H. H. Gandhi, E. Mazur, K. Phillips, and S. K. Sundaram. 2015. “Ultrafast Laser Processing Of Materials: A Review.” Advances In Optics and Photonics, 7, Pp. 684–712. Publisher's VersionAbstract
    We present an overview of the different processes that can result from focusing an ultrafast laser light in the femtosecond–nanosecond time regime on a host of materials, e.g., metals, semiconductors, and insulators. We summarize the physical processes and surface and bulk applications and highlight how femtosecond lasers can be used to process various materials. Throughout this paper, we will show the advantages and disadvantages of using ultrafast lasers compared with lasers that operate in other regimes and demonstrate their potential for the ultrafast processing of materials and structures.
    S. Kang, K. Vora, and E. Mazur. 2015. “One-step direct-laser metal writing of sub-100 nm 3D silver nanostructures in a gelatin matrix.” Nanotechnology, 26, Pp. 1–6. Publisher's VersionAbstract
    Developing an ability to fabricate high-resolution, 3D metal nanostructures in a stretchable 3D matrix is a critical step to realizing novel optoelectronic devices such as tunable bulk metaldielectric optical and THz metamaterial devices that are not feasible with alternate techniques. We report a new chemistry method to fabricate high-resolution, 3D silver nanostructures using a femtosecond-laser direct metal writing technique. Previously, only fabrication of 3D polymeric structures or single/few-layer metal structures was possible. Our method takes advantage of unique gelatin properties to overcome these previous limitations such as limited freedom in 3D material design and short sample lifetime. We fabricate more than 15 layers of 3D silver nanostructures with a resolution of less than 100 nm in a stable dielectric matrix that is flexible and has high large transparency well- matched for potential applications in the optical and THz metamaterial regimes. This is a single-step process that does not require any further processing. This work will be of interest to those interested in the fabrication methods that utilize nonlinear light-matter interactions and the realization of future metamaterials.
    M. Gerhard Moebius, K. Vora, S. Kang, P. Munoz, G. Deng, and E. Mazur. 2015. “Direct Laser Writing of 3D Gratings and Diffraction Optics.” In . CLEO: Science and Innovations Laser-Induced Structuring in Bulk Material (SW1K). Publisher's VersionAbstract
    We fabricate 3D gratings and diffraction optics using direct laser writing. Diffraction patterns of gratings agree with Laue theory. We demonstrate zone plates for visible wavelengths. Direct laser writing is promising for integrated diffraction optics.
    K. Vora. 2014. “Three-dimensional nanofabrication of silver structures in polymer with direct laser writing”. Publisher's VersionAbstract
    This dissertation describes methodology that significantly improves the state of femtosecond laser writing of metals. The developments address two major shortcomings: poor material quality, and limited 3D patterning capabilities. In two dimensions, we grow monocrystalline silver prisms through femtosecond laser irradiation. We thus demonstrate the ability to create high quality material (with limited number of domains), unlike published reports of 2D structures composed of nanoparticle aggregates. This development has broader implications beyond metal writing, as it demonstrates a one-step fabrication process to localize bottom-up growth of high quality monocrystalline material on a substrate. In three dimensions, we direct laser write fully disconnected 3D silver structures in a polymer matrix. Since the silver structures are embedded in a stable matrix, they are not required to be self-supported, enabling the one-step fabrication of 3D patterns of 3D metal structures that need-not be connected. We demonstrate sub- 100-nm silver structures. This latter development addresses a broader limitation in fabrication technologies, where 3D patterning of metal structures is difficult. We demonstrate several 3D silver patterns that cannot be obtained through any other fabrication technique known to us. We expect these advances to contribute to the development of new devices in optics, plasmonics, and metamaterials. With further improvements in the fabrication methods, the list of potential applications broadens to include electronics (e.g. 3D microelectronic circuits), chemistry (e.g. catalysis), and biology (e.g. plasmonic biosensing).
