Femtosecond laser microfabrication

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.
L. Tong, R. R. Gattass, I. Zaharieva Maxwell, J. B. Ashcom, and E. Mazur. 2006. “Optical loss measurements in femtosecond laser written waveguides in glass.” Opt. Commun., 259, Pp. 626–630. Publisher's VersionAbstract
The optical loss is an important parameter for waveguides used in integrated optics. We measured the optical loss in waveguides written in silicate glass slides with high repetition-rate (MHz) femtosecond laser pulses. The average transmission loss of straight waveguides is about 0.3 dB/mm at a wavelength of 633 nm and 0.05 dB/mm at a wavelength of 1.55 m. The loss is not polarization dependent and the waveguides allow a minimum bending radius of 36 mm without additional loss. The average numerical aperture (NA) of the waveguides is 0.065 at a wavelength of 633 nm and 0.045 at a wavelength of 1.55 m. In straight waveguides more than 90% of the transmission loss is due to scattering.
J. B. Ashcom, R. R. Gattass, C. B. Schaffer, and E. Mazur. 2006. “Numerical aperture dependence of damage and supercontinuum generation from femtosecond laser pulses in bulk fused silica.” J. Opt. Soc. Am. B, 23, Pp. 2317–2322. Publisher's VersionAbstract
Competing nonlinear optical effects are involved in the interaction of femtosecond laser pulses with transparent dielectrics: supercontinuum generation and multiphoton-induced bulk damage. We measured the threshold energy for supercontinuum generation and bulk damage in fused silica using numerical apertures ranging from 0.01 to 0.65. The threshold for supercontinuum generation exhibits a minimum near 0.05 NA, and increases quickly above 0.1 NA. For numerical apertures greater than 0.25, we observe no supercontinuum generation. The extent of the blue broadening of the supercontinuum spectrum decreases significantly as the numerical aperture is increased from 0.01 to 0.08, showing that loose focusing is important for generating the broadest supercontinuum spectrum. Using a light scattering technique to detect the onset of bulk damage, we confirmed bulk damage at all numerical apertures studied. At high numerical aperture, the damage threshold is well below the critical power for self-focusing.
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.
C. B. Schaffer, J. Aus der Au, E. Mazur, and J. A. Squier. 2002. “Micromachining and material change characterization using femtosecond laser oscillators.” In . Photonics West. Publisher's VersionAbstract
We use third harmonic generation (THG) microscopy to image waveguides and single-shot structural modifications produced in bulk glass using femtosecond laser pulses. THG microscopy reveals the internal structure of waveguides written with a femtosecond laser oscillator, and gives a three-dimensional view of the complicated morphology of the structural changes produced with single, above-threshold femtosecond pulses. We find that THG microscopy is as sensitive to refractive index change as differential interference contrast microscopy, while also offering the three-dimensional sectioning capabilities of a nonlinear microscopy technique. It is now possible to micromachine three- dimensional optical devices and to image these structures in three dimensions, all with a single femtosecond laser oscillator.
R. R. Gattass, L. R. Cerami, and E. Mazur. 2005. “Optical waveguide fabrication for integrated photonic devices.” In . Nano photonics and functional device technology. Publisher's VersionAbstract
The dynamic nature of future optical networks requires high levels of integration, fast response times, and adaptability of the optical components. Laser micromachining circumvents the limitations of planar integration, allowing both three-dimensional integration and dense packaging of optical devices without alignment requirements. Femtosecond micromachining enables the analog of circuit printing by wiring light between photonic devices in addition to printing the actual photonic device into a single or multiple substrates. Femtosecond laser oscillator-only micromachining has several advantages over amplified femtosecond laser micromachining: easy control over the size of the structures without changing focusing, polarization-independent structures, lower initial investment cost and higher-speed manufacturing. In this paper we review recent results obtained in the field of femtosecond micromachining. Keywords: Femtosecond, micromachining, nonlinear absorption.
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.
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.
