Submicrometer 3D structures fabrication enabled by one-photon absorption direct laser writing

SU-8-meta-phenylenediamine-conjugated thin film for temperature sensing

Polymers have distinctive optical properties and facile fabrication methods that have been well-established. Therefore, they have immense potential for nanophotonic devices. Here, we demonstrate the temperature-sensing potential of SU8-meta-phenylenediamine (SU8-mPD), produced by epoxy amination of the SU-8 polymer. Its properties were examined through a series of molecular structural techniques and optical methods. Thin layers have demonstrated optical emission and absorption in the visible range around 420 and 520 nm, respectively, alongside a strong thermal responsivity, characterized by the 18 ppm °C-1 expansion coefficient. A photonic chip, comprising a thin 5-10 μm SU8-mPD layer, encased between parallel silver and/or gold thin film mirrors, has been fabricated. When pumped by an external light source, this assembly generates a pronounced fluorescent signal that is superimposed with the Fabry-Pérot (FP) resonant response. The chip undergoes mechanical deformation in response to temperature changes, thereby shifting the FP resonance and encoding temperature information into the fluorescence output spectrum. The time response of the device was estimated to be below 1 s for heating and a few seconds for cooling, opening a new avenue for optical sensing using SU8-based polymers. Thermoresponsive resonant structures, encompassing strong tunable fluorescent properties, can further enrich the functionalities of nanophotonic polymer-based platforms. This article is part of the theme issue 'Celebrating the 15th anniversary of the Royal Society Newton International Fellowship'.

High-directivity far-field radiation of quantum dot-based single-photon emitter coupled to polymeric circular waveguide resonant grating

Solid-state single-photon emitters (SPEs) commonly encounter the limitation of quasi-omnidirectional radiation patterns, which poses challenges in utilizing their emission with conventional optical instruments. In this study, we demonstrate the tailoring of the far-field radiation patterns of SPEs based on colloidal quantum dots (QDs), both theoretically and experimentally, by employing a polymer-based dielectric antenna. We introduce a simple and cost-effective technique, namely low one-photon absorption direct laser writing, to achieve precise coupling of a QD into an all-polymer circular waveguide resonance grating. By optimizing the geometry parameters of the structure using 3D finite-difference time-domain simulations, resonance at the emission wavelength of QDs is achieved in the direction perpendicular to the substrate, resulting in photon streams with remarkably high directivity on both sides of the grating. Theoretical calculations predict beam divergence values below 2°, while experimental measurements using back focal plane imaging yield divergence angles of approximately 8°. Our study contributes to the evaluation of concentric circular grating structures employing low refractive index polymer materials, thereby expanding the possibilities for their application.

Plasmonic direct-writing lithography via high numerical aperture objectives

The exploration of light-matter interactions at the sub-wavelength scale requires advanced nano-patterning tools with low cost and high flexibility. Plasmonic lithography as a promising candidate receives much attention owing to its ability to confine ultraviolet light sources into an extremely tiny volume. To date, most plasmonic patterning schemes utilize metallic nano-structures to achieve tight focusing. The drawback is that the plasmonic structures need, however, to be pre-defined, usually accompanied with the expense of complex fabrication processes. Here we numerically and experimentally report an antenna-free plasmonic lithography technique using high numerical aperture (NA) objectives as the scanning head. Minimum feature sizes of 0.36λ/NA and 0.46λ/NA are numerically and experimentally demonstrated, respectively, under the linearly polarized continuous-wave illumination at 457 nm with no involvement of nonlinear effects. Back-focal-plane imaging is used to visualize surface-plasmon excitations, acting as a viable way of adjusting focus precisely. Our method can serve as a candidate for laser processing at the sub-wavelength scale, and offers a truly convenient and economical way of nano-patterning.

