The article comments on a significant breakthrough in detecting mid-infrared light at low photon counts.

The emerging discipline of picophotonics explores events on a scale thousands of times smaller than the wavelength of light. A recent work introduces a phase singularity ruler that allows picometer-scale localization metrology in three dimensions.

The commentary discusses work recently published in Physical Review Letters, which reported a nondestructive approach to realizing precise control of p-p stacking at single molecule scale by laser illumination.

The advancement of ultrafast science necessitates diagnostic techniques capable of higher precision and increased dimensionality for few-cycle pulses. As pulses continue to shorten temporally and broaden spectrally, the temporal and spatial components become inseparable. Consequently, many established techniques fall short of accurately diagnosing both the temporal and spatial characteristics of pulses. We propose an all-optical spatiotemporal oscilloscope to comprehensively characterize the waveform of few-cycle pulses. By introducing a spatiotemporal perturbing pulse to influence high-harmonic (HH) generation, the frequency of the radiating HHs oscillates with variations in the delay between the pulses. This spatially dependent frequency oscillation of the HHs enables the reconstruction of the temporal and spatial details of the perturbing pulse. This method provides a straightforward and reliable strategy for multidimensional waveform characterization of few-cycle pulses, with potential applications in probing ultrafast dynamical processes carrying spatiotemporal information.

Dense waveguides are the basic building blocks for photonic integrated circuits (PICs). Due to the rapidly increasing scale of PIC chips, high-density integration of waveguide arrays working with low crosstalk over broadband wavelength range is highly desired. However, the subwavelength regime of such structures has not been adequately explored in practice. We propose a waveguide superlattice design leveraging the artificial gauge field mechanism, corresponding to the quantum analog of field-induced n-“photon” resonances in semiconductor superlattices. This approach experimentally achieves -24 dB crosstalk suppression with an ultrabroad transmission bandwidth more than 500 nm for dual polarizations on the Si3N4 platform. The fabricated waveguide superlattices support high-speed signal transmission of 112 Gbit/s with high-fidelity signal-to-noise ratio profiles and bit error rates. This design, featuring a silica upper cladding, is compatible with standard metal back-end-of-the-line processes. Based on such a fundamental structure, which is readily transferable to other platforms, passive and active devices over versatile platforms can be realized with a significantly shrunk on-chip footprint, thus it holds great promise for significant reduction of the power consumption and cost in PICs.

Ultracompact metasurfaces have gained a high reputation for manipulating light fields precisely within a subwavelength scale, bringing great development to the fields of nanophotonics, integrated optics, and quantum technology. There is broad interest in expanding the working band of metasurfaces to expand functionalities and the scope of applications. However, increasing the number of working wavelengths multiplexed in a single holographic metasurface is always complicated by two vital issues, i.e., spectral cross talk and the efficiency imbalance between different wavelength channels. Therefore, holographic metasurfaces with multiplexed working wavelengths over three are seldom reported. To address these two issues, we present a design strategy based on unevenly distributed pixels (UEDPs). As a proof of concept, a UEDP-based metasurface is designed to offer a camouflage method to hide four encrypted holographic images in a multicolor printed image. Our results not only demonstrate the idea of UEDP as an easy-to-implement and effective way for strengthening the wavelength multiplexing of metasurfaces but also give rise to a camouflage metasurface by integrating high-capacity and high-security encrypted holographic information with a single printed image. We believe that the generic UEDP-based metasurface design strategy can be readily extended to the realization of artificial functional structures in various disciplines, such as optics, thermology, and acoustics.

Optics is an exciting route for the next generation of computing hardware for machine learning, promising several orders of magnitude enhancement in both computational speed and energy efficiency. However, reaching the full capacity of an optical neural network (NN) necessitates that the computing be implemented optically not only for inference but also for training. The primary algorithm for network training is backpropagation, in which the calculation is performed in the order opposite to the information flow for inference. Although straightforward in a digital computer, the optical implementation of backpropagation has remained elusive, particularly because of the conflicting requirements for the optical element that implements the nonlinear activation function. We address this challenge for the first time, we believe, with a surprisingly simple scheme, employing saturable absorbers for the role of activation units. Our approach is adaptable to various analog platforms and materials and demonstrates the possibility of constructing NNs entirely reliant on analog optical processes for both training and inference tasks.

Recent years have seen significant advances in the study of dissipative soliton molecules in ultrafast lasers, driven by their remarkable connections to a wide range of physical systems. However, understanding and controlling the underlying physics of soliton molecules remain elusive due to the absence of a universal physical model that adequately describes intramolecular motion. We demonstrate that resonant excitation generates breather soliton molecules, with their resonance susceptibility exhibiting high amplitude-driven operations that can be well understood within the framework of the Duffing model. Harnessing powerful experiment techniques and detailed numerical simulations, we reveal the fundamental resonant mode within intrapulse separation constrained to the 100 fs level as well as the presence of the subharmonic and overtones. Additionally, we observe chaotic dynamics arising from the multiple-frequency nonlinear interactions in a strongly dissipative regime. Our work provides a perspective on the complex interactions of dissipative optical solitons through the lens of nonlinear physics. This approach offers a simple test bed for complex nonlinear physics research, with ultrafine scanning of temporal separations of ultrashort laser pulses demonstrating significant potential for applications requiring high detection sensitivity.

Photonic circuits, engineered to couple optical modes according to a specific map, serve as processors for classical and quantum light. The number of components typically scales with that of processed modes, thus correlating system size, circuit complexity, and optical losses. We present a photonic-circuit technology implementing large-scale unitary maps in free space, coupling a single input to hundreds of output modes in a two-dimensional compact layout. The map corresponds to a quantum walk of structured photons, realized through light propagation in three liquid-crystal metasurfaces, having their optic axes artificially patterned. Theoretically, the walk length and the number of connected modes can be arbitrary while keeping losses constant. The patterns can be designed to replicate multiple unitary maps. We also discuss limited reconfigurability by adjusting the overall birefringence and the relative displacement of the optical elements. These results lay the basis for the design of low-loss nonintegrated photonic circuits, primarily for manipulating multiphoton states in quantum regimes.

Compact, single-shot, and accurate Stokes polarimetric imagers are highly desirable for imaging at all scales, from remote sensing to biological diagnosis. Recently, polarimetric imaging demonstrated on the metasurface platform is accelerating its realization and revolutionizing associated techniques and imagers. These breakthroughs, however, are greatly limited by the single operating wavelength and the complexity of metasurfaces. We present a minimalist yet powerful cascaded metasurface strategy to realize wavelength-insensitive snapshot Stokes polarimetric imaging. Two cascaded metasurface polarization gratings built into the 4f imaging system enable optical spin Hall momentum shifts and cross-polarization interference of incident light, which are wavelength-robust and free of polarization cross talk, allowing the 4f system to perform accurate and single-shot polarimetric imaging at an arbitrary wavelength and even low-coherence light. We demonstrate the feasibility and robustness of this cascaded metasurface architecture by characterizing diverse polarization objects. We open an avenue for polarimetric imaging and exhibit promising potential in emerging areas of applications such as biological diagnosis.