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.
    T. Shih, R. R. Gattass, C. R. Mendonca, and E. Mazur. 2007. “Faraday rotation in femtosecond laser micromachined waveguides.” Opt Express, 15, Pp. 5809–5814. Publisher's VersionAbstract
    We demonstrate magneto-optic switching in femtosecond-laser micromachined waveguides written inside bulk terbium-doped Faraday glass. By measuring the polarization phase shift of the light as a function of the applied magnetic field, we find that there is a slight reduction in the effective Verdet constant of the waveguide compared to that of bulk Faraday glass. Electron Paramagnetic Resonance (EPR) measurements confirm that the micromachining leaves the concentration of the terbium ions that are responsible for the Faraday effect virtually unchanged.
    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.
    C. R. Mendonca, P. Tayalia, T. Baldacchini, and E. Mazur. 2006. “Three-dimensional microfabrication for photonics and biomedical applications.” In . Macro 2006 - 41st International Symposium on Macromolecules Proceedings. Publisher's VersionAbstract
    We use two-photon absorption polymerization to fabricate microstructures containing compounds with interesting properties for optical and biomedical applications. Our investigations open the door to new applications in data storage, waveguides manufacturing, organic LEDs, optical circuitry and scaffold for bio-applications.
    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, D. S. Correa, T. Baldacchini, P. Tayalia, and E. Mazur. 2006. “A novel photoinitiator for microfabrication via two-photon polymerization.” In . CLEO 2006. Publisher's VersionAbstract
    We measured the two-photon absorption cross-section of the photoinitiator Lucirin TPO-L and fabricated complex microstructures using this photoinitiator in an acrylate resin. Using quantum chemistry calculations, we relate the nonlinear optical properties of the photoinitiator it to its molecular structure.
    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.
    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.
    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.
    M. Kamata, M. Obara, R. R. Gattass, L. R. Cerami, and E. Mazur. 2005. “Optical vibration sensor fabricated by femtosecond laser micromachining.” Appl. Phys. Lett., 87, Pp. 051106-1–051106-3. Publisher's VersionAbstract
    We fabricated an optical vibration sensor using a high-repetition rate femtosecond laser oscillator. The sensor consists of a single straight waveguide written across a series of three pieces of glass. The central piece is mounted on a suspended beam to make it sensitive to mechanical vibration, acceleration, or external forces. Displacement of the central piece is detected by measuring the change in optical transmission through the waveguide. The resulting sensor is small, simple, and requires no alignment. The sensor has a linear response over the frequency range 20 Hz 2 kHz, can detect accelerations as small as 0.01 m/s2, and is nearly temperature independent.
    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. B. Schaffer, A. O. Jamison, and E. Mazur. 2004. “Morphology of femtosecond laser-induced structural changes in bulk transparent materials.” Appl. Phys. Lett., 84, Pp. 1441–1443. Publisher's VersionAbstract
    Using optical and electron microscopy, we analyze the energy and focusing angle dependence of structural changes induced in bulk glass by tightly-focused femtosecond laser pulses. We observe a transition from small density variations in the material to void formation with increasing laser energy. At energies close to the threshold for producing a structural change, the shape of the structurally changed region is determined by the focal volume of the objective used to focus the femtosecond pulse, while at higher energies the structural change takes on a conical shape. From these morphological observations, we infer the role of various mechanisms for structural change.
