Silicon nitride waveguide devices based on gradient-index lenses implemented by subwavelength silicon grating metamaterials

Integrated polarization-free Bragg filters with subwavelength gratings for photonic sensing

We present polarization-free Bragg filters having subwavelength gratings (SWGs) in the lateral cladding region. This Bragg design expands modal fields toward upper cladding, resulting in enhanced light interaction with sensing analytes. Two device configurations are proposed and examined, one with index-matched coupling between transverse electric (TE) and transverse magnetic (TM) modes and the other one with hybrid-mode (HM) coupling. Both configurations introduce a strong coupling between two orthogonal modes (either TE-TM or HM1-HM2) and rotate the polarization of the input wave through Bragg reflection. The arrangements of SWGs help to achieve two configurations with different orthogonal modes, while expanding modal profiles toward the upper cladding region. Our proposed SWG-assisted Bragg gratings with polarization independency eliminate the need for a polarization controller and effectively tailor the modal properties, enhancing the potential of integrated photonic sensing applications.

Multilayer Bolometric Structures for Efficient Wideband Communication Signal Reception

It is known that the dielectric layer (resonator) located behind the conducting plate of the bolometer system can significantly increase its sensitivity near the resonance frequencies. In this paper, the possibility of receiving broadband electromagnetic signals in a multilayer bolometric meta-material made of alternating conducting (e.g., silicon semiconductor) and dielectric layers is demonstrated both experimentally and numerically. It is shown that such a multilayer structure acts as a lattice of resonators and can significantly increase the width of the frequency band of efficient electromagnetic energy absorption. The parameters of the dielectric and semiconductor layers determine the frequency bands. Numerical modeling of the effect has been carried out under the conditions of our experiment. The numerical results show acceptable qualitative agreement with the experimental data. This study develops the previously proposed technique of resonant absorption of electromagnetic signals in bolometric structures.

High NA and polarization-insensitive ultra-broadband achromatic metalens from 500 to 1050 nm for multicolor two-photon endomicroscopy imaging

Multicolor two-photon endomicroscopy has become a highly competitive tool for functional imaging in biomedical researches. However, to make the imaging system miniature and applicable for freely behaving animal brain activity, metalenses have received much attention in compact imaging systems. For high resolution multicolor imaging and maximizing fluorescence collection, there is a challenge metalenses faced to achieve large numerical aperture (NA) and focus the NIR excitation and VIS emission lights of multiple fluorophores to the same distance simultaneously because of the limitation of the group delay range of the meta-units. In this paper, we proposed a high NA and polarization-insensitive ultra-broadband achromatic metalens specifically for achromatically focusing the excitation and emission light of multiple fluorophores commonly used in neuroscience studies. TiO2 and Si meta-unit libraries composed of heights, widths and the corresponding phase and group delay were constructed, and the optimal meta-units were selected by particle swarm optimization algorithm to engineer the dispersion of metalens in the VIS band and NIR band, respectively. Combining dispersion engineering with spatial multiplexing, the proposed metalens achieved the maximal effective NA up to 0.8 and large achromatic bandwidth ranging from 500 nm to 1050 nm, which exhibited the coefficient of variation of focal lengths was only 3.41%. The proposed achromatic metalens could successfully achromatically focus different fluorescence with any polarization, which was suitable for most fluorophores. Our results firmly establish that the proposed metalens can open the door to high resolution and minimally invasive multicolor two-photon functional imaging in intravital deep brain.

Compact integrated mode-size converter using a broadband ultralow-loss parabolic-mirror collimator

In this Letter, we propose and demonstrate an integrated mode-size converter (MSC) with a compact footprint, low losses, and a broad bandwidth. By exploiting a parabolic mirror, the divergent light from a narrow waveguide (450 nm) is collimated to match the mode size of a wide waveguide (10 µm). The measured insertion loss (IL) is ≈ 0.15 dB over a 100-nm bandwidth. The mode-size conversion is achieved with a footprint as small as ≈ 20 × 32 µm², which is much shorter than the linear taper length required to attain the same level of losses.

