Volume 12, Issue 22

Polarization processors with versatile functionalities are needed in optical systems, which use or manipulate polarized light. In this paper, we propose and realize an integrated polarization processor based on a coherent 4-port micro-ring resonator. The arbitrary unknown polarization state is input to the polarization processor via a 2-dimensional grating coupler (2DGC), which serves as a polarization beam splitter. The coherent 4-port micro-ring resonator (MRR) operates as a unitary processor and is formed by one crossbar micro-ring resonator and two thermally tunable phase shifters, one of which tunes the micro-ring while the other tunes the coherent interference between the two inputs from the 2DGC. The 4-port system can be used to control the input polarization states that appear at the two output ports and, therefore, can be used to implement a multi-function polarization processor, including polarization descrambler, polarization switch, polarizers, and polarization analyzer (both division of space (DOS) and division of time (DOT)). In this paper, we experimentally demonstrate the use of coherent 4-port MRR for polarization mode switching and for polarization mode unscrambling. The polarization unscrambler was capable of separating two polarization-multiplexed 40 GHz data lanes from the input fiber with crosstalk levels below −21 dB and is suitable for use in the receiver for polarization-multiplexed direct-detection optical communications systems. The same photonic circuit may be used as a polarization analyzer, either as a DOS polarization analyzer or a DOT polarization analyzer. The DOS polarization analyzer measured the polarization with measured deviation of the orientation angle (2 ψ ) varying from −0.5° to 1.3°and deviation of ellipticity angle (2 χ ) varying from −0.98° to 7.27°. The DOT polarization analyzer measured the polarization with a deviation of the orientation angle (2 ψ ) that varied from −2.93° to 3.49° and deviation of ellipticity angle (2 χ ) that varied from −3.5° to 3.05°.

In this paper, we combine a Content-Addressable Memory (CAM) encoding scheme previously proposed for analog electronic CAMs (E-CAMs) with optical multiplexing techniques to create two new photonic CAM architectures—wavelength-division multiplexing (WDM) optical ternary CAM (O-TCAM) and time-division multiplexing (TDM) O-TCAM. As an example, we show how these two O-TCAM schemes can be implemented by performing minor modifications in microring-based silicon photonic (SiPh) circuits originally optimized for exascale interconnects. Here, our SiPh O-TCAM designs include not only the actual search engine, but also the transmitter circuits. For the first time, we experimentally demonstrate O-TCAM functionality in SiPh up to ∼ 4 Gbps ${\sim} 4\,\,\text{Gbps}$ and we prove in simulation feasibility for speeds up to 10 Gbps, 10 times faster than typical E-TCAMs at the expense of higher energy consumption per symbol of our O-TCAM Search Engine circuits than the corresponding E-TCAMs. Finally, we identify which hardware and architecture modifications are required to improve the O-CAM’s energy efficiency towards the level of E-CAMs.

Metasurface design tends to be tedious and time-consuming based on sweeping geometric parameters. Common numerical simulation techniques are slow for large areas, ultra-fine grids, and/or three-dimensional simulations. Simulation time can be reduced by combining the principle of the discrete dipole approximation (DDA) with analytical solutions for light scattered by a dipole near a flat surface. The DDA has rarely been used in metasurface design, and comprehensive benchmarking comparisons are lacking. Here, we compare the accuracy and speed of three DDA methods—substrate discretization, two-dimensional Cartesian Green’s functions, and one-dimensional (1D) cylindrical Green’s functions—against the finite difference time domain (FDTD) method. We find that the 1D cylindrical approach performs best. For example, the s -polarized field scattered from a silica-substrate-supported 600 × 180 × 60 nm gold elliptic nanocylinder discretized into 642 dipoles is computed with 0.78 % pattern error and 6.54 % net power error within 294 s, which is 6 times faster than FDTD. Our 1D cylindrical approach takes advantage of parallel processing and also gives transmitted field solutions, which, to the best of our knowledge, is not found in existing tools. We also examine the differences among four polarizability models: Clausius–Mossotti, radiation reaction, lattice dispersion relation, and digitized Green’s function, finding that the radiation reaction dipole model performs best in terms of pattern error, while the digitized Green’s function has the lowest power error.

Combining integrated photonics and radiofrequency (RF) signals in the optical domain can help overcome the limitations of traditional RF systems. However, it is challenging to achieve environmentally insensitive filtering in wireless communications using integration schemes. In this report, the performance of robust RF filters based on a single silicon in-phase/quadrature modulator with significantly improved temperature and optical carrier wavelength sensitivities, which were suppressed by more than three orders of magnitude compared with those of silicon resonators, was experimentally evaluated. Upconversion and the processing of signals were simultaneously realized on the modulator by setting the relative phases of the arms and the bias voltages. Moreover, the filters can be reconfigured as low-pass, high-pass, band-pass, or band-stop filters. From 25 to 75 °C, the center frequency variation was within 0.2 GHz. From 1500 to 1600 nm, the center frequency variation was within 2 GHz. The proposed scheme allows for filtering and reconfiguration without the use of optical processing modules such as resonators or delay lines, thus providing a novel approach to signal processing and a new robust filter for scenarios with dynamic environments.

