Stimulated photons absorption

If cof i is positive (i.e., in the photon absorption ease), the above expression will yield a non-zero eontribution when multiplied by exp(-i cot) and integrated over positive covalues. If cOf j is negative (as for stimulated photon emission), this expression will eontribute, again when multiplied by exp(-i cot), for negative co-values. In the latter situation, pi is the equilibrium probability of finding the moleeule in the (exeited) state from whieh emission will oeeur this probability ean be related to that of the lower state pf by... 

The radiative transitions of the previous descriptions have all been spontaneous Relaxation from the excited state to the ground state and emission of photons occur without external aid. In contrast, a stimulated emission occurs when the half-life of the excited state is relatively long, and relaxation can occur only through the aid of a stimulating photon. In stimulated emission, the emitted photon has the same direction as, and is in phase with, the stimulating photon. The example of Cr +-doped AI2O3 that we utilized earlier for our description of the color of ruby works equally well for a description of stimulated emission. Recall that the presence of chromium in alumina alters the electronic structure, creating a metastable state between the valence and conduction bands. Absorption of a blue-violet photon results in the excitation of an electron from... 

The first panel of Figure 5.12 shows the bichromatic control scenario. The sec panel shows the simplest path to the continuum, consisting of one-photon absorpt of CO]. The subsequent panels show the three-photon process to the contir (absorption of a> followed by stimulated emission and reabsorption of coj, ctc ... 

Figure 5.12 Interfering pathways from Et) to the continuum associated with the scenario in Figure 5.11. The frequency and phase of the lasers are co, and (a) Bichromatic control, (b) One-photon absorption, (c) Three-photon process in which initially unpopulated state Ej) is coupled to the continuum at energy E and interferes with one-photon absorption from state ] ,). (d) Same as in (c) but for a five-photon process. Notice that in processes depicted in (c) and (d) the phase

The results displayed in Figures 13.2, 13.3, and 13.4 show that the most efficient result occurred when a relatively narrow-band laser pulse of moderate intensity was i applied at center frequency near the peak of the absorption spectrum of the dye -molecule. The most effective result occurred when a positively chirped broadband M laser pulse was applied. The positive chirp (i.e., an upward drift of the laser s central j frequency with time) is helpful because stimulated emission back to tire ground stale, which diminishes the number of excited state molecules, invariably occurs to the red of the absorbed photon. By rapidly shifting the laser center frequency more unci more to the blue, one can successively eliminate the frequencies causing stimulated emission from the excited state shortly after it is formed by photon absorption. - f ... 

When a femtosecond laser pulse passes through nearly any medium, coherent vibrational excitation (in general, initiation of coherent wavepacket propagation) is likely [33, 34]. One- or two-photon absorption of a visible or ultraviolet pulse into an electronic excited state can result in phase-coherent motion in the excited-state potential [35]. Impulsive stimulated Raman scattering can initiate phase-coherent vibrational motion in the electronic... 

In photochemical processes for solar energy storage, photon absorption either creates a molecular excited state or stimulates an interband electronic transition in a semiconductor, which induces a molecular change. Comprehensive reviews, including those by Gratzel (1980), Kalyanasundaram (1982) and Harriman (1986-7), have discussed various aspects of photochemical energy conversion. A photoactivated molecular excited state can drive either i) photodissociation ii) photoisomerisation or iii) photoredox reactions. Processes based on semiconductors may involve photovoltaic or photoelectrochemical systems. 

Three-photon active (SPA) materials have been studied extensively over the last few years owing to their potential applications in the fields of telecommunications and biophotonics [26-28, 30], Two major advantages of these materials—longer excitation wavelengths and much better spatial confinement—make them attractive in comparison with two-photon absorption (2PA) based materials [29], One of the most important applications of SPA materials is three-photon pumped frequency-upconversion cavity-less lasing [26, 30], Short infra-red (IR) pulses induce the ASE process via 3-photon absorption followed by fast non-radiative decay to a long-lived state which collects population. Conventional experiments with a pulsed longitudinal pump [27, 28, 30] show that stimulated emission occurs in both forward and backward directions with respect to the pump pulse. 

Kozhekin et al. [38] proposed a method of mapping of quantum states onto an atomic system based on the stimulated Raman absorption of propagating quantum light by a cloud of three-level atoms. Hald et al. [40] have experimentally observed the squeezed spin states of a system of three-level atoms driven by a squeezed held. The observed squeezed spin states have been generated via entanglement exchange with the squeezed held completely absorbed in the process. Fleishhauer et al. [39] have considered a similar system of three-level atoms and have found that quantum states of single-photon helds can be mapped onto collective states of the atomic system. In this case the quantum state of the held is stored in a dark state of the collective states of the system. 

Laser-induced reaction has been widely used to stimulate gas-surface interaction. Lasers are also used to probe molecular dynamics in heterogeneous systems as well. In the applied area, the laser photochemical techniques are successfully applied to produce well defined microstructures and new materials for microelectronic devices (1). Enhanced adsorption and chemical reaction on surfaces can be achieved by a photoexcitation of gaseous molecules, adsorbed species as well as solid substrates. The modes of the excitation include vibrational and electronic states of the gaseous species and of the adsorbates surface complexes. Both a single and a multiple photon absorption may be involved in the excitation process. 

Vibrationally excited processes can be applied in the formation of semiconductor films. One of the examples to demonstrate the photoenhanced chemisorption and reaction due to the vibrational excitation is the interaction of SF molecules with silicon (2). In this case, SFg molecules can be chemically activated by multiple photon absorption of CO2 laser either in the gas phase or in the adsorbed state. Deposition of Si on quartz or glass surface can also be stimulated by the decomposition of SiH enhanced by the irradiation of CO2 laser to the gas phase (3). 

Thus, it has been studied azoethane decomposition [21] stimulated by ruby laser (/.=694.3nm) with power density equal to 70+175 MW/cm2. It has been established that the N2 yield is proportional to the order 2.2 0.1 with respect to the light intensity. The authors of [22] suppose that after photon absorption the excited molecules from the first excited singlet state A are transfered to an other excited singlet state B from which fluorescence is forbidden. The both states are at close range. The excited molecules which are on B level may be decomposed with a rate which depends on the nature of radicals linked with azo group. The authors of [23] assume that both mechanisms (thermo-and photo) are identic, but only in the case of photolysis transfer to triplet (T) state is possible. 

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