LASER. Talián Csaba Gábor Univ. Pécs, Dept. Biophysics,

Transcription

1 LASER Talián Csaba Gábor Univ. Pécs, Dept. Biophysics,

2 LASER = light amplification by stimulated emission of radiation Light generally electromagnetic wave (IR, UV, X-ray) Amplification generation and amplification of light Outer energy + positive feedback Stimulated emission Oscillator LOSER

3 HISTORY Einstein: Zur Quantentheorie der Strahlung, 1917 theoretical foundation of stimulated emission Rudolf Ladenburg ( , GER-USA) experimental prove for stimulated emission, 1928 Alfred Kastler ( FRA, Nobel-prize, 1966), Jean Brossel ( , FRA) discovery of optical pumping, 1952 Charles Hard Townes (1915-, USA, Nobel-prize, 1964), Nyikolaj Baszov ( , USSR, Nobel-prize, 1964), Alekszandr Prokhorov ( , USSR, Nobel-prize, 1964) maser microwave laser

4 HISTORY Gordon Gould ( , USA), first mension of laser and one of its theoretical elaborations

5 HISTORY Theodore Maiman ( , USA), building the first pulsed mode (ruby) laser, 1960 Ali Javan (1926-, IRN-USA), the first continuous gas (He-Ne, IR) laser, 1960 Gábor Dénes ( , HUN-GBR, Nobe-prize, 1971), holography Claude Cohen-Tannoudji (1933-, FRA, Nobel-prize, 1997) atom cooling and trapping with laser

6 PROPERTIES OF LASER 1. Small divergence <10-3 rad or 0,01 almost perfectly parallel beam e.g. good laser: 2mm thick beam in 1km distance has 3,5cm width (extreme good laser: spreads in a 50m wide spot on the Moon, reputedly) good focusing spatial guidance of high accuracy huge spatial energy density achieved

7 PROPERTIES OF LASER 2. Coherence (phase identity, interference-ability) Spatial: the same phase in a given time at several places along the beam length Temporal: the same phase in several different time points at a given cross-section of the beam

8 A PROPERTIES LÉZER TULAJDONSÁGAI OF LASER 3. Small spectral width (monochromacity) for good lasers Δλ = nm 4. Polarised 5. Large power acetilene welding flame: ~10 3 W/cm 2 focused laser: W/cm 2 6. Short impulses (ps-fs)

9 BASES OF LASER FUNCTION 1. Stimulated or induced emission h*ν = E 2 E 1 h*ν = E 2 E 1 h*ν = E 2 E 1 2x h*ν = E 2 E 1 disordered photons! ordered photons! Same direction, energy, wavelength, phase, polarisation!

10 BASES OF LASER FUNCTION

11 BASES OF LASER FUNCTION 2. Population inversion Electrons remain ~10-9 s in excited state 20 C N exc /N ground C N exc /N ground 10-2 The relativ number of atoms at the excited level must be increased It is possible only in multi-state systems Pumping: heat, optical (flashlight, other laser), electric discharge, chemical reaction

12 BASES OF LASER FUNCTION 3. Optical resonance Two parallel, plane or convex mirrors Feeds back the greatest part of energy continuous induced emission one mirror ~ 99,99% reflectivity other one ~ 99% The waves can keep in phase where: d = n*λ d

13 TYPES OF LASER Gas lasers He-Ne CO 2 first He atoms are excited, then they transmit their energy to Ne; cheap, highly coherent; optics resarch main λ = 633nm high power: several 100W; when focused, huge energy density cutting and welding metals main λ = 10,6µm Ar several wavelengths: 351, 458, 488, 515nm etc. microscopes laser printer steel cutting

14 TYPES OF LASER Solid-state lasers (craytal or glass doped with metal ions) Ruby Al2O3 + Cr crystal, optical pumping (helical flashbulb) smaller wavelength, than that of laser holography Sapphire Al2O3 + Ti crystal, well tunable wavelength ( nm), ultrashort (10-100fs) impulses spectroscopy (with an appropriate amplifier the instantaneous power output [50GW] is greater than that of all the electric power plants on Earth) early ruby laser laser pointer