Design and Realization of Polymeric Waveguide/Microring Structures for Telecommunication Domain

Polymer-based micro-optical components are very important for applications in optical communication. In this study, we theoretically investigated the coupling of polymeric waveguide and microring structures and experimentally demonstrated an efficient fabrication method to realize these structures on demand. First, the structures were designed and simulated using the FDTD method. The optical mode and loss in the coupling structures were calculated, thereby giving the optimal distance for optical mode coupling between two rib waveguide structures or for optical mode coupling in a microring resonance structure. Simulations results then guided us in the fabrication of the desired ring resonance microstructures using a robust and flexible direct laser writing technique. The entire optical system was thus designed and manufactured on a flat base plate so that it could be easily integrated in optical circuits.

Operando surface optical nanometrology reveals diazonium salts' visible photografting mechanism

High resolution and quantitative surface modification through photografting is a highly desirable strategy towards the preparation of smart surfaces, enabling chemical functions to be precisely located onto specific regions of inert surfaces. Although promising, the mechanisms leading to direct (without the use of any additive) photoactivation of diazonium salts using visible wavelengths are poorly understood, precluding the generalization of popular diazonium-based electrografting strategies into high resolution photografting ones. In this paper, we employ quantitative phase imaging as a nanometrology tool for evaluating the local grafting rate with diffraction-limited resolution and nanometric precision. By carefully measuring the surface modification kinetics under a range of different conditions, we reveal the reaction mechanism while evaluating the influence of key parameters, such as the power density, the radical precursor concentration and the presence of side reactions.

On-Chip 3D Printing of Polymer Waveguide-Coupled Single-Photon Emitter Based on Colloidal Quantum Dots

In the field of quantum technology, there has been a growing interest in fully integrated systems that employ single photons due to their potential for high performance and scalability. Here, a simple method is demonstrated for creating on-chip 3D printed polymer waveguide-coupled single-photon emitters based on colloidal quantum dots (QDs). By using a simple low-one photon absorption technique, we were able to create a 3D polymeric crossed-arc waveguide structure with a bright QD on top. These waveguides can conduct both excitation laser and emitted single photons, which facilitates the characterization of single-photon signals at different outputs with a conventional confocal scanning system. To optimize the guiding effect of the polymeric waveguide structures, comprehensive 3D finite-difference time-domain simulations were performed. Our method provides a straightforward and cost-effective way to integrate high-performance single-photon sources with on-chip photonic devices, enabling scalable and versatile quantum photonic circuits for various applications.

Design and fabrication of Mach-Zehnder interferometers in soda-lime glass for temperature sensing applications

High sensitivity represents one of the main goals that sensing devices need to satisfy for their applications. This work presents to the best of our knowledge the first integrated Mach-Zehnder interferometer (MZI) embedded in soda-lime glass with comparable sensitivity to silicon-on-insulator (SOI) devices. We manufactured the MZIs by the femtosecond direct laser writing (FDLW) technique and characterized them with temperature. Four buried MZIs were manufactured by slightly increasing the optical path due to separation between the arms of the interferometer (Δ s). We achieved a fringe shift of ∼8n m for an increase of 0.18 µm. We have characterized one of these devices with temperature from 30°C to 70°C obtaining a sensitivity of ∼28p m/ ^∘ C. We improved the sensitivity of the device to ∼54p m/ ^∘ C due to the advantage of the unique three-dimensional (3D) capabilities that FDLW provides, overcoming the characteristically low thermo-optic coefficient of soda-lime glass just by rotating the MZI structure 11°.

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.

