Publications

In recent years, femtosecond lasers have been used for a multitude of micromachining tasks. Several groups have shown that femtosecond laser pulses cleanly ablate virtually any material with a precision that consistently meets or exceeds that of other laser-based techniques, making the femtosecond laser an extremely versatile surface micromachining tool. For large bandgap materials, where laser machining relies on nonlinear absorption of high intensity pulses for energy deposition, femtosecond lasers offer even greater benefit. Because the absorption in a transparent material is nonlinear, it can be confined to a very small volume by tight focusing, and the absorbing volume can be located in the bulk of the material, allowing three-dimensional micromachining. The extent of the structural change produced by femtosecond laser pulses can be as small as or even smaller than the focal volume. Recent demonstrations of three-dimensional micromaching of glass using femtosecond lasers include three-dimensional binary data storage, and the direct writing of optical waveguides and waveguide splitters. The growing interest in femtosecond laser micromachining of bulk transparent materials makes it more important than ever to uncover the mechanisms responsible for producing permanent structural changes.

The short duration of femtosecond laser pulses allows high laser intensities to be reached with low pulse energies. Focusing pulses through high numerical aperture microscope objectives leads to intensities high enough to induce plasma formation and photodisruption of matter at the laser focus through nonlinear mechanisms. These studies investigate the potential for using femtosecond lasers for photodisruptive surgery on the surface and in the bulk of turbid tissue. As our tissue model, we use mouse skin tissue. Our experiments demonstrate that incisions are made on the tissue surface by translating the laser focus across the sample while irradiating with a continuous train of pulses. Subsurface cavities are made in the tissue bulk by focusing the laser beneath the surface. Images of histological sections of samples show that damage microstructures are created with high precision and minimal collateral damage outside the focal region. We find that there is a maximum depth at which subsurface cavities can be made; placing the laser focus below this depth results in filament formation through the nonlinear optical phenomenon of selffocusing. Studies of photodisruption in fixed cell samples show that damage morphologies are made on the size scale of subcellular structures. We demonstrate that cells can be fluorescently labeled for specific cellular structures and that photodisruption of these samples can be effectively discerned from photobleaching effects. Subcellular damage is also shown to be localized completely within the cell. We demonstrate the feasibility of using femtosecond laser pulses for manipulation of subcellular structures in living cells, and we show that this can be done without compromising cell membrane integrity.

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.

We measure the ordinary and extraordinary dielectric function of Te with femtosecond time resolution after coherent phonons are generated with an ultrashort laser pulse. The results reveal oscillatory behavior in the bonding-antibonding split at   3 THz.

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.

We discuss the properties of microstructures obtained by texturing a silicon wafer using femtosecond laser-assisted chemical etching. The texturing process drastically changes the silicons optical and electronic properties allowing for novel use in commercial applications.

High quality Y2O3-ZrO2 single crystal rectangular waveguides had been developed for high-temperature sensing applications. The waveguides were fabricated from bulky Y2O3 stabilized ZrO2 single crystal by precise cut and fine polish. Three rectangular waveguides with cross-section larger than 1mmx1mm and length of 45mm 65mm were obtained. They showed much better optical properties than Y2O3-ZrO2 single crystal fibers grown for fiber-optic sensing in previous work, optical losses of these waveguides were lower than 0.03dB/cm at wavelength of 900nm, and they were able to endure temperature as higher as 2300 degree(s)C. All of them survived a 10g vibration test with average STF (strain to failure) of about 0.25%. Experimental results show that, these waveguides are promising for fiber-optic sensing for temperature above 2000 degree(s)C.

AlthoughY2O3-ZrO2 fiber-optic sensor has been developed for contact measurement of temperature higher than 2000 degree(s)C, its performance is not as good as that of sapphire fiber-optic sensor below 1900 degree(s)C due to the large optical loss of the Y2O3-ZrO2 fiber. In order to improve the Y2O3- ZrO2 fiber-optic sensor for ultra-high-temperature applications, in this work, based on a newly developed rectangular Y2O3-ZrO2 single-crystal waveguide with much lower optical loss, an improved Y2O3- ZrO2 waveguide- fiber-optic sensor has been developed. The sensor has been tested up to near 2300 degree(s)C, we estimate that, the improved sensor has similar performance as the sapphire fiber-optic sensor in accuracy and resolution, except the disadvantage of relatively short waveguide. In addition, in this work, instead of the previous volatile and toxic BeO-coated probe, we use a multi-ions-doped sensor head, which is much stable and safe.

