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Lasers Physics for Biomedical Sciences Dr Irene Polycarpou 1 Laser Lasers are important milestone in the field of science, medicine, industry. Intruder detection in security areas Removing tattoo Eye surger...

Lasers Physics for Biomedical Sciences Dr Irene Polycarpou 1 Laser Lasers are important milestone in the field of science, medicine, industry. Intruder detection in security areas Removing tattoo Eye surgery Laser Light Amplification by Stimulated Emission of Radiation It is a concentrated narrow beam of coherent, monochromatic light, travelling in a particular direction. The laser’s light waves travel together with their peaks lined up, or in phase. This is why laser beams are very narrow, very bright, and can be focused into a tiny spot. Because laser light stays focused and does not spread out much (like a flashlight would), laser beams can travel long distances. They can also concentrate a lot of energy on a very small area. Laser Laser is a narrow beam of light of a single wavelength (monochromatic) in which each wave is in phase (coherent) with other near it. Laser apparatus is a device that produce an intense concentrated, and highly parallel beam of coherent light. Coherence Spatial coherence allows a laser to be focused to a tight spot, enabling applications like laser cutting and lithography. ✓ Spatial coherence also allows a laser beam to stay narrow over long distances (collimation),enabling applications such as laser pointers. Lasers can also have high temporal coherence which allows them to have a very narrow spectrum, i.e., they only emit a single color of light. ✓ Temporal coherence can be used to produce pulses of light as short as a femtosecond. Laser Physics Laser science or laser physics is a branch of optics that describes the theory and practice of lasers. Laser science is principally concerned with quantum electronics, laser construction, optical cavity, design, the physics of producing a population inversion in laser media, and the temporal evolution of the light field in the laser. Historical Overview The working principle of a Laser is based on: Spontaneous Emission Stimulated Emission Population Inversion Basic Theory of a Laser (Einstein 1917) Atom composed of a nucleus and electron cloud If an incident photon is energetic enough, it may be absorbed by an atom, raising the latter to an excited state. It was pointed out by Einstein in 1917 that an excited atom can be revert to a lowest state via two distinctive mechanisms: ✓spontaneous emission and ✓stimulated emission. Spontaneous Emission Electrons can spontaneously transition from an excited state to the ground state, a process during which photons are emitted. The emitted photons are incoherent. This means they are emitted randomly without a fixed phase relationship. → Their phase varies spatially and temporally. e.g. light emission from a tungsten lamp. Laser vs. Ordinary Light Production of Ordinary Light Atom consists of a nucleus and an electron cloud with electrons in different energy states. The electrons can be excited to higher energy states by absorbing energy. Once electron reaches an excited state, it eventually decays to its ground state by giving off a photon in a mechanism known as spontaneous emission. ❖Spontaneous emission is the mechanism in which the ordinary light bulbs give off photons. Laser vs. Ordinary Light Production of Ordinary Light The emitted photon is emitted in some random direction and of a particular frequency and wavelength that corresponds to the difference in energy between the two energy states. The lifetime of the excited atom is the time that stays in the excited state and is approximately 10−8 sec Stimulated Emission Each electron is triggered into emission by the presence of electromagnetic radiation of the proper frequency. The emitted photons are coherent, i.e. they travel in phase. e.g. emission from a laser. Laser vs. Ordinary Light Production of Laser Light If an electron or a photon with a proper energy interacts with an atom in the lower energy state, the photon is absorbed and the atom becomes excited (i.e. moves into a higher energy state). The excited atom may undergo spontaneous emission and emit a photon in some random direction. If energy in the form of an electron or a photon is pumped into the excited atom and the energy is equal to the difference between the two energy levels, the excited atom, instead of absorbing the energy, is stimulated to make a transition to the lower energy state and to emit a second photon of the same frequency (color) and travelling in the same direction as the incident photon. This mechanism is known as stimulated emission. Laser vs. Ordinary Light Production of Laser Light The incident photon stimulates the atom to emit another photon. The second photon has the same energy, i.e. the same wavelength and color as the first one ✓ Laser has a pure color The second photon travels in the same direction and exactly in the same phase with the first photon ✓ Laser has temporal coherence Spontaneous vs. Stimulated Emission Coherent radiation is produced when an atom undergoes stimulated emission. Spontaneous emission occurs