October, 2008 J. Brnjas-Kraljević Laser Light amplification by stimulated emission of radiation laser functioning is based on quantum processes energy states of atom are quantized atoms are emitting or absorbing visible light due to transition of electron between quantum states of atom electron's transition is connected with emission or absorption of quantum of electromagnetic radiation and its energy is equal to energy difference of two states 1917 year Einstein had postulated that the probabilities for spontaneous emission and stimulated emission are connected light photon which excites atom and induces the transition of electron in higher state with equal probability induces the relaxation of excited atom to ground state absorption of photon – transition to higher state stimulated emission spontaneous emission light photons do not posses enough energy to induce the transitions of electrons to excited state, the process which results in inversion of population. laser mechanism is not possible in system with only two energy states; at least three states are needed for such process Stimulated emission if the photon with energy equal to the energy difference of excited and ground state interacts with electron in excited state, there is probability that photon will induce the transition of electron into ground state and simultaneously the emission of another photon of equal energy in the condition of population inversion, significantly higher number of electrons are in excited state and it enables the stimulated emission of more photons the light amplification – emitted photons are correlated in phase and time - the emitted light is coherent in processes of absorption, emission and stimulated emission, photon must have energy exactly equal to energy difference of two states Inversion of population the inversion of population in favor of higher state is requirement for laser transition the inversion of population is the result of subsequent absorptions of photons and a large number of transitions of electrons to higher state inversion could not be achieved in system with one excited state, because it is short-lived, only 10-8 s, and atom relaxes to ground state by spontaneous or stimulated emission the appropriate system should have two excited states, one of them is long-lived this state is metastable one, with different spin orientation of electron, and the transition from this state to ground state is of very low probability forbidden transition He-Ne laser is the example of such three state system He-Ne laser He-Ne laser is most frequent and rather low cost gas type laser, which emits red light of 632.8 nm. it can also produce the beam of green light, 543.5 nm and infrared, 1523 nm. the excitation energy of He is 20.61 eV, very close to metastable state of Ne - 20.66 eV in collisions of atoms, energy is transferred from He to Ne and it is switched to excited state laser can be dangerous, it is out of focus, power is 1 mW - Sun brightness (0.1 watt/cm2) generation of laser beam He-Ne laser it is “pumped” to higher level by energy of electric field Energy is transferred by collisions from He to Ne and raises it to metastable state spontaneous emission decreases the population of lower excited level and keeps up population inversion Properties of laser radiation 1. coherence. Different parts of laser beam are in phase. This phase state is long enough for detection and measurement of interference effects 2. monochromatic. Laser beam has only one wavelength, because the photons are generated by stimulated emission from one atomic state. 3. narrow beam. Inside laser cavity, photons are reflected many times from end mirrors. Photons are perpendicular to mirrors, so the laser beam is narrow and almost not diverging Coherent light coherence is special property of laser beam it is the consequence of stimulated emission – process which enables the light amplification emitted photons are propagating together coherence is present in time and space domain natural sources do not produce the coherent light Parallel beam the emitted beam is extremely narrow parallel mirrors – the front one transmits 99% of light and the back one reflects 100% of light. parallel beam can be dangerous, but also very useful it is not allowed to look directly in laser beam, the damage to eye retina can be immediate and serious Monochromatic light the result of one atomic transition – one wavelength it is not totally monochromatic - Doppler effect of moving atoms and molecules in laser the wavelength is small in comparison with laser cavity– a lot of resonant modules Basic data for laser Cw - continuous wave - emission is constant pulse laser Parameters : wavelength: 488 nm – 1000 nm pulse duration: 100 ns – 10 fs energy density: 1 Jcm-2 – 1000 Jcm-2 Types of lasers CO2 laser cw mode - power over 10 kW pulse mode – high power emission in infrared range pumping by energy of electric field, using nitrogen to excite CO2 efficiency is high - 30% (more than ordinary bulb, where 90% of radiation is in the form of heat) Argon Laser 25 wavelengths in visible range 408.9 - 686.1 nm, best for green light 488 nm and 514.5 nm power 30 - 100 W cw mode 9 - 12 kW Neodium-YAG Laser solid-state laser, crystal of itrium-aluminium-garnet (YAG) doped with Nd3+ ion in cw mode the power is over 1 kW at 1065 nm in pulse mode -very short pulses are produced with resolution of 1 fs Ruby Laser the fist one 1960g - T. H. Maiman. pulse mode at 694.3 nm. Diode laser p-n semiconductor - galium doped with arsenic 5 mW at 840 nm; 50 mW at 760nm; 20 mW at 1300 nm Eximer Lasers Eximer means "excited dimer", laser system is excited twoatomic molecule emission in ultraviolet range gasses of lantanides and halogen elements excited by electrons molecules as XeF are stable in excited state and rapidly are dissociated by transition in ground state. It is possible to achieve the population inversion because the ground state becomes less populated due to dissociation excited states are not long-lived so the pumping must be very quick eximer lasers emit pulses of high power in blue and ultraviolet range Dye Lasers they emit almost continuous spectrum molecules of organic dyes – large number of spectral lines, each with characteristical frequency distribution – overlapping of lines enables laser effect Laser – Tissue Interactions Application in Medicine M.Balarin Interaction with tissues visible radiation excitation of electrons – strongly absorbed, specially higher frequencies – induces heating - not ionization ultraviolet strongly absorbed in surface skin layer higher energies, equal to ionization energies of molecules - photoionization To determine possible interactions we need to know: Optical properties of tissue (coefficients of reflection, absorption and scattering) Thermal properties of tissue (specific heat capacity, heat conductivity) Properties of laser radiation (wavelength, exposure time, applied energy, focal spot size, energy density and power density) Optical properties of tissues parameters: intensity of transmitted, reflected and scattered radiation measured by spectroscopy parameters depend on temperature – it is necessary to measure simultaneously incident radiation detector Tc transmitted and front scattered radiation detector Rd detector Td reflected and back scattered radiation Interaction types: Photochemical interactions Thermal interactions Photoablation Mechanical interactions Plasma induced ablation Photodisruption All these different interaction types share a single common datum: the characteristic energy density ranges from approximately 1 J/cm2 – 1000 J/cm2, and the power density varies over 1015 orders of magnitude, thus the duration of laser exposure, distinguishes and controls all these processes. < 1 ns From 1 ns – 1 ms From 1 ms – 1 min Continuous wave >1s The duration of laser exposure is mainly identical with the interaction time itself. Depending on laser power different effects on tissue are obtained. Low to medium power will induce certain chemical and metabolic reactions which are called biostimulations. The enhancement of power results in thermal effects which cause coagulation, cutting and melting. Wavelengths for which the main interaction process is transmittion are used for biostimulation. For cutting, coagulation and treatment of defects one have to choose the wavelengths for which absorption is dominant. Photochemical Interactions Chemical interactions induced by light. Take place at very low power density (1 W/cm2), and long exposure times (10 min) Wavelengths in the visible range (Rhodamine dye lasers 630 nm) are used because of their efficiency and their high optical penetration depth. Specially adapted chromophores which are capable of causing light-induced reactions are injected into the body. After resonant excitation by laser irradiation these molecules perform a series of chemical reactions resulting in release of highly cytotoxic reactants which cause irreversible oxidation of essential cell structure. Photodynamic Therapy Photosensitive drug (photosenzitizer, hematoporphyrin derivate) is injected into a vein of the patient. Within next few hours it is distributed among all soft tissue except the brain. It remains inactive until irradiated. After 48 – 72 hours most of it is cleared from healthy tissue while the concentration in tumor cells remains constant even after period of 7 – 10 days. After resonant excitation by laser irradiation, the photosenzitizer is first transferred to an excited state and then by relaxation to ground state transfers energy to neighbor oxygen which transfer to very reactive state and oxidize all tissue they are in contact with. Application: oncology – treatment of esophageal tumors dermatology – skin cancer neurosurgery, ophthalmology, gynecology Treatment of viral lesions (HPV, herpes) and psoriasis Thermal Interactions Thermal effects can be induced by either continuous wave or pulsed laser radiation when the power density is > 10 W/cm2 Primary heating Heat transfer Tissue reaction Mechanism of interaction At microscopic level, thermal effects have their origin in bulk absorption occurring in molecular vibration-rotation bands followed by a nonradiative decay. Absorption: A + hn A* Relaxation: A* + M(Ekin) A +M(Ekin + DEkin) Absorption of a photon promotes the molecule to an excited state. Inelastic collision with some partner M of the surrounding medium lead to a deactivation and a simultaneous increase in the kinetic energy of M. Therefore the temperature rises. Water has high absorption at 3 mm, so used lasers are Er:YAG(2,94 mm), Er:YLF (2,8 mm) Primary heating The source is the transfer of laser light into heat. Which percent of radiation will penetrate the tissue depends on tissue reflectivity. Scattering determines the path of radiation through the tissue. For l > visible light reflectivity is low, absorption is low and radiation penetrates deep. Most of organic molecules have high absorption in UV region so radiation does not penetrate deeply. Heat transfer Heat is transferred mostly by conduction while convection and radiation can be neglected. Coefficient of thermal conductivity influence the transfer of heat. That parameter as well as the exposure time influence the depth of penetration. In 1 ms heat will penetrate in water up to 0,7 mm in depth. Tissue reaction Final result of heating is tissue necrosis. Depending on the duration and peak value of the tissue temperature achieved, different effects can be distinguished: Hyperthermia Coagulation Vaporization Carbonization Melting Photoablation Is caused by dissociation at small l (190 – 300 nm); molecular bonds are braking and fragments of tissue are ejected Excitation: AB + hn (AB)* Dissociation: (AB)* A + B + Ekin Acts on surface (few mm) Power density 107 – 1010 W/cm2 Lasers: excimer; mostly ArF(193 nm) Pulse duration: 10 – 100 ns It is used for non bleeding tissue for example in ophthalmology for refractive corneal surgery. Mechanical interactions Plasma formation – usage of big power on small area causes ionization of atoms and formation of plasma. There is a big difference in pressure at the boundary of ionized area which causes shock wave generation. Propagation of this shock waves causes tissue disruption. Lasers: Nd:YAG, Ti:Saphire Pulse duration 100 fs – 500 ps Power density 1011 – 1013 W/cm2 Application in ophthalmology for braking the membranes after the lens implantation, for refractive surgery, in dentistry for caries therapy …. Mechanical interactions Cavitations – occurs when in soft tissue, due to mechanical and thermal interactions, an explosive vaporization does not happen but the gas bubble is formed. This gas bubble then implodes. Laser beam is focused inside the tissue and the tissue is split by mechanical force. Lasers: solid state lasers; Nd:YAG, Nd:YLF Pulse duration 100 fs – 100 ns Power density 1011 – 1016 W/cm2 Application: posterior capsulotomy of the lens after the cataract surgery, laser-induced lithotripsy of urinary calculi.