arXiv subjects

Marco Piccardo

Publications and source records attributed to Marco Piccardo.

At least 19 recordsLinked to original sources

arXiv2026

Space-time beams with tunable orbital group velocity toward plasma superradiance

Light springs are space-time beams that have a helical wavepacket. Due to this special property, light springs result into a rotating pulse when intercepting a plane lying orthogonal to their propagation direction. Associated to this, we introduce here the orbital group velocity, an additional tunable property of light springs. The orbital group velocity quantifies the speed of the light spring intensity rotation, distinctly from the conventional longitudinal group velocity, which describes the motion of the wavepacket envelope along its propagation axis. We demonstrate experimentally by tunable Fourier synthesis that the orbital group velocity can assume sub- and superluminal values, thus becoming a new platform for synthetic motion studies and control of laser-matter interactions. Particularly, in the superluminal regime, when interacting with a thin overdense plasma, we reveal by particle-in-cell simulations that the light spring unlocks superradiant radiation, due to the coherent excitation of the electrons in the plasma acting as a quasiparticle. This superradiant source inherits the ultrafast temporal dynamics of the light springs while emitting in the terahertz region, thus creating a new source of terahertz radiation controlled by the properties of spatiotemporal coupling of the laser. Therefore, spatiotemporal tuning of light springs is at the frontier of controlling laser-matter interaction and generating new tunable sources of radiation.

physics.optics↗

arXiv2026

Quantum cascade laser roadmap

Quantum cascade lasers (QCLs) are unipolar semiconductor lasers first demonstrated in 1994. Since then, they have played a central role in advancing mid-infrared and terahertz photonics, becoming among the most reliable light sources in these regions of the electromagnetic spectrum. Their importance is further reinforced by their ability to generate self-starting optical frequency combs, whose investigation is motivated both by fundamental physics and by a wide range of applications, including molecular spectroscopy and free-space optical communications. This Roadmap provides a unified overview of current advances and emerging directions in QCL research. The chapters are organized into three main sections: device design and technology; frequency combs and pulse formation; and applications of QCLs. Each chapter reviews the relevant background, summarizes the current state of the art, and identifies key challenges and future directions within its specific research area.

physics.optics↗

arXiv2025

High-aspect-ratio silica meta-optics for high-intensity structured light

Structured light and high-intensity ultrafast lasers are two rapidly advancing frontiers in photonics, yet their intersection remains largely unexplored. While ultrafast lasers continue to push the boundaries of peak intensities, structured light has enabled unprecedented control over light's spatial, temporal, and polarization properties. However, the lack of robust optical devices capable of bridging structured light with the high-intensity domain has constrained progress in combining these directions. Here, we demonstrate high-aspect-ratio silica meta-optics, which close this gap by combining silica's extraordinary damage resistance with the advanced phase and polarization control offered by metasurfaces. By leveraging anisotropic etching techniques, we fabricate nanopillars exceeding 3 $μ$m in height with aspect ratios up to 14, enabling precise manipulation of complex light fields at intensities far beyond the thresholds of conventional metasurfaces. We showcase their functionality in generating vortex beams and achieving polarization manipulation with large phase retardance at challenging long-visible wavelengths. High-aspect-ratio silica meta-optics unlock structured laser-matter interactions in extreme regimes, that will surpass plasma ionization thresholds and enable applications such as relativistic particle acceleration and high-harmonic generation with structured beams, for both tabletop ultrafast systems and large-scale laser facilities.

physics.optics↗

arXiv2025

Incoherent Diffraction Imaging with a Pseudo-Thermal Light Source

Incoherent Diffraction Imaging - IDI - is a diffraction-based imaging technique that has been recently proposed to exploit the partial coherence of incoherently scattered light to retrieve structural information from the scattering centers. Similar to the stellar intensity interferometry of Hanbury Brown and Twiss, the signal builds up on the second-order spatial correlations of the emitted light. The complex spatial distribution of the target is thereby encoded in the spatial intensity fluctuations of the scattered light. The first experimental realisations of this imaging technique have been realised using the fluorescence excited by an ultra-short X-ray pulse at Free Electron Laser (FEL) facilities. Here, we propose an alternative set-up based on a table-top Pseudo-Thermal Light Source. This set-up allows us to explore IDI under a wide range of physically relevant conditions as well as to benchmark numerical and analytical models currently used to determine the imaging capabilities of this technique.

