High density molecular jets of complex neutral organic molecules with Tesla valves.

Supersonic jets of gas-phase atoms and small molecules have enabled a variety of ultrafast and ultracold chemical studies. However, extension to larger, more complex neutral molecules proves challenging for two reasons: (i) Complex molecules, such as cis-stilbene, exist in a liquid or solid phase at room temperature and ambient pressure and (ii) a unidirectional flow of high-density gaseous beams of such molecules to the interaction region is required. No delivery system currently exists that can deliver dense enough molecular jets of neutral complex molecules without ionizing or exciting the target for use in gas-phase structural dynamics studies. Here, we present a novel delivery system utilizing Tesla valves, which generates more than an order-of-magnitude denser gaseous beam of molecules compared to a bubbler without Tesla valves at the interaction region by ensuring a fast unidirectional flow of the gaseous sample. We present combined experimental and flow simulations of the Tesla valve setup. Our results open new possibilities of studying large complex neutral molecules in the gas-phase with low vapor pressures in future ultrafast and ultracold studies.

Imaging an isolated water molecule using a single electron wave packet.

Observing changes in molecular structure requires atomic-scale Ångstrom and femtosecond spatio-temporal resolution. We use the Fourier transform (FT) variant of laser-induced electron diffraction (LIED), FT-LIED, to directly retrieve the molecular structure of H2O+ with picometer and femtosecond resolution without a priori knowledge of the molecular structure nor the use of retrieval algorithms or ab initio calculations. We identify a symmetrically stretched H2O+ field-dressed structure that is most likely in the ground electronic state. We subsequently study the nuclear response of an isolated water molecule to an external laser field at four different field strengths. We show that upon increasing the laser field strength from 2.5 to 3.8 V/Å, the O-H bond is further stretched and the molecule slightly bends. The observed ultrafast structural changes lead to an increase in the dipole moment of water and, in turn, a stronger dipole interaction between the nuclear framework of the molecule and the intense laser field. Our results provide important insights into the coupling of the nuclear framework to a laser field as the molecular geometry of H2O+ is altered in the presence of an external field.

Measuring the lifetime of intense-laser generated relativistic electrons in solids via gating their Cherenkov emission.

Optical Kerr gating technique has been employed to investigate the life history of relativistic electrons in solids by temporally gating their Cherenkov emission. Mega-ampere currents of relativistic electrons are created during ultra-intense (2 × 1019 W/cm2) laser-solid interactions. In order to measure the lifetime of these relativistic electrons in solids, we temporally gate their Cherenkov emission using an optical Kerr gate (OKG). The OKG is induced in a nonlinear medium, namely, carbon-di-sulphide (CS2), with a measured gate-width (FWHM) of 2 ps. The gate femtosecond laser pulse is synchronized with the intense interaction pulse generating relativistic electrons. The arrival time of the gate laser pulse on the CS2 cell is varied with the help of a delay stage. We find that Cherenkov emission from relativistic electrons created with a ultra-short laser pulse (25 fs) lives as long as 120 ps, a few thousand times that of the incident light pulse.

Towards Remote Lightning Manipulation by Meters-long Plasma Channels Generated by Ultra-Short-Pulse High-Intensity Lasers.

Remote manipulation (triggering and guiding) of lightning in atmospheric conditions of thunderstorms has been the subject of intense scientific research for decades. High power, ultrashort-pulse lasers are considered attractive in generating plasma channels in air that could serve as conductors/diverters for lightning. However, two fundamental obstacles, namely the limited length and lifetime of such plasma channels prevented their realization to this date. In this paper, we report decisive experimental results of our multi-element broken wire concept that extends the generated plasma channels to the required tens of meters range. We obtain 13-meter-long plasma wire, limited only by our current experimental setup, with plasma conditions that could be sufficient for the leader initiation. This advance, coupled with our demonstrated method of laser heating for long time sustenance of the plasma channel, is a major, significant step towards controlling lightning.

Mapping the Damping Dynamics of Mega-Ampere Electron Pulses Inside a Solid.

We report the lifetime of intense-laser (2×10^{19} W/cm^{2}) generated relativistic electron pulses in solids by measuring the time evolution of their Cherenkov emission. Using a picosecond resolution optical Kerr gating technique, we demonstrate that the electrons remain relativistic as long as 50 picoseconds-more than 1000 times longer than the incident light pulse. Numerical simulations of the propagation of relativistic electrons and the emitted Cherenkov radiation with Monte Carlo geant4 package reproduce the striking experimental findings.

Highly efficient broadband terahertz generation from ultrashort laser filamentation in liquids.

Generation and application of energetic, broadband terahertz pulses (bandwidth ~0.1-50 THz) is an active and contemporary area of research. The main thrust is toward the development of efficient sources with minimum complexities-a true table-top setup. In this work, we demonstrate the generation of terahertz radiation via ultrashort pulse induced filamentation in liquids-a counterintuitive observation due to their large absorption coefficient in the terahertz regime. The generated terahertz energy is more than an order of magnitude higher than that obtained from the two-color filamentation of air (the most standard table-top technique). Such high terahertz energies would generate electric fields of the order of MV cm-1, which opens the doors for various nonlinear terahertz spectroscopic applications. The counterintuitive phenomenon has been explained via the solution of nonlinear pulse propagation equation in the liquid medium.

Intense femtosecond laser driven collimated fast electron transport in a dielectric medium-role of intensity contrast.

Ultra-high intensity (> 1018 W/cm2), femtosecond (~30 fs) laser induced fast electron transport in a transparent dielectric has been studied for two laser systems having three orders of magnitude different peak to pedestal intensity contrast, using ultrafast time-resolved shadowgraphy. Use of a 400 nm femtosecond pulse as a probe enables the exclusive visualization of the dynamics of highest density electrons (> 7 × 1021 cm-3) observed so far. High picosecond contrast (~109) results in greater coupling of peak laser energy to the plasma electrons, enabling long (~1 mm), collimated (divergence angle ~2°) transport of fast electrons inside the dielectric medium at relativistic speeds (~0.66c). In comparison, the laser system with a contrast of ~106 has a large pre-plasma, limiting the coupling of laser energy to the solid and yielding limited fast electron injection into the dielectric. In the lower contrast case, bulk of the electrons expand as a cloud inside the medium with an order of magnitude lower speed than that of the fast electrons obtained with the high contrast laser. The expansion speed of the plasma towards vacuum is similar for the two contrasts.