Broad Pulse Research Articles

The role of transrectal ultrasound in diagnosing prostate cancer.

The availability of the prostate-specific antigen test to screen for prostate cancer has caused a significant workload burden for urologists and radiologists alike. Prostate cancer is common in our aging population and most cancers are now definitively detected by transrectal ultrasound (TRUS)-guided prostatic needle biopsy. TRUS alone has limited potential to identify prostatic cancer because of frequent multifocality of cancer within the prostate, the variable sonographic appearance of prostatic tumors, the poor specificity of focal ultrasonic abnormalities, and the substantial percentage of isoechoic prostate cancers (which cannot be differentiated from adjacent benign tissues with imaging). Developments in TRUS equipment over the past decade include the use of color and power Doppler, higher frequencies, broad bandwidth technologies, and harmonic, contrast harmonic, and pulse inversion imaging. All of these improvements may enhance detection of subtle focal sonographic abnormalities within the prostate. Ultrasonic contrast agents can aid the visualization of subtle alterations in prostatic echotexture by highlighting changes in microvasculature. It is possible that Doppler techniques and contrast agents have the potential to reveal prognostic information about cancer in individual patients.

Ballistic phonon production in photoexcited Ge, GaAs, and Si

Phonon imaging and photoluminescence measurements are used to determine the frequency and spatial distribution of optically generated nonequilibrium phonons in Si, Ge, and GaAs at 1.7 K. At low excitation levels the thermalization of photoexcited carriers and the subsequent phonon down-conversion produce a broad frequency distribution of acoustic phonons that ``quasidiffuse'' in the crystal. These phonons produce a temporally broad heat pulse when detected at a distance from the excitation point. At moderate excitation levels (typically a 10-nS pulse with a power density of \ensuremath{\sim}20 ${\mathrm{W}/\mathrm{m}\mathrm{m}}^{2}$), the laser pulse produces a dense electron-hole plasma that can radically change the frequency distribution of nonequilibrium phonons. The plasma is a potentially rich source of low-frequency acoustic phonons, characterized by a temporally sharp heat pulse at a remote detector. The fraction of low-frequency phonons in the heat pulses is smallest in the direct-gap semiconductor GaAs, where rapid recombination depletes the populations of electrons and holes in just a few nanoseconds. More noticeable low frequency phonon components are seen in heat pulses in the indirect-gap semiconductors Ge and Si. At sufficiently high excitation densities (\ensuremath{\sim}60 ${\mathrm{W}/\mathrm{m}\mathrm{m}}^{2}$) in Ge, there is a suppression of the low-frequency phonon signal, which may result from phonon absorption within a cloud of electron hole droplets. An interesting alternative hypothesis is that the acoustic phonons created in the plasma are sufficiently dense to initiate phonon coalescence, whereby phonons are localized by phonon-phonon scattering over a relatively long period (500 ns). This localized ``hot spot'' could provide the phonon wind that drives the initial rapid expansion of the electron-hole plasma into the crystal.

Quantum control of coherent anti-Stokes Raman processes

Using femtosecond pulse-shaping techniques, we are able to control several useful parameters of coherent anti-Stokes Raman spectroscopy (CARS). By shaping both the pump and the Stokes pulses with an appropriate spectral phase function, we eliminated the nonresonant CARS background. In addition, we designed pulses which selectively excite one of two neighboring Raman levels, even when both are well within the excitation spectrum. High-resolution CARS spectra were recorded in spite of the broad femtosecond pulse spectrum.

Narrow-band coherent anti-stokes Raman signals from broad-band pulses.

By tailoring the phase of a 100 femtosecond probe pulse we are able to obtain a narrow-band coherent anti-Stokes Raman spectroscopy (CARS) resonant signal with a width of less than 15 cm(-1), which is an order of magnitude narrower than the CARS signal from a transform limited pulse. Thus, by measuring the spectrum of the CARS signal we are able to obtain a high-resolution energy level diagram of the probed sample in spite of the broad femtosecond pulse spectrum.

The anomalous X–ray pulsar 1E 1048.1–5937: Phase resolved spectroscopy with the XMM-Newton satellite

We report on an observation of the anomalous X-ray pulsar 1E 1048.1-5937 performed with the XMM-Newton satellite. The phase averaged spectrum of 1E 1048.1-5937 is well described by the sum of a power law with photon index∼2.9 and a blackbody with temperature ∼0.6 keV, without evidence for significant absorption or emission lines. The above spectral parameters do not vary during the phases corresponding to the broad pulse, while the off pulse emission shows a different spectrum characterized by a soft excess at energies below ∼1.5 keV. The XMM-Newton observation and a re-analysis of archival BeppoSAX data show that the spectral parameters and flux of 1E 1048.1-5937 did not change significantly during observations spanning the last four years. All the data are consistent with a 2-10 keV luminosity varying in the range ∼(5-7) ×10 3 3 erg s - 1 (for a distance of 3 kpc).