physics.optics↗

arXiv2024

Temporal solitons in active optical resonators

Solitons, as coherent structures that maintain their shape while traveling at constant velocity, are ubiquitous across various branches of physics, from fluid dynamics to quantum fields. However, it is within the realm of optics where solitons have not only served as a primary testbed for understanding solitary wave phenomena but have also transitioned into applications ranging from telecommunications to metrology. In the optical domain, temporal solitons are localized light pulses, self-reinforcing via a delicate balance between nonlinearity and dispersion. Among the many systems hosting temporal solitons, active optical resonators stand out due to their inherent gain medium, enabling to actively sustain solitons. Unlike conventional modelocked lasers, active resonators offer a richer landscape for soliton dynamics through hybrid driving schemes, such as coupling to passive cavities or under external optical injection, affording them unparalleled control and versatility. We discuss key advantages of these systems, with a particular focus on quantum cascade lasers as a promising soliton technology within the class of active resonators. By exploring diverse architectures from traditional Fabry-Perot cavities to racetrack devices operated under external injection, we present the current state-of-the-art and future directions for soliton-based sources in the realm of semiconductor lasers and hybrid integrated photonic systems.

physics.optics↗

arXiv2024

Lasing on hybridized soliton frequency combs

Coupling is an essential mechanism that drives complexity in natural systems, transforming single, non-interacting elements into intricate networks with rich physical properties. Here, we demonstrate a chip-scale coupled laser system that exhibits complex optical states impossible to achieve in an uncoupled system. We show that a pair of coupled semiconductor ring lasers spontaneously forms a frequency comb consisting of the hybridized modes of its coupled cavity, exhibiting a large number of phase-locked tones that anticross with one another. Experimental coherent waveform reconstruction reveals that the hybridized frequency comb manifests itself as pairs of bright and dark picosecond-long solitons circulating simultaneously. The dark and bright solitons exit the coupled cavity at the same time, leading to breathing bright solitons temporally overlapped with their dark soliton counterparts - a state inaccessible for a single, free-running laser. Our results demonstrate that the rules that govern allowable states of light can be broken by simply coupling elements together, paving the way for the design of more complex networks of coupled on-chip lasers.

physics.optics↗

arXiv2024

Driven bright solitons on a mid-infrared laser chip

Despite the ongoing progress in integrated optical frequency comb technology, compact sources of short bright pulses in the mid-infrared wavelength range from 3 μm to 12 μm so far remained beyond reach. The state-of-the-art ultrafast pulse emitters in the mid-infrared are complex, bulky, and inefficient systems based on the downconversion of near-infrared or visible pulsed laser sources. Here we show a purely DC-driven semiconductor laser chip that generates one picosecond solitons at the center wavelength of 8.3 μm at GHz repetition rates. The soliton generation scheme is akin to that of passive nonlinear Kerr resonators. It relies on a fast bistability in active nonlinear laser resonators, unlike traditional passive mode-locking which relies on saturable absorbers or active mode-locking by gain modulation in semiconductor lasers. Monolithic integration of all components - drive laser, active ring resonator, coupler, and pump filter - enables turnkey generation of bright solitons that remain robust for hours of continuous operation without active stabilization. Such devices can be readily produced at industrial laser foundries using standard fabrication protocols. Our work unifies the physics of active and passive microresonator frequency combs, while simultaneously establishing a technology for nonlinear integrated photonics in the mid-infrared.

physics.optics↗

arXiv2023

Nozaki-Bekki optical solitons

Recent years witnessed rapid progress of chip-scale integrated optical frequency comb sources. Among them, two classes are particularly significant -- semiconductor Fabry-Perót lasers and passive ring Kerr microresonators. Here, we merge the two technologies in a ring semiconductor laser and demonstrate a new paradigm for free-running soliton formation, called Nozaki-Bekki soliton. These dissipative waveforms emerge in a family of traveling localized dark pulses, known within the famed complex Ginzburg-Landau equation. We show that Nozaki-Bekki solitons are structurally-stable in a ring laser and form spontaneously with tuning of the laser bias -- eliminating the need for an external optical pump. By combining conclusive experimental findings and a complementary elaborate theoretical model, we reveal the salient characteristics of these solitons and provide a guideline for their generation. Beyond the fundamental soliton circulating inside the ring laser, we demonstrate multisoliton states as well, verifying their localized nature and offering an insight into formation of soliton crystals. Our results consolidate a monolithic electrically-driven platform for direct soliton generation and open a door for a new research field at the junction of laser multimode dynamics and Kerr parametric processes.

