Lecture 17: Laser Fundamentals 1 ME 645: MEMS: ME 645: MEMS: Design Fabrication Design Fabrication Design, Fabrication Design, Fabrication and Characterization and Characterization P.S. Gandhi P.S. Gandhi Mechanical Engineering Mechanical Engineering IIT Bombay IIT Bombay Acknoledgments: Prof S S Joshi, Mukul Tikekar PRASANNA S GANDHI gandhi@me.iitb.ac.in PRASANNA S GANDHI gandhi@me.iitb.ac.in Motivation Motivation � Why study lasers?? What is their Why study lasers?? What is their importance?? importance?? � MEMS context?? MEMS context?? � Other micromachining techniques: Other micromachining techniques: compared with laser. compared with laser. 2 1
Capabilities of Various Capabilities of Various Micro Micro- -machining machining Technologies Technologies Technology / Minimum Feature Material Materials Feature Feature Positional Removal Rate G Geometry t Si Size/Feature /F t T l Tolerance Tolerance 5 μ m 3 /s FIB 2D/3D 200 nm / 20 nm 100 nm Any Micro- 25 μ m / 2 μ m 3 μ m 10,400 μ m 3 /s PMMA, Al, milling/Micro- Brass, mild turning 2D/3D steel 40, 000 μ m 3 /s Excimer laser 6 μ m submicron Polymers, 2D/3D /submicron ceramics, metals to a l lesser degree d Femtosecond 1 μ m / submicron 13,000 μ m 3 /s Any laser/ 2D/3D submicron 25 million μ m 3 /s Micro-EDM 25 μ m / 3 μ m 3 μ m Conductive (Sinker or wire) materials 2D / 3D LIGA / 2D Sub micron / ~0.3 μ m NA Cu, Ni, 0.02 μ m ~ 0.5 polymers, 3 ceramics μ m Light Light 1 mm Basics Basics (far - IR) 50 µm IR - C (mid - IR) infrared 3 µm (IR) E IR - B IR B 1.4 µm (near - IR) electric field strength IR - A x 780 nm wavelength λ red visible green (VIS) blue 380 nm UV - A (near - UV) 315 nm ( (mid - UV) d ) UV - B 280 nm (far - UV = FUV) ultraviolet 200 nm UV - C (UV) (vacuum UV = VUV) H 100 nm (extreme UV = EUV) 30 nm 4 optical spectrum 2
Light Basics Light Basics The Optical Range 1.2 1.2 1.2 1.2 1.2 1.2 µ eV MeV keV eV meV neV Photon energy 300 300 300 300 300 300 FHz PHz THz GHz MHz kHz Frequency ultra-violet visible γ rays x rays infrared microwaves radiowaves Wavelength 1 pm 1 nm 1 µ m 1 mm 1 m 1 km 1.7 eV 3.3 eV h=4.14x10 -15 eVs Orange Green Yellow Visible Violet Indigo Blue Red Spectrum 5 400 500 600 700 nm Emission Basics Emission Basics Light is considered to Light is considered to be consisting of QUANTA be consisting of QUANTA of energy, of energy, � known as PHOTONS. Each photon carries with it known as PHOTONS. Each photon carries with it amount amount of of energy which depends upon its wavelength or frequency energy which depends upon its wavelength or frequency – – / λ / λ W ph W ph = = hf = hf hf = hc hf = = hc hc/ hc/ 36 J s and c is the velocity where h is Planck's constant = 663x10 where h is Planck's constant = 663x10 -36 J s and c is the velocity free space = 300x10 6 m/s of propagation of the light in of propagation of the light in free space = 300x10 m/s Absorption and emission of light between atomic levels: The Absorption and emission of light between atomic levels: The � excited atom can SPONTANEOUSLY (randomly) de excited atom can SPONTANEOUSLY (randomly) de- -excite to a excite to a lower level if a vacant site permits. The lower level if a vacant site permits. The average average length of time length of time an atom stays in excited state is tens of nanoseconds an atom stays in excited state is tens of nanoseconds Other ways of excitation possible Other ways of excitation possible W u Absorbed Photon Emitted light ∆ W ∆ W=h c/ λ W l 6 Absorption Spontaneous emission 3
