The Infrared Array Camera (IRAC) is one of three focal plane instruments in the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broad-band images at 3.6, 4.5, 5.8, and 8.0 µm. Two nearly adjacent 5.2×5.2 arcmin fields of view in the focal plane are viewed by the four channels in pairs (3.6 and 5.8 µm; 4.5 and 8 µm). All four detector arrays in the camera are 256×256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. IRAC is a powerful survey instrument because of its high sensitivity, large field of view, and four-color imaging. This paper summarizes the in-flight scientific, technical, and operational performance of IRAC.
It is often desirable in laser spectroscopy and isotope separation to extract as much as possible of an atomic or molecular population that is distributed among a number of ground-state sublevels and low-lying metastable levels.We describe a form of coherent trapping that occurs when multiple resonant laser beams are used to couple the various ground states to a common upper level. This effect prevents the extraction of the entire population. We then study the effect with two dye lasers and an atomic beam and suggest possible ways to maximize the pumping efficiency.When two cw lasers are tuned so that they couple two different ground-state sublevels to a common upper level, there is no steady-state population in the upper This effect, which holds even for intense laser fields, might be termed coherent trapping of atomic populations. It is due to optical pumping of the ground-state sublevels into a coherent superposition state that is decoupled from the laser fields. The theory of the effect has been worked out by Arimondo and Orriols' and by Whitley.2 The source of the effect is easily seen by writing the wavefunction in the interaction picture, (1) and substituting it into Schrddinger's equation. Here we have taken the atomic-energy eigenfunctions as Oi(r) and the energy of the various states as Ei = hw 1 , i = 1,2,3. The amplitudes then satisfy the equations of motionwhere we have taken the applied field to beand we have neglected the counterrotating terms. The two dipole matrix elements, which are assumed real for convenience, are denoted by gj, i = 1,2, while the detuning of the two lasers from resonance, b6 and 5,, are defined in Fig 1. Examination of Eqs. (2a) and (2c) shows that there is a constant of the motion when Ba = 6b =_ 6, i.e., when the lasers are tuned to the two-photon resonance. The effect of the constant is most easily seen if we introduce two new amplitudes, r(t) = ai(t)cos 0 -a3(t)sin 0, (4a) s(t) = a,(t)sin 0 + a 3 (t)cos 0,where the angle 0 is defined byThe equations of motion for the amplitudes of the two ground states are then expressible in terms of r(t) andwhere R is the generalized Rabi frequency(5b) (6) One linear combination of the ground states is coupled to the excited state by the applied fields, and the other linear combination is decoupled entirely. The population initially in the linear combination,remains there. If the initial states are not prepared coherently, this means that half of the population remains in the ground state and nothing is gained by the application of two lasers rather than one. We have not included spontaneous or collisional damping in the calculations, but these are included in the calculations of Refs. 1 and 2. The result of including damping is that the population initially in the linear combination of states s is optically pumped in a few lifetimes into the decoupled linear combination r. In the steady state there is no population in level 2 or in the linear combination s. All of it is in the linear combination r. The atom is decoupled from the...
Abstract. Analytic expressions are obtained for the spectrum of the light scattered when a collimated atomic beam is illuminated at right angles to its path by a cw monochromatic laser beam tuned to resonance with a two-level transition. The spectra. as would be determined by a Fabry-Perot interferometer, vary depending on the portion of the interaction region from which the scattered light emanates. The spectrum of the light from a finite sub-region of the interaction volume is described as a function of the location and length of the sub-region, and of the intensity and frequency of the laser. Several interesting features related to the turn-on of the interaction as well as to the finite observation interval are found.
We present a Heisenberg picture QED treatment of the resonance fluorescence of a two-level atom. It is shown that the equations of motion for first-order correlation functions have an asymmetry with respect to the two time arguments. This asymmetry arises in a correct and careful evaluation of the unequal-time commutator between free-field and atomic operators. General solutions of these equations are found without going to the stationary limit.
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