Inverse photonic design of functional elements that focus Bloch surface waves

Generation of Bloch surface beams with arbitrarily designed phases

We proposed a new manipulation method for Bloch surface waves that can almost arbitrarily modulate the lateral phase through in-plane wave-vector matching. The Bloch surface beam is generated by a laser beam from a glass substrate incident on a carefully designed nanoarray structure, which can provide the missing momentum between the two beams and set the required initial phase of the Bloch surface beam. An internal mode was used as a channel between the incident and surface beams to improve the excitation efficiency. Using this method, we successfully realized and demonstrated the properties of various Bloch surface beams, including subwavelength-focused, self-accelerating Airy, and diffraction-free collimated beams. This manipulation method, along with the generated Bloch surface beams, will facilitate the development of two-dimensional optical systems and benefit potential applications of lab-on-chip photonic integrations.

Retrieving the subwavelength cross-section of dielectric nanowires with asymmetric excitation of Bloch surface waves

Optical microscopy with a diffraction limit cannot distinguish nanowires with sectional dimensions close to or smaller than the optical resolution. Here, we propose a scheme to retrieve the subwavelength cross-section of nanowires based on the asymmetric excitation of Bloch surface waves (BSWs). Leakage radiation microscopy is used to observe the propagation of BSWs at the surface and to collect far-field scattering patterns in the substrate. A model of linear dipoles induced by tilted incident light is built to explain the directional imbalance of BSWs. It shows the potential capability in precisely resolving the subwavelength cross-section of nanowires from far-field scattering without the need for complex algorithms. Through comparing the nanowire widths measured by this method and those measured by scanning electron microscopy (SEM), the transverse resolutions of the widths of two series of nanowires with heights 55 nm and 80 nm are about 4.38 nm and 6.83 nm. All results in this work demonstrate that the new non-resonant far-field optical technology has potential application in metrology measurements with high precision by taking care of the inverse process of light-matter interaction.

Spectral tuning of Bloch Surface Wave resonances by light-controlled optical anisotropy

Fostered by the recent advancements in photonic technologies, the need for all-optical dynamic control on complex photonic elements is emerging as more and more relevant, especially in integrated photonics and metasurface-based flat-optics. In this framework, optically-induced anisotropy has been proposed as powerful mean enabling tuning functionalities in several planar architectures. Here, we design and fabricate an anisotropic two-dimensional bull's eye cavity inscribed within an optically-active polymeric film spun on a one-dimensional photonic crystal sustaining Bloch surface waves (BSW). Thanks to the cavity morphology, two surface resonant modes with substantially orthogonal polarizations can be coupled within the cavity from free-space illumination. We demonstrate that a dynamic control on the resonant mode energies can be easily operated by modulating the orientation of the optically-induced birefringence on the surface, via a polarized external laser beam. Overall, reversible blue- and red-shifts of the resonant BSWs are observed within a spectral range of about 2 nm, with a moderate laser power illumination. The polymeric structure is constituted by a novel blend of an azopolymer and a thermally-sensitive resist, which allows a precise patterning via thermal scanning probe lithography, while providing a significant structural integrity against photo-fluidization or mass-flow effects commonly occurring in irradiated azopolymers. The proposed approach based on tailored birefringence opens up new pathways to finely control the optical coupling of localized surface modes to/from free-space radiation, particularly in hybrid organic-inorganic devices.

Multifunctional on-chip directional coupler for spectral and polarimetric routing of Bloch surface wave

Integration of multiple diversified functionalities into an ultracompact platform is crucial for the development of on-chip photonic devices. Recently, a promising all-dielectric two-dimensional platform based on Bloch surface waves (BSWs) sustained by dielectric multilayer has been proposed to enable various functionalities and provide novel approach to photonic devices. Here, we design and fabricate a multifunctional directional coupler to achieve both spectral and polarimetric routing by employing asymmetric nanoslits in a dielectric multilayer platform. Due to the dispersion property of BSWs, the directional coupling behavior is sensitive to wavelength and polarization. We demonstrate numerically and experimentally the wavelength selective directional coupling of TE BSW mode with an intensity ratio of the BSW excitation in opposite directions reaching 10 dB. Polarization selective directional coupling is also achieved at specific operating wavelength due to different response to a nanoantenna for TE and TM BSWs. The proposed two-dimensional photonic device opens new pathway for a wide range of practical applications such as molecular sensing, imaging with different polarization, and spectral requirements.

Total Internal Reflection Ellipsometry Approach for Bloch Surface Waves Biosensing Applications

A one-dimensional photonic crystal with an additional TiO₂ layer, supporting Bloch surface waves (BSW), was used for enhanced signal sensitivity for the detection of protein interaction. To compare the optical response of BSW and photonic crystals (PC), bovine serum albumin and specific antibodies against bovine serum were used as a model system. The results obtained show the enhanced sensitivity of p- and s-BSW components for the 1D PC sample with an additional TiO₂ layer. Furthermore, a higher sensitivity was obtained for the BSW component of p-polarization in the PC sample with an additional TiO₂ layer, where the sensitivity of the ellipsometric parameter Ψ was five times higher and that of the Δ parameter was eight times higher than those of the PC sample. The capabilities of BSW excitations are discussed from the sensitivity point of view and from the design of advanced biosensing.

