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.

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.

Photonic Ising machines are leading key advancements in solving large combinatorial problems, leveraging large-scale platforms with parallel computing capabilities. A well-known bottleneck of complex problems is the appearance of multiple minima in the energetic landscape that attract Metropolis-based iterations in suboptimal solutions, thus hindering the performance of standard optical solvers in large systems. By introducing a double single-pixel detection scheme based on intensity and field averages in an optical-based Ising machine, we effectively implement local and nonlocal nonlinear Hamiltonians, representing a complex and simple state, respectively. Transitioning from nonlocal to local nonlinear detection enables to adiabatically morph the energetic landscape, enhancing the success rate of finding the optimal solution compared to standard isothermal approaches.

Nonlinear Ghost Imaging (NGI) offers a versatile route to implementing time-domain terahertz imaging with a resolution that exceeds the typical diffraction limit [1], [2]. By transforming the detected field data, NGI can effectively “refocus” any chosen plane in the near-field region, thereby unveiling depth information in microscopic volumes that are relevant to both biological and technological applications [3]. Here, we develop a transformation-approach of NGI (Fig. 1) to retrieve the spectral response of two closely spaced subwavelength structures (λ/10) in a microscopic environment. We show how the contribution from the first plane (green curve in Fig. 1a) is progressively isolated with the separation in the sub-wavelength domain from the reconstructed spectrum of the second-plane feature (blue curve in Fig. 1c). Fig. 1b shows an example of our method's ability to resolve two metallic letters positioned on separate planes. Meanwhile, Figs. 1e and 1d compare the reconstructed images before and after addressing the spatio-temporal coupling introduced by near-field propagation. A full account of our hyperspectral tomography results will be discussed in detail.

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.

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.

Photonic Ising machines leverage large-scale parallelism for solving large combinatorial problems, yet multiple minima hamper Metropolis-based algorithm. A double single-pixel detection approach enables energetic transitions from nonlocal to local Hamiltonians, finding the ground state of complex landscapes.

Photonic Ising machines leverage large-scale parallelism for solving large combinatorial problems, yet multiple minima hamper Metropolis-based algorithm. A double single-pixel detection approach enables adiabatic energetic transitions from nonlocal to local Hamiltonians, finding the ground state of complex landscapes.

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.

Complex media have emerged as a powerful and robust framework to control light-matter interactions designed for task-specific optical functionalities. Studies on wavefront shaping through disordered systems have demonstrated optical wave manipulation capabilities beyond conventional optics, including aberration-free and subwavelength focusing. However, achieving arbitrary and simultaneous control over the spatial and temporal features of light remains challenging. In particular, no practical solution exists for field-level arbitrary spatiotemporal control of wave packets. A new paradigm shift has emerged in the terahertz frequency domain, offering methods for absolute time-domain measurements of the scattered electric field, enabling direct field-based wave synthesis. In this work, we report the experimental demonstration of field-level control of single-cycle terahertz pulses on arbitrary spatial points through complex disordered media.

Metasurfaces represent a new frontier in materials science paving for unprecedented methods of controlling electromagnetic waves, with a range of applications spanning from sensing to imaging and communications. For pulsed terahertz generation, metasurfaces offer a gateway to tuneable thin emitters that can be utilised for large-area imaging, microscopy and spectroscopy. In literature THz-emitting metasurfaces generally exhibit high absorption, being based either on metals or on semiconductors excited in highly resonant regimes. Here we propose the use of a fully dielectric semiconductor exploiting morphology-mediated resonances and inherent quadratic nonlinear response. Our system exhibits a remarkable 40-fold efficiency enhancement compared to the unpatterned at the peak of the optimised wavelength range, demonstrating its potential as scalable emitter design.

In the framework of optical frequency conversion, metasurfaces have elevated the potential for effective interfacial nonlinear coefficients through various modes of field localization. For the generation of pulsed ultrafast terahertz (THz) signals, metasurfaces present a viable alternative in the domain of surface-scalable sources driven by low-power oscillators (using nJ pulses). However, recent innovations have predominantly relied on surface plasmons (metals) and, more broadly, on excitations within non-transparency windows—conditions that typically impose limitations on applications and the choice of platforms. Here, we demonstrate the utilization of a fully-dielectric, fully transparent semiconductor that exploits surface-nano-structure-mediated resonances alongside its inherent quadratic nonlinear response. Our system exhibits a remarkable 40-fold efficiency enhancement in comparison to the non-decorated substrate.

Terahertz time-domain imaging aims at reconstructing the complete electromagnetic morphology. We elaborate our theoretical-experimental route to exploit sparse near-field spatio-temporal illumination to enable three-dimensional microscopy.

Plasmonic metasurfaces are widely proposed as means to enhance nonlinear light-matter interactions in optical devices, mediated by an extreme form of field localisation. In the domain of optical-to-terahertz conversion, metastructures demonstrated the ability to enhance the local optical conversion or even to elicit conversion in cases where the native material does not support effective conversion mechanisms [1]. One of the most modern approaches towards ultrafast terahertz sources has been recently enabled by the development of spintronic emitters, in which an ultrafast thermal transient combined with an inverse-spin-Hall effect is responsible for the formation of intense terahertz current sources. A recent seminal work explored this aspect by directly depositing gold nanorods on the spintronic structure [2], showing field enhancement despite the structure being seemly an optically-opaque multiple-layer assembly. An interesting question is whether sparser distributions of plasmonic structures exhibiting stronger local enhancement can be used to enhance the spintronic conversion. In this work, we demonstrate that the distribution of core-shell plasmonic nano-resonator electromagnetically coupled with the surface via a thin silica layer interface introduces a substantial enhancement of the local ultrafast heating process, boosting the spintronic terahertz emission [3].

Laser cavity-solitons can appear in a microresonator-filtered laser when judiciously balancing the slow nonlinearities of the system. Under certain conditions, such optical states can be made to self-emerge and recover spontaneously, and the understanding of their robustness is critical for practical applications. Here, we study the formation of a bonded state comprising a soliton and a blue-detuned continuous wave, whose coexistence is mediated by dispersion in the nonlinear refractive index. Our real-time dispersive Fourier transform measurements, supported by comprehensive theoretical analysis, reveal the presence of an elastic bonding between the two states, resulting in an enhancement of the soliton’s robustness.

Cavity Solitons are optical structures forming in nonlinear cavities when specific conditions of balance are verified [1]. In their temporal version, such solitary waves have been shown to improve the phase coherence properties of microcombs [2] (i.e., optical frequency combs in a microresonator).

Terahertz time-domain imaging targets the reconstruction of the full electromagnetic morphology of an object. In this spectral range, the near-field propagation strongly affects the information in the space-time domain in items with microscopic features. While this often represents a challenge, as the information needs to be disentangled to obtain high image fidelity, here, we show that such a phenomenon can enable three-dimensional microscopy. Specifically, we investigate the capability of the time-resolved nonlinear ghost imaging methodology to implement field-sensitive microvolumetry by plane decomposition. We leverage the temporally resolved, field-sensitive detection to "refocus" an image plane at an arbitrary distance from the source, which defines the near-field condition, and within a microscopic sample. Since space-time coupling rapidly evolves and diffuses within subwavelength length scales, our technique can separate and discriminate the information originating from different planes at different depths. Our approach is particularly suitable for objects with sparse micrometric details. Building upon this principle, we demonstrate complex, time-domain volumetry resolving internal object planes with subwavelength resolution, discussing the range of applicability of our technique.