Ultra-fast dynamics of coherent lattice and spin excitations at the Gd(0001) surface

The Gd(0001) surface is investigated by pump–probe experiments using femtosecond laser pulses at 740–860 nm wavelength. Employing optical second-harmonic generation, spin and lattice dynamics are separated through the symmetry of optical field contributions that are even and odd with respect to magnetization reversal. A coherent phonon–magnon mode at a frequency of 3 THz that is excited through the exchange-split surface state is observed in the time domain. A magneto-elastic phonon–magnon interaction based on spin–orbit coupling is weak for Gd and a modulation of the exchange interaction mediated by the lattice vibration is proposed as a microscopic interaction mechanism of this coupled mode. In parallel, electron–electron and electron–phonon interactions and their magnetic counterparts lead to incoherent dynamics of the electron, lattice, and spin subsystems. Variation of the optical wavelength shows that for longer wavelengths up to 860 nm the coherent mode dominates, while for shorter ones (≥740 nm) incoherent contributions prevail. This dependence indicates that selective depopulation of the occupied surface state component drives the coherent excitation. However, temperature-dependent studies show that the oscillation amplitude of even and odd contributions scales with the spin polarization of the surface state, suggesting that the spin dependence of the ion potentials contributes as well. Furthermore, the frequency of the coherent mode presents a blue shift with a delay of 0.17 THz/ps that saturates at the static frequency of the respective bulk phonon. This behavior is a consequence of equilibration of the screened ion potential at the surface subsequent to the intense laser excitation. PACS 78.47.+p; 63.22.+m; 63.20.Ls; 75.30.Ds     

Interaction of phonons and dirac fermions on the surface of Bi2Se3: a strong Kohn anomaly

We report the first measurements of phonon dispersion curves on the (001) surface of the strong three-dimensional topological insulator Bi2Se3. The surface phonon measurements were carried out with the aid of coherent helium beam surface scattering techniques. The results reveal a prominent signature of the exotic metallic Dirac fermion quasiparticles, including a strong Kohn anomaly. The signature is manifest in a low energy isotropic convex dispersive surface phonon branch with a frequency maximum of 1.8 THz and having a V-shaped minimum at approximately 2kF that defines the Kohn anomaly. Theoretical analysis attributes this dispersive profile to the renormalization of the surface phonon excitations by the surface Dirac fermions. The contribution of the Dirac fermions to this renormalization is derived in terms of a Coulomb-type perturbation model.     

Femtosecond formation of coupled phonon-plasmon modes in InP: Ultrabroadband THz experiment and quantum kinetic theory

The ultrafast transition of an optical phonon resonance to a coupled phonon-plasmon system is studied. After 10-fs photoexcitation of i-InP, the buildup of coherent beats of the emerging hybrid modes is directly monitored via ultrabroadband THz spectroscopy. The anticrossing is mapped out as a function of time and density. A quantum kinetic theory of microscopic carrier-carrier and carrier-LO-phonon interactions explains the delayed formation of the collective modes. The buildup time is quantitatively reproduced to scale with the oscillation cycle of the upper branch of the coupled resonance.     

Asynchronous sampling for ultrafast experiments with low momentum compaction at the ANKA ring

A high‐repetition‐rate pump–probe experiment is presented, based on the asynchronous sampling approach. The low‐α mode at the synchrotron ANKA can be used for a time resolution down to the picosecond limit for the time‐domain sampling of the coherent THz emission as well as for hard X‐ray pump–probe experiments, which probe structural dynamics in the condensed phase. It is shown that a synchronization of better than 1 ps is achieved, and examples of phonon dynamics of semiconductors are presented.     

Generation mechanism for coherent LO phonons in surface-space-charge fields of III-V-compounds

We report on details of coherent LO phonon generation in surface-space-charge regions of III-V-compounds by optical injection of free carriers with laser pulses of 50 fs duration at 2 eV. Both the dynamics of the transient surface field, as well as the coherent lattice vibration, are measured via electro-optic sampling techniques under different experimental conditions. The driving force for the coherent phonon vibration is the sudden depolarization of the crystal lattice due to ultrafast screening of the intrinsic electrical surface field by photoexcited free carriers.     

