INTERACTION OF 3 µm RADIATION WITH MATTER

Transcription

1 INTERACTION OF 3 µm RADIATION WITH MATTER J. Frauchiger, W. Lüthy To cite this version: J. Frauchiger, W. Lüthy. INTERACTION OF 3 µm RADIATION WITH MATTER. Journal de Physique Colloques, 1987, 48 (C7), pp.c7-97-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1987 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C7, suppl6ment au n012, Tome 48, dgcembre 1987 INTERACTION OF 3 pm RADIATION WITH MATTER J. FRAUCHIGER and W. LUTHY Tnstitute of Applied Physics, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland Summary Drilling of small holes into media containing OH or CH bonds with a 2.92 um YA10,:Er laser is studied. In materials containing OH bonds strong absorption allows to heat small volumina with only marginal heat diffusion. Drilling with nearly diffraction limited lateral resolution is achieved. Introduction Interaction of laser radiation with matter has been studied for a long time. Today, lasers are used not only in industry for applications such.as drilling, cutting, or welding but also for medical applications like ophthalmology and surgery [ll. In surgery excimer lasers, Nd:YAG lasers or CO,-lasers are commonly used. For materials containing water or OH groups, a dominant constituent of living tissue, the lasers can still be improved by choosing a wavelength that fits the absorption maximum of water [21. For precise drilling or cutting it is necessary to absorb the laser energy in a volume that is as small as possible in order to minimize thermal damage in neighboring tissue. Si.nce at 3 um the spread of the beam by scattering is only marginal the irradiated volume is proportional to the penetration depth times the spot size. These parameters depend on the focusing optics and the laser wavelength. If the heat conduction of the target material is low and the pulse duration short enough, it is possible to obtain high temperatures within the irradiated volume with minimum energy loss and heat diffusion into the surrounding material. Most of the irradiated energy is then dissipated in the removed material. The absorption spectra of water and blood plasma and for comparison also for "Perspex" are shown in Fig. 1. Fig. 1: Optical density A = -loiog T (T=transmission of a 1 cn layer) and peneration depth (at l/e) of light in the wavelength range from 0.9 to 3.2 um in distilled water HzO, blood plasma BP and P?l?lA together with the emission wavelength at 2.92 vn of the YAlO,: Er laser. Article published online by EDP Sciences and available at

3 C7-98 JOURNAL DE PHYSIQUE It is seen, that a very high absorption in water is found either in the mid infrared at 2.93 vm 131 or in the UV-region where an equivalent value of the absorption is found at wavelengths below 180 nm i21. The 2.92 vm wavelength has several advantages compared to the VUV radiation. (i) 2.92 vm radiation is not absorbed In air and no special optical components are used and (ii) for 2.92 um low loss waveguides exist t41. Therefore a laser source in the 3 vm range t51 is preferred. In our experiments we investigated the possibilities of drilling holes in materials with different penetration depths for 3 vm radiation. Table 1 lists the relevant properties [61 of the investigated target materials. I Material Ibondsl MP I c I I 7 I d ( t0cl J - t I "C - kg Teflon [61 Perspex t61 C-F C-H * CuS04-5(H20)[81 0-H 11 0 * Hair 0-H, C-H Table 1: Properties of the investigated materials with melting-point MP (values denoted with * indicate temperatures of decomposition), heat capacity c, heat conductivity 2, density Y and penetration depth d. Experimental arrangement In the experiments a YA103:Er laser with 50% Er doping was used. At this dopant level the laser emits only one line at 2.92 vm close to the maximum IR absorption in H20. The laser rod was pumped with a linear Xenon flashlamp with typical pump energies of 100 J. The laser pulses were monitored with an InAs photodiode for the measurement of pulse shape and with an energy meter. For precision processing the typical output energy of 20 mj was reduced by neutral density filters. F/5 optics yeald for a Gaussian beam a minimum focus diameter of 5 urn. In our experiments we achieved 15 vm (see Fig 3 b ) Experiments In a first experiment materials with a large penetration depth in the order of 1 mm such as "Teflon" and "Perspex" were irradiated. In "Teflon" no remaining change in surface morphology was found after laser irradiation with energies up to 1 mj. This is due to the properties of "Teflon" given in Tab. 1, especially to the penetration depth of 1.6 mm and the melting temperature of 325 OC. It should be noted, that with respect to the visible wavelength range at 2.92 scattering is strongly reduced. In "Perspex" the penetration depth is 510 vm. The drilling mechanism is based on boilingof the material or its fragments with expulsion of liquid and gaseous material. The drilling process is very much the same as it was observed in silicon t71. In a second experiment we irradiated inorganic material containing crystal water. In the case of CuSOa(-5H20) the density of crystal water is 0.83 g/cm3. Therefore we assume, that the absorption at