when an electron makes an unprovoked transition to a lower energy level. Stimulated emission occurs when an incoming photon induces the electron to change energy levels. If the incoming photon has the right energy, it induces the electron to decay and gives off a new photon. Spontaneous vs. Stimulated Emission Spontaneous Emission Stimulated emission It is a natural, spontaneous process. Not a natural process. Does not require an external photon It requires an external photon as a as a stimulus to initiate the process. stimulus to start the process. Light output Is unpolarized and not Light output is polarized and coherent. coherent. Intensity of light is lesser. Highly intense light. Light is not monochromatic. Light is monochromatic. It is an uncontrolled process. It can be controlled. Production of Laser Light The chance of stimulated emission of an excited atom is the same as the chance of absorption by a ground state atom. To increase the chance of stimulated emission, one should increase the excited atoms relative to the atoms in the ground state. Absorption Let us consider an atom that is initially in level 1 and interacts with an electromagnetic wave of frequency n. The atom may now undergo a transition to level 2, absorbing the required energy from the incident radiation. This is well-known phenomenon of absorption. E2 hn=E2 – E1 E1 Population Inversion Under normal circumstances 1. the higher an energy level is, the less it is populated by thermal energy 2. the number of atoms in the ground state is more than in the excited state. ✓ This is because of the tendency of electrons to stay in the ground state. Under some circumstances (for example, the presence of an upper energy level that has a relatively long lifetime), a system can be constructed so that there are more atoms/molecules in an upper energy level than is allowed under conditions of normal thermodynamic equilibrium. Such an arrangement is called a population inversion. The process by which atoms are raised from lower energy level to the higher energy level is called pumping. Population Inversion & Laser Production When a population inversion exists, an upper- state system will eventually give off a photon of the proper wavelength and drop to the ground state. This photon can stimulate the production of other photons of exactly the same wavelength because of stimulated emission of radiation. Thus, many photons of the same wavelength (and phase, and other similar characteristics) can be generated in a short time. This is light amplification by stimulated emission of radiation, or LASER. Population Inversion Population inversion requires 3 different energy states where the electrons are moved from the ground state into the excited state and spontaneously decay to another state, known as metastable state. A metastable state is a state in which electrons remain longer than usual (about 10-3 sec) so that the transition to the ground state occurs by stimulated rather than spontaneous emission. When one of the produced two photons interacts with another excited atom, another identical photon is produced. The process is repeated resulting in photon multiplication. Mirrors Two mirrors, one of which is partially transparent, are placed at the ends of a long narrow tube which contains the lasing (laser) medium – optical cavity. The mirrors are used to bounce the photons back and forth in a direction parallel to the tube axis. As the photons pass through the atoms of the lasing material, they greatly increase photon multiplication by stimulated emission. All the produced photons are the same (frequency, phase, direction of travel). Some photons leak through the partially reflective mirror and make up the narrow coherent external laser beam. Conditions for Production of Laser Light In order to obtain laser from stimulated emission, 3 conditions must be satisfied: 1. Stimulated Emission 2. Population Inversion 3. Mirrors Properties of Ordinary Light vs. Laser Ordinary Light ✓High Divergence ✓Incoherence ✓Very low Power and Intensity Laser Light ✓Coherent ✓Collimated ✓Monochromatic ✓Unidirectional ✓Highly focused ✓High intensity Properties of Ordinary Light vs. Laser Ordinary light: High divergence An ordinary light source emits light photons uniformly in all directions. Because the area covered by the emitted photons increases with the radius squared, the intensity (power per unit area) decreases rapidly with distance according the inverse square law. I/IO=R02/R2 Ordinary light: Incoherence The excited atoms in an ordinary light bulb act independently and produce light photons that bear no phase relation to one another and are of different frequencies → incoherent light beam. Addition of 2 waves (a and b) of different phases may result in a wave of very low amplitude (c).  Ordinary light: Low Power and Intensity A beam of ordinary light is of very low power and intensity because: ❑The photons are emitted equally in all directions causing the intensity to decay with the distance squared. ❑The emitted photons are not in phase and are of different frequencies and superposition of their wave amplitudes form lower peaks and hence lower intensity and power. Addition of 2 waves (a and b) of Addition of 2 waves (a and b) of different phases may result in a different frequency may result in wave of very low amplitude (c). a wave of higher amplitude (c). Properties of a Laser Light 1. Narrowness The emitted light photons pass through a very narrow outlet and travel in a very tight beam in the same direction with very small angle of divergence. 