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Complex hardware systems based on quantum optics 1 Basics of Quantum Mechanics - Why Quantum Physics? • Classical mechanics (Newton's mechanics) and Maxwell's equations (electromagnetics theory) can explain MACROSCOPIC phenomena such as motion of billiard balls or rockets. • Quantum mechanics is used to explain microscopic phenomena such as photon-atom scattering and flow of the electrons in a semiconductor. • QUANTUM MECHANICS is a collection of postulates based on a huge number of experimental observations. • The differences between the classical and quantum mechanics can be understood by examining both – The classical point of view – The quantum point of view 2 Basics of Quantum Mechanics - Quantum Point of View • Quantum particles can act as both particles and waves WAVE-PARTICLE DUALITY • Quantum state is a conglomeration of several possible outcomes of measurement of physical properties Quantum mechanics uses the language of PROBABILITY theory (random chance) • An observer cannot observe a microscopic system without altering some of its properties. Neither one can predict how the state of the system will change. • QUANTIZATION of energy is yet another property of "microscopic" particles. 3 Christiaan Huygens Dutch 1629-1695 light consists of waves The idea of duality is rooted in a debate over the nature of light and matter dating back to the 1600s, when competing theories of light were proposed by Huygens and Newton. Sir Isaac Newton 1643 1727 light consists of particles 4 If h is the Planck constant Then Louis de BROGLIE French (1892-1987) Max Planck (1901) Göttingen 5 Soon after the electron discovery in 1887 - J. J. Thomson (1887) Some negative part could be extracted from the atoms - Robert Millikan (1910) showed that it was quantified. -Rutherford (1911) showed that the negative part was diffuse while the positive part was concentrated. 6 Quantum numbers In mathematics, a natural number (also called counting number) has two main purposes: they can be used for counting ("there are 6 apples on the table"), and they can be used for ordering ("this is the 3rd largest city in the country"). 7 Atomic Spectroscopy Absorption or Emission Johannes Rydberg 1888 Swedish n1 → n2 name Converges to (nm) 1 → ∞ Lyman 91 2 → ∞ Balmer 365 3→ ∞ Pashen 821 4 → ∞ Brackett 1459 5 → ∞ Pfund 2280 6→ ∞ Humphreys 3283 8 Bohr and Quantum Mechanical Model Wavelengths and energy • Understand that different wavelengths of electromagnetic radiation have different energies. • c=vλ – c=velocity of wave – v=(nu) frequency of wave – λ=(lambda) wavelength 9 • Bohr also postulated that an atom would not emit radiation while it was in one of its stable states but rather only when it made a transition between states. • The frequency of the radiation emitted would be equal to the difference in energy between those states divided by Planck's constant. 10 E2-E1= hv h=6.626 x 10-34 Js = Plank’s constant E= energy of the emitted light (photon) v = frequency of the photon of light • This results in a unique emission spectra for each element, like a fingerprint. • electron could "jump" from one allowed energy state to another by absorbing/emitting photons of radiant energy of certain specific frequencies. • Energy must then be absorbed in order to "jump" to another energy state, and similarly, energy must be emitted to "jump" to a lower state. • The frequency, v, of this radiant energy corresponds exactly to the energy difference between the two states. 11 • In the Bohr model, the electron is in a defined orbit • Schrödinger model uses probability distributions for a given energy level of the electron. Orbitals and quantum numbers • Solving Schrödinger's equation leads to wave functions called orbitals • They have a characteristic energy and shape (distribution). 