    C. B. Schaffer, T. N. Kim, J. F. Garcia, E. Mazur, A. Groisman, and D. Kleinfeld. 2004. “Micromachining of bulk transparent materials using nanojoule femtosecond laser pulses.” In . Boulder Damage Symposium. Publisher's VersionAbstract
    Femtosecond lasers are very effective tools for three-dimensional micromachining of transparent materials. Nonlinear absorption of tightly focused femtosecond laser pulses allows energy to be deposited in a micrometer-sized volume in the bulk of the sample. If enough energy is deposited, localized changes in the material are produced (a change in refractive index, for example). These localized changes are the building blocks from which three-dimensional structures can be produced. With sufficiently tight focusing, the threshold for producing these changes can be achieved with pulse energies that are available directly from laser oscillators, offering greatly increased machining speeds and simpler, cheaper technology compared to using amplified lasers. In addition, the inter-pulse spacing from a laser oscillator is much shorter than the time required for energy deposited by one pulse to diffuse out of the focal volume. As a result, irradiation with multiple pluses on one spot in the sample leads to an accumulation of heat around the focal region. This localized heating provides another mechanism by which material properties can be altered. We demonstrate the three- dimensional fabrication of optical waveguides and microfluidic channels using pulse energies of only a few nanojoules to tens of nanojoules.
    R. R. Gattass and E. Mazur. 2004. “Wiring light with femtosecond laser pulses.” In Photonics Spectra, 12: Pp. 56–60. Publisher's VersionAbstract
    Shortly after the invention of the laser, researchers discovered that intense laser pulses can cause dielectric breakdown and structural change in materials. This breakdown was generally considered a tremendous nuisance, hindering both research and the development of more powerful lasers. Several decades later, however, laser-induced dielectric breakdown inside materials is wiedely used to create internal structural change. It is in this arena that lasers really stand out, as they afford the opportunity that no mechanical tool can: the processing of the bulk of a material without affecting its surface. Recent advances in this area of research make it possible to wire light from one point to another inside a transparent material, opening the door to the manufacturing of entirely monolithic, integrated optical circuitry.
    D. B. Wolfe, J. B. Ashcom, J. C. Hwang, C. B. Schaffer, E. Mazur, and G. M. Whitesides. 2003. “Customization of poly(dimethylsiloxane) stamps by micromachining using a femtosecond-pulsed laser.” Adv. Mater., 15, Pp. 62–65. Publisher's VersionAbstract
    A femtosecond-pulsed Ti:sapphire laser is used to generate surface features in slabs of poly(dimethylsiloxane) with minimum dimensions of 1 mum-smaller than those available by rapid-prototyping techniques using transparency masks. The fabrication of magnetic field concentrators and the addition of custom features to a generic microfluidic channel (see Figure) demonstrate the utility of the technique.
    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.
    S. K. Sundaram, C. B. Schaffer, and E. Mazur. 2003. “Microexplosions in Tellurite glasses.” Appl. Phys. A, 76, Pp. 379–384. Publisher's VersionAbstract
    Femtosecond laser pulses were used to produce localized damage in the bulk and near the surface of baseline, Al2O3-doped, and La2O3-doped sodium tellurite glasses. Single or multiple laser pulses were nonlinearly absorbed in the focal volume by the glass, leading to permanent changes in the material at the focal volume. These changes are caused by an explosive expansion of the ionized material in the focal volume into the surrounding material, i.e., a microexplosion. Writing of simple structures (periodic array of voxels, as well as lines) was demonstrated. The regions of microexplosion and writing were characterized using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and atomic force microscopy (AFM) postmortem. Fingerprints of microexplosions (concentric lines within the region and a concentric ring outside the region), due to the shock wave generated during microexplosions, were evident. In the case of the baseline glass, no chemistry change was observed within the region of the microexplosion. However, Al2O3-doped and La2O3-doped glasses showed depletion of the dopant from the edge to the center of the region of the microexplosions, indicating chemistry gradient within the regions. Interrogation of the bulk- and laser-treated regions using micro- Raman spectroscopy revealed no structural change due to the microexplosions and writing within these glasses.
    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.
    J. B. Ashcom, R. R. Gattass, E. Mazur, and Y. Chay. 2002. “Laser induced microexplosions and applications in laser micromachining.” In . 13th International Meeting on Ultrafast Phenomena. Technical Digest. Publisher's VersionAbstract
    After reviewing some of the fundamentals of surface and bulk damage in transparent materials, we will present an overview of work being done in out laboratory on tighly focusing femtosecond pulses into the bulk of transparent materials with an emphasis on materials processing and micromachining.