Monochromatic Pulses Research Articles

Nonselective excitation of pulsed ELDOR using multi-frequency microwaves

The use of a polychromatic microwave pulse to expand the pumping bandwidth in pulsed electron–electron double resonance (PELDOR) was investigated. The pumping pulse was applied in resonance with the broad (∼100 mT) electron paramagnetic resonance (EPR) signal of the manganese cluster of photosystem II in the S 2 state. The observation pulses were in resonance with the narrow EPR signal of the tyrosine radical, Y D . It was found that in the case of the polychromatic pumping pulse containing five harmonics with the microwave frequencies between 8.5 and 10.5 GHz the PELDOR effect corresponding to the dipole interaction between the Mn cluster and Y D was about 2.9 times larger than that achieved with a monochromatic pulse. In addition to the dipolar modulation, the nuclear modulation effects were observed. The effects could be suppressed by averaging the PELDOR trace over the time interval between the observation microwave pulses. The polychromatic excitation technique described will be useful for improving the PELDOR sensitivity in the measurements of long distances in biological samples, where the pair consists of a radical with a narrow EPR spectrum and slow phase relaxation, and a metal center that has a broad EPR spectrum and a short phase relaxation time.

Modeling of electromagnetic waves in axisymmetric geometry using a Laguerre spectral method

A spectral method for modeling high-frequency electromagnetic waves in axisymmetric geometry is proposed.The method is based on the expansion of the solutions of Maxwell’s equations in Laguerre functions in the time region. The spectral method is used to solve Maxwell’s equations for both 2D media and stratified media. In the case of stratified media, a Fourier–Bessel expansion in the radial variable is used. The effectiveness of the spectral and finite-difference methods is compared. Harmonic solutions and solitary solutions by the Laguerre method are considered, and the dynamics of monochromatic and broadband electromagnetic pulses are examined.

On the Peculiarities of THz Radiation Generation in a Laser Induced Plasmas

Experimental data on terahertz generation from laser spark created by linearly polarized femtosecond pulses focused by spherical lenses are presented. Specific features of generated terahertz radiation registered in the latest experiments for the case of monochromatic laser pulse definitely demonstrate severe deviation from the axial symmetry, which results from conventionally used model for low-frequency current excitation in a laser spark due to the ponderomotive action from fs laser pulse. The new approach to the theoretical interpretation basing on kinetic description for electron distribution function has been developed with taking into account peculiarities of the tunnel ionization under the action of linearly polarized laser field. A simple generalization of developed model for the case of applying external dc electric field to the region of gas breakdown allowed to describe the modification of terahertz radiation pattern depending on the value of external dc field.

Femtosecond Radiation Experiment Detector for X-Ray Free-Electron Laser (XFEL) Coherent X-Ray Imaging

A pixel array detector (PAD) module has been developed at Cornell University for the collection of diffuse diffraction data in anticipation of coherent X-ray imaging experiments that will be conducted at the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory. The detector is designed to collect X-rays scattered from monochromatic femtosecond pulses produced by the LCLS X-ray laser at framing rates up to 120 Hz. Because X-rays will arrive on femtosecond time scales, the detector must be able to deal with instantaneous count-rates in excess of 1017 photons per second per pixel. A low-noise integrating front-end allows the detector to simultaneously distinguish single photon events in low-flux regions of the diffraction pattern, while recording up to several thousand X-rays per pixel in more intense regions. The detector features a per-pixel programmable two-level gain control that can be used to create an arbitrary 2-D, two-level gain pattern across the detector; massively parallel 14-bit in-pixel digitization; and frame rates in excess of 120 Hz. The first full-scale detector will be 1516 x 1516 pixels with a pixel size of 110 X 110 microns made by tiling CMOS ASICs (Application Specific Integrated Circuits) that are bump-bonded to high-resistivity silicon diodes. X-ray testing data of the first 185 X 194 pixel bump-bonded ASICs is presented. These are tiled to make the final detector. The measurements presented include confirmation of single photon sensitivity, pixel response profiles indicating a nearly single-pixel point spread function, radiation damage measurements and noise performance.

