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IEEE Spectrum

1964 Nobel lecture: Production of coherent radiation by atoms and molecules

Charles Hard Townes

Published Aug 1, 1965• 27 citations
Publisher

Abstract

The techniques of quantum electronics allow interesting new ways to generate and explore most of the acoustic spectrum and much of the electromagnetic domain. We can look forward to another decade of rapid development in this field From the time when man first saw the sunlight until very recently, the light which he has used has come predominantly from spontaneous emission, like the random emission of incandescent sources. So have most other types of electromagnetic radiation — infrared, ultraviolet, or gamma rays. The maximum radiation intensities, or specifically the power radiated per unit area per unit solid angle per unit frequency bandwidth, have been controlled by Planck's black-body law for radiation from hot objects. This sets an upper limit on radiation intensity — a limit which increases with increasing temperature, but we have had available temperatures of only a few tens of thousands or possibly a few millions of degrees.

DOI
10.1109/mspec.1965.6501319

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The techniques of quantum electronics allow interesting new ways to generate and explore most of the acoustic spectrum and much of the electromagnetic domain. We can look forward to another decade of rapid development in this field From the time when man first saw the sunlight until very recently, the light which he has used has come predominantly from spontaneous emission, like the random emission of incandescent sources. So have most other types of electromagnetic radiation — infrared, ultraviolet, or gamma rays. The maximum radiation intensities, or specifically the power radiated per unit area per unit solid angle per unit frequency bandwidth, have been controlled by Planck's black-body law for radiation from hot objects. This sets an upper limit on radiation intensity — a limit which increases with increasing temperature, but we have had available temperatures of only a few tens of thousands or possibly a few millions of degrees.
Twentieth century physics began exactly in the year 1900 when the German physicist Max Planck proposed a possible solution to a problem that had been haunting physicists for years. The problem concerned the light emitted by any object that is heated to a certain temperature, and also the softer infrared radiation emitted, with less intensity, by colder objects. It had become well accepted that all this radiation had an electromagnetic origin, and the laws of Nature for these electromagnetic waves were already known. Also known were the laws for heat and cold, so-called ‘thermodynamics’. Or so it seemed. But if we use thermodynamic laws to compute the intensity of the radiation in question, the outcome does not make any sense at all. Such a calculation would tell us that an infinite amount of radiation should be emitted in the very far ultraviolet. Of course, this is not what happens. What does happen is that the intensity of the radiation shows a peak at a certain characteristic wavelength, and diminishes both at wavelengths longer and shorter than this. This characteristic wavelength is inversely proportional to the absolute temperature of the radiating object (absolute temperature is defined by a temperature scale that begins at 273°C below zero or 459.6 OF below zero). At 1000°C or 1800°F an object is ‘red hot’; at this point the object is radiating in the visible light region.
Introduction The preceding chapters have focused on atomic transitions induced by applied radiation. That is, there is an optical field illuminating the atoms, and their response is calculated. It is implicitly assumed that an excited atom left alone will spontaneously return to the ground state, and conserve energy by emitting light that satisfies E 2 − E 1 = ħ ω 21 in accordance with the Planck and Bohr pictures. But this spontaneous emission process is not addressed quantitatively in these earlier chapters. Spontaneous emission is one of the most pervasive phenomena of atomic physics, and yet its origins remain among the least well-understood. The number of misconceptions is enormous, and they are to be found in textbooks, journal articles, and lecture notes. Because the natural decay of an excited atom adds energy to what is usually an empty mode of the electromagnetic field, the process is fundamentally quantum mechanical. Classical or even semi-classical descriptions cannot be assured to give correct answers. This is not to say that such discussion is to be avoided, but only that caution is needed in interpreting the results. Toward the end of the nineteenth century, many experimental results made it clear that classical mechanics needed to be supplemented by new theories. The first venture into the new physics was Planck's hypothesis that the energy of classical radiating oscillators like those discussed in Chap. 1 would have to be quantized