Design of a confocal dispersion objective lens based on the GRIN lens

Spectral confocal technology utilizes the principle of dispersion to establish the precise coding relationship between spatial position and wavelength in the axial focal point. The axial dispersion produced by the dispersion lens will affect the measurement range and resolution of the system. Taking into account the above advantages of the GRIN lens, the dispersion objective lens of spectral confocal displacement sensor based on the radial GRIN lens in this paper is proposed. The imaging characteristics of the GRIN lens are analyzed. By deducing the refractive index distribution and optical properties of the radial GRIN lens, the optical focal length and axial dispersion models of the GRIN lens are established. Then, based on the optical focus and dispersion function of the GRIN lens, the calculation of refractive index distribution is completed by MATLAB. The simulation design of the GRIN dispersion objective lens is completed by ZEMAX. Finally, the optimization design of the GRIN dispersion objective lens is completed. The designed results indicate that the dispersion objective lens based on radial GRIN lens can achieve axial dispersion of 1215 µm in the wavelength range of 420 nm ∼ 620 nm as well as the linear correlation coefficient between wavelength and axial dispersion is 99.69%. The resolution of GRIN dispersion objective lens is about 6.075 nm. The focusing effect of the lens at each wavelength is good, and the measurement range and dispersion linearity of the lens are better than those of the same kind of traditional dispersion objective lens. Compared with the same kind of traditional dispersion objective lens, the dispersion objective lens based on GRIN lens has compact structure and small diameter. And the measurement range and resolution of the system are improved. So it is easier to realize precise measurement. The research results of this paper have certain guiding significance and reference value for the application of the GRIN lens in the spectral confocal system.

Compact and broadband design of an 850 nm 2 × 2 3-dB directional coupler with a shallowly etched SWG gap

A compact and broadband 2×2 3 dB directional coupler (DC) is designed at the 850 nm wavelength region based on the silicon nitride platform. The proposed DC is equipped with a shallowly etched subwavelength gratings (SWG) gap so that the length of the coupling region is effectively reduced to 7.8 µm for equally splitting the fundamental TE polarization state. Such a DC coupling region is much more compact than that using empty and pure SWG gaps. Meanwhile, the DC working bandwidth is determined to be broader than 106 nm, and the insertion loss is always kept under an acceptable level of 0.6 dB over the entire wavelength region. Considering that the shallowly etched device requires two photoetching processes in manufacturing, we did a numerical analysis to characterize the fabrication tolerance under a minimum overlay accuracy of 20 nm in both x and y directions. Such overlay mismatch only increases the maximal imbalance from 0.55 to 0.6 dB at the center wavelength. In addition, we discussed the impact of the proposed DC in its potential application, i.e., optical coherence tomography. The results show that the proposed DC is able to support a high axial resolution of 3.77 µm, and the 20 nm overlay mismatch leads to neglected axial resolution degradation.

Kerr nonlinear medium assisted double-face absorbers for differential manipulation via an all-optical operation

Recently, light absorbers have attracted great attentions due to their promising in applications in functional optoelectronic devices. Herein, we theoretically propose and numerically demonstrate a new absorber platform, which consists of a 280-nm-thick photonic nonlinear waveguide film covering on the metal grating structure. Strong reflection inhibition and absorption enhancement is achieved in both the forward and backward directions, which indicates potential novel performances since the previous reports only achieved absorption in one side due to the using of opaque metal film substrate or the reflective mirror. The anti-reflection bands or the absorption peaks at the shorter and longer wavelength ranges are related to the excitation of the propagating surface plasmon resonance by the slit-assisted grating and the cavity mode by the slit in the metal film. Strong differential manipulation is realized for the double-face absorbers via the all-optical operation. Moreover, the operation wavelengths for the double-face light absorber can be modified strongly via using an asymmetric dielectric medium for the coating films. These new findings pave approaches for subtractive lightwave modulation technology, selective filtering, multiplex sensing and detection, etc.

Broadband achromatic longitudinal bifocal metalens in the visible range based on a single nanofin unit cell

Metasurfaces provide a remarkable platform to manipulate over phase, amplitude, and polarization flexibly and precisely. Bifocal metalens draws great research interest due to its ability of converging wavefronts to different focal positions horizontally and longitudinally. However, as wavelength of incident light changes, chromatic aberration will cause the focal lengths reliance on the incident wavelength, which will affect the performance of metasurface, especially for longitudinal bifocal metalens. In this work, a broadband achromatic longitudinal bifocal metalens (BALBM) based on single nanofin unit cell is demonstrated. Pancharatnam-Berry (PB) phase is used to converge the incident light. Cross commixed sequence distribution (CCSD) is introduced to control the positions of focal points F_Land F_Rwhen left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) incident. Propagation phase is used to compensate the phase difference caused by chromatic aberration. Simulation results show that in the continuous wavelength range from 500 nm to 700 nm, the focal point shifts caused by chromatic dispersion are reduced 81% for F_L and 83% for F_R, respectively. The focal length variations are stabilized to 6.21% for F_Land 4.8% for F_Rcomparing with the focal lengths at the initial wavelength 500 nm. The proposed BALBM brings advances to bifocal metasurfaces in versatile application areas including machine vision, optical computed tomography and microimaging.

