A high-power 2-$$\mu$$m phase-locked femtosecond source is demonstrated by a degenerate doubly resonant optical parametric oscillator (DROPO), which is synchronously pumped by a home-built Yb:YAG Kerr-lens mode-locked thin-disk laser. A dither-free scheme, incorporating an intracavity ’parasitic’ sum-frequency of signal and pump as an error signal, has been used to stabilize the DROPO at degeneracy. To our knowledge, with a pump power of 15.8 W, this system achieves the highest output power (5.6 W) and conversion efficiency (35 %) for an actively stabilized, degenerate BBO-based DROPO operating at 2 $$\mu$$m. The long-term stability measurement of the power over 90 minutes shows a root mean square (RMS) power noise of 0.78%, demonstrating excellent stability and reliable performance.

In optics, temporal mirrors can be realized using solitons - stable refractive index barriers propagating at the speed of light that can be probed by weak waves reflecting off the barrier. This approach offers an alternative method for achieving temporal analogues of reflection.Unlike time-varying material approaches, temporal reflection in soliton-based propagation models has already been demonstrated experimentally. This mechanism exists across different physical systems, enabling laboratory investigations of otherwise inaccessible phenomena such as event horizon physics, rogue wave dynamics, and negative mass effects. This capability to simulate extreme physical conditions makes the approach particularly valuable for advancing efficient light-matter interactions in optical technologies and modern photonics. While such optical temporal mirrors are experimentally feasible, existing implementations face significant technical challenges: they require the generation of femtosecond pulses at precisely tuned incommensurate frequencies. Here, we demonstrate a practical scheme that inherently generates a temporal mirror over extended propagation distances, with back-reaction continuously modifying the conditions required to sustain the mirror. This process autonomously generates both the soliton and test wave at precisely controlled positions in time, space, and frequency. Our scheme provides access to intriguing physics phenomena using modest resources available in standard optical laboratories, leveraging well-established optical effects.

We study photonic meta-atoms in nonlinear waveguides. They shed resonant radiation upon propagation, inspiring an analogy to radiative broadening in atomic spectra. Meta-atoms consist of a soliton and weak trapped states, bound together by cross-phase modulation. Higher-order dispersion enables phase-matching resonances that couple trapped states to the continuum, generating resonant radiation with a Lorentzian lineshape. We demonstrate that their linewidth follows from the finite lifetime of the corresponding trapped-state and discuss prospects for observing the reported effect in photonic-crystal fibres.

Abstract A strong optical pulse in a nonlinear medium can serve as a trap for photons. Photon confinement in such a trap moving at the speed of light is best described as time-trapping. Here we reveal unusual properties that fundamentally distinguish it from conventional trapping. Time-trapped states can exhibit zero leakage through the trap walls and possess extremely broad spectra while remaining genuine single modes with well-defined phase. Linear beatings between modes enable waveform reshaping in both the temporal and spectral domains. Time-trapped states support efficient coupling to external radiation, allowing fundamentally more efficient loading of energy than stationary cavities and surpassing the conventional Lorentz time-bandwidth limit linking lifetime and bandwidth in conventional resonant systems. Time-traps support exotic phenomena including halo states and anomalously large transmission delays. These properties apply equally to weak classical wavepackets and single-photon states, opening new opportunities for ultrafast classical and quantum photonics.

Abstract As photonic systems grow more complex, it becomes increasingly difficult to capture their behaviour within the conventional four dimensions of space and time, particularly for systems operating at the nanometer scale, where strong confinement effects, near-field interactions, and subwavelength structuring introduce additional layers of complexity. The concept of 5D photonics reflects this shift by incorporating additional physical, material, computational, adaptive, and quantum degrees of freedom as active components in design, control, and function. Rather than defining a single extra coordinate, higher-dimensional photonics is about operating photonic systems within expanded, dynamically accessible state spaces where multiple dimensions can interact and evolve together. This roadmap brings together perspectives ranging from modeling and design concepts to experimental platforms, materials, components, and system-level implementations. It covers a wide spectrum of synthetic and structured dimensions, nonlinear and strong-field regimes, adaptive and reconfigurable architectures, cyber-physical and engineering approaches, as well as inherently high-dimensional quantum and excitonic systems. Across all these areas, higher-dimensional thinking emerges not as an abstract construct but as a practical tool for enabling new functionalities, overcoming conventional design limitations, and bridging physical systems with digital and AI-driven layers. By framing these diverse developments within a shared higher-dimensional perspective, the roadmap aims to provide orientation in a rapidly expanding field, reveal conceptual connections between traditionally separate areas of photonics, and highlight common challenges and opportunities. In doing so, it positions higher-dimensional photonics as a central paradigm for developing future photonic technologies that are increasingly adaptive, intelligent, and integrated across physical and virtual domains.