12 • The lowest energy orbital of the hydrogen atom has an energy of -2.18 x 1018 J and the shape in the above figure. Note that in the Bohr model we had the same energy for the electron in the ground state, but that it was described as being in a defined orbit. • The Bohr model used a single quantum number (n) to describe an orbit, the Schrödinger model uses three quantum numbers: n, l and ml to describe an orbital 13 The principle quantum number 'n' • Has integral values of 1, 2, 3, etc. • As n increases the electron density is further away from the nucleus • As n increases the electron has a higher energy and is less tightly bound to the nucleus 14 The azimuthal or orbital (second) quantum number 'l' • Has integral values from 0 to (n-1) for each value of n • Instead of being listed as a numerical value, typically 'l' is referred to by a letter ('s'=0, 'p'=1, 'd'=2, 'f'=3) • Defines the shape of the orbital 15 The magnetic (third) quantum number 'ml' Has integral values between 'l' and -'l', including 0 Describes the orientation of the orbital in space For example, the electron orbitals with a principle quantum number of 3 n l Subshell ml Number of orbitals in subshell 1 3 3 0 1 3s 3p 0 -1,0,+1 2 3d -2,1,0,+1,+2 5 16 • the third electron shell (i.e. 'n'=3) consists of the 3s, 3p and 3d subshells (each with a different shape) • The 3s subshell contains 1 orbital, the 3p subshell contains 3 orbitals and the 3d subshell contains 5 orbitals. (within each subshell, the different orbitals have different orientations in space) • Thus, the third electron shell is comprised of nine distinctly different orbitals, although each orbital has the same energy (that associated with the third electron shell) Note: remember, this is for hydrogen only. 17 Subshell Number of orbitals s 1 p 3 d 5 f 7 18 19 Practice: • What are the possible values of l and ml for an electron with the principle quantum number n=4? • If l=0, ml=0 • If l=1, ml= -1, 0, +1 • If l=2, ml= -2,-1,0,+1, +2 • If l=3, ml= -3, -2, -1, 0, +1, +2, +3 20 Problem #2 • Can an electron have the quantum numbers n=2, l=2 and ml=2? • No, because l cannot be greater than n-1, so l may only be 0 or 1. • ml cannot be 2 either because it can never be greater than l 21 In order to explain the line spectrum of hydrogen, Bohr made one more addition to his model. He assumed that the electron could "jump" from one allowed energy state to another by absorbing/emitting photons of radiant energy of certain specific frequencies. Energy must then be absorbed in order to "jump" to another energy state, and similarly, energy must be emitted to "jump" to a lower state. The frequency, v, of this radiant energy corresponds exactly to the energy difference between the two states. Therefore, if an electron "jumps" from an initial state with energy Ei to a final state of energy Ef, then the following equality will hold: (delta) E = Ef - E i = hv To sum it up, what Bohr's model of the hydrogen atom states is that only the specific frequencies of light that satisfy the above equation can be absorbed or emitted by the atom. 22 Atomic Spectroscopy Absorption or Emission -R/72 -R/62 -R/52 -R/42 Johannes Rydberg 1888 Swedish -R/32 IR -R/22 VISIBLE -R/12 UV Emission Quantum numbers n, levels are not equally spaced R = 13.6 eV 23 Photoelectric Effect (1887-1905) discovered by Hertz in 1887 and explained in 1905 by Einstein. I Albert EINSTEIN (1879-1955) Heinrich HERTZ (1857-1894) Vacuum Vide e i e e 24 I T (énergie cinétique) Kinetic energy 0 0 25 Compton effect 1923 playing billiards assuming l=h/p h ' h h/ l h/ l' 2 p /2m p Arthur Holly Compton American 1892-1962 26 Davisson and Germer 1925 d Clinton Davisson Lester Germer In 1927 2d sin = k l Diffraction is similarly observed using a monoenergetic electron beam Bragg law is verified assuming l=h/p 27 Wave-particle Equivalence. •Compton Effect (1923) •Electron Diffraction Davisson and Germer (1925) •Young's Double Slit Experiment Wave–particle duality In physics and chemistry, wave–particle duality is the concept that all matter and energy exhibits both wave-like and particle-like properties. A central concept of quantum mechanics, duality, addresses the inadequacy of classical concepts like "particle" and "wave" in fully describing the behavior of small-scale objects. Various interpretations of quantum mechanics attempt to explain this apparent paradox. 