Introduction

In 2020, the laser, an acronym for “Light Amplification by Stimulated Emission of Radiation” celebrated its 60th anniversary. The concept of stimulated emission, which forms the basis of lasers, was initially proposed by Albert Einstein in 1917 [1]. Einstein’s theory laid the foundation for understanding how electromagnetic fields could be amplified through population inversion. Rudolf Ladenburg’s work in 1928 [2] provided indirect evidence of stimulated emission, although, at the time, physicists referred to this effect as “negative absorption.” In 1951, Charles H. Townes took a significant conceptual leap by proposing that stimulated emission at microwave frequencies could lead to oscillations within a resonant cavity, ultimately producing coherent output. Then, in 1954, Charles H. Townes and his student James P. Gordon demonstrated the first microwave maser.