Polymer-Based Device Fabrication and Applications Using Direct Laser Writing Technology

Polymer and Hybrid Optical Devices Manipulated by the Thermo-Optic Effect

The thermo-optic effect is a crucial driving mechanism for optical devices. The application of the thermo-optic effect in integrated photonics has received extensive investigation, with continuous progress in the performance and fabrication processes of thermo-optic devices. Due to the high thermo-optic coefficient, polymers have become an excellent candidate for the preparation of high-performance thermo-optic devices. Firstly, this review briefly introduces the principle of the thermo-optic effect and the materials commonly used. In the third section, a brief introduction to the waveguide structure of thermo-optic devices is provided. In addition, three kinds of thermo-optic devices based on polymers, including an optical switch, a variable optical attenuator, and a temperature sensor, are reviewed. In the fourth section, the typical fabrication processes for waveguide devices based on polymers are introduced. Finally, thermo-optic devices play important roles in various applications. Nevertheless, the large-scale integrated applications of polymer-based thermo-optic devices are still worth investigating. Therefore, we propose a future direction for the development of polymers.

Fabrication of waveguide directional couplers using 2-photon lithography

Advances in 2-photon lithography have enabled in-lab production of sub-micron resolution and millimeter scale 3D optical components. The potential complex geometries are well suited to rapid prototyping and production of waveguide structures, interconnects, and waveguide directional couplers, furthering future development and miniaturization of waveguide-based imaging technologies. System alignment is inherent to the 2-photon process, obviating the need for manual assembly and allowing precise micron scale waveguide geometries not possible in traditional fused fiber coupler fabrication. Here we present the use of 2-photon lithography for direct printing of multi-mode waveguide couplers with air cladding and single mode waveguide couplers with uncured liquid photoresin cladding. Experimental results show reproducible coupling which can be modified by selected design parameters.

Dynamic Mechanical Properties of PVC Plastics in the Formation of Microstructures with Novel Magnetostrictor

Molding in thermoplastic polymers using ultrasonic hot embossing technology is promising due to its high precision reproducibility. To understand, analyze and apply the formation of polymer microstructures by the ultrasonic hot embossing method, it is necessary to understand dynamic loading conditions. The Standard Linear Solid model (SLS) is a method that allows analyzing the viscoelastic properties of materials by representing them as a combination of springs and dashpots. However, this model is general, and it is challenging to represent a viscoelastic material with multiple relaxations. Therefore, this article aims to use the data obtained from dynamic mechanical analysis for extrapolation in a wide range of cyclic deformations and to use the obtained data in microstructure formation simulations. The formation was replicated using a novel magnetostrictor design that sets a specific temperature and vibration frequency. The changes were analyzed on a diffractometer. After the diffraction efficiency measurement, it was found that the highest quality structures were formed at a temperature of 68 °C, a frequency of 10 kHz, a frequency amplitude of 1.5 µm and a force of 1 kN force. Moreover, the structures could be molded on any thickness of plastic.

Low-Divergence hBN Single-Photon Source with a 3D-Printed Low-Fluorescence Elliptical Polymer Microlens

Efficiently collecting light from single-photon emitters is crucial for photonic quantum technologies. Here, we develop and use an ultralow fluorescence photopolymer to three-dimensionally print micrometer-sized elliptical lenses on individual precharacterized single-photon emitters in hexagonal boron nitride (hBN) nanocrystals, operating in the visible regime. The elliptical lens design beams the light highly efficiently into the far field, rendering bulky objective lenses obsolete. Using back focal plane imaging, we confirm that the emission is collimated to a narrow low-divergence beam with a half width at half-maximum of 2.2°. Using photon correlation measurements, we demonstrate that the single-photon character remains undisturbed by the polymer lens. The strongly directed emission and increased collection efficiency is highly beneficial for quantum optical experiments. Furthermore, our approach paves the way for a highly parallel fiber-based detection of single photons from hBN nanocrystals.

