In a recent article in The Physics Teacher, Michael Sobel claims (as do many teachers) that physics is in a "special category of hard" and is usually taken only by a "certain sort of very bright student." The appealing, yet suspiciously conceited, notion that physics is only for smart or industrious people is questionable. We offer this response as a means to initiate a dialogue on how we engage with students in our physics courses.
Using a broadband dual-angle pump-probe reflectometry technique, we obtained the ultrafast dielectric function dynamics of bulk ZnO under femtosecond laser excitation. We determined that multiphoton absorption of the 800-nm femtosecond-laser excitation creates a large population of excited carriers with excess energy. Screening of the Coulomb interaction by the excited free carriers, causes damping of the exciton resonance and renormalization of the bandgap, causing broadband (2.33.5 eV) changes in the dielectric function of ZnO. From the dielectric function, many transient material properties, such as the index of refraction of ZnO under excitation, can be determined to optimize ZnO-based devices.
Discussions of education are generally predicated on the assumption that we know what education is. I hope to convince you otherwise by recounting some of my own experiences. Download a podcast of this article narrated by Samuel Smith of the Center for Teaching and Learning at Brigham Young University.
Femtosecond laser ablation selectively dissects subcellular components of the C. elegans neuronal circuit with submicrometer precision for studying the neuronal origins of behavior. We describe the theoretical basis for the high precision of femtosecond laser ablation in the target bulk. Next, we present the experimental setup and a worm rotation technique to facilitate imaging and surgery. We describe the damage caused by different pulse energies on cell bodies and neuronal fibers. Finally, we discuss the regrowth of neuronal fibers after surgery and its impact on behavioral study.
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.
We present a new type of surface-enhanced Raman scattering (SERS) substrate that exhibits extremely large and uniform cross-section enhancements over a macroscopic (greater than 25 mm2) area. The substrates are fabricated using a femtosecond laser nanostructuring process, followed by thermal deposition of silver. SERS signals from adsorbed molecules show a spatially uniform enhancement factor of approximately 107. Spectroscopic characterization of these substrates suggests their potential for use in few or single-molecule Raman spectroscopy.
We perform a transmission experiment on a ZnO nanowire waveguide to study its transmission characteristics under nonlinear femtosecond-pulse excitation. We ﬁnd that both the second harmonic and the photoluminescence couple into low-order waveguide modes of the nanowires but with distinctly different efﬁciencies. We measure the transmission spectrum of a single ZnO nanowire waveguide for near-UV light generated by interband recombination processes. The transmission spectrum allows us to determine the absorption edge of the excited nanowire and to study the temperature proﬁle of the nanowire under femtosecond-pulse excitation.
We present a method for improving femtomole-level trace detection (10^9 molecules) using large-area surface-enhanced Raman scattering (SERS) substrates. Using multiphoton-induced exposure of a commercial photoresist, we physically limit the available molecular adsorption sites to only the electromagnetic hot spots on the substrate. This process prevents molecules from adsorbing to sites of weak SERS enhancement, while permitting adsorption to sites of extraordinary SERS enhancement. For a randomly adsorbed submonolayer of benzenethiol molecules the average Raman scattering cross section of the processed sample is 27 times larger than that of an unprocessed SERS substrate.
This thesis explores the structure and properties of silicon doped with chalcogens beyond the equilibrium solubility limit, with a focus on the potential presence of an impurity band and its relevance to photovoltaics. The investigations that we report here shed new light on the electronic role of sulfur dopants in particular, and also provide new evidence of a semiconductor-to-metal transition consistent with the formation of an electron-conducting impurity band. The thesis is divided into three primary studies. First, we describe doping silicon with a single fs-laser pulse. We ﬁnd that irradiation above the melting threshold is suﬃcient for doping a thin layer of silicon to non-equilibrium sulfur concentrations. Next, we explore the interaction of many fs-laser pulses with a silicon substrate. Temperature-dependent electronic transport measurements indicate metallic conduction, while a form of Fermi level spectroscopy and optical absorption data indicate the presence of an impurity band located 200 − 300 meV below the conduction band edge. Third, we investigate silicon doped to non- equilibrium concentrations using a diﬀerent technique: ion-implantation followed by pulsed laser melting and crystal regrowth. We determine one of the sulfur states present at low sulfur dose. Additional transport measurements point to the presence of a semiconductor-to- metal transition at sulfur doses corresponding to implanted sulfur concentrations just above 10^20 cm−3 . Finally, in the appendices of this thesis, we describe methods to laser-dope silicon while avoiding the development of signiﬁcant surface roughness that typically characterizes such samples. Additionally, we present the status of investigations into laser-doping silicon with selenium to non- equilibrium concentrations.
