ACS Photonics最新文献

Understanding the mechanism of nonlinear second harmonic generation (SHG) in 2D magnetic materials is of great physics interest, with profound implications for both fundamental research and technological applications. Here, we report the first observation of abnormal thickness dependence of SHG in centrosymmetric 2D antiferromagnets: clear SHG in ultrathin 2D Cobalt Monoxide (CoO) nanoflakes and a systematic decrease of SHG with increasing thickness. As centrosymmetric crystals generally have no SHG signal, the observed SHG in ultrathin CoO nanoflakes originates from the antiferromagnetic spin orders that break the time-reversal symmetry and from defect states that break the spatial inversion symmetry. Temperature-dependent and polarization-resolved SHG studies reveal that the abnormal SHG in ultrathin CoO nanoflakes mainly originates from surface antiferromagnetic spin-ordered surface magnetic-dipole-contributed SHG, stronger than bulk magnetic-dipole-contributed SHG and surface electric-dipole-contributed SHG. Our work further suggests that polarization-resolved SHG provides a sensitive probe for studying surface magnetic properties in 2D magnetic materials.

Small-molecule donor:polymer acceptor (SMD:PA) organic solar cells have garnered attention due to their excellent active layer stability, yet their efficiency remains significantly lower than other OSC types. This study addresses the challenge of morphology control in SMD:PA systems via a layer-by-layer (LBL) process to optimize the donor–acceptor interpenetrating network. Using small-molecule donor B1 and polymer acceptor PY-IT with chloroform as a universal solvent, we systematically investigated the impact of LBL processing on the active layer morphology and device performance. The inverted LBL device (ITO/ZnO/PY-IT/B1/MoO₃/Ag) achieved a power conversion efficiency of 8.6%, significantly outperforming the bulk heterojunction devices (inverted 2.91% and normal 6.11%) and previously reported LBL SMD:PA cells (1.12%). Static and femtosecond transient absorption spectra, time-resolved photoluminescence, and grazing incidence X-ray diffraction analyses revealed that the LBL and nonorthogonal solvent strategy facilitated effective B1 infiltration into the PY-IT layer, forming an optimized active layer with refined phase separation and improved donor/acceptor interfaces, thus resulting in enhanced exciton dissociation and charge transport while reducing recombination losses. This work validates the feasibility of LBL processing for high-efficiency SMD:PA OSCs, offering a novel strategy to overcome the efficiency limitations of this class of OSCs.

Edge detection is a fundamental operation for data compression, feature recognition, and structural analysis, underpinning a wide range of scientific and technological applications. Despite recent advances, most optical analogue edge detection methods based on compact metalenses suffer from a lack of tunable directional selectivity, posing challenges for their deployment in real-world scenarios. Here, we present a compact vector vortex metalens composed of a single-layer silicon carbide metasurface for real-time, broadband, direction-selective edge detection. By engineering the superposition of spin-dependent vortex and antivortex beams, the metalens generates a point spread function with radially varying polarization states. Directional edge features are selectively extracted by introducing a linear analyzer after the metalens without requiring external Fourier optics or computational reconstruction. This directional selectivity offers the key advantage of effectively eliminating directional defects in the observed objects, which allows the contours of the objects to be better identified. We experimentally demonstrate high-resolution edge detection across a broadband spectrum for both amplitude-type and phase-type objects such as biological samples. This approach offers an ultrathin and integrable solution for next-generation optical systems that demand real-time orientation-dependent feature analysis within a minimal footprint.

Color centers in hexagonal boron nitride (hBN) emerge as promising quantum light sources at room temperature, with potential applications in quantum communications, among others. The temporal coherence of emitted photons (i.e., their capacity to interfere and distribute photonic entanglement) is essential for many of these applications. Hence, it is crucial to study and determine the temporal coherence of this emission under different experimental conditions. In this work, we report the coherence time of the single photons emitted by an hBN defect in a nanocrystal at room temperature, measured via Michelson interferometry. The visibility of this interference vanishes when the temporal delay between the interferometer arms is a few hundred femtoseconds, highlighting that the phonon dephasing processes are 4 orders of magnitude faster than the spontaneous decay time of the emitter. We also analyze the single photon characteristics of the emission via correlation measurements, defect blinking dynamics, and its Debye–Waller factor. Our room temperature results highlight the presence of a strong electron–phonon coupling, suggesting the need to work at cryogenic temperatures to enable quantum photonic applications based on photon interference.

We propose a compact strategy for the experimental synthesis of randomly structured sources with robust higher-order Poincaré polarization states by manipulating the second-order spatial coherence structure through nonuniform optical coherence engineering. This approach employs a pseudomode representation of the cross-spectral density matrix and utilizes a single amplitude-only digital micromirror device, combined with a common-path interferometer system, to construct patterns, enabling the near-real-time synthesis of sources. Experimental results demonstrate that nonuniform optical coherence engineering significantly enhances the robustness of higher-order Poincaré sphere beams, allowing them to preserve polarization integrity throughout propagation and overcoming the limitations of conventional scalar and vector uniform optical coherence engineering approaches. Furthermore, the synthesized beams exhibit strong resilience to turbulence, with the encoded polarization states preserved with high quality in the far field (focal plane). We believe that optical coherence engineering has the potential to extend the capabilities of existing resilient optical systems, offering a promising solution for compensation-free optical communication systems.

