P.S. Banks

Ultrashort-pulse laser machining of dielectric materials

There is a strong deviation from the usual τ1/2 scaling of laser damage fluence for pulses below 10 ps in dielectric materials. This behavior is a result of the transition from a thermally dominated damage mechanism to one dominated by plasma formation on a time scale too short for significant energy transfer to the lattice. This new mechanism of damage (material removal) is accompanied by a qualitative change in the morphology of the interaction site and essentially no collateral damage. High precision machining of all dielectrics (oxides, fluorides, explosives, teeth, glasses, ceramics, SiC, etc.) with no thermal shock or distortion of the remaining material by this mechanism is described.

High-intensity third-harmonic generation in beta barium borate through second-order and third-order susceptibilities

The azimuthal dependence of third-order and cascaded second-order nonlinear coupling was used to measure the relative contributions of each to direct third-harmonic generation in beta-barium borate. This permitted the measurement of the values of tensor elements chi(3)(10) , chi(3)(11) , and chi(3)(16) relative to the known chi(2)(ij) . Finally, conversion efficiencies to 3 omega of up to 6% were achieved with a femtosecond chirped-pulse amplification laser with 200 GW/cm(2) in collimated beams.

High-intensity third-harmonic generation

The azimuthal dependence of third-order and cascaded second-order nonlinear coupling are used to measure the relative contributions of each to direct third-harmonic generation in β-barium borate. This enabled the measurement of the values of χ10(3),χ11(3), and χ16(3) relative to the known χij(2). Finally, conversion efficiencies to 3ω of up to 6% from a single crystal were achieved with a femtosecond chirped-pulse-amplification laser with 200 GW/cm2 in collimated beams.

Novel all-reflective stretcher for chirped-pulse amplification of ultrashort pulses

We describe the design and operation of an all-reflective on-axis pulse stretcher (with only four simple elements) that has achieved stretching ratios of over 40000. The design is insensitive to alignment errors and is simple to align. Optical aberrations in the system have an effect on the fourth-order phase term, but this can be used to compensate for material dispersion, resulting in a quintic-phase-limited chirped-pulse-amplification system.

Femtosecond laser materials processing

Femtosecond lasers enable materials processing of most any material with extremely high precision and negligible shock or thermal loading to the surrounding area. Applications ranging from drilling teeth to cutting explosives to making high-aspect ratio cuts in metals with no heat-affected zone are made possible by this technology. For material removal at reasonable rates, we developed a fully computer-controlled 15-Watt average power, 100-fs laser machining system.

Metal processing with ultrashort laser pulses

Femtosecond laser ablation has been shown to produce well-defined cuts and holes in metals with minimal heat effect to the remaining material. Ultrashort laser pulse processing shows promise as an important technique for materials processing. We will discuss the physical effects associated with processing based experimental and modeling results. Intense ultra-short laser pulse (USLP) generates high pressures and temperatures in a subsurface layer during the pulse, which can strongly modify the absorption. We carried out simulations of USLP absorption versus material and pulse parameters. The ablation rate as function of the laser parameters has been estimated. Since every laser pulse removes only a small amount of material, a practical laser processing system must have high repetition rate. We will demonstrate that planar ablation is unstable and the initially smooth crater bottom develops a corrugated pattern after many tens of shots. The corrugation growth rate, angle of incidence and the polarization of laser electric field dependence will be discussed. In the nonlinear stage, the formation of coherent structures with scales much larger than the laser wavelength was observed. Also, there appears to be a threshold fluence above which a narrow, nearly perfectly circular channel forms after a few hundred shots. Subsequent shots deepen this channel without significantly increasing its diameter. The role of light absorption in the hole walls will be discussed.

High intensity direct third harmonic generation in BBO

Generation of the third harmonic (THG) in a single process is an idea that has been around since the inception of nonlinear optics. Efficiencies for THG in solids have been limited to less than a percent with the highest conversion achieved using BBO. It has been suggested that unphasematched second-order processes can contribute significantly to THG, similar to the use of cascaded second order processes giving rise to an effective nonlinear refractive index. However, experimental uncertainties in efforts to assess this contribution by measuring conversion efficiency were too large to determine the relative contributions of the several processes. We were able to vary the azimuthal angle in BBO and measured directly the relative contributions of X(2) and X(3) to THG in BBO for both type I and type II phasematching.

Prepulse effects in short pulse laser machining

Summary form only given. The use of femtosecond lasers allows materials processing of practically any material with extremely high precision and minimal collateral damage. Advantages over conventional laser machining (using pulses longer than a few tens of picoseconds) are realized by depositing the laser energy into the electrons of the material on a time scale short compared to the transfer time of this energy to the bulk of the material, resulting in increased ablation efficiency and negligible shock or thermal stress. High average power, chirped-pulse amplification (CPA) systems required for short pulse materials processing typically use a regenerative amplifier. This component, in particular, can produce prepulses from leakage at each round trip as well as from spectral modulation. We examined the effect that prepulses can have on both drilling rates and hole quality for drilling holes in stainless steel, using a 1 kHz, 4 W Ti:sapphire CPA laser.