Publications

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.

Subwavelength- and nanometer-diameter glass nanofibers have been fabricated using a high-temperature taper-drawing process. As-drawn nanofibers show extraordinary uniformities in terms of diameter variation and surface roughness and are suitable for single-mode optical wave guiding. Measured optical losses of these nanofibers are typically below 0.1 dB/mm. Photonic devices such as linear waveguides, optical couplers and microrings assembled with nanofibers are also demonstrated. Our results show that taper-drawn glass nanofibers are promising building blocks for micro- and nano-scale photonic devices.

We report on the formation of high-density regular arrays of nanometer-scale rods using femtosecond laser irradiation of a silicon surface immersed in water. The resulting surface exhibits both micrometer-scale and nanometer-scale structures. The micrometer-scale structure consists of spikes of 5-10 µm width, which are entirely covered by nanometer-scale rods that are roughly 50 nm wide and that protrude perpendicularly from the micrometer-scale spikes. The formation of the nanometer-scale rods involves several processes: refraction of laser light in highly excited silicon, interference of scattered and refracted light, rapid cooling in water, and capillary instabilities.

Room temperature visible and near-infrared photoluminescence from black silicon has been observed. The black silicon is manufactured by shining femtosecond laser pulses on silicon wafers in air, which were later annealed in vacuum. The photoluminescence is quenched above 120 K due to thermalization and competing nonradiative recombination of the carriers. The photoluminescence intensity at 10K depends sublinearly on the excitation laser intensity confirming band tail recombination at the defect sites.

Nanophotonics is a fusion of photonics and nanotechnology, and is defined as nanoscale optical science and technology that includes nanoscale confinement of radiation, nanoscale confinement of matter, and nanoscale photoprocesses for nanofabrication [1.], [2.] and [3.]. While photonics has been widely used for fiber-optic data communication for decades, the application of nanotechnology for optical communication is an emerging technology. The basic motivation for incorporating photonics with nanotechnology is spurred by the requirement of increased integration of photonic devices for a variety of applications such as higher data transmission rates, faster response, lower energy consumption, and denser data storage [2]. For example, to reach an optical data transmission rate as high as 10Tb/s, the size of photonic matrix switching devices should be reduced to 100-nm scale [4].

We have fabricated silica fibers with diameters less than one micrometer and molecularly smooth side walls. We modeled the waveguide properties and demonstrated low-loss light propagation over tens of millimeters, sufficient for microphotonic device applications. We manipulated silica nanowires into geometries to demonstrate waveguiding around tight bends as small as 5 micrometers, evanescent coupling, and wavelength filtering as a ring resonator. The linear waveguiding properties produced by the large index contrast between silica and air yield a tight confinement of the mode, which, when combined with the high electric field intensity in an ultrashort laser pulse, can produce significant nonlinear effects over lengths of about one millimeter. We studied nonlinear optical properties by observing the spectral broadening as a probe for the diameter-dependent nonlinearity of the silica nanowire. Our measurements confirmed the dependence of the generated spectra on the theoretically calculated effective nonlinearity and diameter-dependent dispersion. Our results reveal a diameter range for silica nanowires with an enhanced nonlinearity which may be employed in nonlinear devices. We fabricate a nonlinear Sagnac interferometer using silica nanowires for optical switching and discuss possibilities for optical logic. Our results confirm light-by-light modulation, with pulse energies less than a couple of nJ. Combining top-down and bottom-up fabrication techniques, we use tapered silica fibers to couple light directly into the waveguiding modes of ZnO nanowires. We experimentally confirm simulations for the coupling efficiencies and propagation of higher order modes. We also excite ZnO nanowires with ultrashort laser pulses and compare the spectrum transmitted along the nanowire waveguide with the spectrum at the excitation spot. Our results show a shifting of the band edge that indicates a local heating of the nanowire by a few hundred degrees, which is confirmed by finite-element simulation. Finally, we discuss the outlook for nanoscale nonlinear optical devices.

