Publications

The interactions between the electrons and the lattice in solids define many basic properties of these materials. Exciting the electrons in a solid with intense ultrashort laser pulses is one way to induce non-equilibrium processes, observe the resulting dynamics and obtain direct information on electron-lattice interactions. An ultrashort laser pulse can rapidly heat electrons to temperatures on the order of 103 K, while leaving the lattice near room temperature.

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This dissertation o ers a study of femtosecond laser disruption in single cells. Cells and tissues do not ordinarily absorb light in the near-IR wavelength range of femtosecond lasers. However, the peak intensity of a femtosecond laser pulse is very high and material disruption is possible through nonlinear absorption and plasma generation. Because the pulse duration is very short, it is possible to reach the intensity of optical breakdown at only nanojoules of energy per pulse. The low energy deposition and the high spatial localization of the nonlinear absorption, make femtosecond laser pulses an ideal tool for minimally disruptive subcellular nanosurgery. We show definitively that there can be bulk ablation within a single cell by studying the disrupted region under a transmission electron microscope. The width of the ablated area can be as small as 250 nm in diameter at energies near the ablation threshold. We also studied the e ect of the laser repetition rate on the subcellular disruption threshold. We compared the pulse energies for kHz and MHz pulse trains, and found that in the MHz regime heat accumulation in the focal volume needs to be accounted for. For this repetition rate the minimum pulse energy necessary for disruption depends on the laser irradiation time. We used femtosecond laser nanosurgery to probe tension in actin stress fibers in living endothelial cells. By severing an individual stress fiber and visualizing its retraction, we showed that actin carries prestress in adherent, non-contractile cells. By plating the cells on softer, more compliant substrates, we measured the deflection of the substrate and extrapolated the force contribution of a stress filament on total amount of force exerted by the cell.

We have previously shown that doping silicon with sulfur via femtosecond- laser irradiation leads to near-unity absorption of radiation from ultraviolet wavelengths to below band gap short-wave infrared wavelengths. Here, we demonstrate that doping silicon with two other group VI elements (chalcogens), selenium and tellurium, also leads to near-unity broadband absorption. A powder of the chalcogen dopant is spread on the silicon substrate and irradiated with femtosecond-laser pulses. We examine and compare the resulting morphology, optical properties, and chemical composition for each chalcogen-doped substrate before and after thermal annealing. Thermal annealing reduces the absorption of below-band gap radiation by an amount that correlates with the diffusivity of the chalcogen dopant used to make the sample. We propose a mechanism for the absorption of below band gap radiation based on defects in the lattice brought about by the femtosecond laser irradiation and the presence of a supersaturated concentration of chalcogen dopant atoms. The selenium and tellurium doped samples show particular promise for use in infrared photodetectors as they retain most of their infrared absorptance even after thermal annealinga necessary step in many semiconductor device manufacturing processes.

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.

A processing technique using femtosecond laser pulses to microstructure the surface of a silicon ava- lanche photodiode (APD) has been used to enhance its near-infrared (near-IR) response. Experiments were performed on a series of APDs and APD arrays using various structuring parameters and post- structuring annealing sequences. Following thermal annealing, we were able to fabricate APD arrays with quantum efficiencies as high as 58% at 1064 nm without degradation of their noise or gain performance. Experimental results provided evidence to suggest that the improvement in charge collec- tion is a result of increased absorption in the near-IR.

The nematode C. elegans, a millimeter-long roundworm, is a well-established model organism for studies of neural development and behavior, however physiological methods to manipulate and monitor the activity of its neural network have lagged behind the development of powerful methods in genetics and molecular biology. The small size and transparency of C. elegans make the worm an ideal test-bed for the development of physiological methods derived from optics and microscopy. We present the development and application of a new physiological tool: femtosecond laser dissection, which allows us to selectively ablate segments of individual neural fibers within live C. elegans. Femtosecond laser dissection provides a scalpel with submicrometer resolution, and we discuss its application in studies of neural growth, regenerative growth, and the neural basis of behavior.

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.

