ACS Photonics最新文献

Fluorescence-based single-molecule and fluctuation spectroscopy in the near-IR can open avenues for biomolecular dynamic studies in biological media with suppressed autofluorescence and scattering background. However, further implementation is limited by the lower brightness of NIR fluorophores and available single-photon detector technologies that are still to be explored and adapted. Superconducting nanowire single-photon detectors (snSPDs) have found increasing use in quantum optics and optical communication applications thanks to high sensitivity in the near-infraed (NIR), low dark-counts, no after-pulsing, and high time resolution. Here, we present characterization of fluorescence intensity fluctuations from single vesicles and NIR fluorophores based on fluorescence correlation spectroscopy (FCS), specifically taking advantage of these snSPD properties. We present a concept allowing multiplexed readouts based on only one snSPD, in which the emitted photons are separated by their emission wavelength into different optical paths, thereby translating the emission wavelengths into different arrival times onto the snSPD. This concept allows one-laser-one-detector, dual-color fluorescence cross-correlation spectroscopy (FCCS) measurements, with fluorescence intensity fluctuations of two fluorophore species separately analyzed and cross-correlated. It is shown how two fluorophore species in a sample can be distinguished by their different blinking kinetics, fluorescence lifetimes, and/or diffusion properties. Apart from differences in emission spectra, the presented concept for multiplexing using a single detector can also be applied to distinguish emitters by properties such as polarization, coherence lengths, and fluorescence bunching and antibunching signatures. It can also be generalized to other modalities than FCS, including single-molecule detection, confocal microscopy, and imaging.

In a moiré structure formed by isotropic semiconducting transition metal dichalcogenide (TMD), tuning the isotropic excitons to be anisotropic is quite challenging. Herein, by introducing controllable in-plane interlayer uniaxial strain to a MoS₂ monolayer (ML) and stacking it to another unstrained ML MoS₂, we successfully prepared an artificial bilayer (BL) moiré structure with heterostrain. The intralayer direct and indirect excitons within this structure demonstrate pronounced anisotropic photoluminescence (PL) emissions at both ambient and low temperatures. At 80 K, the PL anisotropic ratios of indirect and direct excitons can be enhanced to 1.45 and 1.31, respectively, in samples subjected to tensile heterostrain. These findings significantly advance our understanding of the anisotropic optical behaviors exhibited by moiré excitons of TMD materials, potentially paving the way for the design of future anisotropic twistronic devices.

Today, upconverting luminescent particles are routinely used as accurate and reliable probes to remotely measure the temperature of minute volumes of matter on the order of attoliters. Lanthanide-doped particles exhibit adaptability as optical nanothermometers within biological systems, aiding the understanding of cellular dynamics, pathology, and physiology. Herein, we investigate the intrinsic optical response of Er/Yb-doped single particles levitating in a vacuum and compare it with the collective response of ensembles of particles embedded in application-relevant wet and dry environments. We make use of a quadrupole Paul trap that employs a time-varying electric field to confine single Er/Yb-doped particles in a vacuum and a thermal bath module to study particles in the above-mentioned environments. Both subsystems use twin-excitation/detection setups that allow us to record luminescence spectra, covering 4 orders of magnitude in laser intensity (e.g., 10^–1–10³ W/cm² at 980 nm) and temperatures from 20 up to 200 °C. We revisit the well-established reliability of ratiometric measurements to accurately measure temperature. We find an almost perfect overlap of the experimental Boltzmann factor as a function of temperature for water, ethanol, and air–substrate environments, which is then used to retrieve the temperature of particles levitating in vacuum. We also explored the influence of the surrounding environment for increasing laser intensities by numerically and experimentally examining the balance among relevant emission bands. Our simulations qualitatively reproduce the experimentally measured luminescence in different environments, yielding a single model to simultaneously explain the laser intensity dependence of UV–NIR transitions for both the low and strong laser excitation regimes. Our findings hold great potential to expand the range of applicability of upconverting particles as dual sensors of temperature and laser intensity in different media relevant to biological and nanophotonic applications.

The development of multifunctional photodetectors that integrate sensing, storage, and computing to mimic the human visual system for efficient image processing is a key area of research. In particular, retina-inspired optoelectronic devices with multispectral information preprocessing capabilities are critical for constructing neuromorphic visual systems; however, achieving this in traditional photodetectors is challenging due to the lack of suitable photoresponse modes. Herein, a graphene/organic photodetector (GOP) with a spectral-tunable photoresponse memory mode switching feature is demonstrated. Benefiting from the unique photogenerated charge transfer and trapping behavior in the heterojunction, the device exhibits memory-free (with recovery times of a few milliseconds) and long-memory (with recovery times of several hundred seconds) photoresponse modes under long-wavelength (650–1064 nm) and short-wavelength (370–520 nm) light stimulation, respectively. Furthermore, the device supports spectral-tunable dual-mode switching between photosynaptic and photodetection under multiple light pulse stimulations, enabling real-time preprocessing of images with mixed green and red dual-wavelength information using a GOP-based 3 × 3-pixel image sensor. We also demonstrate a GOP-constructed neuromorphic visual system for efficient image processing, where the front-end GOP-based image sensor filters out background noise in the input images, significantly improving the image recognition accuracy of the back-end GOP-connected artificial neural network (from 40 to 93%).

In recent years, advancements in optical scattering of nanostructures have significantly driven the development of telecommunications, medical imaging, detection, and novel light sources. However, due to the structural complexity of nanostructures, particularly metasurfaces and metamaterials, traditional methods of full-wave modeling for simulating optical scattering face substantial challenges due to increased degrees of freedom. In this work, we propose a symmetry-adapted finite element method to reduce the computational domain and enhance the efficiency of optical scattering simulations. By introducing the concepts of symmetry group and projection operator, we offer a formal and rigorous framework for decomposing the original problem, i.e., the incident condition, boundary constraints, and the finite element method implementation in decoupled subtasks. To demonstrate its broad applicability, we present three numerical examples: the enhancement of light confinement via quasi-bound states in the continuum in a photonic crystal slab, the scattering cross sections of incident configurations, and the calculation of transmission spectra in the metasurface. These examples illustrate the use of the symmetry finite element method under different symmetry conditions, including mirror symmetry, rotational symmetry, and the combination of Bloch’s theorem. Our method significantly reduces computation time and memory usage, thereby greatly improving the computational efficiency. Given the universality of symmetry principles, our method has important applications in the optical analysis and design of symmetric photonic devices, especially for symmetric yet large-sized optical structures.