Laser Nanolithography Research Articles

Single‐Step 3D Printing of Micro‐Optics with Adjustable Refractive Index by Ultrafast Laser Nanolithography

AbstractLaser exposure defines voxel's dimensions as essential building blocks in direct write 3D nanolithography. However, the exposure conditions not only influence the size of the produced features but also their optical properties. This empowers the realization of an adjustable refractive index out of single material by varying the writing strategy while preserving laser 3D nanolithography's flexibility in geometry and high resolution. Here, the refractive index for the 450–1600 nm spectral range of the micro‐optics out of SZ2080 hybrid polymer is systematically studied by applying ray and wave optics approaches followed by optical resolution analysis. It reveals the exact value of the laser‐printed components instead of the determination assessed by other techniques measuring thin films or bulky volumes of the investigated substance. The studied micro‐lenses are of below 100 µm in dimensions and a clear distinction in their performance on low and high exposure doses is found by analyzing it in all different approaches and validating using different lithography setups. Findings reveal the complexity of the refractive index of the 3D micro‐optics which is influenced by the material density and morphology. A route for freedom in 3D printing shape and refractive index can be realized by the technological optimization of delicate exposure control in ultrafast laser nanolithography.

Microsphere femtosecond laser sub-50 nm structuring in far field via non-linear absorption

Creation of arbitrary features with high resolution is critically important in the fabrication of nano-optoelectronic devices. Here, sub-50 nm surface structuring is achieved directly on Sb₂S₃ thin films via microsphere femtosecond laser irradiation in far field. By varying laser fluence and scanning speed, nano-feature sizes can be flexibly tuned. Such small patterns are attributed to the co-effect of microsphere focusing, two-photons absorption, top threshold effect, and high-repetition-rate femtosecond laser-induced incubation effect. The minimum feature size can be reduced down to ~30 nm (<italic>λ</italic>/26) by manipulating film thickness. The fitting analysis between the ablation width and depth predicts that the feature size can be down to ~15 nm at the film thickness of ~10 nm. A nano-grating is fabricated, which demonstrates desirable beam diffraction performance. This nano-scale resolution would be highly attractive for next-generation laser nano-lithography in far field and in ambient air.

Super-regular femtosecond laser nanolithography based on dual-interface plasmons coupling

Abstract Advances in femtosecond laser-material interaction facilitate the extension of maskless optical processing to the high efficiency and deep-subwavelength scale. Here, a hybrid plasmon lithography technique has been demonstrated by irradiating near-infrared femtosecond laser pulses onto the Si material coated with thin Cr films in a vacuum chamber, and superior nanograting structures are found to deeply penetrate through the thin Cr film into the underlying Si substrate. In stark contrast to the common ripple structures formed on the Si surface, the Cr-layer mediated Si nanograting structures not only exhibit the spatially super-regular arrangements with a deep-subwavelength period of 355 nm but also present the nonsinusoidal sharp-edged groove geometry with a large depth-to-width aspect ratio of 2.1. Theoretical analyses and calculations reveal that the anomalous structure characterizations are physically ascribed to the excitation of dual-interface coupled plasmons in the thin metal layer, which possess the squeezed spatial wavelength and the periodic columnar intensity distributions. Moreover, the further deepening of periodic nanostructures into the Si substrate is also elucidated by the simulation of electric field enhancements at the bottom of shallow grooves under irradiation of subsequent laser pulses. In combination with a wet etching process, the Si nanograting structures can be modified into the smooth and narrow-mouthed V-profiles, whose optical measurements show a near omnidirectional antireflection especially in the visible range of 565–750 nm, which is expected for the design of advanced photonic devices.

High-efficiency-and-quality nanostructuring of molybdenum surfaces by orthogonally polarized blue femtosecond lasers

Femtosecond laser-induced periodic surface structures featured by a mask-free and one-step procedure provide a new fascinating prospect for the high precision manufacture applicable to diverse materials, but still face challenges in both the fabrication efficiency and the structure quality. Here, a homogenous large-area nanolithography is implemented on molybdenum surface, through using cylindrical focusing of double temporally delayed blue femtosecond lasers with orthogonal polarizations. The achieved grating structures present some unique features including the ablated narrow groove width of 100 nm, the small period of 245 nm, the smooth ridge profiles, and especially the uniform distribution in a mm-scaled range without bending, spitting and interruption. The calculated structure orientation angle shows an unprecedentedly minimum dispersion value of 5°, indicting a significant improvement on the structure uniformity compared with the observations of the single-beam femtosecond laser irradiation. The simulation results reveal that the transient spatiotemporal correlations between the localized electromagnetic field distributions from the double orthogonally polarized laser pulses constitutes a positive feedback mechanism to regularize the growth of the surface structures. A macro-sized surface pattern of such uniform structures displays good monochromatic colors and excellent broadband antireflection performances. This efficient laser nanolithography facilitates the applications in the fields of nanooptics.