15 TYPES OF LASER Dye lasers Dissolved or crystallic organic compounds: rhodamine, fluoresceine, coumarin etc. Can fit any shape easy handling Broad range of wavelength, well tunable; pulse-mode function, high power output Astronomy, medicine, spectroscopy Atomic vapour isotop selection uranium enrichment

16 TYPES OF LASER Semiconductor lasers Laser diodes n- and p-type doped semiconductors Typically electric pumping Recombination of electron and hole elicits photon emission, which can also be induced Internal reflection, no need of mirrors CD, DVD, Blue-ray readers, Bar-code reader, scanner, Optical telecommunication, Microscopy, Gas sensing, Photodynamic treatment etc.

17 TYPES OF LASER

18 MEDICAL APPLICATIONS Interaction of laser with matter: Reflection Transmission Absorption Scattering Advategeous properties of laser: Focusability (spatial, temporal) Regulated wavelength Regulated power

19 MEDICAL APPLICATIONS Effect of temperature Laserthermy heat treatment (~40 C), more intensive metabolism, muscle crick, tendon injury, painkilling Coagulation C, protein precipitaion, scar formation, antibleeding effect, treatment of retinal peeling Vaporisation over 100 C, µm scale focusing, expensive surgical incision, gall or kidney stone fragmentation Carbonification over 300 C, tissue removal

20 MEDICAL APPLICATIONS Dermatology Epilation Removal of tattoos Removal of naevi Removal of dermal vessels Wrinkle removal

21 MEDICAL APPLICATIONS Ophthalmology Correction of the refractive errors of the eye by laser surgery Shortsightedness (myopia): decrease of the refractive index of the cornea, flattening of the middle of the cornea. Laser beam evaporates the amount of tissue according to the diopter value. Longsightedness (hypermetropia): increase of depression around the central part of the cornea. the refractive index, forming a Astigmia (irregular curvature): treatment of the more domed and flatter curvatures, so that only a single focal point is made.

22 MEDICAL APPLICATIONS PRK (PhotoRefractive Keratectomy) The good regenerating epithelium of cornea is scraped off, and the laser treatment is performed on the connective tissue layer surface of the cornea. The treated area is covered with contact lens. Epithelial cells recover in 2-3 days. Advantages: entirely safe, with excellent results since 20 years. In about 10 days the patient can go back to work. After the operation the eye must be protected with UV-filter sunglasses and anti-inflammatory drugs. LASEK (Laser Assisted Sub-Epithelial Keratectomy) The superficial epithelial layer is loosened with dilute alcohol, and is folded aside, then after treatment it is reset.

23 MEDICAL APPLICATIONS

24 MEDICAL APPLICATIONS LASIK (Laser Assisted in Situ Keratomyelusis) The surgeon forms a protective lobe from the upper layers of the cornea with a specific computer-guided mechanical device (microkeratom). It is lifted aside, then he carries out the laser treatment in the layers under the lobe. The lobe is resettled, and it sits back to the operated area but can never attach completely any more. Painless procedure but with medical complications. INTRA-LASIK The incision is made by laser, too.

25 MEDICAL APPLICATIONS Photodynamic therapy Hematoporphyrin derivatives accumulate in the tumour cells at a much higher rate than in the healthy cells. They are excited by light absorbtion, and can react with molecular oxygen. Atomic, nascent oxygen is generated. Hematoporphyirin Photofrin Bacteriochlorin

26 MEDICAL APPLICATIONS Administration of fotosensitising precursor (aminolevulinic, acid, ALA) Several hours of incubation, while ALA tarnsforms into protoporphyrin IX. Illumination of the target area with diode laser (couple of minutes) Protoporphyrin absorbs light Excited singlet state Excited triplet state Energy tranfer to triplet oxygen Excited, reactive singlet oxygen Tissue damage The area necrotises in some days ALA

27 MEDICAL APPLICATIONS

28 MEDICAL APPLICATIONS Treatment of malignous tumours; dermatology, operative dentistry, gynecology, esophagus, lung, bladder Corroborates immune system Hard to find or develop chemically appropriate molecules No perfect tumour-selectivity Penetration depth depends on wavelength

29 THANK YOU FOR ATTENTION!