We compare the effectiveness of a first implementation of peer instruction (PI) in a two-year college with the first PI implementation at a top-tier four-year research institution. We show how effective PI is for students with less background knowledge and what the impact of PI methodology is on student attrition in the course. Results concerning the effectiveness of PI in the college setting replicate earlier findings: PI-taught students demonstrate better conceptual learning and similar problem-solving abilities than traditionally taught students. However, not previously reported are the following two findings: First, although students with more background knowledge benefit most from either type of instruction, PI students with less background knowledge gain as much as students with more background knowledge in traditional instruction. Second, PI methodology is found to decrease student attrition in introductory physics courses at both four-year and two-year institutions.

Background Egg laying in Caenorhabditis elegans has been well studied at the genetic and behavioral levels. However, the neural basis of egg-laying behavior is still not well understood; in particular, the roles of specific neurons and the functional nature of the synaptic connections in the egg- laying circuit remain uncharacterized. Results We have used in vivo neuroimaging and laser surgery to address these questions in intact, behaving animals. We have found that the HSN neurons play a central role in driving egg-laying behavior through direct excitation of the vulval muscles and VC motor neurons. The VC neurons play a dual role in the egg-laying circuit, exciting the vulval muscles while feedback-inhibiting the HSNs. Interestingly, the HSNs are active in the absence of synaptic input, suggesting that egg laying may be controlled through modulation of autonomous HSN activity. Indeed, body touch appears to inhibit egg laying, in part by interfering with HSN calcium oscillations. Conclusions The egg-laying motor circuit comprises a simple three-component system combining feed-forward excitation and feedback inhibition. This microcircuit motif is common in the C. elegans nervous system, as well as in the mammalian cortex; thus, understanding its functional properties in C. elegans may provide insight into its computational role in more complex brains.

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.

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.

We present femtosecond time-resolved broadband measurements of the reflectivity of aluminum during the laser-induced solid-to-liquid phase transition. Our experiments show that this transition is a thermal process, settling an existing controversy.

We present femtosecond time-resolved measurements of the reflectivity of aluminum during the laser-induced solid-to-liquid phase transition over the spectral range 1.73.5 eV. Previous optical and electron diffraction studies have shown discrepancies on the order of picoseconds in the timescale of the solid- to-liquid phase change. As a result, it is not clear if the transition mechanism is thermal or non-thermal. Our experiments conclusively show that this transition is a thermal process mediated through the transfer of heat from the photoexcited electronic population to the lattice. Our findings agree with the results of the electron diffraction study and rule out the non-thermal mechanism proposed by the optical study.

In this thesis, we investigate the irradiation of silicon, in a background gas of near atmospheric pressure, with intense femtosecond laser pulses at energy densities exceeding the threshold for ablation (the macroscopic removal of material). We study the resulting structure and properties of the material ejected in the ablation plume as well as the laser irradiated surface itself. The material collected from the ablation plume is a mixture of single crystal silicon nanoparticles and a highly porous network of amorphous silicon. The crystalline nanoparticles form by nucleation and growth; the amorphous material has smaller features and forms at a higher cooling rate than the crystalline particles. The size distribution of the crystalline particles suggests that particle formation after ablation is fundamentally different in a background gas than in vacuum. We also observe interesting structures of coagulated particles such as straight lines and bridges. The laser irradiated surface exhibits enhanced visible and infrared absorption of light when laser ablation is performed in the presence of certain elements–-either in the background gas or in a film on the silicon surface. To determine the origin of this enhanced absorption, we perform a comprehensive annealing study of silicon samples irradiated in the presence of three different elements (sulfur, selenium and tellurium). Our results support that the enhanced infrared absorption is caused by a high concentration of dopants dissolved in the lattice. Thermal annealing reduces the infrared absorptance of each doped sample at the same rate that dopants diffuse from within the polycrystalline grains in the laser irradiated surface layer to the grain boundaries. Lastly, we measure the photovoltaic properties of the laser irradiated silicon as a function of several parameters: annealing temperature, laser fluence, background gas, surface morphology and chemical etching. We explore the concept of using thin silicon films as the irradiation substrate and successfully enhance the visible and infrared absorption of films < 2 micrometers thick. Our results suggest that the incorporation of a femtosecond laser modified region into a thin film silicon device could greatly enhance its photovoltaic efficiency.