physics.optics↗

arXiv2022

Semiconductor ring laser frequency combs with active directional couplers

Rapid development of Fabry-Perot quantum cascade laser frequency combs has converted them from laboratory devices to key components of next-generation fast molecular spectrometers. Recently, free-running ring quantum cascade lasers allowed generation of new frequency comb states induced by phase turbulence. In absence of efficient light outcoupling, ring quantum cascade lasers are not suited for applications as they are limited in their power output to microwatt levels. Here we demonstrate electrically pumped ring quantum cascade lasers with integrated active directional couplers. These devices generate self-starting frequency combs and have output power above ten milliwatts at room temperature. We study the transmission of the ring-waveguide resonator system below the lasing threshold, which reveals the ability to individually control the mode indices in the coupled resonators, their quality factors, and the coupling coefficient. When the ring resonator is pumped above the lasing threshold, the intracavity unidirectional single-mode field parametrically amplifies an externally injected signal tuned into one of the ring resonances, generating an idler sideband via four-wave mixing. The ability to inject external optical signals into integrated laser cavities brings into reach coherent control of frequency comb states in ring semiconductor lasers. Furthermore, tunable coupled active resonators pumped below the lasing threshold enable a versatile platform for the studies of resonant electromagnetic effects, ranging from strong coupling to parity-time symmetry breaking.

physics.optics↗

arXiv2022

Radially and azimuthally pure vortex beams from phase-amplitude metasurfaces

To exploit the full potential of the transverse spatial structure of light using the Laguerre-Gaussian basis, it is necessary to control the azimuthal and radial components of the photons. Vortex phase elements are commonly used to generate these modes of light, offering precise control over the azimuthal index but neglect the radially dependent amplitude term which defines their associated corresponding transverse profile. Here we experimentally demonstrate the generation of high purity Laguerre-Gaussian beams with a single step on-axis transformation implemented with a dielectric phase-amplitude metasurface. By vectorially structuring the input beam and projecting it onto an orthogonal polarisation basis, we can sculpt any vortex beam in phase and amplitude. We characterize the azimuthal and radial purity of the generated vortex beams, reaching a purity of 98% for a vortex beam with $\ell=50$ and $p=0$. Furthermore, we comparatively show that the purity of the generated vortex beams outperform those generated with other well-established phase-only metasurface approaches. In addition, we highlight the formation of 'ghost' orbital angular momentum orders from azimuthal gratings (analogous to ghost orders in ruled gratings), which have not been widely studied to date. Our work brings higher-order vortex beams and their unlimited potential within reach of wide adoption.

physics.optics↗

arXiv2022

Resonators with tailored optical path by cascaded-mode conversions

Optical resonators enable the generation, manipulation, and storage of electromagnetic waves. They are widely used in technology and fundamental research, in telecommunications, lasers and nonlinear optics, ultra-sensitive measurements in cavity optomechanics, and the study of light-matter interactions in the context of cavity QED. The physics underlying their operation is determined by the constructive interference of electromagnetic waves at specific frequencies, giving rise to the resonance spectrum. This mechanism causes the limitations and trade-offs of resonator design, such as the difficulty of confining waves larger than the resonator and the fixed relationship between free spectral range, modal linewidth, and the resonator's refractive index and size. Here, we introduce a new class of optical resonators, generating resonances by designing the optical path through transverse mode coupling in a cascaded process created by mode-converting mirrors. The generalized round-trip phase condition leads to resonator characteristics that are markedly different from Fabry-Perot resonators and can be tailored over a wide range, such as the largest resonant wavelength, the free spectral range, the linewidth, and the quality factor. We confirm the existence of these modes experimentally in an integrated waveguide cavity with mode converters coupling two transverse modes into one supermode. The resonance signature of the cascaded-mode resonator is a spectrum resulting from the coherent superposition of the coupled transverse modes. We also demonstrate a transverse mode-independent transmission through the resonator and show that its engineered spectral properties agree with theoretical predictions. Cascaded-mode resonators introduce properties not found in traditional resonators and provide a mechanism to overcome the existing trade-offs in the design of resonators in various application areas.

physics.optics↗

arXiv2021

Optical vortex crystals with dynamic topologies

Vortex crystals are geometric arrays of vortices found in various physics fields, owing their regular internal structure to mutual interactions within a spatially confined system. In optics, vortex crystals may form spontaneously within a nonlinear resonator but their usefulness is limited by the lack of control over their topology. On the other hand, programmable devices used in free space, like spatial light modulators, allow the design of nearly arbitrary vortex distributions but without any intrinsic dynamics. By combining non-Hermitian optics with on-demand topological transformations enabled by metasurfaces, we report a solid-state laser that generates vortex crystals with mutual interactions and actively-tunable topologies. We demonstrate 10x10 coherent vortex arrays with nonlocal coupling networks that are not limited to nearest-neighbor coupling but rather dictated by the crystal's topology. The vortex crystals exhibit sharp Bragg diffraction peaks, witnessing their coherence and high topological charge purity, which we resolve spatially over the whole lattice by introducing a parallelized analysis technique. By structuring light at the source, we enable complex transformations that allow to arbitrarily partition the orbital angular momentum inside the cavity and to heal topological charge defects, making these resonators a robust and versatile tool for advanced applications in topological optics.