28 Young's Double Slit Experiment F1 Source F2 Ecranwith Mask 2 slits Screen photo Plaque 29 Young's Double Slit Experiment This is a typical experiment showing the wave nature of light and interferences. What happens when we decrease the light intensity ? If radiation = particles, individual photons reach one spot and there will be no interferences If radiation particles there will be no spots on the screen The result is ambiguous There are spots The superposition of all the impacts make interferences 30 Young's Double Slit Experiment Assuming a single electron each time What means interference with itself ? What is its trajectory? If it goes through F1, it should ignore the presence of F2 F1 Source F2 Mask Ecran Plaque photo Screen with 2 slits 31 Young's Double Slit Experiment There is no possibility of knowing through which split the photon went! If we measure the crossing through F1, we have to place a screen behind. Then it does not go to the final screen. We know that it goes through F1 but we do not know where it would go after. These two questions are not compatible Two important differences with classical physics: F1 Source F2 • measurement is not independent from observer • trajectories are not defined; h goes through F1 and F2 both! or through them with equal probabilities! Mask Ecran Plaque photo Screen with 2 slits 32 de Broglie relation from relativity Popular expressions of relativity: m0 is the mass at rest, m in motion E like to express E(m) as E(p) with p=mv Ei + T + Erelativistic + …. 33 de Broglie relation from relativity Application to a photon (m0=0) To remember To remember 34 Useful to remember to relate energy and wavelength Max Planck 35 A New mathematical tool: Wave functions and Operators Each particle may be described by a wave function Y(x,y,z,t), real or complex, having a single value when position (x,y,z) and time (t) are defined. If it is not time-dependent, it is called stationary. The expression Y=Aei(pr-Et) does not represent one molecule but a flow of particles: a plane wave 36 Wave functions describing one particle To represent a single particle Y(x,y,z) that does not evolve in time, Y(x,y,z) must be finite (0 at ∞). In QM, a particle is not localized but has a probability to be in a given volume: dP= Y* Y dV is the probability of finding the particle in the volume dV. Around one point in space, the density of probability is dP/dV= Y* Y Y has the dimension of L-1/3 Integration in the whole space should give one Y is said to be normalized. 37 Normalization An eigenfunction remains an eigenfunction when multiplied by a constant O(lY)= o(lY) thus it is always possible to normalize a finite function Dirac notations <YIY> 38 Mean value • If Y1 and Y2 are associated with the same eigenvalue o: O(aY1 +bY2)=o(aY1 +bY2) • If not O(aY1 +bY2)=o1(aY1 )+o2(bY2) we define ō = (a2o1+b2o2)/(a2+b2) Dirac notations 39 Introducing new variables Now it is time to give a physical meaning. p is the momentum, E is the Energy H=6.62 10-34 J.s 40 Plane waves This represents a (monochromatic) beam, a continuous flow of particles with the same velocity (monokinetic). k, l,w, , p and E are perfectly defined R (position) and t (time) are not defined. YY*=A2=constant everywhere; there is no localization. If E=constant, this is a stationary state, independent of t which is not defined. 41 Correspondence principle 1913/1920 For every physical quantity one can define an operator. The definition uses formulae from classical physics replacing quantities involved by the corresponding operators Niels Henrik David Bohr Danish 1885-1962 QM is then built from classical physics in spite of demonstrating its limits 42 Operators p and H We use the expression of the plane wave which allows defining exactly p and E. 43 Momentum and Energy Operators Remember during this chapter 44 Stationary state E=constant Remember for 3 slides after 45 Kinetic energy Classical quantum operator In 3D : Calling the laplacian Pierre Simon, Marquis de Laplace (1749 -1827) 46 Correspondence principle angular momentum Classical expression Quantum expression lZ= xpy-ypx 47 48 49 50 51 Time-dependent Schrödinger Equation Without potential E = T With potential E = T + V Erwin Rudolf Josef Alexander Schrödinger Austrian 1887 –1961 52 Schrödinger Equation for stationary states Potential energy Kinetic energy Total energy 53 Schrödinger Equation for stationary states Remember H is the hamiltonian Half penny bridge in Dublin Sir William Rowan Hamilton Irish 1805-1865 54 Chemistry is nothing but an application of Schrödinger Equation (Dirac) < YI Y> <Y IOI Y > Dirac notations Paul Adrien Dirac 1902 – 1984 Dirac’s mother was British and his father was Swiss. 