Lasers ….how old are they? Peter Blood Cardiff University.

Lasers

….how old are they?

Peter Blood

Cardiff University

Absorption of light

When light passes through materials it is usually absorbed.

In certain circumstances light may be amplified.

This was called “negative absorption”

It is the basis of laser action

Physics: Photons and matter

Physics: Photons and matter

Light and an atom

Electron transitions between energy levels

Lower state fullUpper state emptyLight absorbed

Lower state emptyUpper state fullMORE light emitted

Light amplification by stimulated emission of radiation

Einstein 1917

Can light amplify light?

Amplification:Need more electrons at high energy than at low energy.

Physics: Photons and matter

No one thought this could be done

stimulated emission just a theoretical curiosity for about 30 years!

Microwave Amplification by Stimulated Emission of Radiation

• Townes, 1951, devised method to amplify microwavesWavelength about 1 cm. (Radio 1 FM: about 3 m)

….while sitting in Franklin Park, Washington on a Saturday morning!

Physics: Photons and matter

•Townes with PhD student Gordon demonstrated this using ammonia molecules in April 1953.

This was the first MASER

“How the laser happened” C H Townes,

Oxford University Press, 1999

But visible light has a wavelength of ?????

The laser inventors

Charles Townes and Art Schawlow speaking at the 40th anniversary of the invention of the laser,

San Francisco 1998.

The patent

The Invention

mirror mirror

Cavity in one direction only

Length ~10,000 wavelengths

Light output

The LASER maker

16 May, 1960,

Theodore H Maiman, Hughes Labs, generated a laser beam

wavelength 694 nm

from a ruby rod in a coiled flash lamp.

Paper submitted for publication Rejected.

Results announced in New York Times, 8 July 1960.

Paper accepted by “Nature”, appeared 6 August 1960.

Maiman’s laser

Ruby rod (pink) 3 cm long.

Electrons excited to upper level by the flashlamp

Wikipedia

Maiman’s Press ConferenceHotel Delmonico, New York 7 July 1960

• “in the upper microwave region we are dealing with dimensions of an eighth of an inch or so: imagine the problem in the optical region where the wavelengths are only a few ten-millionths of an inch”.

• “..it radiates and almost perfectly parallel beam. When reaching the moon nearly a quarter of a million miles away [it] would illuminate a lunar area less than ten miles wide…a searchlight would spread its beam over 25,000 miles”.

• “The laser’s use in radar and communications for space work is obvious since there is no atmosphere …to absorb or scatter the beams.”

The Photographs

Theodore H Maiman

Laser diodes: Nobel Prize: Physics 2000

Jack Kilby "for his part in the invention of

the integrated circuit"

Zhores Alferov Herbert Kromer*

"for basic work …for developing semiconductor heterostructures used in high-speed- and opto-electronics“

Kroemer’s original paper (1962) was also rejected for publication.

Laser diodes made: October 1962

* “Experts said it could not be done”

Lasers for optical disc technologies

Light from diode lasers used to read information from the disc.

Make “pits” smaller: store more information, achieved by:

reducing the laser wavelength

First CD laser diodes achieved this by changing the chemical composition in the heterostructure.

..Now we use “quantum dots”…a kind of nanostructure.

Observation of amplification

Physics: Photons and matter

current

current

camera

meter

Photo detector

amplification

Layer of dots

1 2

How old are they?

•Lasers now commonplace: many important uses.

……but it took a long time to get there from Einstein's idea of stimulated emission in 1917.

•“they said it couldn’t be done”: Townes and Kromer challenged “accepted wisdom”.

•Science and invention:

a mix of imagination and rigour

Physics: Photons and matter

50 years

endnotesReadingThe history of the laser

Mario Bertolotti, Institute of Physics Publishing, 1999

How the laser happened

Charles H Townes, Oxford University Press, 1999

Thanks to……Engineering and Physical Sciences Research Council, and

Cardiff University for research funding

Matt Hutchings and Ian O’Driscoll for research collaboration and practical skills.

Physics: Photons and matter