Showing papers on "High harmonic generation published in 2004"

Nonlinear optics in the extreme ultraviolet

Abstract: Nonlinear responses to an optical field are universal in nature but have been difficult to observe in the extreme ultraviolet (XUV) and soft X-ray regions owing to a lack of coherent intense light sources. High harmonic generation is a well-known nonlinear optical phenomenon and is now drawing much attention in attosecond pulse generation. For the application of high harmonics to nonlinear optics in the XUV and soft X-ray regime, optical pulses should have both large pulse energy and short pulse duration to achieve a high optical electric field. Here we show the generation of intense isolated pulses from a single harmonic (photon energy 27.9 eV) by using a sub-10-femtosecond blue laser pulse, producing a large dipole moment at the relatively low (ninth) harmonic order nonadiabatically. The XUV pulses with pulse durations of 950 attoseconds and 1.3 femtoseconds were characterized by an autocorrelation technique, based on two-photon above-threshold ionization of helium atoms. Because of the small cross-section for above-threshold ionization, such an autocorrelation measurement of XUV pulses with photon energy larger than the ionization energy of helium has not hitherto been demonstrated. The technique can be extended to the characterization of higher harmonics at shorter wavelengths.

Single attosecond pulse and xuv supercontinuum in the high-order harmonic plateau

Abstract: It is proposed that single attosecond pulses be generated by gating high-order harmonic emission with fields whose ellipticity varies rapidly with time. The laser pulse with a time-dependent ellipticity is created from two 5-fs laser pulses centered at 750 nm and separated by 5 fs. One of the laser pulses is left-circularly polarized and the other is right-circularly polarized. Numerical simulations show that when neon atoms are driven by such laser pulses, the generated high-order harmonic spectrum in the 25th to the 85th orders are a supercontinuum that corresponds to single attosecond pulses. A simple analytical expression is derived for estimating the high-harmonic radiation time.

Strong field quantum path control using attosecond pulse trains.

Abstract: We show that attosecond pulse trains have a natural application in the control of strong field processes. In combination with an intense infrared laser field, the pulse train can be used to microscopically select a single quantum path contribution to a process that would otherwise consist of several interfering components. We present calculations that demonstrate this by manipulating the time-frequency properties of high order harmonics at the single atom level. This quantum path selection can also be used to define a high resolution attosecond clock. It has recently been experimentally demonstrated that a train of pulses as short as a few hundred attoseconds is produced when several odd harmonics of an intense infrared (IR) laser field are phase locked [1]. The periodicity of the resulting attosecond pulse train (APT) is half the fundamental IR cycle [2,3]. In this Letter we show that this periodicity makes the APT a natural tool for controlling strong field processes driven by the IR laser. The efficacy of this control mechanism can best be appreciated in the framework of the successful semiclassical description of intense laser-matter interactions [4]. In this picture, the amplitude for any strong field process can be expressed as a coherent sum over only a few quantum orbits [5].These space-time trajectories follow a sequence of release into the continuum (ionization), acceleration in the IR field, and return to the ion core, where the electron can either rescatter or recombine.When an APTis used in combination with an IR laser, the short duration of the attosecond pulses fixes the ionization to a particular point in each IR half cycle and allows us to select which quantum paths are available for the electron to follow. In this Letter we demonstrate the control of high harmonic generation via an APT. The quantum orbits contributing to each harmonic are characterized by their time of release into the continuum and their kinetic energy upon return to the ion core. The two most important orbits are those that have travel times ! 1 and ! 2 less than one optical cycle. Each orbit contributes to the dipole moment with a phase ! j !" " j# ~ "$# Up=!$ , where " j# ~

In situ diagnostics of the crystalline nature of single organic nanocrystals by nonlinear microscopy.

Abstract: We elucidate the crystalline nature and the three-dimensional orientation of isolated organic nanocrystals embedded in a sol-gel matrix, using a polarized nonlinear microscopy technique that combines two-photon fluorescence and second harmonic generation. This technique allows the distinction between monocrystalline structures and nanoscale polycrystalline aggregates responsible for incoherent second harmonic signals.

Second harmonic generation and optical parametric amplification in the mid-IR with orthorhombic biaxial crystals LiGaS2 and LiGaSe2

Abstract: By second harmonic generation in and out of the principal planes we estimated the three nonlinear coefficients of the Li-containing mid-IR crystals LiGaS2 and LiGaSe2 which are isostructural to LiInS2 and LiInSe2 and possess even larger bandgaps First results on optical parametric amplification reveal their advantages for direct down conversion to the mid-IR

Optical second harmonic generation properties of BiB3O6.

