# In a metal, how does the probability distribution of an electron look like at absolute zero?

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1 1

2 Lecture 6 Laser 2

3 In a metal, how does the probability distribution of an electron look like at absolute zero? 3

4 (Atom) Energy Levels For atoms, I draw a lower horizontal to indicate its lowest energy state, and another horizontal line higher in energy to indicate its excited state. Energy 4

5 What is the probability distribution at zero Kelvin? At finite temperature? 5

6 6 Occupation of Energy Levels in Thermal Equilibrium = T k E E n n B exp Boltzmann Distribution At 300 K, for E 2 E 1 = 1 ev, n 2 /n 1 = 1.7 x ev 2%

7 spontaneous emission Optical Processes absorption stimulated emission Before After Light Amplification by Stimulated Emission Radiation 7

8 In a two-level system, can one achieve population inversion with sufficiently large pumping? A. Yes B. No C. No definite answer 8

9 Principle of LASER hυ 32 E 3 E 3 E 3 E 3 hυ 13 E 2 Metastable state E 2 E 2 IN E 2 OUT hυ 21 hυ 2 E 1 E 1 E 1 E 1 Coherent photons (a) (b) (c) (d) The principle of the LASER. (a) Atoms in the ground state are pumped up to the energy level E 3 by incoming photons of energy hυ 13 = E 3 E 1. (b) Atoms at E 3 rapidly decay to the metastable state at energy level E 2 by emitting photons or emitting lattice vibrations; hυ 32 = E 3 E 2. (c) As the states at E 2 are long-lived, they quickly become populated and there is a population inversion between E 2 and E 1. (d) A random photon (from a spontaneous decay) of energy hυ 21 = E 2 E 1 can initiate stimulated emission. Photons from this stimulated emission can themselves further stimulate emissions leading to an avalanche of stimulated emissions and coherent photons being emitted S.O. Kasap, Optoelectronics (Prentice Hall) 9

10 Which do you expect to be easier to be a laser? A. 3-level system B. 4-level system C. Does not matter 10

11 Population Inversion No population inversion Mirror Mirror Optical resonant cavity 11

12 What does it take to make a laser? Find an appropriate material Find an appropriate pumping method Find an appropriate optical cavity 12

13 Type of Lasers Doped insulator lasers Ruby laser Nd:YAG laser Gas laser HeNe laser Argon-ion laser Carbon dioxide laser Excimer laser Semiconductor laser 13

14 Ruby Laser 14

15 Ruby Laser Heat Xenonfilled flash tube ~4 millisec 15

16 Nd:YAG Laser neodymium ions in yittrium aluminum garnet How many level (broadly speaking) system is this? Is it easier to achieve population inversion than a ruby laser? Why? 16

17 Nd:YAG Laser neodymium ions in yittrium aluminum garnet Ophthalmology Skin cancer removal Prostate surgery Hair removal Engraving Drilling Rangefinder Spectroscopy 17

18 He-Ne Laser 18

19 He-Ne Laser 19

20 Carbon Dioxide Laser 20

21 Applications of CO 2 Lasers Medical, laser scalpel 80% of soft biological tissue (skin) is water, which absorbs light at ~10 µm wavelength. à Penetration length ~ 50 µm. Vaporizes tissue Industrial Laser cutting (~ W) 21

22 Semiconductor Laser Population Inversion p + Junction n + E c E v E F p E g Holes in V B Electro ns ev o Electrons in C B E F n E c E c E g p + Inv ers ion reg io n n + E c ev E F n E F p (a) E v (b) The energy band diagram of a degenerately doped p-n with no bias. (b) Band diagram with a sufficiently large forward bias to cause population inversion and hence stimulated emission S.O. Kasap, Optoelectronics (Prentice Hall) V 22

23 Cleaved surface mirror Semiconductor Laser Diode Current (001) p + n + L L GaAs GaAs Electrode Electrode (110) Cleavage plane Active region (stimulated emission region) A schematic illustration of a GaAs homojunction laser diode. The cleaved surfaces act as reflecting mirrors S.O. Kasap, Optoelectronics (Prentice Hall) 23

24 Light Output vs. Current (L-I Curve) threshold 24

25 Heterostructure Laser Refractive index Photon density (a) (b) (c) (d) Electrons in CB E c E v 2 ev n AlGaAs p GaAs (~0.1 µm) Active region 1.4 ev Holes in VB p AlGaAs ΔE c Δn ~ 5% 1999 S.O. Kasap, Optoelectronics (Prentice Hall) 2 ev E c E v (a) A double heterostructure diode has two junctions which are between two different bandgap semiconductors (GaAs and AlGaAs). (b) Simplified energy band diagram under a large forward bias. Lasing recombination takes place in the p- GaAs layer, the active layer (c) Higher bandgap materials have a lower refractive index (d) AlGaAs layers provide lateral optical confinement. 25 Nobel Prize in Physics 2000

26 Common Lasers Type Wavelength Power Efficiency Helium-Neon 632.8nm mW <0.1% Ruby 694.3nm J <0.5% Carbon-Dioxide 10.6µm 3-100W 5-15% Nd:YAG 1.064µm W 0.1-2% Argon 488/514nm 5mW-20W <0.1% Dye nm mW 10-20% Hydrogen- Fluoride Gallium- Arsenide 2.6-3µm W 0.1-1% nm 1-40mW 1-20% 26

27 Principles of laser What We Learned Population inversion in a 3-level system Optical cavity Types of laser Ruby laser YAG laser HeNe laser Semiconductor laser 27

28 How a Laser Works 28

29 In a scale of 1 (easy) to 5 (difficult), rate the lecture on laser. A. 1 (easy) B. 2 C. 3 D. 4 E. 5 (difficult) 29

30 QD TV; Detecting Light 30

31 cyanophenylcyclohexanes Twisted Nematic LC ~ 1V 31

32 fl = foot-lambert, a unit of luminance CCFL - cold cathode fluorescent lamp or LED or QD 32

33 TFT/LCD Display 33

34 34

35 Quantum Dot Like a large atom: electrons are confined in all 3 directions à discrete electron energy levels, like 1s, 2s, 2p, of an atom à discrete hole energy levels Colloidal semiconductor nanocrystals (CdS, CdSe, PbS, PbSe, ) Chemical synthesis 2-5 nm 35

36 Quantum Dots Narrower potential well width à larger confinement energy Confine ment energy For both electrons and holes Larger recombination energy (shorter wavelength) 36

37 Quantum Dot Film Sheet for LCD TV Enhances color Liquid Crystal Module Brightness Enhancement 37 Film

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