Applied PhysicsAfter 1945
Transistors and Lasers
Last time…matter sorted out
Matter and anti-matter (1929) Cosmic rays - many new particles identified
(1930-50s) Hadrons = particles interacting by strong force
Quarks make up all hadrons Leptons = particles interacting by weak force
Standard model proposed by Gell-Mann (1963) All matter (3 families) made of 6 quarks and 6 leptons Four fundamental forces, transmitted by exchange of
particles Photons, gauge bosons, gluons, gravitons
Unification of forces (next week)
Task of today’s lecture
From reductionism to complex systems Greek atomists, Newtonian mechanics, QM, particle
physics all reductionist Field theories, kinetic theory of gases, condensed
matter physics, chaos theory all complex systems From the Manhattan Project to interdisciplinary,
applied research after 1945 Nuclear and thermonuclear weapons Bell Labs and the transistor Many industrial labs and lasers and masers
Condensed matter physics
Explaining the physical behavior of solids Magnetism, heat capacity, electrical properties, physical
properties (e.g., hardness, cleaving, transparency)
Conductors, insulators, semiconductors Known since Faraday (1831) Semiconductor conductance affected by temperature and
impurity atoms (1 ppm), unlike conductors & insulators “Electron gas theory” of Thomson (1900) fails
• Assume an electric field gives velocity to free electrons and that kinetictheory of gases can be applied to free electrons
• Could not predict measured conductivites or heat capacities of metals Quantum theory of solids (1928) succeeds
• Assume free electron energies in solids are quantized in bands• Assume electrons move as waves through atomic lattice
Finding an amplifier
Major technological need (reverse salient) Repeaters for long-distance telephone Radio receivers Radar receivers during World War II
Lee De Forest’s triode amplifier, 1906 Employed light-bulb technology Add third element (grid) to cathode ray tube AT&T purchases all patent rights, first
transcontinental phone line in 1915 Makes possible development of sound motion
pictures (many contributions by De Forest)
Triode amplifier (“valve”)Heater
AnodeCathode
Grid
Battery
+-
Battery+
+
VinVout
Currentin
Currentout
Cathode ray tube
Research at Bell Labs
Problems with the triode Large, fragile, short lived, hot, energy needs Ineffective at higher frequencies (microwave)
Cat’s whisker diode, 1874
Bell Lab’s post-war interdisciplinary work Created condensed matter section
Mission: “… to obtain new knowledge that can be used to develop new andimproved components for communication systems.”
Theoretical and experimental physicists, physical chemist, electronicsexpert, technicians worked together in total freedom (Manhattan Proj model)
Many Bell Lab people attended courses in quantum physics at ColumbiaUniversity
Semiconductor
Metal
2 types of semiconductorsfound 1939 at Bell Labs
N-type 28Si14 dopped with arsenic
(75As33) Valence electrons Extra electrons
P-type Silicon dopped with boron
(11B5) Extra ‘holes’ (missing
electrons) N-P junction = diode
1-way current flow
+4
+4 +4+4
+4
+4
+4+4+4
+5
+4
+4 +4+4
+4
+4
+4+4+4
+3
N P+-
(current flow)- +
Shockley’s failure, 1947
Applied quantum theory of electrons andsought a “semiconductor triode” -- failed
Battery
Battery
Vin
Vout
Metal contacts
Electric field acting as“grid”
+Semiconductor
+
Bardeen & Brattain’s pointcontact transistor, 1947
Submerged Shockley’s triode in liquid Found unexpected amplification Tried many liquids, geometries, replaced
liquid with another semiconductor Amplified … they won Nobel Prize in 1956
N-type
P-typeVin Vout
Narrow gap!
+
+ Flow of holes modulatedby Vin regulates flow ofcurrent in Vout
Marketing the transistor
Transistors combine several P-N junctions Slow development to 1952
Transistors 8x more expensive than vacuum tubes Transistors could not be manufactured reliably No civilian applications except hearing aids
Military purchases, 1952-64 ($50 million) Navy study shows 60% tube failure in wartime Nuclear missile program requires miniaturization
• Silicon Valley emerges to meet military need• Manufacturing costs drop• Illustrates role of government orders in civilian economy
First transistor radios, 1959 ($50 each!)
Lasers = coherent light
Light Amplification of Stimulated Emissionof Radiation = LASER
Stimulated emission in excited atoms
Hypothesized by Einstein in 1916, but notexplored further
Ground state
Excited state
Ener
gy
E=hfPhoton in with E=(hf)
2 photons out, in phase (each with hf)
Coherentlight!
Theory of the laserMirror
Optical crystal Partial mirror
Atoms
Pump light in to excite atoms
One excited atom emits photon parallel to axis, starting cascadeof stimulated emission (in phase) as photons move toward 1 end
Cascade amplified as photons are reflected from end mirrors
When amplification is great enough, coherent beam passes throughpartially reflecting end mirror creating “light amplification”
From WWII to masers tolasers
Charles Townes at Bell Labs and Columbia Radar work during World War II (microwaves) Study molecular structure with microwaves Needed shorter-λ microwaves, built MASER, 1954
• Microwave Amplification of Stimulated Emission of Radiation
Race to build lasers, 1954-62 Townes-Gould idea 1958 (consulting for Bell Labs) Ruby crystal laser, Hughes Research Labs, 1960
• Powered by Edgerton flash lamp, gave red light• News releases called it a “killer-ray gun”
Helium-neon gas laser, Bell Labs 1960• Excited helium excites neon, emits red light
Semiconductor laser, IBM, GE, RCA 1962• Used in pocket pointers, DVD-drives, laser printers
Lasers--billion $/yr industry
Six Nobel Prizes in physics have involved lasers Industrial labs developed the technology
Maser patent to Research Corp. of Smithsonian Instwith royalties to Townes
Laser patent to Bell Labs, legal war ensues andGould (the graduate student) wins control
Applications of the laser everywhere Cutting (everything, from metals to eyeballs),
information transfer, light sources Modulated at 1011 cycles/sec in optical cables!