At the fundamental level, full description of light-matter interaction requires quantum treatment of both matter and light. However, for standard light sources generating intense laser pulses carrying quadrillions of photons in a coherent state, the classical description of light during intense laser-matter interaction has been expected to be adequate. Here, we show how nonlinear optical response of matter can be controlled to generate dramatic deviations from this standard picture, including generation of several squeezed and entangled harmonics of the incident laser light. In particular, such nontrivial quantum states of harmonics are generated as soon as one of the harmonics induces a transition between different laser-dressed states of the material system. Such transitions generate an entangled light-matter wave function, which can generate quantum states of harmonics even in the absence of a quantum driving field or material correlations. In turn, entanglement of the material system with a single harmonic generates and controls entanglement between different harmonics. Hence, nonlinear media that are near resonant with at least one of the harmonics appear to be quite attractive for controlled generation of massively entangled quantum states of light. Our analysis opens remarkable opportunities at the interface of attosecond physics and quantum optics, with implications for quantum information science.

An optical soliton, propagating in a waveguide (see Fig. 1a), can, under certain conditions, trap photons [1], [2] at other frequencies. Such a trap can be best described as “time-cavity”, since the corresponding equations describe a trapping in a time-dependent effective potential $V_{\text{eff}}(t)$, making it a part of the newly born field of time-analog optics [3], [4].

We show that nonclassical, massively entangled bright states of light can be obtained during harmonic generation in strong optical fields due to entanglement with matter. The states are highly controllable by the pump pulse shape. © 2024 The Author(s)

Attosecond science operates with strong optical fields, with a lot of photons generated in harmonics; because of this, attosecond light pulses are often believed classical (coherent) states. Even if recent experiments shattered this view [1], [2], it is unclear yet, what resources can be used to make such states quantum.

The polarization states of terahertz (THz) radiation generated in a photoionized gas driven by intense two- or multi-frequency fields with locally controlled polarization are studied. We reveal a universal property of the resulting THz waveforms: the ellipticity of their polarization state increases linearly with frequency. This “linear chirp of ellipticity” makes plasma-based THz generation unique among other THz sources. However, it also imposes some constraints on the polarization properties of the generated THz radiation. We derive a general expression for the THz ellipticity and demonstrate how the polarization states of the generated THz waveforms can be manipulated and controlled by the polarization of the pump pulses.

We consider photon trapping by refractive index steps moving with speed of light. We show that such trapping fundamentally differs from conventional, “standing” cavities, demonstrating zero reflection through the walls and breaking several fundamental limits. © 2024 The Author(s)

Photocurrent-induced harmonics appear in gases and solids due to tunnel ionization of electrons in strong fields and subsequent acceleration. In contrast to three-step harmonic emission, no return to the parent ions is necessary. Here we show that the same mechanism produces harmonics in metallic nanostructures in strong fields. Furthermore, we demonstrate how strong local field gradient, appearing as a consequence of the field enhancement, affects photocurrent-induced harmonics. This influence can shed light at the state of electron as it appears in the continuum, in particular, to its initial velocity.

Here, we report on terahertz (THz) radiation generation in air driven by the fundamental and second harmonic of Yb:KGW laser pulses with durations of a few hundred femtoseconds. It was found that the spectrum of generated THz pulses surprisingly spans up to 50 THz, which is comparable to that usually obtained using much shorter Ti:sapphire laser pulses. The broad bandwidth is attributed to a strong spatiotemporal reshaping of the pump pulses in a filament. The achieved energy conversion efficiency is comparable to the one usually obtained from much shorter pump pulses and could be further improved by an optimized experimental setup. The obtained results indicate that compact Yb-based sources provide an attractive alternative to much larger and expensive laser systems.