Direct fabrication and characterization of gold nanohole arrays

We demonstrate a one-step fabrication method to realize desired gold (Au) nanoholes arrays by using a one-photon absorption based direct laser writing technique. Thanks to the optically induced thermal effect of Au material at 532 nm excitation wavelength, the local temperature at the laser focus area can reach as high as 600°C, which induces an evaporation of the Au thin film resulting in a metallic nanohole. By controlling the laser spot movement and exposure time, different two-dimensional Au nanoholes structures with periodicity as small as 500 nm have been demonstrated. This allows obtaining plasmonic nanostructures in a single step without needing the preparation of polymeric template and lift-off process. By this direct fabrication technique, the nanoholes do not have circular shape as the laser focusing spot, due to the non-uniform heat transfer in a no-perfect flat Au film. However, the FDTD simulation results and the experimental measurement of the transmission spectra show that the properties of fabricated plasmonic nanoholes arrays are very close to those of ideal plasmonic nanostructures. Actually, the plasmonic resonance depends strongly on the periodicity of the metallic structures while the heterogeneous form of the holes simply enlarges the resonant peak. Furthermore, it is theoretically demonstrated that the non-perfect circular shape of the Au hole allows amplifying the electromagnetic field of the resonant peak by several times as compared to the case of perfect circular shape. This could be an advantage for application of this fabricated structure in laser and nonlinear optics domains.

Fabrication of Functional Microdevices in SU-8 by Multi-Photon Lithography

This review surveys advances in the fabrication of functional microdevices by multi-photon lithography (MPL) using the SU-8 material system. Microdevices created by MPL in SU-8 have been key to progress in the fields of micro-fluidics, micro-electromechanical systems (MEMS), micro-robotics, and photonics. The review discusses components, properties, and processing of SU-8 within the context of MPL. Emphasis is focused on advances within the last five years, but the discussion also includes relevant developments outside this period in MPL and the processing of SU-8. Novel methods for improving resolution of MPL using SU-8 and discussed, along with methods for functionalizing structures after fabrication.

Low-cost fabrication of microlasers based on polymeric micropedestals

Our current work exploits direct laser writing (DLW) and low one-photon absorption (LOPA) in a low-cost three-dimensional optical fabrication system designed to print micrometric polymeric structures. Micropedestals were obtained by focusing a laser beam on a photoresist layer deposited on a silica glass substrate. Subsequent coating with rhodamine 6G dye allows these pedestals to function as microlasers upon optical excitation at 532 nm. Our microlasers, with a diameter of ∼53µm and a height of ∼40µm, exhibit a broad fluorescence peak in the spectral range 540-600 nm, in addition to narrow lasing peaks, exhibiting quality factors Q exceeding 2000 and a lasing threshold of ∼5µJcm^-2. The observed free spectral range associated with the lasing peaks of ∼1.3nm is consistent with simulations, which we include in this paper. In addition, we present simulations for the longitudinal shift of the patterning laser spot, which occurs particularly for relatively thick photoresist layers, coupled with a large index contrast at the photoresist top surface. Such a shift could introduce errors in the resulting microfabricated structures if left unaccounted for. We hope that our work will contribute to the development of microlasers for various photonic applications, particularly if dimensions can be reduced, for on-chip optical communications and data processing.

Direct Laser Writing of Fluorescent Silver Nanoclusters: A Review of Methods and Applications

Metal nanoclusters (NCs) are nanomaterials of size of less than 2 nm that exhibit a set of unique physical, chemical, optical, and electronic properties. Because of recent interest in NCs, a great deal of effort is being made to develop synthetic routes that allow control over the NC size, shape, geometry, and properties. Direct laser writing is one of the few synthesis methods that allow the generation of photostable NCs with high quantum yield in a highly controlled fashion. A key advantage of laser-written NCs is the ability to create easy-to-use solid-state devices for a range of applications. This review will present necessary background and recent advances in laser writing of silver NCs and their applications in different solid-state matrixes such as glass, zeolites, and polymer substrate. This topic will be of interest to researchers in the fields of materials science, optics and photonics, chemistry, and biomedical sciences.

