Semiconductor lasers emission from

Figure 9.41 shows a diode junction for generating the inverted population combined with a Fabry-Perot resonant cavity. When the junction is forward biased, one can obtain laser emission from such a structure. To get some of fhe Hght out of the cavity at least one, and often both, of the mirrors are only partially reflecting. It turns out to be easy to fabricate such mirrors in a single crystal material such as a semiconductor... 

The upward arrows in the figure indicate the pumping channels to various high energy levels by flashlamp (0.5 /tim) or semiconductor lasers (0.8 /rm), where Nd + ions display strong absorption transitions. The downward arrow indicates the widely used laser emission at 1.06 /xm, associated with the -> " ln/2 transition. In addition, laser action is also generally possible from the same " F3/2 level to the " 19/2 state at around 0.9 /xm and to the 113/2 state at around 1.3 /xm. 

The methods range from simple, inexpensive absorption spectroscopy to sophisticated tunable-laser-excited fluorescence and ionization spectroscopies. AAS has been used routinely for uranium and thorium determinations (see for example Pollard et al., 1986). The technique is based on the measurement of absorption of light by the sample. The incident light is normally the emission spectrum of the element of interest, generated in a hollow-cathode lamp. For isotopes with a shorter half life than and Th, this requires construction of a hollow-cathode lamp with significant quantities of radioactive material. Measurement of technetium has been demonstrated in this way by Pollard et al. (1986). Lawrenz and Niemax (1989) have demonstrated that tunable lasers can be used to replace hollow-cathode lamps. This avoids the safety problems involved in the construction and use of active hollow-cathode lamps. Tunable semiconductor lasers were used as these are low-cost devices. They do not, however, provide complete coverage of the spectral range useful for AAS and the method has, so far, only been demonstrated for a few elements, none of which were radionuclides. 

Lasers for CDs and DVDs are based on emission from solid state semiconductor materials their operation is described in Chapter 22. 

The physical mechanism by which semiconductor light-emitting diodes (LEDs) emit light is spontaneous recombination of electron-hole pairs and simultaneous emission of photons. The spontaneous emission process is fundamentally different from the stimulated emission process occurring in semiconductor lasers and superluminescent LEDs. The characteristics of spontaneous emission that determine the optical properties of LEDs will be discussed in this section. 

Specific examples of semiconductor laser array pumps include devices made from the AlGaAs material system. The emission wavelength can be varied from 700 to 900 nm by varying the Ga/Al composition. These emission wavelengths can be made to coincide with absorption bands of several important solid state lasing ions, such as neodymium, chromium, holmium, erbium, and promithium. 

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