Ultrafast dynamics of a charge-transfer dimer as a model for the photoinduced phase transition of charge-transfer compounds

By applying ultrafast pump-probe spectroscopy with 15 fs temporal resolution to (TMTTF(+))(2) dimers we provide a full picture of the structural relaxation following photoexcitation of their CT transition. Both population and coherent phonon dynamics allow tracking wave packet motion onto the multidimensional excited state potential energy surface, as obtained by density functional theory calculations. We show that the vibrations that are strongly coupled to the charge-transfer transition of the dimer correspond to those driving the photoinduced phase transition occurring in charge-transfer crystalline solids.     

双能态自旋-晶格声子耦合量子隧道系统的非经典能态和量子相干耗散

采用关联表象变分波函数方案, 介入三个非经典关联效应, 求解有限温度双能态自旋-晶格声子耦合量子隧道系统的非经典态, 着重研究化解由于粒子自旋-单声子相互作用引起的量子涨落导致双能态系统的退相干性量子耗散. 这三个非经典关联效应是: 1) 声子位移-粒子自旋 (σ_z)间强非绝热关联; 2) 声子压缩态效应及其伴随发生的单声子相干态-声子压缩态两过程相干效应; 3) 由关联表象导致的声子位移(U_D)与声子压缩(U_S)的表象关联非绝热修正. 结果表明: 由于引入粒子自旋-双声子相互作用, 大幅度地增强了声子场压缩态, 特别是更进一步极大幅度地增强了非经典压缩-相干态效应. 因此, 由粒子自旋-单声子相互作用产生的Debye-Walle相干弹性散射效应导致量子隧道项(-Δ₀σ_x)的强烈指数衰减及其伴随严重的量子相干损失的极大幅度的抑制, 并且自旋-晶格声子耦合量子隧道系统的非经典态能量大幅度降低. 关键词： 非经典能态 量子隧穿相干损失 自旋-双声子相互作用 压缩相干态效应     

Coherent phonons, nanoseismology and THz radiation in InGaN/GaN heterostructures

The ultrafast optical photoexcitation of hot electrons and holes in semiconductors by femtosecond laser pulses can trigger coherent phonon oscillations. We discuss the huge coherent acoustic phonons which have been generated in InGaN/GaN heterostructures and epilayers and how they might be used in imaging of surfaces and interfaces in nanostructures. We also discuss the THz radiation emitted from these phonons.     

Theoretical and experimental investigations of coherent phonon dynamics in sapphire crystal using femto- second time-resolved coherent anti-Stokes Raman scattering

We report on the theoretical and the experimental investigations of the coherent phonon dynamics in sapphire crystal using the femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) technique. The temporal chirped white-light continuum (WLC) is used for the Stokes pulse, therefore we can perform the selective excitation of the phonon modes without using a complicated laser system. The expected quantum beat phenomenon is clearly observed. The theoretical formulas consist very well with the experimental results. The dephasing times of the excited phonon modes, the wavenumber difference, and the phase shift between the simultaneously excited modes are obtained and discussed. This work opens up a way to study directly high-frequency coherent phonon dynamics in bulk crystals on a femtosecond time scale and is especially helpful for understanding the nature of coherent phonons.     

Resonance effects on coherent phonon generation in lead phthalocyanine crystalline films

We report the first observation of coherent phonons in crystalline lead phthalocyanine (PbPc) films grown on a (0001) sapphire substrate by using a pump–probe technique. Coherent phonon oscillations corresponding to intermolecular (lattice) vibrations in the PbPc film are observed in the frequency region from 1 to 5 THz. It is found that the intensities of coherent phonons are resonantly enhanced at the exciton energies of the Q band observed in the absorption spectrum. PACS 78.47.+p; 78.66.-w; 82.53.Xa; 78.40.Me     

Optical Absorption Spectra and Intraband Dynamics in Terahertz-Driven Semiconductor Superlattice

We have theoretically investigated the optical absorption spectrum and intraband dynamics by subjecting a superlattice to both a terahertz (THz)-frequency driving field and an optical pulse by using an excitonic basis.In the presence of a THz dc field, the satellite structures in the absorption spectra are presented. The satellite structure is a result from the THz nonlinear dynamics of Wannier-Stark ladder excitons. On the other hand, the coherent intraband polarization is investigated. We find that the excitonic Bloch oscillation is driven by the THz field and yields an intraband polarization that continues to oscillate at times much longer than the intraband dephasing time. The temporal evolution of the slowly varying components of the intraband polarization is dependent on the THz frequency.     

