Interaction of femtosecond laser pulses with transparent materials

Chris B. Schaffer

Harvard University Ph.D. Thesis (2001)

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An intense femtosecond laser pulse can have an electric field strength which approaches or even exceeds the strength of the electric field that holds valence electrons in a transparent material to their ionic cores. In this regime, the interaction between the laser pulse and the material becomes highly nonlinear. Laser energy can be nonlinearly absorbed by the material, leading to permanent damage, and the material’s nonlinear response to the laser field can, in turn, induce radical changes in the laser pulse itself. The nature of these nonlinear interactions, the changes produced in the material and to the laser pulse, as well as several practical applications are explored in this thesis. We measure the laser intensity required to damage bulk transparent materials and uncover the dominant nonlinear ionization mechanism for different laser wavelengths and material band gaps. Using optical and electron microscopy, we examine the morphology of the material changes induced by tightly-focused femtosecond laser pulses in bulk transparent materials, and identify several mechanisms by which material changes are produced. We show that a high repetition rate train of femtosecond laser pulses can provide a point source of heat located inside the bulk of a transparent material, an effect which no other technique can achieve. The mechanism for white-light continuum generation is uncovered through measurement of the laser wavelength, the material band gap, and the external focusing angle dependence of the continuum spectrum. Using a time-resolved imaging technique, we follow the dynamics of the laser-produced plasma over eight orders of magnitude in time, revealing picosecond time scale dynamics that have not been previously observed. Finally, we discuss applications in direct writing of optical waveguides and in sub-cellular laser surgery.

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Laser-induced breakdown and damage in bulk transparent materials induced by tightly focused femtosecond laser pulses

Chris B. Schaffer, Andre Brodeur, and Eric Mazur

Measurement Science and Technology (2001)

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Laser-induced breakdown and damage to transparent materials has remained an active area of research for four decades. In this paper we review the basic mechanisms that lead to laser-induced breakdown and damage and present a summary of some open questions in the field. We present a method for measuring the threshold intensity required to produce breakdown and damage in the bulk, as opposed to on the surface, of the material. Using this technique, we measure the material band-gap and laser-wavelength dependence of the threshold intensity for bulk damage using femtosecond laser pulses. Based on these thresholds, we determine the relative role of different nonlinear ionization mechanisms for different laser and material parameters.

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Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy

Chris B. Schaffer, André Brodeur, José F. García, and Eric Mazur

Optics Letters (2001)

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Using tightly focused femtosecond laser pulses of just 5 nJ, we produce optical breakdown and structural change in bulk transparent materials and demonstrate micromachining of transparent materials by use of unamplified lasers. We present measurements of the threshold for structural change in Corning 0211 glass as well as a study of the morphology of the structures produced by single and multiple laser pulses. At a high repetition rate, multiple pulses produce a structural change dominated by cumulative heating of the material by successive laser pulses. Using this cumulative heating effect, we write single-mode optical waveguides inside bulk glass, using only a laser oscillator.

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Micromachining using ultrashort pulses from a laser oscillator

Chris B. Schaffer and Eric Mazur

Optics and Photonics News (2001)

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In recent years femtosecond laser pulses have been used to micromachine a great variety of materials. Ultrashort pulses cleanly ablate virtually any material with a precision that meets or exceeds that of other laser-based techniques, making the femtosecond laser an attractive micromachining tool.1 In transparent materials, where micromachining relies on nonlinear absorption, femtosecond lasers allow three-dimensional microfabrication with submicrometer precision. These lasers can produce three-dimensionally localized refractive index changes in the bulk of a transparent material, opening th door to the fabrication of a wide variety of optical devices. Until recently, micromachining of transparent materials was believed to require amplified laser systems. We have found, however, that transparent materials can also be micromachined using tightly focused trains of femtosecond laser pulses from an unamplified laser oscillator. In addition to reducing the cost and complexity of the laser system, femtosecond laser oscillators enable micromachining using a multipleshot cumulative effect.We have used this new technique to directly write singlemode optical waveguides into bulk glass.

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Time-decorrelated multifocal micromachining and trapping

David N. Fittinghoff, Chris B. Schaffer, Eric Mazur, and Jeffrey A. Squier

IEEE Journal of Selected Topics in Quantum Electronics (2001)

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Temporally decorrelated multifocal arrays eliminate spatial interference between adjacent foci and allow multifocal imaging with the diffraction-limited resolution of a single focus, even for foci spaced by less than the focal diameter. In this paper, we demonstrate a high-efficiency cascaded-beamsplitter array for producing temporally decorrelated beamlets. These beamlets are used to produce a multifocal microscope with which we have demonstrated two-photon fluorescence imaging, multifocal micromachining of optical waveguides, and multifocal optical trapping.

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Programmable shaping of ultrabroad-bandwidth pulses from a Ti:sapphire laser

Anatoly Efimov, Chris Schaffer, David H. Reitze

Journal of the Optical Society of America (1995)

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We have used a commercially available liquid-crystal spatial light modulator within a reflective optics pulse-shaping apparatus to shape ultrashort pulses with temporal resolution approaching 10 fs. Using the spatial light modulator as a phase modulator, we produce a variety of complex ultrafast waveforms, including odd pulses, high repetition rate (.23 THz) pulse trains, and asymmetric pulse trains. We also show that it is possible to compensate for large amounts of high-order phase dispersion (in excess of 60p) by appropriate cubic- and quartic-phase modulations of the pulse. Finally, we examine the limitations of shaping ultrabroad-bandwidth pulses. We find that, for specific classes of waveforms, Fourier-transform pulse-shaping techniques can be used for pulses with 5-fs durations, which exceed the current state of the art in ultrashort pulse generation. However, synthesis of general waveforms with 5-fs resolution will require compensating for nonlinear spatial dispersion of frequency in the masking plane.

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