Thermal Stability Analysis of Surface Wave Assisted Bio-Photonic Sensor

In this paper, the thermal stability of a Bloch Surface Wave (BSW) assisted bio-photonic sensor is investigated. The structural analysis is carried out using the transfer matrix method (TMM). The design comprises a truncated one-dimensional photonic crystal (1D-PhC) structure along with a defective top layer. The structural parameters are optimized to excite a BSW at the top interface for an operating wavelength of 632.8 nm. The mode confinement is confirmed by using wavelength interrogation, angular interrogation and surface electric field profile. Further, the effect of thermal variation on BSW excitation angle and sensitivity is carried out. The analysis shows the average variations in excitation angle and sensitivity of about −0.00096 degree/°C and 0.01046 (degree/RIU)/°C, respectively. Additionally, the analysis is also extended towards different lower wavelengths of 400 nm and 550 nm, which provides average variations in the excitation angles of about −0.0027 degree/°C, and 0.0016 degree/°C. This shows that the structural sensitivity response is more thermally stable at the lower wavelength range. Thus, showing its potential applications in designing thermally stable bio-photonic sensors.

Optical meta-waveguides for integrated photonics and beyond

The growing maturity of nanofabrication has ushered massive sophisticated optical structures available on a photonic chip. The integration of subwavelength-structured metasurfaces and metamaterials on the canonical building block of optical waveguides is gradually reshaping the landscape of photonic integrated circuits, giving rise to numerous meta-waveguides with unprecedented strength in controlling guided electromagnetic waves. Here, we review recent advances in meta-structured waveguides that synergize various functional subwavelength photonic architectures with diverse waveguide platforms, such as dielectric or plasmonic waveguides and optical fibers. Foundational results and representative applications are comprehensively summarized. Brief physical models with explicit design tutorials, either physical intuition-based design methods or computer algorithms-based inverse designs, are cataloged as well. We highlight how meta-optics can infuse new degrees of freedom to waveguide-based devices and systems, by enhancing light-matter interaction strength to drastically boost device performance, or offering a versatile designer media for manipulating light in nanoscale to enable novel functionalities. We further discuss current challenges and outline emerging opportunities of this vibrant field for various applications in photonic integrated circuits, biomedical sensing, artificial intelligence and beyond.

Converting the guided-modes of Bloch surface waves with surface pattern

The guided-modes of Bloch surface waves, such as the transverse electric modes (TE00 and TE01 modes), can simultaneously exist in a low-refractive-index ridge waveguide with subwavelength thickness that are deposited on an all dielectric one-dimension photonic crystal. By using the finite difference frequency domain method, coupled mode theory and finite-difference time-domain method, the conversion between the guided-modes has been investigated. This conversion can be realized in a broadband wavelength with surface pattern of this low-index ridge. This conversion is useful for developing lab-on-a-chip photonic devices, such as a mode converter that can maintain the output mode purity over 90% with working wavelength ranging from 590 to 680 nm, and a power splitter that can maintain the splitting ratio over 8:2 with wavelength ranging from 530 to 710 nm.

Multimode Interference of Bloch Surface Electromagnetic Waves

Integrated photonics aims at on-chip controlling light in the micro- and nanoscale ranges utilizing the waveguide circuits, which include such basic elements as splitters, multiplexers, and phase shifters. Several photonic platforms, including the well-developed silicon-on-insulator and surface-plasmon polaritons ones, operate well mostly in the IR region. However, operating in the visible region is challenging because of the drawbacks originating from absorption or sophisticated fabrication technology. Recently, a new promising all-dielectric platform based on Bloch surface electromagnetic waves (BSWs) in multilayer structures and functioning in the visible range has emerged finding a lot of applications primarily in sensing. Here, we show the effect of multimode interference (MMI) of BSWs and propose a method for implementing the advanced integrated photonic devices on the BSW platform. We determine the main parameters of MMI effect and demonstrate the operation of Mach-Zehnder interferometers with a predefined phase shift proving the principle of MMI BSW-based photonics in the visible spectrum. Our research will be useful for further developing a versatile toolbox of the BSW platform devices which can be essential in integrated photonics, lab-on-chip, and sensing applications.

