Publications

    L. Tong and E. Mazur. 2005. “Subwavelength-diameter silica wires for microscale optical components.” In . SPIE Photonics West 2005. Publisher's VersionAbstract
    Subwavelength-diameter silica wires fabricated using a taper-drawing approach exhibit excellent diameter uniformity and atomic-level smoothness, making them suitable for low-loss optical wave guiding from the UV to the near-infrared. Such air-clad silica wires can be used as single-mode waveguides; depending on wavelength and wire diameter, they either tightly confine the optical fields or leave a certain amount of guided energy outside the wire in the form of evanescent waves. Using these wire waveguides as building blocks we assembled microscale optical components such as linear waveguides, waveguide bends and branch couplers on a low-index, non- dissipative silica aerogel substrate. These components are much smaller than comparable existing devices and have low optical loss, indicating that the wire-assembly technique presented here has great potential for developing microphotonics devices for future applications in a variety of fields such as optical communication, optical sensing and high-density optical integration. Keywords: Subwavelength, silica, nanowire, microphotonics, nanophotonics, optical components
    L. Tong, J. Lou, and E. Mazur. 2005. “Modeling of subwavelength-diameter optical wire waveguides for optical sensing applications.” In . Advanced Sensor Systems and Applications II. Publisher's VersionAbstract
    Low-loss optical wave guiding along a subwavelength-diameter silica wire leaves a large amount of the guided field outside the solid core as evanescent wave and at the same time maintains the coherence of the light, making it possible to develop sensitive and miniaturized optical sensors for physical, chemical and biological applications. Here we introduce, for the first time to our knowledge, a scheme to develop optical sensors based on evanescent-wave- guiding properties of subwavelength-diameter wires. Optical wave guiding properties of these wires that are pertinent to a waveguide sensor, such as single-mode condition, evanescent field, Poynting vector and optical loss are investigated. By measuring the phase shift of the guided light, we propose a Mach-Zehnder-type sensor assembled with two silica wires. The sensitivity and size of the sensor are also estimated, which shows that, subwavelength- diameter silica wires are promising for developing optical sensors with high sensitivity and small size.
    L. Tong, J. Lou, and E. Mazur. 2005. “Waveguide bends from nanometric silica wires.” In . Nanophotonics, Nanostructure, and Nanometrology. Publisher's VersionAbstract
    We propose to use bent silica wires with nanometric diameters to guide light as optical waveguide bend. We bend silica wires with scanning tunneling microscope probes under an optical microscope, and wire bends with bending radius smaller than 5 m are obtained. Light from a He-Ne laser is launched into and guided through the wire bends, measured bending loss of a single bend is on the order of 1 dB. Brief introductions to the optical wave guiding and elastic bending properties of silica wires are also provided. Comparing with waveguide bends based on photonic bandgap structures, the waveguide bends from silica nanometric wires show advantages of simple structure, small overall size, easy fabrication and wide useful spectral range, which make them potentially useful in the miniaturization of photonic devices.
    J. E. Carey. 2004. “Femtosecond-laser Microstructuring of Silicon for Novel Optoelectronic Devices”. Publisher's VersionAbstract
    This dissertation comprehensively reviews the properties of femtosecond- laser microstructured silicon and reports on its first application in optoelectronic devices. Irradiation of a silicon surface with intense, short laser pulses in an atmosphere of sulfur hexafluoride leads to a dramatic change in the surface morphology and optical properties. Following irradiation, the silicon surface is covered with a quasi-ordered array of micrometer-sized, conical structures. In addition, the microstructured surface has near-unity absorptance from the near-ultraviolet (250 nm) to the near-infrared (2500 nm). This spectral range includes below-band gap wavelengths that normally pass through silicon unabsorbed. We thoroughly investigate the effect of experimental parameters on the morphology and chemical composition of microstructured silicon and propose a formation mechanism for the conical microstructures. We also investigate the effect of experimental parameters on the optical and electronic properties of microstructured silicon and speculate on the cause of below-band gap absorption. We find that sulfur incorporation into the silicon surface plays an important role in both the formation of sharp, conical microstructures and the near-unity absorptance at below-band gap wavelengths. Because of the novel optical properties, femtosecond-laser microstructured silicon has potential application in numerous optoelectronic devices. We use femtosecond-laser microstructured silicon to create silicon-based photodiodes that are one hundred times more sensitive than commercial silicon photodiodes in the visible, and five orders of magnitude more sensitive in the near-infrared. We also create femtosecond-laser microstructured silicon solar cells and field emission arrays.