Controllable movement of single-photon source in multifunctional magneto-photonic structures

Quantum dot (QD) coupling in nanophotonics has been widely studied for various potential applications in quantum technologies. Micro-machining has also attracted substantial research interest due to its capacity to use miniature robotic tools to make precise controlled movements. In this work, we combine fluorescent QDs and magnetic nanoparticles (NPs) to realize multifunctional microrobotic structures and demonstrate the manipulation of a coupled single-photon source (SPS) in 3D space via an external magnetic field. By employing the low one photon absorption (LOPA) direct laser writing (DLW) technique, the fabrication of 2D and 3D magneto-photonic devices containing a single QD is performed on a hybrid material consisting of colloidal CdSe/CdS QDs, magnetite Fe3O4 NPs, and SU-8 photoresist. Two types of devices, contact-free and in-contact structures, are investigated to demonstrate their magnetic and photoradiative responses. The coupled SPS in the devices is driven by the external magnetic field to perform different movements in a 3D fluidic environment. The optical properties of the single QD in the devices are characterized.

Synthesis and structural study of organic two-photon-absorbing cycloalkanone chromophores

The three organic two-photon-absorbing cycloalkanone chromophores 2,4-bis[4-(diethylamino)benzylidene]cyclobutanone, C₂₆H₃₂N₂O (I), 2,5-bis[4-(diethylamino)benzylidene]cyclopentanone, C₂₇H₃₄N₂O (II), and 2,6-bis[4-(diethylamino)benzylidene]cyclohexanone, C₂₈H₃₆N₂O (III), were obtained by a reaction between 4-(diethylamino)benzaldehyde and the corresponding cycloalkanone and were characterized by single-crystal X-ray diffraction studies, as well as density functional theory (DFT) quantum-chemical calculations. Molecules of this series have three main fragments, i.e. central acceptor (A) and two terminal donors (D₁ and D₂) and represent examples of the D₁-π-A-π-D₂ molecular design. All three compounds crystallize with two crystallographically independent molecules in the asymmetric unit (A and B) and are distinguished by the conformations of both the molecular Et₂N-C₆H₄-C=C-C(=O)-C=C-C₆H₄-NEt₂ backbone (arcuate or linear) and the terminal diethylamino substituents (syn- or antiperiplanar to the plane of the molecule). The central four- and five-membered rings in I and II are almost planar, and the six-membered ring in III adopts a sofa conformation. In the crystals of I-III, the two independent molecules A and B form hydrogen-bonded [A...B] dimers via intermolecular C-H...O hydrogen bonds. Furthermore, the [A...B] dimers in I are bound by intermolecular C-H...O hydrogen bonds into two-tier puckered layers, whereas in the crystals of II and III, the [A...B] dimers are stacked along the c and a axes, respectively. Taking into account the decreasing steric strain upon expanding the central ring, compound I might be more efficient as a two-photon absorption chromophore than compounds II and III, which corresponds to the results of spectroscopic studies.

Versatile direct laser writing of non-photosensitive materials using multi-photon reduction-based assembly of nanoparticles

Versatile direct laser writing (DLW), not limited by material photosensitivity, offers opportunities for fundamental and technological innovation for micro-/nanofabrication in integrated photonics, electronics and material science. Although DLW has high potential in micro-/nanodevice fabrication, material choice suffers an intrinsic limitation: DLW cannot be applied to non-photosensitive materials. We describe a newly discovered rapid-assembly phenomenon of fine particles based on femtosecond laser multi-photon-reduction in solution. This phenomenon allowed the writing of micropatterns with thick clad layers filled with nanoparticles. We wrote continuous patterns by moving the laser focus even in the case of non-photosensitive material such as SiO₂. By transcending the strict material limitation, this novel laser writing process promises to be a powerful tool in a variety of scientific fields.

An Optimization of Two-Dimensional Photonic Crystals at Low Refractive Index Material

Photonic crystal (PC) is usually realized in materials with high refractive indices contrast to achieve a photonic bandgap (PBG). In this work, we demonstrated an optimization of two-dimensional PCs using a low refractive index polymer material. An original idea of assembly of polymeric multiple rings in a hexagonal configuration allowed us to obtain a circular-like structure with higher symmetry, resulting in a larger PBG at a low refractive index of 1.6. The optical properties of such newly proposed structure are numerically calculated by using finite-difference time-domain (FDTD) method. The proposed structures were realized experimentally by using a direct laser writing technique based on low one-photon absorption method.

