Planar Photonics Research Articles

Fabrication of luminescent disc-shaped microstructures via wet-chemical etching of hybrid sol–gel layers for potential photonic applications

Sol–gel materials based on SiO2 and TiO2 precursors are attractive as a new platform for planar photonics. Particularly interesting are those based on organically modified silica (ORMOSIL), which may improve the luminescent properties of organic dyes. However, their microstructurization remains a challenge as it requires optimization of various technological stages. Here, we report the structurization of thin layers based on ORMOSIL precursor and titanium(IV) ethoxide (TET) containing luminescent rhodamine B (RhB) dye. Films were fabricated using sol–gel synthesis and dip-coating method. Depending on the time of annealing performed at 200 °C, layers with different thicknesses (300–760 nm) and refractive indices (RI) (1.51–1.68) were obtained. Combining photolithography and wet-chemical etching processes made it possible to fabricate well-separated sol–gel waveguides and discs of different diameters. The etching time in buffered hydrofluoric acid (BHF) affected the depth of the etched luminescent microstructures. Additionally, it was found that a longer layer’s annealing time increased the etching selectivity of the substrate over the sol–gel layer. This enabled the obtaining of under-etched sol–gel goblet microstructures. Selected samples were investigated using scanning electron microscopy (SEM). UV–Vis photoluminescence measurements showed that long heat treatment also influenced the emission spectrum’s shape. The stability of the films under ambient conditions was established using spectroscopic ellipsometry. It was proven that films heat-treated at 200 °C did not change their properties during storage time of around 2 months. Relatively high RI, luminescent properties, and structurization potential make these microstructures interesting for application in integrated photonic devices, e.g., light amplifiers or sensing systems.

All‐Dielectric Meta‐Waveguides for Flexible Polarization Control of Guided Light

AbstractGuided waves are the perfect carriers of electromagnetic signals in planar miniaturized devices due to their high localization and controlled propagation direction. However, it is still a challenge to control the polarization of propagating guided waves. In this work, the broadband polarization TE‐TM degeneracy of highly localized guided waves propagating along an all‐dielectric metasurface and a subwavelength chain of dielectric high‐index cylinders is discovered, both theoretically and experimentally. Using the discovered near‐field polarization degree of freedom, the polarization transformation for guided waves propagating along a subwavelength chain of dielectric cylinders at any frequency within the finite spectral range is demonstrated experimentally. Namely, the simplest planar near‐field polarization device – the quarter‐wave‐retardation (linear‐to‐circular) polarization transformer of guided waves is implemented. The results obtained discover the polarization degree of freedom for guided waves paving the way toward numerous applications in flat optics and planar photonics.

Programmable spatially-varying linearly polarized light with high degree of polarization for planar liquid crystal photonics

The use of liquid crystal spatial light modulator (LC-SLM) to manipulate the polarization of the light has been widely explored for different applications, including optical field manipulation, maskless lithography, polarization imaging, and LC planar-optics, among others. Precise polarization manipulation of LC-SLM is therefore highly demanded. Here, we introduce a high-efficiency approach for calibrating the polarization manipulation of a parallel-aligned LC-SLM. The approach revolves around the primary calibration criterion, the degree of polarization (DoP). Through parameter optimization to generate the calibration grayscale, a remarkable DoP as high as 97 % can be attained, with a sustained DoP level of up to 93.7 %. Experimental results showcase outstanding DoP uniformity in the modulated beam achieved with the calibrated LC-SLM. The root mean square error (RMSE) of polarization for modulated light from LC-SLM is about 0.018. A pixelated polarization calibration approach is proposed to enable efficient and precise programming of spatially-varying linearly polarized light (SVLPL), which can be further verified by the implementation of the Pancharatnam-Berry (PB) lens. The resolution of the fabricated LC planar optical elements with our setup can reach 1.54 μm. The programmable SVLPL with high DoP and resolution is potentially useful for many applications in planar optics and vectorial optical field manipulation.