Double-slit vacuum polarization effects in ultraintense laser fields

The influence of the strong laser-driven vacuum on a propagating electromagnetic probe wave has been studied in detail. We investigate two scenarios comprising a focused probe laser beam passing through a region of vacuum polarised by an ultra-intense laser field. By splitting this strong field into two, separated, monochromatic Gaussian pulses counter-propagating in a plane perpendicular to the probe field axis, we demonstrate a leading order light-by-light diffraction effect that generates an interference pattern reminiscent of the classic double-slit experiment. We calculate the total number of probe photons diffracted as well as the number diffracted into regions where the vacuum polarisation signal is higher than the probe background. In addition, we calculate the induced ellipticity and polarisation rotation in the probe beam and show how, in the realistic situation in which the centres of the two strong fields are not exactly aligned, certain ranges of beam separation and observation distance may actually lead to an increase over the idealised case of a single strong laser beam.

Analytic description of high-order harmonic generation by atoms in a two-color laser field

A closed-form analytic formula describing high-order harmonic generation (HHG) in a two-color field of frequencies $\ensuremath{\omega}$ and $2\ensuremath{\omega}$ is derived quantum mechanically in the low-frequency (tunneling) limit for an electron bound by a short-range potential and generalized to the case of an active electron in a neutral atom. The HHG rates are presented as a product of an electron wave packet describing the ionization of an active electron and its propagation in a laser field up to the recombination event and an atom-specific cross section of the electron's photorecombination. In contrast to the case of a monochromatic laser pulse [Frolov et al., Phys. Rev. Lett. 102, 243901 (2009)], the two-color wave packet involves the interference of two terms (involving the Airy function) that describe the emission of harmonics during the first and second half-cycles of the fundamental laser cycle and give rise to the two-plateau structures in the HHG spectra. For the case of the H atom, we show that our analytic results are in good agreement with those obtained from a numerical solution of the three-dimensional time-dependent Schr\odinger equation. The factorization formula is used for describing the dependence of HHG rates for inert gases on the relative phase and intensities of the $\ensuremath{\omega}$ and $2\ensuremath{\omega}$ components of a laser field. It is shown that atomic structure (including electron correlation) effects can modify substantially the two-color HHG spectra of inert gases.

The application of frequency swept pulses for the acquisition of nuclear quadrupole resonance spectra

The acquisition of nuclear quadrupole resonance (NQR) spectra with wideband uniform rate and smooth truncation (WURST) pulses is investigated. 75As and 35Cl NQR spectra acquired with the WURST echo sequence are compared to those acquired with standard Hahn-echo sequences and echo sequences which employ composite refocusing pulses. The utility of WURST pulses for locating NQR resonances of unknown frequency is investigated by monitoring the integrated intensity and signal to noise of 35Cl and 75As NQR spectra acquired with transmitter offsets of several hundreds kilohertz from the resonance frequencies. The WURST echo sequence is demonstrated to possess superior excitation bandwidths in comparison to the pulse sequences which employ conventional monochromatic rectangular pulses. The superior excitation bandwidths of the WURST pulses allows for differences in the characteristic impedance of the receiving and excitation circuits of the spectrometer to be detected. Impedance mismatches have previously been reported by Marion and Desvaux [D.J.Y. Marion, H. Desvaux, J. Magn. Reson. (2008) 193(1) 153–157] and Muller et al. [M. Nausner, J. Schlagnitweit, V. Smrecki, X. Yang, A. Jerschow, N. Muller, J. Magn. Reson. (2009) 198(1) 73–79]. In this regard, WURST pulse sequences may afford an efficient new method for experimentally detecting impedance mismatches between receiving and excitation circuits, allowing for the optimization of solids and solution NMR and NQR spectrometer systems. The use of the Carr–Purcell Meiboom–Gill (CPMG) pulse sequence for signal enhancement of NQR spectra acquired with WURST pulses and conventional pulses is also investigated. Finally, the utility of WURST pulses for the acquisition of wideline NQR spectra is demonstrated by acquiring part of the 63/65Cu NQR spectrum of CuCN.

Review of recent efforts on efficient generation of monochromatic THz pulses based on difference-frequency generation

We review our recent efforts on power scaling of THz pulses generated from several nonlinear-optical crystals. By using a high-resistivity GaP crystal, we have significally increased the output peak power to as high as 722 W. By stacking three GaP wafers, we have further increased the highest output peak power to 2.36 kW. On the other hand, by using CO2 laser pulses, we have obtained the average output power of 260 μW. We have also used these laser pulses to scale up the output power for the THz pulses to 29.8 μW by stacking GaAs wafers. Indeed, by stacking up to ten wafers, we have increased the output power by a factor of 160. Finally, by using ultrafast laser pulses, we have achieved record-high output powers for the THz pulses generated from multi-period periodically-poled LiNbO3 crystals based on a backward configuration. The highest output power obtained by us so far is 10.7 μW.

