Optimization of high-harmonic generation

The dependence of the high-harmonic conversion efficiency in noble gases on experimental parameters is analyzed. The results presented here supply a guideline for the optimization of HHG. The most striking finding of our analysis is the dependence of the harmonic yield on the laser pulse duration. It is generally accepted that the harmonic yield increases for shorter pulse durations. Calculations in neon show that this is only true for harmonics close to the cutoff. Away from the cutoff however, the harmonic yield increases approximately linearly with the pulse duration.     

Efficient output coupling of intracavity high-harmonic generation

We demonstrate a novel technique for coupling extreme-ultraviolet (XUV) harmonic radiation out of a femtosecond enhancement cavity. We use a small-period diffraction grating etched directly into the surface of a dielectric mirror. For the fundamental light, this element acts as a high reflector. For harmonic wavelengths, it acts as a diffraction grating, coupling XUV radiation out of the cavity. Using this method, we observed the third through twenty-first odd harmonics with a dramatic increase in usable power over previous results of high-harmonic generation at high repetition rates.     

Operation of a high-harmonic gyrofrequency multiplier

The operation of a sixth-harmonic gyrofrequency multiplier is described. The initial experimental results produced 3.5 kW of 17.23 GHz radiation from a 260 keV, 0.3 A, =v/v=4.8 axis-encircling electron beam in a near uniform magnetic field. The output cavity conversion efficiency was measured at 4.5% compared to the predicted value of 8% from a nonlinear simulation code. The design for a higher power, higher efficiency device utilizing magnetic tapering is also presented.     

Enhancing the brilliance of high-harmonic generation

Both the gas jet and the capillary setup have remarkable features for high-harmonic generation (HHG). We demonstrated both the ability of the capillary to tailor its spectral XUV output in unique ways, as well as the feasibility of the jet and its ability to produce photon energies up to the keV range. For maximizing the brilliance in both schemes we have successfully applied the concept of self-learning loops to several of our experiments. We present current results on the focusability of capillary high-harmonic radiation, a wave front measurement on gas jet high harmonics and measurements of the influence of gas pressure and laser focus position on the divergence of gas jet high harmonics. This is embedded in an overview of selected previous and current work of our group.     

Lateral shearing interferometry of high-harmonic wavefronts

We present a technique for frequency-resolved wavefront characterization of high harmonics based on lateral shearing interferometry. Tilted replicas of the driving laser pulse are produced by a Mach-Zehnder interferometer, producing separate focii in the target. The interference of the resulting harmonics on a flat-field extreme ultraviolet spectrometer yields the spatial phase derivative. A comprehensive set of spatial profiles, resolved by harmonic order, validate the technique and reveal the interplay of single-atom and macroscopic effects.     

Operation of a large-orbit high-harmonic multicavity gyroklystronamplifier

An experimental four-cavity, fifth-harmonic gyroklystron is described that separates at 11.3 GHz in a 1.2-KG magnetic field. This device used longer drift regions than previous configurations. Principal results reported include 45 dB of small-signal gain and 0.6 kW of output power. Other results include the measurement of phase noise characteristics and the identification of spurious output signals. An improved beam diagnostic which determines the pitch of the electron orbits was used together with a fluorescent uranium-glass witness plate and beam-current diagnostics to characterize the electron beam. This information was used to perform analytic modelling and computer simulation of the amplifier. The experimental results are compared with the predictions of the analytic theory and simulation, and substantial agreement is demonstrated     

Theory of combined AM and high-harmonic FM mode locked laser

Closed form solutions for a simultaneously AM and high-harmonic FM mode locked laser system is presented. Analytical expressions for the pulsewidth and pulsewidth-bandwidth products are derived in terms of the system parameters. The analysis predicts production of 17 ps duration pulses in a Nd:YAG laser mode locked with AM and FM modulators driven at 80 MHz and 1.76 GHz for 1 W modulator input power. The predicted values of the pulsewidth-bandwidth product lie between the values corresponding to the pure AM and FM mode locking values.     

All-optical measurement of high-harmonic amplitudes and phases in aligned molecules

We report a new all-optical approach to measuring the phase and amplitude of high-harmonic emission from aligned molecules. We combine the transient grating technique with a continuous rotation of the molecular alignment axis and develop an analytical model that enables the simultaneous determination of phases and amplitudes. Measurements in N(2) molecules are shown to be in qualitative agreement with the results of ab initio quantum scattering calculations.     

Nonlinear analysis of high-harmonic slotted gyro-TWT amplifier

A nonlinear self-consistent simulation code is employed to investigate the behavior of the slotted gyrotron traveling-wave amplifier (gyro-TWT), in which an axis-encircling electron beam synchronously interacts with a high-order azimuthal mode in a magnetron-type waveguide. The efficiency of a fourth-harmonic device with an ideal 60 kV, 5 A beam is shown to reach 30% for α≡ν_⊥/ν_z=2. The growth rate for the π mode is roughly 25% larger than for the 2π mode. The efficiency increases for lower voltage and the device is found to be moderately sensitive to the radial spread of the beam's guiding center position and extremely sensitive to the axial velocity spread. For an ideal 60 kV, 5 A beam with α=1.5, the efficiency of a second-harmonic gyro-TWT is 42% and falls to 10% for an eighth-harmonic device. The design of a 35 GHz, 60 kV, 5A, α=1.5, eight-vane, fourth-harmonic gyro-TWT with 7% axial velocity spread is presented. It is predicted that this design will yield a peak output power of 90 kW, a peak efficiency of 30%, and 6.3% saturated bandwidth     