55 Uncertainty principle the Heisenberg uncertainty principle states that locating a particle in a small region of space makes the momentum of the particle uncertain; and conversely, that measuring the momentum of a particle precisely makes the position uncertain We already have seen incompatible operators Werner Heisenberg German 1901-1976 56 It is not surprising to find that quantum mechanics does not predict the position of an electron exactly. Rather, it provides only a probability as to where the electron will be found. We shall illustrate the probability aspect in terms of the system of an electron confined to motion along a line of length L. Quantum mechanical probabilities are expressed in terms of a distribution function. For a plane wave, p is defined and the position is not. With a superposition of plane waves, we introduce an uncertainty on p and we localize. Since, the sum of 2 wavefucntions is neither an eigenfunction for p nor x, we have average values. With a Gaussian function, the localization below is 1/2p 57 p and x do not commute and are incompatible For a plane wave, p is known and x is not (Y*Y=A2 everywhere) Let’s superpose two waves… this introduces a delocalization for p and may be localize x At the origin x=0 and at t=0 we want to increase the total amplitude, so the two waves Y1 and Y2 are taken in phase At ± Dx/2 we want to impose them out of phase The position is therefore known for x ± Dx/2 the waves will have wavelengths 58 Superposition of two waves 2 env eloppe Y 1 0 -1 4.95 a (radians) -2 0 1 2 3 Dx/(2x(√2p)) Factor 1/2p a more realistic localization 4 5 Dx/2 59 Uncertainty principle A more accurate calculation localizes more (1/2p the width of a gaussian) therefore one gets Werner Heisenberg German 1901-1976 x and p or E and t play symmetric roles in the plane wave expression; Therefore, there are two main uncertainty principles 60 De Broglie’s postulate: wavelike properties of particles Matter wave: de Broglie the total energy of matter related to the frequency ν of the wave is E=hν the momentum of matter related to the wavelength λ of the wave is p=h/λ Ex: (a) the de Broglie wavelength of a baseball moving at a speed v=10m/s, and m=1kg. (b) for a electron K=100 eV. (a) l h / p 6.6 10 34 /(1 10) 6.6 10 35 m 6.6 10 25 o A (b) l h / p h / 2mK 34 6.6 10 / 2 9.1 10 l (electron) l (baseball ) 31 100 1.6 10 19 o 1.2 A 61 De Broglie’s postulate: wavelike properties of particles The experiment of Davisson and Germer (1) A strong scattered electron beam is detected at θ=50o for V=54 V. (2) The result can be explained as a constructive interference of waves scattered by the periodic arrangement of the atoms into planes of the crystal. (3) The phenomenon is analogous to the Bragg-reflections (Laue pattern). 1927, G. P. Thomson showed the diffraction of electron beams passing through thin films confirmed the de Broglie relation λ=h/p. (Debye-Scherrer method) 62 De Broglie’s postulate: wavelike properties of particles Bragg reflection: constructive interference: 2d sin nl o d 0.91 A , 50 o and 90 o / 2 65 o for n 1 o l 2d sin 2 0.91 sin 65 1.65 A o (X - ray wavelength ) for electron K 54 eV consistent l h / 2mK 34 6.6 10 / 2 9.1 10 (electron wavelength ) 31 54 1.6 10 19 o 1.65 A 63 De Broglie’s postulate: wavelike properties of particles Debye-Scherrer diffraction X-ray diffraction: zirconium oxide crystal electron diffraction : gold crystal Laue pattern of X-ray (top) and neutron (bottom) diffraction for 64 sodium choride crystal De Broglie’s postulate: wavelike properties of particles The wave-particle duality Bohr’s principle of complementarity: The wave and particle models are complementary; if a measurement proves the wave character of matter, then it is impossible to prove the particle character in the same measurement, and conversely Einstein’s interpretation: for radiation (photon) intensity I (1 / 0 c ) 2 hN 2 N 2 is a probability measure of photon density Max Born: wave function of matter is Y ( x , t ) just as satisfies wave equation Y 2 is a measure of the probability of finding a particle in unit volume at a given place and time. Two superposed matter waves obey a principle of superposition of radiation. 