Based on Optical Pumping

There are several different aspects of optical pumping that are related to a number of spectroscopic techniques based on optical pumping. The r.y aspect concerns the increase or decrease of the population in selected levels. At sufficiently high laser intensities the molecular transition can be saturated. This means that a maximum change AN = Nis — Nio of the population densities can be achieved, where A A is negative for the lower level and positive for the upper level of the transition (Sect. 2.1). In case of molecular transitions, where only a small fraction of all excited molecules returns back into the initial level /) by fluorescence, this level may be depleted rather completely. 

V. Diemer, W. Demtrdder, Infrared atomic Cs laser based on optical pumping of Cs2 molecules. Chem. Phys. Lett. 176, 135 (1991)... 

Combined laser-microwave spectroscopy based on optical pumping was also performed in the solid state. Spectral line broadening caused, e.g., by strain and phonon interaction, can be overcome by extreme cooling and specific site selective procedures. Very narrow lines are attainable particularly in the spectra of rare earth ions doped to crystals in low concentration. Rare earth ions, therefore, play an important role in solid state spectroscopy, as will be illustrated in the course of this section. 

Most fundamental studies of the optical gain in semiconductor laser materials and structures are based on the stripe excitation method used to measure the optical gain [13], This relies on optical pumping and a variation of the length of the excited zone. Great care must be taken in order to avoid saturation effects. 

It should be noted that the simple principles of the absorption and emission processes discussed above for the two-, three- and fom-level systems are based on optical photon transitions. However, non-radiative processes need to be considered as well. They can even be the dominant ones, as would be the case for electron impact excitation, pumping in a discharge, or for coUisional energy transfer between levels (e.g. as encountered in the HeNe gas laser or the Nd YAG solid-state laser). Regardless of the way the pumping and relaxation steps are implemented, the overall formalism remains more or less unchanged only the transition probability expressions have to be adapted. 

Another possibility of obtaining line profiles narrower than the natural linewidth of an optical transition is based on optical-optical double-resonance methods (see Sect.10.2.4). Assume that two monochromatic laser waves, a pump wave with frequency Wp and a weak probe wave with interact resonantly with two molecular transitions sharing a common upper level (Fig.13.17). The linewidth of the probe transition is then given by [13.30 10.51]... 

The first method extracts / based on optical transient absorption results using a laser only pump-probe approach, where the photoexcitation conditions are set to be as close as possible to those in the XTA measurements. The/ values can be obtained by comparing AOD (change in optical density) and the total OD in the spectral region of the ground state bleach, as long as minimal interference from other spectral features is present in the same region. 

These limitations have recently been eliminated using solid-state sources of femtosecond pulses. Most of the femtosecond dye laser teclmology that was in wide use in the late 1980s [11] has been rendered obsolete by tliree teclmical developments the self-mode-locked Ti-sapphire oscillator [23, 24, 25, 26 and 27], the chirped-pulse, solid-state amplifier (CPA) [28, 29, 30 and 31], and the non-collinearly pumped optical parametric amplifier (OPA) [32, 33 and 34]- Moreover, although a number of investigators still construct home-built systems with narrowly chosen capabilities, it is now possible to obtain versatile, nearly state-of-the-art apparatus of the type described below Ifom commercial sources. Just as home-built NMR spectrometers capable of multidimensional or solid-state spectroscopies were still being home built in the late 1970s and now are almost exclusively based on commercially prepared apparatus, it is reasonable to expect that ultrafast spectroscopy in the next decade will be conducted almost exclusively with apparatus ifom conmiercial sources based around entirely solid-state systems. 

A small (25-kg), portable apheresis system, available in 1993, is designed to meet a wide variety of blood cell separation needs. The role of the apheresis system is to control the behavior, separation, and collection of blood components from the bowl while maintaining maximum donor safety. The system controls the flow rates of blood and components through variable pump speeds. It directs the flow of components out of the bowl, by fully automatic opening and closing of valves based on the output of the system sensors. The system monitors the separation of blood components in the bowl by an optics system that aims at the shoulder of the bowl. A sensor on the effluent line monitors the flow of components out of the bowl. 

V.G. Kozlov, V. Bulovic, P.E. Burrows, M. Baldo, V.B. Khalfin, G. Pailhasarathy, S.R. Forrest, Y. You, M. E. Thompson, Study of lasing action based on Forster energy transfer in optically pumped otganic semiconductor thin films, J. Appl. Phys. 1998, 4, 4096. 

We have studied, theoretically and experimentally, the characteristics of a novel class of lasers that are based on radial Bragg reflectors. Lasing action with low threshold levels are demonstrated at room temperature under pulsed optical pumping. The observed Q factors are in the order of several thousands. The unique characteristics on these lasers make them promising candidates for numerous applications in telecommunications, sensing, and basic research. 

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