Volume 14, Issue 18

Accurate knowledge of nonlinear optical parameters is essential for optimizing energy deposition in ultrafast laser 3D printing, yet these values remain undetermined for many commonly used materials. In this study, we address this gap by experimentally determining the two-photon absorption (TPA) and non-linear refraction coefficients ( β and n 2 ) of the widely used SZ2080 TM resist with the photo-initiators (PI) IRG369 and BIS (Irgacure 369 and 4,4′ bis(diethylamino)-benzophenone or Michler’s ketone). Using the Z-scan method at 515 nm with a low repetition rate (1 kHz) to avoid thermal accumulation, we found that the nonlinear response of the host polymer has a considerable contribution to energy deposition despite the addition of the PI, as the host polymer makes up the majority of 99 % in the solution. The TPA cross section σ were 5.7 ± 0.4 GM (1 GM = 10 −50  cm 4  s photon −1 ) for pure SZ2080 TM , ∼ 40 $\sim 40$  GM for IRG and ∼ 87 $\sim 87$  GM for BIS at 515 nm. The nonlinear refractive index n 2 for pure polymer was (85.3 ± 6) × 10 −5  cm 2 /TW, favoring a self-focusing, and was larger than that for PIs: ∼ 16 × 1 0 − 5 $\sim 16{\times}1{0}^{-5}$  cm 2 /TW (IRG369) and ∼ 2.8 × 1 0 − 5 $\sim 2.8{\times}1{0}^{-5}$  cm 2 /TW (BIS). Hence, the properties of the host material govern non-linear light propagation, although, in high numerical aperture focusing, self-focusing has a minor contribution to the variation of refractive index. Crucially, the determined TPA coefficients for pure SZ2080 TM provide experimental evidence that it can initiate polymerization without PIs, enabling a more sustainable and environmentally friendly fabrication route by avoiding the use of toxic additive compounds. These findings will allow for the estimation of exact energy deposition in 3D laser printing using ultrashort laser pulses and support the development of an initiator-free additive manufacturing approach.

The formation of isotropic two-dimensional laser-induced periodic surface structures (2D-LIPSS) under circular polarization demonstrates a unique self-organization capability. In this study, we investigate LIPSS formation under both circularly and linearly polarized light, assessing the impact of polarization dynamics, from discrete rotation angles to ultrafast changes within a single optical cycle, on pattern formation. This phenomenon stems from the surface’s ability to temporally integrate all polarization states within an optical cycle, leading to a circularly oriented response. Similarly to linear polarization, the interference between the topography-induced scattered field and the incident laser field guides the process, with feedback-driven topographical evolution sustaining structure growth. However, unlike linearly polarized light, which imposes unidirectional alignment, circular polarization promotes a more symmetric arrangement. Superposed on the hexagonal lattice of pillars with short-range order, near-field interactions generate radially oriented nanostructures that interconnect the pillars via concentric filaments. By investigating these two spatial scales separately, the respective formation mechanisms and their interplay can be clarified. Supported by electromagnetic simulations, these findings offer a comprehensive framework for understanding the mechanisms behind isotropic 2D-LIPSS formation.

Metasurfaces have become significant platforms for optical manipulation, yet unlocking their full potential for nonlinear optics requires novel mechanisms to control and enable frequency conversion processes. This study demonstrates how structural dimerization in plasmonic metasurfaces coupled to waveguides can modify linear and nonlinear optical behavior via Brillouin zone folding. By introducing a centrosymmetric unit cell design featuring two mirrored split-ring resonators, we allow guided modes that were previously below the light line to appear as guided-mode resonances. These resonances facilitate nonlocal modes, which are present as distinct narrow transparency windows. Although centrosymmetric dimerized design typically forbids far-field radiation through quadratic nonlinear interactions, we observe notable second-harmonic generation – not merely through symmetry breaking at oblique incidence, which proves insufficient, but rather with the support of a nonlocal mode. The excitation of a collective mode at the pump frequency provides a strong nonlinear response by mediating the formation of a net dipole moment at the second-harmonic frequency, enabling far-field radiation that is otherwise forbidden. This synchronized action among split-ring resonators leads to observable second-harmonic generation, confirmed by both experimental evidence and simulations. Our results indicate that dimerized metasurfaces represent a versatile platform for harnessing collective modes in nonlinear interactions. This motivates further research and suggests promising applications in advanced nonlinear photonic devices.