Abstract: We present studies of the optical properties of the new nonlinear material BiB3O6 for second harmonic generation from the visible to infrared. We have determined the phase-matching conditions and effective nonlinear coefficients in the three principal optical planes, acceptance bandwidths, spatial and temporal walkoff, group velocity dispersion and double phase-matching behaviour. We also report on experimental studies in this material, where efficient, high-average-power second harmonic generation of femtosecond pulses into the blue is demonstrated. Using 130-fs fundamental pulses at 76 MHz, single-pass second harmonic average powers as much as 830 mW at greater than 50% conversion efficiency have been generated over a tunable range of 375–435 nm. Using cross-correlation measurements in a 100-µm β-BaB2O4 crystal second harmonic pulse durations of 220 fs are obtained. Our theoretical findings are verified by experimental data, where excellent agreement between the calculations and measurements is obtained. Direct comparison of BiB3O6 with β-BaB2O4 also confirms improved performance of this new material for second harmonic generation of femtosecond pulses.

Attosecond localization of electrons in molecules

Abstract: Numerical solutions of the time-dependent Schrodinger equation for a 1D model non-Born–Oppenheimer H are used to illustrate the nonlinear nonperturbative response of molecules to intense (I ≥ 1013 W/cm2), ultrashort (t < 10 fs) laser pulses. Molecular high-order harmonic generation (MHOHG) is shown to be an example of such response and the resulting nonlinear photon emission spectrum is shown to lead to the synthesis of single attosecond (10−18 s) pulses. Application of such ultrashort pulses to the H system results in localized electron wavepackets whose motion can be detected by asymmetry in the photoelectron spectrum generated by a subsequent probe attosecond pulse, thus leading to measurement of electron motion in molecules on the attosecond time scale. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Self-stabilization of carrier-envelope offset phase by use of difference-frequency generation.

Abstract: Self-stabilized carrier-envelope offset phase is achieved by use of difference-frequency (DF) generation. The spectrum from a Ti:sapphire oscillator is broadened in a photonic crystal fiber, and a DF (900 nm) between the blue component (490 nm) and the infrared component (1080 nm) is generated. The beat signal between the fundamental and the DF signal is clearly observed. The wavelength of the DF signal can be tuned down to 780 nm, and hence the signal can be used for injection seeding of a Ti:sapphire oscillator.

Excited states dynamics in time-dependent density functional theory: High-field molecular dissociation and harmonic generation

Abstract: We present numerical simulations of femtosecond laser induced dynamics of some selected simple molecules — hydrogen, singly ionized sodium dimer, singly ionized helium trimer and lithium cyanide. The simulations were performed within a real-space, real-time, implementation of time-dependent density functional theory (TDDFT). High harmonic generation, Coulomb explosion and laser induced photo- dissociation are observed. The scheme also describes non-adiabatic effects, such as the appearance of even harmonics for homopolar but isotopically asymmetric dimers, even if the ions are treated classically. This TDDFT-based method is reliable, scalable, and extensible to other phenomena such as photoisomerization, molecular transport and chemical reactivity. PACS. 33.80.Gj Diffuse spectra; predissociation, photodissociation - 33.80.Wz Other multiphoton processes

High-order harmonic generation up to 250 eV from highly ionized argon.

Abstract: We demonstrate the generation of very high-order harmonics, up to 250 eV, using argon gas. This extends by 100 eV the highest harmonics previously observed using Ar and exceeds the energies observed using any other medium besides helium. This advance is made possible by using a waveguide geometry to limit plasma-induced laser beam defocusing, making it possible to generate high harmonics from Ar ions. This work shows that high harmonic emission from ions can extend laser-based coherent up-conversion into the soft x-ray region of the spectrum.

Single sub − 50 − attosecond pulse generation from chirp-compensated harmonic radiation using material dispersion

Abstract: A method for obtaining a single sub-50-attosecond pulse using harmonic radiation is proposed. For the generation of broad harmonic radiation during a single half-optical cycle, atoms are driven by a femtosecond laser pulse with intensity above the saturation intensity for optical field ionization and hence experience a large nonadiabatic increase of the laser electric field between optical cycles. Although the chirped structure of the harmonic radiation imposes a limit on the minimum achievable pulse duration, we demonstrate that its positive chirp can be compensated by the negative group delay dispersion of an appropriately selected x-ray filter material, used also for the spectral selection, resulting in a single attosecond pulse with a duration less than 50 as.

