飞秒激光脉冲和磁性薄膜中的MnIr层相互作用激发了初始相位为90°，在薄膜中传播速度为4 300 m/s的相干声学声子.该声学声子的振动频率与激光能量密度无关，与该磁性薄膜总厚度成反比.相干声学声子的产生机理可能为飞秒激光激发磁性薄膜的电子，使电子温度急剧上升，随后由于激光吸收而强度减弱形成了一个瞬间的随深度增加而减小的电子温度梯度，使晶格在深度方向相干振荡，即激发了声学声子.另外，外磁场的变化对声学声子的频率的影响在实验误差范围内，说明磁相互作用和晶格中的电相互作用相比是非常弱的.
The interaction between the femtosecond laser pulse and the MnIr layer in the magnetic thin film excites the coherent acoustic phonon with an initial phase of 90° and a propagation velocity of 4 300 m/s. The vibration frequency of the acoustic phonon is independent of the laser energy density and is inversely proportional to the total thickness of the magnetic thin film. And the acoustic phono can propagate in the adjacent metal layers due to the high lattice matching. The electron temperature in the magnetic thin film increases sharply absorbing by femtosecond laser pulse, then an decreasing electron temperature gradient with increasing depth is generated instantaneously by the absorption of the laser which lead to the lattice oscillate coherently in the depth direction, that is, the acoustic phonon. In addition, the frequency of the acoustic phonon changes caused by the applied magnetic field is within the experimental error range, indicating that the magnetic interaction is very weak compared to the electrical interaction in the lattice.
[1] ROSSIGNOL C, RAMPNOUX J M, PERTON M, et al. Generation and detection of shear acoustic waves in metal submicrometric films with ultrashort laser pulses[J]. Physical Review Letters, 2005, 94:166106.
[2] PARK H, WANG X, NIE S, et al. Mechanism of coherent acoustic phonon generation under nonequilibrium conditions[J]. Physical Review B, 2005, 72:100301.
[3] WU S, GEISER P, JUN J, et al. Long-lived, coherent acoustic phonon oscillations in GaN single crystals[J]. Applied Physics Letters, 2006, 88:041917.
[4] ROSSIGNOL C, PERRIN B. Interferometric detection in picoseconds ultrasonics for nondestructive testing of submicrometric opaque multilayered samples:TiN/AlCu/TiN/Ti/Si[J]. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 2005, 52(8):1354-1359.
[5] WANG D, CROSS A, GUARINO G, et al. Time-resolved dynamics of coherent acoustic phonons in CdMnTe diluted-magneticsingle crystals[J]. Applied Physics Letters, 2007, 90:211905.
[6] LIM D, AVERITT R D, DEMSAR J, et al. Coherent acoustic phonons in hexagonal manganite LuMnO3[J]. Applied Physics Letters, 2003, 83(23):4800-4802.
[7] MILLER J K, QI J, XU Y, et al. Near-bandgap wavelength dependence of long-lived traveling coherent longitudinal acousticphonons in GaSb-GaAsheterostructures[J]. Physical Review B, 2006, 74:113313.
[8] WEN Y C, CHOU L C, LIN H H, et al. Efficient generation of coherent acoustic phonons in (111) InGaAs/GaAs multiple quantum wells through piezoelectric effects[J]. Applied Physics Letters, 2007, 90:172102.
[9] THOMSEN C, STRAIT J, VARDENY Z, et al. Coherent phonon generation and detection by picosecond light pulses[J]. Physical Review Letters, 1984, 53(10):989-992.
[10] THOMSEN C, GRAHN H T, MARISETAL H J, et al. Surface generation and detection of phonons by picosecond light pulses[J]. Physical Review B, 1986, 34(6):4129-4138.
[11] OGI H, FUJⅡ M, NAKAMURA N, et al. Resonance acoustic-phonon spectroscopy for studying elasticity of ultrathin films[J]. Applied Physics Letters, 2007, 90:191906.
[12] NAKAMURA N, URANISHI A, WAKITA M, et al. Elastic stiffness of L10 FePt thin film studied by picosecond ultrasonics[J]. Applied Physics Letters, 2011, 98:101911.
[13] HASE M, ISHIOKA K, DEMSAR J, et al. Ultrafast dynamics of coherent optical phonons and nonequilibrium electrons in transition metals[J]. Physical Review B, 2005, 71(18):184301.
[14] ZEIGER H J, VIDAL J, CHENG T K, et al. Theory for displacive excitation of coherent phonons[J]. Physical Review B, 1992, 45(2):768-778.
[15] KUZNETSOV A V, STANTON C J. Theory of coherent phonon oscillations in semiconductors[J]. Physical Review Letters, 1994, 73(24):3243-3246.
[16] 张郑兵, 马小柏, 金钻明, 等. Fe/Si薄膜中相干声学声子的光激发研究[J]. 物理学报, 2012, 61(9):097401.
中国综合性科技类核心期刊(北大核心)