The curved X-ray spectrum of PSR B1509-58 observed with BeppoSAX

We report timing and spectral results for PSR B1509 58 observed by BeppoSAX in February 1998. We obtained pulse proles with high statistical signicance from 0.1 to 300 keV that exhibit the well-known broad asymmetric single pulse. The shape of this pulse does not change across this energy window and can be described as the sum of a narrow and a broader Gaussian component separated0:13 in phase. The spectral distribution can be accurately represented, over the entire BeppoSAX energy range, by a curved function rather than a simple power-law shape. The extrapolation of this model to higher energies is also consistent with the COMPTEL fluxes in the 0.75{30 MeV range, with the maximum luminosity for the broad pulse of PSR B1509 58 reached at 5:4 MeV. The comparison with the Crab pulsar spectrum suggests a possible origin of the X-ray emission in terms of synchrotron radiation from secondary pair particles near the neutron star.

Temporal interference of coherent laser pulses in optical fibers

We report on collective effects for the train of optical pulses when a reproduction of the periodical pulse train occurs at some distance in dispersive medium. The reproduction of the initial temporal distribution can be treated as a temporal analog of the spatial Talbot effect. In experiment with a multimode fiber, the group of seven pulses lost its temporal shape converting in one broad pulse at the distance of 1200 m, while continuous train of pulse preserved its temporal structure.

Mechanism of the reaction, CH4+O(1D2)→CH3+OH, studied by ultrafast and state-resolved photolysis/probe spectroscopy of the CH4⋅O3 van der Waals complex

The mechanism of the reaction CH4+O(1D2)→CH3+OH was investigated by ultrafast, time-resolved and state-resolved experiments. In the ultrafast experiments, short ultraviolet pulses photolyzed ozone in the CH4⋅O3 van der Waals complex to produce O(1D2). The ensuing reaction with CH4 was monitored by measuring the appearance rate of OH(v=0,1;J,Ω,Λ) by laser-induced fluorescence, through the OH A←X transition, using short probe pulses. These spectrally broad pulses, centered between 307 and 316 nm, probe many different OH rovibrational states simultaneously. At each probe wavelength, both a fast and a slow rise time were evident in the fluorescence signal, and the ratio of the fast-to-slow signal varied with probe wavelength. The distribution of OH(v,J,Ω,Λ) states, Pobs(v,J,Ω,Λ), was determined by laser-induced fluorescence using a high-resolution, tunable dye laser. The Pobs(v,J,Ω,Λ) data and the time-resolved data were analyzed under the assumption that different formation times represent different reaction mechanisms and that each mechanism produces a characteristic rovibrational distribution. The state-resolved and the time-resolved data can be fit independently using a two-mechanism model: Pobs(v,J,Ω,Λ) can be decomposed into two components, and the appearance of OH can be fit by two exponential rise times. However, these independent analyses are not mutually consistent. The time-resolved and state-resolved data can be consistently fit using a three-mechanism model. The OH appearance signals, at all probe wavelengths, were fit with times τfast≈0.2 ps, τinter≈0.5 ps and τslow≈5.4 ps. The slowest of these three is the rate for dissociation of a vibrationally excited methanol intermediate (CH3OH*) predicted by statistical theory after complete intramolecular energy redistribution following insertion of O(1D2) into CH4. The Pobs(v,J,Ω,Λ) was decomposed into three components, each with a linear surprisal, under the assumption that the mechanism producing OH at a statistical rate would be characterized by a statistical prior. Dissociation of a CH4O* intermediate before complete energy randomization was identified as producing OH at the intermediate rate and was associated with a population distribution with more rovibrational energy than the slow mechanism. The third mechanism produces OH promptly with a cold rovibrational distribution, indicative of a collinear abstraction mechanism. After these identifications were made, it was possible to predict the fraction of signal associated with each mechanism at different probe wavelengths in the ultrafast experiment, and the predictions proved consistent with measured appearance signals. This model also reconciles data from a variety of previous experiments. While this model is the simplest that is consistent with the data, it is not definitive for several reasons. First, the appearance signals measured in these experiments probe simultaneously many OH(v,J,Ω,Λ) states, which would tend to obfuscate differences in the appearance rate of specific rovibrational states. Second, only about half of the OH(v,J,Ω,Λ) states populated by this reaction could be probed by laser-induced fluorescence through the OH A←X band with our apparatus. Third, the cluster environment might influence the dynamics compared to the free bimolecular reaction.