65 De Broglie’s postulate: wavelike properties of particles The uncertainty principle Heisenberg uncertainty principle: Experiment cannot simultaneously determine the exact value of momentum and its corresponding coordinate. Dp x Dx / 2 DEDt / 2 Bohr’s thought experiment: (1) Dp x 2 p sin ' ( 2h / l ) sin ' Dx l / sin ' ( a diffraction apparatus a l / ) Dp x Dx ( 2h / l ) sin ' l / sin ' 2h / 2 (2) E p x2 / 2m DE 2 p x Dp x / 2m v x Dp x Dx v x Dt v x Dx / Dt DE ( Dx / Dt )Dp x D ED t D p x D x 2 h / 2 Bohr’s thought experiment 66 De Broglie’s postulate: wavelike properties of particles Properties of matter wave wave propagation velocity: h E E mv 2 / 2 v w l ( ) ( ) p h p mv 2 w v Why? a de Broglie wave of a particle Y ( x , t ) sin 2p ( x / l t ) set 1/l Y ( x , t ) sin 2p (x t ) (1) x fixed, at any time t the amplitude is one, frequency is ν. (2) t fixed, Ψ(x,t) is a sine function of x. n 0,1,2,....... (3) zeros of the function are at 2p (xn t ) np xn n / 2 t xn n / 2 ( / )t these nodes move along x axis with a velocity w dxn / dt / l it is the node propagation velocity (the oscillation velocity) 67 De Broglie’s postulate: wavelike properties of particles modulate the amplitude of the waves Y ( x , t ) Y1 ( x , t ) Y2 ( x , t ) Y1 ( x , t ) sin 2p (x t ), Y2 ( x , t ) sin 2p [( d ) x ( d )t ] d d ( 2 d ) ( 2 d ) x t ] sin 2p [ x t] 2 2 2 2 d d for d 2 and d 2 Y ( x , t ) 2 cos[ x t ] sin 2p (x t ) 2 2 (1) the velocity of the individual wave is w / Y ( x , t ) 2 cos[ (2) the group velocity of the wave is g dν / 2 dν dκ / 2 d 68 De Broglie’s postulate: wavelike properties of particles group velocity of waves equal to moving velocity of particles E dE 1 p dp d d h h l h h d dE / h dE g d dp / h dp 1 dE E mv 2 p mv dE mvdv v 2 dp gv The Fourier integral can prove the following universal properties of all wave. DxD 1 / 4p for 1/l , and DtD 1 / 4p for matter wave : p h / l 1 / l p / h DxD DxD ( p / h) (1 / h)DxDp 1 / 4p DpDx / 2 uncertainty principle E h E / h DtD ( E / h) (1 / h)DtDE uncertainty principle DEDt / 2 the consequence of duality 69 De Broglie’s postulate: wavelike properties of particles Ex: An atom can radiate at any time after it is excited. It is found that in a typical case the average excited atom has a life-time of about 10-8 sec. That is, during this period it emit a photon and is deexcited. (a) What is the minimum uncertainty D in the frequency of the photon?o (b) Most photons from sodium atoms are in two spectral lines at about l 5890 A . What is the fractional width of either line, D / ? (c) Calculate the uncertainty D E in the energy of the excited state of the atom. (d) From the previous results determine, to within an accuracy D E , the energy E of the excited state of a sodium atom, relative to its o lowest energy state, that emits a photon whose wavelength is centered at 5890 A (a) DDt 1 / 4p D 1 / 4pDt 8 10 6 sec-1 (b) c / l 3 1010 / 5890 10 8 5.1 1014 sec-1 D/ 8 10 6 / 5.1 1014 1.6 10 8 natural width of the spectral line h / 4p h 6.63 10 34 8 (c) DE 3 . 3 10 eV the width of the state 8 Dt 4pDt 4p 10 (d) D/ hD/h DE / E E DE /( D / ) 2.1 eV 70 De Broglie’s postulate: wavelike properties of particles uncertainty principle in a single-slit diffraction for a electron beam: sin l Dy , py p sin Dp y p y p sin Dp y Dy pl Dy h l Dy h l Dy 2 71 De Broglie’s postulate: wavelike properties of particles Ex: Consider a microscopic particle moving freely along the x axis. Assume that at the instant t=0 the position of the particle is measured and is uncertain by the amount Dx0 . Calculate the uncertainty in the measured position of the particle at some later time t. At t 0 Dp x / 2Dx0 D v x Dp x / m / 