Nonadiabatic quantum path analysis of high-order harmonic generation: Role of the carrier-envelope phase on short and long paths

Abstract: High-order harmonic generation process in the few- and multiple-optical-cycle regime is theoretically investigated, using the saddle-point method generalized to account for nonadiabatic effects The influence of the carrier-envelope phase of the driving pulses on the various electron quantum paths is analyzed We demonstrate that the short and long quantum paths are influenced in different ways by the carrier-envelope phase In particular, we show that clear phase effects are visible on the long quantum paths even in the multiple-optical-cycle regime, while the short quantum paths are significantly influenced by the carrier-envelope phase only in the few-optical-cycle regime

Narrow-linewidth megahertz-rate pulse-burst laser for high-speed flow diagnostics

Abstract: A second-generation pulse-burst laser system for high-speed flow diagnostics is described in detail. The laser can produce a burst of high-energy pulses (of the order of hundreds of millijoules per pulse) with individual pulse durations of less than 10 ns and pulse separations as short as 1 micros. A key improvement is the addition of a phase-conjugate mirror, which effectively isolates the high-intensity, short-duration pulses from the low-intensity, long-duration background illumination. It allows for more-efficient amplification and harmonic generation, with efficiencies exceeding 50% for second-harmonic and 40% for third-harmonic generation. Characteristics of the laser system, including gain narrowing, pulse-burst energy distribution, pulse narrowing, and overall pulse-burst energy, are described. In addition, the applicability of the laser for spectroscopic-based flow diagnostics is demonstrated through the presentation of megahertz-rate planar Doppler velocimetry results.

Focusing coherent soft-x-ray radiation to a micrometer spot size with an intensity of 10(14) W/cm2.

Abstract: We investigate the focusability of intense coherent soft-x-ray radiation generated by high-order harmonic conversion. The 27th-harmonic wave at 29.6 nm is focused by an off-axis parabolic mirror with a SiC/Mg multilayer coating. Focal-spot images are observed from the visible fluorescence induced by the soft-x-ray photons on a Ce:YAG scintillator. We demonstrate focusing of the soft-x-ray beam to a 1-microm spot size with a peak intensity of 1 x 10(14) W/cm2, which is to our knowledge the highest ever reported in the soft-x-ray region.

Nonlinear optical lithography with ultra-high sub-Rayleigh resolution.

Abstract: A nonlinear optical, interferometric method for improving the resolution of a lithographic system by an arbitrarily large factor with high visibility is described. The technique is implemented experimentally for both two-fold and three-fold enhancement of the resolution with respect to the traditional Rayleigh limit. In these experiments, an N-photon-absorption recording medium is simulated by Nth harmonic generation followed by a CCD camera. This technique does not exploit quantum features of light; this fact suggests that the improved resolution achieved through use of “quantum lithography” results primarily from the nonlinear response of the recording medium and not from quantum features of the light field.

Generation of vacuum-ultraviolet light below 160 nm in a KBBF crystal by the fifth harmonic of a single-mode Ti:sapphire laser

Abstract: We have generated a vacuum-ultraviolet light below 160 nm by sum-frequency mixing in an optically contacted, prism-coupled KBe2BO3F2 crystal The vacuum-ultraviolet light was generated as the fifth harmonic of a tunable, single-mode, 1-kHz Ti:sapphire laser system The wavelength of 156 nm, to our knowledge, is the shortest ever obtained by use of nonlinear crystals in a phase-matched process In addition, we demonstrated a 1576-nm light source as an inspection tool for F2 laser lithography with an average power of 08 μW

Measurement and control of the frequency chirp rate of high-order harmonic pulses

Abstract: We measure the chirp rate of harmonics 13 to 23 in argon by cross correlation with a 12 femtosecond probe pulse. Under low ionization conditions, we directly measure the negative chirp due to the atomic dipole phase, and show that an additional chirp on the pump pulse is transferred to the qth harmonic as q times the fundamental chirp. Our results are in accord with simulations using the experimentally measured 815 nm pump and probe pulses. The ability to measure and manipulate the harmonic chirp rate is essential for the characterization and optimization of attosecond pulse trains.

Role of orbital symmetry in high-order harmonic generation from aligned molecules

Abstract: High-order harmonic generation has been explored theoretically and experimentally in ${\text{CO}}_{2}$, a linear molecule characterized by doubly antisymmetric highest-occupied molecular orbitals. For the first time to our knowledge, the high-harmonic yield is studied as a function of the angle between the molecular axis and the polarization direction. A minimum yield is found at $0\ifmmode^\circ\else\textdegree\fi{}$ for all harmonics, which constitutes evidence for destructively interfering terms in the ionization probability amplitude playing an important role in strong-field phenomena in molecules. The maximum yield is always found at intermediate angles, but the detailed behavior is harmonic specific. Numerical simulations reproduce the main features observed in the experiment.

