Multi-cycle Pulses Research Articles

Single attosecond pulse generation from asymmetric molecules with a multicycle laser pulse

A new scheme for single attosecond pulse generation from asymmetric molecules with a multicycle laser pulse is proposed. It is shown that both even and odd harmonics are generated from the asymmetric molecules, and the attosecond pulses are produced every full cycle of the driving laser field rather than each half-cycle. By filtering the harmonics in the cutoff of the spectrum, a single attosecond pulse can be obtained with a multicycle laser pulse with a duration of 2 times longer than the few-cycle pulse conventionally used.

Coupling terahertz radiation onto a metal wire using a subwavelength coaxial aperture

We demonstrate a simple technique for coupling freely propagating broadband THz radiation to multi-cycle THz pulses on a cylindrical metal wire. This is accomplished by inserting the tapered end of a cylindrical wire into the center of a subwavelength circular aperture fabricated in a freestanding metal film, forming an effective coaxial waveguide. By doing so, we convert the transmission properties of THz pulses through the aperture from an evanescent mode to a propagating mode. By fabricating concentric annular grooves about the aperture, multicycle THz pulses are coupled to the wire. The individual groove geometry, number of grooves, and groove spacing surrounding the subwavelength aperture on the metal film determine the shape of THz pulses launched on the waveguide.

Phase-locked high-order-harmonic and sub-100−aspulse generation from stretched molecules

High harmonic generation from diatomic molecules in a linearly polarized intense laser field is investigated and the emission time of the harmonics is discussed with the time-frequency analysis method. It is shown that high harmonic generation from molecules at equilibrium distance is similar to that from atoms. Only the harmonics in the cutoff are synchronized, i.e., well phase-locked, whereas the other harmonics are not phase-locked. For the molecule stretched well beyond its equilibrium distance, the harmonics exhibit distinct time-frequency characteristics. The harmonic spectrum can be extended to ${I}_{p}+8{U}_{p}$, where ${I}_{p}$ and ${U}_{p}$ are the ionization and ponderomotive potential, and the harmonics with energies below ${I}_{p}+3.17{U}_{p}$ are not phase-locked and the harmonics with energies beyond ${I}_{p}+3.17{U}_{p}$ are well phase-locked. Thus a large range of harmonics which are well phase-locked are produced, and a train of clean attosecond (as) pulses with a single $90\text{\ensuremath{-}}\mathrm{as}$ pulse in each half optical cycle can be generated with a multicycle laser pulse. Using a few-cycle laser pulse, an isolated attosecond pulse with a duration of about $95\phantom{\rule{0.3em}{0ex}}\mathrm{as}$ is obtained.

Generation of multicycle terahertz pulses via optical rectification in periodically inverted GaAs structures

The authors demonstrate the generation of multicycle narrow-bandwidth terahertz pulses in periodically inverted GaAs structures using optical rectification of 2μm, 100fs pump pulses. Three different types of orientation-inverted samples are employed: optically contacted multilayer, orientation-patterned, and diffusion-bonded GaAs. The terahertz pulses are characterized by two-color (pump at 2μm and probe at 0.8μm) terahertz time-domain spectroscopy and terahertz Michelson interferometry.

Detonation Tube Impulse in Subatmospheric Environments

Thrust from a multicycle pulse detonation engine operating at practical flight altitudes will vary with surrounding environment pressure. We have carried out the first experimental study using a detonation tube hung in a ballistic pendulum arrangement within a large pressure vessel to determine the effect that the environment has on the single-cycle impulse. Air pressure decreased below 100 kPa, whereas initial pressure of the stoichiometric ethylene-oxygen mixture inside the tube varied between 100 and 30 kPa. The original impulse model (Wintenberger et al., Journal of Propulsion and Power, Vol. 19, No. 1, 2002, pp. 22-38) was modified to predict the observed increase in impulse and blowdown time as the environmental pressure decreased below 1 atm. Comparisons between the impulse from detonation tubes and ideal steady-flow rockets indicate incomplete expansion of the detonation tube exhaust, resulting in a 37% difference in impulse at a pressure ratio (ratio of pressure behind the Taylor wave to environmental pressure) of 100.