A Bio-Based Resin for a Multi-Scale Optical 3D Printing

Materials obtained from renewable sources are emerging to replace the starting materials of petroleum-derived plastics. They offer easy processing, fulfill technological, functional and durability requirements at the same time ensuring increased bio-compatibility, recycling, and eventually lower cost. On the other hand, optical 3D printing (O3DP) is a rapid prototyping tool (and an additive manufacturing technique) being developed as a choice for efficient and low waste production method, yet currently associated with mainly petroleum-derived resins. Here we employ a single bio-based resin derived from soy beans, suitable for O3DP in the scales from nano- to macro-dimensions, which can be processed even without the addition of photoinitiator. The approach is validated using both state-of-the art laser nanolithography setup as well as a widespread table-top 3D printer - sub-micrometer accuracy 3D objects are fabricated reproducibly. Additionally, chess-like figures are made in an industrial line commercially delivering small batch production services. Such concept is believed to make a breakthrough in rapid prototyping by switching the focus of O3DP to bio-based resins instead of being restricted to conventional petroleum-derived photopolymers.

Prospects for creating a laser nanolithography system for tasks of diffractive optics and nanophotonics

This paper considers the basic concepts of constructing an advanced direct laser writing nanolithographic system dedicated to a high-resolution synthesis of large-sized planar elements of diffractive optics and nanophotonics. The issues of the rational implementation of optical, mechanical and electronic units of the system are solved together with considering the functional structure for a precise computer control of the mentioned units. The proposed design of the opto-mechanical unit aims to reduce significantly the distortions of fabrication of the optical elements due to the use of a new symmetrical two-channel construction and a differential laser interferometer for the radial positioning of a focused laser beam. A complex of issues on constructing a long-working distance superresolving focusing system are also considered alongside. The complex modernization of the nanolithographic system software and hardware complex based on system-on-chip (SoC) using hybrid processors (FPGA+ARM) is considered, which will make the system more universal and portable to various IT platforms.

Maskless laser nano-lithography of glass through sequential activation of multi-threshold ablation

Controllable nanofabrication is at the very foundation of nano-science and nano-technology. Today, ultrafast laser writing has been broadly adopted for micro-fabrication because of its ability to make precise and rapid processing of almost all types of materials in an ambient environment. However, direct laser writing is typically unsuitable for high-quality 2D nano-patterning. In this work, we introduce a maskless laser nano-lithographic technique that allows us to create regular 2D periodic nanopatterns on glass. Glass is a particularly challenging material since it does not absorb light readily. Our strategy starts with a glass sample being coated with a thin layer of metal, and then irradiated with a series of pulse bursts at progressively increasing fluence levels. This process allows us to sequentially activate a series of tailored physical processes that lead to the formation of regular 2D periodic nanopatterns on glass. The formation mechanism of this nano-patterning is also simulated numerically and further corroborated by a series of control experiments. We also show controllability in forming various shapes and sizes of nanopatterns through tailored fluence doses. Our technique provides a high-speed and low-cost method for glass nanofabrication.

Three-dimensional femtosecond laser nanolithography of crystals

Nanostructuring hard optical crystals has so far been exclusively feasible at their surface, as stress induced crack formation and propagation has rendered high precision volume processes ineffective. We show that the inner chemical etching reactivity of a crystal can be enhanced at the nanoscale by more than five orders of magnitude by means of direct laser writing. The process allows to produce cm-scale arbitrary three-dimensional nanostructures with 100 nm feature sizes inside large crystals in absence of brittle fracture. To showcase the unique potential of the technique, we fabricate photonic structures such as sub-wavelength diffraction gratings and nanostructured optical waveguides capable of sustaining sub-wavelength propagating modes inside yttrium aluminum garnet crystals. This technique could enable the transfer of concepts from nanophotonics to the fields of solid state lasers and crystal optics.

Pore Shape Defines Paths of Metastatic Cell Migration.

Invasion of dense tissues by cancer cells involves the interplay between the penetration resistance offered by interstitial pores and the deformability of cells. Metastatic cancer cells find optimal paths of minimal resistance through an adaptive path-finding process, which leads to successful dissemination. The physical limit of nuclear deformation is related to the minimal cross section of pores that can be successfully penetrated. However, this single biophysical parameter does not fully describe the architectural complexity of tissues featuring pores of variable area and shape. Here, employing laser nanolithography, we fabricate pore microenvironment models with well-controlled pore shapes, through which human breast cells (MCF10A) and their metastatic offspring (MCF10CA1a.cl1) could pervade. In these experimental settings, we demonstrate that the actual pore shape, and not only the cross section, is a major and independent determinant of cancer penetration efficiency. In complex architectures containing pores demanding large deformations from invading cells, tall and narrow rectangular openings facilitate cancer migration. In addition, we highlight the characteristic traits of the explorative behavior enabling metastatic cells to identify and select such pore shapes in a complex multishape pore environment, pinpointing paths of least resistance to invasion.