On the synergistic effects of laser/phonon-stimulated processes: A master equation approach

The synergistic effects of an adspecies/surface system influenced by a coherent laser field and an incoherent thermal phonon field are theoretically investigated. A generalized master equation is derived in the Schrödinger-Markoff picture and the total stimulated transition rates are decomposed into three parts: thermal phonon, laser and the interference terms. The energy transfer dynamics is pictured via the evolution equation of the average excitation of the active mode. A random phase of the off-diagonal matrix elements induced by the laser-stimulated surface processes is introduced. Finally, possible applications of the laser/phonon-stimulated master equation are discussed.     

太赫兹表面极化激元

作为束缚于表面或界面的电磁波与极性元激发的耦合模量子,表面极化激元是克服衍射极限的核心物理.在紫外、可见以及近红外波段,表面等离子极化激元展现出了亚波长特性,具有高分辨成像等应用,并发展成为"表面等离子极化激元亚波长光学"学科;在中红外波段,表面声子极化激元发挥着同样的作用.太赫兹波段曾是人类认识的空白区域,近三十年来得以高速发展,其战略意义重大.具有克服衍射极限能力的太赫兹表面极化激元同样是小型化与集成化太赫兹器件,以及太赫兹超高分辨成像的重要物理基础.近几年来,对以石墨烯为代表的二维材料的研究突飞猛进,诞生了"石墨烯表面等离子极化激元亚波长光学"这门学科,并贡献于太赫兹领域.本文对可在太赫兹波段工作的人工超构材料、掺杂半导体、二维电子气、二维材料、拓扑绝缘体等结构材料的表面极化激元进行了较为全面的总结与介绍,为研制克服衍射极限的太赫兹集成光子学器件提供可资借鉴的物理基础.     

金刚石氮-空位色心的原子自旋声子耦合机理

金刚石氮-空位色心结构因在量子精密测量领域的高灵敏度优势而备受关注.本文引入耦合声子场对氮-空位色心原子自旋进行共振调控,以提高氮-空位色心的自旋跃迁效率.首先,基于波函数和晶格的点阵位移矢量关系,分析了声子与晶格能量交互作用,研究了基于声子共振调控的氮-空位色心的自旋跃迁机理,建立了基于应变诱导的能量转移声子-自旋交互耦合激发模型.其次,基于氮-空位色心晶格振动理论,引入满足布洛赫定理的系数矩阵,建立了不同轴向氮-空位色心第一布里渊区特征区域的声子谱模型.同时,基于德拜模型,考虑热膨胀效应,解析该声子共振系统的声子热平衡性质,并对其比热模型进行研究.最后,基于分子动力学仿真软件CASTEP和密度泛函理论进行第一性原理研究,构建了声子模式下不同轴向氮-空位色心的结构优化模型,并分析了其结构特性、声子特性和热力学特性.研究结果表明,系统声子模式的演化依赖于氮-空位的占位,声子模式强化伴随着热力学熵的降低.含氮-空位色心金刚石的共价键较纯净无缺陷金刚石更弱,热力学性质更不稳定.含氮-空位色心金刚石的声子主共振频段处于THz量级,次共振频率约为[800,1200]MHz.根据次共振频段设计叉指宽度为1.5μm的声表面波共振机构,其中心频率约为930 MHz.在该声子共振调控参数条件下,声子共振调控方法可有效增大氮-空位色心的自旋跃迁概率,实现氮-空位色心原子自旋操控效率的提高.     

Characteristics of coherent transverse optical phonon in CuI thin films on Au nano-films

We have investigated the coherent phonon in CuI thin films deposited on Au films with nanoscale roughness. It is found that the coherent transverse optical (TO) phonon in the CuI thin film on the Au film is dramatically enhanced, whereas that in the CuI thin film without the Au film is hardly observed. The enhancement of the coherent TO phonon in the CuI thin film on the Au film will originate from the surface enhanced electric field around the surface of the Au film with nanoscale roughness. To clarify the properties of the enhanced coherent phonon, we have investigated the pump-power dependence and pump-polarization dependence of the coherent phonon. We discuss the generation process of the coherent phonon in the CuI thin film on the Au film as compared with the pump-power dependence and the pump-polarization dependence.     