Single attosecond pulse generation in the multicycle-driver regime by adding a weak second-harmonic field

We present a method of producing single attosecond pulses by high-harmonic generation with multicycle driver laser pulses. This can be achieved by tailoring the driving pulse so that attosecond pulses are produced only every full cycle of the oscillating laser field rather than every half-cycle. It is shown by classical and quantum-mechanical model calculations that even a minor addition (1%) of phase-locked second-harmonic light to the 800 nm fundamental driver pulse for high-harmonic generation leads to a major (15%) difference in the maximum kinetic energies of the recombining electrons in adjacent half-cycles.

Manipulation of terahertz waveforms in nonlinear optical crystals by shaped optical pulses

We manipulate terahertz (THz) waveforms by optical rectification in nonlinear optical crystals using a pair of temporally separated femtosecond pulses. Two single-cycle THz pulses coherently control rotational transitions in gas-phase HCl molecules. Continuously tunable narrowband THz pulses are generated in a fanned-out, periodically poled lithium niobate crystal. The shape of the multicycle THz pulses is controlled by adjusting the relative time delay and intensity between the two optical pulses.

Generation of terahertz pulses with arbitrary elliptical polarization

We employ two different methods to generate controllable elliptical polarization of teraherz (THz) pulses. First, THz pulses are generated via optical rectification in nonlinear crystals using a pair of temporally separated and perpendicularly polarized optical pulses. The THz ellipticity is controlled by adjusting the relative time delay and polarization of the two optical pulses. We generate mixed polarization states of single-cycle THz pulses using ZnTe, and elliptically polarized multicycle THz pulses in periodically poled lithium niobate crystals. Second, we generate elliptically polarized THz pulses by making a THz “wave plate” using a combination of a wire-grid THz polarizer and a mirror to transform linearly polarized multicycle THz pulses into elliptical polarization.

Coupling of terahertz pulses onto a single metal wire waveguide using milled grooves

We demonstrate a novel approach for coupling freely propagating THz pulses onto a metal wire waveguide utilizing grooves fabricated directly into the metal. Using broadband THz pulses incident on the wire, we use metal segments containing zero, one, three, and eight uniformly spaced grooves to launch surface propagating multi-cycle pulses along the wire. We observe a one-to-one correspondence between the groove number and the number of oscillations in the THz waveform radiated from the end of the waveguide. We further demonstrate that this coupled radiation is radially polarized. Although the cross-sectional parameters of the grooves are identical in the present measurements, alteration of the individual grooves in a controlled manner should allow for arbitrarily shaped THz pulses to be launched on the waveguide.

Control of inertial cavitation in shockwave lithotripsy

The role of inertial cavitation in stone comminution and tissue damage during extracorporeal shock-wave lithotripsy (ESWL) became appreciated several years after the successful introduction of ESWL into routine clinical use in the early 1980s. It is only relatively recently, however, that attempts have been made to enhance or suppress the cavitation field in lithotripsy by adapting the acoustic field generated by the shock-wave source. The single-cycle acoustic pulses used in commercial ESWL generate a cavitation field in which the bubbles may remain in an expanded phase for many times the pulse duration. Some control of cavitation can be gained by timing an additional pulse from a second source to coincide, for example, with the bubbles expanded phase [M. R. Bailey, Tech. Rep. No. ARL-TR-97-1, Univ. of Texas, Austin (1997)]. This can generate an earlier and more violent bubble collapse. A single source generating multiple-cycle pulses can, similarly, provide a means of controlling cavitation activity in lithotripsy. This approach has been examined using an electromagnetic-type shock-wave source to generate high-amplitude, multicycle pulses in water. By tailoring the pressure waveform specifically to control cavitation, it may be possible to refine medical ultrasound applications, such as ESWL, where cavitation plays a role.

