Efficient integrated tri-modal coupler for few-mode fibers.

This paper demonstrates a high-efficiency vertical grating coupler for the LP01x, LP11ax, and LP11bx modes of a graded-index few-mode fiber. The coupler is composed of a non-uniform straight bidirectional grating that was inverse-designed to address the desired fiber modes, combined with two mode-selective directional couplers and two tapers. The device was fabricated by e-beam lithography with a minimum feature size of 100 nm and presented coupling efficiencies of -3.0 dB, -3.6 dB, and -3.4 dB for the LP01x, LP11ax, and LP11bx modes, respectively. The high efficiency of the proposed CMOS-compatible coupler demonstrates its potential as a key device for high-capacity networks exploiting space division multiplexing on few-mode fibers.

Compact dual-polarization silicon integrated couplers for multicore fibers.

Compact fiber-to-chip couplers play an important role in optical interconnections, especially in data centers. However, the development of couplers has been mostly limited to standard single-mode fibers, with few devices compatible with multicore and multimode fibers. Through the use of state-of-the-art optimization algorithms, we designed a compact dual-polarization coupler to interface chips and dense multicore fibers, demonstrating, for the first time, coupling to both polarizations of all the cores, with measured coupling efficiency of -4.3dB and with a 3 dB bandwidth of 48 nm. The dual-polarization coupler has a footprint of 200µm2 per core, which makes it the smallest fiber-to-chip coupler experimentally demonstrated on a standard silicon-on-insulator platform.

Optimization of the electromagnetic scattering problem based on the topological derivative method.

A new optimization method based on the topological derivative concept is developed for the electromagnetic design problem. Essentially, the purpose of the topological derivative method is to measure the sensitivity of a given shape functional with respect to a singular domain perturbation, so that it has applications in many relevant fields such as shape and topology optimization for imaging processing, inverse problems, and design of metamaterials. The topological derivative is rigorously derived for the electromagnetic scattering problem and used as gradient descent direction to find local optima for the design of electromagnetic devices. We demonstrate that the resulting topology design algorithm is remarkably simple and efficient and naturally leads to binary designs, while depending only on the solution of the conventional finite element formulation for electrodynamics. Finally, several numerical experiments in two and three spatial dimensions are presented to illustrate the performance of the proposed formulation.

Dielectric barrier discharge plasma treatment of modified SU-8 for biosensing applications.

In this work we demonstrate the use of a dielectric barrier discharge plasma for the treatment of SU-8. The resulting hydrophilic surface displays a 5° contact angle and (0.40 ± 0.012) nm roughness. Using this technique we also present a proof of concept of IgG and prostate specific antigen biodetection on a thin layer of SU-8 over gold via surface plasmon resonance detection.

Design of a compact CMOS-compatible photonic antenna by topological optimization.

Photonic antennas are critical in applications such as spectroscopy, photovoltaics, optical communications, holography, and sensors. In most of those applications, metallic antennas have been employed due to their reduced sizes. Nevertheless, compact metallic antennas suffer from high dissipative loss, wavelength-dependent radiation pattern, and they are difficult to integrate with CMOS technology. All-dielectric antennas have been proposed to overcome those disadvantages because, in contrast to metallic ones, they are CMOS-compatible, easier to integrate with typical silicon waveguides, and they generally present a broader wavelength range of operation. These advantages are achieved, however, at the expense of larger footprints that prevent dense integration and their use in massive phased arrays. In order to overcome this drawback, we employ topological optimization to design an all-dielectric compact antenna with vertical emission over a broad wavelength range. The fabricated device has a footprint of 1.78 µm × 1.78 µm and shows a shift in the direction of its main radiation lobe of only 4° over wavelengths ranging from 1470 nm to 1550 nm and a coupling efficiency bandwidth broader than 150 nm.

Side-lobe level reduction in bio-inspired optical phased-array antennas.

Phased arrays are expected to play a critical role in visible and infrared wireless systems. Their improved performance compared to single element antennas finds uses in communications, imaging, and sensing. However, fabrication of photonic antennas and their feeding network require long element separation, leading to the appearance of secondary radiation lobes and, consequently, crosstalk and interference. In this work, we experimentally show that by arranging the elements according to the Fermat's spiral, the side lobe level (SLL) can be reduced. This reduction is proved in a CMOS-compatible 8-element array, revealing a SLL decrement of 0.9 dB. Arrays with larger numbers of elements and inter-element spacing are demonstrated through an spatial light modulator (SLM) and an SLL drop of 6.9 dB is measured for a 64-element array. The reduced SLL, consequently, makes the proposed approach a promising candidate for applications in which antenna gain, power loss, or information security are key requirements.

Full three-dimensional isotropic carpet cloak designed by quasi-conformal transformation optics.

A fully three-dimensional carpet cloak presenting invisibility in all viewing angles is theoretically demonstrated. The design is developed using transformation optics and three-dimensional quasi-conformal mapping. Parametrization strategy and numerical optimization of the coordinate transformation deploying a quasi-Newton method is applied. A discussion about the minimum achievable anisotropy in the 3D transformation optics is presented. The method allows to reduce the anisotropy in the cloak and an isotropic medium could be considered. Numerical simulations confirm the strategy employed enabling the design of an isotropic reflectionless broadband carpet cloak independently of the incident light direction and polarization.