Focusing of mid-infrared polaritons through patterned graphene on van der Waals crystals

Abstract Manipulating the propagation of mid-infrared (mid-IR) light is crucial for optical imaging, biosensing, photocatalysis, and guiding photonic circuits. Artificially engineered metamaterials were introduced to comprehensively control optical waves. However, fabrication challenges and optical losses have impeded the progress. Fortunately, two-dimensional van der Waals (vdW) materials are alternatives because of their inherent optical properties, such as hyperbolic behavior, high confinement, low loss, and atomic-scale thickness. In this research, we conducted theoretical and numerical investigations on the α-phase molybdenum trioxide, a biaxial vdW material, with patterned graphene to assess the potential of the tunable focusing of mid-IR light. Our proposed method directly alters the path of excited light to focus mid-IR light by negative refraction. Further, the patterned graphene in our design offers enhanced focusing characteristics, featuring a significantly reduced waist diameter with 1/92 of the free-space wavelength, an enhanced beam quality without pronounced field ripples, and a fivefold increase in field intensity. Moreover, our approach significantly preserves the waist diameter of the focused beam while facilitating directional steering. Thus, the focused beam can propagate in a canalized manner toward the desired direction. These advancements lay the foundation for promising applications in planar photonics.

Focused Electron Beam Micro- and Nanopatterning of Thin Films Derived from Sol–Gels Based on TiO2 Precursors for Planar Photonics

Focused Electron Beam Micro- and Nanopatterning of Thin Films Derived from Sol–Gels Based on TiO₂ Precursors for Planar Photonics

Broadband multifunctional metasurfaces for concentric perfect vortex beam generation via trigonometric functions

Metasurfaces are engineered structures that offer an unprecedented nanoscale solution for precisely manipulating light’s magnitude, phase, and polarization, exhibiting the potential to reduce the size of optical configurations significantly. However, these structures often encounter performance limitations and are typically operational for a single design wavelength. Integrating multiple optical functionalities into a single-layered structure and achieving a highly efficient broadband response is an active metasurface research topic. In this study, we showcased a single-cell driven broadband metasurface capable of generating concentric perfect vortex (PV) beams, where each PV beam is realized by multiplexing the phase profiles of multiple optical components and trigonometric phase features to achieve infinite topological charges. The proposed all-silicon metasurface demonstrates broadband functionality in “E” and “S” optical communication bands spanning from 1460 nm to 1565 nm, covering critical telecom wavelengths, and this characteristic facilitates seamless integration of our work with the existing ecosystem. Our multifunctional approach prioritizes simplicity that yields maximum polarization conversion efficiency and produces different outcomes for different handedness of incident light without introducing complexity. To prove the concept, we numerically simulated multiple metasurfaces for different trigonometric functions, with periods of axicon of 4 µm and 6 µm. The results indicate that this work will pave the way for diverse applications like optical trapping, particle manipulation, quantum processing, optical communication, and high-capacity information processing.

Stimuli‐responsive active materials for dynamic control of light field

AbstractThe increasing demand for the multidimensional and dynamic control of light has spurred the development of stimuli‐responsive, reconfigurable, and programmable optical systems. Liquid crystals (LCs), which combine liquid‐like stimuli‐responsiveness and crystal‐like orientational ordering, have emerged as highly appealing soft materials. Owing to their exceptional optical performance and programmable functionalities, they are becoming incredibly important materials in active planar optics and photonics. Additionally, silk proteins, luminescent materials, and metasurfaces exhibit dynamic optical properties, enabling remarkable multifunctional applications. This review focuses on the advancements in stimuli‐responsive materials, including LCs, silk proteins, luminescent materials, and active metasurfaces as well as some of these materials paired with LCs. Their attractive tunable applications in optics and photonics, along with the great potential for the future development of active optical systems, are also emphasized.

