Band 14, Heft 9

Photonic integrated circuits (PICs) are transforming optical technology by miniaturizing complex photonic elements and systems onto single chips. However, scaling PICs to higher densities is constrained by optical crosstalk and device separation requirements, limiting both performance and size. Recent advancements in anisotropic metamaterials, particularly subwavelength gratings (SWGs), address these challenges by providing unprecedented control over evanescent fields and anisotropic perturbations in PICs. Here we review the role of anisotropic SWG metamaterials in enhancing integration density, detailing two foundational mechanisms – skin depth engineering and anisotropic perturbation – that mitigate crosstalk and enable advanced modal controls. We summarize their applications within four key functions: confinement manipulation, hetero-anisotropy and zero-birefringence, adiabatic mode conversion, and group velocity and dispersion control, showing how each benefits from distinct SWG properties. Finally, we discuss current limitations and future directions to expand the full potentials of anisotropic SWG metamaterials, toward highly dense and scalable PICs.

Four-wave mixing is a widely used nonlinear process for wavelength conversion, parametric amplification and signal regeneration in various Kerr devices, which enables wavelength-tunability and lower-power operation in compact optical systems. Here, we demonstrate low-power continuous-wave four-wave mixing in an ultra-silicon-rich nitride topological waveguide leveraging the strong confinement of the Su–Schrieffer–Heeger topological structure and ultra-silicon-rich nitride platform’s high Kerr nonlinearity and negligible nonlinear loss. We experimentally observe continuous-wave four-wave mixing at an ultra-low pump power of 510 µW, and wavelength tunability of 54 nm with on/off conversion efficiency of −57 dB at a pump power of 3 mW. We further investigate the efficiency of the four-wave mixing process when disorder is introduced into the Su–Schrieffer–Heeger waveguide array resulting in ±80 % randomness in the coupling coefficients. It is experimentally shown that similar conversion efficiencies are achieved in the presence and absence of disorder, indicating robustness against potential fabrication errors. We expect that this work can be applied to develop compact, tunable wavelength conversion systems operating at very low power levels which are robust against certain types of disorder.

The focused vortex beam generates a hollow beam, which has been widely used for size-controlled nanoparticle formation on various materials. However, the size variation of the vortex beam is limited by the integral order of the 2π phase wrap, while the waste is caused by the large side lobe around the center. In this study, we propose a method for hollow beam generation by splitting a femtosecond laser and imparting opposite phases to the outer annular region and the central Gaussian region. After focusing, these two regions overlap at the focal spot, resulting in a hollow beam due to phase cancellation. By modulating the relative dimensions of these two regions, the hollow center can be continuously varied. When such a hollow beam is used for surface processing, the thermal capillary effect facilitates the convergence of the molten material toward the center, ultimately leading to the formation of nanoparticles. This ability to control size allows precise control of nanoparticle size with a diameter range from 140 nm to 940 nm. This method holds great promise for guiding research into nanoparticle properties that are influenced by size effects.

We investigated the generation and control of fast photoelectrons (PEs) by exposing plasmonic nanoparticles (NPs) to short infrared (IR) laser pulses with peak intensities between 10 12 and 3 × 10 13  W/cm 2 . Our measured and numerically simulated PE momentum distributions demonstrate the extent to which PE yields and cutoff energies are controlled by the NP size, material, and laser peak intensity. For strong-field photoemission from spherical silver, gold, and platinum NPs with diameters between 10 and 100 nm our results confirm and surpass extremely high PEs cutoff energies, up to several hundred times the incident laser-pulse ponderomotive energy, found recently for gold nanospheres [Saydanzad et al., Nanophotonics 12 , 1931 (2023)]. As reported previously for dielectric NPs [Rupp et al., J. Mod. Opt. 64 , 995 (2017)], at higher intensities the cutoff energies we deduce from measured and simulated PE spectra tend to converge to a metal-independent limit. We expect these characteristics of light-induced electron emission from prototypical plasmonic metallic nanospheres to promote the understanding of the electronic dynamics in more complex plasmonic nanostructures and the design of nanoscale light-controlled plasmonic electron sources for photoelectronic devices of applied interest.

