A transient photocurrent model is developed to explain coherent terahertz emission from air irradiated by a symmetry-broken laser field composed of the fundamental and its second harmonic laser pulses. When the total laser field is asymmetric across individual optical cycles, a nonvanishing electron current surge can arise during optical field ionization of air, emitting a terahertz electromagnetic pulse. Terahertz power scalability is also investigated, and with optical pump energy of tens of millijoules per pulse, peak terahertz field strengths in excess of 150 kV/cm are routinely produced.

No abstract

No abstract

The two- and three-photon spectrum of expansion-cooled ammonia has been recorded in the 380–500 nm region using multiphoton ionization (MPI) detection. In addition, the vacuum ultraviolet spectrum of gas phase ammonia has been measured photoelectrically in the 125–160 nm region. Features due to transitions to eleven electronic states are assigned in the 5.7–9.3 eV energy range, where only five band systems were previously assigned. By utilizing the spectral simplification provided by expansion cooling, the different selection rules for multiphoton absorption, and the differences between the MPI and VUV spectra, assignments can be made with much more certainty. Three previously assigned band systems have been reassigned, and seven additional electronic states identified. It is found that states arising from nd orbitals are responsible for the most intense features in the VUV spectrum, in contrast to previous experimental and theoretical work where they were excluded from consideration. Evidence is found for direct competition between photodissociation and photoionization of NH3 excited states. Earlier photochemical studies of ammonia are reinterpreted in light of the new spectroscopic assignments.

Over the past decade, pioneering and innovative experiments using subpicosecond lasers have demonstrated the generation and detection of acoustic and shock waves in materials with terahertz frequencies, the highest possible frequency acoustic waves 1-5 . In addition to groundbreaking demonstrations of acoustic solitons, these experiments have led to new techniques for probing the structure of thin films 6-8 . Terahertzfrequency electromagnetic radiation has been used in applications as diverse as molecular and material excitations 9,10 , charge transfer 11,12 , imaging 13 and plasma dynamics 14 . However, at present, existing approaches to detect and measure the time dependence of terahertz-frequency strain waves in materials use direct optical probes-time-resolved interferometry or reflectrometry 2,15,16 . Piezoelectric-based strain gauges have been used in acoustic shock and strain wave experiments for decades, but the time resolution of such devices is limited to ∼100 ps and slower, the timescale of electronic recording technology. We have recently predicted that terahertz-frequency acoustic waves can be detected by observing terahertz radiation emitted when the acoustic wave propagates past an interface between materials of differing piezoelectric coefficients 17,18 . Here, we report the first experimental observation of this fundamentally new phenomenon and demonstrate that it can be used to probe structural properties of thin films.As an acoustic wave traverses an interface between materials with differing piezoelectric response, polarization currents and concurrent radiation are predicted to be generated at the boundary. These currents radiate on the timescale of the strain changes 17 . For acoustic waves with characteristic frequencies of terahertz (corresponding to picosecond timescales), terahertz radiation is emitted. This phenomenon bears a close resemblance to the so-called transition radiation phenomenon that occurs when a charged particle propagates past an interface between two dielectric materials, generating polarization currents and radiation from the interface. It is distinct from a narrowband, coherent terahertz emission mechanism that has been predicted to occur when a planar shock wave propagates through an ionic crystal [19][20][21] . When the interface is flat and some propagation properties of the strain wave are known, the time dependence of the strain can be computed from the time dependence of the radiated electric field. This enables the observation of ultrafast strain profiles in regions of a material not accessible to active probes such as those used in interferometryor reflectometry-based methods 2,[22][23][24] . This acoustic transition radiation phenomenon has been predicted theoretically 17 , but not yet observed experimentally. Some experimental evidence exists for other forms of electromagnetic radiation that may