2 m D x 0 At time t Dx tDv x t / 2m Dx0 Dx0 Dx or t Dx 72 De Broglie’s postulate: wavelike properties of particles Some consequences of the uncertainty principle: (1) Wave and particle is made to display either face at will but not both simultaneously. Dirac’s relativistic of electron: E ofc 2radiation; p 2 m02c 4 (2) We can observequantum either themechanics wave or the particle behavior but assumption: the uncertainty principle prevents from observing Dirac’s a vacuum consists of aus sea of electrons inboth together. (3) Uncertainty principle makes predictions onlyatofallprobable negative energy levels which are normally filled points inbehavior space. of the particles. The philosophy of quantum theory: (1) Neil Bohr: Copenhagen interpretation of quantum mechanics. (2) Heisenberg: Principally, we cannot know the present in all details. (3) Albert Einstein: “God does not play dice with the universe” The belief in an external world independent of the perceiving subject is the basis of all natural science. 73 How laser works Spontaneous emission and stimulated emission An excited electron may gives off a photon and decay to the ground state by two processes: •spontaneous emission: neon light, light bulb •stimulated emission : the excited atoms interact with a pre-existing photon that passes by. If the incoming photon has the right energy, it induces the electron to decay and gives off a new photon. Ex. Laser. Optical pumping many electrons must be previously excited and held in an excited state without massive spontaneous emission: this is called population inversion. The process is called optical pumping. Example of Ruby laser. Optical pumping Only those perpendicular to the mirrors will be reflected back to the active medium, They travel together with incoming photons in the same direction, this is the directionality of the laser. Characteristics of laser • The second photon has the same energy, i.e. the same wavelength and color as the first – laser has a pure color • It travels in the same direction and exactly in the same step with the first photon – laser has temporal coherence Comparing to the conventional light, a laser is differentiated by three characteristics. They are: Directionality, pure color, temporal coherence. Characteristics of laser Pure color Directionality Temporal coherence The power and intensity of a laser The power P is a measure of energy transfer rate; Total energy output (J) Power(W) exposurte time (s) where the unit of power is Joules/s or W. The energy encountered by a particular spot area in a unit time is measured by the intensity (or power density): Power (W) Intensity (W/cm ) spot area (cm 2 ) 2 laser versus ordinary lights: The directionality of laser beam offers a great advantage over ordinary lights since it can be concentrate its energy onto a very small spot area. This is because the laser rays can be considered as almost parallel and confined to a welldefined circular spot on a distant object. Sample problem: we compare the intensity of the light of a bulb of 10 W and that of a laser with output power of 1mW (10-3 W). For calculation, we consider an imagery sphere of radius R of 1m for the light spreading of the bulb, laser beams illuminate a spot of circular area with a radius r = 1mm. Pbulb 10W 10W 5 2 I bulb 8 10 W / cm A 4pR 2 4p (100cm)2 Plaser 103W 103W 2 2 I laser 3 10 W / cm A pr 2 p (0.1cm)2 I laser 3 102W / cm2 400 5 2 I bulb 8 10 W / cm Fluence, F is defined as the total energy delivered by a laser on an unit area during an expose time TE, F(J/cm2)=I(watts/cm2) x TE(s) The advantage of directionality of a laser : we can focus or defocus a laser beam using a lens. This can be used to vary the intensity of the laser. f Incoming parallel ray Focused spot Diverged beam continuous wave (CW) lasers versus pulsed lasers • CW lasers has a constant power output during whole operation time. • pulsed lasers emits light in strong bursts periodically with no light between pulses usually T>>Tw • The tw may vary from milliseconds (1ms=10-3 s) to femtoseconds (1fs=10-15s), but typically at nanoseconds (1ns=10-9s). • energy is stored up and emitted during a brief