Thermoelectric photodetectors are robust alternatives to photodiodes with applications in extreme environments; however, the poor absorptivity of thermoelectric materials limits their photosensitivity. Here, we take a new look at the traditional thermoelectric materials Sb 2 Te 3 and Bi 2 Te 3 in their recently discovered ability to support interband plasmonic resonances in the visible spectrum. We fabricated nanoresonators directly into the thermoelectric materials to improve their optical absorptance through plasmonic field enhancements, leading to improved photo-thermoelectric conversion. A thermoelectric detector with Sb 2 Te 3 and Bi 2 Te 3 nanostructures demonstrated ∼90 % optical absorptance across the visible spectrum, more than twice that of unpatterned materials. The solid-state device was fabricated on a substrate and exhibited a response time of 160 µs and a specific detectivity of 3.2 × 1 0 6 cm H z 1 2 W − 1 $\left.3.2{\times}1{0}^{6} \text{cm\,H}{\text{z}}^{1/\right.2} {\text{W}}^{-1}$ . Our demonstration that plasmonic and thermoelectric properties can be exploited within the same material could advance photodetectors and other optoelectronic technologies, such as biosensors, solar cells, and integrated spectrometers.

In this paper, we propose for the first time 100 GHz intelligent reflective surface (IRS) using screen-printable, high phase changing ratio vanadium dioxide (VO 2 ). Sub-THz communications offer advantages such as ultra-high speed and ultra-low latency, it increases communication challenges due to path losses and non-line-of-sight (NLOS) problems. IRS is a representative solution to this NLOS problem. Conventional IRSs using PIN and varactor diodes have difficulty covering the sub-THz band due to the operating frequency limitations of these tuning elements. In this study, we successfully applied screen-printed VO 2 switches to sub-THz IRS to control the reflection angle. The screen-printed VO 2 achieved the highest reported available phase-changing-ratio (PCR) of 1,000, increasing design freedom. The measurement results are consistent with the numerically calculated values and EM simulation results. We believe that this solution will open new avenues and potential for practical applications in low-cost, large-area tunable RF electronics in the sub-THz band.

Metasurfaces have attracted considerable interest in optical encryption due to their remarkable ability to manipulate light at subwavelength scales, however the aspect of encryption security remains an area requiring deeper exploration. Here, we propose and demonstrate metasurface-enabled optical encryption and steganography that provides dual-layer information protection. A secret information is embedded within multiple carrier images using a run-length encoding algorithm, dispersing the data to safeguard it against direct observation and brute-force attacks, thereby establishing the first layer of security. The second layer is achieved by encoding the multiple carrier images onto a silicon metasurface, leveraging light wavelength and polarization to generate diverse optical keys post-steganography. To validate the proposed scheme, several silicon metasurface samples are fabricated and characterized in the visible spectrum. By adjusting various combinations of optical keys, three encrypted carrier images are retrieved with high fidelity and negligible crosstalk, and the concealed secret information is successfully extracted through a corresponding decryption algorithm. The proposed approach enhances optical information security at the hardware level, making it less susceptible to leakage. It is anticipated that the demonstrated advancement will hold significant potential for applications in information security and optical anti-counterfeiting.

This article presents a control method for radial cell-pair rotations using a single-fiber manipulation technique that combines microcavity cascade optical tweezers with optical fiber mode coupling technology. It explores the mechanisms of cell manipulation under the influence of mode coupling and capillary fluid forces. By controlling the angle of fiber twisting and utilizing the birefringence effect along with the principle of beam mode coupling, it is possible to achieve precise and regular variations in the energy of the LP21 mode beam spot, thereby altering the magnitude and direction of the forces acting on the cell-pair, which induces a tendency for rotational motion. The microcavity cascade optical tweezers provide a small capillary fluid force and serve to isolate the cell-pair from the external environment, allowing it to respond to changes in beam spot energy within a stable microcavity space, thus enabling controllable rotations in both direction and angle. The combination of microcavity cascade optical tweezers with beam mode coupling technology achieves, for the first time, radial cell-pair rotations driven by a single fiber, which holds significant implications for the study of polarized cell migration as well as the investigation of tissue fluidity and connectivity dynamics in cancer prediction.

3D additive manufacturing enables the fabrication of nanophotonic structures with subwavelength features that control light across macroscopic scales. Gradient-based optimization offers an efficient approach to design these complex and non-intuitive structures. However, expanding this methodology from 2D to 3D introduces complexities, such as the need for structural integrity and connectivity. This work introduces a multi-objective optimization method to address these challenges in 3D nanophotonic designs. Our method combines electromagnetic simulations with an auxiliary heat-diffusion solver to ensure continuous material and void connectivity. By modeling material regions as heat sources and boundaries as heat sinks, we optimize the structure to minimize the total temperature, thereby penalizing disconnected regions that cannot dissipate thermal loads. Alongside the optical response, this heat metric becomes part of our objective function. We demonstrate the utility of our algorithm by designing two 3D nanophotonic devices. The first is a focusing element. The second is a waveguide junction, which connects two incoming waveguides for two different wavelengths into two outgoing waveguides, which are rotated by 90° to the incoming waveguides. Our approach offers a design pipeline that generates digital blueprints for fabricable nanophotonic materials, paving the way for practical 3D nanoprinting applications.