Power scaling of widely-tunable monochromatic terahertz radiation by stacking high-resistivity GaP plates

A high-resistivity GaP crystal was used to generate monochromatic THz pulses with peak output powers reaching 722 W at 108.1 μm by mixing two coherent beams at about 1 μm based on phase-matched difference-frequency generation. By stacking two and three GaP plates with their second-order nonlinear coefficients being switched between the adjacent ones, we have increased the peak power at 120.3 μm from 433.4 W to 1.36 and 2.36 kW, respectively. 2.36 kW corresponds to the photon conversion efficiency of 25%, which is two orders of magnitude higher than our previous result. In contrast, if they are stacked for having the same sign of the nonlinear coefficients, the wavelength corresponding to the highest peak power is red-shifted to 204.8 and 303.9 μm, respectively. Such a result indicates that there is an optimal interaction length for each specific output wavelength.

Time domain normal mode analysis of underwater acoustic wave propagation for a single layered acoustic channel in two dimensional Cartesian coordinates

A new analytical time domain Normal Mode solution in a single layer acoustic waveguide in Cartesian coordinates is presented in the thesis. The method is based on the separation of variables technique for the two dimensional time domain wave equation. After separation, Sturm-Liouville type ordinary differential equation for depth dependency and inhomogeneous Klein-Gordon type partial differential equation for time and range dependency are obtained. Whenever the ordinary differential equation is solved in a classical manner with the application of Dirichlet boundary condition on the upper and lower boundaries of the waveguide, Inhomogeneous Klein-Gordon type partial differential equation is solved by using the Green function technique in time domain. Then the exact solution satisfying the causality principle is constructed for the acoustic pressure. The main advantage of the proposed causal solution is not necessary to use the Fourier Transformation to obtain time domain response of arbitrary acoustical source signals having the monochromatic and Gaussian pulse type time dependency in the waveguide. The excellent agreement for the comparisons of the range dependent transmission losses is observed between the proposed solution and the KRAKEN program.

Photoprocesses with the sub-Doppler spectral selectivity excited by the monochromatic optical pulse in a thin gas cell

The method of sub-Doppler spectroscopy is theoretically elaborated, which is based on the specific dynamics of a number of optically excited atomic particles (atoms or molecules) of a rarefied gas medium in a thin cell after the action of the resonance pulse of the monochromatic radiation. Corresponding calculations are carried out on the basis of density matrix equations for the resonance optical transition between Zeeman degenerate ground and excited quantum levels of particles in case of the linear polarization of the laser pulse at its normal incidence on the cell. The situation is considered when the radiative lifetime of the excited level is much more than the characteristic transit time of particles between nearest plane-parallel walls of the cell. Then the distribution of a number of excited particles versus the pulse frequency detuning narrows in the process of particles collisions with walls of the cell after action of the laser pulse. The factor of such a narrowing (in comparison with the Doppler broadening of the spectral line of the resonance transition) may be more than the ratio of the characteristic transverse size of the thin gas cell to its inner thickness. We discuss possible use of given sub-Doppler resonances (of the number of excited particles) in the high-resolution spectroscopy and also in high-selective processes of photo-ionization and photo-dissociation, especially, for isotope (or isomer) separation and detection of rare (in particular single) atoms or molecules of a gas medium.

Quantum interference in laser-assisted photo-ionization excited by a femtosecond x-ray pulse

The photoelectron energy spectra (PESs) excited by monochromatic femtosecond x-ray pulses in the presence of a femtosecond laser are investigated. A PES is composed of a set of separate peaks, showing interesting comb-like structures. These structures result from the quantum interferences between photoelectron wave packets generated at different times. The width and the localization of each peak as well as the number of peaks are determined by all the laser and x-ray parameters. Most of peak heights of the PES are higher than the classical predictions.

