Craig B. Arnold

Subwavelength direct-write nanopatterning using optically trapped microspheres

A number of non-lithographic techniques are now available for processing materials on the nanoscale, including optical techniques capable of producing features that are much smaller than the wavelength of light used. However, these techniques can be limited in speed, ease of use, cost of implementation, or the range of patterns they can write. Here we report how Bessel beam laser trapping of microspheres near surfaces can be used to enable near-field direct-write subwavelength nanopatterning. Using the microsphere as an objective lens to focus the processing laser, we demonstrate arbitrary patterns and individual features with minimum sizes of approximately 100 nm (which is less than one-third the processing wavelength) and a positioning accuracy better than 40 nm in aqueous and chemical environments. Submicron spacing is maintained between the near-field objective and the substrate without active feedback control. If implemented with an array of optical traps, this approach could lead to a high-throughput probe-based method for patterning surfaces with subwavelength features.

Thick film laser induced forward transfer for deposition of thermally and mechanically sensitive materials

Laser forward transfer processes incorporating thin absorbing films can be used to deposit robust organic and inorganic materials but the deposition of more delicate materials has remained elusive due to contamination and stress induced during the transfer process. Here, we present the approach to high resolution patterning of sensitive materials by incorporating a thick film polymer absorbing layer that is able to dissipate shock energy through mechanical deformation. Multiple mechanisms for transfer as a function of incident laser energy are observed and we show viable and contamination-free deposition of living mammalian embryonic stem cells.

Fundamentals of Laser-Material Interaction and Application to Multiscale Surface Modification

Lasers provide the ability to accurately deliver large amounts of energy into confined regions of a material in order to achieve a desired response. For opaque materials, this energy is absorbed near the surface, modifying surface chemistry, crystal structure, and/or multiscale morphology without altering the bulk. This chapter covers a brief introduction to the fundamental principles governing laser propagation and absorption as well as the resulting material responses. We then highlight two case studies of improving efficiency in photovoltaic and optoelectronic devices as well as optimizing cell-surface interactions in biological interfaces.

Ultrastable nanostructured polymer glasses

Owing to the kinetic nature of the glass transition, the ability to significantly alter the properties of amorphous solids by the typical routes to the vitreous state is restricted. For instance, an order of magnitude change in the cooling rate merely modifies the value of the glass transition temperature (T(g)) by a few degrees. Here we show that matrix-assisted pulsed laser evaporation (MAPLE) can be used to form ultrastable and nanostructured glassy polymer films which, relative to the standard poly(methyl methacrylate) glass formed on cooling at standard rates, are 40% less dense, have a 40 K higher T(g), and exhibit a two orders of magnitude enhancement in kinetic stability at high temperatures. The unique set of properties of MAPLE-deposited glasses may make them attractive in technologies where weight and stability are central design issues.

Laser processing of nanocrystalline TiO2 films for dye-sensitized solar cells

Pulsed-laser deposition and laser direct-write have been applied to deposit dense (30nm thick) and porous nanocrystalline TiO2 (nc-TiO2, 5–20μm thick) layers incorporated in dye-sensitized solar cells. Laser direct-write is a laser-induced forward transfer technique that enables the fabrication of conformal structures containing metals, ceramics, polymers, and composites on rigid and flexible substrates without the use of masks or additional patterning steps. A pulsed UV laser (355nm) was used to forward transfer a suspension of TiO2(P25) nanopowder onto a F-doped SnO2 coated glass substrate. In this letter we demonstrate the use of laser transfer techniques to produce porous nc-TiO2 films required for dye-sensitized solar cells. The dye solar cells fabricated with the laser processed TiO2 layers on glass showed a power conversion efficiency of ∼4.3% under an illumination of 10mW∕cm2.

