Dip-In Photoresist for Photoinhibited Two-Photon Lithography to Realize High-Precision Direct Laser Writing on Wafer

Universal Intermolecular Energy Transfer Strategy for Extending Initiator Libraries of Photoinhibited Multiphoton Lithography

Photoinhibited multiphoton lithography allows for high-precision nano additive manufacturing of arbitrary three-dimensional (3D) microstructures. However, its application is severely limited by the scarcity of effective initiators, as most conventional multiphoton lithography initiators lack photoinhibition capacity due to unavoidable single-photon absorption. Herein, we report a universal intermolecular energy transfer strategy that can deplete S1 state initiator molecules excited by single-photon absorption, thereby mitigating single-photon absorption-induced unwanted photopolymerization and shifting the role of the initiator from photopromotion to photoinhibition. As a result, the initiators for photoinhibited multiphoton lithography can be easily extended. Utilizing multispectral analysis and density functional theory calculations, we have elucidated the underlying mechanisms of photopromotion to photoinhibition by exploring intramolecular charge transfer and intermolecular energy transfer. This strategy has been validated with a range of commercial initiators, showcasing its versatility and effectiveness. Furthermore, the resolution of two-dimensional (2D) and 3D structures created through this strategy has been greatly improved while maintaining a high writing speed. These advancements have propelled the field of high-precision nano additive manufacturing, opening up new possibilities for the fabrication of intricate nanostructures.

Three-Dimensional High-Resolution Laser Lithography of CsPbBr3 Quantum Dots in Photoresist with Sub-100 nm Feature Size

Perovskite quantum dots (PQDs), with their excellent optical properties, have become a leading semiconductor material in the field of optoelectronics. However, to date, it has been a challenge to achieve the three-dimensional high-resolution patterning of perovskite quantum dots. In this paper, an in situ femtosecond laser-direct-writing technology was demonstrated for three-dimensional high-resolution patterned CsPbBr3 PQDs using a two-photon photoresist nanocomposite doped with the CsPbBr3 perovskite precursor. By adjusting the laser processing parameters, the minimum line width of the PQDs material was confirmed to be 98.6 nm, achieving a sub-100 nm PQDs nanowire for the first time. In addition, the fluorescence intensity of the laser-processed PQDs can be regulated by the laser power. Our findings provide a new technology for fabricating high-resolution display devices based on laser-direct-writing CsPbBr3 PQDs materials.

Recent Advances in 3D Printed Electrodes - Bridging the Nano to Mesoscale

3D architected electrodes offer inherent physicochemical advantages for energy storage, conversion, and sensing. 3D printing methods such as stereolithography and two photon polymerization are uniquely capable of fabricating these architected electrodes with a high degree of geometric complexity impossible to achieve with other methods at the mesoscale (10 µm-1 mm). The material set for 3D printing traditionally is focused on structural materials rather than functional materials suitable for electronic and electrochemical applications. In this review the fundamental challenges are considered for transforming 3D printed materials into conductive, multifunctional electrodes suitable for electrical and electrochemical devices by printing nanocomposites, infusing molecular precursors and post-processing these structures via carbonization. To understand the design of 3D electrodes toward their use in both sensors and electrochemical devices such as catalysts, this review summarizes recent advances in hierarchical design of porous metastructures, the engineering of mass transport and electronic transport in 3D structures, and the application of high-throughput materials design by machine learning and artificial intelligence. These emerging approaches to 3D electrode design and architecture promise to expand the capabilities of additive manufacturing beyond structural materials and bring its advantages to bear on modern devices such as sensors, batteries, supercapacitors, and electrocatalysts.

Ultra-high precision nano additive manufacturing of metal oxide semiconductors via multi-photon lithography

As a basic component of the versatile semiconductor devices, metal oxides play a critical role in modern electronic information industry. However, ultra-high precision nanopatterning of metal oxides often involves multi-step lithography and transfer process, which is time-consuming and costly. Here, we report a strategy, using metal-organic compounds as solid precursor photoresist for multi-photon lithography and post-sintering, to realize ultra-high precision additive manufacturing of metal oxides. As a result, we gain metal oxides including ZnO, CuO and ZrO2 with a critical dimension of 35 nm, which sets a benchmark for additive manufacturing of metal oxides. Besides, atomic doping can be easily accomplished by including the target element in precursor photoresist, and heterogeneous structures can also be created by multiple multi-photon lithography, allowing this strategy to accommodate the requirements of various semiconductor devices. For instance, we fabricate an ZnO photodetector by the proposed strategy.

