Search

Extreme ultraviolet (EUV) fluorescence emitted from Ar clusters irradiated by intense EUV free electron laser (FEL) pulses has been investigated. The EUV fluorescence spectra display rich structure at wavelengths shorter than the incident FEL wavelength of 51 nm. The results suggest that multiply-charged ions are produced following the ion-electron recombination processes which occur in the nanoplasma created by multi-photon excitation during the intense EUV-FEL pulses.

We theoretically investigated sequential double ionization (SDI) of Ar by the nearly circularly polarized laser pulses with a fully correlated classical ensemble model. The ion momentum distributions of our numerical results at various laser intensities and pulse durations agree well with the experimental results. The experimentally observed multi-electron effects embodied in the joint momentum spectrum of the two electrons is also reproduced by our correlated classical calculations. Interestingly, our calculations show that the angular distribution of the first photoelectron from the trajectories which eventually suffer SDI differs from the distribution of the photoelectrons from above-threshold ionization trajectories. This observation provides additional evidence of multi-electron effects in strong field SDI.

We present systematic investigations of harmonic generation in Argon, driven in the vicinity of a five-photon resonance by intense, tunable picosecond radiation pulses. When properly matching the laser frequency with the Stark-shifted multi-photon resonance, we observe a pronounced enhancement not only of the 5th, but also the 7th and 9th harmonic of the driving laser (i.e. at orders higher than the involved multi-photon resonance). We study the harmonic yield at different intensities and wavelengths of the driving laser to determine optimal conditions for resonantly-enhanced harmonic generation.

We numerically investigated the femtosecond laser filamentation in a cell filled with argon of non-uniform density distribution. By comparison with the case of uniform density distribution, we demonstrated the crucial differences in the dynamics between the two cases. We found that the pulse-splitting appeared earlier due to the sensitivity of rear-part refocusing to the plasma density and a double Λ shape appeared in the spatio-temporal intensity profile in the non-uniform density case., (© 2012 Optical Society of America)

We observed that increasing the clusters size and laser pulse contrast can enhance the X-ray flux emitted by femtosecond-laser-driven-cluster plasma. By focusing a high contrast laser (10(-10)) on large argon clusters, high flux Kα-like X-rays (around 2.96 keV) is generated with a total flux of 2.5 × 10(11) photons/J in 4π and a conversion efficiency of 1.2 × 10-4. In the case of large Kr clusters, the best total flux for L-shell X-rays is 5.3 × 1011 photons/J with a conversion efficiency of 1.3 × 10-4 and, for the Kα X-ray (12.7 keV), it is 8 × 10(8) photons/J with a conversion efficiency of 1.6 × 10-6. Using this X-ray source, a single-shot high-performance X-ray imaging is demonstrated.

We report on the fabrication of planar waveguide in Nd:YVO(4) crystal by using swift Ar(8+) ion irradiation. At room temperature continuous wave (cw) laser oscillation at wavelength of ~1067 nm has been realized through the optical pump at 808 nm with a low threshold of 9.3 mW. The slope efficiency of the waveguide laser system is of 8.5%. The optical-to-optical conversion efficiency is 6.6%., (© 2011 Optical Society of America)

The Boltzmann constant can be optically determined by measuring the Doppler width of an absorption line of molecules at gas phase. We propose to apply a near infrared cavity ring-down (CRD) spectrometer for this purpose. The superior sensitivity of CRD spectroscopy and the good performance of the near-ir lasers can provide ppm (part-per-million) accuracy which will be competitive to present most accurate result obtained from the speed of sound in argon measurement. The possible influence to the uncertainty of the determined Doppler width from different causes are investigated, which includes the signal-to-noise level, laser frequency stability, detecting nonlinearity, and pressure broadening effect. The analysis shows that the CRD spectroscopy has some remarkable advantages over the direct absorption method proposed before. The design of the experimental setup is presented and the measurement of C2H2 line near 0.8 μm at room temperature has been carried out as a test of the instrument.