All-silicon, low-cross-talk terahertz waveguide crossing based on effective medium

All-silicon effective-medium-clad waveguides are a promising candidate for an integrated terahertz platform with high efficiency and broad bandwidth. Waveguide crossings are essential circuit components, allowing for wave routing over shorter paths to increase circuit density. However, the simple intersection of two orthogonal effective-medium-clad waveguides results in terahertz wave scattering, leading to relatively high cross talk. In this work, a low-loss, 40% fractional bandwidth crossing utilizing Maxwell-Garnet effective-medium theory and wavefront planarization techniques is proposed. This monolithic structure is fabricated on a single high-resistivity float-zone silicon wafer using a deep reactive ion etching process with a modest 4.4 mm diameter (4.03λ₀) structure footprint. Experimentally verified results show low insertion loss, less than 1 dB, and average cross talk level of -39dB for both E11x and E11y operating modes, over 220-330 GHz with a 40% fractional bandwidth. This waveguide crossing can be foreseen as a useful routing component for terahertz all-silicon integrated circuits. The proposed techniques are applicable to other dielectric waveguide platforms at infrared and optical frequencies.

Compact silicon-based on-chip wavelength triplexer using directional couplers with double bridged subwavelength gratings

A compact silicon on-chip wavelength triplexer engineered by double bridged subwavelength gratings (BSWGs) is proposed based on cascaded symmetric directional couplers (SDCs). The SDC consists of a pair of symmetric and identical BSWGs with another SWG waveguide implemented in the middle of the SDC section. It functions well provided that the coupling lengths can match even or odd times of beat lengths of the three wavelengths. Relying on deliberately tailoring the structural dimensions of SWGs at different positions of SDC, the two lowest-mode refractive indices and beat lengths can be efficiently tuned, drastically reducing device footprint. The results show a total device length of 42.2 µm, which is ∼10% of its conventional counterpart based on SDCs. An insertion loss (IL) lower than 0.67 dB, reflection loss (RL) below -21.4dB, and extinction ratio (ER) as high as 34.5 dB are also obtained in the results. The bandwidths around wavelength bands and fabrication tolerances to dimensional variations are investigated and analyzed. The field evolutions for the three injected wavelength bands are presented.

Discretization of two-dimensional Luneburg lens based on the correctional effective medium theory

The Luneburg lens is widely applied in both the optical and microwave regimes because it offers high gain and a wide beam-scanning range. However, Luneburg lens typically suffer from low efficiency which is caused by the dielectric loss of medium employed. To address this issue, we propose herein a general method for discretization of two-dimensional Luneburg lens based on correctional effective-medium theory. In discrete Luneburg, the efficiency is not dependent on the employed medium roughly because that the main component in the lens is air, resulting into a significant improvement of efficiency. Subsequently, a systemic study of lens discretization is presented, which is validated by a discrete Luneburg lens easily fabricated by using 3D printing. In addition, a novel wave-patch reduction feature allows the discrete lens to function as well. This work presents a fundamental theory for lens discretization, which is valid not only for the Luneburg lens but also for other types of lenses. It can be applied in imaging, antennas, or phase manipulation in both the optical and microwave bands.

Hybrid plasmonic slot waveguide with a metallic grating for on-chip biosensing applications

Designing reliable and compact integrated biosensors with high sensitivity is crucial for lab-on-a-chip applications. We present a bandpass optical filter, as a label-free biosensor, based on a hybrid slot waveguide on the silicon-on-insulator platform. The designed hybrid waveguide consists of a narrow silicon strip, a gap, and a metallic Bragg grating with a phase-shifted cavity. The hybrid waveguide is coupled to a conventional silicon strip waveguide with a taper. The effect of geometrical parameters on the performance of the filter is investigated by 3D finite-difference time-domain simulations. The proposed hybrid waveguide has potential for sensing applications since the optical field is pulled into the gap and outside of the silicon core, thus increasing the modal overlap with the sensing region. This biosensor offers a sensitivity of 270 nm/RIU, while it only occupies a compact footprint of 1.03µm×17.6µm.

