This chapter focuses on absorption. Absorption has been called the “death of photons.” While the energy of a photon is never truly lost, most people find the conversion of photons into heat and chemical reactions less appealing than their original emission. However, without absorption, the earth would be a far less colorful place, with no paintings, flowers, leopard spots, or stained-glass windows. There would still be some colors due to interference and scattering, but people would be blind to them because human vision is based on absorption. More importantly, if nothing on earth absorbed light, the planet would be lifeless and cold. Understanding absorption helps one understand all the interactions of light with matter.
This chapter focuses on absorption. Absorption has been called the “death of photons.” While the energy of a photon is never truly lost, most people find the conversion of photons into heat and chemical reactions less appealing than their original emission. However, without absorption, the earth would be a far less colorful place, with no paintings, flowers, leopard spots, or stained-glass windows. There would still be some colors due to interference and scattering, but people would be blind to them because human vision is based on absorption. More importantly, if nothing on earth absorbed light, the planet would be lifeless and cold. Understanding absorption helps one understand all the interactions of light with matter.
Introduction The emission and absorption of radiation, as well as the conversion of radiation into other modes of energy such as heat or electricity, all involve interaction between electromagnetic waves and atoms, molecules, or free electrons. Such daily phenomena as the radiative emission by the sun, the shielding of earth from harmful UV radiation by the ozone layer, the blue color of the sky, and red sunsets are all – despite their celestial magnitude – generated by microscopic particles. Most lasers depend on emission by excited atoms (e.g. the He-Ne laser), ionized atoms (the Ar + laser), molecules (CO or CO 2 lasers), impurities trapped in crystal structures (Nd: YAG or Ti:sapphire lasers), or semiconductors (GaAs diode lasers). Similarly, many scattering processes of interest (e.g., Rayleigh or Mie scattering) result from the exchange of energy and momentum between incident radiation and atomic or molecular species. In the previous chapter we saw that the energy of microscopic particles is quantized: their energy can be acquired, stored, or released only in fixed lumps called quanta . The example of the “particle in the box” (eqn. 8.19) illustrated that these energy quanta are specific not only to the particle itself but to the system to which it belongs. Thus, in the box, the energy of the particle is specified by its own mass and by the dimension of the box; in a different box, the same particle will have an entirely different system of energy levels and the quanta will have different magnitudes.
“Why is the sky blue?” or “why are clouds white?” are examples of basic questions that always intrigued the mind of young, and not-so-young, students before their exposure to the fundamentals of light–matter interaction, a phenomenon that occurs anytime and anywhere in our universe and constitutes the main thrust in this chapter. It starts by highlighting the basic aspects of absorption and spontaneous and stimulated emission, described (in terms of quantum mechanics) by using Einstein coefficients and transition moments ; further along, the (classical) macroscopic connection with the Beer–Lambert law is derived. The journey continues by visiting elastic scattering and its two modalities, Rayleigh and Mie scattering , depending on the scattering particle size. In the final step, inelastic Brillouin and Raman scattering are discussed, placing emphasis on the laser-spectroscopic techniques underlying these types of photon–matter interactions. The chapter ends with a brief description how prehistoric humans already knew how to prepare colors for paintings (as exemplified in the Altamira cave in Cantabria, Spain); and a short exposé into state-of-the-art single-molecule spectroscopy using a single photon. This thread of spanning an arc between (historical) breakthroughs and cutting-edge research will be common to all chapters of the book.
Ever splitting the light! How often do they strive to divide that which, despite everything, would always remain single and whole. Goethe (quoted in Zajonc 1993) Introduction The use of electromagnetic wave theory and geometrical optics to describe propagation of light is satisfactory when there is no exchange of energy between radiation and matter. All observable characteristics – direction of propagation, polarization, diffraction, interference, the energy of radiation, and so forth – are accurately described by one of these so-called classical theories. However, with the exception of absorption by lossy media (eqn. 4.36) or the effects of dispersion (eqn. 4.22), classical theories fail to give reasons for many phenomena that have practical consequences in modern technology and that involve interaction between radiation and matter. Even the partially successful descriptions of dispersion or loss cannot explain the existence of the multitude of spectrally distinct absorption lines; cannot predict the wavelengths for absorption or the wavelengths for anomalous dispersion; cannot account for all the absorbed energy; and, most importantly, cannot predict or describe the effect of optical gain, which is closely related to absorption. Thus, the principles of operation of many important devices of modern optics such as lasers, photodiodes, electronic cameras, and television screens cannot be explained by classical theories. Even the emission by the sun or an incandescent lamp are beyond the scope of these theories. It is evident therefore that a new, modern theory is necessary – one that is either more complete than, or simply complements, the classical theories.
