Inverse design of nanophotonic devices with structural integrity

Computational inverse design has been a driving force behind the development of compact and highly efficient nanophotonic devices. However, due to fabrication constraints, devices have so far mostly been restricted to planar geometries. With recent developments, additive manufacturing techniques are poised to open up a vast design space for free-form nanophotonic devices, bringing with them a new set of inverse design challenges. The most urgent one is structural integrity. With a technique such as 3D laser lithography (nearly) every structure can be written, but not every structure is self supported and is with that feasible; free-floating elements are simply not an option. To address this challenge, we present here a method for the inverse design of nanophotonic devices that combines electromagnetic and structural topology optimization. To illustrate the proposed method, we present designs for a nanolens and a mode converter with structural integrity. We show that some of these designs achieve efficiencies comparable to those of conventional nanophotonic inverse design while maintaining structural integrity; and even slightly surpass them. This opens up new possibilities for photonic device design and may lead to the development of novel photonic devices for additive manufacturing

A neural operator-based surrogate solver for free-form electromagnetic inverse design

Neural operators have emerged as a powerful tool for solving partial differential equations in the context of scientific machine learning. Here, we implement and train a modified Fourier neural operator as a surrogate solver for electromagnetic scattering problems and compare its data efficiency to existing methods. We further demonstrate its application to the gradient-based nanophotonic inverse design of free-form, fully three-dimensional electromagnetic scatterers, an area that has so far eluded the application of deep learning techniques

Inverse design of cavities for Bloch Surface Waves interfaced to integrated waveguides

The design of functional elements for Bloch Surface Waves (BSW) is challenging because of the relatively low index contrast offered by the respective platforms. Here, we design a supporting photonic nanostructure that extracts as much light as possible from a quantum emitter into a waveguide in an integrated BSW architecture. The inverse problem is solved using topology optimization. Emphasis is put on discussing the algorithm’s emerging strategies for the design to enhance the Purcell factor, the coupling efficiency, or both for different index contrasts. Fully three-dimensional simulations of an explicit device show the benefits of our devices and pave the way for integrating such unconventional photonic elements into future fully-integrated BSW devices

Identifying regions of minimal back-scattering by a relativistically-moving sphere

The far-field back-scattering amplitude of an electric field from a relativistically-moving sphere is analyzed. Contrary to prior research, we do so by expressing the fields in the helicity basis, and we highlight here its advantages when compared to the commonly-considered parity basis. With the purpose of exploring specific scattering phenomena considering relativistic effects, we identify conditions that minimize the back-scattered field, leading to a relativistic formulation of the first Kerker condition. The requirements to be satisfied by the sphere are expressed in terms of Mie angles, which constitute an effective parametrization of any possible optical response a sphere might have. We are able to identify multiple combinations of Mie angles up to octupolar order via gradient-based optimization that satisfy our newly formulated relativistic Kerker condition, yielding minima for the back-scattered energy as low as 0.016% of the average scattered energy. Our results can be extended to involve multiple particles forming a metasurface, potentially having direct implications on the design of light sails as considered by the Breakthrough Starshot Initiative.Comment: 4 figures, 1 table, 9 pages + appendix. Link to code used: https://github.com/tfp-photonics/Jorkle.j

Identifying regions of minimal back-scattering by a relativistically-moving sphere

The far-field back-scattering amplitude of an electric field from a relativistically-moving sphere is analyzed. Contrary to prior research, we do so by expressing the fields in the helicity basis, and we highlight here its advantages when compared to the commonly-considered parity basis. With the purpose of exploring specific scattering phenomena considering relativistic effects, we identify conditions that minimize the back-scattered field, leading to a relativistic formulation of the first Kerker condition. The requirements to be satisfied by the sphere are expressed in terms of Mie angles, which constitute an effective parametrization of any possible optical response a sphere might have. We are able to identify multiple combinations of Mie angles up to octupolar order via gradient-based optimization that satisfy our newly formulated relativistic Kerker condition, yielding minima for the back-scattered energy as low as 0.016% of the average scattered energy. Our results can be extended to involve multiple particles forming a metasurface, potentially having direct implications on the design of light sails as considered by the Breakthrough Starshot Initiative

On the physical significance of non-local material parameters in optical metamaterials