Photostability and long-term preservation of a colloidal semiconductor-based single photon emitter in polymeric photonic structures

Colloidal semiconductor quantum dots (QDs) are promising candidates for various applications in electronics and quantum optics. However, they are sensitive and vulnerable to the chemical environment due to their highly dynamic surface with a large portion of exposed atoms. Hence, oxidation and detrimental defects on the nanocrystal (NC) interface dramatically deteriorate their optical as well as electrical properties. In this study, a simple strategy is proposed not only to obtain good preservation of colloidal semiconductor QDs by using a protective polymer matrix but also to provide excellent accessibility to micro-fabrication by optical lithography. A high-quality QD-polymer nanocomposite with mono-dispersion of the NCs is synthesized by incorporating the colloidal CdSe/CdS NCs into an SU-8 photoresist. Our approach shows that the oxidation of the core/shell QDs embedded in the SU-8 resist is completely avoidable. The deterministic insertion of multiple QDs or a single QD into photonic structures is demonstrated. Single photon generation is obtained and well-preserved in the nanocomposite and the polymeric structures.

Deterministic Insertion of KTP Nanoparticles into Polymeric Structures for Efficient Second-Harmonic Generation

We investigate theoretically and experimentally the creation of virtually any polymer-based photonic structure containing individual nonlinear KTiOPO 4 nanoparticles (KTP NPs) using low one-photon absorption (LOPA) direct laser writing (DLW) technique. The size and shape of polymeric microstructures and the position of the nonlinear KTP crystal inside the structures, were perfectly controlled at nanoscale and on demand. Furthermore, we demonstrated an enhancement of the second-harmonic generation (SHG) by a factor of 90 when a KTP NP was inserted in a polymeric pillar. The SHG enhancement is attributed to the resonance of the fundamental light in the cavity. This enhancement varied for different KTP NPs, because of the random orientation of the KTP NPs, which affects the light/matter interaction between the fundamental light and the NP as well as the collection efficiency of the SHG signal. The experimental result are further supported by a simulation model using Finite-Difference Time-Domain (FDTD) method.

Large thermal tuning of a polymer-embedded silicon nitride nanobeam cavity

Tunable silicon nitride nanophotonic resonators are a critical building block for integrated photonic systems in the visible wavelength range. We experimentally demonstrate a thermally tunable polymer-embedded silicon nitride nanobeam cavity with a tuning efficiency of 44 pm/°C and 0.13 nm/mW in the near-visible wavelength range. The large tuning efficiency comes from the high thermo-optic coefficient of the SU-8 polymer and the "air-mode" cavity design, where a large portion of the cavity field is confined inside the polymer region. The demonstrated resonator will enable locally tunable cavity quantum electrodynamic experiments in the silicon nitride platform.

Mesoscale laser 3D printing

3D meso scale structures that can reach up to centimeters in overall size but retain micro- or nano-features, proved to be promising in various science fields ranging from micro-mechanical metamaterials to photonics and bio-medical scaffolds. In this work, we present synchronization of the linear and galvanometric scanners for efficient femtosecond 3D optical printing of objects at the meso-scale (from sub-μm to sub-cm spanning five orders of magnitude). In such configuration, the linear stages provide stitch-free structuring at nearly limitless (up to tens-of-cm) working area, while galvo-scanners allow to achieve translation velocities in the range of mm/s-cm/s without sacrificing nano-scale positioning accuracy and preserving the undistorted shape of the final print. The principle behind this approach is demonstrated, proving its inherent advantages in comparison to separate use of only linear stages or scanners. The printing rate is calculated in terms of voxels/s, showcasing the capability to maintain an optimal feature size while increasing throughput. Full capabilities of this approach are demonstrated by fabricating structures that reach millimeters in size but still retain sub-μm features: scaffolds for cell growth, microlenses, and photonic crystals. All this is combined into a benchmark structure: a meso-butterfly. Provided results show that synchronization of two scan modes is crucial for the end goal of industrial-scale implementation of this technology and makes the laser printing well aligned with similar approaches in nanofabrication by electron and ion beams.