We report on a high-power self-phase-locked doubly resonant optical parametric oscillator (DROPO), which is synchronously-pumped by a Kerr-lens mode-locked thin-disk laser. The DROPO delivers a 4.9 W, 229 fs pulse train at 2060 nm.

Nonlinear waveguides with two distinct domains of anomalous dispersion can support the formation of molecule-like two-color pulse compounds. They consist of two tightly bound subpulses with frequency loci separated by a vast frequency gap. Perturbing such a two-color pulse compound triggers periodic amplitude and width variations, reminiscent of molecular vibrations. With increasing strength of perturbation, the dynamics of the pulse compound changes from harmonic to nonlinear oscillations. The periodic amplitude variations enable coupling of the pulse compound to dispersive waves, resulting in the resonant emission of multi-frequency radiation. We demonstrate that the location of the resonances can be precisely predicted by phase-matching conditions. If the pulse compound consists of a pair of identical subpulses, inherent symmetries lead to degeneracies in the resonance spectrum. Weak perturbations lift existing degeneracies and cause a splitting of the resonance lines into multiple lines. Strong perturbations result in more complex emission spectra, characterized by well separated spectral bands caused by resonant Cherenkov radiation and additional four-wave mixing processes.

We present a detailed overview of the physics of two-color soliton molecules in nonlinear waveguides, i.e. bound states of localized optical pulses which are held together due to an incoherent interaction mechanism. The mutual confinement, or trapping, of the subpulses, which leads to a stable propagation of the pulse compound, is enabled by the nonlinear Kerr effect. Special attention is paid to the description of the binding mechanism in terms of attractive potential wells, induced by the refractive index changes of the subpulses, exerted on one another through cross-phase modulation. Specifically, we discuss nonlinear-photonics meta atoms, given by pulse compounds consisting of a strong trapping pulse and a weak trapped pulse, for which trapped states of low intensity are determined by a Schrödinger-type eigenproblem. We discuss the rich dynamical behavior of such meta-atoms, demonstrating that an increase of the group-velocity mismatch of both subpulses leads to an ionization-like trapping-to-escape transition. We further demonstrate that if both constituent pulses are of similar amplitude, molecule-like bound-states are formed. We show that z-periodic amplitude variations permit a coupling of these pulse compound to dispersive waves, resulting in the resonant emission of Kushi-comb-like multi-frequency radiation.

We study incoherently coupled two-frequency pulse compounds in waveguides with single zero-dispersion and zero-nonlinearity points. In such waveguides, supported by a negative nonlinearity, soliton dynamics can be obtained even in domains of normal dispersion. We demonstrate trapping of weak pulses by solitary-wave wells, forming nonlinear-photonics meta-atoms, and molecule-like bound-states of pulses. We study the impact of the Raman effect on these pulse compounds, finding that, depending on the precise subpulse configuration, they decelerate, accelerate, or are completely unaffected. Our results extend the range of systems in which two-frequency pulse compounds can be expected to exist and demonstrate further unique and unexpected behavior.

We present a detailed overview of the physics of two-color soliton molecules in nonlinear waveguides, i.e. bound states of localized optical pulses which are held together due to an incoherent interaction mechanism. The mutual confinement, or trapping, of the subpulses, which leads to a stable propagation of the pulse compound, is enabled by the nonlinear Kerr effect. Special attention is paid to the description of the binding mechanism in terms of attractive potential wells, induced by the refractive index changes of the subpulses, exerted on one another through cross-phase modulation. Specifically, we discuss nonlinear-photonics meta atoms, given by pulse compounds consisting of a strong trapping pulse and a weak trapped pulse, for which trapped states of low intensity are determined by a Schrödinger-type eigenproblem. We discuss the rich dynamical behavior of such meta-atoms, demonstrating that an increase of the group-velocity mismatch of both subpulses leads to an ionization-like trapping-to-escape transition. We further demonstrate that if both constituent pulses are of similar amplitude, molecule-like bound-states are formed. We show that z-periodic amplitude variations permit a coupling of these pulse compound to dispersive waves, resulting in the resonant emission of Kushi-comb-like multi-frequency radiation.