    K. Vora, S. Kang, M. Gerhard Moebius, and E. Mazur. 2014. “Femtosecond laser direct writing of monocrystalline hexagonal silver prisms.” Appl. Phys. Lett., 105, Pp. 141114–. Publisher's VersionAbstract
    *Kevin Vora and SeungYeon Kang have made equal contributions to the present work. Bottom-up growth methods and top-down patterning techniques are both used to fabricate metal nanostructures, each with a distinct advantage: One creates crystalline structures and the other offers precise positioning. Here, we present a technique that localizes the growth of metal crystals to the focal volume of a laser beam, combining advantages from both approaches. We report the fabrication of silver nanoprisms— hexagonal nanoscale silver crystals—through irradiation with focused femtosecond laser pulses. The growth of these nanoprisms is due to a nonlinear optical interaction between femtosecond laser pulses and a polyvinylpyrrolidone film doped with silver nitrate. The hexagonal nanoprisms have bases hundreds of nanometers in size and the crystal growth occurs over exposure times of less than 1 ms (8 orders of magnitude faster than traditional chemical techniques). Electron backscatter diffraction analysis shows that the hexagonal nanoprisms are monocrystalline. The fabrication method combines advantages from both wet chemistry and femtosecond laser direct-writing to grow silver crystals in targeted locations. The results presented in this letter offer an approach to directly positioning and growing silver crystals on a substrate, which can be used for plasmonic devices.
    G. Obara, H. Shimizu, T. Enami, E. Mazur, M. Terakawa, and M. Obara. 2013. “Growth of high spatial frequency periodic ripple structures on SiC crystal surfaces irradiated with successive femtosecond laser pulses.” Optics Express, 21, Pp. 26323–26334. Publisher's VersionAbstract
    We present experimentally and theoretically the evolution of high spatial frequency periodic ripples (HSFL) fabricated on SiC crystal surfaces by irradiation with femtosecond laser pulses in a vacuum chamber. At early stages the seed defects are mainly induced by laser pulse irradiation, leading to the reduction in the ablation threshold fluence. By observing the evolution of these surface structures under illumination with successive laser pulses, the nanocraters are made by nanoablation at defects in the SiC surface. The Mie scattering by the nanoablated craters grows the periodic ripples. The number of HSFL is enhanced with increasing pulse number. At the edge of the laser spot the Mie scattering process is still dominant, causing the fabrication of HSFL. On the periphery of the spot SiC substrate remains a semiconductor state because the electron density in the SiC induced by laser irradiation is kept low. The HSFL observed is very deep in the SiC surface by irradiating with many laser pulses. These experimental results are well explained by 3D FDTD (three-dimensional finite-difference time- domain) simulation.
    A. Hu, G. Deng, S. Denis Courvoisier, O. Reshef, C. C. Evans, E. Mazur, and Y. Norman. Zhou. 2013. “Femtosecond laser induced surface melting and nanojoining for plasmonic circuits”. Publisher's VersionAbstract
    Femtosecond laser induced nonthermal processing is an emerging nanofabrication technique for delicate plasmonic devices. In this work we present a detailed investigation on the interaction between ultra-short pulses and silver nanomaterials, both experimentally and theoretically. We systematically study the laser-silver interaction at a laser fluent from 1 J/m2 to 1 MJ/m2. The optimal processing window for welding of silver nanowires occurs at fluences of 200-450 J/m2. The femtosecond laser-induced surface melting allows precise welding of silver nanowires for "T” and “X” shape circuits. These welded plasmonic circuits are successfully applied for routining light propagation.
    H. Shimizu, G. Obara, M. Terakawa, E. Mazur, and M. Obara. 2013. “Evolution of Femtosecond Laser-Induced Surface Ripples on Lithium Niobate Crystal Surfaces.” Appl. Physics Express, 6, Pp. 112701-1–3. Publisher's VersionAbstract
    We fabricated periodic ripple structures on the surface of a lithium niobate crystal by irradiation with femtosecond laser pulses and observed the evolution of these structures under irradiation with successive laser pulses. After just a few laser pulses we observed nanorod-shaped craters, aligned with each other but randomly distributed over the surface. The nanocraters are caused by nanoablation at defects in the crystal surface. With increasing pulse number, side-lobed nanocraters appear and light scattered from the initial nanorod- shaped craters at the crystal surface interferes with the incident light, causing the formation of periodic structures.