Plasmonic waveguides have attracted tremendous interest due to efficiently confining photons on the subwavelength spatial scale to be beating the propagation diffraction limit. Transition metal molybdenum (Mo) exhibits outstanding properties in light trapping and electromagnetic field confining, making it potentially valuable in 1.55 μm plasmonic waveguide applications. However, the reliable fabrication of high-quality Mo plasmonic waveguides is a significant challenge. A real-space micro-imaging study of the surface plasmon on Mo structures is still absent. In this study, we successfully prepared a single-crystalline Mo microrod waveguide structure and fabricated subwavelength gratings on it. The diffraction gratings were designed, optimized, and etched to excite the surface plasmon polariton behaviour of Mo for the first time. The grating-optimized Mo microrod single-crystal reveals highly efficient waveguide performance around near-infrared spectroscopy, exhibiting a long propagation length of 32 μm and a low transmission loss of 0.067 dB μm −1 . The results provide an alternative to advanced materials research and optical device applications of plasmonic waveguide systems.

Coherent perfect absorption (CPA), as time-reversed lasing, arises from appropriate wave interference within absorbers, offering flexible control over wave absorption. Typically, this control involves tuning the phase difference between two counter-propagating incident beams. Here, we elucidate the critical role of defect connectivity within three-dimensional zero-index media for realizing and controlling CPA. Specifically, the realization of CPA critically depends on the establishment of long-range connectivity of defects in a specific direction. Once the long-range connectivity is established, the CPA exhibits remarkable resilience against defects’ deformation, changes in size and shape of the zero-index media, as well as variations in number and orientation of incident channels. Notably, a minor disruption to this connectivity will result in a complete reduction of absorption to zero, highlighting an ultra-sensitive absorption property in response to connectivity perturbations. Our findings not only unveil a physical mechanism for realizing CPA but also open up promising avenues for advanced CPA control with versatile functionalities.

Due to the unique self-acceleration, self-healing, and non-diffraction properties, Airy beams have been explored extensively and found applications in various fields. It has been proven as an essential aspect to tune the trajectory of Airy beams for extensive applications. In this paper, we propose a method based on liquid crystal (LC) alignment with metasurfaces, which enables dynamic tuning of the trajectory of Airy beams. Benefiting from both the tunable property of LCs and the compact alignment of metasurfaces, we achieve a sizeable linear potential in a short distance, which leads to the effective tuning of the trajectory of Airy beams dynamically. The introduction of metasurfaces into the alignment of LCs provides a promising method to manipulate the planar optical field.

Perfect vortices, whose ring profile is independent of the topological charge, play a key role in telecommunications and particle micro-manipulation. In this work, we report the compact generation of a new kind of double-ring perfect vortices, called double-ring perfect vector beams, by exploiting dual-functional silicon metaoptics. In particular, we develop and test a new paradigm to generate those beams with the possibility of selecting different topological charges between the two rings. The generated beams are characterized through a filtering method, proving that the two rings have a vectorial nature with the same magnitude and either the same or different topological charges. Their unique properties suggest promising applications for optical tweezing and manipulation of low refractive-index particles, trapping of cold atoms, and high-capacity communications.

Optical waveguides covered with thin films, which transmittance can be controlled by external action, are widely used in various applications from optical modulators to saturable absorbers. It is natural to suggest that the losses through such a waveguide will be proportional to the absorption coefficient of the covering material. In this letter, we demonstrate that under certain conditions, this simple assumption fails. Instead, we observe that the reduction of the material loss of the film can lead to an increase in the propagation losses through the waveguide. For this, we use a side polished fiber covered with a single-walled carbon nanotube thin film whose absorption can be attenuated either by a short pulse illumination (due to absorption saturation) or with electrochemical gating. For the films thicker than 50 nm, we observe saturable absorption to turn into optical limiting with nonmonotonic dependence on the incident power. With a numerical simulation, we identify that this nontrivial behavior comes from mode reshaping due to changes in the absorption coefficient of the covering film. We demonstrate the applicability of the observed effect by fabricating the device which nonlinear optical response can be controllably switched between saturable absorbing and optical limiting. Finally, we utilize an analytical approach to predict the required parameters and corresponding nontrivial shapes of the nonlinear absorbance curves. These results provide new perspectives for engineering complex reconfigurable nonlinear optical responses and transmittance dependences of nanomaterial covered waveguides.

Optical vortices (OVs) have rapidly varying spatial phase and optical energy that circulates around points or lines of zero optical intensity. Manipulation of OVs offers innovative approaches for various fields, such as optical sensing, communication, and imaging. In this work, we demonstrate the correlation between OVs and absorption enhancement in two types of structures. First, we introduce a simple planar one-dimensional (1D) structure that manipulates OVs using two coherent light sources. The structure shows a maximum of 6.05-fold absorption gap depending on the presence of OVs. Even a slight difference in the incidence angle can influence the generation/annihilation of OVs, which implies the high sensitivity of angular light detection. Second, we apply inverse design to optimize two-dimensional (2D) perfect ultrathin absorbers. The optimized free-form structure achieves 99.90 % absorptance, and the fabricable grating structure achieves 97.85 % at 775 nm wavelength. To evaluate OV fields and their contribution to achieving absorption enhancement, we introduce a new parameter, OV circularity. The optimized structures generate numerous OVs with a maximum circularity of 95.37 % (free-form) and 96.14 % (grating), superior to our 1D structure. Our study reveals the role of high-circularity localized OVs in optimizing nano-structured absorbers and devices for optical sensing, optical communication, and many other applications.