Mechanism of Reversible Conductance Transitions in a Crystalline Thin-Film

Abstract Recently there has been considerable interest in developing nanometer- and sub-nanometer-scale electronic devices. Of particular interest in this regard is whether individual molecules or molecular complexes can be employed as electronic device elements. Aviram et al. have reported switching and rectification in an organic thin film. More recently, Potember et al. have shown a field-induced conductance transition on a 500 nm scale, but did not demonstrate local reversibility of the transition. The reverse transition was induced only by application of a broad laser pulse or heat. We have observed and replicated reversible conductance transitions in a fully-organic crystalline complex, on a scale close to the dimensions of the unit cell. A crystalline thin-film organic complex of 3-nitrobenzal malononitrile and 1,4-phenylenediamine (NBMN-pDA), exhibits reversible conductance transitions on the sub-nanometer scale when exposed to local electric field pulses from an STM tip.

Soliton formation from a pulse passing the zero-dispersion point in a nonlinear Schrodinger equation

We consider in detail the self-trapping of a soliton from a wave pulse that passes from a defocusing region into a focusing one in a spatially inhomogeneous nonlinear waveguide, described by a nonlinear Schrodinger equation in which the dispersion coefficient changes its sign from normal to anomalous. The model has direct applications to dispersion-decreasing nonlinear optical fibers, and to natural waveguides for internal waves in the ocean. It is found that, depending on the (conserved) energy and (nonconserved) "mass" of the initial pulse, four qualitatively different outcomes of the pulse transformation are possible: decay into radiation; self-trapping into a single soliton; formation of a breather; and formation of a pair of counterpropagating solitons. A corresponding chart is drawn on a parametric plane, which demonstrates some unexpected features. In particular, it is found that any kind of soliton(s) (including the breather and counterpropagating pair) eventually decays into pure radiation with an increase of energy, the initial "mass" being kept constant. It is also noteworthy that a virtually direct transition from a single soliton into a pair of symmetric counterpropagating ones seems possible. An explanation for these features is proposed. In two cases when analytical approximations apply, viz., a simple perturbation theory for broad initial pulses and the variational approximation for narrow ones, comparison with direct simulations shows reasonable agreement.

Propagation of the electromagnetic field in optical-limiting reverse-saturable absorbers

Reverse-saturable absorbers are of considerable interest for optical limiting. Using the electric dipole perturbation, we derived the rate equation for a five-level system describing reverse-saturable absorbers. Traditional theories for the propagating laser beam in these materials are expressed in terms of the optical intensity. However, with the introduction of high-power short-pulsed lasers, the propagation of light in these materials may be subject to nonlinear phenomena such as self-focusing and self-phase modulation. Furthermore, conventional theories treat the laser light as a continuous wave or as a very broad temporal pulse in which dispersive effects are neglected. In order to incorporate these other nonlinear or dispersive effects, and therefore determine their influence in reverse-saturable absorbers, we derived an equation for the propagation of the electromagnetic field, rather than the intensity, coupled to the rate equations for a five-level system. We also coupled our theory to experimentally measurable parameters for these materials and detailed the various physical approximations used to obtain the rate equations.

Generation of Low-Frequency Electromagnetic Waves by Spectrally Broad Intense Laser Pulses in a Plasma

A new mechanism for the emission of low-frequency electromagnetic (EM) waves, including the generation of a quasistatic magnetic field, by a relativistically intense laser pulse with a wide spectrum is presented. The emission is due to modulational and filamentational instabilities of the photon gas in a plasma. The generation of the magnetic field is associated with a significant change in the laser pulse shape during the propagation. This process is identified in our 2D particle-in-cell (PIC) simulations with a high intensity (1019 < I < 1020 W/cm2) laser pulse.

Studies of binary sources with the Indian X-ray Astronomy Experiment

Studies of several x-ray binaries have been made with the Indian X-ray Astronomy Experiment on the IRS-P3 satellite. We present here results from timing analysis of four of the neutron star and black hole binaries; namely GRS 1915+105, the recently discovered x-ray transient XTE J1748-288 and the x-ray pulsars Cep X-4 and 4U1907+09. New results include detection of a variety of regular and irregular bursts from GRS 1915+105 and detection of irregular intensity variations in the transient source XTE J1748-288. The x-ray pulse profile of 4U1907+09 was found to have changed from a double-peak pulse observed in August 4–7, 1996 to a broad single-peak pulse in May 26 – June 3, 1998 period. Details of these results are presented.