into integer multiples of ħ ω. This notion led to his famous formula for the spectrum of thermal radiation from a non-reflective object (black body), and this formula describes a radiation field in thermodynamic equilibrium with its environment. The result of Planck's hypothesis in the context of thermal equilibrium plays a vital role in the discussion of spontaneous emission. Einstein A - and B -coefficients Soon after Bohr proposed a model of atomic structure consisting of discrete energy levels, connected by radiation whose energy satisfied Δ E = ħω when atoms made transition between them, Einstein published the first serious attempt to connect the absorption and emission processes. Absorption, he reasoned, could occur only in the presence of applied radiation, whereas spontaneous decay occurred without the application of any field.
The phenomena which are principally responsible for the interaction of radiation with matter in everyday life are absorption and spontaneous emis­ sion. Light falling on matter is absorbed, leaving atoms in excited states which spontaneously emit radiation with a spread of frequency inversely proportional to the decay time. The situation is modified when the atomic decay times are sufficiently long and the radiation sufficiently intense, for then radiation may fall on excited atoms, resulting in stimulated emission rather than absorption. We saw in Chap. 2 that this third process was required to obtain the Planck blackbody radiation law in Einstein’s derivation. It is stimulated emission which enables an atomic medium to amplify incident radiation, and therefore is an essential ingredient in light amplifiers and oscillators. In fact, the word LASER is an acronym for Light Amplification by Stimulated Emission of Radiation.
The field of quantum optics describes the interaction of electromagnetic radiation with matter whereby both are described quantum mechanically. So far the used frequencies of the radiation, were restricted to the operation range of lasers, which is between microwave to ultraviolet radiation. With the availability of new radiation sources such as sychrotrons, the field of X-ray quantum optics could be established. The high energy of the X-rays offer new possibilities, such as the examination of the interaction of radiation the nuclei of atoms. Unfortunately, X-ray source limitations still prohibit the direct transfer of control or detection schemes. One approach to overcome such limitations are mechanical motions of resonant absorber, which induces an additional phase at nuclear forward scattering. In this thesis the use of oscillating resonant absorber is examined theoretically. The phase offset of the oscillation offers an additional and controllable degree of freedom, which in uences the time and energy spectrum of a scattered wave packet. One problem occurring due to the high energy, is the lack of methods to measure the energy spectrum. To solve this problem, a new approach is developed, using the phase offset to measure the energy spectrum and the complex phase in the energy domain.
We introduce a unique technique for generating directional coherent emissions that could be utilized to create coherent sources in a wide range of frequencies from the extreme ultraviolet (XUV) to the deep infrared. This is accomplished without population inversion by pumping a two-level system with a far-detuned strong optical field that induces the splitting of the two-level system. A nonlinear process of four-wave mixing then occurs across the split system, driving coherent emission at sidebands both red- and blue-detuned from the pump frequency, and propagates both forward and backward along the pump beam path. We observed this phenomenon in dense rubidium vapor along both the D1 and D2 transitions. The sideband emission exhibits a short pulse duration (<1 ns) with threshold-like behavior dependent on both the pump intensity and Rb vapor density. This technique offers a new capability for manipulating the emission frequency simply through intensity-induced atomic modulation that can be scaled to most frequency regimes using various atomic/molecular ensembles and pump energies. A scheme for generating coherent directional emission without the need for population inversion could be a valuable alternative to the laser. Developed by scientists at Texas A&M University in the USA, the approach can in principle be applied anywhere in the electromagnetic spectrum—from the extreme ultraviolet to the far infrared. It involves pumping a two-level system with a strong detuned optical signal, which induces transient AC Stark splitting of the energy levels and four-wave mixing. Initial tests with a dense rubidium vapour indicated that the near-infrared sidebands generated in the process are coherent pulses that are less than 1 nanosecond in duration. The approach should work with other atomic and molecular systems at other wavelengths and may be an attractive way for generating coherent extreme ultraviolet or X-ray radiation as well as a novel approach for remote sensing.