A Spherical Array Approach for Simulation of Binaural Impulse Responses using the Finite Difference Time Domain Method

The finite difference time domain (FDTD) method has direct applications in musical instrument modeling, simulation of environmental acoustics, room acoustics and sound reproduction paradigms, all of which benefit from auralization. However, rendering binaural impulse responses from simulated
data is not straightforward to accomplish as the calculated pressure at FDTD grid nodes does not contain any directional information. This paper addresses this issue by introducing a spherical array to capture sound pressure on a finite difference grid, and decomposing it into a plane-wave density
function. Binaural impulse responses are then constructed in the spherical harmonics domain by combining the decomposed grid data with free field head-related transfer functions. The effects of designing a spherical array in a Cartesian grid are studied, and emphasis is given to the relationships
between array sampling and the spatial and spectral design parameters of several finite-difference
schemes.

Binaural Reproduction of Finite Difference Simulations Using Spherical Array Processing

Due to its efficiency and simplicity, the finite-difference time-domain method is becoming a popular choice for solving wideband, transient problems in various fields of acoustics. So far, the issue of extracting a binaural response from finite difference simulations has only been discussed in the context of embedding a listener geometry in the grid. In this paper, we propose and study a method for binaural response rendering based on a spatial decomposition of the sound field. The finite difference grid is locally sampled using a volumetric array of receivers, from which a plane wave density function is computed and integrated with free-field head related transfer functions, in the spherical harmonics domain. The volumetric array is studied in terms of numerical robustness and spatial aliasing. Analytic formulas that predict the performance of the array are developed, facilitating spatial resolution analysis and numerical binaural response analysis for a number of finite difference schemes. Particular emphasis is placed on the effects of numerical dispersion on array processing and on the resulting binaural responses. Our method is compared to a binaural simulation based on the image method. Results indicate good spatial and temporal agreement between the two methods.

Quasi-isochoric ion beam heating using dynamic confinement in spherical geometry for X-ray scattering experiments in WDM regime

Extreme states of matter such as Warm Dense Matter “WDM” and Dense Strongly Coupled Plasmas “DSCP” play a key role in many high energy density experiments, however creating WDM and DSCP in a manner that can be quantified is not readily feasible. In this paper, isochoric heating of matter by intense heavy ion beams in spherical symmetry is investigated for WDM and DSCP research: The heating times are long (100 ns), the samples are macroscopically large (mm-size) and the symmetry is advantageous for diagnostic purposes. A dynamic confinement scheme in spherical symmetry is proposed which allows even ion beam heating times that are long on the hydrodynamic time scale of the target response. A particular selection of low Z-target tamper and x-ray probe radiation parameters allows to identify the x-ray scattering from the target material and use it for independent charge state measurements Z* of the material under study.

Attosecond phase locking of harmonics emitted from laser-produced plasmas

Laser-driven coherent extreme-ultraviolet (XUV) sources provide pulses lasting a few hundred attoseconds(1,2), enabling real-time access to dynamic changes of the electronic structure of matter(3,4), the fastest processes outside the atomic nucleus. These pulses, however, are typically rather weak. Exploiting the ultrahigh brilliance of accelerator-based XUV sources(5) and the unique time structure of their laser-based counterparts would open intriguing opportunities in ultrafast X-ray and high-field science, extending powerful nonlinear optical and pump-probe techniques towards X-ray frequencies, and paving the way towards unequalled radiation intensities. Relativistic laser-plasma interactions have been identified as a promising approach to achieve this goal(6-13). Recent experiments confirmed that relativistically driven overdense plasmas are able to convert infrared laser light into harmonic XUV radiation with unparalleled efficiency, and demonstrated the scalability of the generation technique towards hard X-rays(14-19). Here we show that the phases of the XUV harmonics emanating from the interaction processes are synchronized, and therefore enable attosecond temporal bunching. Along with the previous findings concerning energy conversion and recent advances in high-power laser technology, our experiment demonstrates the feasibility of confining unprecedented amounts of light energy to within less than one femtosecond.

Controlling the divergence of high harmonics from solid targets: a route toward coherent harmonic focusing

Harmonic generation from relativistically oscillating plasma surfaces formed during the interaction of high contrast lasers with solid-density targets has been shown to be an efficient source of extreme ultraviolet (XUV) and X-ray radiation. Recent work has demonstrated that the exceptional coherence properties of the driving laser can be mirrored in the emitted radiation, permitting diffraction limited performance and attosecond phase locking of the harmonic radiation. These unique properties may allow the coherent harmonic focusing (CHF) of high harmonics generated from solid density targets to intensities on the order of the Schwinger limit of 10(29) W cm(-2) with laser systems available in the near future [Phys. Rev. Lett. 93, 115002 (2004)] and thus pave the way for unique experiments exploring the nonlinear properties of vacuum on ultra-fast timescales. In this paper we investigate experimentally as well as numerically the prospect of focusing high harmonics under realistic experimental conditions and demonstrate, using particle in cell (PIC) simulations, that precise control of the wavefronts and thus the focusability of the generated harmonics is possible with pre-shaped targets.

High brightness keV harmonics from relativistically oscillating plasma surfaces

X-ray harmonic radiation extending to 3.3 angstrom, 3.8 keV from Petawatt class laser-solid interactions is presented. The harmonic spectra display a relativistic limit scaling up to similar to 3000th order, above which an intensity dependent scaling roll-over is observed. Highly directional beamed emission for harmonic photon energy h nu > 1 keV is found to be into a cone angle

Diffraction-limited performance and focusing of high harmonics from relativistic plasmas

When a pulse of light reflects from a mirror that is travelling close to the speed of light, Einstein's theory of relativity predicts that it will be up-shifted to a substantially higher frequency and compressed to a much shorter duration. This scenario is realized by the relativistically oscillating plasma surface generated by an ultraintense laser focused onto a solid target. Until now, it has been unclear whether the conditions necessary to exploit such phenomena can survive such an extreme interaction with increasing laser intensity. Here, we provide the first quantitative evidence to suggest that they can. We show that the occurrence of surface smoothing on the scale of the wavelength of the generated harmonics, and plasma denting of the irradiated surface, enables the production of high-quality X-ray beams focused down to the diffraction limit. These results improve the outlook for generating extreme X-ray fields, which could in principle extend to the Schwinger limit.

Efficient extreme UV harmonics generated from picosecond laser pulse interactions with solid targets

The generation of high harmonics created during the interaction of a 2.5 ps, 1053 nm laser pulse with a solid target has been recorded for intensities up to 10(19) W cm(-2). Harmonic orders up to the 68th at 15.5 nm in first order have been observed with indications up to the 75th at 14.0 nm in second-order diffraction. No differences in harmonic emission between s and p polarization of the laser beam were observed. The power of the 38th high harmonic at 27.7 nm is estimated to be 24 MW.