Direct Laser Writing of Micro-Nano Filters Based on Three-Photon Polymerization

Direct laser writing (DLW) enables the manufacturing of functional quantum dot (QD)-polymer nanostructures with the special performance desired for technological applications. However, most papers fabricate the QD-polymer photoresist based on the principle of two-photon polymerization using laser wavelengths of 750-800 nm, which cannot effectively fabricate the near-infrared QD-polymer photoresist with absorption wavelengths above 800 nm due to linear absorption. Moreover, most papers report a relatively low doping concentration of QDs. To address these issues, this study introduces three-photon DLW technology using a near-infrared 1035 nm laser to effectively avoid the linear absorption of the near-infrared PbS/CdS QD-polymer photoresist. Three kinds of QD-polymer photoresists with concentrations up to 150 mg mL-1 are prepared through surface modification of QDs. We demonstrate that three-photon DLW is feasible to fabricate high-concentration QD-polymer photoresist to produce micro/nano high-performance QD-polymer filters of visible and near-infrared light absorption. This study provides materials and process guidance for the fabrication and application of visible and near-infrared optical filters through three-photon DLW processing of various kinds of functional nanoparticles-polymer photoresist.

Light and matter co-confined multi-photon lithography

Mask-free multi-photon lithography enables the fabrication of arbitrary nanostructures low cost and more accessible than conventional lithography. A major challenge for multi-photon lithography is to achieve ultra-high precision and desirable lateral resolution due to the inevitable optical diffraction barrier and proximity effect. Here, we show a strategy, light and matter co-confined multi-photon lithography, to overcome the issues via combining photo-inhibition and chemical quenchers. We deeply explore the quenching mechanism and photoinhibition mechanism for light and matter co-confined multiphoton lithography. Besides, mathematical modeling helps us better understand that the synergy of quencher and photo-inhibition can gain a narrowest distribution of free radicals. By using light and matter co-confined multiphoton lithography, we gain a 30 nm critical dimension and 100 nm lateral resolution, which further decrease the gap with conventional lithography.

Sub-diffraction optical beam lithography based on a center-non-zero depletion laser

Photoinhibition (PI) mechanisms have been introduced in nanofabrication which allows breaking the diffraction limit by large factors. Donut-shaped laser is usually selected as a depletion beam to reduce linewidth, but the parasitic process has made the results of the experiment less than expected. As a result, the linewidth is difficult to achieve below 50 nm with 780 nm femtosecond and 532 nm continuous-wave lasers. Here, we propose a new, to the best of our knowledge, method based on a center-non-zero (CNZ) depletion laser to further reduce linewidth. By constructing a smaller zone of action under the condition of keeping the maximum depletion intensity constant, a minimum linewidth of 30 nm (λ / 26) was achieved. Two ways to construct CNZ spots were discussed and experimented, and the results show the advantages of our method to reduce the parasitic process to further improve the writing resolution.

High NA and polarization-insensitive ultra-broadband achromatic metalens from 500 to 1050 nm for multicolor two-photon endomicroscopy imaging

Multicolor two-photon endomicroscopy has become a highly competitive tool for functional imaging in biomedical researches. However, to make the imaging system miniature and applicable for freely behaving animal brain activity, metalenses have received much attention in compact imaging systems. For high resolution multicolor imaging and maximizing fluorescence collection, there is a challenge metalenses faced to achieve large numerical aperture (NA) and focus the NIR excitation and VIS emission lights of multiple fluorophores to the same distance simultaneously because of the limitation of the group delay range of the meta-units. In this paper, we proposed a high NA and polarization-insensitive ultra-broadband achromatic metalens specifically for achromatically focusing the excitation and emission light of multiple fluorophores commonly used in neuroscience studies. TiO2 and Si meta-unit libraries composed of heights, widths and the corresponding phase and group delay were constructed, and the optimal meta-units were selected by particle swarm optimization algorithm to engineer the dispersion of metalens in the VIS band and NIR band, respectively. Combining dispersion engineering with spatial multiplexing, the proposed metalens achieved the maximal effective NA up to 0.8 and large achromatic bandwidth ranging from 500 nm to 1050 nm, which exhibited the coefficient of variation of focal lengths was only 3.41%. The proposed achromatic metalens could successfully achromatically focus different fluorescence with any polarization, which was suitable for most fluorophores. Our results firmly establish that the proposed metalens can open the door to high resolution and minimally invasive multicolor two-photon functional imaging in intravital deep brain.

High-resolution 3D nanoprinting based on two-step absorption via an integrated fiber-coupled laser diode

Three-dimensional (3D) laser nanoprinting with high resolution and low cost is highly desirable for fabricating arbitrary 3D structures with fine feature size. In this work, we use a 405-nm integrated fiber-coupled continuous wave (cw) laser diode to establish an easy-to-build 3D nanoprinting system based on two-step absorption. Two-dimensional (2D) gratings with a sub-150-nm period and 3D woodpile nanostructures with a lateral period of 350 nm have been printed at a low speed. At a faster scan velocity of 1000 µm/s, 2D gratings with sub-200-nm resolution and sub-50-nm linewidth can still be fabricated with laser power less than 1 mW. The two-step absorption of the used benzil initiator enables us to use a second cw laser with 532-nm wavelength to enhance the polymerization with sub-100-nm feature size when starting with insufficient 405-nm laser power, which possess the potential to find applications in high-speed high-resolution parallel-writing and in situ manipulation.