4 3 um is similar to that of pure water with a penetration depth of 0,78 vm. Then the heated volume is in the order of cm3. Expulsion of matter was found to start already at a laser energy of about 1 uj. Figure 2a) shows the characteristic shape of holes drilled with different pulse energies, as obtained by microscopic inspection of cleaved samples. Figure 2b) shows a plot of the expulsed mass as a function of laser energy. The four measured values are denoted by + and the solid line is a linear approximation. Its slope gives the efficiency of the drilling process (0.32 mg/j). Similar results have been obtained in CaS04-(2H,O). Assuming the same efficiency in living tissue, a cut of 20um wldth and a depth of 1 mm might be performed at a speed of 1 cm/s with a 1 W Erbium laser. t a /-+'zq Y i - C+/+ I I ELI] 0 1- E Fig. 2a) Shape of holes drilled with 750 uj, 400 uj, 113 uj and 32 yj respectively. a) Expulsed mass as a function of irradiated laser energy. The solid line represents a linear approximation of the values + for CuSO,( -5H,O) In a third experiment biological material was irradiated to demonstrate the lateral resolution of the drilling process in complex structures and the thickness of the thermally damaged wall material. Human hair was used as target material. As shown in Fig. 3 irradiation with pulse energies of a) 90 UJ produces holes with about 25 ym diameter of 30um depth and b) with 2 pulses of 90 UJ energy each the hair is shot through. 700 C b Fig. 3: Holes in human hair, produced by single laser pulses: a) Irradiation with 90 uj energy each. The depth of the holes is about 30 ym. b) A gold wire of 12 um diameter is passed through a hair, drilled through by two laser pulses of 90 UJ energy each.

5 C7-100 JOURNAL DE PHYSIQUE In conclusion we have shown that the strong absorption at 2.92 vm in H20 leads to heated volumina, so small, that a laser energy of few uj leads to evaporation. Even with relatively long laser pulses of 80 us only marginal heat diffusion occurs. In interaction with organic material containing H20 a lateral resolution close to the diffraction limit can be achieved. Acknowledgments We thank H.P. Weber for helpful discussions and P.Herren and M. Frey for their support.this work was supported in part by the Swiss Commission for the Encouragement of Scientific Research. References 1. M.L. Wolbarsht "Laser Surgery : CO, or HF" IEEE J. of Quantum Electron. QE-20 (12) W.L. Wolfe and G.J. Zissis Editors "The Infrared Handbook" ' Office of Naval Research, Department of the Navy, Washington DC p (1987) 3. V.M. Zolotarev, B.A. Mikhailov, L.I. Alperovich, and S.I. Popov "Disperision and Absorption of Liquid Water in the Infrared and Radio Regions of the Spectrum" Opt. and Spectrosc. 27 [ (1969) 4. D.C: Tran "Advances in mid -infrared fibres" in IOOC-ECOC 85, Proceedings of the 5th International Conference on Integrated Optics and Optical Fiber Communication and the llth.european Cqnference on Optical Communication. Technical Digest Vol 2. p Institute Internazionale delle Communicazioni, Compagnia dei Librai Editrice, Genova, Italy (1985) 5. H.P. Weber and W. Luthy "The YAl0,:Er laser" in Tunable Solid-State Lasers 11. Editors: A.B.Budgor, L.Esterowitz and L.G. DeShazer Springer Optical. Sciences Volume 52. pp Landolt-Bornstein, "Zahlenwerte und Funktionen" Vol.IV, Part 1 p , Springer-Verlag, Heidelberg (1955) 7. W. Luthy, K. Affolter and M. Fuhrer "Laser Induced Surface Deformations on Silicon" Phys. Lett. 2 (1) (1979) 8. Handbook of Chemistry and Physics, R.C. Weast Ed. The Chemical Rubber Company, B-89, 55th Ed (1975)