Thermoelectric Energy Harvesting Research at the Assist NSF Nano-Systems Engineering Research Center

Advanced Self-Powered Systems of Sensors and Technologies (ASSIST) is a Nano-Systems Engineering Research Center (NERC) sponsored by the National Science Foundation. The center uses nano-enabled energy harvesting, energy storage, nanodevices and sensors to create innovative battery-free, body-powered, wearable, health monitoring systems.The thermoelectricseffort focuses on combining nanostructured, high figure of merit materials with advances in material and device design to minimize parasitic electrical and thermal resistances. The three key challenges are: 1) Materials with high figures of merit, ZT; 2) Flexible yet efficient packaging that reduces the parasitic thermal resistances at the skin / device and device / ambient interfaces; 3) A device with a small form factor that is comfortable to wear.The experimental work is supported by a modeling effort that focuses on achieving comprehensive understanding of fundamental electrical and thermal transport properties of lead and bismuth chalcogenide heterostructures. The theoretical treatment starts from a first principles approach based on density functional theory to extract the energy band details of electrons and phonons as well as the interaction between them in the bulk materials and in nanostructures. Then, the macroscopic electrical and thermal transport properties are evaluated by solving the Boltzmann transport equation in the diffusive regime, including those for the structures with dimensional constraints. A Monte Carlo approach is chosen to be used for both electrons and phonons that can accurately describe nonlinear effects. The key focus is placed on the nanoscaled superlattices as well as more complex quantum well quantum wires.Different integration approaches are being considered including a) integration of epitaxial materials using conventional pick-and-place tooling and thermal compression bonding into a flexible polyamide package and b) electrochemical deposition of thermoelectric materials within the flexible package.It is essential for the flexible package to conduct heat extremely well. Furthermore, the package has to be stable up to at least 300oC needed for solid diffusion bonding of the thermoelectric legs. In our work, we are exploring polyamide with embedded metal studs (Figure 1) to satisfy both the flexibility and temperature stability requirements.The small form factor requires a small and flexible heatsink to effectively dissipate the heat via convection. Together with the thermal resistance of the skin, the thermal resistance of the heat sink limits the achievable temperature difference across the thermoelectric device. As shown in Figure 2, the performance of a generator capable of delivering 1 mW output power can be severely hampered by these thermal resistances. The calculations were carried out for heat sink and skin conductance ranges of 20-500 and 20-100 W/m2K respectively assuming a device thermal resistance of ~500 K/W. The total device area is 4 cm2. We are exploring millimeter tall carbon nanotubes to construct highly efficient, small-profile heatsinks (Figure 3). The advantages of CNTs include their large surface area, high thermal conductivity and their ability to grow as fins on patterned catalysts.Another effort in the center is aiming at passive heat pipes using novel superhydrophilic hierarchical nanostructures. The goal is to enable a flexible, integrated two-phase heat transfer device to effectively transport heat to and from the thermoelectric generator (TEG). Heat pipes can offer thermal conductivity figures that are orders of magnitude higher than those offered by even the best metals. The approach is based on surface nanostructures with two-tier length scales to design both the fluid transport via improved wicking and thermal transfer properties independently. The devices are fabricated using laser nanolithography, atomic layer deposition, and nanowire synthesis. The key challenges are optimizing viscous fluid friction and thermal contact area in the nanostructures, as well as device packaging using thin metals sheets into a flexible platform.

Применение лазеров в нанотехнологии: получение наночастиц и наноструктур методами лазерной абляции и лазерной нанолитографии

Применение лазеров в нанотехнологии: получение наночастиц и наноструктур методами лазерной абляции и лазерной нанолитографии

Array-based optical nanolithography using optically trapped microlenses

Current demands on optical nanolithography require the ability to rapidly and cost-effectively write arbitrary patterns over large areas with sub-diffraction limit feature sizes. The challenge in accomplishing this with arrays of near-field probes is maintaining equal separations between the substrate and each probe, even over non-planar substrates. Here we demonstrate array-based laser nanolithography where each probe is a microsphere capable of fabricating 100 nm structures using 355 nm light when self-positioned near a surface by Bessel beam optical trapping. We achieve both a feature size uniformity and relative positioning accuracy better than 15 nm, which agrees well with our model. Further improvements are possible using higher power and/or narrower Bessel beam optical traps.

Apertureless near-field optics on commercial AFM: Tip to sample gap control

Novel mode of AFM operation is proposed providing the small, few nanometers tip to sample gap, appropriate for the ANSOM experiments. A set-up open for the run-time adjustments, working at ambient conditions is considered. Efficiency of a method is demonstrated by applying it to the laser nanolithography on different materials with a regular AFM tip.