Laser confinement

The laser control of the velocity distribution of atoms or molecules at particular quantum levels that emerged in the course of development of saturation spectroscopy free of Doppler broadening (Lamb 1964) is fairly close to the ideas considered in this book. I myself started to work on the problem of laser elimination of Doppler broadening as far back as 1965 and gradually progressed to ideas of laser confinement of atomic motion within a volume of about A . Therefore, I have decided to include a brief description of the ideas of laser velocity-selective control of atoms and molecules. 

A logical consequence of this trend is a quantum w ell laser in which tire active region is reduced furtlier, to less tlian 10 nm. The 2D carrier confinement in tire wells (fonned by tire CB and VB discontinuities) changes many basic semiconductor parameters, in particular tire density of states in tire CB and VB, which is greatly reduced in quantum well lasers. This makes it easier to achieve population inversion and results in a significant reduction in tire tlireshold carrier density. Indeed, quantum well lasers are characterized by tlireshold current densities lower tlian 100 A cm . ... 

A laser pulse strikes the surface of a sample (a), depositing energy, which leads to melting and vaporization of neutral molecules and ions from a small, confined area (b). A few nanoseconds after the pulse, the vaporized material is either pumped away or, if it is ionic, it is drawn into the analyzer of a mass spectrometer (c). 

Fundamentally, introduction of a gaseous sample is the easiest option for ICP/MS because all of the sample can be passed efficiently along the inlet tube and into the center of the flame. Unfortunately, gases are mainly confined to low-molecular-mass compounds, and many of the samples that need to be examined cannot be vaporized easily. Nevertheless, there are some key analyses that are carried out in this fashion the major one i.s the generation of volatile hydrides. Other methods for volatiles are discussed below. An important method of analysis uses lasers to vaporize nonvolatile samples such as bone or ceramics. With a laser, ablated (vaporized) sample material is swept into the plasma flame before it can condense out again. Similarly, electrically heated filaments or ovens are also used to volatilize solids, the vapor of which is then swept by argon makeup gas into the plasma torch. However, for convenience, the methods of introducing solid samples are discussed fully in Part C (Chapter 17). 

Emission spectroscopy is confined largely to the visible and ultraviolet regions, where spectra may be produced in an arc or discharge or by laser excitation. Absorption spectroscopy is, generally speaking, a more frequently used technique in all regions of the spectrum and it is for this reason that we shall concentrate rather more on absorption. 

Because there are two changes ia material composition near the active region, this represents a double heterojunction. Also shown ia Figure 12 is a stripe geometry that confines the current ia the direction parallel to the length of the junction. This further reduces the power threshold and makes the diffraction-limited spreading of the beam more symmetric. The stripe is often defined by implantation of protons, which reduces the electrical conductivity ia the implanted regions. Many different stmctures for semiconductor diode lasers have been developed. 

J. R. Murray, J. H. CampbeU, and D. N. Frank, Beamlet Project Technology Demonstration fora National Inertial Confinement Fusion Ignition Facility, Paper CTuCl, 1993 Conference on Lasers and Electro-Optics, Baltimore, Md., May 2—7, 1993. 

Epitaxial crystal growth methods such as molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) have advanced to the point that active regions of essentially arbitrary thicknesses can be prepared (see Thin films, film deposition techniques). Most semiconductors used for lasers are cubic crystals where the lattice constant, the dimension of the cube, is equal to two atomic plane distances. When the thickness of this layer is reduced to dimensions on the order of 0.01 )J.m, between 20 and 30 atomic plane distances, quantum mechanics is needed for an accurate description of the confined carrier energies (11). Such layers are called quantum wells and the lasers containing such layers in their active regions are known as quantum well lasers (12). 

Several heterostructure geometries have been developed since the 1970s to optimize laser performance. Initial homojunction lasers were advanced by the use of heterostmctures, specifically the double-heterostmcture device where two materials are used. The abiUty of the materials growth technology to precisely control layer thickness and uniformity has resulted in the development of multiquantum well lasers in which the active layer of the laser consists of one or mote thin layers to allow for improved electron and hole confinement as well as optical field confinement. 

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