Point source in a phononic grating: stop bands give rise to phonon-focusing caustics

Abstract

We use locally-excited gigahertz surface phonon wavepackets in microscopic line structures of different pitches to reveal profound anisotropy in the radiation pattern of a point source in a grating. Time-domain data obtained by an ultrafast optical imaging technique and by numerical simulations are Fourier transformed to obtain frequency-filtered real-space acoustic field patterns and k-space phononic band structure. The numerically-obtained k-space images are processed to reveal an intriguing double-horn structure in the lowest-order group-velocity surface, which explains the observed non-propagation sectors bounded by caustics, noted at frequencies above the bottom of the first stop band. We account for these phonon-focusing effects, analogous to collimation effects previously observed in two-and three-dimensional lattices, with a simple

Watching surface waves in phononic crystals

“…This temporal and spatio-temporal Fourier transform technique has also been applied to timedomain simulations of the wave fields in this one-dimensional phononic-crystal structure. The simulations were found to agree well with experiment [37,38].…”

“…Time-domain imaging of acoustic wave propagation in phononic crystals was first demonstrated in millimetre-scale structures and liquid/solid systems without the use of optics by means of focused transducers at megahertz frequencies [30][31][32]. More recently, the versatility of time-domain optical interferometry for ultrafast measurements of surface acoustic waves (SAWs) on micrometre-scale solid and solid/air phononic crystals has been demonstrated up to approximately 1 GHz [33][34][35][36][37][38][39]. In this paper, we review results obtained using such ultrafast time-domain imaging of the acoustic field on the surface of microscopic phononic crystals in two spatial dimensions in the range around 100 MHz-1 GHz.…”

Коэффициенты Усиления Потока Фононов В Кристаллах С Различным Типом Анизотропии Упругой Энергии

“…[7] таб-лицы 4,6). Для устранения расходимости фактора уси-ления в точках нулевой кривизны на поверхности мед-ленности в работах [5,[10][11][12] плотность потока фононов усреднялась по площади детектора S. Трудности таких расчетов, а также анализа связи величин коэффициентов усиления с кривизной изоэнергетических поверхностей, как показывают работы [2][3][4][5][7][8][9][10][11][12][13][14][15][16], обусловлены тем, что, хотя исходные выражения для них были сформу-лированы почти 50 лет назад, однако аналитического выражения не было получено. Нами решена эта задача: предложен новый метод расчета коэффициента A λ и получены простые аналитические выражения для него.…”

Numerical and analytical investigation of the influence of porosity on the frequency response of GLARE composite

“…As well as effective-medium models, also numerical simulation can be used to study wave-propagation problems in inhomogeneous media, such as periodic structures [10] or random media [11]. In the presented work, both techniques are applied to model porosity.…”