Proposed beatwave experiment at RAL with the Vulcan CPA laser

A chirped pulse amplification (CPA) laser configuration capable of driving a plasma beat wave into saturation before modulation instabilities can grow is reported. The proposal is based on generating a single sub-ps, broad bandwidth pulse (/spl sim/16 nm) and stretching and filtering to select two wavelength components (separation /spl sim/7 nm). The two spectral components are temporally stretched (to >100 ps) and separated (by /spl sim/1 ns). The pulses are then amplified sequentially in a single Nd:glass chain to greater than 15 J per pulse. Using a single-pass reflective grating compressor, the pulses are compressed (from 2 to 5 ps) and automatically synchronized. The system is capable of introducing a chirp, such that the compressed pulses can compensate for relativistic detuning of the plasma wave. The optimum laser amplification and recombination configuration for generating a saturated laser-driven beatwave is presented, and options for future work are discussed.

Effect of guiding filters on the behavior of dispersion-managed solitons

We show that guiding filters fundamentally alter the behavior of dispersion-managed soltions by making the pulse energy nearly independent of path-average dispersion (D?) in the neighborhood of D?=0 . This fact enables one to design maps permitting adequate pulse energy with narrow-bandwidth, temporally broad pulses for the attainment of high spectral efficiency and reduced nonlinear penalties in wavelength-division multiplexing.

Design of short dispersion decreasing fibre forenhanced compression of higher-ordersoliton pulses around 1550 nm

Compression of higher-order (N ≃ 3) soliton pulses in standard singlemode fibre followed by compression in short dispersion decreasing fibre (DDF) can produce ultra-short fundamental solitons. This design scheme uniquely enables high-quality compression of broad pulses (< 30 ps) down to < 142 fs over a wide input wavelength range using a remarkably short DDF (< 455 m).

A two component model of the crab pulsar emission based on the BeppoSAX pulse profiles

We propose a two component model for the emission of PSR B0531+21 (Crab pulsar) to describe the energy dependence of the pulse profile and, in particular, of the relative peak (and interpeak) intensities. We show that the energy change of the X-ray pulse profile can be well reproduced by adding to the optical profile a unique broad pulse component whose intensity scales with the energy.

Dynamical properties of trions and excitons in modulation doped CdTe/CdMgZnTe quantum wells

We report on the population and phase relaxation of neutral excitons (X) and positively charged excitons (X +), called trions, in p modulation doped CdTe/CdMgZnTe multiple quantum wells. Time-resolved photoluminescence (PL) measurements which investigate the population dynamics show that trions and excitons are in thermal equilibrium with each other and allow to determine the lifetimes of trions and excitons. The coherent dephasing of both quasiparticles is studied by degenerate four-wave mixing (FWM). The slower dephasing of trions compared with excitons is interpreted in terms of trion and hole localization. Under excitation with spectrally broad femtosecond laser pulses, the FWM traces show modulations due to quantum beats between the trion and exciton transitions.

Seismic evidence for an inner core transition zone

Seismic waves that traverse Earth's inner core along north-south paths produce unusually broad pulse shapes at long periods (compared with waves along east-west paths) and reflections from below the inner core boundary at short periods. The observations provide compelling evidence for a seismic velocity discontinuity along north-south paths about 200 kilometers below the inner core boundary separating an isotropic upper inner core from an anisotropic lower inner core. The triplication associated with such a structure might be responsible for reported waveform complexity of short-period inner core arrivals along north-south paths and, if the depth of the boundary is laterally variable, their large travel-time variation.

Nonlinear propagation of laser-generated acoustical pulses in granulated media

Laser generation of high-intensity broad frequency band acoustical pulses in different fluids and their propagation in water-saturated cobalt-manganese crust (CMC) samples are investigated. Acoustic pulses of amplitude up to 3 MPa and duration about 1 s are generated in water, oil, and benzene by the action of CO2 on a laser pulse. The propagation of the short acoustic pulses through the water-saturated CMC samples is considered. The complexity of the CMC structure leads to specific nonlinear acoustical features of the process—nonlinear distortion of the acoustic pulse and amplitude-dependent attenuation. Abnormal sound attenuation in the high-frequency band is observed with low-energy probing sound pulses. This effect could be connected with the sound energy capture and its localization as oscillating modes of the structure. This effect vanishes with increasing intensity of the probing pulses, apparently due to the changes in crust structure. At the same time, specific nonlinear distortion of the sound pulse takes place—increasing of the acoustic pulse amplitude leads to decreasing of the negative phase of the passed acoustic pulse. [Work supported in part by Russian Fund for Fundamental Research.]