Parametric amplification of optical phonons.

We use coherent midinfrared optical pulses to resonantly excite large-amplitude oscillations of the Si-C stretching mode in silicon carbide. When probing the sample with a second pulse, we observe parametric optical gain at all wavelengths throughout the reststrahlen band. This effect reflects the amplification of light by phonon-mediated four-wave mixing and, by extension, of optical-phonon fluctuations. Density functional theory calculations clarify aspects of the microscopic mechanism for this phenomenon. The high-frequency dielectric permittivity and the phonon oscillator strength depend quadratically on the lattice coordinate; they oscillate at twice the frequency of the optical field and provide a parametric drive for the lattice mode. Parametric gain in phononic four-wave mixing is a generic mechanism that can be extended to all polar modes of solids, as a means to control the kinetics of phase transitions, to amplify many-body interactions or to control phonon-polariton waves.

Generation of narrowband, high-intensity, carrier-envelope phase-stable pulses tunable between 4 and 18 THz.

We report on the generation of high-energy (1.9 µJ) far-infrared pulses tunable between 4 and 18 THz frequency. Emphasis is placed on tunability and on minimizing the bandwidth of these pulses to less than 1 THz, as achieved by difference-frequency mixing of two linearly chirped near-infrared pulses in the organic nonlinear crystal DSTMS. As the two near-infrared pulses are derived from amplification of the same white light continuum, their carrier envelope phase fluctuations are mutually correlated, and hence the difference-frequency THz field exhibits absolute phase stability. This source opens up new possibilities for the control of condensed matter and chemical systems by selective excitation of low-energy modes in a frequency range that has, to date, been difficult to access.

Narrowband carrier-envelope phase stable mid-infrared pulses at wavelengths beyond 10 µm by chirped-pulse difference frequency generation.

We report on the generation of narrowband carrier-envelope phase stable mid-infrared (MIR) pulses between 10 and 15 µm. High pulse energies and narrow bandwidths are required for the selective nonlinear excitation of collective modes of matter that is not possible with current sources. We demonstrate bandwidths of <2% at 12.5 µm wavelength through difference frequency generation between two near-infrared (NIR) pulses, which are linearly chirped. We obtain a reduction in bandwidth by one order of magnitude, compared to schemes that make use of transform-limited NIR pulses. The wavelength of the narrowband MIR pulse can be tuned by changing the optical delay between the two chirped NIR pulses.

THz-Frequency Modulation of the Hubbard U in an Organic Mott Insulator.

We use midinfrared pulses with stable carrier-envelope phase offset to drive molecular vibrations in the charge transfer salt ET-F_{2}TCNQ, a prototypical one-dimensional Mott insulator. We find that the Mott gap, which is probed resonantly with 10 fs laser pulses, oscillates with the pump field. This observation reveals that molecular excitations can coherently perturb the electronic on-site interactions (Hubbard U) by changing the local orbital wave function. The gap oscillates at twice the frequency of the vibrational mode, indicating that the molecular distortions couple quadratically to the local charge density.

Effect of cimetropium bromide and other antispasmodic compounds on in vitro guinea-pig gallbladder.

The effect of cimetropium bromide, a new antimuscarinic compound, on bethanechol- and electrically-induced contractions was studied on isolated guinea-pig gallbladder. Atropine and two other widely employed antispasmodics (i.e., rociverine and octylonium bromide) were employed as reference compounds. Cimetropium and atropine proved to be competitive antimuscarinics, their pA2 being 7.77 +/- 0.14 and 8.31 +/- 0.14, respectively. On the contrary, rociverine displayed a dual effect being a competitive antagonist at low (up to 10(-5) mol/l) and a mixed one at high (greater than 10(-5) mol/l) concentrations. When tested against bethanechol- and electrically-induced contractions, all the compounds, with the exception of octylonium bromide, showed a concentration-dependent relaxant effect. In both experimental conditions, the potency of cimetropium was of the same order of magnitude as that of atropine and 150-200 times higher than that of rociverine. These data, together with the reported activity on human gallbladder in vivo and the well known involvement of cholinergic system in the control of gallbladder motility, could represent the rationale for the clinical use of cimetropium in the treatment of biliary colics as well as spasms of biliary tree.

Action of pirenzepine on the human urinary bladder in vitro.

The novel compound pirenzepine was tested for its antimuscarinic effect on the human urinary bladder "in vitro." Its behavior towards the contractions induced by acetylcholine or bethanechol and towards electrically induced contractions was identical to that of atropine. However, its potency was 100 to 300 times lower than that of atropine. Results obtained with ganglion blocking agents, tetrodotoxin and cooled preparations of urinary bladder seem to indicate the virtually total absence of ganglionic cells. On the other hand they point out the fundamental role of post-synaptic muscarinic M2 receptors as the most important component of the cholinergic system in the bladder. Of course the existence of other transmitters released at the cholinergic nerve endings after electrical field stimulation cannot be excluded on the basis of our experiments.