Spatiotemporal structure of isodiffracting ultrashort electromagnetic pulses

We present a model of isodiffracting single-cycle and few-cycle ultrashort electromagnetic pulses. The model is based on exact solutions of the time-dependent paraxial wave equation with space-time coupling effects included. The spatiotemporal structure of these pulses is characterized by a scaling parameter which relates off-axis pulse shapes to the axial temporal waveforms. Depending on the spectrum a pulse may transform itself from a single-cycle pulse to a multicycle pulse along the radial coordinate. This model is also used to describe recirculating pulses in a curved mirror cavity resonator. The Gouy phase shift contributes an absolute phase that results in a pulse-to-pulse temporal instability.

Diffraction-induced transformation of near-cycle and subcycle pulses

An analytical approach to the theory of diffraction transformation of pulses with superbroad spectra and arbitrary time dependence, in particular half-cycle (unipolar), single-cycle, and multicycle pulses, was developed. Closed-form solutions were found for on-axis propagation of half-cycle pulses with initially Gaussian spatial profiles that have either cosh-1-like or Gaussian time dependence, for single-cycle pulses based on higher modes of these functions, and for multicycle pulses. The far-field propagation demonstrates common patterns of time-derivative behavior regardless of the initial spatiotemporal profile. It is also shown that the time width of an off-axis pulse increases with the angle of observation. Owing to time–space reciprocity, the pulse transformation that is due to diffraction can be reversed, e.g., by reflection of a pulse from a spherical concave mirror.

Automatic analysis of single motor unit potentials for the diagnosis of central motor disorders

Previous studies have shown that the efficacy of transcranial magnetic stimulation (TMS) to excite corticospinal neurons depends on pulse waveform.In this study, we examined whether the effectiveness of polyphasic TMS can be increased by using a pulse profile that consists of multiple sine cycles.In eight subjects, single-pulse TMS was applied to the left primary motor hand area through a round coil attached to a stimulator device that generated polyphasic pulses consisting of one to six full-sine cycles with a cycle length of 86 μs. In different blocks, we varied the number of sine cycles per pulse and recorded the motor-evoked potential (MEP) from the right first dorsal interosseus muscle. For each stimulus type, we determined resting motor threshold (RMT), stimulus-response curve (SRC), and mean MEP amplitude evoked at maximal stimulator output to assess the efficacy of stimulation.Multicycle pulses were more effective than a single full-sine cycle in exciting corticospinal neurons. TMS with multicycle pulses resulted in lower RMT, larger MEP amplitudes at maximal stimulator output and a steeper slope of the SRC relative to a TMS pulse consisting of a single-sine cycle. The increase in efficacy was already evident when two full-sine cycles were used and did not increase further by adding more cycles to the TMS pulse.Increasing the number of full-sine cycles per pulse can improve the efficacy of TMS to excite corticospinal neurons, but there is no simple linear relationship between the number of cycles and TMS efficacy.

Laboratory measurements of shear wave attenuation in saturated sand

Laboratory measurements of shear wave attenuation as a function of frequency were made using recently developed ceramic bimorph bender transducers to excite transverse particle motion in a medium grain water-saturated sand. The measurements were made at 13 frequencies from 450 to 7000 Hz. Multicycle sine-wave pulses were used to insure steady-state vibration at the measurement frequency. Attenuation was determined from the slope of a linear least-squares fit to the maximum received level versus transducer separation data. This not only affords an estimate of the attenuation but allows confidence intervals to be placed around that estimate. The attenuation values’ which increased with frequency from 5 dB/m at 450 Hz to 120 dB/m at 7000 Hz’ did not exhibit a simple first-power frequency dependence. The results were compared with predictions based on a two-component model developed by Stoll and were consistent in both amplitude and frequency dependence.