Optics letters最新文献

Fluorescence interference is a pervasive challenge in Raman spectroscopy, often limiting its broader application. Time-gated Raman spectroscopy offers a more universal solution by temporally separating Raman signals from fluorescence; however, it faces significant challenges when dealing with samples that exhibit short fluorescence lifetimes. Achieving high time resolution to effectively distinguish these signals typically requires advanced detectors that are not only costly but also difficult to source commercially, often resulting in substantial residual fluorescence that diminishes overall signal quality. In this work, we identified that the dominant noise in time-gated Raman spectroscopy is wavelength-to-wavelength fluctuation noise, which cannot be reduced by simply extending the collection time. Through our analysis, we discovered that this noise is linearly proportional to the fluorescence background and remains consistent across different time windows when collected using the time-correlated single-photon counting (TCSPC) technology. Recognizing this consistent pattern, we developed a novel, to the best of our knowledge, method to effectively remove this noise by leveraging the time-resolved fluorescence spectrum. For example, in the case of sesame oil excited with a 532 nm laser, it is typically difficult to obtain a recognizable Raman spectrum when the gate width exceeds 300 ps. However, using our method, we were able to achieve a decent signal even with a gate width of 4 ns. By correcting the Raman spectrum using the captured pure fluorescence spectrum, we achieve up to a 23-fold improvement in the signal-to-noise ratio (SNR). This innovation significantly reduces the dependence on high-cost, high-time-resolution detectors, potentially expanding the adoption and applicability of time-gated Raman spectroscopy across various fields.

Erbium-doped thin-film lithium niobate (TFLN) lasers have attracted great interest in recent years due to their compatibility with high-speed electro-optic (EO) modulation on the same platform. In this work, high-efficiency single-mode erbium-doped microring lasers with milliwatt output powers were demonstrated. Monolithic lithium niobate microring resonators using pulley-waveguide-coupling were fabricated by the photolithography assisted chemo-mechanical etching (PLACE) technique. The maximum single-mode laser power of 1.26 mW with the side-mode suppression ratio (SMSR) of 50 dB was achieved around the wavelength of 1562 nm, as well as the maximum laser slope efficiency of 2.51% and the minimum laser linewidth of 30 kHz. Besides, the lasing band was easily switched by the pulley-coupler with variable waveguide widths. The demonstrated milliwatt-level on-chip microlasers hold great promise as bright light sources for various integrated devices on the TFLN platform such as EO modulators and combs.

In this Letter, we present an experimental demonstration of an all-fiber ring Mamyshev oscillator (MO) that delivers a 300 mW optical power at a repetition rate of 45 MHz. Numerical design has shown that more than 70 MHz repetition rate is achievable in a ring cavity configuration using standard step-index fibers and components. Interest of WDM type filtering on pulse duration is especially shown. This demonstration of a high repetition rate ring Mamyshev oscillator working with the simplest transmission filter paves the way to very simple and cost-effective all-standard-PM-fiber Mamyshev oscillators.

In computational imaging, getting better imaging quality with shorter time usage is always a challenging problem. The powerful compressed sensing functions as a backend algorithm, which leaves room for us to develop a methodology of compression in imaging systems. Optical differentiation was widely utilized in direct imaging to highlight the features of an image. We apply optical differentiation to compress information in the correlated imaging system. The experimental results indicate a significant improvement in the signal-to-noise ratio and imaging speed. In addition, this scheme enables phase imaging from the second-order correlation. Our work can spark potential applications in biological microscopic and scattering media imaging.

Multiple-input multiple-output (MIMO) technology, a core component of 6G, has been widely adopted in optical wireless communication (OWC) systems. Accurate recognition of different MIMO types is essential for MIMO selection and demodulation. In this Letter, we propose an open-set MIMO recognition method for OWC systems using a Siamese neural network (SNN). Simulation results show that the SNN significantly outperforms other recognition approaches, including convolutional neural networks (CNNs) and traditional machine learning techniques. For SNN-based recognition, over 90% accuracy is achieved with training based on only nine fixed sampling points in both 2 × 2 and 4 × 4 MIMO-OWC systems.

