Laser Active Ions

Laser active ions are those that have energy level structures and transition probabilities, thus limiting to those with ground configuration of 3d or 4f . In other words, laser active ions are from transitional elements. 

Rare earth atoms are characterized with a ground-state electronic configuration, which consists of a core that is identical to that of xenon (Xe), while the remaining electrons occupy higher orbital levels. The shells of Xe atom, with quantum numbers n= 1,2, and 3 are completely filled, while in the shell with n = 4, s, p, and d subsheUs are fiiUy filled, whereas the 4f subshell capable of accommodating up to 14 electrons is empty. However, in the shell of n = 5, the 5s and 5p orbits are fiiUy filled with eight electrons. 

The elements after Xe have this electronic configuration, plus electrons in the orbits of 4f, 5d, 6s, and so on. There are three elements. Cesium (Cs), barium (Ba), and lanthanum (La), between Xe and the RE elements. Cs has one 6s electron and Ba has two 6s electrons, while La has two 6s and one 5d electrons. When it comes to RE elements, the electrons start to fill the inner vacant 4f orbits. For instance, the first RE element Ce has only one electron in the f orbit, so that its configuration is [Xe]6s 4f 5d, while neodymium (Nd) has four electrons in the f orbit, i.e., its electron structure is [Xe]6s 4f.  

RE ions are usually trivalent or occasionally divalent at a certain special environment. For RE ions, outermost 6s electrons are aU given up and the 5d electron is lost, if there is. If there is no 5d electron, one of the 4f electrons will be lost. For example, Ce ion has an electronic configuration of [Xe]4f, while the electronic configuration of Nd ion is [Xe]4f. Therefore, after becoming ions, the electronic configurations of the RE elements become simpler. As listed in Table 1.1, the RE ions are only different in the number of electrons in the 4f shell [78]. IF the RE elements are present as divalent ion, only the two outermost 6s electrons are lost. 

Element No. RE element Trivalent ions 4f electrons Ground state 

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The dominant single crystal for solid-state lasers is YAG, which is produced using the Cz melt-growth process [29]. Transition metal elements or lanthanide rare earth elements are used as laser active ions that are doped in YAG host material. Due to its narrow spectral width and high quantum efficiency, Nd ion, a four-level laser system has been acknowledged to be the most popular active ion. 

Pr Ion Due to the manifold Pq located in the 20,500 cm region, Pr ion has been demonstrated to exhibit laser transitions in the visible, from blue, green, orange to red, as well as near-infrared region. The manifold Pq has a large emission cross-section, especially for the red transition of Pq p2. Therefore, Pr " has been extensively studied as a laser active ion. However, Pr " ion has a relatively... 

Usually, pumping sources with broadband or multiple emissions, e.g., flash-lamps and solar radiations, exhibit less superposition with the absorption lines of ions, so that a large portion of the pump radiation would oscillate inside the laser materials, without being absorbed by the laser-active ions. Furthermore, this type of pumping has a variety of absorption transitions, thus leading to various quantum defects. In this case, an average quantum defect is used. 

Considerable interest in the subject of C-H bond activation at transition-metal centers has developed in the past several years (2), stimulated by the observation that even saturated hydrocarbons can react with little or no activation energy under appropriate conditions. Interestingly, gas phase studies of the reactions of saturated hydrocarbons at transition-metal centers were reported as early as 1973 (3). More recently, ion cyclotron resonance and ion beam experiments have provided many examples of the activation of both C-H and C-C bonds of alkanes by transition-metal ions in the gas phase (4). These gas phase studies have provided a plethora of highly speculative reaction mechanisms. Conventional mechanistic probes, such as isotopic labeling, have served mainly to indicate the complexity of "simple" processes such as the dehydrogenation of alkanes (5). More sophisticated techniques, such as multiphoton infrared laser activation (6) and the determination of kinetic energy release distributions (7), have revealed important features of the potential energy surfaces associated with the reactions of small molecules at transition metal centers. 

In our laboratory we have utilized multiphoton infrared laser activation of metal ion-hydrocarbon adducts to probe the lowest energy pathways of complex reaction systems (6). Freiser and co-workers have utilized dispersed visible and uv radiation from conventional light sources to examine photochemical processes involving organometallic fragments... 

If the EDA and CT pre-equilibria are fast relative to such a (follow-up) process, the overall second-order rate constant is k2 = eda c e In this kinetic situation, the ion-radical pair might not be experimentally observed in a thermally activated adiabatic process. However, photochemical (laser) activation via the deliberate irradiation of the charge-transfer absorption (hvct) will lead to the spontaneous generation of the ion-radical pair (equations 4, 5) that is experimentally observable if the time-resolution of the laser pulse exceeds that of the follow-up processes (kf and /tBet)- Indeed, charge-transfer activation provides the basis for the experimental demonstration of the viability of the electron-transfer paradigm in Scheme l.21... 

A wide variety of process-induced defects in Si are passivated by reaction with atomic hydrogen. Examples of process steps in which electrically active defects may be introduced include reactive ion etching (RIE), sputter etching, laser annealing, ion implantation, thermal quenching and any form of irradiation with photons or particles wih energies above the threshold value for atomic displacement. In this section we will discuss the interaction of atomic hydrogen with the various defects introduced by these procedures. 

Figure 2.18 shows the range covered by different tunable solid state laser systems based on transition metal ions. As observed, a good variety of matrices have shown tunable laser action on the basis of Cr + as an active ion. The fundamental aspects determining the tunability of those Cr + based systems will be the subject of Section 6.4 in Chapter 6. 

Transition metal (TM) ions are frequently used as optically active dopants in commercial phosphors and in tunable sohd state lasers. TM ions are formed from atoms in the... 

The power outputs of the lasers will be actively stabilized and matched using a system of thermopile power meters (PM) and acousto-optic modulators (AOM s). Spatial filters (SF) will be used to ensure a well defined laser mode at the interaction region. A system of beam scanners will be used to accurately characterize the laser and ion beams at the interaction region and to measure the intersection angles 61. ... 

Solid state lasers CW or pulsed lasers in which the active medium is a sohd matrix (crystal or glass) doped with an ion (e. g., Nd, Cr, Er ). The emitted wavelength depends on the active ion, the selected optical transition, and the matrix. Some of these lasers are tunable within a very broad range (e.g., from 700 to 1000 run for Ti doped sapphire). 

A list of sensitized lanthanide lasers is given in Table IV. The laser transitions are shown in the next section for figures of the energy level s and transition of the sensitizer and activator ions and the original references see Refs. 21 and 34. Other sensitization schemes are known, but only those actually used for lasers are included. These have most commonly used f-f transitions of lanthanides. Possible d-d sensitization schemes have also been noted (35). 

An important feature in the spectroscopic behavior is that of fluorescence or luminescence of certain lanthanide ions, notably Y and Eu, when used as activators in lanthanide oxide, silicate, or transition-metal oxide lattices. Oxide phosphors are used in color television tubes. Certain of the +2 ions trapped in CaF2 lattices, as well as organic cation salts of complex anions such as [Eu /J-diket4] , show laser activity. 

S3/2— I15/2) of erbium ion have a laige emission section and are easy to achieve upconversion, so erbium ion is a good active ion as upconversion phosphors materials. The energy level transition( l]3/2— I15/2) can emit out 1.5 pm eye-safe laser radiation and the ion is also a well active ion for the eye-safe laser material. Gd203 powder with a cubic structure is a good host material for its excellent photics and thermal properties. It not only can be used as upconversion phosphors material but also can be used to prepare transparent ceramics as a host material. 

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