    K. Vora, S. Kang, S. Shukla, and E. Mazur. 2012. “Fabrication of disconnected three-dimensional silver nanostructures in a polymer matrix.” Appl. Phys. Lett., 100, Pp. 063120–063120-3. Publisher's VersionAbstract
    We present a simple, one-step technique for direct-writing of a structured nanocomposite material with disconnected silver nanostructures in a polymer matrix. A nonlinear optical interaction between femtosecond laser pulses and a composite material creates silver structures that are embedded inside a polymer with submicrometer resolution (300 nm). We create complex patterns of silver nanostructures in three dimensions. The key to the process is the chemical composition of the sample that provides both a support matrix and controlled growth. The technique presented in this letter may offer a cost-effective approach for the fabrication of bulk optical devices with engineered dispersion. Copyright (2012) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
    K. Vora, S. Kang, S. Shukla, and E. Mazur. 2012. “Three-dimensional silver nanostructure fabrication through multiphoton photoreduction.” In . Proceedings of the SPIE. Publisher's VersionAbstract
    Metal nanofabrication techniques have become increasingly important for photonic applications with rapid developments in plasmonics, nanophotonics and metamaterials. While two-dimensional (2D) techniques to create high resolution metal patterns are readily available, it is more difficult to fabricate 3D metal structures that are required for new applications in these fields. We present a femtosecond laser technique for 3D direct-writing silver nanostructures embedded inside a polymer. We induce the photoreduction of silver ions through non-linear absorption in a sample doped with a silver salt. Utilizing nonlinear optical interactions between the chemical precursors and femtosecond pulses, we limit silver-ion photoreduction processes to a focused volume smaller than that of the diffraction-limit. The focal volume is scanned rapidly in 3D by means of a computer-controlled translation stage to produce complex patterns. Our technique creates dielectric-supported silver structures, enabling the nanofabrication of silver patterns with disconnected features in 3D. We obtain 300 nm resolution. © 2012 COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
    K. Vora, S. Kang, and E. Mazur. 2012. “A Method to Fabricate Disconnected Silver Nanostructures in 3D.” J. Vis. Exp., 69, Pp. e4399–. Publisher's VersionAbstract
    A Method to Fabricate Disconnected Silver Nanostructures in 3D The standard nanofabrication toolkit includes techniques primarily aimed at creating 2D patterns in dielectric media. Creating metal patterns on a submicron scale requires a combination of nanofabrication tools and several material processing steps. For example, steps to create planar metal structures using ultraviolet photolithography and electron-beam lithography can include sample exposure, sample development, metal deposition, and metal liftoff. To create 3D metal structures, the sequence is repeated multiple times. The complexity and difficulty of stacking and aligning multiple layers limits practical implementations of 3D metal structuring using standard nanofabrication tools. Femtosecond-laser direct-writing has emerged as a pre-eminent technique for 3D nanofabrication. Femtosecond lasers are frequently used to create 3D patterns in polymers and glasses. However, 3D metal direct-writing remains a challenge. Here, we describe a method to fabricate silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm. The method enables the fabrication of patterns not feasible using other techniques, such as 3D arrays of disconnected silver voxels.8 Disconnected 3D metal patterns are useful for metamaterials where unit cells are not in contact with each other, such as coupled metal dot or coupled metal rod resonators. Potential applications include negative index metamaterials, invisibility cloaks, and perfect lenses. In femtosecond-laser direct-writing, the laser wavelength is chosen such that photons are not linearly absorbed in the target medium. When the laser pulse duration is compressed to the femtosecond time scale and the radiation is tightly focused inside the target, the extremely high intensity induces nonlinear absorption. Multiple photons are absorbed simultaneously to cause electronic transitions that lead to material modification within the focused region. Using this approach, one can form structures in the bulk of a material rather than on its surface. Most work on 3D direct metal writing has focused on creating self-supported metal structures. The method described here yields submicrometer silver structures that do not need to be self-supported because they are embedded inside a matrix. A doped polymer matrix is prepared using a mixture of silver nitrate (AgNO3), polyvinylpyrrolidone (PVP) and water (H2O). Samples are then patterned by irradiation with an 11-MHz femtosecond laser producing 50-fs pulses. During irradiation, photoreduction of silver ions is induced through nonlinear absorption, creating an aggregate of silver nanoparticles in the focal region. Using this approach we create silver patterns embedded in a doped PVP matrix. Adding 3D translation of the sample extends the patterning to three dimensions.
