We present the results of a systematic study of photoelectron emission from gasphase dielectric nanoparticles (SiO 2 ) irradiated by intense 25 fs, 780 nm linearly polarized laser pulses as a function of particle size (20 nm to 750 nm in diameter) and laser intensity. We also introduce an experimental technique to reduce the effects of focal volume averaging. The highest photoelectron energies show a strong size dependence, increasing by a factor of six over the range of particles sizes studied at a fixed intensity. For smaller particle sizes (up to 200 nm), our findings agree well with earlier results obtained with few-cycle, ~4 fs pulses. For large nanoparticles, which exhibit stronger near-field localization due to field-propagation effects, we observe the emission of much more energetic electrons, reaching energies up to ~200 times the ponderomotive energy. This strong deviation in maximum photoelectron energy is attributed to the increase in ionization and charge interaction for many-cycle pulses at similar intensities.
For the last several decades, the wavelength range accessible for strong-field, few-cycle studies has remained limited to the visible, near infrared and mid-wave infrared regimes. In particular, sources in the long-wave infrared have been lacking. We report the development of a 1 kHz, few-cycle laser source with up to a 9 μ m central wavelength and gigawatt peak powers. When focused, this source can ionize gas targets, which we demonstrate here through the ionization of atomic xenon at wavelengths ranging from 5 μ m to 9 μ m. This opens up new opportunities for fundamental atomic and molecular physics, enabling experimental tests of strong-field ionization theories in the extreme long-wavelength, few-cycle limit and the direct excitation of vibrational transitions in organic molecules.
Carrier-envelope phases (CEPs) from a kHz repetition rate, non-CEP stabilized laser system are measured and tagged with two different methods: an f–2f interferometer and a stereo-above-threshold-ionization carrier-envelope-phase-meter. Both methods utilize the octave spanning spectrum generated in the hollow-core fiber (HCF) that broadens the laser spectrum to produce few-cycle pulses. Phases from both methods are carefully synchronized and compared on a single shot level. The results show that the CEPs measured by both methods are in good agreement and demonstrate that a HCF based f–2f interferometer is well suited for CEP tagged experiments.
Ultrashort, intense light pulses permit the study of nanomaterials in the optical non-linear regime, potentially leading to optoelectronics that operate in the petahertz domain. These non-linear regimes are often present just below the damage threshold thus requiring the careful tuning of laser parameters to avoid the melting and disintegration of the materials. Detailed studies of the damage threshold of nanoscale materials are therefore needed. We present results on the damage threshold of Au nanowires when illuminated by intense femtosecond pulses. These nanowires were synthesized with the directed electrochemical nanowire assembly (DENA) process in two configurations: (1) free-standing Au nanowires on W electrodes and (2) Au nanowires attached to fused silica slides. In both cases the wires have a single-crystalline structure. For laser pulses with durations of 108 fs and 32 fs at 790 nm at a repetition rate of 2 kHz, we find that the free-standing nanowires melt at intensities close to 3 TW/cm 2 and 7.5 TW/cm 2 , respectively. The Au nanowires attached to silica slides melt at slightly higher intensities, just above 10 TW/cm 2 for 32 fs pulses. Our results can be explained with an electron-phonon interaction model that describes the absorbed laser energy and subsequent heat conduction across the wire. 4702-4705 (1995). 30. G. Easley, "Generation of nonequilibrium electron and lattice temperatures in copper by picosecond laser pulses," Phys. Rev. B 33, 2144-2145 (1986). 31. Z. Lin, L. Zhigilei, and V. Celli, "Electron-phonon coupling and electron heat capacity of metals under conditions of strong electron-phonon nonequilibrium," Phys. Rev. B 77, 075133 (2008). 32. S. I. Anisimov, B. Kapeliovich, and T. Perel'man, "Electron emission from metal surfaces exposed to ultrashort laser pulses," Sov. Phys. JETP 39, 375-377 (1975
We present a compact, simple design for the generation and tuning of both the spot size and effective focal length of Bessel-like beams. In particular, this setup provides an important tool for the use of Bessel-like beams with high-power, femtosecond laser systems. Using a shallow angle axicon in conjunction with a spherical lens, we show that it is possible to focus Bessel-like modes to comparable focal spot sizes to sharp axicons while maintaining a long effective focal length. The resulting focal profiles are characterized in detail using an accurate high dynamic range imaging technique. Quantitatively, we introduce a metric (R0.8) which defines the spot-size containing 80% of the total energy. Our setup overcomes the typical compromise between long working distances and small spot sizes. This is particularly relevant for strong-field physics where most experiments must operate in vacuum.
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