time tw, • this results in a very high instantaneous power Pi • the average power Pave delivered by a pulsed laser is low. Instantaneous power Average power Pave Pi Pi E pulse tw Pave E pulse T E pulse R Where R is the repetition rate Example A pulsed laser emits 1 milliJoule (mJ) energy that lasts for 1 nanoseconds (ns), if the repetition rate R is 5 Hz, comparing their instantaneous power and average power. (The repetition rate is the number of pulses per second, so the repetition rate is related to the time interval by R=1/T). Pi E pulse Pave tw 1mlliJoule 103 J 9 106W 1ns 10 s E pulse T E pulse( J ) R( Hz ) 103 J 5Hz 5 103W Mechanisms of laser interaction with human tissues When a laser beam projected to tissue Five phenomena: •reflection, •transmission, •scattering, •re-emission, •absorption. Laser light interacts with tissue and transfers energy of photons to tissue because absorption Photocoagulation What is a coagulation? • A slow heating of muscle and other tissues is like a cooking of meat in everyday life. • The heating induced the destabilization of the proteins, enzymes. • This is also called coagulation. • Like egg whites coagulate when cooked, red meat turns gray because coagulation during cooking. ALsrhtigotissusbov50oCbutblow 100oC iducsdisordrigoprotisdothr bioolculs,thisprocssisclld photocogultio. Consequence of photocoagulation When lasers are used to photocoagulate tissues during surgery, tissues essentially becomes cooked: • they shrink in mass because water is expelled, • the heated region change color and loses its mechanical integrity • cells in the photocoagulated region die and a region of dead tissue called photocoagulation burn develops • can be removed or pull out, Applications of photocoagulation • destroy tumors • treating various eye conditions like retinal disorders caused by diabetes • hemostatic laser surgery - bloodless incision, excision: due to its ability to stop bleeding during surgery. A blood vessel subjected to photocoagulation develops a pinched point due to shrinkage of proteins in the vessel’s wall. The coagulation restriction helps seal off the flow, while damaged cells initiate clotting. Photo-vaporization With very high power densities, instead of cooking, lasers will quickly heat the tissues to above 100o C , water within the tissues boils and evaporates. Since 70% of the body tissue is water, the boiling change the tissue into a gas. This phenomenon is called photovaporization. Photo- vaporization results in complete removal of the tissue, making possible for : • hemostatic incision,or excision. • complete removal of thin layer of tissue. Skin rejuvenation, Conditions for photo-vaporization 1. the tissue must be heated quickly to above the boiling point of the water, this require very high intensity lasers, 2. a very short exposure time TE, so no time for heat to flow away while delivering enough energy, highly spatial coherence (directionality) of lasers over other light sources is responsible for providing higher intensities Intensity requirement Intensity (W/cm2) Low (<10) Moderate (10 – 100) High (>100) Resulting processes General heating Photocoagulation Photo-vaporization Photochemical ablation When using high power lasers of ultraviolet wavelength, some chemical bonds can be broken without causing local heating; this process is called photo-chemical ablation. The photo-chemical ablation results in clean-cut incision. The thermal component is relatively small and the zone of thermal interaction is limited in the incision wall. Selective absorption of laser light by human tissues Selective absorption Selective absorption occurs when a given color of light is strongly absorbed by one type of tissue, while transmitted by another. Lasers’ pure color is responsible for selective absorption. The main absorbing components of tissues are: • Oxyhemoglobin (in blood): the blood’s oxygen carrying protein, absorption of UV and blue and green light, • Melanin (a pigment in skin, hair, moles, etc): absorption in visible and near IR light (400nm – 1000nm), • Water (in tissues): transparent to visible light but Selective absorption Applications of lasers