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!