Soon after it was discovered that intense laser pulses of nanosecond duration from a ruby laser could anneal the lattice of silicon, it was established that this so-called pulsed laser annealing is a thermal process. Although the radiation energy is transferred to the electrons, the electrons transfer their energy to the lattice on the timescale of the excitation. The electrons and the lattice remain in equilibrium and the laser simply heats the solid to the melting temperature within the duration of the laser pulse. For ultrashort laser pulses in the femtosecond regime, however, thermal processes (which take several picoseconds) and equilibrium thermodynamics cannot account for the experimental data. On excitation with femtosecond laser pulses, the electrons and the lattice are driven far out of equilibrium and disordering of the lattice can occur because the interatomic forces are modified due to the excitation of a large (10% or more) fraction of the valence electrons to the conduction band. This review focuses on the nature of the non-thermal transitions in semiconductors under femtosecond laser excitation.

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.

We use third harmonic generation (THG) microscopy to image waveguides and single-shot structural modifications produced in bulk glass using femtosecond laser pulses. THG microscopy reveals the internal structure of waveguides written with a femtosecond laser oscillator, and gives a three-dimensional view of the complicated morphology of the structural changes produced with single, above-threshold femtosecond pulses. We find that THG microscopy is as sensitive to refractive index change as differential interference contrast microscopy, while also offering the three-dimensional sectioning capabilities of a nonlinear microscopy technique. It is now possible to micromachine three- dimensional optical devices and to image these structures in three dimensions, all with a single femtosecond laser oscillator.

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.

In recent years, femtosecond lasers have been used for a multitude of micromachining tasks. Several groups have shown that femtosecond laser pulses cleanly ablate virtually any material with a precision that consistently meets or exceeds that of other laser-based techniques, making the femtosecond laser an extremely versatile surface micromachining tool. For large bandgap materials, where laser machining relies on nonlinear absorption of high intensity pulses for energy deposition, femtosecond lasers offer even greater benefit. Because the absorption in a transparent material is nonlinear, it can be confined to a very small volume by tight focusing, and the absorbing volume can be located in the bulk of the material, allowing three-dimensional micromachining. The extent of the structural change produced by femtosecond laser pulses can be as small as or even smaller than the focal volume. Recent demonstrations of three-dimensional micromaching of glass using femtosecond lasers include three-dimensional binary data storage, and the direct writing of optical waveguides and waveguide splitters. The growing interest in femtosecond laser micromachining of bulk transparent materials makes it more important than ever to uncover the mechanisms responsible for producing permanent structural changes.

Based on the exact solutions of Maxwells equations, we have studied the basic theoretical properties of submicron and nano-diameter air-cladding silica-wire waveguides. The single-mode condition and the modal field of the fundamental modes have been obtained. Silica wires with diameters of 100-1000nm and lengths ranging from hundreds of micrometer to over 1 millimeter have been fabricated. SEM examination shows that these wires have uniform diameters and smooth surfaces, which are favorable for optical wave guiding. Light has been sent into these wires by optical coupling, and guiding light through a bent wire has also been demonstrated. These wires are promising for assembling photonic devices on a micron or submicron scale.

We measure the ordinary and extraordinary dielectric function of Te with femtosecond time resolution after coherent phonons are generated with an ultrashort laser pulse. The results reveal oscillatory behavior in the bonding-antibonding split at   3 THz.

Arrays of sharp, conical microstructures were obtained by structuring the surface of a silicon wafer using femtosecond laser-assisted etching. Analysis of the arrays shows high, stable field emission without any further processing steps and turn-on fields as low as 1.3 V/um

Arrays of sharp, conical microstructures were obtained by structuring the surface of a silicon wafer using femtsecond laser assisted etching. Analysis of the arrays shows high, stable field emission without any further processing steps.

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 materials 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.

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.

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. In transparent materials, where micromachining relies on nonlinear absorption, femtosecond lasers allow three-dimensional microfabrication with sub-micrometer precision. These lasers can produce three-dimensionally localized refractive index changes in the bulk of a transparent material, opening the door to the fabrication of a wide variety of optical devices. Until now micromachining of transparent materials required amplified laser systems. We recently found 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 multiple-shot cumulative effect. We have used this new technique to directly write single-mode optical waveguides into bulk glass.