We propose a surface-normal dual-polarization in-phase and quadrature modulator (DP-IQM) that employs a thin dielectric metasurface (MS) layer inserted on a high-speed electro-absorptive modulator array. The metasurface provides the functionalities of all the passive components necessary for a DP-IQM, including a polarization beam splitter/combiner and an interferometric circuit, to a normal-incident beam. A dielectric metasurface composed of silicon nanoposts is designed and fabricated to experimentally demonstrate polarization and beam splitting functionalities with a phase error of less than 0.08 rad and a power imbalance of less than 0.9 dB. These small errors correspond to an error-vector magnitude of less than 7% when it is used to generate dual-polarization quaternary phase-shift-keying (DP-QPSK) or 16 quadrature amplitude modulation (DP-16QAM) signals. The surface-normal configuration of our scheme allows scaling to a large two-dimensional (2D) array to generate spatially parallelized DP coherent signals for various applications, including optical communication, computing, and sensing.

Localized symmetry has been shown to significantly impact the luminescence behavior of Mn⁴⁺ ions through the electron-phonon coupling process. Building on this characteristic, three types of inverse spinel structure oxides (Mg₂XO₄, where X = Ti, Ti/Ge, Ge) doped with Mn⁴⁺ were developed, exhibiting strong red emission when exposed to UV and blue light. A thorough examination reveals that the symmetric improvement of the Mn⁴⁺ sites within the octahedral environment leads to significant changes in their luminescence behavior, including a suppression of zero-phonon-line (ZPL) emission, a blueshift, and an extension of the luminescence lifetime. Moreover, variable-temperature PL spectra of phosphors are carefully measured. Low-temperature PL spectra demonstrate three distinct sharp emission peaks for Mg₂GeO₄:Mn⁴⁺, while Mg₂TiO₄:Mn⁴⁺ exhibits a broad emission band. The average primary phonon energies involved in the vibronic processes are determined through theoretical fitting of temperature-dependent PL intensities. Lastly, the luminescence dynamics associated with anti-Stokes, ZPL, and Stokes emissions are analyzed. The observed increase in luminescence lifetime indicates a significant impact of the localized environment on luminescence properties.

Penicillin G detection is of great significance in medical research and disease diagnosis. Liquid crystal (LC), as a branch of sensitive materials, has a broad application prospect in the field of biosensing. Herein, a liquid crystal-coated silica microbubble resonator (LC-MBR), with high sensitivity for penicillin G detection, has been proposed and demonstrated. Whispering gallery mode (WGM) spectra with a Q factor exceeding 10⁶ were excited through the coupling between tapered fiber and LC-MBR. The orientation transition of the LC molecules, combined with WGM resonance, amplified target information and triggered wavelength blueshifts. LC-MBR was then applied in pH detection, achieving a sensitivity of 2.51 nm/pH within a pH range of 3.8-8.5. This resonator was also successfully employed to detect enzymatic reactions of penicillinase, with a detection limit of 6 × 10³ U/mL. The fabricated sensor offers significant advantages, including a simple structure, improved stability, and higher sensitivity across a wide pH detection range, making it a promising candidate for future biomedical applications.

Metasurfaces have demonstrated significant potential in optical encryption and anti-counterfeiting due to their incredible capability of manipulating various light properties. However, previous metasurface-encryption methods did not sufficiently explore the spatial frequency aspect, particularly regarding evanescent waves. Here, we propose an encryption scheme by introducing evanescent waves into the encoding and decoding processes. Different parts of the target image are individually encoded at distinct spatial frequencies within the evanescent-wave region. Only when an evanescent wave with a specific transverse wave vector (TWV) serves as the key can the complete target image be retrieved. Our work exemplifies a thorough exploration of the mathematical framework underlying metasurface-based holography. It enables the feasibility of a single metasurface in concealing and retrieving information in the evanescent-wave region, thus opening up a new, to our knowledge, methodology for high-security optical information encryption.

Enhancing the skin effect in non-Hermitian photonic crystals has traditionally required alternating materials with different gain and loss characteristics inside the basic unit, which not only increases fabrication complexity but also faces limitations due to different material compatibility and material integrations. In this paper, a novel method is proposed that requires only one material but enables significant enhancement of the skin effect intensity by introducing spatial rotational symmetry breaking in two-dimensional reciprocal photonic crystals. Our result has shown a 306.09% improvement in skin effect intensity with just one material, if compared to previous designs. Our finding not only may broaden the theoretical framework of non-Hermitian optics but also demonstrates significant potential for practical applications in photonic integration, optical sensing, and laser design, thus opening up new possibilities for future photonic device innovations.