The introductory chapter of this book recounts the evolution of classical and EM theory, discovery of photoelectric effect, and quantum concept of light, leading up to the point when Einstein planted the seed of lasers by conceptualizing the stimulated emission. It then uses revealing illustrations to introduce the concept of population inversion and explains why stimulated emission in conjunction with a population inverted medium only can result in the amplification of light. This chapter also captures the great excitement that followed the birth of laser in 1960 and the controversies that prevail even today, with regard to the allocation of credit on the invention of lasers. Although two (1964 and 1981) Nobel Prizes have been awarded to works leading to the development of lasers, it is strange that Theodore Maiman, credited with the invention of lasers, was ignored by the Nobel committee. The birth of the laser was indeed a giant leap for mankind, and the rest of the chapter provides a glimpse of some of the mind-boggling effects that they exhibit. This book is intended to reflect, in its two parts, the epic journey that lasers have undergone to date.

1. 1.

   E = Mc² is regarded as the most famous equation in physics and perhaps in science. It underlines the equivalence of mass and energy and their interconvertibility. This seemingly simple looking elegant equation, however, has ramifications of colossal magnitude – it planted the seed for the atom bomb, the most devastating weapon produced by humans.

2. 2.

   By Einstein’s own recollection, the nascent thought of invariance of the speed of light that would later give birth to the theory of relativity and change the world forever struck him at his early teen.

3. 3.

   A solid angle is a three-dimensional analog of an angle. It can be readily understood by considering the cases of a circle and a sphere. A circle of radiusr has a perimeter 2πr which subtends an angle2π at the center, while a sphere of radiusr has a surface area of4πr² which subtends a 3D angle or solid angle4π at the center of the sphere.

4. 4.

   A scientifically correct statement would, however, be that theseN photons are being emitted by the source over an infinitesimally small duration of time.

5. 5.

   A resonant photon in this case is the one whose energy equals the energy of excitation of the atom.

6. 6.

   In the normal parlance, the lifetime of an excited energy state can be defined as the time over which ̴66% of its population decays to the ground energy state.

7. 7.

   On December 5, 2022, the researchers at the NIF achieved what eluded them for decades- ‘Self-sustaining fusion that can yield more energy than goes into it’. In a nutshell, this feat, regarded as the most remarkable scientific achievement of the 21st century, forms a crucial milestone in humans’ quest to replicate the process that powers the Sun and the stars of the universe.

8. 8.

   As a femtosecond laser pulse is focused on a material surface or a human tissue, the electrons acquire energy from the incident radiation, but the time is too short for them to transfer it to the lattice. The electrons, in turn, become extremely hot and get out of the surface taking along a chunk of material through Coulomb attraction. As there is no local heating, there is no heat affected zone, and consequently, such a process is termed as cold ablation. Such ultrashort pulses are therefore ideal for microsurgery and micromachining applications.

9. 9.

   This is a direct consequence of Heisenberg’s uncertainty principle that can be mathematically expressed as Δν × τ ≈ 1, whereτ is the duration of the pulse andΔν is its bandwidth. Thus, the shorter the pulse, the broader the bandwidth.

10. 10.

    The distinction of being the oldest person to be awarded the Nobel Prize was, however, extremely short-lived for Ashkin as in the following year (2019) Nobel Prize in Chemistry was awarded to 97-year-old John Bannister Goodenough (b–1922).

11. 11.

    LIGO is a large-scale physics observatory located in the USA with primary aim of detecting gravitational waves by employing the technique of laser interferometry. It is of interest to note here that 2017 Nobel Prize in Physics was awarded to Rainer Weiss (b–1932), Barry C. Barish (b–1936), and Kip S. Thorne (b–1940) for the very first LIGO-based detection in 2015 of the Universe’s gravitational wave that originated from a collision between two black holes.

12. 12.

    Einstein died in the early hours of April 18, 1955, and on the previous night before he went to sleep that last time, he persevered with his obsession, a unified field theory that would tie together all the forces of nature. Twelve pages filled with equations found on the table adjacent to his hospital bed bore testimony to this.

Authors and Affiliations

1. Former Head, Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Mumbai, India

   Dhruba J. Biswas

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