IntroductionPulsed LasersRuby LaserNd: YAG LaserLasersIntroductionLASER – Light Amplification by Stimulated Emission of RadiationFor stimulated emission to occur, medium must have some molecules in an excited state. Butconsider the thermal population of the excited state at room temperature if the wavelength of emission is 694 nm.31in6.2 10N No molecules of the medium are in the excited state. Therefore to create stimulated emission, a large number of molecules must be promoted to an excited state. The process of creating a large number of molecules in the excited state is called population inversion. Since excited state molecules are being continually stimulated to release a photon and drop tothe ground state, a laser must have a way of keeping the excited state continually populated. Population inversion is created using a flash lamp or sometimes another laser. Parts of a LaserA laser has two main parts.1.) Gain medium- The medium that will be emitting the stimulated emission.2.) Resonance cavity- It is important to have a long gain medium to build intensity of light.- Intensity of light is increased by being reflected back and forth through the gain medium.g a i n m e d i u mm i r r o rp a r t i a l m i r r o rl a s e r l i g h tr e s o n a n c e c a v i t yf l a s h l a m pLight comes out of resonance cavity via “leaking” through mirror.1Resonance ModesLight emitted via spontaneous emission can have any direction.Spontaneously emitted photons cause stimulated emission.A stimulated emission photon has the same phase, frequency and direction as the photon that stimulated it.Light with its photons having the same phase, frequency and direction is called coherent light.Recall that electronic transitions have a finite linewidth. (Natural broadening, etc…)However within the resonance cavity, the photons that do not have an integral number of wavelengths between mirrors destructively interfere with each other.Only those photons with an integral number of wavelengths between the mirrors survive.The wavelengths of light that transmit from the cavity are called resonance modes.Surprise! Laser light is not really monochromatic, but …- For 694.4 nm light, the modes are 0.016 nm apart- The number of modes is approximately 10 – 20; therefore, laser light is effectively monochromatic.These resonance modes are one the reasons laser light is not infinitely coherent.Two Operational Modes of LasersContinuous Wave Lasers (CW lasers)Laser light is continuously emitted from resonance cavity.Most applications of lasers use CW lasers.- Raman spectroscopy- Laser Pointer- CD drive- Construction levels- Medical lasers- Etching of semiconductor chipsBecause gain medium is constantly being excited, the power of CW lasers is limited since thegain medium can overheat.Pulsed LasersLaser light is emitted very briefly. 10 ms to 10 fs (that is 10-15 s!)Applications of pulsed lasers include- Cutting and welding of materials- Studying transition state chemistry- Studying nonlinear opticsMuch more power can be delivered to a pulse.- World record (2002): Petawatt laser at Lawrence Livermore National Laboratory.1513600J 600J1.25PW 1.25 10 W500fs 5 10 s   - Power capacity of electric utilities in United States: 0.7 PW- Used for attempts to induce nuclear fusion.2Specific Lasers and Their Lasing MechanismsRuby Laser- Gain medium is Cr3+ ions in Al2O3 (garnet) matrix.  0.05% by weight.Energy levels of Cr3+ ions4F14A24F12En o n r a d i a t i v e d e c a yb l u e g r e e n6 9 4 3 Å ( r e d )- The nonradiative decay must occur quickly before spontaneous emission.- The spontaneous emission lifetime of 2E state is  2 msec (spin-forbidden transition).- The ruby laser is an example of a three-level system.- Disadvantage of three level system is that the many ions in the ground state must be “optically pumped to excited state, i.e., population inversion is difficult to maintain.- As a consequence, the power (intensity) of the ruby laser is limited.- Population inversion is created with Xenon flash lamp.3Nd: YAG Laser- Gain medium is Nd3+ ions in Y3Al5O12 (Yttrium Aluminum Garnet) matrix. Energy levels of Nd3+ ions4F5 / 24I9 / 22H9 / 2n o n r a d i a t i v e d e c a y1 . 0 6 4 m  ( i n f r a r e d )4I1 1 / 24F3 / 2- Example of a four-level system.- Population inversion for a four-level system is easier to maintain.- Thus, power of YAG laser can be much greater than three-level system.- High thermal conductivity of YAG allows thermal energy to be dissipated quickly.- Emission of light is in the infrared region.- Light can be made green, by adding ‘frequency doubling’ crystal (potassium titanyl phosphate) in front of beam.Helium-Neon (HeNe) Laser- Gain medium is neon gas- Helium gas is necessary for collisional energy transfer and to inhibit collisions between neon atoms.2 S3c o l l i s i o n a le n e r g y t r a n s f e r3 . 3 9 m 2 S13 S ( 2 p 5 s5 1)2 S ( 2 p 4 s5 1)3 P ( 2 p 4 p5 1)3 P ( 2 p 3 p5 1)6 3 2 8 Å ( r e d )1 . 1 5 m h e l i u m n e o n- Three lasing frequencies possible: 6328 A, 1.15 m, 3.39 m- The gain medium can flow since it is a gas.- Flow allows gas to be cooled, thus the intensity of the laser can be increased.- HeNe lasers are used barcode scanners.4CO2 Laser- Gain medium is carbon dioxide gas.- Resonance cavity contains 5 times as much N2 as CO2.- Population inversion occurs when vibrationally excited N2 collides with CO2 molecule and transfers energy.- Notation: (#, †, ‡)  (sym  = #, antisym  = †, bending  = ‡).c o l l i s i o n a le n e r g y t r a n s f e r9 . 6 m  = 11 0 . 6 m n i t r o g e n C O2 = 0( 0 0 0 )( 0 0 1 )( 1 0 0 )( 0 1 0 )( 0 0 2 )6 6 7 c m- 1- Three lasing frequencies possible: 10.6 m, 9.6 m and 667 cm-1- CO2 lasers are high intensity lasers used for metal cutting and welding.Nitrogen Laser- Gain medium is N2 gas.C 3u3 3 7 . 1 n m B 3gX 1 +u- Since population inversion is difficult to maintain, the nitrogen laser is strictly a pulsed laser.- Commonly used for spectroscopic applications.Dye Lasers5- Gain medium is an organic dye such as Rhodamine-6G.- Dye has a “broad spectrum”- Rhodamine-6G 570 nm – 610 nm.OHNH+NOOCl-- Dye is placed in solvent (often ethanol).- Dye flows through chamber to allow cooling of gain medium.- Another laser is used to create population