Generation of strong optical field in soft X-ray region by using high-order harmonics

Abstract: For further development of a variety of applications, one of the most important issues is the improvement of output harmonic energy and conversion efficiency We review our work on the energy scaling of high-order harmonic generation based on phase-matching using a loosely focused beam Our harmonic energy scaling method can be universally applied to harmonic generation in the neutral rare gas target In addition, we demonstrate a new concept for spatial separation between the high-energy pump and harmonic beam to increase the available soft X-ray energy for the applications This method is very simple and highly useful for not only high-order harmonic generation but also longitudinally pumped X-ray lasers By combining the high-energy high harmonic source and new spatial separation method, we successfully demonstrate focusing of the soft X-ray beam with a peak intensity of 1/spl times/10/sup 14/ W/cm/sup 2/, which is to our knowledge the highest ever reported in the soft X-ray region

Enhancement of high-order harmonic generation at tuned wavelengths through adaptive control.

Abstract: An adaptive learning loop enhances the efficiency and tuning of high-order harmonic generation. In comparison with simple chirp tuning, we observe a broader tuning range and a twofold to threefold enhancement in integrated photon flux in the cutoff region. The driving pulse temporal phase varies significantly for different tunings and is more complicated than a simple chirp. We compare our experimental results with a one-dimensional, time-dependent model that incorporates the intrinsic atomic response, the experimental pulse temporal phase, ionization effects, and transverse coherence of the spatial mode of the laser. The model agrees with our experimental results and indicates that a specific quantum path coupled with ionization effects determines the optimized harmonic spectrum.

Interferometric second harmonic generation microscopy.

Abstract: We describe a novel second harmonic generation (SHG) microscope that employs heterodyne detection by interfering the epidirected SHG from a sample with SHG from a reference crystal. In addition, the microscope provides complementary reflectance information based on optical coherence microscopy (OCM). The instrument features dual balanced detection to minimize the effect of source fluctuations, and polarization-sensitive detection to measure the nonlinear susceptibility of the sample. Interferometric detection can potentially improve the sensitivity and thus extend the imaging depth as compared with direct detection of SHG.

High harmonic generation in a semi-infinite gas cell.

Abstract: Ten-millijoule 35-femtosecond laser pulses interact with a cell of helium or neon that extends from a focusing lens to an exit foil near the laser focus. High harmonic orders in the range of 50 to 100 are investigated as a function of focal position relative to the exit foil. An aperture placed in front of the focusing lens increases the brightness of observed harmonics by more than an order of magnitude. Counter-propagating light is used to directly probe where the high harmonics are generated within the laser focus. In neon, the harmonics are generated in the last few millimeters before the exit foil, limited by absorption. In helium, the harmonics are produced over a much longer distance.

Observation of Carrier-Envelope Phase Phenomena in the Multi-Optical-Cycle Regime

Abstract: So far the role of the carrier-envelope phase of a light pulse has been clearly experimentally evidenced only in the sub-6-fs temporal regime Here we show, both experimentally and theoretically, the influence of the carrier-envelope phase of a multi-optical-cycle light pulse on high-order harmonic generation For the first time, we demonstrate that the short and long electron quantum paths contributing to harmonic generation are influenced in a different way by the pulse carrier-envelope phase

Influence of large-scale installation of energy saving lamps on the line Voltage distortion of a weak network supplied by photovoltaic station

Abstract: The aim of this research is to calculate and analyze the harmonic distortion extent caused in weak low-voltage networks due to a possible large-scale installation of the widely spread compact fluorescent lamps (CFLs). The research uses data from laboratory measurements of the produced harmonic currents by CFLs, as well as field measurements of the harmonic content and other characteristic measurements of the electric network of a small Greek island, which is supplied by an autonomous photovoltaic (PV) station. The PV station is also a harmonic generator by itself. This fact combined with other highly nonlinear loads-such as CFLs-causes serious problems at the network power quality as undesirable harmonic components are induced. The whole network is simulated and three scenarios of CFL installation extent are considered, so that the harmonic flow analysis reveals the influence in the power quality. It is examined whether this weak low-voltage electric network is able to keep the total harmonic distortion factor at all nodes under 8%, as more and more CFLs are installed. The conclusions drawn from this analysis are important and must be taken into account in every electric network design.

Observation of two-photon above-threshold ionization of rare gases by XUV harmonic photons

Abstract: We have observed two-photon above-threshold ionization in Ar, Xe and He by the fifth harmonic (25 eV photon energy) of a KrF laser.

Optimization of high-order harmonic generation by adaptive control of a sub-10-fs pulse wave front.

Abstract: We present a method for the optimization of high-order harmonic generation based on wave-front correction of the driving laser beam. The technique exploits wave-front adaptive control by means of a deformable mirror, governed by an optimization procedure.