Publications

A. P. Fagen. 2003. “Assessing and Enhancing the Introductory Science Course in Physics and Biology: Peer Instruction, Classroom Demonstrations, and Genetics Vocabulary”. Publisher's VersionAbstract
Most introductory college science courses in the United States are taught in large lectures with students rarely having the opportunity to think critically about the material being presented nor to participate actively. Further, many classes focus on teaching rather than learning, that is, the transfer of information as opposed to actual student understanding. This thesis focuses on three studies about the assessment and enhancement of learning in undergraduate science courses. We describe the results of an international survey on the implementation of Peer Instruction (PI), a collaborative learning pedagogy in which lectures are interspersed with short conceptual questions designed to challenge students to think about the material as it is being presented. We present a portrait of the many instructors teaching with PI and the settings in which it is being used as well as data on the effectiveness of PI in enhancing student learning in diverse settings. The wide variety of implementations suggests that PI is a highly adaptable strategy that can work successfully in almost any environment. We also provide recommendations for those considering adopting PI in their classes. Classroom demonstrations are an important aspect of many introductory science courses, but there is little evidence supporting their educational effectiveness. We explore the effect of different modes of presentation on enhancing student learning from demonstrations. Our results show that students who actively engage with a demonstration by predicting the outcome before it is conducted are better able to recall and explain the scenario posed by that demonstration. As preliminary work for the creation of an inventory of conceptual understanding in introductory biology, we discuss results from a survey of vocabulary familiarity and understanding in an undergraduate genetics course. Students begin introductory classes with significant gaps in their understanding, some of which are retained beyond instruction. Further, they overstate their knowledge, and the degree to which they exhibit overconfidence increases over the period of instruction.
C. A. D. Roeser. 2003. “Ultrafast Dynamics and Optical Control of Coherent Phonons in Tellurium”. Publisher's VersionAbstract
This dissertation reports the ultrafast dynamics of tellurium after excitation by one or more intense femtosecond laser pulses. Irradiation of tellurium by femtosecond pulses is known to excite coherent phonons, but the nature of the excitation process and the details of the material dynamics under intense excitation are, as of yet, not precisely determined. We investigate these dynamics by monitoring the response of tellurium using an optical pump– probe technique designed to measure the dielectric tensor across the visible spectrum with femtosecond time resolution. The observed dynamics are similar to the ultrafast dynamics of molecules, where photoexcitation of electrons establishes a new potential surface on which the nuclei move. The time-resolved dielectric tensor measurements provide a "snapshot" of the material in a particular lattice configuration. From the observed changes in the optical properties, we infer the underlying changes in the lattice, and thereby develop a picture of the nuclear motion. We find that the main resonance for interband electronic transitions in tellurium shifts to lower photon energy due to the lattice displacement that results from photoexcitation. Under single pulse excitation, a rapid change in the equilibrium lattice configuration leads to a long-lived shift in the resonance energy along with fast oscillations around this value. Under double pulse excitation, the lattice dynamics can be controlled; we achieve both enhancement and cancellation of coherent phonons for excitation strengths up to the damage threshold.
J. E. Carey, E. N. Glezer, and E. Mazur. 2003. “Propagation and Characterization of Ultrashort Laser Pulses.” In Spectroscopy of Systems with Spatially Confined Structures, edited by Rino Di Bartolo, Pp. 213–242. Kluwer Academic Publishers.Abstract
These lectures provide an introduction to the techniques and measurements of ultrashort laser pulses. We begin with a review of the electrical and optical properties of solids and of the propagation of electromagnetic waves through a medium. We then discuss methods for the temporal and spectral characterization of ultrashort laser pulses. Finally we discuss the effects of dispersion and filtering on the temporal and spectral resolution of measurements with ultrashort laser pulses.
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.
