Assembly of alignment-sensitive optical systems remains challenging because even small positional or angular errors can measurably affect system performance. In solid-state lasers, such errors can lead to changes in output power, beam quality, and stability. As a result, assembly of these systems remains largely manual, which increases production effort and cost, limits scalability, and often results in bulky designs constrained by large kinematic optic mounts. To address this challenge, this paper presents a feedback-guided robotic assembly workflow for compact, alignment-sensitive optical systems and validates it experimentally with a diode-pumped ruby laser. The workflow combines vision-guided pickup and passive placement with fluorescence-based axial positioning of the crystal, optical-feedback-based cavity alignment, stepwise optimization of the laser output, and permanent fixation via UV-curable adhesive. The process was implemented on a commercial precision optics assembly platform and executed without manual adjustment. As a representative validation case, the assembled ruby laser produced diffraction-limited output at 694.3 nm with an output power of 30 mW. The results show that passive placement, in situ optical feedback, optimization, and bonding can be integrated into a repeatable robotic process for laser assembly. The demonstrated workflow provides a practical route toward automated assembly of optical systems whose performance depends critically on precise alignment and on preserving that alignment during and after bonding.

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.

The nonlinear interaction of near-infrared ultrashort laser pulses with bulk fused silica at intensities below the catastrophic damage threshold is the birthplace of local electronic excitation events that may relax as point defects. The accumulation of defect centers creates additional sources of electronic excitation, gradually lowering the ionization threshold upon multi-pulse irradiation. However, the use of the near single-cycle sub-4 fs pulses erases in a surprising manner the contribution of defects to the damage threshold. This is the result of increasing the strong intensity clamping that limits energy density and therefore carrier collisional multiplication. Thus, the role of additional defect sources in the overall carrier population becomes secondary. In addition, the significant broadening of the near single-cycle pulse may enable impulsive depletion of defect levels, providing a possible additional pathway contributing to the suppression of nonlinear ionization memory.

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.

Q-switched lasers are compact, cost-effective, and highly pulse energy-scalable sources for nanosecond-scale laser pulses. The technology has been developed for many decades and is widely used in scientific, industrial and medical applications. However, their inherently narrow bandwidth imposes a lower limit on pulse duration-typically in the few-hundred-picosecond range-limiting the applicability of Q-switched technology in fields that require ultrafast laser pulses in the few-picosecond or femtosecond regime. In contrast, mode-locked lasers can produce broad-band, ultrafast ( <1 ps) pulses, but are complex, expensive, and typically require a large footprint. To bridge the parameter gap between these two laser platforms-in terms of pulse duration and achievable peak power-we here propose a Herriott-type multi-pass cell (MPC) based post-compression scheme for shortening the pulse durations of Q-switched lasers down to the ultrafast, picosecond regime. We experimentally demonstrate post-compression of 0.5 ns, 1 mJ pulses from a Q-switched laser to 24 ps using a compact glass-rod MPC for spectral broadening. We verify this result numerically and show that compression down to a few picoseconds is possible using the nanosecond MPC (nMPC). Through spectral filtering approaches, the nMPC suppresses detrimental nonlinear processes such as stimulated Raman scattering, which have set severe limitations for fiber-based post-compression of Q-switched lasers until today. Our results pave the way to cost-efficient and compact ultrafast laser platforms based on Q-switched laser technology.

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.

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.

Nonlinear fiber amplifiers allow for scaling the pulse energy, while the pulse properties can be tailored by the interplay of dispersion and Kerr-nonlinearity, aiming to significantly shorten the input pulses. Here, we describe a monolithic polarization-maintaining nonlinear fiber amplifier based on thulium, relying solely on single-mode step-index fused silica fibers. During the amplification of up-chirped pulses with an initial transform-limited pulse duration of 2.3 ps in an anomalous dispersive gain fiber, monotonic spectral broadening occurs and is subsequently extended in a normal dispersive fiber. The setup generates pulses with ultra-broadband spectra of up to 119 nm at pulse energies of up to 74 nJ. The pulses can be well compressed to sub-100 fs duration, equivalent to a compression factor of 24. Even though massive nonlinear spectral broadening is observed, the noise performance characterized by the relative intensity noise remains almost unaffected. Fundamentals for design and limitations of the output parameters are discussed by a numerical model.

Understanding the gain dynamics of amplification in rare-earth-doped silica glass fibers is crucial for developing sophisticated low-noise amplifiers and frequency combs. Recent and ongoing research efforts in these areas have specifically targeted the spectral region around 2 µm and the use of thulium-doped silica glass fibers as a broadband, power-scalable gain medium. We present a comprehensive characterization of the transfer functions of thulium ions in silica glass for emission in the spectral 2 µm region, including the magnitude and phase information. We investigated the most widespread pumping schemes: in-band pumping at 1550 nm, as well as out-of-band pumping at 790 nm. A semi-analytical model and a numerical model are developed to describe the complex energy level system of thulium ions and their associated ion-ion interactions, providing deeper insight into the influence of various parameters on the transfer functions of thulium ions in silica glass. The presented results of the transfer functions support the choice of fiber amplifier geometries and electronics to optimize noise performance.

A monolithic PM nonlinear fiber amplifier based on thulium is presented, which allows to generate ultrashort pulses with over 50 nJ pulse energy, corresponding to wide spanning spectra and compressed pulse durations of 121 fs.

Mamyshev oscillators have pushed the frontiers in output parameters of ultrafast fiber oscillators within recent years tremendously. Various configurations have been presented, reaching pulse energies as high as $1.1 \mu \mathrm{J}$ [1] and pulse durations as short as 17 fs [2]. Many of these setups compromise the stable, alignment free operation of monolithic fiber oscillators, since free-space components were used. Furthermore, pulses shorter than 100 fs are usually only achieved in cladding-pumped systems, alongside with high pulse energies, requiring several watts of pump power. Some setups even incorporate two gain sections, which increases the amount of needed components and complexity.

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].

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.

The two most notable methods for producing short laser pulses are mode-locked and Q-switched lasers. While mode-locked lasers generate pulses in the pico- (ps) to femtosecond (fs) range, Q-switched lasers typically produce pulses at the nanosecond (ns) or 100 ps level [1]. Thus, mode-locking has been the superior technology when ultrashort pulses or high peak powers are required. On the other hand, Q-switched lasers can have great advantages owing to their simplicity, compactness, low costs, and the large pulse energies that can be generated [2]. To bridge the parameter gap between mode-locked and Q-switched lasers, pulse post-compression appears attractive. One possible method is self-phase-modulation (SPM) in optical fibers [3]. However, stimulated-Raman-scattering limits the possible peak powers to far below 1 kW and thus pulse energies to lower than μJ level for ns pulses [3]. Another SPM-based method are Multi-pass cells (MPCs), which have become a standard technology for post-compression of mode-locked lasers [4]. However, MPCs have not yet been used to compress Q-switched laser pulses due to their inherently low peak powers. Here, we demonstrate a new MPC type, facilitating post-compression of pulses with order-of-magnitude lower peak power than typical bulk-MPCs [4]. We use a fused silica block as the nonlinear medium and compress 1 mJ pulses from the nanosecond down to the picosecond regime.

In this work, we develop adaptive schemes using goal-oriented error control for a highly nonlinear flow temperature model with temperature dependent density. The dual-weighted residual method for computing error indicators to steer mesh refinement and solver control is employed. The error indicators are used to employ adaptive algorithms, which are substantiated with several numerical tests. Therein, error reductions and effectivity indices are consulted to establish the robustness and efficiency of our framework.