in OFC science [ 5 ]. They demonstrated that a coherent OFC source with a large spectrum – covering at least one octave – can be stabilised with a self-referenced approach, where the frequency and the phase do not vary and are completely determined by the source physical parameters. These fully stabilised OFCs solved the challenge of directly measuring optical frequencies and are now exploited as the most accurate time references available, ready to replace the current standard for time. Very recent advancements in the fabrication technology of optical micro-cavities [ 6 ] are contributing to the development of OFC sources. These efforts may open up the way to realise ultra-fast and stable optical clocks and pulsed sources with extremely high repetition-rates, in the form of compact and integrated devices. Indeed, the fabrication of high-quality factor (high-Q) micro-resonators, capable of dramatically amplifying the optical field, can be considered a photonics breakthrough that has boosted not only the scientific investigation of OFC sources [ [8] , [13] , [11] , [12] , [10] , [7] , [9] ] but also of optical sensors and compact light modulators [ [14] , [6] ]. In this framework, the demonstration of planar high-Q resonators, compatible with silicon technology [ [14] , [13] , [11] , [12] , [10] ], has opened up a unique opportunity for these devices to provide entirely new capabilities for photonic-integrated technologies. Indeed, it is well acknowledged by the electronics industry that future generations of computer processing chips will inevitably require an extremely high density of copper-based interconnections, significantly increasing the chip power dissipation to beyond practical levels [ [15] , [16] , [17] ]; hence, conventional approaches to chip design must undergo radical changes. On-chip optical networks, or optical interconnects, can offer high speed and low energy per-transferred-bit, and micro-resonators are widely seen as a key component to interface the electronic world with photonics. Many information technology industries have recently focused on the development of integrated ring resonators to be employed for electrically-controlled light modulators [ [14] , [15] , [16] , [17] ], greatly advancing the maturity of micro-resonator technology as a whole. Recently [ [13] , [11] , [12] ], the demonstration of OFC sources in micro-resonators fabricated in electronic (i.e. in complementary metal oxide semiconductor (CMOS)) compatible platforms has given micro-cavities an additional appeal, with the possibility of exploiting them as light sources in microchips. This scenario is creating fierce competition in developing highly efficient OFC generators based on micro-cavities which can radically change the nature of information transport and processing. Even in telecommunications, perhaps a more conventional environment for optical technologies, novel time-division multiplexed optical systems will require extremely stable optical clocks at ultra-high pulse repetition-rates towards the THz scale. Furthermore, arbitrary pulse generators based on OFC [ [18] , [19] ] are seen as one of the most promising solutions for this next generation of high-capacity optical coherent communication systems. This review will summarise the recent exciting achievements in the field of micro-combs, namely optical frequency combs based on high-Q micro-resonators, with a perspective on both the potential of this technology, as well as the open questions and challenges that remain.

We experimentally demonstrate the generation of highly coherent Type-II micro-combs based on a microresonator nested in a fiber cavity loop, known as the filter-driven four wave mixing (FD-FWM) laser scheme. In this system, the frequency spacing of the comb can be adjusted to integer multiples of the free-spectral range (FSR) of the nested micro-resonator by properly tuning the fiber cavity length. Sub-comb lines with single FSR spacing around the primary comb lines can be generated. Such a spectral emission is known as a “Type-II comb.” Our system achieves a fully coherent output. This behavior is verified by numerical simulations. This study represents an important step forward in controlling and manipulating the dynamics of an FD-FWM laser.

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.

Surface-Terahertz generation at high excitation is dominated by both the nonlinear response and surface field dynamics. Our experimental characterization sheds light on this synergy revealing that no hard saturated limit for the THz emission exists.

Reading a mathematical problem can be often challenging for students. To be successful, they need to understand the words, transposing their meaning on an abstract level, to identify the relations stated in the problem and the task requested. Even if the meaning of the words is known, the case can be given that the student cannot grab the global meaning, nor inferring useful implications to solve the problem. This article offers an overview on reading comprehension issues linked to mathematical texts, and then proposes a set of common mistakes made by young readers, emerged in a first exploratory field experience carried out with grade 7 students. Given that learning activities were carried out on the Pythagorean Theorem, specific attention was devoted to the transposition of words into geometrical figures.

We propose a scheme for bifurcation control in micro-cavities based on the interplay between the ultrafast Kerr effect and a slow nonlinearity, such as thermo-optical, free-carriers-induced, or opto-mechanical one. We demonstrate that Hopf bifurcations can be efficiently controlled with a low energy signal via four-wave mixing. Our results show that new strategies are possible for designing efficient micro-cavity-based oscillators and sensors. Moreover, they provide new understanding of the effect of coherent wave mixing in the thermal stability regions of optical micro-cavities, fundamental for micro-resonator-based applications in communications, sensing, and metrology, including optical micro-combs.

We introduce a method for diagnosing the electric surface potential of a semiconductor based on THz surface generation. In our scheme, that we name Optical Pump Rectification Emission, a THz field is generated directly on the surface via surface optical rectification of an ultrashort pulse after which the DC surface potential is screened with a second optical pump pulse. As the THz generation directly relates to the surface potential arising from the surface states, we can then observe the temporal dynamics of the static surface field induced by the screening effect of the photo-carriers. Such an approach is potentially insensitive to bulk carrier dynamics and does not require special illumination geometries.

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].

In this work, we study the nonlinear dynamics of micro-resonators in the presence of a thermo-optic effect, showing that four-wave mixing can trigger and control periodic thermal instabilities and chaotic regimes.

Supplemental document

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.