N. Shen. 2003. “Photodisruption in biological tissues using femtosecond laser pulses”. Publisher's VersionAbstract
Transparent materials do not ordinarily absorb visible or near-infrared light. However, the intensity of a tightly focused femtosecond laser pulse is great enough that nonlinear absorption of the laser energy takes place in transparent materials, leading to optical breakdown and permanent material modification. Because the absorption process is nonlinear, absorption and material modification are confined to the extremely small focal volume. Optical breakdown in transparent or semi-transparent biological tissues depends on intensity rather than energy. As a result, focused femtosecond pulses induce optical breakdown with significantly less pulse energy than is required with longer pulses. The use of femtosecond pulses therefore minimizes the amount of energy deposited into the targeted region of the sample, minimizing mechanical and thermal effects that lead to collateral damage in adjacent tissues. We demonstrate photodisruptive surgery in animal skin tissue and single cells using 100-fs laser pulses. In mouse skin, we create surface incisions and subsurface cavities with much less collateral damage to the surrounding tissue than is produced with picosecond pulses. Using pulses with only a few nanojoules of energy obtained from an unamplified femtosecond oscillator, we destroy single mitochondria in live cells without affecting cell viability, providing insights into the structure of the mitochondrial network. An apparatus is constructed to perform subcellular surgery and multiphoton 3D laser scanning imaging simultaneously with a single laser and objective lens.
A. M.-T. Kim, C. A. D. Roeser, and E. Mazur. 2003. “Modulation of the Bonding-Antibonding Splitting in Te by Coherent Phonons.” Phys. Rev. B, 68, Pp. 012301–012304. Publisher's VersionAbstract
We present femtosecond time-resolved measurements of the dielectric tensor of tellurium following intense photoexcitation. Strong impulsive photoexcitation of crystalline tellurium weakens the covalent bonds between atoms, which undergo coherent oscillations (at > 3 THz) as they relax to new equilibrium positions. As this photoexcitation drives the lattice toward a band-crossing transition, we track the decrease and oscillation of the bonding-antibonding splitting. The reduction of the bonding-antibonding splitting exceeds the band gap for 100 fs, indicating a transient state with crossed bands.
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.
C. B. Schaffer, J. F. Garcia, and E. Mazur. 2003. “Bulk heating of transparent materials using a high repetition-rate femtosecond laser.” Appl. Phys. A, 76, Pp. 351–354.Abstract
Femtosecond laser pulses can locally induce structural and chemical changes in the bulk of transparent materials, opening the door to the three-dimensional fabrication of optical devices. We review the laser and focusing parameters that have been applied to induce these changes and discuss the different physical mechanisms that play a role in forming them. We then describe a new technique for inducing refractive index changes in bulk material using a high repetition-rate femtosecond oscillator. The changes are caused by a localized melting of the material which results from an accumulation of thermal energy due to nonlinear absorption of the high repetition-rate train of laser pulses.
C. A. D. Roeser, A. M.-T. Kim, J. Paul Callan, L. Huang, E. N. Glezer, Y. Siegal, and E. Mazur. 2003. “Femtosecond time-resolved dielectric function measurements by dual-angle reflectometry.” Rev. Sci. Instrum., 74, Pp. 3413–3422. Publisher's VersionAbstract
We present a technique to measure the dielectric function of a material with femtosecond time resolution over a broad photon energy range. The absolute reflectivity is measured at two angles of incidence, and the dielectric function is calculated by numerical inversion of Fresnel-like formulas. Using white-light generation, the single-color probe is broadened from the near IR to the near UV, but femtosecond time resolution is maintained. Calibration of the apparatus and error analysis are discussed. Finally, measurements of isotropic, thin film, and uniaxial materials are presented and compared to reflectivity-only studies to illustrate the merit of the technique.
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.
C. A. D. Roeser, A. M.-T. Kim, and E. Mazur. 2003. “Ultrafast Lattice-Bonding Dynamics of Tellurium.” In . Ultrafast Electronics and Optoelectronics. Publisher's VersionAbstract
A pump-probe technique measuring the dielectric function is presented and applied to the ultrafast dynamics of coherent phonons in Te. Oscillations in the bonding-antibonding splitting are revealed, allowing for THz modulation of a semiconductor-semimetal transition.
