We devise a new high order local absorbing boundary condition (ABC) for radiating problems and scattering of time-harmonic acoustic waves from obstacles of arbitrary shape. By introducing an artificial boundary S enclosing the scatterer, the original unbounded domain Ω is decomposed into a bounded computational domain Ω − and an exterior unbounded domain Ω + . Then, we define interface conditions at the artificial boundary S , from truncated versions of the well-known Wilcox and Karp farfield expansion representations of the exact solution in the exterior region Ω + . As a result, we obtain a new local absorbing boundary condition (ABC) for a bounded problem on Ω − , which effectively accounts for the outgoing behavior of the scattered field. Contrary to the low order absorbing conditions previously defined, the order of the error induced by this ABC can easily match the order of the numerical method in Ω − . We accomplish this by simply adding as many terms as needed to the truncated farfield expansions of Wilcox or Karp. The convergence of these expansions guarantees that the order of approximation of the new ABC can be increased arbitrarily without having to enlarge the radius of the artificial boundary. We include numerical results in two and three dimensions which demonstrate the improved accuracy and simplicity of this new formulation when compared to other absorbing boundary conditions.

We demonstrate the strong coupling of both magnons and phonons to terahertz (THz) frequency electromagnetic (EM) waves confined to a photonic crystal (PhC) cavity. Our cavity consists of a two-dimensional array of air-holes cut into a hybrid slab of ferroelectric lithium niobate (LiNbO3) and erbium orthoferrite (ErFeO3), a canted antiferromagnetic crystal. The phonons in LiNbO3 and the magnons in ErFeO3 are strongly coupled to the electric and magnetic field components of the confined EM wave, respectively. This leads to the formation of new cavity magnon-phonon-polariton modes, which we experimentally observe as a normal-mode splitting in the frequency spectrum and an avoided crossing in the temperature-frequency plot. The cavity also has a mode volume of V = 3.4 × 10 −3 λ 3 ≃ 0.5(λ/n) 3 µm 3 and can achieve a Q-factor as high as 1000. These factors facilitate the pursuit of the fields of THz cavity spintronics and quantum electrodynamics. arXiv:1707.03503v1 [physics.optics]

We present a direct comparison between resonant terahertz (THz) and nonresonant impulsive stimulated Raman scattering (ISRS) excitation of phonon-polaritons in ferroelectric lithium niobate. THz excitation offers advantages of selectively driving only the forward propagating phonon-polariton mode to exceedingly high amplitudes, without complications due to nonlinear processes at the high 800 nm pump fluences used in Raman excitation. At peak-to-peak THz electric field strengths exceeding 1 MV/cm, the ferroelectric lattice is driven into the anharmonic regime, allowing experimental determination of the shape of the potential energy surface.Ultrafast control over a crystalline lattice is of interest in developing a basic understanding of how light can influence material properties, as well as potential practical applications such as the development of ultrafast switches and a variety of optoelectronic applications [1][2][3][4]. Strong THz radiation, with frequencies resonant to the modes of interest, has been promoted as a preferred means of lattice control [1] over two common routes: displacive excitation [5,6] and nonresonant Raman excitation of vibrations [7][8][9]. In typical displacive-type excitation, energy is deposited into the material's electronic subsystem, transiently distorting the lattice. Such excitation may coherently drive many modes simultaneously, but potentially leaves large amounts of (unwanted) incoherent thermal energy. Nonresonant ultrafast Raman excitation, termed impulsive stimulated Raman scattering (ISRS) [7-9], can coherently excite lattice modes without the excess thermal energy, but it is an inefficient process and thus extremely high-fluence laser pulses are required to a

To realize the full promise of terahertz polaritonics (waveguide-based terahertz field generation, interaction, and readout) as a viable spectroscopy platform, much stronger terahertz fields are needed to enable nonlinear and even robust linear terahertz measurements. We use a novel geometric approach in which the optical pump is totally internally reflected to increase the distance over which optical rectification occurs. Velocity matching is achieved by tuning the angle of internal reflection. By doing this, we are able to enhance terahertz spectral amplitude by over 10x compared to conventional single-pass terahertz generation. An analysis of the depletion mechanisms reveals that 3-photon absorption and divergence of the pump beam are the primary limiters of further enhancement. This level of enhancement is promising for enabling routine spectroscopic measurements in an integrated fashion and is made more encouraging by the prospect of further enhancement by using longer pump wavelengths.

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