Focusing of Hyperbolic Phonon Polaritons by Bent Metal Nanowires and Their Polarization Dependence

Manipulating in-plane hyperbolic phonon polaritons on biaxial van der Waals crystals provides an opportunity for the development of planar photonic devices. Recent studies show that nanometal disks constructed on the surface of hyperbolic media can focus phonon polaritons, which is a remarkable breakthrough in planar photonics. Here, we demonstrate that using bent metal nanowires made by strain engineering not only produces the focusing of hyperbolic phonon polaritons but also introduces a new modulation factor, the polarization of incident light, which shows a clear affection on focusing. In this study, we investigate the operating mechanism and applicability of this nanolens from the perspective of device size, incident light frequency, and polarization, where the effect of polarization is unprecedented for this type of surface engineering. With the support of super-resolution technology, the foci with high confinement are clearly observed and the focal length from 0.7 to 2.01 μm is achieved. Moreover, the angle between the in-plane polarization direction and the axial direction of the nanowires directly affects the intensity of the local field. This approach provides a new idea to control electromagnetic fields in a two-dimensional plane, which is beneficial to further promote the design of planar optical functional devices.

Planar Photonic Chips with Tailored Dispersion Relations for High-Efficiency Spectrographic Detection

Planar Photonic Chips with Tailored Dispersion Relations for High-Efficiency Spectrographic Detection

Integrating planar photonics for multi-beam generation and atomic clock packaging on chip

The commercialization of atomic technologies requires replacing laboratory-scale laser setups with compact and manufacturable optical platforms. Complex arrangements of free-space beams can be generated on chip through a combination of integrated photonics and metasurface optics. In this work, we combine these two technologies using flip-chip bonding and demonstrate an integrated optical architecture for realizing a compact strontium atomic clock. Our planar design includes twelve beams in two co-aligned magneto-optical traps. These beams are directed above the chip to intersect at a central location with diameters as large as 1 cm. Our design also includes two co-propagating beams at lattice and clock wavelengths. These beams emit collinearly and vertically to probe the center of the magneto-optical trap, where they will have diameters of ≈100 µm. With these devices we demonstrate that our integrated photonic platform is scalable to an arbitrary number of beams, each with different wavelengths, geometries, and polarizations.

Room‐Temperature Exceptional‐Point‐Driven Polariton Lasing from Perovskite Metasurface

AbstractExcitons in lead bromide perovskites exhibit high binding energy and high oscillator strength, allowing for a strong light‐matter coupling regime in the perovskite‐based cavities localizing photons at the nanoscale. This opens up the way for the realization of exciton‐polariton Bose–Einstein condensation and polariton lasing at room temperature – the inversion‐free low‐threshold stimulated emission. However, polariton lasing in perovskite planar photon cavities without Bragg mirrors has not yet been observed and proved experimentally. In this study, perovskite metasurface is employed, fabricated with nanoimprint lithography, supporting so‐called exceptional points to demonstrate the room‐temperature polariton lasing. The exceptional points in exciton‐polariton dispersion of the metasurface appear upon optically pumping in the nonlinear regime in the spectral vicinity of a symmetry‐protected bound state in the continuum providing high mode confinement with the enhanced local density of states beneficial for polariton condensation. The observed lasing emission possesses high directivity with a divergence angle of 1° over one axis. The employed nanoimprinting approach for solution‐processable large‐scale polariton lasers is compatible with various planar photonic platforms suitable for on‐chip integration.

