Corrugation-assisted metal-coated angled fiber facet for wavelength-dependent off-axis directional beaming

Ceramic surface relief gratings imprinted on an optical fiber tip

We report on the fabrication, experimental measurement, and numerical simulation of sol-gel diffraction grating structures deposited on the end-face of a single mode optical fiber. Using the imprint method, we manufactured surface relief grating structures in four configurations with different grating-relative-to-fiber arrangements. We demonstrate the high quality of the fabricated structures based on atomic force microscopy imaging and their operational characteristics, presenting measured and simulated far-field intensity distributions. Using a numerical model, we simulated the diffraction patterns in the far-field. We obtained strong agreement between the results of the simulations and the experiments in terms of the angular positions of the diffraction peaks. We also investigated the tolerance of fabricated structures to high-power lasers. Among the proposed structures, the most intriguing is the grism fabricated on a fiber end-face using sol-gel imprint technology for the first time, to the best of our knowledge.

Active optical metasurfaces: comprehensive review on physics, mechanisms, and prospective applications

Optical metasurfaces with subwavelength thickness hold considerable promise for future advances in fundamental optics and novel optical applications due to their unprecedented ability to control the phase, amplitude, and polarization of transmitted, reflected, and diffracted light. Introducing active functionalities to optical metasurfaces is an essential step to the development of next-generation flat optical components and devices. During the last few years, many attempts have been made to develop tunable optical metasurfaces with dynamic control of optical properties (e.g., amplitude, phase, polarization, spatial/spectral/temporal responses) and early-stage device functions (e.g., beam steering, tunable focusing, tunable color filters/absorber, dynamic hologram, etc) based on a variety of novel active materials and tunable mechanisms. These recently-developed active metasurfaces show significant promise for practical applications, but significant challenges still remain. In this review, a comprehensive overview of recently-reported tunable metasurfaces is provided which focuses on the ten major tunable metasurface mechanisms. For each type of mechanism, the performance metrics on the reported tunable metasurface are outlined, and the capabilities/limitations of each mechanism and its potential for various photonic applications are compared and summarized. This review concludes with discussion of several prospective applications, emerging technologies, and research directions based on the use of tunable optical metasurfaces. We anticipate significant new advances when the tunable mechanisms are further developed in the coming years.

Femtosecond laser induced concentric semi-circular periodic surface structures on silicon based on the quasi-plasmonic annular nanostructure

In this study, we report polarization-dependent concentric circular periodic surface structures on Si induced by a single shot femtosecond (fs) laser pulse based on pre-processed quasi-plasmonic annular-shaped nanostructure. An abnormal annular-shaped energy deposition of the fundamental fs laser pulse can be found by using dual-wavelength superposition of the fundamental frequency (ω) and the second-harmonic frequency (2ω) of an fs Ti:sapphire laser, which is confirmed by real beam shape detection. Based on the annular-shaped energy distribution of dual-wavelength fs laser, a concentric quasi-plasmonic corral nanostructure can be imprinted on the Au thin film. Surface plasmon polaritons (SPPs) excitations on the planar metallic nanostructures enable the manipulation of light on subwavelength scales. Thus, the pre-processed concentric quasi-plasmonic corral nanostructure can act as a precursor for the subsequent SPPs excitation and propagation by the fs laser irradiation. Using this technique, polarization-dependent semi-circular periodic surface structures on silicon can be found by the irradiation of fs laser pulse with only one shot. This research provides an additional freedom for the laser induced periodic surface structure (LIPSS) modulation based on the modulation of SPPs excitation and propagation, which plays an important role in the formation of LIPSS.

Metallic Fresnel zone plate implemented on an optical fiber facet for super-variable focusing of light

We propose and investigate a metallic Fresnel zone plate (FZP/MFZP) implemented on a silver-coated optical fiber facet for super-variable focusing of light, the focal point of which can be drastically relocated by varying the wavelength of the incident light. We numerically show that when its nominal focal length is set to 20 μm at 550 nm, its effective focal length can be tuned by ~13.7 μm for 300-nm change in the visible wavelength range. This tuning sensitivity is over 20 times higher than that of a conventional silica-based spherical lens. Even with such high tuning sensitivity with respect to the incident wavelength change, the effective beam radius at the focal point is preserved nearly unchanged, irrespective of the incident wavelength. Then, we fabricate the proposed device, exploiting electron- and focused-ion-beam processes, and experimentally verify its super-variable focusing functionality at typical red, green, and blue wavelengths in the visible wavelength range, which is in good agreement with the numerical prediction. Moreover, we propose a novel MFZP structure that primarily exploits the surface-plasmon-polariton-mediated, extra-ordinary transmission effect. For this we make all the openings of an MFZP, which are determined by the fundamental FZP design formula, be partitioned by multi-rings of all-sub-wavelength annular slits, so that the transmission of azimuthally polarized light is inherently prohibited, thereby leading to super-variable and selective focusing of radially polarized light. We design and fabricate a proof-of-principle structure implemented on a gold-coated fused-silica substrate, and verify its novel characteristics both numerically and experimentally, which are mutually in good agreement. We stress that both the MFZP structures proposed here will be very useful for micro-machining, optical trapping, and biomedical sensing, in particular, which invariably seek compact, high-precision, and flexible focusing schemes.