When an intense laser pulse is focused into a gas, the light-atom interaction that occurs as atoms are ionized results in an extremely nonlinear optical process--the generation of high harmonics of the driving laser frequency. Harmonics that extend up to orders of about 300 have been reported, some corresponding to photon energies in excess of 500 eV. Because this technique is simple to implement and generates coherent, laser-like, soft X-ray beams, it is currently being developed for applications in science and technology; these include probing the dynamics in chemical and materials systems and imaging. Here we report that by carefully tailoring the shapes of intense light pulses, we can control the interaction of light with an atom during ionization, improving the efficiency of X-ray generation by an order of magnitude. We demonstrate that it is possible to tune the spectral characteristics of the emitted radiation, and to steer the interaction between different orders of nonlinear processes.

We present spatial coherence measurements of extreme ultraviolet (EUV) light generated through the process of high-harmonic up-conversion of a femtosecond laser. With a phase-matched hollow-fiber geometry, the generated beam was found to exhibit essentially full spatial coherence. The coherence of this laser-like EUV source was shown by recording Gabor holograms of small objects. This work demonstrates the capability to perform EUV holography with a tabletop experimental setup. Such an EUV source, with low divergence and high spatial coherence, can be used for experiments involving high-precision metrology, inspection of optical components for EUV lithography, and microscopy and holography with nanometer resolution. Furthermore, the short time duration of the EUV radiation (a few femtoseconds) will enable EUV microscopy and holography to be performed with ultrahigh time resolution.

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We demonstrate experimentally how the time-dependent phase modulation induced by molecular rotational wave packets can manipulate the phase and spectral content of ultrashort light pulses. Using impulsively excited rotational wave packets in CO2, we increase the bandwidth of a probe pulse by a factor of 9, while inducing a negative chirp. This chirp is removed by propagation through a fused silica window, without the use of a pulse compressor. This is a very general technique for optical phase modulation that can be applied over a broad spectral region from the IR to the UV.

Line imaging of fluorescent and absorptive objects with a single-pixel imaging technique that acquires one-dimensional cross-sections through a sample by imposing a spatially-varying amplitude modulation on the probing beam is demonstrated. The fluorophore concentration or absorber distribution of the sample is directly mapped to modulation frequency components of the spatially-integrated temporal signal. Time-domain signals are obtained from a single photodiode, with object spatial frequency correlation encoded in time-domain bursts in the electronic signal from the photodiode.

The testing of aeroelastically and aerothermoelastically scaled wind-tunnel models in hypersonic flow is not feasible; thus, computational aeroelasticity and aerothermoelasticity are essential to the development of hypersonic vehicles. Several fundamental issues in this area are examined by performing a systematic computational study of the hypersonic aeroelastic and aerothermoelastic behavior of a three-dimensional configuration. Specifically, the flutter boundary of a low-aspect-ratio wing, representative of a fin or control surface on a hypersonic vehicle, is studied over a range of altitudes using third-order piston theory and Euler and Navier-Stokes aerodynamics. The sensitivity of the computational-fluid-dynamics-based aeroelastic analysis to grid resolution and parameters governing temporal accuracy are considered. In general, good agreement at moderate-to-high altitudes was observed for the three aerodynamic models. However, the wing flutters at unrealistic Mach numbers in the absence of aerodynamic heating. Therefore, because aerodynamic heating is an inherent feature of hypersonic flight and the aeroelastic behavior of a vehicle is sensitive to structural variations caused by heating, an aerothermoelastic methodology is developed that incorporates the heat transfer between the fluid and structure based on computational-fluid-dynamics-generated aerodynamic heating. The aerothermoelastic solution procedure is then applied to the low-aspect-ratio wing operating on a representative hypersonic trajectory. In the latter study, the sensitivity of the flutter margin to perturbations in trajectory angle of attack and Mach number is considered. Significant reductions in the flutter boundary of the heated wing are observed. The wing is also found to be susceptible to thermal buckling. Nomenclature a 1 = speed of sound C L , C M , C D = coefficients of lift and moment about the elastic axis and drag C p = coefficient of pressure CFL = Courant-Friedrichs-Lewy three-dimensional input parameter regulating pseudo-time-step size C w = Chapman-Rubesin coefficient c = reference chord length of the double-wedge airfoil c pw = specific heat of the wall h ht = heat-transfer coefficient k ! = reduced frequency M = freestream Mach number M, K = generalized mass and stiffness matrices of the structure M f = flutter Mach number n = normal vector n m= number of modes p = pressure p 1 = freestream pressure Q = generalized force vector for the structure= heat-transfer rate due to aerodynamic heating, radiation, conduction, and stored energy q i = modal amplitude of mode i q vf = virtual-flutter dynamic pressure q 1 = dynamic pressure Re = Reynolds number S = surface area of the structure T = temperature T AW = adiabatic-wall temperature T E = kinetic energy of the structure T R = radiation equilibrium wall temperature= potential energy of the structure V = freestream velocity v n = normal velocity of airfoil surfaces w = displacement of the surface of the structure x, y, z = spatial coordinates y = law-of-the-wall coordinate Zx; y;...

An evolutionary learning algorithm in conjunction with an ultrafast optical pulse shaper was used to control vibrational motion in molecular gases at room temperature and high pressures. We demonstrate mode suppression and enhancement in sulfur hexa¯uoride and mode selective excitation in carbon dioxide. Analysis of optimized pulses discovered by the algorithm has allowed for an understanding of the control mechanism. Ó 2001 Elsevier Science B.V. All rights reserved.Controlling atoms and molecules with coherent optical ®elds has been a longstanding goal in chemical physics [1]. Recently, several experiments in this ®eld have demonstrated successful control of molecular ionization and dissociation [2,3], atomic and molecular¯uorescence [4,5], the shape of electronic and molecular wave packets [6±9], and currents in semiconductors [10,11]. Many of these experiments have employed learning control loops and programmable optical pulse shapers to discover optimal pulse shapes for control [12]. There has been increasing interest in not only achieving control in quantum systems, but also in understanding the control mechanism, which has proven to be a challenging task. Here we demonstrate two experimental advances in coherent control of quantum systems. First, we demonstrate selective control over molecular motion in gases at STP using very short, shaped excitation pulses. Coherent vibrational excitation corresponding to thermal temperatures over 2000 K is achieved. This is important in order to reach the goal of laser selective chemistry [13] on a macroscopic scale, because it extends cryogenic and molecular beam experiments to high temperatures and densities. Second, using a special cost function [14] incorporated into an evolutionary learning algorithm [15], we obtain a clear interpretation of the control mechanism, which is based on impulsive stimulated Raman scattering [16]. This serves as a demonstration of a general technique for systematically gaining insight from solutions found by learning algorithms. Furthermore, as there is no molecular resonance exploited in this excitation scheme, and the

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