A. Ishizawa

High-order harmonic generation from a solid surface plasma by using a picosecond laser

Investigations have been performed on harmonic generation from solid-vacuum interfaces using a prepulse-free 2.2-ps 1054-nm laser pulse at maximum intensities of /spl sim/1/spl times/10/sup 15/ W/spl middot/cm/sup -2/. The harmonic intensities were strongest for p-polarized pump and dropped to about 50% in the case of s-polarized pumps. The ratio I/sub 4w/I/sub 2w/ between the intensities of the fourth and second harmonics was 0.25 at I/spl lambda//sup 2/=6/spl times/10/sup 14/ W/spl middot/cm/sup -2//spl middot//spl mu/m/sup 2/, which is relatively large when compared with the value predicted by simulations. The harmonics are collimated in the specular direction, with a transverse mode and divergence comparable to that of the driving laser.

Enhancement of high-order harmonic generation efficiency from solid-surface plasma by controlling the electron density gradient of picosecond laser-produced plasmas

We have investigated the efficiency of generating the fifth-order harmonic from plasma at the surface of a solid by varying the electron density gradient of the picosecond laser-produced plasmas. In order to modify the plasma scale length, we carried out experiments in which a 2.2 ps prepulse irradiates the solid target 0-20 ps prior to the arrival of the main Nd:glass laser pulse with the same pulse duration. The chirped pulse amplification Nd:glass laser system delivered pulses up to 100 mJ energy at 1054 nm wavelength. Although third-harmonic radiation can also be observed when a single beam is used, a /spl sim/5 ps time delay results in an increase in the harmonic signal by a factor of 2-3, which then decreases after 10 ps. We also have carried out more detailed model-calculations using the simulations. As a result, it is found that it is possible to generate high harmonics efficiency by controlling the density gradient using two laser pulses.

The spatial distribution of high-order harmonics from solid surface plasmas

Investigations have been performed on harmonic generation from solid-vacuum interfaces using a 2.2-ps 1054-nm laser pulse at maximum intensities of /spl sim/2/spl times/10/sup 17/ W/spl middot/cm/sup -2/. It is found that the divergence of the harmonics gradually increases at intensities between I/spl lambda//sup 2/=2/spl times/10/sup 15/ and 2/spl times/10/sup 16/ W/spl middot/cm/sup -2//spl middot//spl mu/m/sup 2/. We experimentally observe that the divergence of the harmonics decreases as the order of the harmonic increases. We also found that the divergence of the third harmonics emission is smaller for 2.2-ps pulsewidth for the driving lasers as compared with those for 100-ps pulsewidth lasers.

Studies of advanced soft X-ray laser and coherent higher harmonic generation using intense subpicosecond laser produced high density plasma

Abstract Current activities on our research of soft X-ray lasers and higher harmonic generations using intense, subpicosecond lasers are reviewed. Especially for soft X-ray lasers we experimentally demonstrate that by longitudinally pumping 2-mm-long molybdenum preformed plasma with high-intensity 475 fs duration laser pulse, a highly directive soft-X-ray laser at 18.9 nm wavelength is generated. The divergence of the beam is evaluated to be of the submilliradian order, and only requires pump laser energy of 150 mJ . Simulations show that the pedestal in the main pump pulse can generate electron density and gain profiles with large spatial gradients, which result in the selective amplification of low-order transverse modes. The present result is the demonstration of an efficient and alternative method of improving the spatial coherence of X-ray lasers with amplified spontaneous emission medium, with possibilities of becoming an excellent tool to explore various application experiments. In higher harmonic generation, topics related to observation of blue shift due to collisionless absorption process is described.

Aspect of Harmonic Generation from Solid Surface Plasma by Using Picosecond Laser

Investigations have been performed on harmonic generation from solid-vacuum interfaces using a prepulse-free 2.2 ps, 1054 nm laser pulse at maximum intensities of ~ 2 × 1017W · cm−2. We observed, for the first time, that the divergence of harmonics decreases as the order of the harmonic increases when irradiated at the same pump intensity. We also measured that the third harmonics emission is more specular when using a shorter pulse. The harmonic intensities were stronger for p-polarized pump than s-polarized pumps. It has been found that the conversion efficiency of higher harmonic decreases gradually.

Generation of harmonics of femtosecond radiation from the surface of aluminum targets

The results of a study of the generation of harmonics from a laser plasma resulting from the interaction of radiation of femtosecond duration (λ=1.06 μm, t=475 fs, and I∼2×1017 W cm−2) with aluminum targets are presented. The observed frequency shift of harmonics to the short-wavelength region (1.6 and 5.1 nm for the second and fifth harmonics, respectively) is determined by a collisionless absorption resulting from an anomalous skin effect. The efficiencies of conversion into the second and fifth harmonics in an s-polarized pumping field were lower than the conversion efficiencies in a p-polarized pumping field by a factor of eight and a factor of two, respectively (for intensities I<1017 W cm−2). With a further increase in the pumping intensity, these values decreased to 0.8 and 0.5, respectively. The mechanisms of such behavior of the conversion process are considered.

Coherent soft X-ray (λ=18.9 nm) generation in a transient gain scheme in molybdenum plasma

We investigate the threshold conditions for coherent soft x-ray generation using a compact hybrid (neodymium-doped glass-sapphire titanate) laser with an output energy of several hundred millijoules. Collisional excitation of molybdenum ions in a longitudinal-pumping scheme was used for x-ray (λ=18.9 nm) generation. The generated radiation exhibited a low divergence and a strong dependence on the delay between picosecond and femtosecond pumping radiations.

Coherent short-wavelength radiation via picosecond Nd:glass lasers

We review the present status of our research on the generation of coherent soft x-ray radiation using high- intensity picosecond Nd:glass laser pulses. We adopt two completely different methods for this purpose, that is high- harmonic generation from solid-vacuum interfaces, and longitudinally pumped transient collisional excitation Ni- like Mo x-ray laser. Unexpected jetlike structures are observed in the visible emission of the longitudinally- pumped molybdenum plasma, extending over a length of several millimeters. Efficiency of harmonic generation from near- solid density plasma is found to increase by a factor of 2 to 3 when using prepulses.

Submilliradian Divergence Longitudinally Pumped Nickellike Molybdenum X‐Ray Laser

We experimentally demonstrate an 18.9 nm nickellike molybdenum x‐ray laser with a very small divergence of less than one milliradian, using the longitudinally pumping technique. Simulations show that the pedestal of the longitudinal pump beam produces a region within the preplasma with large electron density and gain gradients. It is shown that this can lead to the selective amplification of transverse modes with high spatial coherence.

Observation of Blueshift on Harmonic Generation from Subpicosecond Laser Produced Solid Surface Plasmas

Harmonic generation from solid surface plasmas is studied using a 475 fs Nd: Glass laser system. We observe that for a 45 degrees angle of incidence, the second harmonic is blue shifted by 1.6 nm, and the fifth harmonic is blue shifted by 5.1 nm for p‐polarization at the intensity of 1 × 1017 Wcm−2. Furthermore, we find out that the blueshift distance is increased with orders of harmonic. The blueshift is interpreted as the collisionless absorption due to the anomalous skin effect. We also observe that the ratio between the harmonics intensities produced by p‐polarized and s‐polarized pump were 8 and 2 for the second and fifth harmonics. respectively at pump intensities 1 below 1017 W cm−2. The same ratio decreased to 0.8 and 0.5 for 1 > 2 × 1017 W cm−2.