principles of lasers激光原理第1章 激光的发展简介

13
Lasers and LEDs on p–n junctions
• • January 1962: observations of superlumenscences in GaAs p-n junctions (Ioffe Institute, USSR). Sept.-Dec. 1962: laser action in GaAs and GaAsP p-n junctions (General Electric , IBM (USA); Lebedev Institute (USSR).
14
Light intensity
Wavelength
“+”
p n
EF
GaAs “–”
n
Eg
p LD
LD
n
h EF
p
Cleaved mirror
Condition of optical gain:
Ref: Z. Alferov, Semiconductor Revolution in the 20th Century, St Petersburg Academic University
Electromagnetic spectrum
3
Electromagnetic spectrum
4
Ref: Wikipedia
Photons
A photon is an elementary particle, the quantum of all forms of
electromagnetic radiation including light.
Maser in 1950s
11
Charles Hard Townes
Jim Gordon
First Laser in 1960 (Ruby)
12
@694 nm https://www.youtube.com/watch?v=h5ZgJBuq4XY
Nobel prize in physics in 1964
Cross section relation
N2 dN 2 nr dt nr
Stimulated emission
g2 21 g112
N X : Carrier population (number) Fs : Photon flux (number)
dN 2 21Fs N 2 dt st
Quantized, Quantum mechanics Classical, Maxwell’s equations middle
Quantized, Quantum mechanics Quantized, Quantum field thoery complex
Chapter 1 Introduction_L1
15
"for basic work on information and communication technology"
Zhores I. Alferov
b. 1930
Herbert Kroemer
b. 1928
1923–2005
Jack S. Kilby
17
Laser-related Nobel prizes in Physics
E hv hc / 4. Photon momentum p k wave number k 2 /
3. Photon energy 5. Photon has 2 possible polarization states 6. Photons obey the Bose-Einstein statistics, rather than the Fermi Dirac statistics (electrons, neutron, proton>> Pauli exclusion principle: two identical fermions cannot occupy the same quantum sate simultaneously)
EnF – EpF > Eg
1
The Nobel Prize in Physics 2000
“for developing semiconductor heterostructures used in high-speed- and opto-electronics” “for his part in the invention of the integrated circuit”
Absorption Spontaneous emission
22
E2
hv hv
E2
E1
E1 hv=E2-E1 Stimulated emission produces
hv=E2-E1
Stimulated emission E2 hv
photons in the same phase and direction, different to spontaneous emission Non-radiative decay…
16
Laser-related Nobel prizes in Physics
17
Laser-related Nobel prizes in Physics
18
Charles H. Townes, How the Laser Happened, Oxford, 1999
International Year of Light
E1 hv=E2-E1
Probability of the processes
Spontaneous emission
23
N2 dN 2 sp dt sp
Non-radiative decay (no photon)
A
1
Rates relation
sp
g2W21 g1W12
Classical, Newtonian mechanics Classical, Maxwell’s equations Simple
Semi-classical Semi-classical Appr.theory Quantum Quantum Appr. theory
(Quantum Electrodynamics)
Absorption
W21 21Fs
sp : Spontaneous emission lifetime nr : Nonradiative decay lifetime 21 : Cross section of stimulated emission 12 : Cross section of absorption
Chapter 1 Introduction_L1
20
Laser源自文库history Laser concept
Laser properties
Absorption of light
When light passes through materials it is usually absorbed.
N(0)e
-1
sp
Time t
Carrier decay due to spontaneous emission
Boltzman distribution
25
Thermal equilibrium: A system is said to be in thermal equilibrium if the temperature within the system is spatially and temporally uniform (constant), where the motion of atoms reach a steady state, and the atom fluctuations are, on average, invariant to time.
9
Laser history Laser concept
Laser properties
Discovery of stimulated emission in 1917
10
Ref: S. Domsch, Basics of Laser Physics, at Univeritaetsmedizin Mannheim
19
In proclaiming an International Year focusing on the topic of light science and its applications, the UN has recognized the importance of raising global awareness about how light-based technologies promote sustainable development and provide solutions to global challenges in energy, education, agriculture and health. Light plays a vital role in our daily lives and is an imperative cross-cutting discipline of science in the 21st century. It has revolutionized medicine, opened up international communication via the Internet, and continues to be central to linking cultural, economic and political aspects of the global society.
6
LASER:
Light Amplification of Stimulated Emission of Radiation
Lasers in our life
7
https://youtu.be/D0DbgNju2wE
Approaches
8
Approach Classical Classical Appr. thoery Matter Light Complexity
21
In certain circumstances light may be amplified. This was called “gain” (negative absorption) It is the basis of laser action
Interaction of light and an atom
Principles of Lasers
Cheng Wang
Phone: 20685263 Office: SEM 318 wangcheng1@shanghaitech.edu.cn
The course
2
4 credits, 64 credit hours, 16 weeks, 32 lectures 70% exame, 30% project including lab
Reference:
O. Svelto, Principles of Lasers, Springer, 2010 (main) B. Saleh and M. Teich, Fundamentals of Photonics, Wiley, 2007 William T. Silfvast, Laser Fundamentals, Cambridge, 2004
5
1. Photons exhibit wave-particle duality, both waves and particles http://actu.epfl.ch/news/the-first-ever-photograph-of-light-as-both-a-parti/
2. Photon has zero rest mass
dN1 12 Fs N1 dt a
W12 12 Fs
g X : Degeneracy of the energy level
Population vs. time
24
N2 (t )sp N2 (0)e
N(0)
t / sp
Population N 2