Optical Gain at 637 nm Wavelength in Polymer Waveguide Amplifier Under Commercial LED Pumping for Planar Photonic Integration (Advanced Optical Materials 13/2022)

Optical Gain at 637 nm Wavelength in Polymer Waveguide Amplifier Under Commercial LED Pumping for Planar Photonic Integration (Advanced Optical Materials 13/2022)

Encrypted holographic metasurface based on Pancharatnam–Berry phase superposition

As a new type of planar micro-nano photonics device, metasurfaces can precisely modulate light at the subwavelength scale, which are considered as an ideal computational holographic encoding carrier. In this paper, based on the Pancharatnam–Berry (PB) phase, 2D arrayed metasurface with nanopillar cells is designed by superimposing the phase of focus and holographic imaging, and its focusing and holographic properties are investigated. Different from the method of alternately arranging nanopillars to achieve functional superposition, this method can further reduce the size of the integrated device and improve the functional density of the optical system greatly, which has broad development prospects in the fields of encryption and anti-counterfeiting.

Optical Gain at 637 nm Wavelength in Polymer Waveguide Amplifier Under Commercial LED Pumping for Planar Photonic Integration

AbstractOptical gains at 637 nm wavelength using light‐emitting diodes (LEDs) instead of traditional semiconductor lasers as pumping sources are demonstrated in the organic molecule 2,6‐bis[4‐(diphenylamino)phenyl]‐9,10‐anthracenedione (AQ(PhDPA)2)‐doped polymethylmethacrylate (PMMA) and SU‐8 polymer waveguides. Under excitation of four blue‐violet LEDs with different central wavelengths, fluorescence in the red band is observed owing to the transition of AQ(PhDPA)2 from the excited states to the ground state based on the thermally activated delayed fluorescence (TADF) mechanism. Channel waveguides with a cross‐section of 6 µm × 5 µm are fabricated. The relative gains of 5.0 and 4.0 dB cm−1 are obtained in rectangular waveguides with active core layers as AQ(PhDPA)2 doped PMMA and SU‐8, respectively. Optical gains have also been achieved in waveguides based on evanescent‐wave coupling. By using the vertical top pumping mode of a 450 nm LED, a gain of 2.6 dB cm−1 is obtained in a passive SU‐8 waveguide with AQ(PhDPA)2 doped PMMA as the upper cladding. In contrast, the replacement of SU‐8 with AQ(PhDPA)2 doped SU‐8 as the active material, triples the optical gain, which reaches to 7.8 dB cm−1 owing to the presence of stimulated excitation in both channel waveguide and upper cladding.

Generating ultraviolet perfect vortex beams using a high-efficiency broadband dielectric metasurface.

Due to the topological charge-independent doughnut spatial structure as well as the association of orbital angular momentums, perfect vortex beams promise significant advances in fiber communication, optical manipulation and quantum optics. Inspired by the development of planar photonics, several plasmonic and dielectric metasurfaces have been constructed to generate perfect vortex beams, instead of conventional bulky configuration. However, owing to the intrinsic Ohmic losses and interband electron transitions in materials, these metasurface-based vortex beam generators only work at optical frequencies up to the visible range. Herein, using silicon nitride nanopillars as high-efficiency half-wave plates, broadband and high-performance metasurfaces are designed and demonstrated numerically to directly produce perfect vortex beams in the ultraviolet region, by combining the phase profiles of spiral phase plate, axicon and Fourier transformation lens based on geometric phase. The conversion efficiency of the metasurface is up to 86.6% at the design wavelength. Moreover, the influence of several control parameters on perfect vortex beam structures is discussed. We believe that this ultraviolet dielectric generator of perfect vortex beams will find many significant applications, such as high-resolution spectroscopy, optical tweezer and on-chip communication.

Black Phosphorus Photodetector Enhanced by a Planar Photonic Crystal Cavity

Black Phosphorus Photodetector Enhanced by a Planar Photonic Crystal Cavity

Lithography-free IR polarization converters via orthogonal in-plane phonons in \u03b1-MoO3 flakes