Sulfur doping of silicon beyond the solubility limit by femtosecond laser irradiation leads to near-unity broadband absorption of visible and infrared light and the realization of silicon-based infrared photodetectors. The nature of the infrared absorption is not yet well understood. Here we present a study on the reduction of infrared absorptance after various anneals of different temperatures and durations for three chalcogens (sulfur, selenium, and tellurium) dissolved into silicon by femtosecond laser irradiation. For sulfur doping, we irradiate silicon in SF6 gas; for selenium and tellurium, we evaporate a film onto the silicon and irradiate in N2 gas; lastly, as a control, we irradiated untreated silicon in N2 gas. Our analysis shows that the deactivation of infrared absorption after thermal annealing is likely caused by dopant diffusion. We observe that a characteristic diffusion lengthcommon to all three dopantsleads to the reduction of infrared absorption. Using diffusion theory, we suggest a model in which grain size of the re-solidiﬁed surface layer can account for this characteristic diffusion length, indicating that deactivation of infrared absorptance may be caused by precipitation of the dopant at the grain boundaries.
Femtosecond laser ablation permits non-invasive surgeries in the bulk of a sample with submicrometer resolution. We briefly review the history of optical surgery techniques and the experimental background of femtosecond laser ablation. Next, we present several clinical applications, including dental surgery and eye surgery. We then summarize research applications, encompassing cell and tissue studies, research on C. elegans, and studies in zebrafish. We conclude by discussing future trends of femtosecond laser systems and some possible application directions.
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
Two-photon polymerization is a powerful tool for fabricating three-dimensional micro/nano structures for applications ranging from nanophotonics to biology. To tailor such structure for specific purposes it is often important to dope them. In this paper we report on the fabrication of structures, with nanometric surface features (resolution of approximately 700 nm), using two-photon polymerization of an acrylic resin doped with the biocompatible polymer chitosan using a guest-host scheme. The fluorescence background in the Raman spectrum indicates the presence of chitosan throughout the structure. Mechanical characterization reveals that chitosan does not affect the mechanical properties of the host acrylic resin and, consequently, the structures exhibit excellent integrity. The approach presented in this work can be used in the fabrication of micro- and nanostructures containing biopolymers for biomedical applications.
We report a pump-probe study of the two-photon induced reflectivity changes in bis (n-butylimido) perylene thin films. To enhance the two-photon excitation we deposited bis (n-butylimido) perylene films on top of gold nano-islands. The observed transient response in the reflectivity spectrum of bis (n-butylimido) perylene is due to a depletion of the molecule�s ground state and excited state absorption.
eer Instruction (PI) is an interactive teaching technique that promotes classroom interaction to engage students and address difficult aspects of the material (Crouch, Watkins, Fagen, & Mazur, 2007; Crouch & Mazur, 2001; Mazur, 1997). By providing opportunities for students to discuss concepts in class, PI allows students to learn from each other. However, for this method to be most effective, students need to come to class with some basic understand- ing of the material. Just-in-Time Teaching (JiTT) is an ideal complement to PI, as JiTT structures students' reading before class and provides feedback so the instructor can tailor the PI questions to target student difficulties. Separately, both JiTT and PI provide students with valuable feedback on their learning at different times in the process – JiTT works asynchronously out of class, and PI gives real-time feedback. Together, these methods help students and instructors monitor learning as it happens, strengthening the benefits of this feedback. As this chapter details, the combination of these methods is useful for improving student learning and skill development.
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 growing number of physics teachers are currently turning to instructional technologies such as wireless handheld response systemscolloquially called clickers. Two possible rationales may explain the growing interest in these devices. The first is the presumption that clickers are more effective instructional instruments. The second rationale is somewhat reminiscent of Martin Davis' declaration when purchasing the Oakland Athletics: As men get older, the toys get more expensive. Although personally motivated by both of these rationales, the effectiveness of clickers over inexpensive low-tech flashcards remains questionable. Thus, the first half of this paper presents findings of a classroom study comparing the differences in student learning between a Peer Instruction group using clickers and a Peer Instruction group using flashcards. Having assessed student learning differences, the second half of the paper describes differences in teaching effectiveness between clickers and flashcards.
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.
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.