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 report on a novel approach to the realization of nematic liquid-crystal (LC) phase correctors to form spherical and cylindrical wave fronts. A LC cell with a distributed reactive electrical impedance was driven by an ac voltage applied to the cell boundary to yield the desired spatial distribution of the refractive index. The two-dimensional function of the phase delay introduced into the light beam depends on the frequency of the ac control voltage, the geometry of the boundary electrode surrounding the LC cell, and the electrical parameters of the cell. We realized a cylindrical adaptive lens with a clear aperture of 15 mm 3 4 mm and a spherical adaptive lens with circular aperture of 6.5 mm. Both devices are capable of focusing collimated light in the range`. . . 0.5 m. © 1998 Optical Society of America OCIS codes: 010.1080, 160.3710, 220.3620.Liquid-crystal (LC) phase modulators have a great potential for use as adaptive optics 1 because of their light-transmitting operation, simple control, reliability, low power consumption, and low control voltage. In many applications the correction of low-order aberrations such as of defocus and astigmatism is of primary importance. The adaptive lenses described in Refs. 2 -4 and references therein are controlled by arrays of individual electrodes, approximating the wave front by step functions by means of the zonal correction principle. Good approximation to a continuous wavefront prof ile in these modulators can be achieved only with a large number of discrete control electrodes.We suggest a novel approach to forming a smooth continuous distribution of the refractive index in a nematic LC layer over the entire aperture of LC cell, realizing a modal control principle. 5The spatial modulation of the wave front is def ined by the geometry of the contacts located at the periphery of the modulator aperture, by the frequency spectrum of the control voltage, and by the electric characteristics of the LC cell. Ultimately we need a single circular contact to control a spherical adaptive lens and two linear equidistant contacts for a cylindrical lens.Let us consider the LC cell configuration shown in Fig. 1. The LC layer is sandwiched between two transparent plate electrodes deposited upon glass substrates. The distributed resistance of the control electrode is much greater than that of the ground electrode. A control voltage is applied to the contacts deposited at the periphery of the highly resistive electrode. The initial LC layer alignment is determined by the alignment coating, and its thickness is set by dielectric spacers.When an ac control voltage is applied to the peripheral contacts (Fig. 1) the active impedance of the highresistance control electrode and the reactive impedance of the capacitor formed by the LC layer sandwiched between the control and ground electrodes form a distributed voltage divider, resulting in a nearly parabolic distribution of the ac voltage over the LC layer. A simplified equivalent circuit corresponding to the situation described is shown...

An electrostatically deformable, gold-coated, silicon nitride membrane mirror was used as a phase modulator to compress pulses from 92 to 15 fs. Both an iterative genetic algorithm and single-step dispersion compensation based on frequency-resolved optical gating calibration of the mirror were used to compress pulses to within 10% of the transform limit. Frequency-resolved optical gating was used to characterize the pulses and to test the range of the deformable-mirror-based compressor.

Off-axis aberrations in a beam-scanning multiphoton confocal microscope are corrected with a deformable mirror. The optimal mirror shape for each pixel is determined by a genetic learning algorithm, in which the second-harmonic or two-photon fluorescence signal from a reference sample is maximized. The speed of the convergence is improved by use of a Zernike polynomial basis for the deformable mirror shape. This adaptive optical correction scheme is implemented in an all-reflective system by use of extremely short (10-fs) optical pulses, and it is shown that the scanning area of an f:1 off-axis parabola can be increased by nine times with this technique.

An electrostatically controlled flexible mirror has been fabricated on a silicon chip by means of bulk micromachining. The mirror has a 10.5 mm × 10.5 mm square aperture and consists of a 0.5-µm-thick tensile-stressed silicon-nitride diaphragm coated with a 0.2-µm-thick reflective aluminum layer. The reflecting surface is initially plane with a mean-square deviation of ~λ/8 for λ = 633 nm. The shape of the reflecting surface is controlled electrostatically by an array of integrated actuators. Good initial optical quality and the possibility of electrostatic control of the reflecting surface make the on-chip mirror useful for various electro-optical applications.

We demonstrate the use of a deformable-mirror pulse shaper, combined with an evolutionary optimization algorithm, to correct high-order residual phase aberrations in a 1-mJ, 1-kHz, 15-fs laser amplif ier. Frequencyresolved optical gating measurements reveal that the output pulse duration of 15.2 fs is within our measurement error of the theoretical transform limit. This technique signif icantly reduces the pulse duration and the temporal prepulse energy of the pulse while increasing the peak intensity by 26%. It is demonstrated, for what is believed to be the f irst time, that the problem of pedestals in laser amplif iers can be addressed by spectral-domain correction.  2000 Optical Society of America OCIS codes: 140.7090, 140.3590, 320.5540, 320.5520.The past five years have seen considerable improvements in the capabilities of high-power ultrafast lasers. Ti : sapphire-based oscillator-amplifier systems can generate peak powers of ϳ100 TW at 10 Hz, or 0.3-1 TW at kilohertz repetition rates.

Wave-front correction and focal spot improvement of femtosecond laser beams have been achieved, for the first time to our knowledge, with a deformable mirror with an on-line single-shot three-wave lateral shearing interferometer diagnostic. Wave-front distortions of a 100-fs laser that are due to third-order nonlinear effects have been compensated for. This technique, which permits correction in a straightforward process that requires no feedback loop, is also used on a 10-TW Ti:sapphire-Nd:phosphate glass laser in the subpicosecond regime. We also demonstrate that having a focal spot close to the diffraction limit does not constitute a good criterion for the quality of the laser in terms of peak intensity.

We have optimized the design and imaging procedures, to clearly resolve the malaria parasite in Giemsa-stained thin blood smears, using simple low-cost cellphone-based microscopy with oil immersion. The microscope uses a glass ball as the objective and the phone camera as the tube lens. Our optimization includes the optimal choice of the ball lens diameter, the size and the position of the aperture diaphragm, and proper application of immersion, to achieve diagnostic capacity in a wide field of view. The resulting system is potentially applicable to low-cost in-the-field optical diagnostics of malaria as it clearly resolves micron-sized features and allows for analysis of parasite morphology in the field of 50 × 50 μm, and parasite detection in the field of at least 150 × 150 μm.