Abstract:We study THz-emission from a plasma driven by an incommensurate-frequency two-colour laser field. A semi-classical transient electron current model is derived from a fully quantum-mechanical description of the emission process in terms of sub-cycle field-ionization followed by continuum-continuum electron transitions. For the experiment, a CEP-locked laser and a near-degenerate optical parametric amplifier are used to produce two-colour pulses that consist of the fundamental and its near-half frequency. By choosing two incommensurate frequencies, the frequency of the CEP-stable THz-emission can be continuously tuned into the mid-IR range. This measured frequency dependence of the THz-emission is found to be consistent with the semi-classical transient electron current model, similar to the Brunel mechanism of harmonic generation.
We describe a terahertz time-domain-spectroscopy system that is based on photoconductive components fabricated from low-temperature-grown GaBiAs epitaxial layers and activated by femtosecond 1.03 μm pulses emitted by an Yb:fiber laser. Experiments performed with the laser pulses of different durations have evidenced that the spectral range of the system is limited by the photoexcited carrier lifetime. The optical to terahertz conversion efficiency of the system exceeds 10−4 with the usable spectral range of about 3 THz in the case of 170 fs excitation pulses.
Treatment of the
laser borane anti-B18H22 (compound 1) with iodine in ethanol gives the monoiodinated derivative
7-I-anti-B18H21 (compound 2) in 67% yield, or, by reaction with iodine or ICl in the
presence of AlCl3 in dichloromethane, the diiodinated derivative
4,4′-I2-anti-B18H20 (compound 3) in 85% yield. On excitation with
360 nm light, both compounds 2 and 3 give
strong green phosphorescent emissions (λmax = 525
nm, ΦL = 0.41 and λmax = 545 nm,
ΦL = 0.71 respectively) that are quenched by dioxygen
to produce O2(1Δg) singlet
oxygen with quantum yields of ΦΔ = 0.52 and
0.36 respectively. Similarly strong emissions can be stimulated via
the nonlinear process of two-photon absorption when exciting with
720 or 800 nm light. The high quantum yields of singlet-oxygen production,
coupled with the option of two-photon excitation, make compounds 2 and 3 promising O2(1Δg) photosensitizers. The molecular structures of compounds 2 and 3 were determined by single-crystal X-ray
crystallographic studies as well as multinuclear NMR spectroscopy
and mass spectrometry. Time-resolved UV–vis spectroscopy was
used to delineate their photophysical properties, and the electronic-structure
properties of the emitting species were determined by means of multiconfigurational
quantum-chemistry computations.
A highly birefringent silicate glass photonic-crystal fiber (PCF) is employed for polarization-controlled nonlinear-optical frequency conversion of femtosecond Cr: forsterite laser pulses with a central wavelength of 1.24 mum to the 530--720-nm wavelength range through soliton dispersion-wave emission. The fiber exhibits a modal birefringence of 1.2.10(-3) at the wavelength of 1.24 mum due to a strong form anisotropy of its core, allowing polarization switching of the central wavelength of its blue-shifted output by 75 nm. Polarization properties and the beam quality of the blue-shifted PCF output are shown to be ideally suited for polarization-sensitive nonlinear Raman microspectroscopy.
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