δf / δx = σ F (N 2 -N 1 ) ΔF~N 2 -N 1

Transcription

1 LASER Light Amplification by Stimulated Emission of Radiation BASIC PROPERTIES O LASER RADIATION Spontaneous emission Incoherence in time Incoherence in space Polychromatic light Small energy density Non-polarized Coherence in space Coherence in time Stimulated Emission Monochromatic light (small bandwidth) High energy density /Polarized/ SPONTANEOUS VS. STIMULATED EMISSION E light electric kinetic Einstein, 97 Random process CONDITIONS O LIGHT AMPLIICATION (LA) δ / δx = σ (N 2 -N ) N 2 Δ~N 2 -N + Δ Initially: Boltzmann N 2 /N = e - ΔE/kT N >>N 2 Δ << 0 Absorption dominates. Interaction 2. E = hf=- 3. Polarity (p ~ cos 2 φ) N Absorption Amplification Amplification: Δ > 0 N < N 2 population inversion

2 CONDITION OR POPULATION INVERSION CREATING POPULATION INVERSION Δ~N 2 -N + Δ N >>N 2 (Boltzmann) Δ << 0 Absorption dominates E3 Minimum condition: 3 energy levels ast, spontaneous transition Inversion: N 2 =N + Abs ~ N Em ~ N 2 Optical pumping: N 2 increases, N decreases N = N 2 Equilibrium, Δ = 0 Pumping Impossible in a system with 2 levels N < N 2 population inversion, amplification P spontaneus relax. < P stimulated em. CREATING POPULATION INVERSION ROM LASER AMPLIIER TO OSCILLATOR Optimal condition: 4 energy levels E3 ast, spontaneous transition Inversion: N 2 = E3 LASER PLASMA Pumping Pumping E0 E0 ast, spontaneous transition Rear mirror 99,9 % RESONATOR: Standing wave, L = n λ / 2 Modes: transversal and axial ront mirror (outcoupling) %

3 Some important events in the history of lasers TYPES O LASERS 97 Einstein predicts stimulated emission and that probabilities of excitation and stimulated emission (B 2 és B 2 ) are equal 954 irst microwave laser (MASER) 960 irst visible laser (ruby laser) 966 irst gas laser 984 irst X-ray laser State Material pumping λ (nm) output E (W) t (ns) gas He-Ne electric pulse 633 c.w. 0. Ar++ electric pulse 488, 54 c.w. 00 Kr++ electric pulse 657,752 c.w. 3 CO2 electric pulse 0600 c.w. 30 Excimer (Ar, XeCl) electric pulse 93, 308 pulsed 0 MW -3 liquid dye laser light various pulsed/c.w. 3 solid Ruby (Cr+++ & Al2O3) flashlamp 694 pulsed 200 MW 00 Nd-YAG flashlamp 065 pulsed MW 0 Nd-YAG Xe lamp 065 c.w. 60 Er-YAG flashlamp 2900 pulsed MW 0 Dióde (e.g. GaAs) current > 500 c.w./pulsed 5 H Visible laser: Guiding laser light mirrors, fiberoptics (coupling with lenses) MEDICAL APPLICATIONS O LASERS LOW POWER flow cytometry laser nephelometry lab diagnostics correlation spectosc. microscopies, tweezer endoscopy clinical diagnostics laser doppler fotodynamic diag. hyperemisation sof laser therapy laserthermy diagnostics therapy HIGH POWER coagulation (60-90 C) laser surgery vaporization (00-50 C) excision (300 C) fotodynamic therapy

4 Reflection, scatter Interaction of lasers with tissues - Absorption Excitation atomization ionization SHOCK WAVE Heating Photochem. reactions luorescence Photodissociation Shock wave 40 o C Laserthermy o C Coagulation o C Vaporisation / Cutting 300 o C Carbonisation / Excision PHOTODYNAMIC DIAGNOSIS PHOTODYNAMIC THERAPY luorescent dye Tumor takes up dye selectively Ar laser Excited dye produces free radicals Dye laser Ar laser iberoptics /endoscope Tumor cells with dye iberoptics Kr laser

5 luorescence correlation spectroscopy (CS) Interactions to be seen with CS luorescence fluctuation Protein - DNA Hybridization of nucleic acids 0,3 μm (t) t emtoliter (confocal) volume,5 μm G(τ ) Autokorrelációs függvény δ ( t) δ ( t + τ ) G ( τ ) = 2 δ ( t) = ( t) Antigen-antibody Receptor Ligand luorescent labeling of target molecule (protein, lipid, etc.) Illuminating a small, <μm3 volume with focussed laser beam Sensitive detection of fluorescence fluctuation in time If concentration of molecules is low, fluctuation will be high G diff τ ( τ) = N + τ τ D + τ 2 S τ D G(τ) Systems with several components I Determining the concentration and diffusion constant of bound and free molecules /N,9,7,5,3, 0,9 0,0 0, τ free τ bound τ (ms) Systems with several components II - crosscorrelation Determine how the fluctuation of two spectrally different labeles correlate with each other: Can reveal molecular associations even if diffusion constant does not change significantly upon association G ( τ ) = V eff G c ab ( τ ) = a + b ab δ ( t) δ ( t + τ ) a a b ( ca tot )( c ) τ, b, tot γ xy γ + z γ xy τ τ + 2 ab γ zsa τ ab b

6 Plasma X-ray lasers CREATION Holography RETRIEVAL Laser pulse (GW) X-ray emission Into both directions Usage: Bio-holography Nanoelectronics Nano technology (robotics) Laser Photo plate, to make hologram interference Hologram Laser Observer Divergent beam Original beam Convergent beam NOVA (LLNL) 053 nm Nd-glass 20 TW 0 x 70 cm dia beam 00 KJ 20 ps 0 ns Nano-flowmeter Object Photons scattered from object Virtual 3D image Real 3D image Laser tweezer P body P photon P photon Body transparent to applied laser Refraction of photons changes their momentum which pushes the body towards higher photon densities (the optical axis) Photonpressure keeps equilibrium with gravity Photon density Optical axis Distance Micromanipulation Selection, cell fusion, insemination measuring molecular forces of pn and distances of nm magnitude