J. B. Ashcom, C. B. Schaffer, and E. Mazur. 2002. “Numerical aperture dependence of damage and white light generation from femtosecond laser pulses in bulk fused silica.” In . Photonics West. Publisher's VersionAbstract
The femtosecond laser has become an important tool in the micromachining of transparent materials. In particular, focusing at high numerical aperture enables structuring the bulk of materials. At low numerical aperture and comparable energy, focused femtosecond pulses result in white light or continuum generation. It has proven difficult to damage transparent materials in the bulk at low NA. We have measured the threshold energy for continuum generation and for bulk damage in fused silica for numerical apertures between 0.01 and 0.65. The threshold for continuum generation exhibits a minimum near 0.05 NA, and increases quickly near 0.1 NA. Greater than 0.25 NA, no continuum is observed. The extent of the anti-stokes pedestal in the continuum spectrum decreases strongly as the numerical aperture is increased to 0.1, emphasizing that slow focusing is important for the broadest white light spectrum. We use a sensitive light scattering technique to detect the onset of damage. We are able to produce bulk damage at all numerical apertures studied. At high numerical aperture, the damae threshold is well below the critical power for self-focusing, which allows the breakdown intensity to be determined. Below 0.25 NA, the numerical aperture dependence suggests a possible change in damage mechanism.
E. Mazur, H. J. M. Hijnen, L. J. F. Hermans, and J. J. M. Beenakker. 1984. “Experiments on the influence of a magnetic field on diffusion in N2-noble gas mixtures.” Physica, 123A, Pp. 412–427. Publisher's VersionAbstract
Experimental results are presented for the magnetic field effect on diffusion in N2-noble gas mixtures at 300 K. The data show that the polarization produced by a concentration gradient is different from the one produced in a temperature gradient and that this difference is due to a different scalar part of the polarizations.
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.
R. R. Gattass. 2006. “Femtosecond-laser interactions with transparent materials: applications in micromachining and supercontinuum generation”. Publisher's VersionAbstract
Femtosecond-lasers represent a source for electric field pulses which can have field intensities approaching and even exceeding the atomic binding field. For an electric field of this order, the polarization response of the medium changes from linear to nonlinear. For transparent media, depending on the field intensity, the laser pulse is either nonlinearly absorbed or, at lower field intensities, modifies the medium as it propagates, modulating its own spectrum. Nonlinear absorption has direct applications to the micromachining of photonic devices. We discuss the effect of different laser parameters such as the repetition rate and number of pulses in the femtosecond-laser generated structures. Additionally, we investigate the transmission losses, bending loss, supported electromagnetic modes and index of refraction profiles of optical interconnects fabricated through femtosecond micromachining. This dissertation also covers experiments on the propagation of femtosecond pulses confined in structures whose diameter is below the wavelength of the incident light, silica based nanowires. We demonstrate the possibility of making sub-micrometer diameter silica fibers and discuss the effects of their diameter-dependent dispersion and enhanced nonlinearity for femtosecond laser pulse propagation. The nonlinearity and dispersion are presented as a function of the nanowire diameter and our results confirm the theoretical predictions for the enhancement of the nonlinearity and the effect of high dispersion. Both technologies, nanowires and femtosecond manufactured waveguides, represent alternatives for photonic circuits interconnects, but at nanometer and micrometer scales, respectively.
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, 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.
A. Ben-Yakar, A. Harkin, J. Ashmore, M. Shen, E. Mazur, R. L. Byer, and H.A. Stone. 2003. “Thermal and fluid processes of a thin melt zone during femtosecond laser ablation of glass.” In . Photon Processing in Microelectronics and Photonics II. Publisher's VersionAbstract
Microfluidic channels on borosilicate glass are machined using femtosecond lasers. The morphology of the ablated surface is studied using scanning microscopy. The results show micron scale features inside the channels. The formation mechanism of these features is investigated by additional experiments accompanied by a theoretical analysis of the thermal and fluid processes involved in the ultrafast laser ablation process. These studies indicate the existence of a very thin melting zone on glass and suggest that the surface morphology is formed by the plasma pressure-driven fluid motion of the melting zone during the ablation process.

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