Allowed and forbidden Raman scattering mechanisms for detection of coherent LO phonon and plasmon-coupled modes in GaAs

Abstract

Detection mechanisms of coherent phonons in variously doped GaAs are investigated by transient reflectivity method with photoexcitation near the E0 gap and probe near either the E0 or E1 gaps. By varying the probe light polarization angle, the coherent amplitudes of both the LO phonon and the LO phonon-plasmon coupled (LOPC) modes show evidence for an interference between their anisotropic and isotropic dielectric response components. We attribute the anisotropic and isotropic components to the reflectivity modulation by lattice and electronic polarizations via dipole-allowed and dipole-forbidden Raman scattering processes. The forbidden processes are resonantly enhanced at the E0 and E1 critical points due to the relaxation of momentum conservation and the strong built-in electric field near the surface. The relative contribution of the anisotropic and isotropic components depends on the modes (LO or LOPC) as well as the probe wavelength (E0 or E1), because of the different Raman scattering mechanisms involved. Reflectivity measurements with the near ultraviolet light, which is used to probe the E1 gap, also enable highly surface sensitive detection of ultrafast LO phonon-plasmon dynamics, which is strongly depth-dependent in the depletion layer of n-doped GaAs.

Ultrafast changes in lattice symmetry probed by coherent phonons

“…3c is not the result of a damped oscillation of the M 1 phase. Such a feature may correspond to plasmons 28 or even squeezed phonons 29 that cannot be induced by the weaker second pulse. However, at these high fluences, the intensity in the wings of P 1 , which would otherwise be negligible, may become significant and could also affect the optical signal.…”

Chiral Phase Transition in Charge Ordered1T−TiSe2

“…Recently, it was discovered that the breakdown of inversion symmetry in the charge ordered phase of 1T-TiSe 2 leads to the presence of a chiral structure at low temperatures [3][4][5][6][7][8][9]. In this phase, a helical charge density distribution arises from a rotation of the dominant charge density wave component as one progresses through consecutive atomic layers.…”

“…Additional symmetries may be broken through the coupling to atomic displacements, and these are often key in understanding the material properties of charge ordered systems. The broken rotational symmetry in 2H-TaSe 2 , for example, yields a reentrant phase transition under pressure [1], while the broken inversion symmetry in rare-earth nickelates RNiO 3 , renders them multiferroic [2].Recently, it was discovered that the breakdown of inversion symmetry in the charge ordered phase of 1T-TiSe 2 leads to the presence of a chiral structure at low temperatures [3][4][5][6][7][8][9]. In this phase, a helical charge density distribution arises from a rotation of the dominant charge density wave component as one progresses through consecutive atomic layers.…”

Comment on “Charge-parity symmetry observed through Friedel oscillations in chiral charge-density waves”

“…This particular description of the chiral charge density wave is invoked by Ishioka et al in their equation (1). 4 Because both the non-chiral and the chiral charge density wave consist of a superposition of the same three propagation vectors, it is immediately clear that a parameter which only involves these vectors cannot distinguish between these states. Instead, it is essential to take into account the relative phase differences between the charge density wave components.…”

“…[1][2][3] In this state, the direction of the dominant component of a triple-q charge density wave rotates as one progresses in the direction perpendicular to the atomic layers. In their recent publication, 4 Ishioka et al analyze scanning tunneling microscopy images of the chiral phase of 1T-TiSe 2 in terms of a parameter H CDW , whose sign is suggested to reflect the handedness of the chiral state. Here we point out that the sign of this parameter is not well defined in 1T-TiSe 2 , and that consequently H CDW cannot be used as an order parameter for the chiral state.…”