J. B. Ashcom. 2003. “The role of focusing in the interaction of femtosecond laser pulses with transparent materials”.Abstract
Femtosecond lasers can generate extremely intense optical pulses, where the electric field is comparable to or greater than the field that binds electrons to their parent ions. In this regime, the interaction of light with transparent materials can become strongly nonlinear. Nonlinear absorption of the pulse can create an energetic plasma leading to damage in the material. The material, too, can modify the propagation of the pulse, leading to self- focusing and white light continuum generation, a dramatic broadening of the pulse spectrum. This dissertation identifies the numerical aperture (NA), or the strength of the external focusing as a critical parameter in controlling the interaction between short pulses and transparent materials. Using fused silica as a model optical material, we show that at high NA, single shot, catastrophic damage occurs and continuum generation is not observed. At low NA, continuum generation is produced, but bulk material modification accumulates over time at energies above the continuum generation threshold. The continuum spectrum decreases with increasing NA. Bulk micromachining using femtosecond lasers is practical for numerical apertures of 0.25 NA and above, where self-focusing effects are minimal. The bulk modification of natural diamond and surface machining of transparent polymers for microprinting and microfluidic channel fabrication are presented as applications of these results at high NA.
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.
J. E. Carey, C. H. Crouch, and E. Mazur. 2003. “Femtosecond-Laser-Assisted Microstructuring of Silicon Surfaces.” In Optics and Photonics News, 14: Pp. 32–36.Abstract
The authors present a technique for microstructuring silicon surfaces using femtosecond-laser-assisted chemical etching. The microstructuring process dramatically changes both the surface morphology and the optoelectronic properties. The authors discuss the reasons behind these changes and the possibilities for new uses in photodetector, sensor and display applications.
L. Tong, R. R. Gattass, J. B. Ashcom, S. He, J. Lou, M. Shen, I. Zaharieva Maxwell, and E. Mazur. 2003. “Subwavelength-diameter silica wires for low-loss optical wave guiding.” Nature, 426, Pp. 816–819. Publisher's VersionAbstract
Silica waveguides with diameters larger than the wavelength of transmitted light are widely used in optical communications, sensors and other applications. Minimizing the width of the waveguides is desirable for photonic device applications, but the fabrication of low-loss optical waveguides with subwavelength diameters remains challenging because of strict requirements on surface roughness and diameter uniformity. Here we report the fabrication of subwavelength-diameter silica wires for use as low-loss optical waveguides within the visible to near-infrared spectral range. We use a two-step drawing process to fabricate long free-standing silica wires with diameters down to 50 nm that show surface smoothness at the atomic level together with uniformity of diameter. Light can be launched into these wires by optical evanescent coupling. The wires allow single-mode operation, and have an optical loss of less than 0.1 dB/mm. We believe that these wires provide promising building blocks for future microphotonic devices with subwavelength-width structures.
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.
C. Wu, C. H. Crouch, L. Zhao, and E. Mazur. 2002. “Visible luminescence from silicon surfaces microstructured in air.” Appl. Phys. Lett., 81, Pp. 1999–2001.Abstract
We report visible luminescence from SiOx formed by microstructuring silicon surfaces with femtosecond laser pulses in air. Incorporation of oxygen into the silicon lattice occurs only where the laser beam strikes the surface. Laser microstructuring therefore offers the possibility of writing submicrometer luminescent features without lithographic masks. The amount of oxygen incorporated into the silicon surface depends on the laser fluence; the peak wavelength of the primary luminescence band varies between 540 and 630 nm and depends on the number of laser shots. Upon annealing, the intensity of the primary luminescence band increases significantly without any change in the luminescence peak wavelength, suggesting that the luminescence comes from defects rather than quantum confinement.
A. M.-T. Kim, J. Paul Callan, C. A. D. Roeser, and E. Mazur. 2002. “Ultrafast dynamics and phase changes in crystalline and amorphous GaAs.” Phys. Rev. B, 66, Pp. 245203–245203.Abstract
The femtosecond time-resolved response of the spectral dielectric function provides a great wealth of information on carrier and lattice dynamics in highly photo-excited semiconductors. We present measurements of the dielectric function of crystalline and amorphous GaAs over a broad spectral energy range (1.7–3.4 eV), with sub- 100 fs time resolution. A detailed analysis of the data reveals many new insights into the dynamics and phase changes in semiconductors at high excitation fluences up to and beyond the damage threshold.
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.
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.

Pages