Emission Basics Emission Basics BOLTZMANN BOLTZMANN Distribution Distribution Energy = Δ W = |W W ph ph = W = |W u u - W W l l | PHOTON PHOTON � Thermal equilibrium ENERGY ENERGY The population density of atoms, N The population density of atoms, N u , in , in � Δ W4 an excited state, W u , in relation to an excited state, W , in relation to those, those, N N l l , in a lower energy state , in a lower energy state W W l l is is Δ W3 exp(- ∆ W/kT) given by the Boltzmann relationship, given by the Boltzmann relationship, Δ W2 Δ W2 as as as, as, N u /N N l l = exp[ = exp[- -(W (W u -W W l l )/ )/kT kT] ] - Δ W/ = = exp exp[ [- W/kT kT] ] Equilibrium between Absorption Equilibrium between Absorption vs vs � Δ W1 spontaneous emission spontaneous emission N u /N l at a given temperature T 7 Principles of Laser Principles of Laser � Thermal equilibrium Thermal equilibrium � balance q balance of Absorption vs of Absorption vs spontaneous spontaneous Stimulated photon emission emission W2 W = hf � A third mechanism also exists: A third mechanism also exists: crucial to the formation of crucial to the formation of LASER action this process is LASER action this process is Incident photon W = hf known as known as STIMULATED STIMULATED EMISSION EMISSION W1 � Not a dominant process in Not a dominant process in Stimulated emission thermal systems at room thermal systems at room temperatures. temperatures. 8 4
Principles of Laser Principles of Laser � LASER: LASER: Light S i h i h ight Amplification by mplification by S lifi lifi i i b b Stimulated S i S timulated i l l d d � Emission of E mission of Radiation, has become adiation, has become synonymous with everything that is synonymous with everything that is high high- -tech and futuristic tech and futuristic � Lasers work on the basic principle of � Lasers work on the basic principle of Lasers work on the basic principle of Lasers work on the basic principle of � � converting electrical energy into a high converting electrical energy into a high energy density beam of light through energy density beam of light through “stimulated emission” leading to “stimulated emission” leading to amplification amplification 9 Principles of Laser Principles of Laser � Stimulated emission: Stimulated emission: atoms in an upper energy level atoms in an upper energy level can be triggered or stimulated can be triggered or stimulated by incident photon by incident photon � Incident photon must have an energy corresponding Incident photon must have an energy corresponding to the energy difference between the upper and to the energy difference between the upper and lower lower states states � The The incident photon is not absorbed by the atom incident photon is not absorbed by the atom. . atomic model (Bohr) energy-level diagram of an atom electron ionization y E atomic E 4 E 4 energy nucleus excited states E 3 E 2 electron shell E 1 ground state absorption spontaneous emission stimulated emission A B A B A B E 2 E 2 E 2 h ν = E 2 – E 1 h ν = E 2 – E 1 h ν = E 2 – E 1 10 10 2h ν = 2(E 2 – E 1 ) E 1 E 1 E 1 5
Principles of Laser Principles of Laser � The stimulated photon has unique properties: � The stimulated photon has unique properties: The stimulated photon has unique properties: The stimulated photon has unique properties: � It is in phase with the incident photon It is in phase with the incident photon � It has the same wavelength as the incident photon It has the same wavelength as the incident photon � Travels in same direction as incident photon Travels in same direction as incident photon � It is possible that the rate at which atoms are PUMPED into It is possible that the rate at which atoms are PUMPED into one of these states exceed the rate at which they leave. A one of these states exceed the rate at which they leave. A large number of atoms can be excited into, and held in, the large number of atoms can be excited into, and held in, the upper state upper state � Atoms can stay in this Atoms can stay in this metastable metastable state (de state (de- -exciting only on exciting only on stimulation by another photon) while the population is being stimulation by another photon) while the population is being built up this is known as a POPULATION INVERSION built up this is known as a POPULATION INVERSION 11 11 Principles of Laser Principles of Laser Typical scenario Q Q: Laser corresponding to what wavelength?? Q: Laser corresponding to what wavelength?? Q p p g g g g Energy exp(- ∆ W/kT) W4 De- W3 excitation Population inversion Population inversion Between W1 and W2 W2 Pumping action LASER ACTION W1 Population of atoms 12 12 6
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