Ultrasonic absolute ranging is desirable for the applications in profilometry, robot obstacle avoidance, and resource exploration, whose performances are determined by the bandwidth, noise floor, and stability of the ultrasound detection system. Here, an ultrasonic absolute ranging system with a 20 kHz update rate and micrometer-level precision is proposed by optical ultrasound detection technology based on an ultrafast time-stretched swept laser. Benefiting from the high coherence and stability of the swept laser, the noise equivalent pressure reaches 100 mPa/√Hz order in the MHz region with a linear response degree of 0.9996. Different from utilizing the single-frequency laser source, the optical ultrasound detection with the swept laser need not require any servo-feedback setups to lock the linear work region. Besides, the maximum detection bandwidth can reach 50 MHz when the sweep rate of the laser is 100 MHz, which ensures a high-speed ultrasonic ranging system. A repetition-rate-tunable ultrasonic frequency comb with a pulse duration of about 0.8 μs and a carry frequency of 2 MHz is further induced to achieve the underwater absolute ranging. The ranging precision with an update frame of about 20 kHz reaches 10 μm in order. The Allan deviation shows that the highest precision reaches ∼2.5 μm at an average time of 6 ms. This work provides new insights for large-bandwidth and high-stability optical ultrasound detection as well as fast and high-precision underwater ranging.

This study investigates the influence of fumaric acid on the optical and microphysical properties of aqueous FeCl₃ microdroplets and how aging affects them. This process replicates a pathway for brown carbon (BrC) formation in the atmosphere. The experiment combines a Paul electrodynamic trap (PET), which captures a single particle, and a dual-wavelength cavity ring-down spectroscopy (CRDS) system. Initially, measurements were conducted under controlled humidity cycling, obtaining the particle phase function at a 532 nm wavelength. Retrievals reveal an irreversible increase in particle radius and complex refractive index (m_λ = n_λ + ik·_λ) after a dehydration–hydration cycle. The second part involves measuring a single particle trapped from the FeCl₃ + fumaric acid solution after 24 h in darkness. Instrumental flexibility enabled complementary measurements of the particle phase function at 473, 532, and 660 nm wavelengths and the extinction cross-section (σ_ext,λ) at 405 and 532 nm wavelengths. The most significant result was the retrieval of multiwavelength m_λ, revealing a strong spectral dependence of k_λ, which decreased from 0.014 at 405 nm to 0.000 at 660 mn. Radiative effects were evaluated and compared with other oxidation pathways of fresh biomass tar proxies, highlighting the need for precise BrC characterization in climate models, particularly in the UV range.

A common approach to detecting weak signals or minute quantities involves leveraging the localized spectral features of resonant modes, whose sharper lines (i.e., high Q-factors) enhance transduction sensitivity. However, maximizing the Q-factor often introduces technical challenges in fabrication and design. In this work, we propose an alternative strategy to achieve sharper spectral features by using interference and nonlinearity, all while maintaining a constant dissipation rate. Using far-infrared thermomechanical detectors as a test case, we demonstrate that signal transduction along an engineered response curve slope effectively reduces the detector’s noise equivalent power (NEP), achieving $\sim 30\mathrm{pW}/\sqrt{\mathrm{Hz}}$ NEP for electrical read-out, sub-THz detectors with an optimized absorbing layer.

Electromagnetically induced absorption (EIA) enables subnatural spectral features and enhanced light–matter interaction, yet its implementation in integrated photonics has been hampered by the challenge of maintaining coherent phase control in high-Q microcavities. Here, we report a magnetically tunable photonic platform in which whispering-gallery microspheres are functionalized with magnetorheological elastomers (MrEs), providing anisotropic permeability and reconfigurable optical responses while preserving ultrahigh quality factors (Q >10⁷). This hybrid architecture supports the first demonstration of magnetically controlled photonic EIA, exhibiting tunable Fano-like asymmetries, Boltzmann-distributed spectral detuning, and a record magnetic sensitivity of −598.02 MHz/mT─exceeding prior cavity-based magnetometers by over an order of magnitude. Moreover, the EIA detuning traces reproduce the hysteresis behavior of the nanoparticle–elastomer composite, enabling direct optical quantification of magnetic squareness ratios (S_r >0.7) with submillitesla resolution. By combining coherent photonic interference with soft-matter magnetism, this work establishes magnetically reconfigurable EIA as a new route for cavity-based dispersion engineering, with implications for quantum metrology, precision spectroscopy, and magneto-optical photonic devices.