Partial Maxwell fish-eye lens inspired by the Gutman lens and Eaton lens for wide-angle beam scanning

This paper presents a novel two-dimensional (2-D) partial Maxwell fish-eye (PMFE) lens with the capability of wide-angle beam scanning inspired by the Gutman lens and Eaton lens, which is obtained by cutting a part from the 2-D Maxwell fish-eye (MFE) lens along a straight line. In terms of the refractive index profile, the MFE lens is similar to the Gutman lens near the center and the Eaton lens near the edge, respectively. We demonstrate the potential of the PMFE lens in wide-angle beam scanning based on its Gutman-like focusing and Eaton-like rotating characteristics corresponding to different feed points. As an example, a fully metallic PMFE lens antenna in the Ka-band composed of a bed of nails and a series of linearly arranged waveguide feeders is designed and experimentally verified. The measured results reveal wide-angle scanning ranges, especially about ±90° at 36 GHz, low reflections and low mutual couplings. The frequency scanning due to the dispersion of the lens is also discussed.

CMOS-compatible integrated 4-f system for mode-transparent spatial manipulation

To exploit spatial dimension, on-chip optical modes with various spatial profiles have been utilized in optical interconnects and spatial analog computing. An integrated Fourier optical system is able to perform spatial operations. However, the reported schemes based on a subwavelength structure pose difficulty in fabrication, and the fabrication-friendly structure has been investigated only with a fundamental mode. With the complementary metal-oxide-semiconductor process, we propose an integrated 4-f system with simple geometry and a moderate minimum feature size to manipulate the mode's spatial size and position in a mode-transparent way. A size magnification of 2.5 and center-to-center position offset of 7 µm are experimentally demonstrated. Reasonable insertion loss and low inter-mode crosstalk are measured over a 30 nm bandwidth. The work in this Letter paves the way for an on-chip Fourier optical system with convenient fabrication and broadband operation.

Subwavelength grating waveguide filter based on cladding modulation with a phase-change material grating

Subwavelength engineering and utilizing phase-change materials with large contrast in their optical properties have become powerful design tools for integrated silicon photonics. Reversible phase-transition of phase-change materials such as Ge₂Sb₂Te₅ (GST) provide a new degree of freedom and open up the possibility of adding new functionalities to the designed devices. We present an optical filter based on a silicon subwavelength grating (SWG) waveguide evanescently coupled to phase-change material loading segments arranged periodically around the SWG core. The effect of the GST loading segments' geometry and their distance from the SWG core on the filter's central wavelength and bandwidth are studied with three-dimensional finite-difference time-domain simulations. The employment of GST in the structure adds a switching functionality with an extinction ratio of 28.8 dB. We also examine the possibility of using the proposed structure as a reconfigurable filter by controlling the partial crystallization of the GST offering a blueshift of more than 4 nm.

Reflectionless plasmonic right-angled waveguide bend and divider using graphene and transformation optics

In this work, a plasmonic right-angled waveguide bend and divider are proposed. Using the Transformation Optics (TO) approach the transformation media of a bend and a T-shaped divider are obtained. Such media with continuous refractive index are realized with the help of graphene in the terahertz frequency range, key to effectively guiding the surface plasmon polariton (SPP) propagation on the 90 degree curves. Components with such capability are promising for THz device applications.

Metamaterial-based ultrashort multimode waveguide taper with low intermodal crosstalk

We propose and theoretically demonstrate an ultrashort multimode waveguide taper based on the all-dielectric metamaterial. Attributed to the gradient index distribution of the metamaterial, the spot sizes of the four lowest-order transverse magnetic (TM) modes can be expanded in a short distance of 6 μm with negligible mode conversions. Numerical results prove that the insertion losses of the taper are lower than 1 dB, 1.12 dB, 1.26 dB and 1.66 dB for the TM₀ - TM₃ modes, respectively, and the intermodal crosstalk values are below -15 dB for the four modes, both in the wavelength range of 1.5 μm - 1.6 μm. To the best of our knowledge, this is the first multimode waveguide taper that has low intermodal crosstalk of < -15 dB over a 100-nm bandwidth.

