Femtosecond laser microfabrication

R. R. Gattass, L. R. Cerami, and E. Mazur. 2006. “Micromachining of bulk glass with bursts of femtosecond laser pulses at variable repetition rates.” Opt. Exp., 14, Pp. 5279–5284. Publisher's VersionAbstract
Oscillator-only femtosecond laser micromachining enables the manufacturing of integrated optical components with circular transverse profiles in transparent materials. The circular profile is due to diffusion of heat accumulating at the focus. We control the heat diffusion by focusing bursts of femtosecond laser pulses at various repetition rates into sodalime glass. We investigate the effect the repetition rate and number of pulses have on the size of the resulting structures. We identify the combinations of burst repetition rate and number of pulses within a burst for which accumulation of heat occurs. The threshold for heat accumulation depends on the number of pulses within a burst. The burst repetition rate and the number of pulses within a burst provide convenient control of the morphology of structures generated with high repetition rate femtosecond micromachining.
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
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.
A. Ben-Yakar, R. L. Byer, A. Harkin, J. Ashmore, H.A. Stone, M. Shen, and E. Mazur. 2003. “Morphology of femtosecond-laser-ablated borosilicate glass surfaces.” Appl. Phys. Lett., 83, Pp. 3030–3032. Publisher's VersionAbstract
We study the morphology of borosilicate glass surface machined by femtosecond laser pulses. Our observations show that a thin rim is formed around ablated craters after a single laser pulse. When multiple laser pulses are overlapped, the crater rims also overlap and produce a surface roughness. The rim appears to be a resolidified splash from a molten layer generated during the ablation process. We estimate that this molten layer is a few micrometers thick and exists for a few microseconds. During this melt lifetime, forces acting on the molten layer move it from the center to the edge of the crater.
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.
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.
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.
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).
D. Souza Correa, P. Tayalia, E. Mazur, and C. R. Mendonca. 2006. “Complex microstructures fabricated via two-photon absorption polymerization.” In . Macro 2006 - 41st Symposium on Macromolecules. Publisher's VersionAbstract
Using acrylic resin and Lucirin TPO-L as photoinitiator, we fabricated complex microstructures via the process of two photon absorption (2PA) polymerization. We measured the 2PA cross-section of Lucirin TPO-L, which is the parameter responsible for the nonlinear process, and the value found is among the ones reported in the literature for common photoinitiators. We also carried out quantum chemistry calculation in order to correlate the nonlinear optical properties of this photoinitiator to its molecular structure.
C. R. Mendonca, T. Baldacchini, P. Tayalia, and E. Mazur. 2007. “Reversible birefringence in microstructures fabricated by two-photon absorption polymerization.” J. Appl. Phys., 103, Pp. 013109-1–013109-4. Publisher's VersionAbstract
This paper reports the fabrication of birefringent microstructures using two- photon absorption polymerization. The birefringence is caused by a light-driven molecular orientation of azoaromatic molecules (Disperse Red 13) upon excitation with an Ar+ laser at 514.5 nm. For an azoaromatic dye content of 1% by weight we obtain a birefringence of 5x10-5. This birefringence can be completely erased by overwriting the test spot with circularly polarized laser light or by heating the sample. Our results open the door to the development of new applications in optical data storage, wave guiding, and optical circuitry.
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.
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.
C. B. Schaffer, A. Brodeur, J. F. Garcia, and E. Mazur. 2001. “Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy.” Opt. Lett., 26, Pp. 93–95. Publisher's VersionAbstract
Using tightly-focused femtosecond laser pulses of just 5 nJ we produce optical breakdown and structural change in bulk transparent materials, and demonstrate micromachining of transparent materials using unamplified lasers. We present measurements of the threshold for structural change in Corning 0211 glass, as well as a study of the morphology of the structures produced by single and multiple laser pulses. At high repetition-rate, multiple pulses produce a structural change dominated by cumulative heating of the material by successive laser pulses. Using this cumulative heating effect, we write single-mode optical waveguides inside bulk glass using only a laser oscillator.
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.

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