Recent Advances in One-Dimensional Micro/Nanomotors: Fabrication, Propulsion and Application

Due to their tiny size, autonomous motion and functionalize modifications, micro/nanomotors have shown great potential for environmental remediation, biomedicine and micro/nano-engineering. One-dimensional (1D) micro/nanomotors combine the characteristics of anisotropy and large aspect ratio of 1D materials with the advantages of functionalization and autonomous motion of micro/nanomotors for revolutionary applications. In this review, we discuss current research progress on 1D micro/nanomotors, including the fabrication methods, driving mechanisms, and recent advances in environmental remediation and biomedical applications, as well as discuss current challenges and possible solutions. With continuous attention and innovation, the advancement of 1D micro/nanomotors will pave the way for the continued development of the micro/nanomotor field.

Polarization-Sensitive Patterning of Azopolymer Thin Films Using Multiple Structured Laser Beams

The polarization sensitivity of azopolymers is well known. Therefore, these materials are actively used in many applications of photonics. Recently, the unique possibilities of processing such materials using a structured laser beam were demonstrated, which revealed the key role of the distribution of polarization and the longitudinal component of light in determining the shape of the nano- and microstructures formed on the surfaces of thin azopolymer films. Here, we present numerical and experimental results demonstrating the high polarization sensitivity of thin azopolymer films to the local polarization state of an illuminating structured laser beam consisting of a set of light spots. To form such arrays of spots with a controlled distribution of polarization, different polarization states of laser beams, both homogeneous and locally inhomogeneous, were used. The results obtained show the possibility of implementing a parallel non-uniform patterning of thin azopolymer films depending on the polarization distribution of the illuminating laser beam. We believe that the demonstrated results will not only make it possible to implement the simultaneous detection of local polarization states of complex-shaped light fields but will also be used for the high-performance fabrication of diffractive optical elements and metasurfaces.

Investigation on mode dispersion and lamination stability of multimode polymer waveguides for an optical backplane

In this paper, two noteworthy issues of mode dispersion and lamination stability of multimode polymer waveguides for optical backplane are investigated. In the case of center launching by 50-µm graded-index (GI) multimode fiber (MMF), mode dispersion of polymer waveguides with different widths is analyzed theoretically and measured in the view of bit error rate (BER) curves. Compared with the waveguide with the width of 40 µm, 1-dB power penalty is observed by the 70-µm-width waveguide due to its larger mode dispersion. On the other hand, waveguide stability after laminating process with high temperature and pressure is measured experimentally. No significant changes in core shape and size are observed. The average insertion loss of 80 channels before and after lamination are 0.137 dB/cm and 0.192 dB/cm, respectively. Error-free transmission at 25 Gb/s is obtained by laminated waveguides. The results imply the feasibility and potential of multimode waveguides for optical backplane.

Enhanced poling and infiltration for highly efficient electro-optic polymer-based Mach-Zehnder modulators

An ultra-narrow 40-nm slotted waveguide is fabricated to enable highly efficient, electro-optic polymer modulators. Our measurement results indicate that V_πL's below ∼ 1.19 V.mm are possible for the balanced Mach-Zehnder modulators using this ultra-narrow slotted waveguide on a hybrid silicon-organic hybrid platform. Our simulations suggest that V_πL's can be further reduced to ∼ 0.35 V.mm if appropriate doping is utilized. In addition to adapting standard recipes, we developed two novel fabrication processes to achieve miniaturized devices with high modulation sensitivity. To boost compactness and decrease the overall footprint, we use a fabrication approach based on air bridge interconnects on thick, thermally-reflowed, MaN 2410 E-beam resist protected by an alumina layer. To overcome the challenges of high currents and imperfect infiltration of polymers into ultra-narrow slots, we use a carefully designed, atomically-thin layer of TiO₂ as a carrier barrier to enhance the efficiency of our electro-optic polymers. The anticipated increase in total capacitance due to the TiO₂ layer is negligible. Applying our TiO₂ surface treatment to the ultra-narrow slot allows us to obtain an improved index change efficiency (∂n/∂V) of ∼ 22% for a 5 nm TiO₂ layer. Furthermore, compared to non-optimized cases, our peak measured current during poling is reduced by a factor of ∼ 3.