Subwavelength grating based mode (de)multiplexer for 3D photonic integrated circuits

A broadband three-dimensional (3D) mode (de) multiplexer [(De)MUX] is proposed based on the subwavelength grating (SWG) for 3D photonic integrated circuits (PICs). The proposed 3D mode (De)MUX consists of three SWG waveguides on two vertical layers. The coupling strength and operating bandwidth can be increased benefitting from both the subwavelength structure and offset between bus and access SWGs. The proposed 3D mode (De)MUX is optimized based on the 3D full-vectorial finite difference time domain method. The 1-dB bandwidths of the optimized device are over >300, 107, and 128 nm for demultiplexing TE₀, TE₁, and TE₂ modes, respectively. The coupling lengths are only 5.0 and 1.75 µm for demultiplexing the TE₁ and TE₂ modes, respectively. The insertion losses are 0.12, 0.27, and 0.29 dB, respectively. The proposed 3D mode (De)MUX is also fabrication tolerant.

Compact, broadband, and low-loss silicon photonic arbitrary ratio power splitter using adiabatic taper

The arbitrary ratio power splitter is widely used in photonic integrated circuits (PICs), for signal monitoring, power equalization, signal feedback, and so on. Here we designed a fabrication-tolerant, compact, broadband, and low-loss arbitrary ratio power splitter. The proposed arbitrary ratio power splitter was realized with an adiabatically tapered silicon rib waveguide with 70 nm shallow etches and an Si₃N₄ waveguide. The fabrication analysis confirmed that both of them are robust to fabrication errors. 3D finite-difference time-domain simulations show a very low excess loss (less than 0.02 dB for Si₃N₄ waveguide and 0.05 dB for Si rib waveguide), and a broadband operating wavelength range (100 nm). Good fabrication tolerance and standard critical dimensions make the arbitrary ratio power splitter compatible with the standard fabrication process of commercial silicon photonic foundries.

Multiple drains in generalized Maxwell's fisheye lenses

The subwavelength imaging phenomenon in Maxwell's fisheye lens with one drain has been reported previously. In this paper, we theoretically find that coherent perfect absorbers (CPAs) perform well in generalized Maxwell's fisheye (GMFE) lenses. Such CPAs are embedded inside the GMFE lenses to absorb the incoming coherent waves. They can be served as drains and dramatically improve the resolution of images in the GMFE lenses. In particular, they can be applied to realize the subwavelength imaging. We also study the multiple imaging characteristics of GMFE lenses with several CPAs in wave optics. Full-wave simulations were performed to verify the imaging functionalities.

Magnifying lens designed by optical conformal mapping

We proposed an alternative method to design a magnifying lens by optical conformal mapping. Different from previous hyperlens or superlens, the proposed lens needs no materials with negative or anisotropic refractive index. The lens has better photonic transporting efficiency than conventional a solid immersion lens due to impedance matching. The proposed lenses have many other advantages, such as broadband, low loss, and no need to redesign the sizes and material parameters when another magnifying ratio is required. Both numerical simulations and experimental demonstrations are implemented to verify the performance of the lens.

Deterministic design of focusing apodized subwavelength grating coupler based on weak form and transformation optics

The focusing apodized subwavelength grating coupler (F-ASGC) has advantages of high coupling efficiency, small footprint and simple fabrication process, which make it a popular component for chip-scale coupling and testing of integrated optical circuit. However, the design of F-ASGC based on effective medium theory lacks accuracy, causing the drawbacks of peak wavelength deviation and performance degradation. In this work, we propose a deterministic design method of F-ASGC. Our grating coupler is formed by assembling various subwavelength grating units according to their complex effective indexes. The complex effective indexes of these grating units are accurately obtained by the weak form calculation. Then combining with transformation optics, we strictly analyze the F-ASGC for the first time. The simulation results show that the deterministically designed F-ASGC has high coupling efficiency of -2.51 dB, 3 dB bandwidth of 51 nm, and accurate central wavelength of 1553.1 nm. And we also fabricated it on the commercial SOI wafer. The measured maximum efficiency is -3.10 dB, the 3 dB bandwidth is 55 nm, and the central wavelength is 1551.5 nm.

Theoretical model of a subwavelength grating polarization beam splitter

This study presents a theoretical model of a subwavelength grating polarization beam splitter (SWGPBS) using a combination of the theory of thin-film interference and the effective medium theory for guided-mode resonance. The structural parameters of SWGPBSs at oblique incidence calculated by our theoretical models and electromagnetic wave simulation were in good agreement within the range of the predicted approximation error. Feasibility of the oblique incidence SWGPBSs was verified, and the physical limitations of the SWGPBSs were clarified. Because the design procedure of SWGPBSs was simplified with our theoretical analysis, the range of their applications can be expanded to other fields.