High-Precision and Rapid Direct Laser Writing Using a Liquid Two-Photon Polymerization Initiator

Two-photon polymerization based direct laser writing (DLW) is an emerging micronano 3D fabrication technology wherein two-photon initiators (TPIs) are a key component in photoresists. Upon exposure to a femtosecond laser, TPIs can trigger the polymerization reaction, leading to the solidification of photoresists. In other words, TPIs directly determine the rate of polymerization, physicochemical properties of polymers, and even the photolithography feature size. However, they generally exhibit extremely poor solubility in photoresist systems, severely inhibiting their application in DLW. To break through this bottleneck, we propose a strategy to prepare TPIs as liquids via molecular design. The maximum weight fraction of the as-prepared liquid TPI in photoresist significantly increases to 2.0 wt %, which is several times higher than that of commercial 7-diethylamino-3-thenoylcoumarin (DETC). Meanwhile, this liquid TPI also exhibits an excellent absorption cross section (64 GM), allowing it to absorb femtosecond laser efficiently and generate abundant active species to initiate polymerization. Remarkably, the respective minimum feature sizes of line arrays and suspended lines are 47 and 20 nm, which are comparable to that of the-state-of-the-art electron beam lithography. Besides, the liquid TPI can be utilized to fabricate various high-quality 3D microstructures and manufacture large-area 2D devices at a considerable writing speed (1.045 m s^-1). Therefore, the liquid TPI would be one of the promising initiators for micronano fabrication technology and pave the way for future development of DLW.

Low-Fluorescence Starter for Optical 3D Lithography of Sub-40 nm Structures

Stimulated emission depletion (STED) has been used to break the diffraction limit in fluorescence microscopy. Inspired by this success, similar methods were used to reduce the structure size in three-dimensional, subdiffractional optical lithography. So far, only a very limited number of radical polymerization starters proved to be suitable for STED-inspired lithography. In this contribution, we introduce the starter Michler's ethyl ketone (MEK), which has not been used so far for STED-inspired lithography. In contrast to the commonly used 7-diethylamino-3-thenoylcoumarin (DETC), nanostructures written with MEK show low autofluorescence in the visible range. Therefore, MEK is promising for being used as a starter for protein or cell scaffolds in physiological research because the autofluorescence of DETC so far excluded the use of the green emission channel in multicolor fluorescence or confocal microscopy. In turn, because of the weak transitions of MEK in the visible spectrum, STED, in its original sense, cannot be applied to deplete MEK in the outer rim of the point spread function. However, a 660 nm laser can be used for depletion because this wavelength is well within the absorption spectrum of transient states, possibly of triplet states. We show that polymerization can be fully stopped by applying transient state absorption at 660 nm and that structure sizes down to approx. 40 nm in the lateral and axial directions can be achieved, which means 1/20 of the optical wavelength used for writing.

Efficient fabrication method for non-periodic microstructures using one-step two-photon lithography and a metal lift-off process

This paper presents a mask-less, flexible, efficient, and high-resolution fabrication method for non-periodic microstructures. Sub-wavelength micro-polarizer arrays, (MPAs) which are the most essential part of the focal plane polarimeters, are typical non-periodic structures. The grating ridges of each polarizer were oriented in four different directions offset by 45°, corresponding to different polarization directions. The finite element method was introduced to optimize the structural parameters of the MPA in the far-infrared region. The numerical results demonstrated that the designed MPA had a TM transmittance of more than 55% and an extinction ratio no less than 7 dB. An aluminum MPA that operates in the 8-14 µm infrared region was prepared by one-step two-photon lithography (TPL) and the metal lift-off process. The femtosecond laser exposed the photoresist with only a single scan, making TPL very efficient. The fabricated single-layer sub-wavelength MPAs with a period of 3 µm, a duty cycle of 0.35-0.5, and a height of 150 nm, were analyzed by an optical microscope and an atomic force microscope. The successful fabrication of the MPA indicated that one-step TPL could be a viable and efficient method for pattern preparation in the fabrication of non-periodic microstructures.

A Power Compensation Strategy for Achieving Homogeneous Microstructures for 4D Printing Shape-Adaptive PNIPAM Hydrogels: Out-of-Plane Variations

In the last decade, 3D printing has attracted significant attention and has resulted in benefits to many research areas. Advances in 3D printing with smart materials at the microscale, such as hydrogels and liquid crystalline polymers, have enabled 4D printing and various applications in microrobots, micro-actuators, and tissue engineering. However, the material absorption of the laser power and the aberrations of the laser light spot will introduce a decay in the polymerization degree along the height direction, and the solution to this problem has not been reported yet. In this paper, a compensation strategy for the laser power is proposed to achieve homogeneous and high aspect ratio hydrogel structures at the microscale along the out-of-plane direction. Linear approximations for the power decay curve are adopted for height steps, discretizing the final high aspect ratio structures. The strategy is achieved experimentally with hydrogel structures fabricated by two-photon polymerization. Moreover, characterizations have been conducted to verify the homogeneity of the printed microstructures. Finally, the saturation of material property is investigated by an indirect 3D deformation method. The proposed strategy is proved to be effective and can be explored for other hydrogel materials showing significant deformation. Furthermore, the strategy for out-of-plane variations provides a critical technique to achieve 4D-printed homogeneous shape-adaptive hydrogels for further applications.