NMR and the uncertainty principle: How to and how not to interpret homogeneous line broadening and pulse nonselectivity. IV. (Un?)certainty

AbstractFollowing the treatments presented in Parts I, II, and III, I herein address the popular notion that the frequency of a monochromatic RF pulse as well as that of a monochromatic FID is “in effect” uncertain due to the (Heisenberg) Uncertainty Principle, which also manifests itself in the fact that the FT‐spectrum of these temporal entities is spread over a nonzero frequency band. I will show that the frequency spread should not be interpreted as “in effect” meaning a range of physical driving RF fields in the former, and “spin frequencies” in the latter case. The fact that a shorter pulse or a more quickly decaying FID has a wider FT‐spectrum is in fact solely due to the Fourier Uncertainty Principle, which is a less well known and easily misunderstood concept. A proper understanding of the Fourier Uncertainty Principle tells us that the FT‐spectrum of a monochromatic pulse is not “broad” because of any “uncertainty” in the RF frequency, but because the spectrum profile carries all of the pulse's features (frequency, phase, amplitude, length, temporal location) coded into the complex amplitudes of the FT‐spectrum's constituent eternal basis harmonic waves. A monochromatic RF pulse's capability to excite nonresonant magnetizations is in fact a purely classical off‐resonance effect that has nothing to do with “uncertainty”. Analogously, “Lorentzian lineshape” means exactly the same thing physically as “exponential decay,” and all inferences as to the physical reasons for that decay must be based on independent assumptions or observations. © 2008 Wiley Periodicals, Inc. Concepts Magn Reson Part A 32A: 373–404, 2008.

Distributed Strain Measurement in Steel Bridge with Fiber Optic Sensors: Validation through Diagnostic Load Test

Fiber optic sensing technologies are emerging as valid alternatives for the health monitoring of civil structures. Distributed sensors based on Brillouin scattering add the unique capability of measuring strain and temperature profiles along optical fibers. Measurement is performed by establishing the correlation between fiber strain and temperature, and the frequency shift of the Brillouin backscattered light induced by a monochromatic light pulse. The technology holds potential for use on large structures and integrated transportation infrastructure. Its effectiveness has been assessed through scaled laboratory experiments, whereas field validation is limited to very few demonstration projects conducted to date. This paper presents a pilot application of Brillouin optical time domain reflectometry to measure strain profiles along the steel girders of a continuous slab-on-girder bridge subjected to diagnostic load testing. One of the exterior continuous girders required heat-straightening after falling during construction due to wind. The significance of applying a distributed measurement technique lies in the potential to assess the global girder response, which would be impractical and uneconomical using discrete measurement techniques. A 1.16km long sensing circuit was installed onto the web of four girders. The circuit comprises bare optical fiber sensors, and a novel adhesively bonded fiberglass tape with embedded sensing fibers for strain measurement and thermal compensation. The strain profiles were first converted into deflection profiles and validated against discrete deflection measurements performed with a high-precision total station system. Structural assessment based on comparison of the strain profiles with the results of three-dimensional finite-element analysis of the bridge superstructure, and with specification mandated criteria, indicated that the response of the girder under investigation was within the design limits, and did not pose serviceability concerns. Factors that may affect measurement accuracy are finally discussed on the basis of the experimental and numerical results.

NMR and the uncertainty principle: How to and how not to interpret homogeneous line broadening and pulse nonselectivity. III. Uncertainty?

AbstractFollowing the treatments presented in Parts I and II, I herein discuss in more detail the popular notion that the frequency of a monochromatic RF pulse as well as that of a monochromatic FID is “in effect” uncertain due to the (Heisenberg) Uncertainty Principle, which also manifests itself in the fact that the FT‐spectrum of these temporal entities is spread over a nonzero frequency band. In Part III, I continue my preliminary review of some further fundamental concepts, such as the Heisenberg and Fourier Uncertainty Principles, that are needed to understand whether or not the NMR linewidth and the RF excitation bandwidth have anything to do with “uncertainty”. The article then culminates in re‐addressing our Two NMR Problems in a more conscientious frame of mind by using a more refined formalism. The correct interpretation of these problems will be discussed in Part IV. © 2008 Wiley Periodicals, Inc. Concepts Magn Reson Part A 32A: 302–325, 2008.