2. Monochromaticity All the emitted light photons are of the same frequency and wavelength. The wavelength of the laser light is determined by the amount of energy released which is equal to the energy difference. 3. Coherence The emitted photons are in phase and travel in the same direction. 4. Very high power and intensity A beam of laser is of very high power and intensity because: 1) the photons are emitted in the same direction (i.e. very small angle of divergence), limiting the coverage area and loss of intensity with distance 2) the emitted photons are in phase and of the same frequency and superposition of their waves amplitudes forms higher peaks. Types of Lasers According to the excitation process i. Pulsed laser: The atoms are excited by periodic inputs of energy. During and after excitation, the photons continue to multiply until all the atoms have been stimulated to move to the ground energy state. The process of excitation then is repeated with each input pulse of energy. ii. Continuous laser: The atoms are excited by continuously inputs of energy. Excitation and stimulated emission occur simultaneously with continuous photons multiplication and output laser beam. Types of Lasers According to the lasing media i. Gas laser: The material is gaseous, such as helium and neon and carbon dioxide. ii. Dye (fluid) laser: The material a liquid, complex organic dye such as rhodamine 6G. iii. Metal – vapor laser: Ion lasers are based on vaporization of a solid or liquid metal, such as copper, vaporized with a buffer gas such as helium. iv. Solid laser: The material is distributed on a solid matrix such as the ruby. v. Semiconductor laser: The material is a semiconductor such as compounds based on gallium arsenide. They are sometimes called diode lasers. Interaction of Laser with Tissue a b LASER BEAM REFLECTION SCATTERING TARGET TISSUE Transmitting Interaction of Laser with Tissue c e g d f e- e- HEAT PHOTO- SHOCK WAVE FLUORESCENCE PHOTO- DISSOCIATION (Breaks For diagnostic CHEMISTRY (Break molecular bond) mineralized Destroy the target deposits) Interaction of Laser with Tissue Absorption: when laser beam interacts with tissues it is absorbed and scattered (i.e. loss of intensity of the beam and energy transfer from the beam to the tissues in the form of heat). Applications: destroy tissues in localized areas, break stones in the gallbladder or kidneys). Photocoagulation: At temperatures higher than 37 C proteins destabilize and, in temperatures higher than 50 C begin to lose their natural order and ability to perform their biological functions. Photocoagulation is the heat-induced denaturation of proteins. Applications: bleeding-free laser surgery. blood vessel subjected to photocoagulation Interaction of Laser with Tissue Photovaporisation: Rapid heating of the tissue to temperatures above the boiling point of water using lasers, boils the water within the tissues. Boiling changes the tissues into gas. Photovaporization results in the complete removal of the vaporized tissue, making it possible to delicately remove thin layers of tissue. Photocoagulation which occurs around the edges of a photovaporized region makes it a blood-free procedure. Applications: treatment for enlarged prostate etc. Heat by Laser Destructive effects can be extremely selective and precisely controlled Reversible Protein Coagulation Vaporization effect Denaturation and ablation 37 C 60 C 80 C 100 C Homeostasis Welding Cutting Laser Penetration Depth Penetration depth is a measure of how deep light or any electromagnetic radiation can penetrate into a material. It is defined as the depth at which the intensity of the radiation inside the material falls to 1/e (about 37%) of its original value at (or more properly, just beneath) the surface. For a given material, penetration depth will generally be a function of wavelength. Applications of Lasers in Medicine General “Bloodless” knife – sealing of blood vessels, burning of diseased tissue Destroy cancer cells and to deliver energy to tissue. ✓ Hence, requires the wavelength of laser to be efficiently absorbed. ✓ Good absorption in the blue-green region of the spectrum (400-600 nm). ✓ Less efficient in red/infrared. Dermatology Hair removal and cosmetic surgeries. Ophthalmology Photocoagulation - sealing of blood vessels in the retina, to repair tears, holes which occur prior to retinal detachment Dentistry Surgeries on the soft tissues of the mouth and jaw as well as drill out cavities, bleach and reshape teeth with laser. Applications of Lasers in Treatment Treatment Laser Systems Tissue heating (Skin rejuvenation & tissue welding) Coagulation Vaporization Fragmentation of tattoo pigment Cold cutting Photoacoustic (lithotripsy) Photodissociation (non-thermal ablation of the cornea in ophthalmology). Laser in Surgery Laser surgery is surgery using a laser (instead of a scalpel) to cut tissue. Examples include the use of a laser scalpel in otherwise conventional surgery, and soft tissue laser surgery, in which the laser beam vaporizes soft tissues with high water content. Laser resurfacing is a technique in which molecular bonds of a material are dissolved by a laser. Laser surgery is commonly used on the eye. Laser in Surgery Techniques used include LASIK, which is used to correct near and far- sightedness in vision, and phorefractive keratectomy, a procedure which permanently reshapes the cornea using an excimer laser to remove a small amount of tissue. Types of surgical lasers include carbon- dioxide, argon, neodymium-doped yttrium aluminium garnet (Nd:YAG), and potassium titanyl phosphate (KTP, also known as green-light laser). Laser in Surgery A. Lymphoangioma of the tongue with recurrent bleeding. B. Three months following ND:YAG Laser therapy. Marginal 3cm of tongue treated by spot technique. Central portion of tongue untreated. Laser in Surgery A. A 5cm diameter superficial hemangioma of lesser curvature of the stomach with recurrent bleeding B. Immediately following Argon Laser photocoagulation on hemangioma. No recurrence after 3-years follow up. Photocoagulation of the Retina Heating a blood vessel to a point where the blood coagulates and blocks the vessel. Photocoagulation can be done by: 1. Xenon lamp 2. Laser Treatment of the Retina The dark brown melanin pigment of the retina absorbs the green beam of the argon laser. The argon laser can destroy specific regions of the retina without harming the other area of the eye, which absorb different wavelength of light. Laser Angioplasty The removal of plaque in obstructed vessel by laser, administrated through a fiber optics. Fluorescence characterization of the vessel wall could be performed via the same fiber as that used for the delivery of high-power pulses for plaque removal. Photodynamic Therapy (PDT) for Cancer A dye selectively concentrates in cancerous tissue 48 to 72 hours after it is injected. Blue-violet light from krypton laser, administrated through an optical fiber, causing dye to fluorescence, so it can easily be observed and diagnosed. The optical fiber then drives laser light of another wavelength, which destroys the tumor. Laser in Dermatology Skin rejuvenation and resurfacing techniques work by targeting water absorption in the mid-IR. They heat and ablate tissue or exert subtle thermal effects, stimulating a wound healing response. These applications cover a variety of conditions, including wrinkle removal, sun damaged skin, age and acne spots. Advantages and Disadvantages Disadvantages The heat generated by the beam can sometimes spread to parts of the skin other than the abnormal blood vessels and cause scarring or loss of pigments. R. Rox Andrson and A. Jhon (1983) (Harvard University) suggested that short exposure less than 1 ms – to intense light would destroy the absorption site but produce little or no damage to adjacent tissue. Advantages Wide damage caused by the longer slower heating of tissue can be turned advantageous. Removing of a damaged portion of the liver cause extensive bleeding. The long exposure to a continuous wave laser reduces bleeding because heat spreads to the capillaries nearby. A CO2 laser with a wavelength 10.6 microns may be used because it is absorbed by the compound most common to tissue: water Applications of Lasers in Diagnostics Diagnostic Laser Systems Several factor have to be consider in designing a diagnostic laser system: 1. A suitable excitation wavelength. 2. Knowledge about fluorescence properties of different chromospheres in tissue is needed. 3. Origin of the fluorescence spectra must be identified. 4. Tumor seeking drugs (e,g. hemato-porphoryin) is used to enhance the optical demarcation of malignant tumors. Laser Doppler Imaging (LDI) The laser Doppler velocimeter sends a monochromatic laser beam toward the target and collects the reflected radiation. According to the Doppler effect, the change in wavelength of the reflected radiation is a function of the targeted object's relative velocity. Thus, the velocity of the object can be obtained by measuring the change in wavelength of the reflected laser light, which is done by forming an interference fringe pattern. Laser Doppler Imaging (LDI) Used in hemodynamics research as a technique to partially quantify blood flow in human tissues such as skin or the eye fundus. Blood flow pulse wave in the central retinal artery (red) and vein (blue), measured by laser Doppler holography in the eye fundus of a healthy volunteer. Laser Spectrum Lasers can have high temporal coherence, which permits them to emit light with a very narrow frequency spectrum. Ultraviolet radiation for lasers consists of wavelengths between 180 and 400 nanometers (nm). The visible region consists of radiation with wavelengths between 400 and 700 nm. Laser Safety Users of Class 3B and/or Class 4 lasers and laser systems must register all new lasers at the time of purchase, create and annually update a laser safety Standard Operating Procedure (SOP), and complete laser safety training biennially. Revision Revision Revision Revision Revision Revision