Theory of high-harmonic gyrotron oscillators with slottedcross-section structure

A linear-theory analysis of gyrotron oscillators with slotted cross section is used to calculate the net change in beam energy. In this formalism, geometric factors are clearly distinguished from the geometry-independent harmonic resonance terms due to the fundamental electron cyclotron maser and peniotron interactions. This separation of the interaction terms from the geometric factors simplifiers the physical analysis, and leads to a very compact form for the net change in beam energy. The theory is applied to slotted rectangular oscillators and to slotted cylindrical oscillators to show that a unified expression can be obtained for the start-oscillation condition. In sample applications of the theory, it is demonstrated that slots lower the start-oscillation condition in both cylindrical and rectangular geometries, and can lead to a decrease in this condition as harmonic number is increased in the rectangular geometry. It is also found that the peniotron interaction, which is easily identified in this formalism, may be very strong in slotted cavities     

Role of the intramolecular phase in high-harmonic generation

We study numerically the generation of high-order harmonics by two-center molecules for arbitrary angles between the molecular axis and the laser polarization axis. For fixed angle, the harmonic spectrum exhibits a minimum at a frequency which is independent of the laser parameters. The amplitude of each harmonic is strongly angle dependent, and a pronounced minimum is found at the same angle where a sudden jump in the harmonic phase occurs. By calculating the spatial dependence of the harmonic amplitudes and phases, we are able to explain these effects in terms of interfering contributions from various regions within the molecule.     

Energy transfer equation for high-harmonic cyclotron maser and peniotron interactions

The high-harmonic cyclotron maser and peniotron interactions both appear as special cases of a general gyrotron energy transfer equation derived from the linearized Vlasov equation of motion.     

Spatial control of high-harmonic generation in hollow fibers

We demonstrate the control of high-harmonic generation in a hollow fiber by shaping the spatial structure of the generating laser pulse. We use a liquid-crystal-based two-dimensional spatial light modulator to control the spatial phase of the driver pulse. An evolutionary algorithm finds the spatial laser phase distribution that is optimal for reaching maximum total harmonic yield and for selectively enhancing the cutoff region of the spectrum. We show that enhacement of harmonic generation is related to coupling into a single fiber mode. Our results directly show that spatial properties of the laser are important parameters in fully controlling the high-harmonic spectrum. It is thus not possible to derive the controllability of the high-harmonic generation from the single-atom response only.     

Attosecond probing of vibrational dynamics with high-harmonic generation

The numerical solution of the time-dependent Schrodinger equation for vibrating hydrogen molecules in few-cycle laser pulses shows that high-harmonic generation is sensitive to the laser-induced vibrational motion. More intense harmonics are generated in heavier isotopes, the difference increasing with the harmonic frequency. Analytical theory reveals a dependence of the harmonics on the vibrational autocorrelation function. With the help of a genetic algorithm, the nuclear motion can be reconstructed from the harmonic spectra with sub-fs time resolution.     

Enhancement of high-harmonic generation by laser-induced cluster vibration

Sharp periodic enhancement of high-harmonic generation synchronized to prepulse-induced cluster vibration is observed. Tenfold enhancement of high-harmonic-generation efficiency is achieved by optimal selection of the prepulse-pump delay. This strong correlation between high-harmonic generation and cluster vibration also provides a new tool for studying the vibrational dynamics of nanometer atomic clusters.     

Controlling aspect ratio of focal spots of high numerical aperture objective lens in multi-photon absorption process

Three types of objective lens (OL), i.e., circular, annular and one-dark-ring, are presented to control the ratio, called aspect ratio (AR), of the longitudinal size and the transverse size of focal spots of micro-focus-region of high numerical aperture (NA) OL. The AR can be varied from 7.1 to 2.7 in the case of using a circular OL with NA changes from 0.7 to 1.4. By employing an annular OL, the transverse size of the central lobe of micro-focus-region decreases but its longitudinal size increases, so that the AR increases several times with respect to the case of using a circular OL. However, using a one-dark-ring OL, one can reduce either both transverse and longitudinal sizes or only the longitudinal size of focal spot, so that the AR obtained with a one-dark-ring OL is about 70% of that obtained with a circular OL. Such lenses can be useful for many applications such as sub-microfabrication and three-dimensional data storage using multi-photon absorption process.     

Control of laser high-harmonic generation with counterpropagating light

Relatively weak counterpropagating light is shown to disrupt the emission of laser high-harmonic generation. Harmonic orders ranging from the teens to the low thirties produced by a 30-femtosecond pulse in a narrow argon jet are "shut down" with a contrast as high as 2 orders of magnitude by a chirped 1-picosecond counterpropagating laser pulse (60 times less intense). Alternatively, under poor phase-matching conditions, the counterpropagating light boosts harmonic production by similar contrast through quasiphase matching where out-of-phase emission is suppressed.     

Minimum structure of high-harmonic spectrafrom aligned O2 and N2 molecules

We experimentally investigated the high-order harmonic generation (HHG) from aligned O₂ and N₂ molecules in a linearly polarized laser field, and presented the dependence of the harmonic spectrum on the driving laser intensity and molecular alignment angle. The minimum position of HHG of O₂ varies with changing the laser intensity, which is caused by multi-orbital interference. However, the location of the observed minimum structure in N₂ harmonic spectrum remained unchanged upon changing the laser intensity. The mechanism of the spectral minimum for N₂ case is regarded as a Cooper-like minimum in HHG associated with the molecular electronic structure. This work indicates that harmonic spectroscopy can effectively uncover information about molecular structure and electron dynamics.