Experimental control of excitation flow produced by delayed pulses in a ladder of molecular levels.

We study a method for controlling the flow of excitation through decaying levels in a three-level ladder excitation scheme in Na(2) molecules. Like the stimulated Raman adiabatic passage (STIRAP), this method is based on the control of the evolution of adiabatic states by a suitable delayed interaction of the molecules with two radiation fields. However, unlike STIRAP, which transfers a population between two stable levels g and f via a decaying intermediate level e through the interaction of partially overlapping pulses (usually in a Lambda linkage), here the final level f is not long lived. Therefore, the population reaching level f decays to other levels during the transfer process. Thus, rather than controlling the transfer into level f, we control the flow of the population through this level. In the present implementation a laser P couples a degenerate rovibrational level in the ground electronic state X 1Sigma(g)+, v" = 0, j" = 7 to the intermediate level A 1Sigma(u)+, v' = 10, J' = 8, which in turn is linked to the final level 5 1Sigma(g)+, v = 10, J = 9 by a laser S, from which decay occurs to vibrational levels in the electronic A and X states. As in STIRAP, the maximum excitation flow through level f is observed when the P laser precedes the S laser. We study the influence of the laser parameters and discuss the consequences of the detection geometry on the measured signals. In addition to verifying the control of the flow of population through level f we present a procedure for the quantitative determination of the fraction kappa(f) of molecules initially in the ground level which is driven through the final level f. This calibration method is applicable for any stepwise excitation.

Steering population flow in coherently driven lossy quantum ladders.

We present a detailed theory of a technique for the adiabatic control of the population flow through a preselected decaying excited level in a three-level ladder quantum system, as was experimentally demonstrated recently by Garcia-Fernandez et al. [Phys. Rev. Lett. 95, 043001 (2005)]. Specifically, we consider a three-state excitation chain of bound states, 1-2-3, of successively increasing excitation energy, in which probability loss via fluorescence occurs from states 2 and 3. We describe a laser excitation scheme that can, by adjustment of laser parameters, alter at will the relative fraction of population that, starting from state 1, is ultimately lost through states 2 and 3. We present analytical results for the conditions under which quasiadiabatic passage can take place.

Experimental demonstration of selective coherent population transfer via a continuum.

We report the first experimental demonstration of coherent population transfer, induced by stimulated Raman adiabatic passage, via continuum states. Population is transferred from the metastable state 2s(1)S(0) to the excited state 4s(1)S(0) in helium atoms in a two-photon process mediated by coherent interaction with the ionization continuum. While incoherent techniques usually do not permit any population transfer in such a process, we show that stimulated Raman adiabatic passage allows significant population transfer to take place also via ultrafast decay channels.

Control of population flow in coherently driven quantum ladders.

A technique for adiabatic control of the population flow through a preselected decaying excited level in a three-level quantum ladder is presented. The population flow through the intermediate or upper level is controlled efficiently and robustly by varying the pulse delay between a pair of partly overlapping coherent laser pulses. The technique is analyzed theoretically and demonstrated in an experiment with Na2 molecules.

Interferometric analysis of nanostructured surface profiles: correcting material-dependent phase shifts.

We present a technique to correct interferometry for the material-dependent phase shift that accompanies reflection. Such corrections are needed for nanometer accuracy of surfaces that are not of homogeneous composition. We adapt the general theory of reflection from surfaces in which there are irregular and unresolved areas of several materials to treat the specific case in which only two materials are present, as is the case for many practical applications. We show, for the approximation of a large numerical aperture that collects all reflected light, how measurements of three quantities, together with known values of the optical constants, allow determination of the material-dependent phase shift at each position on the surface. We demonstrate with numerical simulation, appropriate to measuring a surface of alumina in which optically unresolved titanium carbide granules are embedded, that our approach also succeeds, with nanometer accuracy, when the numerical aperture is small. The method is discussed for use with a miniature interferometric phase sensor, but it has application to any interferometer.