Electromagnetic spectrum and the LASER

Electromagnetic spectrum and
the LASER
September 2011
Zoltán Ujfalusi
University of Pécs, Faculty of Medicine,
Dept. Biophysics
Scientists
physicists, chemists, astronomers
• Sir Isaac Newton
• Sir William Herschel
• Johann Wilhelm Ritter
• Joseph von Fraunhofer
• Robert Wilhelm Bunsen
• Gustav Robert Kirchhoff
• Albert Einstein
• Louis-Victor de Broglie
• James Clerk Maxwell
• Heinrich Rudolf
- dispersion (1664)
- IR (1800)
- UV (1801)
- lines in the solar spectra (1814)
- interpretation of lines (1861)
- interpretation of lines (1861)
- light quantum (photon) (1904)
- matter-waves (1924)
- EM radiation theoretically (1864)
- EM radiation pragmatically (1888)
Huygens-Fresnel principle
1. All points on a wave front can be considered as
point sources for the production of spherical
secondary wavelets.
2. The interference of the secondary wavelets
determines the further behaviour of the wave.
Transversal
wave
E
B
x
x
The light
Electromagnetic wave
magnetic field
strength- vector
electric field strength -
vector
wavelength
The vectors of the electric and the magnetic gradients
are perpendicular to each other and to the direction of
the propagation of the wave.
• James Clerk Maxwell (1864)
verified their existence theoretically.
• Heinrich Rudolf (1888) confirmed
their existence experimentally.
Interaction of the light with matter
• Quanted energy uptaking (photon)
• Interaction of electromagnetic wave
with atomic system (matter):
• reflection
• absorption
• transmission
• (scattering)
1

The dual nature of the light
Wave
(propagation)
• Diffraction
• Interference
• Polarization
Particle
(interaction)
• photoeffect
• Compton-effect
Albert Einstein (1905) : photoelectric effect
photon (light quantum), its energy: E = h·n (or E = h·f)
Louis-Victor de Broglie (1924) : Matter-waves theory
(All materials have wave nature as well.)
λ = h/p, where p is the impulse => λ = h/m·v
Polarization
Important physical quantities and
relations
Frequency: n (1/s)
Wavelength:
� (m)
Wavenumber: n (cm-1 )
c
v
1
�
Energy: E (J) h . n
Extinct. coeff.:� (M -1 cm -1 or (mg/ml) -1 cm -1 )
a
Interference
�x � s � s � a�
sin�
1
2
Einstein: energy of a photon (light-quantum)
To achieve max. gain: To achieve max. weakening:
1
a� sin� � n�
� a�
� � ( n � ) ��
sin 2
Linearly polarized light
2

Praya dubia
Joseph von Fraunhofer
(1787–1826)
Types of the luminescence
Bathocyroë
Atolla vanhoeffeni
Line spectra (emission) of some elements
He
Hg
Na
Ne
Ar
LASER HISTORY IN A NUTSHELL
Light Amplification by Stimulated Emission of Radiation
1917 - Albert Einstein: theoretical prediction of stimulated emission
1946 - G. Meyer-Schwickerather: first eye surgery with light
1950 - Arthur Schawlow and Charles Townes: emitted photons may be in the
visible range
1954 - N.G. Basow, A.M. Prochorow, and C. Townes: ammonia maser
1960 - Theodore Maiman: first laser (ruby laser)
1964 - Basow, Prochorow, Townes (Nobel prize): quantum electronics
1970 - Arthur Ashkin: laser tweezers
1971 - Dénes Gábor (Nobel prize): holography
1997 - S. Chu, W.D. Phillips and C. Cohen-Tanoudji (Nobel prize): atom cooling
with laser
3

E 2
r(n)
E 1
Laser principles I. Stimulated emission
Elementary radiative processes:
1. Absorption 2. Spontaneous emission 3. Stimulated emission
B 12
•Frequency of transition:
n 12=N 1B 12r(n)
•�E= E 2-E 1=hn
energy quantum
is absorbed.
N 2
N 1
Explanation: two-state atomic or molecular system
E 1, E2 : energy levels, E2>E1
r(n) : spectral power density of external field
N 1, N2 : number of atoms, molecules on the given energy level
B 12, A21, B21: transition probabilities between energy levels (Einstein coefficients), B12 = B21
A 21
•Frequency of transition:
n 21=N 2A 21
•E 2-E 1 photons
radiate independently
in all directions.
r(n)
B 21
•Frequency of transition:
n 21=N 2B 21r(n)
•Upon external stimulation.
•Field energy increases.
•Phase, direction and
frequency of emitted and
external photons are identical.
Laser principles III. Optical resonance
End mirror
Pumping
Active medium
d=n�/2
Partially
transparent mirror
Resonator:
•two, parallel planar (or concave) mirrors
•Couples part of the optical power back in the active medium
•Positive feedback -> self-excitation -> resonance
Types of laser
Based on active medium:
1. Solid state lasers
Crystals or glasses doped with metal ions; Ruby, Nd-YAG, Ti-zaphire
Red - infrared spectral range; CW, Q-switched modes, high power
2. Gas lasers
Best known: He-Ne laser (10 He/Ne). Small energy, wide use
CO 2 laser: CO 2-N 2-He mixture; �~10 µm; enormous power (100 W)
Laser beam
3. Dye lasers
Dilute solution of organic dyes (e.g., rhodamine, coumarine); pumped with another laser
Large power (in Q-switched mode); Tunable
4. Semiconductor lasers
No need for resonator mirrors (internal reflection)
Red, IR spectral range. Large CW power (up to 100 W)
Beam characteristics not ideal. Wide use due to small size.
Laser principles II. Population inversion
•Amplification depends
on the relative population
of energy levels
E 1
E 0
A
F
Active
medium F+dF
dz
Thermal equilibrium Population inversion
•Population inversion only
in multiple-state systems!
•Pumping: electric,
optical, chemical energy
1. Small divergence
Parallel, collimated beam
E 2
Pumping
E 0
E 1
E 0
E 1
Fast relaxation
Metastable state
Properties of laser
2. High power
In continuous (CW) mode: tens, hundreds of watts (e.g., CO 2)
Q-switched mode: instantaneous power is enormous (GW)
Large spatial power density due to small divergence
3. Small spectral width
“Monochrome”
Large spectral power density
4. Polarized
5. Possibility of very short pulses
ps, fs
Laser transition
6. Coherence
phase equivalence, ability for interference
Temporal coherence (phase equivalence of photons emitted at different times)
Spatial coherence (phase equivalence across beam diameter)
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