Turbulence measurements using a nanoscale thermal anemometry probe

A nanoscale thermal anemometry probe (NSTAP) has been developed to measure velocity fluctuations at ultra-small scales. The sensing element is a free-standing platinum nanoscale wire, 100 nm × 2 µm × 60 µm, suspended between two currentcarrying contacts and the sensor is an order of magnitude smaller than presently available commercial hot wires. The probe is constructed using standard semiconductor and MEMS manufacturing methods, which enables many probes to be manufactured simultaneously. Measurements were performed in grid-generated turbulence and compared to conventional hot-wire probes with a range of sensor lengths. The results demonstrate that the NSTAP behaves similarly to conventional hot-wire probes but with better spatial resolution and faster temporal response. The results are used to investigate spatial filtering effects, including the impact of spatial filtering on the probability density of velocity and velocity increment statistics.

High-speed varifocal imaging with a tunable acoustic gradient index of refraction lens

Fluidic lenses allow for varifocal optical elements, but current approaches are limited by the speed at which focal length can be changed. Here we demonstrate the use of a tunable acoustic gradient (TAG) index of refraction lens as a fast varifocal element. The optical power of the TAG lens varies continuously, allowing for rapid selection and modification of the effective focal length at time scales of 1 mus and shorter. The wavefront curvature applied to the incident light is experimentally quantified as a function of time, and single-frame imaging is demonstrated. Results indicate that the TAG lens can successfully be employed to perform high-rate imaging at multiple locations.

Laser processing of polymer thin films for chemical sensor applications

Contemporary and next-generation commercial and defense-related platforms offer countless applications for thin-film polymer coatings, including the areas of microelectronics, optoelectronics, and miniature chemical and biological sensors. In many cases, the compositional and structural complexity, and the anisotropy of the material properties preclude the processing of many of these polymers by conventional physical or chemical vapor deposition methods. The Naval Research Laboratory has developed several advanced laser-based processing techniques for depositing polymer thin films for a variety of structures and devices. The two techniques detailed in this work, matrix-assisted pulsed laser evaporation (MAPLE) and MAPLE direct-write (MAPLE DW), are based on the concept of laser absorption by a matrix solution consisting of a solvent and the desired polymer. MAPLE is a physical vapor deposition process that takes place inside a vacuum chamber, while MAPLE DW is a laser forward-transfer process that is carried out under atmospheric conditions. Both processes have been successfully used in the fabrication of thin films and structures of a range of organic materials and systems. Examples of their use in the fabrication of two types of chemical sensors, together with a comparison of the performance of these laser-processed sensors and that of similar sensors made by traditional techniques are provided.

Time-resolved study of polyimide absorption layers for blister-actuated laser-induced forward transfer

Blister-actuated laser-induced forward transfer (BA-LIFT) is a versatile, direct-write process capable of printing high-resolution patterns from a variety of sensitive donor materials without damage to their functionality. In this work, we use time-resolved imaging to study the laser-induced formation of blisters on polyimide films in order to understand and optimize their role in BA-LIFT. We find that the initial blister expansion occurs very rapidly (<100 ns), followed by a brief oscillation (100–500 ns), and then a longer time contraction to steady-state dimensions (0.5–50 μs). This behavior is explained by kinetic and thermal effects that occur during the process. We further probe the influence of polyimide thickness, laser beam diameter, and laser fluence on blister formation characteristics. Results indicate that the presence of a thin layer of donor material on the polyimide surface does not have a significant effect on the size and shape of the blisters which form.

Solution-processed chalcogenide glass for integrated single-mode mid-infrared waveguides.

Chalcogenide glass materials exhibit a variety of optical properties that make them desirable for near- and mid-infrared communications and sensing applications. However, processing limitations for these photorefractive materials have made the direct integration of waveguides with sources or detectors challenging. Here we demonstrate the viability of two complementary soft lithography methods for patterning and integrating chalcogenide glass waveguides from solution. One method, micro-molding in capillaries (MIMIC), is shown to fabricate multi-mode As(2)S(3) waveguides which are directly integrated with quantum cascade lasers (QCLs). In a second method, we demonstrate the ability of micro-transfer molding (µTM), to produce arrays of single mode rib waveguides (2.5 µm wide and 4.5 µm high) over areas larger than 6 cm(2) while maintaining edge roughness below 5.1 nm. These methods form a suite of processes that can be applied to chalcogenide solutions to create a diverse array of mid-IR optical and photonic structures ranging from <5 to 10s of µm in dimension.