The fine structure in atomic spectral lines is of great significance for spectroscopy. Most of the elements found on this planet were formed during the initial few minutes right after the Big Bang, and these heavy elements (compared to hydrogen) can be sensed through their different spectral lines (and also through nuclear reactions). These studies show us how the elements and the atoms, as a whole, were formed. The structure of atoms has been studied for almost 120 years and has given us a better understanding of the finer details within atomic spectra. These fine structures have helped scientists learn more about Quantum Electrodynamics (QED). This chapter explains different spectra, how they are formed, the factors they depend on, the role of electron spin and energy, electronic configurations, Pauli exclusion, Aufbau principles, and more about fine structures. The fine structure also aids in the understanding of how atoms are formed through electron sharing between nuclei. Fine structures have been very useful in Quantum Physics because they have allowed us to learn more about how electrons behave within atoms, and from there, we can form a better understanding of how atoms come together in molecules.

Light–matter interaction plays a vital role in spectroscopy, and quantum information processing, as well as in sensing and laser also. In many fields of its application, light is treated as an electromagnetic plane wave that is propagated at the speed of light in vacuum. As a result, light–matter interaction can be considered as very weak and captured at the lowest order in quantum dynamics. However, we provide a very effective description of the relevant background physics of light, such as optical resonators and the interaction of light and matter in classical and quantum mechanics. A detailed explanation of the coherence domain that is led toward strong interaction of electromagnetic radiations and matter in which their phases themselves are arranged in a periodic manner. Moreover, the concept of machine learning provides great innovation in various kinds of fields, such as life sciences, instrumental development, the latest computer vision, molecular structures, and also in medical sciences. Finally, this article is concluded by adding a few applications and future aspects of light–matter interaction, which is expected to evolve further to encompass more intriguing physics and its novel applications.

This chapter is devoted to the discussion of many electron quantum systems. Most of the systems existing in nature comes under this category. Although the theory of many particle systems is complicated, this chapter presents the introductory description of such systems. In first part of this chapter, we present the general quantum mechanical description of such systems which is followed by various approximations needed to obtain some acceptable solution. Hartree and Hartree-Fock approach to compute the electron-electron interaction has been discussed. This is followed by the discussion of other small but important components of interactions such as spin–orbit interaction and relativistic effects. Towards the end, we have briefly mentioned the density functional-based Thomas-Fermi model of an atom which is the basis of density functional-based approaches for treating the many electron systems.

Spectroscopy deals with the interaction of light radiation with matter, which provides information on the structure and properties of matter (solids, liquids, and gasses). If laser light is used in place of the light radiation, then the spectroscopy is known as laser spectroscopy. Laser spectroscopy has emerged as a tool in many scientific techniques like tracking air quality, process control, medical research, national security, agriculture, artwork authentication, and many more. This is due to the special characteristics of lasers as compared to ordinary light. Although the emission of laser radiation is governed by the same rules and principles as that of any other light sources, laser light is not like any other ordinary source of radiation found in nature. It is a much more powerful technological tool than light from ordinary sources. Its features like coherence, monochromaticity, and collimation (directionality or low-beam divergence) make it special. The laser beam emerging from the output mirror of the resonant cavity is highly parallel, and its divergence (the spread in a beam of light) is typically a few milliradians, that is, negligibly small. The photons emitted even at a slight angle with respect to the tube axis bounce back into the walls of the tube and do not contribute to the output beam (not 100% true due to diffraction). The laser cavity is resonant only for the frequencies ν=nc/2d, where d is the separation between the mirrors of the resonant cavity adjusted as an integral of half of the wavelength, limiting the wavelength range (production of laser of well-defined wavelength). The intensity of the laser, defined as the power emitted per unit area of the output mirror per unit solid angle, is extremely high compared with that of a conventional source. The conventional sources of radiation are incoherent in nature, which means that any two photons of the electromagnetic waves of the same wavelength are out of phase, while the laser is both temporally and spatially coherent, which means that the coherence of the laser medium exists for a relatively long time and over a relatively large distance. Laser, by virtue of its coherent nature, is used for local heating, as in metal cutting, metal welding, and for holography. The coherent nature of the laser is by virtue of the mechanism through which it is produced, that is, the process of stimulated emission where photons are essentially copied or exactly in phase. The production of laser in the same phase takes place as all emitted photons are at exactly the same wavelength due to the transition between two fixed energy levels (the amplification mechanism of the laser). The simplest explanation for these properties of the laser is in the mechanism of the laser itself. The process mainly includes the stimulated emission, which takes place in the amplifying medium contained by the laser. This is done with the application of a set of mirrors used for feeding the light back to the amplifying medium so that the developed beam is grown continuously. The key concept for the realization of the laser operation is the principle of coherence accompanying stimulated emission. This stimulated emission needs the process of population inversion, for which the lasing medium must have at least three energy levels.