Real-Time Measurement of the Hygroscopic Growth Dynamics of Single Aerosol Nanoparticles with Bloch Surface Wave Microscopy

The growth in aerosol particles caused by water uptake during increasing ambient relative humidity alters the physical and chemical properties of aerosols, which then affects public health, atmospheric chemistry, and the Earth's climate. The temporal resolution and sensitivity of current techniques are not sufficient to measure the growth dynamics of single aerosol nanoparticles. Additionally, the specific time required for phase transition from solid to aqueous has not been measured. Here, we describe a label-free photonic microscope that uses the Bloch surface waves as the illumination source for imaging and sensing to provide real-time measurements of the hygroscopic growth dynamics of a single aerosol (diameter <100 nm) containing the main components of air pollution. This specific time can be measured for both pure and mixed aerosols, showing that organics will delay the phase transition. This photonic microscope can be extended to investigate physicochemical reactions of various aerosols, and then knowing this specific time will be favorable for understanding the reaction kinetics among single aerosols and the surrounding medium.

Trapping metallic particles using focused Bloch surface waves

Metallic particles are promising for applications in various areas, including optical sensing, imaging and electric field enhancement-induced optical and thermal effects. The ability to trap or transport these particles stably will be important in these applications. However, while traditional optical tweezers can trap metallic Rayleigh particles easily, it is difficult to trap metallic mesoscopic/Mie particles because of the strong scattering forces that come from the far-field trapping laser beam. Here we demonstrate that metallic particles can be trapped stably using focused Bloch surface waves that propagate in the near-field region of a dielectric multilayer structure with a photonic band gap. Focused Bloch surface waves can be excited efficiently using an annular beam with azimuthal polarization and a high-numerical-aperture objective. Numerical simulations were performed to calculate the optical forces loaded on a gold particle by focused Bloch surface waves and the results were consistent with those of the experimental observations.

Switchable Assembly and Guidance of Colloidal Particles on an All-Dielectric One-Dimensional Photonic Crystal

Dielectric multilayer photonic-band-gap structures, called one-dimensional photonic crystals (1DPCs), have drawn considerable attention in the fields of physics, chemistry, and biophotonics. Here, experimental results verify the feasibility of a 1DPC working as a substrate for switchable manipulations of colloidal microparticles. The optically induced thermal convective force on a 1DPC can assemble colloidal particles that are dispersed in a water solution, while the photonic scattering force on the same 1DPC caused by propagating evanescent waves can guide these particles. Additionally, in the 1DPC, one internal mode can be excited that has seldom been noticed previously. This mode shows an ability to assemble particles over large areas even when the incident power is low. The assembly and guidance of colloidal particles on the 1DPC are switchable just through tuning the polarization and angle of the incident laser beam. Numerical simulations are carried out, which are consistent with these experimental observations.

All-dielectric concentration of electromagnetic fields at the nanoscale: the role of photonic nanojets

The photonic nanojet (PNJ) is a narrow high-energy beam that was originally found on the back side of all-dielectric spherical structures. It is a unique type of energy concentration mode. The field of PNJs has experienced rapid growth in the past decade: nonspherical and even pixelized PNJ generators based on new physics and principles along with extended photonic applications from linear optics to nonlinear optics have driven the re-evaluation of the role of PNJs in optics and photonics. In this article, we give a comprehensive review for the emerging sub-topics in the past decade with a focus on two specific areas: (1) PNJ generators based on natural materials, artificial materials and nanostructures, and even programmable systems instead of conventional dielectric geometries such as microspheres, cubes, and trihedral prisms, and (2) the emerging novel applications in both linear and nonlinear optics that are built upon the specific features of PNJs. The extraordinary features of PNJs including subwavelength concentration of electromagnetic energy, high intensity focusing spot, and lower Joule heating as compared to plasmonic resonance systems, have made PNJs attractive to diverse fields spanning from optical imaging, nanofabrication, and integrated photonics to biosensing, optical tweezers, and disease diagnosis.

Laser-Inscribed Stress-Induced Birefringence of Sapphire

Birefringence of 3 × 10 - 3 is demonstrated inside cross-sectional regions of 100 μ m, inscribed by axially stretched Bessel-beam-like fs-laser pulses along the c-axis inside sapphire. A high birefringence and retardance of λ / 4 at mid-visible spectral range (green) can be achieved using stretched beams with axial extension of 30-40 μ m. Chosen conditions of laser-writing ensure that there are no formations of self-organized nano-gratings. This method can be adopted for creation of polarization optical elements and fabrication of spatially varying birefringent patterns for optical vortex generation.

Analytical level set fabrication constraints for inverse design

Inverse design methods produce nanophotonic devices with arbitrary geometries that show high efficiencies as well as novel functionalities. Ensuring fabricability during optimization of these unrestricted device geometries is a major challenge for these design methods. In this work, we construct a fabrication constraint penalty function for level set geometry representations of these devices. This analytical penalty function limits both the gap size and boundary curvature of a device. We incorporate this penalty in a fully automated optical design flow using a quasi-Newton optimization method. The performance of our design method is evaluated by designing a series of waveguide demultiplexers (WDM) and mode converters with various footprints and minimum feature sizes. Finally, we design and experimentally characterize three WDMs with a 80 nm, 120 nm and 160 nm feature size.