    P. Tayalia, E. Mazur, and D. J. Mooney. 2011. “Controlled architectural and chemotactic studies of 3D cell migration.” Biomaterials, 32, Pp. 2634–2641. Publisher's VersionAbstract
    Chemotaxis plays a critical role in tissue development and wound repair, and is widely studied using ex vivo model systems in applications such as immunotherapy. However, typical chemotactic models employ 2D systems that are less physiologically relevant or use end-point assays, that reveal little about the stepwise dynamics of the migration process. To overcome these limitations, we developed a new model system using microfabrication techniques, sustained drug delivery approaches, and theoretical modeling of chemotactic agent diffusion. This model system allows us to study the effects of 3D architecture and chemotactic agent gradient on immune cell migration in real time. We find that dendritic cell migration is characterized by a strong interplay between matrix architecture and chemotactic gradients, and migration is also influenced dramatically by the cell activation state. Our results indicate that Lipopolysaccharide-activated dendritic cells studied in a traditional transwell system actually exhibit anomalous migration behavior. Such a 3D ex vivo system lends itself for analyzing cell migratory behavior in response to single or multiple competitive cues and could prove useful in vaccine development.
    M. Shinoda, R. R. Gattass, and E. Mazur. 2009. “Femtosecond laser-induced formation of nanometer-width grooves on synthetic single-crystal diamond surfaces.” J. Appl. Phys., 10, Pp. 053102-1–053102-4. Publisher's VersionAbstract
    We form periodic linear grooves in synthetic single-crystal diamond with femtosecond pulses at 800 nm. The grooves are 40 nm wide, 500 nm deep, up to 0.3 mm long, and have an average spacing of 146 7 nm. The grooves are perpendicular to the direction of the laser polarization and are formed below the ablation threshold. The submicrometer periodicity is caused by interference between a laser-induced plasma and the incident laser beam, which locally enhances the field at the surface so the ablation threshold is exceeded. Using Raman spectroscopy we find that the structures retain the original diamond composition.
    C. R. Mendonca, D. S. Correa, F. Marlow, T. Voss, P. Tayalia, and E. Mazur. 2009. “Three-dimensional fabrication of optically active microstructures containing an electroluminescent polymer.” Appl. Phys. Lett., 95, Pp. 113309–113311. Publisher's VersionAbstract
    Microfabrication via two-photon absorption polymerization is a technique to design complex microstructures in a simple and fast way. The applications of such structures range from mechanics to photonics to biology, depending on the dopant material and its specific properties. In this paper, we use two-photon absorption polymerization to fabricate optically active microstructures containing the conductive and luminescent polymer poly(2-methoxy-5-(2'-ethylhexyloxy)- 1,4-phenylenevinylene) (MEH-PPV). We verify that MEH-PPV retains its optical activity and is distributed throughout the microstructure after fabrication. The microstructures retain the emission characteristics of MEH-PPV and allow waveguiding of locally excited fluorescence when fabricated on top of low refractive index substrates
    D. S. Correa, P. Tayalia, G. Cosendey, D. S. Dos Santos, R. F. Aroca, E. Mazur, and C. R. Mendonca. 2009. “Two-Photon Polymerization for Fabricating Structures Containing the Biopolymer Chitosan.” Journal of Nanoscience and Nanotechnology, 9, Pp. 5845–5849. Publisher's VersionAbstract
    Two-photon polymerization is a powerful tool for fabricating three-dimensional micro/nano structures for applications ranging from nanophotonics to biology. To tailor such structure for specific purposes it is often important to dope them. In this paper we report on the fabrication of structures, with nanometric surface features (resolution of approximately 700 nm), using two-photon polymerization of an acrylic resin doped with the biocompatible polymer chitosan using a guest-host scheme. The fluorescence background in the Raman spectrum indicates the presence of chitosan throughout the structure. Mechanical characterization reveals that chitosan does not affect the mechanical properties of the host acrylic resin and, consequently, the structures exhibit excellent integrity. The approach presented in this work can be used in the fabrication of micro- and nanostructures containing biopolymers for biomedical applications.