Low-threshold organic lasing from a square optical microcavity fabricated by imaging holography

We propose and demonstrate the versatile fabrication of optical subwavelength microcavities by using imaging holography. As a demonstration, a peculiar square optical microcavity with a periodicity of 400 nm is imaged from a micrometer-scale diffractive optical element, attributing to the interference by the refocusing of the multiple diffractive beams. By spin-coating an active conjugated polymer onto the microcavity, highly directional laser emission with a low pumping threshold of 0.5 kW/cm² is achieved. The effect of the film thickness on the lasing performance is also investigated. This imaging holography technique can enable convenient and easy fabrication of optical microcavities with subwavelength features, hence providing significant flexibility and richness on engineering the optical response of photonic nanostructures.

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

Polymer materials exhibit unique properties in the fabrication of optical waveguide devices, electromagnetic devices, and bio-devices. Direct laser writing (DLW) technology is widely used for micro-structure fabrication due to its high processing precision, low cost, and no need for mask exposure. This paper reviews the latest research progresses of polymer-based micro/nano-devices fabricated using the DLW technique as well as their applications. In order to realize various device structures and functions, different manufacture parameters of DLW systems are adopted, which are also investigated in this work. The flexible use of the DLW process in various polymer-based microstructures, including optical, electronic, magnetic, and biomedical devices are reviewed together with their applications. In addition, polymer materials which are developed with unique properties for the use of DLW technology are also discussed.

Scaffolds in a shell-a new approach combining one-photon and two-photon polymerization

We report on a laser system combining one-photon and two-photon polymerization for precise and fast fabrication of macroscopic three-dimensional structures featuring microscale and nanoscale characteristics. This single-stage process significantly reduces the production time as demonstrated by scaffolds in a shell application. Porous scaffolds with different pore sizes are surrounded by a ring so that cells can be seeded directly to the scaffolds kept in a shell and do not spread over the whole substrate expecting a saving of cell suspension, faster growth on the scaffolds, and a more controllable environment. Compared to a two-photon polymerization process, the ring is fabricated about 500 times faster using one-photon polymerization. The presented hybrid process qualifies for further applications illustrated by a fluidic system.

Low-loss flexible Parylene photonic waveguides for optical implants

We demonstrate compact, low-loss (<5  dB/cm) Parylene C photonic waveguides in a flexible, biocompatible, all-polymer platform suitable for implantable applications. The scattering loss due to the sidewall roughness resulting from the reactive ion etching of Parylene C was identified as the primary source of propagation loss. A fabrication process utilizing the conformal coating of Parylene C was developed to significantly reduce waveguide propagation loss (by more than 30 dB/cm). We also performed thermal annealing at 300°C to smoothen the sidewalls; however, it was found to adversely affect the waveguide performance.

Laser direct-writing lithography equipment system for rapid and μm-precision fabrication on curved surfaces with large sag heights

A novel laser direct writing lithography equipment system was proposed to realize rapid and μm-precision fabrication on curved surfaces with large curvatures and large sag heights. Through detailed design, analysis, simulation, and measurement, it showed that the system was able to continuously, linearly and rapidly change the focus position in z coordinate. For demonstration, a concentric-circular photoresist grating with a sag height of 5.2 mm and a designed line width/space of 12.5 μm/25 μm was fabricated within 40 s on a convex K9 glass substrate surface with a curvature radius of 32.77 mm. When combining with the movement of the x-y-z stage, the system could fabricate the micropatterns on curved surfaces with larger dimensions. Further, the system possessed the general principles of optics, mechanics, and automation for μm-precision 3D microfabrication.

