Magnetic field effects photon absorption

The possibility of two-photon absorption (TPA) due to non-linear effects has been mentioned in Sect. 4.1. The magnetic-field-tuned LHeT absorption by n-type GaAs of a laser line at 20.2 cm-1 (2.50 meV) has been reported by [28] for B = 1.15 T and attributed to a two-photon Is —> 2s transition at 40.4cm 1 (5.00meV). The fact that the initial and final states of this transition have the same parity can be explained by assuming an odd-parity... 

SMART MATERIALS. From a technical and simple point of view, a smart material is a material that responds to its environment in a timely manner. To expand on this definition, a smart material is one that receives, transmits, or processes a stimulus and responds by producing a useful effect, which may include a signal that the material is acting upon it. Stimuli may include strain, stress, temperature, chemicals, an electric field, a magnetic field, hydrostatic pressures, different types of radiation, and other forms of stimuli. Transmission or processing of the stimulus may be in the form of an absorption of a photon, of a chemical reaction, of an... 

All photoeffects involve the absorption of photons to produce an excited state in the absorber or liberate electrons directly. With the direct release of electrons, photoemission may occur from the surface of solids. While the excited state may revert to the ground state, it may proceed further to a photochemical reaction to provide an electron-hole pair (exciton) as the primary photoproduct. The exciton may dissociate into at least one free carrier, the other generally remaining localized. In an externally applied electric field, photoconduction occurs. Photomagnetic effects arise in a magnetic field. Absorption of photons yield photoelectric action spectra which resemble optical absorption spectra. Photoeffects are involved in many biological systems in which charge transfer takes place (e.g., as observed in the chlorophylls and carotenoids) [14]. 

Figure 4 7. Effect of light on a pair of electrons in a molecular orbital. Hie arrows indicate the directions of the election spins with respect to the local magnetic field, (a) In the ground state (a singlet), the two elections in a filled orbital have their spins in opposite directions, (b) The absorption of a photon can cause the molecule to go to an excited singlet state where the spins of the electi ons are still in opposite directions, (c) In an excited triplet state, the spins of the two electrons are in the same direction (but in different orbitals).

If a Josephson junction is irradiated with microwaves of frequency /, the I-V behavior shows a series of steps, called Shapiro steps, as shown in Figure 30.32. These steps correspond to supercurrents across the junction when the condition for the absorption of microwave photons is satisfied (this is called the ac Josephson effect). Similar behavior is seen when we expose the junction to a magnetic field. How Josephson junctions can be used to detect very small magnetic fields is described in Chapter 33. 

Besides absorption and emission measurements, other techniques (optical, magnetic or magneto-optical) can also be used for determination and assignment of crystal-field energy levels inside the 4f shell two-photon absorption (TPA), Zeeman effect spectroscopy, electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD). 

---