When light interacts with a material made from subwavelength periodically arranged constituents, non-local effects can emerge. They occur because of either a complicated response of the constituents or possible lattice interactions. In lowest-order approximations of a general non-local response function, phenomena like an artificial magnetism and a bi-anisotropic response emerge. However, investigations beyond these lowest-order descriptions of non-local effects are needed for optical metamaterials (MMs) where a significant long-range interaction becomes evident. This highlights the need for additional material parameters to account for spatial non-locality in an effective medium description. These material parameters emerge from a Taylor expansion of the general and exact non-local response function. Even though these non-local parameters improve the effective description, their physical significance is yet to be understood. To improve the situation, we consider a conceptional MM consisting of scatterers characterized by a prescribed multipolar response arranged on a square lattice. Lorentzian polarizabilities describe the scatterers in the electric dipolar, electric quadrupolar, and magnetic dipolar terms. A slab of such a MM is homogenized while considering an increasing number of non-local terms in the constitutive relations at the effective level. We show that the effective permittivity and permeability are linked to the electric and magnetic dipole moments of the scatterers. The non-local material parameters are related to the higher-order multipolar moments and their interaction with the dipolar terms. Studying the effective material parameters with the knowledge of the induced multipolar moments in the lattice facilitates our understanding of the significance of each material parameter. Our insights aid in deciding on the order to truncate the Taylor expansion of the considered constitutive relations for a given MM

Inverse photonic design of functional elements that focus Bloch surface waves

Bloch surface waves (BSWs) are sustained at the interface of a suitably designed one-dimensional (1D) dielectric photonic crystal and an ambient material. The elements that control the propagation of BSWs are defined by a spatially structured device layer on top of the 1D photonic crystal that locally changes the effective index of the BSW. An example of such an element is a focusing device that squeezes an incident BSW into a tiny space. However, the ability to focus BSWs is limited since the index contrast achievable with the device layer is usually only on the order of Δn≈0.1 for practical reasons. Conventional elements, e.g., discs or triangles, which rely on a photonic nanojet to focus BSWs, operate insufficiently at such a low index contrast. To solve this problem, we utilize an inverse photonic design strategy to attain functional elements that focus BSWs efficiently into spatial domains slightly smaller than half the wavelength. Selected examples of such functional elements are fabricated. Their ability to focus BSWs is experimentally verified by measuring the field distributions with a scanning near-field optical microscope. Our focusing elements are promising ingredients for a future generation of integrated photonic devices that rely on BSWs, e.g., to carry information, or lab-on-chip devices for specific sensing applications

Inverse design of cavities for Bloch Surface Waves interfaced to integrated waveguides

International audienceThe design of functional elements for Bloch Surface Waves (BSW) is challenging because of the relatively low index contrast offered by the respective platforms. Here, we design a supporting photonic nanostructure that extracts as much light as possible from a quantum emitter into a waveguide in an integrated BSW architecture. The inverse problem is solved using topology optimization. Emphasis is put on discussing the algorithm’s emerging strategies for the design to enhance the Purcell factor, the coupling efficiency, or both for different index contrasts. Fully three-dimensional simulations of an explicit device show the benefits of our devices and pave the way for integrating such unconventional photonic elements into future fully-integrated BSW devices

Inverse photonic design of functional elements that focus Bloch surface waves

Bloch surface waves (BSWs) are sustained at the interface of a suitably designed one-dimensional (1D) dielectric photonic crystal and an ambient material. The elements that control the propagation of BSWs are defined by a spatially structured device layer on top of the 1D photonic crystal that locally changes the effective index of the BSW. An example of such an element is a focusing device that squeezes an incident BSW into a tiny space. However, the ability to focus BSWs is limited since the index contrast achievable with the device layer is usually only on the order of Δn≈0.1 for practical reasons. Conventional elements, e.g., discs or triangles, which rely on a photonic nanojet to focus BSWs, operate insufficiently at such a low index contrast. To solve this problem, we utilize an inverse photonic design strategy to attain functional elements that focus BSWs efficiently into spatial domains slightly smaller than half the wavelength. Selected examples of such functional elements are fabricated. Their ability to focus BSWs is experimentally verified by measuring the field distributions with a scanning near-field optical microscope. Our focusing elements are promising ingredients for a future generation of integrated photonic devices that rely on BSWs, e.g., to carry information, or lab-on-chip devices for specific sensing applications