Chiral Plasmonic Hydrogen Sensors

In this article, a chiral plasmonic hydrogen-sensing platform using palladium-based nanohelices is demonstrated. Such 3D chiral nanostructures fabricated by nanoglancing angle deposition exhibit strong circular dichroism both experimentally and theoretically. The chiroptical properties of the palladium nanohelices are altered upon hydrogen uptake and sensitively depend on the hydrogen concentration. Such properties are well suited for remote and spark-free hydrogen sensing in the flammable range. Hysteresis is reduced, when an increasing amount of gold is utilized in the palladium-gold hybrid helices. As a result, the linearity of the circular dichroism in response to hydrogen is significantly improved. The chiral plasmonic sensor scheme is of potential interest for hydrogen-sensing applications, where good linearity and high sensitivity are required.

Single-photon three-dimensional microfabrication through a multimode optical fiber

Two-photon polymerization (TPP) processes have enabled the fabrication of advanced and functional microstructures. However, most TPP platforms are bulky and require the use of expensive femtosecond lasers. Here, we propose an inexpensive and compact alternative to TPP by adapting an endoscopic imaging system for single-photon three-dimensional microfabrication. The wavefront of a visible continuous-wave laser beam is shaped so that it focuses into a photoresist through a 5 cm long ultra-thin multimode optical fiber (∅70 μm, NA 0.64). Using this device, we show that single-photon polymerization can be confined to the phase-controlled focal spot thanks to the non-linearity of the photoresist, likely due to oxygen radical scavenging. Thus, by exploiting this non-linearity with a specific overcuring method we demonstrate single-photon three-dimensional fabrication of solid and hollow microstructures through a multimode fiber with a 1.0-μm lateral and 21.5-μm axial printing resolution. This opens up new possibilities for advanced and functional microfabrication through endoscopic probes with inexpensive laser sources.

Fabrication of helical photonic structures with submicrometer axial and spatial periodicities following "inverted umbrella" geometry through phase-controlled interference lithography

In this Letter we report for the first time, to the best of our knowledge, a phase spatial light modulator (SLM)-based interference lithography (IL) approach for the realization of hexagonally packed helical photonic structures with a submicrometer scale spatial, as well as axial, periodicity over a large area. A phase-only SLM is used to electronically generate six phase-controlled plane beams. These six beams from the front side and a direct central backside beam are used together in an "inverted umbrella" geometry setup to realize the desired submicrometer axial periodic chiral photonic structures through IL. The realized structures with 650 nm spatial and 353 nm axial periodicities on negative photoresist can be used as an optical filter and refractive index sensor, as evidenced from the FDTD-based simulation study on its optical properties. Further, the fabricated templates can be transferred to metals such as silver or aluminum for the realization of a metamaterial-based broadband circular polarizer ranging from 1 to 3.5 μm of near-infrared spectra.

Direct Laser Writing of Magneto-Photonic Sub-Microstructures for Prospective Applications in Biomedical Engineering

We report on the fabrication of desired magneto-photonic devices by a low one-photon absorption (LOPA) direct laser writing (DLW) technique on a photocurable nanocomposite consisting of magnetite ( Fe 3 O 4 ) nanoparticles and a commercial SU-8 photoresist. The magnetic nanocomposite was synthesized by mixing Fe 3 O 4 nanoparticles with different kinds of SU-8 photoresists. We demonstrated that the degree of dispersion of Fe 3 O 4 nanoparticles in the nanocomposite depended on the concentration of Fe 3 O 4 nanoparticles, the viscosity of SU-8 resist, and the mixing time. By tuning these parameters, the most homogeneous magnetic nanocomposite was obtained with a concentration of about 2 wt % of Fe 3 O 4 nanoparticles in SU-8 2005 photoresist for the mixing time of 20 days. The LOPA-based DLW technique was employed to fabricate on demand various magneto-photonic submicrometer structures, which are similar to those obtained without Fe 3 O 4 nanoparticles. The magneto-photonic 2D and 3D structures with sizes as small as 150 nm were created. We demonstrated the strong magnetic field responses of the magneto-photonic nanostructures and their use as micro-actuators when immersed in a liquid solution.