Anharmonic renormalization and non-Markovian decay of coherent optical phonons in polar semiconductors

The influence of anharmonic renormalization effects on the decay dynamics of coherent longitudinal optical phonons is investigated from a microscopic point of view. Time-resolved coherent anti-Stokes Raman signals are calculated for GaP on the basis of a full phonon dispersion calculation, and the relevant decay channels are identified and compared. Anharmonic renormalization effects are found to induce non-Markovian behaviour of the decay dynamics and lead to a decrease of the decay time. The renormalization effects only depend on the special properties of the phonon dispersion of the given material. This underlines the intrinsic nature of the non-Markovian decay dynamics of phonons for any material. Non-Markovian dynamics of the decay of coherent LO-phonons is calculated for GaP and result in a 30% faster decay signal than the corresponding Markovian dynamics.     

Dynamical Properties of Coherent Plasmons Coupled with LO Phonons in an InAs/GaAs Strained Superlattice

We have investigated the coherent plasmon photoexcited in the electron miniband of an InAs/GaAs strained superlattice (SL) using a reflection-type pump-probe technique. Under the condition that the center energy of the pump pulses is tuned to the fundamental transition energy of the SL, the coherent plasmon coupled with the longitudinal optical (LO) phonon of GaAs that is the barrier layer is observed. The time-domain signals of the coherent plasmon coupled with the GaAs-like LO phonon drastically change with varying the generation and detection energies. From the energy dependence of the time-domain signals, we discuss the relaxation process of the carriers photoexcited in the miniband of the SL.     

The dynamics of a THz Rydberg wavepacket

An optically excited Rydberg wavepacket can be generated by exciting the electron from a low-lying state to a coherent superposition of high-lying states with a short broadband optical pulse. A special kind of Rydberg wavepacket is generated in the case of a interaction of a weak THz half cycle pulse with a stationary Rydberg state, called the THz wavepacket. This THz wavepacket is a coherent superposition of the initial Rydberg state and its neighbouring states. We have investigated the time evolution of THz wavepackets by measuring the impact of two in time delayed half cycle pulses ( ≈ 200 V cm^-1) on the population of a stationary (n = 40) Rydberg state in rubidium. The first half cycle pulse creates the THz wavepacket and the second half cycle pulse probes the dynamics of the THz wavepacket. We support our experimental data by numerically solving the Schr?dinger equation and with a semi-classical picture. Whereas an optically excited wavepacket is initially localized, a THz wavepacket is initially delocalized and becomes localized after half a revival time. Received 23 August 2000 and Received in final form 27 March 2001     

Enhanced Terahertz Phonon Polariton in Lithium Niobate Chip

Terahertz (THz) phonon polaritons, fundamental quasi-particles that couple lattice vibrations with electromagnetic fields at THz frequencies, are found in a variety of materials that offer the potential for a wide range of THz optoelectronic devices and on-chip integrated systems. However, these compact devices and on-chip systems are hampered by the absence of on-chip powerful THz phonon polariton sources. In this study, the efficient generation and amplification of THz phonon polaritons are proposed and directly visualized on a lithium niobate (LN) chip via a tilted pulsefront phase matching technique. By combining lateral pumping and phase matching schemes, two orders of magnitude are successfully attained in the interaction distance between the pump light and the LN chip, accompanied by a substantial amplification of generated THz phonon polariton. The results of this study may lead to abundant potential applications in chip scale THz photonic devices and systems based on LN materials and its integrated heterostructures.     

Ultrafast Fano resonance between optical phonons and electron-hole pairs at the onset of quasiparticle generation in a semiconductor

Based on the many-body time-dependent approach applied to the ultrafast time region, we investigate the dynamics of creation of an optical phonon incorporating with the electron-hole continuum in a semiconductor. In the transient Fano resonance, due to an interference between those sharp (optical phonon) and continuum (electron-hole pair) quasiparticles, we find the robust destructive interference at birth of them, i.e., tau approximately 0 if the created phonon is coherent under the irradiation of ultrashort optical pulses. The origin is found to be the potential scattering of the electron-hole pair by the q=0 coherent phonon. This finding agrees well with the recent experiment.