Thin-film lithium tantalate (TFLT) has recently emerged as a promising photonic platform for chip-scale nonlinear optics due to its weaker photorefraction, higher optical damage threshold, broader transparency window, and lower birefringence compared to that of thin-film lithium niobate. Here we report the first functional second harmonic generator achieved through high-fidelity poling of z-cut TFLT waveguides, based on a low-loss lithium tantalate integrated photonic platform. As a result, quasi-phase matching is performed between telecom (1,550 nm) and near-visible (775 nm) wavelengths in a straight waveguide and prompts strong second-harmonic generation with a normalized efficiency of 229 %/(W·cm 2 ). An absolute conversion efficiency of 5.5 % is achieved with a pump power of 700 mW in the waveguide. Such a second-harmonic generator exhibits stable temperature tunability (−0.44 nm/°C), which is important for applications that require precise frequency alignment such as atomic clocks and quantum frequency conversion.

Efficient generation of high-quality photon pairs is essential for modern quantum technologies. Micro-ring resonator is an ideal platform for studying on-chip photon sources due to strong nonlinear effect, resonant-enhanced optical fields, and high integration. Thin-film lithium niobate (TFLN) micro-ring resonators with periodically poled quasi-phase matching have shown high-quality photon pair generation. However, periodic poling technology remains expensive and requires complex fabrication hindering its scalability and capability for practical application in nonlinear photonic devices. To address this, we propose a scalable approach using TFLN micro-ring resonators based on modal phase matching to achieve cost-effective, efficient high-quality photon pair generation, significantly simplifying fabrication. We achieved pair generation rates up to 40.2 MHz/mW through spontaneous parametric down-conversion, with coincidence-to-accidental ratios exceeding 1,200. By combining micro-ring resonance enhancement with modal phase matching, our approach reduces device size and fabrication cost while maintaining high nonlinear efficiency. These results advance the development of compact, efficient on-chip photon sources for next-generation nonlinear and quantum photonic applications.

Electro-optic modulators (EOMs) underpin a wide range of critical applications in both classical and quantum information processing. While these devices have been extensively optimized in a wide range of materials from ferroelectric insulators like lithium niobate to semiconductors like gallium arsenide and indium phosphide, there is a need to explore new design and manufacturing methods with a view towards improving device performance. Here, we demonstrate true push-pull EOMs in a suspended GaAs photonic integrated circuit (PIC) platform by exploiting the orientation induced asymmetry of the Pockels r 41 coefficient, and folding the two arms of a cm-scale Mach–Zehnder interferometer (MZI) modulator along two orthogonal crystal axes. Our work also shows the potential of incorporating ideas from micro-electro-mechanical systems (MEMS) in integrated photonics by demonstrating high-performance active devices built around cm-scale suspended waveguides with sub-µm optical mode confinement.

Future technologies aim to radically increase photonic integration, which can be achieved either by structuring the materials or by cleverly manipulating photonic resonances. The latter method involves several tunable resonant modes in a single simple structure. Here we demonstrate experimentally and theoretically the existence of multiple cascades of quasi-bound states in the continuum in single dielectric resonators with rectangular cross sections – in rings, split rings, and cuboids, which form the basis of modern photonics. The effect is determined by the photonic structure of such resonators: it consists of individual galleries, each starting with a transverse Fabry–Pérot-like resonance in height or width and continuing with an equidistant sequence of longitudinal modes. When only one of the transverse dimensions in the spectrum changes, only one gallery type is predominantly shifted, leading to the avoiding crossings with the other gallery and the formation of multiple cascades of quasi-bound states in the continuum via the Friedrich–Wintgen mechanism. This “Fabry–Pérot-tronic” has an obvious advantage over the “Mie-tronic”, whose only variable geometric parameter is the radius of the sphere. Such single dielectric resonators with cascades of quasi-bound states in the continuum can become building blocks for multichannel sensors, antennas, amplifiers, and lasers with a wide range of equidistant generation frequencies; in addition, such a simple resonator creates a new platform for multifrequency sensing using machine learning.