Introduction Electromagnetic interactions play a central role in low-energy physics, chemistry, and biology. They are responsible for the cohesion of atoms and molecules and are at the origin of the emission and absorption of light by such systems. They can be described in terms of absorptions and emissions of photons by charged particles or by systems of charged particles like atoms and molecules. Photons are the energy quanta associated with a light beam. Since the discoveries of Planck and Einstein at the beginning of the last century, we know that a plane light wave with frequency ν , propagating along a direction defined by the unit vector u , can also be considered as a beam of photons with energy E = hν and linear momentum p = ( h ν/ c ) u . We shall see later on that these photons also have an angular momentum along u depending on the polarization of the associated light wave. Conservation laws are very useful for understanding the consequences of atom–photon interactions. They express that the total energy, the total linear momentum, and the total angular momentum are conserved when the atom emits or absorbs a photon. Consider for example the conservation of the total energy. Quantum mechanics tells us that the energy of an atom cannot take any value. It is quantized, the possible values of the energy forming a discrete set E a , E b , E c , . . . In an emission process, the atom goes from an upper energy level E b to a lower one E a and emits a photon with energy h ν. Conservation of the total energy requires The energy lost by the atom on going from E b to E a is carried away by the photon.
Photon cannot keep itself unchanged from emission to absorption. The information encoded on the photon is also changed due to interaction with environment. There has no definitely demonstration that the photon being absorbed is the original one from ideal light source since the quantum mechanics itself is an indeterminate theory that the physical measurement is only the probability. We divide the change of the photon state into two parts that one can be compensated and the other cannot be compensated. A concept of photon reconstruction is introduced to explain every optical phenomena including Raman scattering, multi-photon absorption, nonlinear phenomena, free electron lasing, quantum entanglement, high order coherence, ghost imaging and the de-phase which result in error bits or information loss in the quantum information process. An experimental result is explained to show that the signal photon can modify the background even the energy of the photon is not enough for absorption in the wide-band gap semiconductor material. The photon-current-voltage curve and dark-current-voltage curve of an absorption, grating, and multiplication InGaAs/InP avalanche photodiode is analyzed to show that 1550nm input light modified the dark background even the applied reverse bias voltage is far below the punch-through voltage. This increase of the dark count directly relates to the input photons at the 1550nm wavelength but is not due to absorption in the absorption layer and insensitive to the applied voltage.
Light emission occurs as a result of interaction between light and matter. Let us assume a two-level atom with levels 1 (ground state) and 2 (excited state). The energies of the ground and excited states are E 1 and E 2 , respectively, and the energy difference is therefore given by the difference ΔE 12 =E 2 −E 1 . When light with photons of energy equal to this difference is absorbed by the atomic or molecular system, an electron will be excited from level 1 to level 2. The energy of the photons is given by ΔE 12 =E 12 =hν 12 (h is Planck's constant and 1→2 is the frequency of light resulting from the level 1→2 transition). This energy is absorbed by an atom or a molecule that has energy levels, separated by ΔE, where these energy levels are the ground and the excited states. This kind of “quantum jump” of an electron between two states occurs in atomic systems between electronic levels; it can be extended to molecular systems, where vibrational and translational energy levels participate in the quantum jump. The interaction between light and matter involves the transition of electrons between different states. This interaction results in the absorption of photons (stimulated absorption) as well as spontaneous and stimulated emission. These processes can be described using Einstein's A and B coefficients, as will be described in the next paragraphs.