Advanced Photonics Research最新文献

Over the last decade, external seeded free-electron lasers (FELs) have achieved significant advancements across various disciplines, progressively establishing themselves as indispensable tools in fields ranging from fundamental science to industrial applications. The performance of seeded FELs is critically dependent on the quality of the frequency up-conversion process. Optimized conditions for seeded FELs are typically considered as the maximization of the bunching factor. This article discusses alternative perspectives on the optimization criteria for seeded FELs by analyzing the impact of dispersion strength on their overall performance. This study investigates the differences among the required dispersion strength for achieving the maximum bunching factor, maximum pulse energy, optimal energy stability through theoretical analysis, simulation calculations, and experimental explorations. The direct observation in experiments emphasizes the consideration of trade-off between pulse energy and energy stability in seeded FELs. These results provide valuable insights and practical guidance for controlling the pulse characteristics of seeded FELs, contributing to the tuning and optimization of FEL facilities.

In recent years, gigahertz (GHz)-burst-mode femtosecond lasers have revolutionized laser processing by significantly improving processing efficiency and quality. However, the underlying mechanisms are still unclear and experimentally optimizing the processing parameters is difficult due to their huge number. This study implements a theoretical approach based on simulations to investigate the underlying mechanisms of the Cu ablation efficiency enhancement by GHz-burst mode. The simulation results obtained using a two-temperature model suggest that the ablation efficiency is improved due to the interaction of subsequent intra-pulses in a GHz burst with Cu melted by the preceding intra-pulses. It is demonstrated that for GHz-burst mode, a 515 nm wavelength achieves higher Cu ablation performance than when using the infrared wavelength and demonstrates 2.8 times higher ablation efficiency than the single-pulse mode. The simulation results well agree with the experimental results. The approach used here represents a major advance not only in understanding the underlying mechanisms but also in determining the optimal processing conditions for practical applications.

Coherent anti-Stokes Raman spectroscopy (CARS) is a nonlinear optical technique widely utilized for vibrational imaging and molecular characterization in fields such as chemistry, biology, medicine, and materials science. Despite the high signal intensity provided by CARS, the nonresonant background (NRB) can obscure valuable molecular fingerprint information. Therefore, effective NRB removal and phase retrieval are essential for achieving precise spectral analysis and accurate material characterization. This review provides a comprehensive overview of the evolution of CARS-NRB removal and phase retrieval methods, tracing the transition from classical experimental techniques and numerical algorithms to cutting-edge deep learning models. The discussion evaluates the strengths and limitations of each approach and explores future directions for integrating deep learning to improve phase retrieval accuracy and NRB removal efficiency in CARS applications.

Thin-film coatings are a versatile option for creating color. These coatings can be made tunable by introducing phase-change materials (PCMs), whose optical response changes due to an external signal. However, commonly used PCMs such as Ge₂Sb₂Te₅ (GST) are limited in their applications for color coatings, due to high absorption of visible light. Here, the alternative PCM Sb₂S₃ is used, and a novel coating design is demonstrated, which incorporates the PCM layer into an asymmetric Fabry–Perot cavity with Ag, SiO₂ and Ti layers. Simulations are used to show that this design yields high-chroma colors in a wide range of hues, which exhibit a large tunable hue shift when the phase of the Sb₂S₃ layer changes. Three coatings are deposited to experimentally verify these results, which give good agreement with the simulations. The phase change of the Sb₂S₃ layer is demonstrated using direct heating using a hotplate, and also using laser annealing, which allows microscale image writing to be realized on the coatings.

Time-multiplexed laser projection is recognized as a promising projection technology owing to its broad color gamut and high energy efficiency. The escalating demands for enhanced laser safety and reduced space occupation necessitate the development of ultrashort throw ratio (UTR) projectors. However, the inherent mechanical deflection limitations and distortions of micro electro mechanical systems (MEMS) scanners pose significant challenges to achieving UTR projection. To address these constraints, trapezoidal distortion is first eliminated by ensuring vertical laser incidence on MEMS mirrors during the initial projection phase. Subsequently, a systematic design methodology for UTR laser MEMS projection systems is proposed, incorporating a miniaturized catadioptric projection objective that maintains uniformly optical étendue distribution. This configuration achieves a throw ratio of 0.27, reduces pillow distortion to 0.4%, and ensures consistent resolution across the entire screen. Segmentation and modular assembly of the catadioptric objective at telecentric intermediate image planes enhances the efficiency of initial structural design phase. The developed projection prototype experimentally validates the efficacy of the projection system designed through the proposed methodology in decreasing the projection ratio, correcting distortion, and maintaining uniform projection resolution across the full field of view.