Transport-of-intensity equation (TIE) as a noninterference method for quantitative phase imaging (QPI) has broad applications in micrographic imaging and optical metrology. Previous TIE-based QPI systems require the axial displacement of the detector to capture the axial intensity distributions, thus limiting the systems’ response speed, integration, and phase retrieval accuracy. Besides, the TIE-based phase imaging for edge positions with large phase gradients remains challenging. In this work, a compact polarization-multiplexed Moiré metalens is proposed to achieve QPI and edge-enhanced imaging for high-precision and unwrapping phase imaging. This Moiré metalens enables continuous zooming from 58.7 μm to 61.8 μm, allowing flexible selection of the detection positions. Under x -polarization light incidence, the metalens can achieve phase retrieval based on the TIE method, with the Root Mean Square Errors (RMSE) reaching 0.015 rad. Under y -polarization light incidence, the metalens realizes varifocal edge-enhanced imaging for amplitude and phase objects, with a minimum spatial resolution of 1.3 μm. This Moiré metalens opens a new avenue to develop compact, integrated, and multifunctional phase imaging devices and has potential applications in optical detection, microscopy, and biomedical imaging.

Optical reservoir computing (ORC) promises fast, energy-efficient temporal inference by harnessing the rich transient dynamics of photonic systems. Yet most ORC demonstrations still depend on fiber delay lines or camera-based spatial multiplexing, which caps the clock rate at a few tens of MSa/s and complicates monolithic integration. Here we introduce a frequency-multiplexed ORC whose nodes are the individual modes of a dissipative Kerr-soliton microcomb generated in a high- Q Si 3 N 4 microresonator. The input signal is encoded as a rapid detuning modulation of the pump laser, so the intracavity dynamics of the microcomb provide both the high-dimensional nonlinear mapping and tens of nanoseconds of memory, while output weighting is realized optically with standard microring arrays. Numerical modeling with 60 comb modes provides a normalized mean-square error (NMSE) of 0.015 on the Santa Fe chaotic time-series task at 50 MSa/s and more than a tenfold reduction in symbol-error rate for nonlinear equalization (NLEQ) at 100 MSa/s. A proof-of-concept experiment using 37 measured modes also confirms the concept on the Santa Fe chaotic time-series and NLEQ benchmarks. Because both the microcomb and weighting network are fabricated by a complementary metal-oxide semiconductor (CMOS)-compatible process, the architecture offers a clear path toward compact, energy-efficient photonic processors operating at greater than 1 GSa/s, directly addressing the scalability and speed challenges of nanophotonic ORC.

Edge states emerging at the boundaries of materials with nontrivial topology are attractive for many practical applications due to their remarkable robustness to disorder and local boundary deformations, which cannot result in scattering of the energy of the edge states impinging on such defects into the bulk of material, as long as forbidden topological gap remains open in its spectrum. The velocity of such states traveling along the edge of the topological insulator is typically determined by their Bloch momentum. In contrast, here, using valley Hall edge states forming at the domain wall between two honeycomb lattices with broken inversion symmetry, we show that by imposing Airy envelope on them one can construct edge states which, on the one hand, exhibit self-acceleration along the boundary of the insulator despite their fixed Bloch momentum and, on the other hand, do not diffract along the boundary despite the presence of localized features in their shapes. We construct both linear and nonlinear self-accelerating edge states, and show that nonlinearity considerably affects their envelopes. Such self-accelerating edge states exhibit self-healing properties typical for nondiffracting beams. Self-accelerating valley Hall edge states can circumvent sharp corners, provided the oscillating tail of the self-accelerating topological state is properly apodized by using an exponential function. Our findings open new prospects for control of propagation dynamics of edge excitations in topological insulators and allow to study rich phenomena that may occur upon interactions of nonlinear envelope topological states.

We experimentally demonstrate compact and broadband rapid adiabatic couplers (RACs) with arbitrary power split ratios, achieved through the combination of translational offset and waveguide width control. Fabricated RACs of four different target split ratios show power splitting within ±3 % of the design target over a 160 nm wavelength range. Using these RACs, we implement an 8-channel dense wavelength division multiplexing (DWDM) interleaver exhibiting < −20 dB crosstalk for the center eight channels with flat-top passbands. Over a broader wavelength range, the design maintains crosstalk below −10 dB across more than 40 channels with 100 GHz spacing, demonstrating the broadband capability and scalability of RAC-based photonic integrated circuits.