    R. R. Gattass and E. Mazur. 2008. “Femtosecond laser micromachining in transparent materials.” Nat. Phot., 2, Pp. 219–225. Publisher's VersionAbstract
    Femtosecond laser micromachining can be used either to remove materials or to change a material's properties, and can be applied to both absorptive and transparent substances. Over the past decade, this technique has been used in a broad range of applications, from waveguide fabrication to cell ablation. This review describes the physical mechanisms and the main experimental parameters involved in the femtosecond laser micromachining of transparent materials, and important emerging applications of the technology.
    C. R. Mendonca, D. S. Correa, T. Baldacchini, P. Tayalia, and E. Mazur. 2008. “Two-photon absorption spectrum of Lucirin TPO-L.” Appl. Phys. A, 90, Pp. 633–636. Publisher's VersionAbstract
    Two-photon absorption induced polymerization provides a powerful method for the fabrication of intricate three-dimensional microstructures. Recently, Lucirin TPO-L was shown to be a photoinitiator with several advantageous properties for two-photon induced polymerization. Here we measure the two-photon absorption cross-section spectrum of Lucirin TPO-L, which presents a maximum of 1.2 GM at 610 nm. Moreover, from the two-photon absorption spectrum we determined that Lucirin TPO-L radical quantum yield is independent of the wavelength. Despite its small two-photon absorption cross- section, it is possible to fabricated excellent microstructures by two-photon polymerization microfabrication due to the high polymerization quantum yield (0.99) of Lucirin TPO-L. These results show that optimization of the two-photon absorption cross-section is not the only factor to be considered when searching for new photoinitiators for microfabrication via two-photon absorption.
    P. Tayalia, C. R. Mendonca, T. Baldacchini, D. J. Mooney, and E. Mazur. 2008. “3D cell migration studies using two-photon engineered polymer scaffolds.” Advanced Materials, 20, Pp. 4494–4498. Publisher's VersionAbstract
    We use two-photon polymerization to fabricate 3D scaffolds with precise control over pore size and shape for studying cell migration in 3D. These scaffolds allow movement of cells in all directions. The fabrication, imaging, and quantitative analysis method developed here can be used to do systematic cell studies in 3D.
    C. R. Mendonca, T. Shih, and E. Mazur. 2008. “Femtosecond laser waveguide micromachining of PMMA films with azoaromatic chromophores.” Opt. Express, 16, Pp. 200–206. Publisher's VersionAbstract
    We report on the femtosecond-laser micromachining of poly(methyl methacrylate) (PMMA) films doped with nonlinear azoaromatic chromophores: Disperse Red 1, Disperse Red 13 and Disperse Orange 3. We study the conditions for controlling chromophore degradation during the micromachining of PMMA doped with each chromophore. Furthermore, we successfully used fs-micromachining to fabricate optical waveguides within a bulk sample of PMMA doped with these azochromophores.
    C. R. Mendonca, S. Orlando, G. Cosendey, M. T. Winkler, and E. Mazur. 2007. “Femtosecond laser micromaching in the conjugated polymer MEH- PPV.” Applied Surface Science, 254, Pp. 1135–1139. Publisher's VersionAbstract
    Femtosecond-laser micromachining of poly[2-methoxy-5-(2'1/2- ethylhexyloxy)- p-phenylene vinylene] (MEH-PPV) films is investigated using 130-fs pulses at 800-nm from a laser oscillator operating at 76-MHz repetition rate. We investigate the effect of pulse energy and translation speed on the depth and morphology of the micromachined regions. We quantified the MEH- PPV photobleaching induced by the fs-laser, and the conditions in which the emission of MEH-PPV is preserved after the micromaching.