We numerically study programmable laser cavity soliton generation within a nested cavity architecture. The nested architecture includes a fibered Fabry-Pérot resonator as well as a photonic device used as programmable delay-line, which enables repetition rate control from GHz to THz. In the proposed approach, the pulse repetition rate is adjusted based on a temporal interleaving process, obtained after propagation through the programmable delay-line structure. Our numerical study demonstrates how a programmable delay-line can be leveraged to obtain a highly-tunable passively mode-locked laser via harmonic mode-locking, featuring the formation of energy efficient yet reconfigurable laser cavity soliton pulses. In particular, we outline practical guidelines for experimental implementation and the proposed architecture is expected to provide enhanced versatility for compact and tunable frequency comb generation.

We propose and numerically study a fully programmable Raman laser, based on a double cavity fiber architecture. The two cavities are coupled by a photonic integrated device used as programmable delay-line and ultimately form a Raman fiber laser controllable in both repetition rate and optical carrier frequency. A first comb is generated in a EDFA based cavity soliton laser (1560 nm), and used as an optical pump for a second fiber loop acting as a tunable Raman laser (1645-1741 nm). The latter provides a Stokes comb whose carrier frequency is readily adjustable by means of the programmable delay-line. The final device allows the generation of two programmable combs, with emission properties that can be tailored in terms of repetition rate, spectral width and optical carrier difference. This work provides the numerical basis and essential features of this tunable Raman laser architecture towards experimental implementation.

Abstract Spintronic emitters promise to revolutionise terahertz (THz) sources by converting ultrafast optical pulses into broadband THz radiation without phase-matching constraints. Because the conversion relies on spin-current injection across a nanometre-thin magnetic layer, its efficiency is ordinarily limited by weak optical coupling. Here, we present a demonstration of a drop-casting based approach to introduce ultrafast plasmonic-mediated coupling: a sparse-layer of silica–gold core–shell nanoparticles is deposited directly onto a W/Fe/Pt spintronic trilayer. This sparse (≈ 6%) decoration leads to a measured enhancement of the emitted THz peak field between 1.1x and 1.6x relative to the bare stack as the angle is increased from 0° to 75°, pointing to a very high local conversion enhancement for this low-coverage spintronic emitter compared with the bare stack, with the maximum emission reached at 0°. This demonstration points to a viable pathway toward highly efficient spintronic terahertz emitters with potential applications in spectroscopy, imaging, and ultrafast technologies.

Fiber Fabry-Perot resonators have proven their ability to generate broad and stable optical frequency combs, and are ideal devices for fiber systems as they are high-Q, compact, and easily integrated with FC/PC connectors. Here, we present an advanced fiber Fabry-Perot resonator designed for multi-frequency comb generation and spatial multiplexing. The resonator is fabricated using a three-core optical fiber and is able to generate two mutually coherent frequency combs while being locked to a driving laser. Multiplexing of the combs is achieved with a fan-in/fan-out system, enabling a fully fiber-based experimental setup. The generated combs, induced by cavity solitons, feature a 1.27 GHz repetition rate and a bandwidth above 40 nm. A slight difference in the group index of each core leads to a 112 kHz repetition rate offset between the combs, enabling dual-comb spectroscopy proof-of-concept measurement of a 0.1 nm absorption band.

We present a physics-guided discrimination approach for the practical identification of laser cavity solitons in a microresonator-filtered fibre laser. We introduce simple experimental observables derived from radio-frequency characterisation of the laser output, such as the low-frequency variance around DC and the normalised AC power. We show that, in conjunction with simple optical-spectrum observables such as the number of comb lines, they are sufficient to discriminate laser cavity solitons within the experimental microcomb landscape defined by the key control parameters of the system—erbium pump power and fibre-cavity length. Based on these quantities, we discuss simple heuristic figures of merit that exhibit a clear minimum for soliton states, paving the way for automated state discrimination.

Core-shell nanoparticles on an ultrathin spintronic trilayer support orientation-independent localized plasmons that mediate local heating. We demonstrate macroscopic terahertz emission enhancement at very low coverage, pointing to strongly boosted local fields at the interface.

Metrological-grade millimetre wave baseband comb sources covering the subterahertz window are a key building block for next-generation wireless communications, precision sensing, and positioning systems. While optical microcombs have set new benchmarks in ultra-low phase noise single-frequency microwave generation, to date, no microcomb source has directly produced a millimetre-wave baseband comb. Here, we present a 50 GHz repetition rate carrier-envelope offset estabilised millimetre-wave baseband comb source covering the sub-terahertz region, generated from an optical microcomb source. Our microresonator-filtered microcomb enables direct, coherent downconversion via photoconductive antennas, even without external amplification. The metrological-grade optical soliton source produces single-cycle, naturally zero carrier-envelope offset millimetrewave baseband combs. It supports time-domain spectroscopy without any need to temporally align the source and detection pulses, as the ultra-high phase coherence allows significant differences between the optical paths of the source and detection pulses, which we tested over 8m, finding no degradation even in freerunning operation. Finally, the multisoliton operation regime provides a simple way of spectrally tailoring the microwave output by selecting different optical soliton states.