    M. A. Sheehy. 2004. “Femtosecond-laser Microstructuring of Silicon: Dopants and Defects”. Publisher's VersionAbstract
    This dissertation deals with the incorporation of elements into silicon using a femtosecond laser in order to understand the source for below-band gap absorptance. Previous experimental results indicate that irradiation of silicon with a femtosecond laser in the presence of sulfur hexafluoride (SF6) leads to unique optical properties. The absorptance for above-band gap radiation is increased to 95%; the more interesting result is that the below-band gap absorptance goes from nearly 0% to 90%. In the first set of experiments performed, we irradiated silicon in the presence of H2S, SiH4, and H2. The absorptance for samples prepared in H2S is identical to that of samples prepared in SF6; the other samples have a trailing edge of absorptance for energies below the band gap. This result indicated that sulfur played a crucial role in the below-band gap absorptance. The next set of experiments involved incorporating selenium and tellurium from a powder source to investigate possible dependence of the optical properties on the size of the dopant (selenium and tellurium have the same valence, but are larger in atomic size than sulfur). Incorporation of these two elements also leads to near-unity absorptance for below-band gap radiation. A comparison of the composition and the optical properties before and after annealing showed that the source for below-band gap absorptance is likely due to both the incorporated chalcogen and defects. The final set of experiments deals with the incorporation of elements from other families. These studies bolster the results of the previous research and provide furtherdetails on the interaction of the dopant with the laser- modified surface. We speculate on some requirements the dopants must satisfy (i.e. atomic size and valence onfiguration) and propose further research that can be done in this area. These experiments provide significant insight into the optical absorption mechanism and show that this material has great potential for devices that operate in the infrared portion of the spectrum, such as infrared photodiodes and solar cells.
    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. H. Crouch, J. E. Carey, J. M. Warrender, M. J. Aziz, E. Mazur, and F. Y. Genin. 2004. “Comparison of structure and properties of femtosecond and nanosecond laser-structured silicon.” Appl. Phys. Lett., 84, Pp. 1850–1852. Publisher's VersionAbstract
    We compare the optical properties, chemical composition, and crystallinity of silicon microstructures formed in the presence of SF6 by femtosecond laser irradiation and by nanosecond laser irradiation. In spite of very different morphology and crystallinity, the optical properties and chemical composition of the two types of microstructures are very similar. The structures formed with femtosecond (fs) pulses are covered with a disordered nanocrystalline surface layer less than 1 m thick, while those formed with nanosecond (ns) pulses have very little disorder. Both ns-laser-formed and fs-laser-formed structures absorb near-infrared (1.12.5 m) radiation strongly and have roughly 0.5% sulfur impurities.
    M. Shen, C. H. Crouch, J. E. Carey, and E. Mazur. 2004. “Femtosecond laser-induced formation of submicrometer spikes on silicon in water.” Appl. Phys. Lett., 85, Pp. 5694–5696. Publisher's VersionAbstract
    We fabricate submicrometer silicon spikes by irradiating a silicon surface that is submerged in water with 400-nm, 100-fs laser pulses. These spikes are less than a micrometer tall and about 200 nm wide one to two orders of magnitude smaller than the microspikes formed by laser irradiation of silicon in gases or vacuum. Scanning electron micrographs of the surface show that the formation of the spikes involves a combination of capillary waves on the molten silicon surface and laser-induced etching of silicon. Chemical analysis and scanning electron microscopy of the spikes shows that they are composed of silicon with a 20-nm thick surface oxide layer.
    L. Tong, J. Lou, and E. Mazur. 2004. “Single-mode guiding properties of subwavelength-diameter silica and silicon wire waveguides.” Opt. Express, 12, Pp. 1025–1035. Publisher's VersionAbstract
    Single-mode optical wave guiding properties of silica and silicon subwavelength-diameter wires are studied with exact solutions of Maxwells equations. Single mode conditions, modal fields, power distribution, group velocities and waveguide dispersions are studied. It shows that air-clad subwavelength-diameter wires have interesting properties such as tight-confinement ability, enhanced evanescent fields and large waveguide dispersions that are very promising for developing future microphotonic devices with subwavelength-width structures.
    C. H. Crouch, J. E. Carey, M. Shen, E. Mazur, and F. Y. Genin. 2004. “Infrared absorption by sulfur-doped silicon formed by femtosecond laser irradiation.” Appl. Phys. A, 79, Pp. 1635–1641. Publisher's VersionAbstract
    We microstructured silicon surfaces with femtosecond laser irradiation in the presence of SF6. These surfaces display strong absorption of infrared radiation at energies below the band gap of crystalline silicon. We report the dependence of this below-band gap absorption on microstructuring conditions (laser fluence, number of laser pulses, and background pressure of SF6) along with structural and chemical characterization of the material. Significant amounts of sulfur are incorporated into the silicon over a wide range of microstructuring conditions; the sulfur is embedded in a disordered nanocrystalline layer less than 1 mm thick that covers the microstructures. The most likely mechanism for the below-band gap absorption is the formation of a band of sulfur impurity states overlapping the silicon band edge, reducing the band gap from 1.1 eV to approximately 0.4 eV.