Three-dimensional quasi-conformal transformation optics through numerical optimization.

In this paper we demonstrate the possibility to achieve 3-dimensional quasi-conformal transformation optics through parametrization and numerical optimization without using sliding boundary conditions. The proposed technique, which uses a quasi-Newton method, is validated in two cylindrical waveguide bends as design examples. Our results indicate an arbitrarily small average anisotropy can be achieved in 3D transformation optics as the number of degrees of freedom provided by the parametrization was increased. The waveguide simulations confirm modal preservation when the residual anisotropy is neglected.

Study of a low-cost trimodal polymer waveguide for interferometric optical biosensors.

A novel evanescent wave biosensor based on modal interaction between the fundamental mode and the second order mode is proposed and numerically demonstrated. By taking advantage of their symmetries, it is possible to design a device where only the fundamental and the second order modes can propagate, without excitation of the first order mode. With this selection of modes it is possible to achieve a high sensitivity behavior in the biosensor configuration, due to the strong interaction between the evanescent field and the outer surface as compared to previous evanescent wave-based biosensor designs.

Anisotropy minimization via least squares method for transformation optics.

In this work the least squares method is used to reduce anisotropy in transformation optics technique. To apply the least squares method a power series is added on the coordinate transformation functions. The series coefficients were calculated to reduce the deviations in Cauchy-Riemann equations, which, when satisfied, result in both conformal transformations and isotropic media. We also present a mathematical treatment for the special case of transformation optics to design waveguides. To demonstrate the proposed technique a waveguide with a 30° of bend and with a 50% of increase in its output width was designed. The results show that our technique is simultaneously straightforward to be implement and effective in reducing the anisotropy of the transformation for an extremely low value close to zero.

WDM-compatible mode-division multiplexing on a silicon chip.

Significant effort in optical-fibre research has been put in recent years into realizing mode-division multiplexing (MDM) in conjunction with wavelength-division multiplexing (WDM) to enable further scaling of the communication bandwidth per fibre. In contrast, almost all integrated photonics operate exclusively in the single-mode regime. MDM is rarely considered for integrated photonics because of the difficulty in coupling selectively to high-order modes, which usually results in high inter-modal crosstalk. Here we show the first microring-based demonstration of on-chip WDM-compatible mode-division multiplexing with low modal crosstalk and loss. Our approach can potentially increase the aggregate data rate by many times for on-chip ultrahigh bandwidth communications.

Transformation inverse design.

We present a new technique for the design of transformation-optics devices based on large-scale optimization to achieve the optimal effective isotropic dielectric materials within prescribed index bounds, which is computationally cheap because transformation optics circumvents the need to solve Maxwell's equations at each step. We apply this technique to the design of multimode waveguide bends (realized experimentally in a previous paper) and mode squeezers, in which all modes are transported equally without scattering. In addition to the optimization, a key point is the identification of the correct boundary conditions to ensure reflectionless coupling to untransformed regions while allowing maximum flexibility in the optimization. Many previous authors in transformation optics used a certain kind of quasiconformal map which overconstrained the problem by requiring that the entire boundary shape be specified a priori while at the same time underconstraining the problem by employing "slipping" boundary conditions that permit unwanted interface reflections.

On-chip transformation optics for multimode waveguide bends.

Current optical communication systems rely almost exclusively on multimode fibres for short- and medium-haul transmissions, and are now expanding into the long-haul arena. Ultra-high bandwidth applications are the main drive for this expansion, based on the ability to spatially multiplex data channels in multimode systems. Integrated photonics, on the other hand, although largely responsible for today's telecommunications, continues to operate almost strictly in the single-mode regime. This is because multimode waveguides cannot be compactly routed on-chip without significant inter-mode coupling, which impairs their data rate and prevents the use of modal multiplexing. Here we propose a platform for on-chip multimode devices with minimal inter-mode coupling, opening up the possibilities for integrated multimode optics. Our work combines a novel theoretical approach--large-scale inverse design of transformation optics to maximize performance within fabrication constraints-with unique grayscale-lithography fabrication of an exemplary device: a low-crosstalk multimode waveguide bend.

Integrated Luneburg lens via ultra-strong index gradient on silicon.

Gradient index structures are gaining increased importance with the constant development of Transformation Optics and metamaterials. Our ability to fabricate such devices, while limited, has already demonstrated the extensive capabilities of those designs, in the forms of invisibility devices, as well as illusion optics and super-lensing. In this paper we present a low loss, high index contrast lens that is integrated with conventional nanophotonic waveguides to provide improved tolerance in fiber-to-chip optical links for future communication networks. This demonstration represents a positive step in making the extraordinary capabilities of gradient index devices available for integrated optics.

Focusing light in a curved-space.

We use transformation optics to demonstrate 2D silicon nanolenses, with wavelength-independent focal point. The lenses are designed and fabricated with dimensions ranging from 5.0 microm x 5.0 microm to 20 microm x 20 microm. According to numerical simulations the lenses are expected to focus light over a broad wavelength range, from 1.30 mum to 1.60 mum. Experimental results are presented from 1.52 microm to 1.61 microm.