Since their first conception in the 60’ s, mode-locked lasers (MLL) [1] have proved their efficiency to provide broad and coherent combs. Based on passive mode-locking strategies [2], ultrashort (sub-picosecond) and stable pulse trains are commonly realized nowadays [3]. Nevertheless, those broadband spectrums are generated at low repetition rate (sub GHz) which constitute the principal limitation of such approach. Over the last decades, Kerr comb regains attention towards providing broad spectrum and high repetition rate [4]. In particular, nested cavity architectures are of prime interest due to their efficiency and self-stabilization capabilities [5], yet the repetition rate of such lasers remains fixed by the dimensions of the resonator employed. Here, we propose a novel approach for fiber Kerr comb laser, including a Programmable Delay Line (PDL) based on photonic integrated circuits (Fig. 1a) [6]. We numerically show that, by selecting the required harmonic of the MLL oscillation, this device allows for controlling the repetition rate of a Kerr comb generated within a nested cavity configuration [7].

Microcombs are optical frequency combs in microresonators [1], [2]. Laser cavity-soliton (LCS) states can be achieved in a system comprising a nonlinear Kerr microresonator nested in a fibre laser [3], resulting in a self-emergent, robust [4], and efficient [5] microcomb. Generally, the platform generally produces a broader variety of states depending on the specific parameter settings of the system. The two critical experimental parameters that need to be adjusted to achieve solitary oscillation via self-emergence [4] are the gain, which controls the state energy, and the length of the main cavity, which governs the group velocity mismatch between the two cavities. When searching for solitary oscillations, these parameters must be spanned over large ranges. While detailed information can be obtained through accurate but time-consuming methods like laser scanning spectroscopy and interferometry, real-time approaches are preferable when handling large datasets. In this work, we introduce a rapid method to distinguish typical lasing states, including soliton states, based on simple experimental properties. Specifically, our approach utilises radiofrequency (RF) data and spectral analysis to map states within nonlinear optical systems in real time, with minimal computational effort when handling large datasets.

Dual-comb (DC) spectroscopy has revolutionized the field of metrology by enabling fast and accurate measurements using low-pass photodetectors [1]. This interferometric technique however requires high mutual coherence between the combs, for which various approaches have been developed, including electronic locking systems between the laser sources or generating the combs within monolithic laser systems [2]. In this work, we report the generation of DC sources within monolithic, high-quality-factor Kerr fiber Fabry-Perot (FFP) resonators by exploiting the spatial light multiplexing capabilities of these fiber systems. Additionally, these passive resonators offer seamless integration into photonic systems through standard FC/PC connectors with low insertion loss, making them practical for generating all-fiber optical frequency combs (OFCs) in the GHz range [3], [4].

Manipulating broadband fields in scattering media is a modern challenge across photonics and other wave domains. Recent studies have shown that complex propagation in scattering media can be harnessed to manipulate broadband light wave packets in space-time for focusing, imaging, and computing applications. Interestingly, while many proposed methodologies operate on intensity-based assessment of scattered fields, often in the spectral domain, from a pure transmission-function perspective, scattering operates as a linear field-level combinatory process, i.e., the superposition of transformation of unit excitations. As a result, we recently demonstrated that gaining experimental access to instantaneous scattered fields, as available through time-domain spectroscopy in the terahertz (THz) spectral range, in conjunction with sparse light excitation typical of ghost imaging, provides a key advantage in enabling the functionalisation of scattering, exposing a novel modelling paradigm. In this paper, we provide experimental proof of reconstructing 1-dimensional object features through a scattering medium using a fully broadband THz time-domain approach.

We numerically study programmable Kerr comb mode-locked fiber laser based on a nested cavities architecture. The nested architecture includes an optical resonator as well as a photonic device used as programmable delay-line, which allows for controlling the pulse repetition rate of a passively mode-locked laser over a frequency range covering the GHz-THz interval. In the proposed approach, the pulse repetition rate is adjusted based on a temporal interleaving process, obtained after propagation through the programmable delay-line structure. Our numerical study demonstrates how a programmable delay-line can be leveraged to obtain a highly-tunable Kerr-comb laser via harmonic mode-locking, featuring the formation of energy efficient yet reconfigurable laser cavity soliton pulses. In particular, basic rules are outlined to facilitate the experimental implementation of such a mode-locked laser. The proposed laser architecture is expected to thus provide enhanced versatility for compact and tunable frequency comb generation.

Metasurfaces have emerged as an innovative platform to overcome the limitations of traditional nonlinear materials, such as phase-matching constraints and low conversion efficiencies [1]–[3]. By enabling light manipulation at the nanoscale, these two-dimensional structures provide new opportunities for efficient frequency conversion and compact, scalable sources of ultrafast terahertz (THz) pulses.