Characterization and Optimization of Real-Time Photoresponsive Gelatin for Direct Laser Writing

There is an abundance of plastic materials used in the widest range of applications, such as packaging, machine parts, biomedical devices and components, etc. However, most materials used today are non-decomposable in the environment, producing a huge burden on ecosystems. The search for better, safer alternatives is still on. Here we present a detailed analysis of a simple, cheap, non-toxic, even edible, eco-friendly material, which can be easily manufactured, laser patterned and used for the fabrication of complex structures. The base substance is gelatin which is made photoresponsive by adding plasticizers and sensitizers. The resulting films were analyzed with respect to their optical, thermal and mechanical properties, which can be modified by a slight variation of chemical composition. The material is optimized for rapid laser-manufacturing of elastic microstructures (lenses, gratings, cantilevers, etc.) without any waste or residues. Overall, the material properties were tailored to increase photothermal responsivity, improve the surface quality and achieve material homogeneity, transparency and long-term stability (as verified using electron microscopy, infrared spectroscopy and differential scanning calorimetry).

Interconnection of Few-Mode Fibers and Photonic Integrated Circuits Using Mode-Field Adapters

We propose a detailed method for the interconnection between optical fibers and waveguides of photonic integrated circuits. Appropriate modal transmission is accomplished by matching the mode field diameters from both waveguide structures. Links from one structure to another are created by an interconnecting waveguide, maintaining a fixed coupling efficiency as its size is modified to adjust to the target waveguide core. This tailored transition acts as a mode field adapter, equalizing the transmission among multiple modes and reducing the mode-dependent losses while coupling. We present an algorithm to design the mode field adapter based on matching the effective mode areas using the power overlap integral. A study case considering a polymer photonic integrated device immediately connected to a few-mode fiber is analyzed. Coupling efficiencies over 90% for every transmitted mode are achieved, showing an evident improvement compared to typical approaches only matching core sizes. Detailed comparison of the results for each transmission mode is presented. This same procedure can be used to interconnect optical waveguides with different refractive index profiles and core geometry.

Organic Bioelectronics for In Vitro Systems

Bioelectronics have made strides in improving clinical diagnostics and precision medicine. The potential of bioelectronics for bidirectional interfacing with biology through continuous, label-free monitoring on one side and precise control of biological activity on the other has extended their application scope to in vitro systems. The advent of microfluidics and the considerable advances in reliability and complexity of in vitro models promise to eventually significantly reduce or replace animal studies, currently the gold standard in drug discovery and toxicology testing. Bioelectronics are anticipated to play a major role in this transition offering a much needed technology to push forward the drug discovery paradigm. Organic electronic materials, notably conjugated polymers, having demonstrated technological maturity in fields such as solar cells and light emitting diodes given their outstanding characteristics and versatility in processing, are the obvious route forward for bioelectronics due to their biomimetic nature, among other merits. This review highlights the advances in conjugated polymers for interfacing with biological tissue in vitro, aiming ultimately to develop next generation in vitro systems. We showcase in vitro interfacing across multiple length scales, involving biological models of varying complexity, from cell components to complex 3D cell cultures. The state of the art, the possibilities, and the challenges of conjugated polymers toward clinical translation of in vitro systems are also discussed throughout.

Two-Photon Polymerization: Functionalized Microstructures, Micro-Resonators, and Bio-Scaffolds

The direct laser writing technique based on two-photon polymerization (TPP) has evolved considerably over the past two decades. Its remarkable characteristics, such as 3D capability, sub-diffraction resolution, material flexibility, and gentle processing conditions, have made it suitable for several applications in photonics and biosciences. In this review, we present an overview of the progress of TPP towards the fabrication of functionalized microstructures, whispering gallery mode (WGM) microresonators, and microenvironments for culturing microorganisms. We also describe the key physical-chemical fundamentals underlying the technique, the typical experimental setups, and the different materials employed for TPP.

