Laser selective cutting by impulsive heat deposition in the ir wavelength range for direct-drive ablation

Abstract

A method of laser processing of materials and specifically laser induced ablation processes for laser removal of material by impulsive heat deposition (IHD) by direct and specific excitation of short lived vibrations or phonons of the material in such a way as to not generate highly reactive and damaging ions through multiphoton absorption. The heat deposition and ensuing ablation is achieved faster than heat transfer to surrounding tissue by either acoustic or thermal expansion or thermal diffusion. The deposited laser energy is channeled into the ablation process to drive the most efficient ablation process possible with minimal damage to surrounding areas by either ionizing radiation or heat effects.

1 .- 38 . (canceled) 
     
     
         39 . An apparatus for laser processing a material, comprising:
 a laser source for generating laser pulses with wavelengths lying between about 1.5 and about 20 microns, and said laser pulses having
 i) an energy sufficient that the light that is absorbed in the laser irradiated volume of material produces superheated temperatures above a vaporization point of at least one component of the material contained in the laser irradiated volume of material, 
 ii) a pulse duration time and wavelength that meets requirements for impulsive heat deposition such that the pulse duration time is shorter than a time required for thermal diffusion out of the laser irradiated volume of material and shorter than a time required for a thermally driven expansion of the laser irradiated volume of material, and 
 iii) the pulse duration time is long enough and the pulse energy low enough so that a peak intensity of said laser pulses is below a threshold for ionization to occur in the material, 
   such that most of the energy contained in said laser pulses will be converted to ablation of material in the laser irradiated volume of the material with any residual energy not being enough to substantially damage regions surrounding the laser irradiated volume of material.   
     
     
         40 . The apparatus according to  claim 39  wherein the laser pulses have pulse durations lying between about 1 and about 1000 ps. 
     
     
         41 . The apparatus according to  claim 40  including non-linear optical crystals, and wherein the laser pulses are generated by three wave mixing in said non-linear optical crystals. 
     
     
         42 . The apparatus according to  claim 40  including a waveguiding element for delivering the laser pulses to said irradiated volume of material. 
     
     
         43 . The apparatus according to  claim 42  wherein said waveguiding element is selected from the group consisting of hollow fibres, holey fibres, photonic crystal fibres, and spatially profiled index of refraction fibre optics, and wherein said waveguiding element is placed close enough to said volume of material being irradiated so that said laser pulses exiting from said waveguiding element are in a near field region having an intensity profile which is approximately flat top and are absorbed by said material before the profile changes to that characteristic of a far field region. 
     
     
         44 . The apparatus according to  claim 40  including pulse shaping means for shaping the laser pulses to give an intensity profile which is approximately flat top. 
     
     
         45 . The apparatus according to any of  claim 40  including pulse shaping means for shaping the laser pulses in temporal and spectral domains in order to reduce effects of bleaching and ionization in the laser irradiated volume of material and thereby reduce damage to surrounding material from excess heat or ionization effects transferred to the surrounding area. 
     
     
         46 . The apparatus according to  claim 45  wherein said pulse shaping means shapes the laser pulses so that they contain spectrally shifted components occurring later in the pulse, corresponding in time to dynamically shifting absorption bands of constituents of the material due to excited state creation and heating of the constituents of the material by said laser pulses. 
     
     
         47 . The apparatus according to  claim 45  wherein said pulse shaping means shapes the laser pulses into a burst of sub-pulses, where a time interval between the sub-pulses is greater than the thermal relaxation time of the excited states, and wherein an envelope of the burst of sub-pulses satisfies the laser pulse duration requirements of steps ii) and iii). 
     
     
         48 . The apparatus according to  claim 46  wherein said pulse shaping means shapes the laser pulses so that they contain spectrally shifted components occurring later in each laser pulse, corresponding in time to dynamically shifting absorption bands in constituents of the material being processed. 
     
     
         49 . The apparatus according to  claim 40  including wavelength tuning means for tuning the wavelength of the laser pulses to water absorption lines in any hydrated constituents present in the material. 
     
     
         50 . The apparatus according to  claim 49  wherein the wavelength tuning means tunes the laser pulse wavelengths to the OH symmetric stretch (3400 cm −1  spectral regions) and/or OH bending motions (1650 cm −1  spectral regions). 
     
     
         51 . The apparatus according to  claim 49  wherein the wavelength tuning means tunes the laser pulse wavelengths to vibrational absorption lines corresponding to amide and phosphate bands. 
     
     
         52 . The apparatus according to  claim 40  wherein said material is tissue. 
     
     
         53 . The apparatus according to  claim 52  wherein said tissue is selected from the group consisting of dental tissue including tooth dentin, enamel and gum tissue, corneal tissue, skin, all human organs, connective tissue, muscular tissue, vascular tissue, nerves, urological tissue, glandular tissue, endocrine tissue, and bone tissue.