We show that the mechanism of nanoparticle formation during femtosecond laser ablation of silicon is affected by the presence of a background gas. Femtosecond laser ablation of silicon in a H2 or H2S background gas yields a mixture of crystalline and amorphous nanoparticles. The crystalline nanoparticles form via a thermal mechanism of nucleation and growth. The amorphous material has smaller features and forms at a higher cooling rate than the crystalline nanoparticles. The background gas also results in the suspension of plume material in the gas for extended periods, resulting in the formation (on a thin film carbon substrate) of unusual aggregated structures including nanoscale webs that span tears in the film. The presence of a background gas provides additional control of the structure and composition of the nanoparticles during short pulse laser ablation. PACS 81.16.-c

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.

Photodetectors fabricated on microstructured silicon are reported. The photodetectors exhibited high photoresponse; at 3 V bias, the responsivities were 92 A/W at 850 nm and 119 A/W at 960 nm. At wavelengths longer than 1.1 m, the photodetectors still showed strong photoresponse. A generation- recombination gain mechanism has been proposed to explain the photoresponse of these photodiodes. From measurements of the noise current density, the calculated gain was approximately 1200 at 3 V bias.

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.

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.

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.

Science is a creative process where the synthesis of new ideas requires discussion and debate. However, the traditional model for teaching assumes that all information presented to students is automatically learned. As a result, most students leave their introductory science courses frustrated and without a solid conceptual understanding. At the same time, instructors feel that students have not lived up to their expectations, yet they cannot identify the problem. Peer Instruction is an interactive approach that was designed to improve the learning process. This approach provides students with greater opportunity for synthesizing the concepts while instructors get timely feedback that can help focus the instruction on the points that are the most difficult for the students. Peer Instruction is flexible and easy to use on its own or in conjunction with other teaching methods. This chapter discusses the motivation for using Peer Instruction and the mechanics of implementing it in the classroom.

German translation of of Chapter 2 of "Peer Instruction: A User's Manual" by Eric Mazur (Prentice Hall, 1997).

We investigate if the gender gap in conceptual understanding in an introductory university physics course can be reduced by using interactive engagement methods that promote in-class interaction, reduce competition, foster collaboration, and emphasize conceptual understanding. To this end we analyzed data from the introductory calculus-based physics course for non- majors at Harvard University taught traditionally or using different degrees of interactive engagement. Our results show that teaching with certain interactive strategies not only yields significantly increased understanding for both males and females, but also reduces the gender gap. In the most interactively taught courses, the pre-instruction gender gap was gone by the end of the semester.

We use two-photon absorption polymerization to fabricate optically active microstructures that exhibit optically-induced birefringence and dichroism. Our results open the door to new applications in data storage, waveguides and optical circuitry.

Background: Caenorhabditis elegans actively crawls down thermal gradients until it reaches the temperature of its cultivation, exhibiting what is called cryophilic movement. Implicit in the worms ability to actively bias its movements down thermal gradients is an ability to detect thermal gradients, and implicit in regulating the display of cryophilic bias is the ability to compare current ambient temperature with a stored memory of cultivation temperature. Several lines of evidence link the AFD sensory neuron to thermotactic behavior, but its exact role is not yet known. A current model contends that AFD is part of a thermophilic mechanism which biases movement up thermal gradients that counterbalances a cryophilic mechanism which biases movement down thermal gradients. Results: We used tightly-focused femtosecond laser pulses to dissect the AFD neuronal cell bodies and the AFD sensory dendrites in C. elegans to investigate their contribution to biased cryophilic movement. We establish that femtosecond laser ablation can exhibit submicrometer precision allowing the severing of individual AFD nerve fibers without causing collateral damage. Severing AFD dendrites in young adult worms permanently abolishes their sensory contribution without functional regeneration. We show that thermosensory input to the AFD neuron is required to activate a mechanism for generating cryophilic bias, but we find no evidence that AFD laser surgery reduces a putative ability to generate thermophilic bias. In addition, although disruption of the AIY interneuron causes worms to exhibit cryophilic bias at all temperatures, we find no evidence that disruption of the AIZ interneuron causes thermophilic bias at any temperature. Conclusions: We conclude that laser surgical analysis of the thermotactic circuit does not support a current model in which AFD opposes cryophilic bias by generating thermophilic bias. Our data supports a model in which a mechanism for generating cryophilic bias is gated by the AFD neurons.

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