Exploiting polaritons in natural vdW materials has been successful in achieving extreme light confinement and low-loss optical devices and enabling simplified device integration. Recently, α-MoO3 has been reported as a semiconducting biaxial vdW material capable of sustaining naturally orthogonal in-plane phonon polariton modes in IR. In this study, we investigate the polarization-dependent optical characteristics of cavities formed using α-MoO3 to extend the degrees of freedom in the design of IR photonic components exploiting the in-plane anisotropy of this material. Polarization-dependent absorption over 80% in a multilayer Fabry-Perot structure with α-MoO3 is reported without the need for nanoscale fabrication on the α-MoO3. We observe coupling between the α-MoO3 optical phonons and the Fabry-Perot cavity resonances. Using cross-polarized reflectance spectroscopy we show that the strong birefringence results in 15% of the total power converted into the orthogonal polarization with respect to incident wave. These findings can open new avenues in the quest for polarization filters and low-loss, integrated planar IR photonics and in dictating polarization control.

Prismatic discontinuous Galerkin time domain method with an integrated generalized dispersion model for efficient optical metasurface analysis

Planar photonics technology is expected to facilitate new physics and enhanced functionality for a new generation of disruptive optical devices. To analyze such planar optical metasurfaces efficiently, we propose a prismatic discontinuous Galerkin time domain (DGTD) method with a generalized dispersive material (GDM) model to conduct the full-wave electromagnetic simulation of planar photonic nanostructures. Prism-based DGTD allows for triangular prismatic space discretization, which is optimal for planar geometries. In order to achieve an accurate universal model for arbitrary dispersive materials, the GDM model is integrated within the prism-based DGTD. As an advantage of prismatic spatial discretization, the prism-based DGTD with GDM has fewer elements than conventional tetrahedral methods, which in turn brings higher computational efficiency. Finally, the accuracy, convergence behavior, and efficiency improvements of the proposed algorithm is validated by several numerical examples. A simulation toolkit with the proposed algorithm has been released online, enabling users to efficiently analyze metasurfaces with customized pixel patterns.

Van der Waals thin films of WTe2 for natural hyperbolic plasmonic surfaces

A hyperbolic plasmonic surface supports highly directional propagating polaritons with extremely large density of states. Such plasmon polaritons have been realized in artificially structured metasurfaces. However, the upper bound of the achievable plasmon wave vector is limited by the structure size, which calls for a natural hyperbolic surface without any structuring. Here, we experimentally demonstrate a natural hyperbolic plasmonic surface based on thin films of WTe2 in the light wavelength range of 16 to 23 microns by far infrared absorption spectroscopy. The topological transition from the elliptic to the hyperbolic regime is further manifested by mapping the isofrequency contours of the plasmon. Moreover, the anisotropy character and plasmon frequency exhibit prominent temperature dependence. Our study demonstrates the first natural platform to host 2D hyperbolic plasmons, which opens exotic avenues for the manipulation of plasmon propagation, light-matter interaction and light emission in planar photonics.

Planar Terahertz Photonics Mediated by Liquid Crystal Polymers

AbstractTerahertz (THz) band is expected to satisfy the ever‐increasing demand for high‐capacity wireless data transfer. Multifunctional photonic devices with compactness and low loss are highly pursued for future THz communications. Here, a strategy for planar THz photonics is proposed that enables free wavefront manipulation with submillimeter thin liquid crystal polymer (LCP) films. Such elements work on the spatial geometric phase modulation, which is accomplished by preprogramming the axis orientations of LCP. The LCP monomers follow the guidance of local photoalignment agent and are further polymerized under UV exposure at the existence of a doped photo‐initializer. Thanks to the high resolution and excellent flexibility of the photopatterning technique, THz elements with versatile functions can be realized. As examples, waveplates, polarization gratings, and lenses, which are suitable for the polarization control, beam deflecting, focusing, or collimating, are demonstrated. Due to the intrinsic flexibility of LCP films, an f‐tunable lens enabled by mechanically induced deformation is exhibited. Specific mode generators for vortices and Bessel beams are also presented, which can function as separate channels for the mode division multiplexing in THz communications. This work provides a robust platform for fabricating integrated, low‐loss, and tunable THz elements suitable for the advanced THz apparatuses.