Nonzero Berry curvature is central to the existence of topological edge states in electronic and photonic valley-Hall systems. While manipulating the Berry curvature in condensed matter systems is challenging, valley-Hall topological photonics offer unprecedented control, where the broken spatial inversion symmetry alters the Berry curvature. Herein, an all-silicon Berry antenna is presented, using a continuously varying geometry corresponding to a gradual change in Berry curvature. The on-chip topological edge mode with a tunable field extent is achieved to enhance effective antenna aperture, creating a high-gain on-chip photonic antenna with perfectly planar wavefronts. Experimentally, a maximum gain of 17 dBi that supports 20 Gbps chip-to-chip wireless communication is demonstrated, with active optical tunability of the antenna gain with modulation depths of 8 dBi. This Berry antenna paves the way for the development of complementary metal-oxide-semiconductor (CMOS) compatible topological Berry devices, with potential applications in integrated micro-/nano-photonics, next-generation wireless communications (6G to Xth generation), and terahertz detection and ranging.

It is a significant challenge to accurately identify and differentiate sample materials in the gas phase, especially when they have closely similar refractive indices. A promising solution in the THz range is to use plasmonic spoof surface structures configured in an Otto arrangement. Here, this study proposes a multilevel meta-grating structure sensor that achieves a remarkable improvement over the conventional binary-grating structure model. The proposed setup has been meticulously designed to maximize reflectance within the sensor's reflectance spectrum. This has been achieved by precisely adjusting the air gap distance, effectively minimizing the impact of sample material density. An in-depth analysis of gas samples with this structure shows a considerable increase in sensitivity with a small refractive index change up to 11.4 (TH/RIU), and the results are validated by simulating the reflection spectrum using the semi-analytical rigorous coupled wave analysis method. Moreover, the eigenmode solver in the CST Studio software is used to generate dispersion curves. The newly proposed design is particularly suitable for effectively detecting different gases with closely spaced refractive indices, making the proposed structure very useful in high-precision sensors that discern small refractive index changes.

A plasmonic reflective filter (PRF) based on monolayer graphene and four nanoscale cylindrical silica grooves inside a silver film is suggested and theoretically analyzed. The designed filter has a dual-band reflection and absorption spectra with a near-unity absorption value at resonance wavelengths located in near-infrared region. Furthermore, the two reflection dips can be tuned via harnessing graphene chemical potential. Application of subwavelength metallic structure as well as graphene layer contributes to the increased light–matter interactions and reinforces the light confinement in the nanoscale regions in the cylindrical grooves. As a result, near-zone electric field is highly enhanced and leads to the strong optical absorption of the filter. Also, the optical response of the proposed PRF is independent from the polarization of the incident light due to the symmetrical geometry of the structure. The suggested filter can be utilized in photonic integrated circuits.

All-Dielectric Metasurfaces

Confinement of light within nanostructures of increasingly smaller sizes has resulted in ever more precise control of light-matter interactions. In article number 2400223, Taisuke Ohta and co-workers demonstrate near-field imaging of volume-type photonic resonances within a dielectric metasurface using photoelectron emission microscopy. This imaging approach, utilizing far-field excitation and sub-optical wavelength spatial resolution, offers a new avenue for examining light–matter interactions within dielectric nanophotonic systems.

Mid-infrared (IR) fiber lasers are crucial for applications in spectroscopy, medical diagnostics, and environmental sensing, owing to their ability to interact with fundamental molecular vibrational bands. However, achieving stable ultrafast pulse generation in this spectral range remains challenging due to the limited availability of robust saturable absorbers. For the first time, we demonstrate Q-switched mode-locking in an all-fiber Er-doped ZrF₄-BaF₂-LaF₃-AlF₃-NaF laser employing an aerosol-synthesized carbon nanotube (CNT) film. Furthermore, we compare the laser performance with pulse generation using a state-of-the-art GaSb-based semiconductor-saturable absorber mirror (SESAM) in an identical cavity design. The CNT-saturable absorber enables pulse generation with a minimum duration of 1.32 μs and a pulse energy of 1.4 μJ at an average output power of 63.1 mW. In contrast, the SESAM-based laser produces 345-ns pulses with a pulse energy reaching 3.84 μJ and an average power of 202 mW. These results provide new insights into the interplay between saturable absorber properties and mid-IR fiber laser performance, paving the way for next-generation compact ultrafast sources for scientific and industrial applications.