Femtosecond-laser micromachining of poly[2-methoxy-5-(2'1/2- ethylhexyloxy)- p-phenylene vinylene] (MEH-PPV) films is investigated using 130-fs pulses at 800-nm from a laser oscillator operating at 76-MHz repetition rate. We investigate the effect of pulse energy and translation speed on the depth and morphology of the micromachined regions. We quantified the MEH- PPV photobleaching induced by the fs-laser, and the conditions in which the emission of MEH-PPV is preserved after the micromaching.

The use of ultrashort laser pulses for microscopy has steadily increased over the past years. In this so-called multiphoton microscopy, laser pulses with pulse duration around 100 femtoseconds (fs) are used to excite fluorescence within the samples. Due to the high peak powers of fs lasers, the absorption mechanism of the laser light is based on nonlinear absorption. Therefore, the fluorescence signal is highly localized within the bulk of biological materials, similar to a confocal microscope. However, this nonlinear absorption mechanism can not only be used for imaging but for selective alteration of the material at the laser focus: The absorption can on one hand lead to the excitation of fluorescent molecules of fluorescently tagged cells by the simultaneous absorption of two or three photons or on the other hand, in case of higher order processes, to the creation of free-electron plasmas and, consequently, plasma-mediated ablation. Typical imaging powers are in the range of tens of milliwatts using 100-fs pulses at a repetition rate of 80-90 MHz, while pulse energies needed for ablation powers are as low as a few nanojoules when using high numerical aperture microscope objectives for focusing the laser radiation into the sample. Since the first demonstration of this technique, numerous applications of fs lasers have emerged within the field of cellular biology and microscopy. As the typical wavelengths of ultrashort laser systems lie in the near infrared between 800 and 1000 nm, high penetration depth can be achieved and can provide the possibility of imaging and manipulating the biological samples with one single laser system.

We use tapered silica fibers to inject laser light into ZnO nanowires with diameters around 250 nm to study their waveguiding properties. We find that high-order waveguide modes are frequently excited and carry significant intensity at the wire surface. Numerical simulations reproduce the experimental observations and indicate a coupling efficiency between silica and ZnO nanowires of 50%. Experimentally, we find an emission angle from the ZnO nanowires of about 90, which is in agreement with the simulations.

We calculated the intraband and interband optical deformation potentials for semiconducting alpha-Te from experimental data of the band gap shrinkage and softening of the A1-optical phonon mode in response to femtosecond laser excitation. These potentials were obtained by applying first- and second-order perturbation theory to the Frlich Hamiltonian describing the carrier-phonon interaction. The intraband optical deformation potential is considerably smaller than previously estimated values; there are no previously reported values for the interband optical deformation potential.

Development of biochips containing living cells for biodetection, drug screening and tissue engineering applications is limited by a lack of reconfigurable material interfaces and actuators. Here we describe a new class of nanostructured magnetizable materials created with a femtosecond laser surface etching technique that function as multiplexed magnetic field gradient concentrators. When combined with magnetic microbeads coated with cell adhesion ligands, a microarray of virtual adhesive islands that can support cell attachment, resist cell traction forces and maintain cell viability. A cell death (apoptosis) response can then be actuated on command by removing the applied magnetic field, thereby causing cell retraction, rounding and detachment. This simple technology may be used to create reconfigurable interfaces that allow users to selectively discard contaminated or exhausted cellular sensor elements, and to replace them with new living cellular components for continued operation in future biomedical microdevices and biodetectors. Keywords: magnetic particles, magnetic gradient concentrator, culture substrate, apoptosis, mechanical force, cell shape

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.

Subpicosecond, picometer atomic displacements in α-Те photoexcited by single femtosecond laser pulses have been measured by means of time-resolved optical reflectometry revealing threshold- like coherent quantum emission of single softened fully symmetrical optical A1-phonons and demonstrating absolute detection capability of this technique in studies of coherent phonon dynamics in solids.