In this article, the characteristics of GaN-based LEDs grown on Ar-implanted GaN templates to form inverted Al0.27Ga0.83N pyramidal shells beneath an active layer were investigated. GaN-based epitaxial layers grown on the selective Ar-implanted regions had lower growth rates compared with those grown on the implantation-free regions. This resulted in selective growth, and formation of V-shaped concaves in the epitaxial layers. Accordingly, the inverted Al0.27Ga0.83N pyramidal shells were formed after the Al0.27Ga0.83N and GaN layers were subsequently grown on the V-shaped concaves. The experimental results indicate that the light-output power of LEDs with inverted AlGaN pyramidal shells was higher than those of conventional LEDs. With a 20 mA current injection, the output power was enhanced by 10% when the LEDs were embedded with inverted Al0.27Ga0.83N pyramidal shells. The enhancement in output power was primarily due to the light scattering at the Al0.27Ga0.83N/GaN interface, which leads to a higher escape probability for the photons, that is, light-extraction efficiency. Based on the ray tracing simulation, the output power of LEDs grown on Ar-implanted GaN templates can be enhanced by over 20% compared with the LEDs without the embedded AlGaN pyramidal shells, if the AlGaN layers were replaced by Al0.5Ga0.5N layers.

Self-compression and spectral supercontinuum (SC) generated by filamentation of femtosecond laser pulses with duration from 45 fs down to 6 fs in argon gas have been numerically investigated. A 45-fs pulse can be self-compressed into a few-cycle pulse with duration of 12 fs at the post-filamentation region. By properly employing a high-pass filter to select the broadening high-frequency spectra which are almost in phase, the pulse can be further shortened to about 7 fs. By contrast, a 6-fs pulse cannot be further self-compressed into a shorter pulse by filamentation although it can generate much broader SC extending from 200 nm to 1300 nm. It is also found that a separate and strong SC in the ultraviolet (UV) region extending from 220 nm to 300 nm and peaked at about 255 nm can be generated at proper propagation distances, which corresponds to a pulse with duration of about 5 fs.

In a holographic recording is advisable that the diffraction efficiency increases linearly with the exposure in a wide zone of the curve of diffraction efficiency versus energetic exposure and the slope of the curve must be approximately constant before saturation in order to improve the energetic sensitivity and to get reproducibility in different recordings with the same kind of photopolymer, although to find examples of deviations to this behavior it is usual. The more important deviation experimentally observed in photopolymers with high thickness happen when the first maximum in the curve is lower than the second one. This effect is opposed to an overmodulation. We present a main hypothesis related to the dye concentration into the layer and with the molecular weight of the polymer chains generated in the polymerization process in order to explain this effect.

The characteristics of THz pulse radiation from argon clusters irradiated with intense femtosecond laser pulses have been investigated. The average power of the THz pulses radiated from argon clusters is approximately 40 times greater than that from argon gas. The enhancement of the THz radiation originates from large laser absorption by the clusters. The polarization state and the angular distribution of the power of the THz pulses show no dependence on laser polarization, which suggests that the radiation originates from laser-induced charge separation along the laser propagation direction.

We investigate both experimentally and theoretically the mechanisms driving the co-filamentation of two ultrashort laser pulses at 800 and 400 nm in Argon. The cross-Kerr lens and cross-phase modulation between the two filaments of different colors bridging both the continuum spectra and the plasma channels induced by the individual pulses. This dual-color filamentation also results in the simultaneous generation of two few-cycle pulses at both 800 and 400 nm, providing a potential way to generate attosecond pulses.

We review recent experimental and theoretical work on the use of counterpropagating light to enhance high-order harmonic generation through all-optical quasi-phase matching. Also presented is a new technique for measuring the coherence of high harmonics in the nonlinear medium. This information is crucial for understanding the process of harmonic generation over extended distances, as well as for effective enhancement using quasi-phase matching techniques.

We report on first proof-of-principles results on non-sequential double ionization of argon and neon achieved by using a newly developed long-cavity Ti:sapphire femtosecond oscillator with a pulse duration of 45 fs and a repetition of 6.2 MHz combined with a dedicated reaction microscope. Under optimized experimental conditions, peak intensities larger than 2.310(14) W/cm(2) have been achieved. Ion momentum distributions were recorded for both rare gases and show significantly different features for single as well as for double ionization. For single ionization of neon a spike of zero-momentum electrons is found when decreasing the laser intensity towards the lowest ionization rate we can measure which is attributed to a non-resonant ionization channel. As to double ionization, the longitudinal momentum distribution for Ne(2+) displays a clear double-hump structure whereas this feature is found to be smoothened out with a maximum at zero momentum for Ar(2+).