Simulation of a multiple-wavelength time-of-flight neutron spectrometer for a long-pulsed spallation source

We have performed design studies and ray-tracing simulations of a direct-geometry spectrometer at the cold moderator of a long-pulsed 5 MW neutron spallation source. A 100 m long guide path makes it possible to achieve 2% elastic resolution at 5 Å incoming neutron wavelength, without a need to shorten the 2 ms long source pulse. Assuming a 16 Hz repetition rate, the 60 ms time between pulses allows us to extract from each source pulse several monochromatic pulses at the sample with a spacing in the range 5–10 ms. These pulses are equidistantly spaced in wavelength with δλ ≈ 0.2–0.4 Å, depending on the spectrometer setting. If all these bands contain useful information, the spectrometer gains up to an order of magnitude in incident flux from the repetition scheme. The incident flux will then reach values of roughly 100 times the present IN5 spectrometer at the ILL at similar resolution and frame length. There are possibilities of optimizing the guide design further, to achieve even higher flux values. Virtual experiments provide us with the final energy resolution of the spectrometer, including the line shapes. The performance of the instrument is found to be excellent, judged by the virtual data. Such an instrument positioned at a long-pulsed neutron spallation source would be very powerful for cold neutron spectroscopy for energy transfers in the range 10 µeV to 10 meV.

NMR and the uncertainty principle: How to and how not to interpret homogeneous line broadening and pulse nonselectivity. II. The Fourier connection

AbstractBy building on the nomenclature introduced and the fundamental ideas discussed in Part I, I now continue my preliminary review of the basic concepts we need for exploring the issues surrounding the uncertainty principles in the context of our “Two NMR Problems” (namely: both the essense of homogeneous NMR line broadening, as well as the fact that a short monochromatic RF pulse exhibits a polychromatic spectral bandwidth, and consequently gives a broad off‐resonance NMR excitation band, are often rationalized in terms of a superficial and/or misguided use of the “uncertainty principle”). To that end, in Part II, I discuss Fourier theory in a way that is hoped to offer a number of delicate, important, and interesting insights that are usually not readily available from more conventional texts. © 2008 Wiley Periodicals, Inc. Concepts Magn Reson Part A 32A: 1–33, 2008.

NMR and the uncertainty principle: How to and how not to interpret homogeneous line broadening and pulse nonselectivity. I. The fundamentals

AbstractBoth the essence of homogeneous NMR line broadening as well as the fact that a short monochromatic radiofrequency pulse exhibits a polychromatic spectral bandwidth and is consequently associated with a broad off‐resonance NMR excitation band are often rationalized in terms of a superficial and/or misguided use of the “Uncertainty Principle.” The problem is of quite fundamental nature, and stems from several deeper “issues” that reach beyond the world of NMR. To explore this topic, we need to go back to some basic concepts including the Fourier transform, time‐frequency analysis, different versions of the “uncertainty principle,” and linear systems theory. My theme will be developed in four subsequent articles. First, by employing an approach that is unconventional in its thematic structure and applied nomenclature, I present an overview of some basic ideas (periodicity, basis functions, harmonic waves, density functions, etc.) that will then pave the way to discussing, in a somewhat avant‐garde manner, the Fourier transform in Part II. © 2007 Wiley Periodicals, Inc. Concepts Magn Reson Part A 30A: 309–348, 2007.

Single-frequency terahertz source pumped by Q-switched fiber lasers based on difference-frequency generation in GaSe crystal

We demonstrate a unique terahertz (THz) source that is compact, utilizes recently developed all-fiber Q-switched lasers, and is based on difference-frequency generation in a GaSe crystal. A single piezo simultaneously Q switched the two fiber lasers by using stress-induced birefringence, to achieve the temporal overlap of pulses from the two fiber lasers. These correlated pulses then combine in the GaSe crystal to produce coherent and highly monochromatic THz pulses. The peak power for this THz source can reach 0.53 mW, corresponding to an average power of 0.43 microW and a conversion efficiency of 4.75 x 10(-7). The estimated linewidth of this THz source can be as narrow as approximately 35 MHz or 1.17 x 10(-3) cm(-1).

On-body propagation loss estimation using method of equivalent sources

A new method for the fast estimation of the on-body propagation loss is proposed. The method is applicable for a variety of propagation models but limited by a far-field zone observation point. The technique is based on the equivalent source theorem. Knowing the propagation losses for an infinitesimal current source, propagation losses of real antennas can be obtained with only the knowledge of their equivalent currents over a surface surrounding the antenna. The method can be applied for any kind of excitation signals and offers reduction in simulation time. It has been verified using both monochromatic and Gaussian pulse excitation and good agreement is obtained compared with a full conventional FDTD model with a simple antenna.