Laser induced breakdown of a material causes chemical and physical changes, chemical and physical breakdown, disintegration, ablation, and vaporization. Lasers provide good control for procedures which require precision such as inscribing a micro pattern. Pulsed rather than continuous beams are more effective for many procedures, including medical procedures. A pulsed laser beam comprises bursts or pulses of light which are of very short duration, for example, on the order of 10 nanoseconds in duration or less. Typically, these pulses are separated by periods of quiescence. The peak power of each pulse is relatively high often on the order of gigawatts and capable of intensity on the order of 1013 w/cm2 or higher. Although the laser beam is focused onto an area having a selected diameter, the effect of the beam extends beyond the focused area or spot to adversely affect peripheral areas adjacent to the spot. Sometimes the peripheral area affected is many times greater than the spot itself. This presents a problem, particularly when high precision is required, or where tissue is affected in a medical procedure. In the field of laser machining, current lasers using nanosecond pulses cannot produce features with a high degree of precision and control, particularly when nonabsorptive wavelengths are used.
In U.S. Pat. No. 5,656,186, Mourou et al., provided a method to localize laser induced breakdown, and provided a method to induce breakdown in a preselected pattern in a material or on a material. In U.S. Pat. No. 5,235,606, Mourou et al., described a CPA (chirped-pulse amplification) system for use in such method.
Mourou et al. showed that when matter is subjected to focused high-power laser pulses localized plasmas are generated by optical breakdown. More specifically, the Mourou U.S. Pat. No. 5,656,186 showed a method for laser induced breakdown of a material with a pulsed laser beam where the material is characterized by a relationship of fluence breakdown threshold (Fth) versus laser beam pulse width (T) that exhibits an abrupt, rapid, and distinct change or at least a clearly detectable and distinct change in slope at a predetermined laser pulse width value. The method comprises generating a beam of laser pulses in which each pulse has a pulse width equal to or less than the predetermined laser pulse width value. The beam is directed to a material where laser induced breakdown is desired. The technique can produce features smaller than the spot size and Rayleigh range due to enhanced damage threshold accuracy in the short pulse regime.
Mourou et al. departs from the conventional thinking concerning optically induced dielectric breakdown relationship to pulse duration, demonstrating the dependence weakening below certain pulse width value. The small pulse energy and short pulse duration associated with optical breakdown according to Mourou prevents collateral damage from heating, and associated undesirable mechanical phenomena.
A major barrier to creating nanotechnology is that fabrication of nanometer scale features requires complex processes. Ultrashort pulsed lasers have demonstrated potential for fabricating sub-micron features in diverse substrates by taking advantage of the sharp boundaries of optical breakdown created by femtosecond pulses of laser light. The present invention reveals a new method for providing a new mechanism for optical breakdown.
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