Light and general radiation laws. Coherence and incoherence. Emission, absorption and amplification of radiation. Units and physical constants

Light and general radiation laws. Coherence and incoherence. Emission, absorption and amplification of radiation. Units and physical constants.
Introduction

The subject of the proposed course is the study of generation and amplification of light by stimulated emission. Therefore, it would seem natural to begin studying this subject with automatic inclusion of the conclusions of classical electromagnetic theory and ordinary optics, which are the foundation for the subject under discussion. We are interested in electromagnetic radiation in the visible or near the spectrum. The wavelength in this region varies from 0.3 to 3 μm, and the frequency varies from 10¹⁴ to 10¹⁵ Hz. However, for us, of course, it is important that we are dealing with electromagnetic radiation, and not that it is visible.

In Fig. 1.1 shows progress over 150 years in the creation of sources of coherent electromagnetic radiation by an ever-increasing frequency of oscillations. As can be seen from the picture in the 1960, laser radiation appeared. Lasers on-ranks with atomic energy and a computer are called one of the three most outstanding technical achievements of the 20th century. It's not for nothing that sometimes lasers are called "Locomotive" of the 21st century. The literal translation of the word "laser" means laser light amplification by stimulated emission. The perfection of lasers and the widening of the field of their application are in full swing. Figure 1.2 shows the reduction in the duration of ultrashort laser pulses, from 10 picoseconds (1 ps = 10^-12 seconds) to several femtoseconds (1fc=10^-15 seconds) for forty years. So, the intensity (power density) of laser radiation of 10²⁰-10²¹ W • sm^-2 has already been achieved. With this intensity, the electric field strength reaches 10¹² V·sm^-1, it is two orders of magnitude stronger than the proton field at the ground level of the hydrogen atom.


Fig.1.1. Progress in creating sources of coherent electromagnetic radiation with an ever-increasing frequency of oscillations.
The magnetic field reaches 10⁹-10¹⁰ ers. The use of very short pulses of up to 10^-15 seconds (that is, to the femtosecond) opens up a number of possibilities, in particular, for obtaining x-ray pulses of duration in the attosecond (10^-18 s).

Before outlining the physical principles and principles of the action of lasers, we will briefly review the basics of the theory of light emission.