physics.optics↗

arXiv2021

Roadmap on multimode light shaping

Our ability to generate new distributions of light has been remarkably enhanced in recent years. At the most fundamental level, these light patterns are obtained by ingeniously combining different electromagnetic modes. Interestingly, the modal superposition occurs in the spatial, temporal as well as spatio-temporal domain. This generalized concept of structured light is being applied across the entire spectrum of optics: generating classical and quantum states of light, harnessing linear and nonlinear light-matter interactions, and advancing applications in microscopy, spectroscopy, holography, communication, and synchronization. This Roadmap highlights the common roots of these different techniques and thus establishes links between research areas that complement each other seamlessly. We provide an overview of all these areas, their backgrounds, current research, and future developments. We highlight the power of multimodal light manipulation and want to inspire new eclectic approaches in this vibrant research community.

physics.optics↗

arXiv2020

Recent twists in twisted light: A perspective on optical vortices from dielectric metasurfaces

Optical vortices are the electromagnetic analogue of fluid vortices studied in hydrodynamics. In both cases the traveling wavefront, either made of light or fluid, is twisted like a corkscrew around its propagation axis - an analogy that inspired also the first proposition of the concept of optical vortex. Even though vortices are one of the most fundamental topological excitations in nature, they are rarely found in their electromagnetic form in natural systems, for the exception of energetic sources in astronomy, such as pulsars, quasars and black holes. Mostly optical vortices are artificially created in the laboratory by a rich variety of approaches. Here we provide our perspective on a technology that shook-up optics in the last decade - metasurfaces, planar nanostructured metamaterials - with a specific focus on its use for molding and controlling optical vortices.

physics.optics↗

arXiv2020

Response to Comment on Widely tunable compact terahertz gas lasers

We recently demonstrated a widely tunable THz molecular laser and reported mathematical formulas and a table for comparing how various molecules would perform as such lasers (Chevalier et al., Science, 15 November 2019, p. 856-860). Here we correct the value of a single parameter used to calculate the table (see Erratum for Chevalier et al.), thereby eliminating the concerns raised by Lampin and Barbieri (Lampin et al., arXiv:2004.04422). We also show that our simplified model for the output THz power is a better approximation than the alternative one proposed in the technical comment.

physics.app-ph↗

arXiv2020

Arbitrary polarization conversion for pure vortex generation with a single metasurface

The purity of an optical vortex beam depends on the spread of its energy among different azimuthal and radial modes. The smaller is this spread, the higher is the vortex purity and the more efficient are its creation and detection. There are several methods to generate vortex beams with well-defined orbital angular momentum but only few exist allowing to select a pure radial mode. These typically consist of many optical elements with rather complex arrangements, including active cavity resonators. Here we show that it is possible to generate pure vortex beams using a single metasurface plate in combination with a polarizer. We generalize an existing theory of independent phase and amplitude control with birefringent nanopillars considering arbitrary input polarization states. The high purity, sizeable creation efficiency and impassable compactness make the presented approach a powerful complex amplitude modulation tool for pure vortex generation, even in the case of large topological charges.

physics.optics↗

arXiv2020

Mode-locked ultrashort pulses from an 8 $μ$m wavelength semiconductor laser

Quantum cascade lasers (QCL) have revolutionized the generation of mid-infrared light. Yet, the ultrafast carrier transport in mid-infrared QCLs has so far constituted a seemingly insurmountable obstacle for the formation of ultrashort light pulses. Here, we demonstrate that careful quantum design of the gain medium and control over the intermode beat synchronization enable transform-limited picosecond pulses from QCL frequency combs. Both an interferometric radio-frequency technique and second-order autocorrelation shed light on the pulse dynamics and confirm that mode-locked operation is achieved from threshold to rollover current. Being electrically pumped and compact, mode-locked QCLs pave the way towards monolithically integrated non-linear photonics in the molecular fingerprint region beyond 6 $μ$m wavelength.

physics.optics↗

arXiv2019

Semiconductor ring laser frequency combs induced by phase turbulence

Semiconductor ring lasers are miniaturized devices that operate on clockwise and counterclockwise modes. These modes are not coupled in the absence of intracavity reflectors, which prevents the formation of a standing wave in the cavity and, consequently, of a population inversion grating. This should inhibit the onset of multimode emission driven by spatial hole burning. Here we show that, despite this notion, ring quantum cascade lasers inherently operate in phase-locked multimode states, that switch on and off as the pumping level is progressively increased. By rewriting the master equation of lasers with fast gain media in the form of the complex Ginzburg-Landau equation, we show that ring frequency combs stem from a phase instability---a phenomenon also known in superconductors and Bose-Einstein condensates. The instability is due to coupling of the amplitude and phase modulation of the optical field in a semiconductor laser, which plays the role of a Kerr nonlinearity, highlighting a connection between laser and microresonator frequency combs.

physics.optics↗