Vegetable Oil-Based Thiol-Ene/Thiol-Epoxy Resins for Laser Direct Writing 3D Micro-/Nano-Lithography

The use of renewable sources for optical 3D printing instead of petroleum-based materials is increasingly growing. Combinations of photo- and thermal polymerization in dual curing processes can enhance the thermal and mechanical properties of the synthesized thermosets. Consequently, thiol-ene/thiol-epoxy polymers were obtained by combining UV and thermal curing of acrylated epoxidized soybean oil and epoxidized linseed oil with thiols, benzene-1,3-dithiol and pentaerythritol tetra(3-mercaptopropionate). Thiol-epoxy reaction was studied by calorimetry. The changes of rheological properties were examined during UV, thermal and dual curing to select the most suitable formulations for laser direct writing (LDW). The obtained polymers were characterized by dynamic-mechanical thermal analysis, thermogravimetry, and mechanical testing. The selected dual curable mixture was tested in LDW 3D lithography for validating its potential in optical micro- and nano-additive manufacturing. The obtained results demonstrated the suitability of epoxidized linseed oil as a biobased alternative to bisphenol A diglycidyl ether in thiol-epoxy thermal curing reactions. Dual cured thermosets showed higher rigidity, tensile strength, and Young’s modulus values compared with UV-cured thiol-ene polymers and the highest thermal stability from all prepared polymers. LDW results proved their suitability for high resolution 3D printing—individual features reaching an unprecedented 100 nm for plant-based materials. Finally, the biobased resin was tested for thermal post-treatment and 50% feature downscaling was achieved.

Two-Photon-Assisted Polymerization and Reduction: Emerging Formulations and Applications

Two-photon lithography (TPL) is an emerging approach to fabricate complex multifunctional micro/nanostructures. This is because TPL can easily develop various 2D and 3D structures on a variety of surfaces, and there has been a rapidly expanding pool of processable photoresists to create different materials. However, challenges in developing two-photon processable photoresists currently impede progress in TPL. In this review, we critically discuss the importance of photoresist formulation in TPL. We begin by evaluating the commercial photoresists to design micro/nanostructures for promising applications in anti-counterfeiting, superomniphobicity, and micromachines with movable parts. Next, we discuss emerging hydrogel/organogel photoresists, focusing on customizing photoresist formulations to fabricate reconfigurable structures that can respond to changes in local pH, solvent, and temperature. We also review the development of metal salt-based photoresists for direct metal writing, whereby various formulations have been developed to enable applications in online sensing, catalysis, and electronics. Finally, we provide a critical outlook and highlight various outstanding challenges in formulating processable photoresists for TPL.

Decimating Spatial Frequency Components in Periodically Modulated Nanoscale Surface Structures for Sensing of Ambient Refractive Index Changes

In our previous study, we developed an array of unique porous structures (an array of barnacle-like porous structures) to apply to biosensing chips. The porous structure was formed by an internal swelling phenomenon of a polystyrene colloidal particle monolayer, which was surrounded by a poly(vinyl alcohol) layer, for the duration of the monolayer's immersion in a toluene bath. Barnacle-like porous structures were formed when polystyrene particles that had rapidly swelled broke the outer layer around the top of the particles. However, after the surface was coated with a thin Ag layer, the porous structure showed a relatively broad extinction spectrum that was undesirable for sensing chips based on both surface plasmon extinction and grating coupling. In this paper, we propose an approach to obtain relatively sharp extinction spectra based on the decimation of the spatial frequencies of the porous structures. This study also investigates formation properties in more detail to control the structural features of the resultant porous structures. A relatively sharp peak in the extinction spectrum was ultimately obtained.

STED Direct Laser Writing of 45 nm Width Nanowire

Controlled fabrication of 45 nm width nanowire using simulated emission depletion (STED) direct laser writing with a rod-shape effective focus spot is presented. In conventional STED direct laser writing, normally a donut-shaped depletion focus is used, and the minimum linewidth is restricted to 55 nm. In this work, we push this limit to sub-50 nm dimension with a rod-shape effective focus spot, which is the combination of a Gaussian excitation focus and twin-oval depletion focus. Effects of photoinitiator type, excitation laser power, and depletion laser power on the width of the nanowire are explored, respectively. Single nanowire with 45 nm width is obtained, which is λ/18 of excitation wavelength and the minimum linewidth in pentaerythritol triacrylate (PETA) photoresist. Our result accelerates the progress of achievable linewidth reduction in STED direct laser writing.