    A. Serpengüzel, T. Bilici, I. Inanc, A. Kurt, J. E. Carey, and E. Mazur. 2004. “Temperature dependence of photoluminescence in non-crystalline silicon.” In . Photonics West. Publisher's VersionAbstract
    Crystalline silicon being ubiquitous throughout the microelectronics industry has an indirect bandgap, and therefore is incapable of light emission. However, strong room temperature visible and near-IR luminescence from non-crystalline silicon, e.g., amorphous silicon, porous silicon, and black silicon, has been observed. These silicon based materials are morphologically similar to each other, and have similar luminescence properties. We have studied the temperature dependence of the photoluminescence from these non-crystalline silicons to fully characterize and optimize these materials in the pursuit of obtaining novel optoelectronic devices.
    C. H. Crouch, A. P. Fagen, J. Paul Callan, and E. Mazur. 2004. “Classroom Demonstrations: Learning Tools or Entertainment?” Am. J. Phys., 72, Pp. 835–838. Publisher's VersionAbstract
    We compared student learning from different modes of presenting classroom demonstrations, in order to determine how much students learn from traditionally presented demonstrations, and whether this learning can be enhanced by simply changing the mode of presentation and thereby increasing student engagement. Students who passively observe demonstrations understand the underlying concepts no better than students who do not see the demonstration at all. Students who predict the demonstration outcome before seeing it, however, display significantly greater understanding.
    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.
    C. A. D. Roeser, M. Kandyla, A. Mendioroz, and E. Mazur. 2004. “Optical control of coherent lattice vibrations in tellurium.” Phys. Rev. B, 70, Pp. 212302–212305. Publisher's VersionAbstract
    We present femtosecond time-resolved measurements of the dielectric tensor of tellurium under single and double pulse excitation. We demonstrate the ability to both enhance and cancel coherent lattice vibrations for large lattice shifts under near-damage threshold excitation. The excitation conditions for which cancellation is achieved in tellurium reveal a departure from the low-excitation strength behavior of similar materials.
    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.
    J. E. Carey and E. Mazur. 2003. “Femtosecond Laser-Assisted Microstructuring of Silicon for Novel Detector, Sensing and Display Technologies.” In . LEOS 2003. Publisher's VersionAbstract
    Arrays of sharp, conical microstructures are obtained by stucturing the surface of a silicon wafer using femtosecond laser-assisted chemical etching. The one step, maskless structuring process drastically changes the optical, material and electronic properties of the original silicon wafer. These properties make microstructured silicon viable for use in a wide range of commercial devices including solar cells, infrared photodetectors, chemical and biological sensors, and field emission devices.
    M. Shen, C. H. Crouch, J. E. Carey, R. J. Younkin, E. Mazur, M. A. Sheehy, and C. M. Friend. 2003. “Formation of regular arrays of silicon microspikes by femtosecond laser irradiation through a mask.” Appl. Phys. Lett., 82, Pp. 1715–1717. Publisher's VersionAbstract
    We report fabrication of regular arrays of silicon microspikes by femtosecond laser irradiation of a silicon wafer covered with a periodic mask. Without a mask, microspikes form, but they are less ordered. We believe that the mask imposes order by diffracting the laser beam and providing boundary conditions for capillary waves in the laser-melted silicon.
    R. J. Younkin, J. E. Carey, E. Mazur, J. A. Levinson, and C. M. Friend. 2003. “Infrared absorption by conical silicon microstructures made in a variety of background gases using femtosecond-laser pulses.” J. Appl. Phys., 93, Pp. 2626–2629. Publisher's VersionAbstract
    We show that the near-unity infrared absorptance of conical microstructures fabricated by irradiating a Si(111) surface with 100-fs laser pulses depends on the ambient gas in which the structures are formed. Of the background gases we investigate, SF6 is the most effective, yielding an absorptance of 0.9 for radiation in the 1.2-2.5 m wavelength range. Use of Cl2, N2, or air produces surfaces with absorptances intermediate between that for microstructures formed in SF6 and that for flat, crystalline silicon, for which the absorptance is roughly 0.05?0.2 for a 260- m thick sample.

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