The light–matter interaction at the metal-dielectric interface lead to the discovery of plasmonics, whose name has its origin in the coherent collective oscillation of free electrons, named plasmons, which appears in this regime. The fundamental and applied research in plasmonics has evolved fantastically, giving rise to other subfields, such as nanoplasmonics, when the interaction occurs in the nanoscale. By starting with photonics and plasmonics as the basis for such modern research areas, this chapter deals with some aspects of modern photonics and plasmonics at the nanoscale, including the optical source of photons based on those processes, as nanolasers. Initially, a review of the basic aspects of plasmonics will be given, building upon the existing literature. Thereafter, recent aspects of the young field of nanoplasmonics, as the modern interface between nanophotonics and plasmonics, will be introduced, followed by some nonlinear nanoplasmonics in gold nanorods, besides applications in surface-enhanced Raman spectroscopy (SERS). In going further, plasmonic nanolasers and random lasers will be reviewed, including a literature update on spasers, a plasmonic laser. The chapter will conclude with an outlook in this field from the author’s point of view.

Nanophotonic materials are driving an open-door consensus to represent novel applications in a large number of possibilities, such as communications, sensors, bioimaging, and energy harvesting technologies. The mechanisms for materials doped with rare-earth (RE) ion emitters are very well recognized. Some strategies have become available to enhance optical emission in upconversion and downconversion, which has largely expanded the scope of nanothermometers, photodynamic therapy, anticounterfeiting, and scintillators. The modulation of luminescent mechanisms opens possibilities for the creation of new technologies, as well as some applications.

In this chapter, we discuss the basic concepts of resonance Raman scattering and photoluminescence and how these optical spectroscopies are key for accessing the electronic states of condensed matter systems. In particular, we used carbon nanotubes as a model system for discussing how Raman spectroscopy and photoluminescence are powerful for determining with high resolution the energy of the confined electronic states. These techniques are among the most promising for advancing the science of nanomaterials because they are noninvasive, contactless, readily available, and allow one to characterize materials incorporated in devices.

After the synthesis of phosphor materials, the main challenge is to characterize them properly for the desired functionality and properties. Characterization techniques could not be counted on fingers as they are many, but one can always select a few by keeping in mind the purpose of the synthesized material. This chapter is dedicated to the widespread characterization techniques used for phosphor materials. In order to consider the interest of researchers working in the field of phosphor materials, only those characterization techniques are discussed here that assess the fundamental properties of phosphors like phase, crystallinity, particle size, bandgap/refractive index, vibrational bands, luminescence efficiency, a lifetime of emission levels, and probability of nonradiative transition, etc. For the confirmation of the phase and crystallinity of the phosphors, the X-ray diffraction (XRD) technique is discussed. A scanning electron microscope (SEM) is included to know the morphology of the prepared phosphor. For measuring the particle size of dispersible phosphors, the scattering technique is discussed. For the determination of the bandgap and refractive index of the phosphors, the UV-Vis spectroscopy technique is discussed. To investigate the luminescence characteristics of the phosphors, photoluminescence, upconversion, and decay measurement techniques are discussed, and last, to know nonradiative transitions in phosphors the photoacoustic technique is discussed.

Metal–dielectric nanocomposites (MDNC)—dielectric crystals, glasses, and liquids, with embedded metal nanoparticles (NPs)—promise important photonic applications. Relevant characteristics of MDNC are their large linear and nonlinear optical properties if a careful selection of host and metal NPs (their nature, shape, density, and spatial arrangement) is made. In this chapter, we present examples of NL spectroscopy of MDNC applying the Z-scan and Kerr shutter techniques. The understanding of the results is based on the classical description of Nonlinear Optics. A nonlinearity management procedure was applied to control the optical response of the MDNC by playing with the volume fraction occupied by metal nanostructures, laser intensity, and wavelength, as well as the morphology of the nanostructures. High-order nonlinearities were measured and exploited to illustrate new phenomena enhanced by the metal NPs.

Understating modern theories of solid-state physics is only possible if we understand quantum mechanics and its applications at the atomic levels. Therefore, this chapter presents the evolution of quantum physics and the chronological overview of the attempts to understand the microscopic picture of the atomic structure. Historically, the roots of atomic physics lie in many thought experiments, which believed that matter is made of tiny indivisible constituents called atoms. One of the oldest references in this context lies in the Indian philosophy by Maharishi Kanad in one of his books, “Vaisesika Sutra,” which propounded the matter as consisting of indestructible particles of indivisible paramanus (the atoms). He believed that these paramanus have their unique identity and specific properties. This idea was further advocated by Democritus, a Greek philosopher, who put forward the theory of the atomic universe (460–370BCE), which states that the world is made of hard (indestructible) indivisible particles (called the atoms) of matter moving through the empty space. The word atom is taken from the Greek word atomos means, literally, “uncuttable,” This thought-based hypothesis of the atoms was first supported experimentally by John Dalton in 1804, who demonstrated that the atoms are indestructible, indivisible, and alike for the same element. This is the first theory proposed on the experimental observations. These theories come over a wide gap of time but established the same concept that there are some indivisible constituents of matter. Another fact associated with the atoms was their charge neutrality.