Spintronic emitters promise to revolutionise terahertz (THz) sources by converting ultrafast optical pulses into broadband THz radiation without phase-matching constraints. Because the conversion relies on spin-current injection across a nanometre-thin magnetic layer, its efficiency is ordinarily limited by weak optical coupling. Here, we present a demonstration of a drop-casting based approach to introduce ultrafast plasmonic-mediated coupling: a sparse-layer of silica-gold core-shell nanoparticles is deposited directly onto a W/Fe/Pt spintronic trilayer. This sparse (six percent) decoration increases the wafer-averaged THz pulse energy, pointing to a very high local conversion enhancement for this low-coverage spintronic emitter compared with the bare stack. This demonstration points to a viable pathway toward highly efficient spintronic terahertz emitters with potential applications in spectroscopy, imaging, and ultrafast technologies.

Microcombs require ultralow-noise repetition rates to enable next-generation applications in metrology, high-speed communications, microwave photonics, and sensing, where spectral purity is a central performance metric. Best-performing sources operate actively locked at "quiet points" in parameter space, fixed by device and material properties. Creating broad, low-noise operating regions with relaxed constraints-especially in simplified free-running architectures that avoid electronics-heavy control-remains an open challenge. Here, we demonstrate a symmetry-protected topological Möbius soliton molecule that enables intrinsically low phase noise in a fully free-running microcomb, operating without any external referencing or control. Using a microresonator-filtered laser, we implement a Möbius geometry via interleaved microcavity modes. Upon the formation of a topological Möbius soliton molecule, the free-running laser exhibits over 15 dB of phase-noise suppression across 10 Hz-10 kHz at a 100 GHz repetition rate, yielding -63 dBc/Hz phase noise at 1 kHz and an Allan deviation of 4x10^-10 at 10 s average time-without any external control. We show that the Möbius structure brings dynamic robustness to the comb, and we demonstrate a symmetry-protected topological regime that enables long-term drift-invariant operation. Our results establish a route to intrinsically noise-quenched microcombs operating in a fully free-running configuration, governed by internal physical principles and suitable for field-deployable, low-noise photonic systems.

A laser cavity-soliton (LCS) state can form in a system where a nonlinear Kerr microresonator is embedded in a fibre laser cavity, to produce an optical frequency comb in a microresonator, [1]–[3], resulting in a self-emergent, stable, and efficient microcomb [4], [5]. In general, the system can generate various states depending on specific parameter settings, with two critical parameters for achieving self-emerging solitary oscillations: gain, which controls the soliton's energy, and main cavity length, which governs the group velocity mismatch between the two nested cavities. Hysteresis is a phenomenon in which a system exhibits memory-like behaviour, allowing it to remain in one of two stable states under identical external conditions. This property is associated with bistability. A fundamental aspect of microresonator fibre laser systems is that the emergence of LCS states is intrinsically linked to slow, energy-dependent nonlinearities within the laser cavity [4]. While bistability due to fast Kerr nonlinearity has been extensively studied in soliton formation, it is less understood how the slow energy-dependent nonlinear processes affect it. Investigating this connection is crucial for optimizing the stability and tunability of microcomb states. To demonstrate hysteresis in LCS, we conducted experiments using a dedicated optical setup (Fig. 1a). By sweeping the pump power in opposite directions, we observed the formation of distinct states with different powers (Fig. 1b). Our focus was on the transition between single-soliton and two-soliton states, where we identified a bistable region exhibiting consistent hysteresis (Fig. 1b).

Microcombs, optical frequency combs generated with integrated high Q-factor optical resonators, have emerged as a transformative tool in optics, offering compact chip-scale frequency comb sources that enable advances in metrology, spectroscopy and communications. Limiting factors include efficiency, robustness against environmental noise and long-term stability. A key requirement for many applications is the locking of two major degrees of freedom of the comb, the carrier offset and repetition rate frequencies, on which the positions of the comb teeth depend.

Terahertz Nonlinear Ghost Imaging introduces a groundbreaking method for object sampling at spatial-temporal levels, achieving super-resolution (i.e., beyond the diffraction limit). Our theoretical and experimental endeavour seeks to leverage this technique, enabling arbitrary field-level waveform manipulation through intricate propagation in scattering environments. This approach facilitates essential agile waveform adjustment, made possible through near-field interactions between terahertz sources and scattering media.

We explore the nonlinear emergence and recovery of a bonded state consisting of a soliton and a continuous wave (CW), with the soliton being red-detuned and the CW blue-detuned relative to the microcavity resonance slopes. Our findings demonstrate that the blue-detuned CW and soliton form in distinct regions of the erbium laser spectrum, where the slow resonant nonlinearity of the amplifier exhibits a different sign. Real-time measurements using Dispersive Fourier Transform (DFT) reveal an elastic bonding between these states, mediated by the system's modal structure and slow nonlinearities. This study provides crucial insights into the dynamic interaction of complex laser states and suggests potential enhancements to the stability of solitary regimes.