Ions, laser-cooled

These ion lasers are very inefficient, partly because energy is required first to ionize the atom and then to produce the population inversion. This inefficiency leads to a serious problem of heat dissipation, which is partly solved by using a plasma tube, in which a low-voltage high-current discharge is created in the Ar or Kr gas, made from beryllium oxide, BeO, which is an efficient heat conductor. Water cooling of the tube is also necessary. 

The photoluminescence measurements of Thewalt et al. (1985) were performed at 4.2 K with 200 mW of 514.5 nm excitation from an Ar-ion laser in a 4 mm-diameter spot. The spectrum was analyzed with a double-grating spectrometer using a cooled photomultiplier operating in the photon-counting mode. 

Using the same PAbs an optical biosensor system has been developed for 2,4,6-TCP [224]. The principle is the detection of laser-induced fluorescence (LIF) in single microdroplets by a homogeneous quenching fluorescence immunoassay (QFIA). The competitive immunoassay occurs in microdroplets (d=58.4 mm) produced by a piezoelectric generator system. A continuous Ar ion laser (488 nm) excites the fluorescent tracer and its fluorescence is detected by a spectrometer attached to a cooled, charge-coupled device (CCD) camera... 

In the second topic, we will describe a very smart new application based on Yb + ion doped glasses laser cooling of condensed matter. 

Raman spectra were measured on fresh, chemically etched surfaces in quasi-backscattering configuration using a triple DILOR XY spectrometer, a liquid nitrogen cooled CCD detector, and a 514.5-nm Ar-ion laser. The laser beam of power level 20 mW was focused on an area of 0.1 mm2 on the mirror-like plane (it was the (ab) plane of the single crystals). The measurements were performed in a cryostat with a helium gas atmosphere in the temperature range 5-295 K below temperature of metal-insulator phase transition. 

A mercury atom that was ionized by a weak electron beam was captured in a miniature Paul (radio frequency) trap that has internal dimensions of rQ s 466 pm and zQ s 330 pm. The rf trapping frequency was 21.07 MHz with a peak voltage amplitude of about 730 V. The ion was laser cooled by a few microwatts of cw laser radiation that was frequency tuned below the 6s Si -6p Pi electric dipole transition near 194 nm. When the Hg+ ion was cold and the 194 nm radiation had sufficient intensity to saturate the strongly allowed S-P transition, 2 x 10 photons/s were scattered. With our collection efficiency, this corresponded to an observed peak count rate of about 10 s-1 against a background of less than 50 s— -. 

Experiments with trapped a cryogenic plasma of electrons at temperatures as low as 50K and densities as high as 1010/cm3 have begun to probe plasma behavior below the classical plasma region in Fig. 2. Experiments with laser-cooled trapped ions have been underway to probe the liquid region.33 Very recently crystalization was observed with only a few ions34 35 and with more than 100 ions.36 A very nice feature of the measurements is that one can learn the positions of shells of ions and even individual ions by imaging the fluorescence from the ions. A numerical simulation produced crystalization very similar to what was observed.37... 

This works only for a dilute concentrations of the atoms, to prevent the absorption of the photons into the gas in the form of heat due to atom-atom collisions. Only certain atoms and ions have optical transitions amenable to laser cooling, since it is extremely difficult to generate the amounts of laser power needed at wavelengths much shorter than 300 nm. The following is a partial list of atoms that have been laser-cooled H, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Cr, Er, Fe, Cd, Ag, Hg (plus metastable Al, Yb, He, Ne, Ar, Kr), and some ions. 

The macromode spectra described here are acquired with an Instruments SA Jobin Yvon Ramanor HG.2S system. Sample excitation is done with either argon or krypton ion lasers. This scanning spectrometer has a thermoelectrically cooled PMT detector and is fitted with a modified Nachet 400 microscope accessory for Raman microprobe work. The microprobe is capable of providing information from domains as small as 1 // in diameter. 

Little basic research has been devoted to Yb3+ ion itself which possesses only one excited state [151]. However, as shown previously, this ion is of special interest for a number of energy transfer and up-conversion processes, which take advantage of the strong 2F7/2 — 2F5/2 absorption band of Yb3+. In addition, efficient lasing around 1.02 can be obtained with this ion and laser cooling effects have been observed. These points are developed in the next section devoted to applications of rare-earth-doped fluoride glasses. 

Bliimel, R., Kappler, C., Quint, W. and Walther, H. (1989a). Chaos and order of laser-cooled ions in a Paul trap, Phys. Rev. A40, 808-823. 

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