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

Light and an atom Electron transitions between energy levels Lower state full Upper state empty Light absorbed Lower state empty Upper state full MORE 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 microwaves Wavelength 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 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 40 th anniversary of the invention of the laser, San Francisco The patent

The Invention 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 Paper accepted by Nature, appeared 6 August 1960.

Maimans laser Ruby rod (pink) 3 cm long. Electrons excited to upper level by the flashlamp Wikipedia

Maimans Press Conference Hotel 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 lasers 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 Kroemers 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 camera meter Photo detector amplification Layer of dots 12

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 they said it couldnt be done: Townes and Kromer challenged accepted wisdom. Science and invention: a mix of imagination and rigour Physics: Photons and matter 50 years

endnotes Reading The 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 ODriscoll for research collaboration and practical skills. Physics: Photons and matter