Performance Characterization of an xy-Stage Applied to Micrometric Laser Direct Writing Lithography

This article concerns the characterization of the stability and performance of a motorized stage used in laser direct writing lithography. The system was built from commercial components and commanded by G-code. Measurements use a pseudo-periodic-pattern (PPP) observed by a camera and image processing is based on Fourier transform and phase measurement methods. The results report that the built system has a stability against vibrations determined by peak-valley deviations of 65 nm and 26 nm in the x and y directions, respectively, with a standard deviation of 10 nm in both directions. When the xy-stage is in movement, it works with a resolution of 0.36 µm, which is an acceptable value for most of research and development (R and D) microtechnology developments in which the typical feature size used is in the micrometer range.

Optically Clear and Resilient Free-Form µ-Optics 3D-Printed via Ultrafast Laser Lithography

We introduce optically clear and resilient free-form micro-optical components of pure (non-photosensitized) organic-inorganic SZ2080 material made by femtosecond 3D laser lithography (3DLL). This is advantageous for rapid printing of 3D micro-/nano-optics, including their integration directly onto optical fibers. A systematic study of the fabrication peculiarities and quality of resultant structures is performed. Comparison of microlens resiliency to continuous wave (CW) and femtosecond pulsed exposure is determined. Experimental results prove that pure SZ2080 is ∼20 fold more resistant to high irradiance as compared with standard lithographic material (SU8) and can sustain up to 1.91 GW/cm² intensity. 3DLL is a promising manufacturing approach for high-intensity micro-optics for emerging fields in astro-photonics and atto-second pulse generation. Additionally, pyrolysis is employed to homogeneously shrink structures up to 40% by removing organic SZ2080 constituents. This opens a promising route towards downscaling photonic lattices and the creation of mechanically robust glass-ceramic microstructures.

Proximity-effect correction for 3D single-photon optical lithography

A proximity-effect-correction (PEC) algorithm for three-dimensional (3D) single-photon gray-scale photolithography is proposed and numerically analyzed in this paper. The gray-scale dose assigned to every point within the photoresist volume is optimized to guarantee that the fabricated 3D patterns are as close to the designed patterns as possible. PEC optimizations for 3D woodpile geometries using low and high absorption photoresist are simulated. Spatial resolution of the proposed PEC algorithm is numerically studied. We also investigated the efficacy of our algorithm on a variety of related 3D geometries.

Controlled coupling of a single nanoparticle in polymeric microstructure by low one-photon absorption-based direct laser writing technique

We investigated the coupling of a single nanoparticle (NP) into a polymer-based photonic structure (PS). The low one-photon absorption microscopy with a two-step technique allowed us first to accurately determine the location of a NP and then to embed it as desired into an arbitrary PS. The coupling of a gold NP and a polymer-based PS was experimentally investigated showing a six-fold photon collection enhancement as compared to that of a NP in unpatterned film. The simulation results based on finite-difference time-domain calculation method confirmed this observation and showed a 2.86-fold enhancement in extraction efficiency thanks to the NP/PS coupling.

Concept for three-dimensional optical addressing by ultralow one-photon absorption method

With respect to experimental condition, we have investigated the point spread function of a high numerical aperture objective lens, taking into account the absorption effect of the studied material. By using a material possessing an ultralow one-photon absorption (LOPA) coefficient at the excitation wavelength, the light beam can penetrate deeply inside the material and be tightly focused into a subwavelength spot, almost the same as in the absence of material. Combining tight focusing and ultralow absorption conditions, we show that LOPA-based microscopy is thus capable of three-dimensional imaging and fabrication with long penetration depth up to 300 μm. As compared to the commonly used two-photon absorption microscope, the LOPA method allows simplification of the experimental setup and also minimization of the photodamaging or bleaching effect of materials.