基于耦合场量子受激拉曼散射的太赫兹波辐射

使用两块长度各为65 mm的MgO:LiNbO3和无掺杂LiNbO3晶体,以1064nm的Q开关Nd:YAG激光器作为抽运光源,在12 mJ/pulse的抽运光能量下得到频率范围为0.34～2.90 THz的电磁辐射.分析表明,激光入射使LiNbO3晶体中具有电磁特性的横光学声子可以和入射光子形成耦合场量子.作为一种电磁特性的元激发,LINbO3晶体的耦合场量子的辐射场频率覆盖部分太赫兹频段范围,并可通过耦合场量子受激拉曼散射过程辐射THz波.根据耦合场量子辐射理论,通过分析晶体的耦合场量子色散特性曲线,可以确定该晶体能否辐射THz波及其带宽范围.     

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.     

Field expulsion and reconfiguration in polaritonic photonic crystals

We uncover a rich set of optical phenomena stemming from the incorporation of polar materials exhibiting transverse phonon polariton excitations into a photonic crystal structure. We identify in the frequency spectrum two regimes in which the dielectric response of the polaritonic medium can induce extreme localization of the electromagnetic energy. Our analysis of the effect of polarization and the interaction between the polariton and photonic band gaps on the Bloch states leads to a pair of mechanisms for sensitive frequency-controlled relocation and/or reconfiguration of the fields.     

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.     

TATB晶体声子谱及比热容的第一性原理研究

利用第一性原理并结合vd W-DF2范德瓦耳斯力校正研究了TATB(C6H6O6N6)晶体声子谱及比热容.采用冷冻声子法计算了TATB晶体声子谱和声子态密度,发现在2.3 THz附近TATB声子态密度最大,证实了太赫兹光谱实验观察到的2.22 THz附近的强吸收峰.基于声子态密度研究了振动模式对比热容的贡献,分析结果表明,常温下0—27.5 THz频段振动模式贡献了比热容的93.7%.同时比较了升温过程中振动模式对比热容的贡献,指出TATB热分解的引发键是C—NO_2键断裂的可能性更大.     

PbB_4O_7晶体的太赫兹光谱和软光学声子

对实验测量的PbB4O7晶体样品的太赫兹(1012Hz)光谱、拉曼光谱以及红外—可见—紫外光谱进行了分析.在0.25—2.5THz波段介电函数随频率变化曲线ε(ν)出现共振型尖峰.四方面的分析表明PbB4O7晶体中存在软光学声子:1)介电函数随频率的变化曲线ε(ν)满足LST(Lyddane-Sachs-Teller)关系;2)在共振峰的频率附近(3.10THz)有很强的拉曼散射峰;3)吸收系数随频率的变化曲线α(ν)满足极化激元的特征;4)透过晶体的光子的色散关系ν(k)发生断开的畸变.PbB4O7晶体中存在软光学声子的意义在于,在满足产生极化激元的条件下,透过晶体的光子的频率会发生劈裂,分为升高和降低的两支,有可能利用这种原理来改变光子的频率.     

KTiOPO4晶体的太赫兹光学声子振荡特性研究

采用超快半导体光电导开关的宽带太赫兹时域频谱系统,研究了 KTiOPO4 晶体在0.5～2.0THz波段的光学声子振荡特性;在晶体z轴平行于THz波电场振动方向时发现了明显的吸收峰.利用晶体中晶格振动模式（光学声子）对光谱的选择性吸收特性和多洛仑兹振子伪谐振介电模型,很好地拟合了KTP晶体的复介电常量曲线,得到了对应晶格弱振动的光学声子的频谱参量值.研究结果表明,位于ω1 /2π=1.76 THz的吸收峰是KTP晶体中沿z轴排列的K+相对于PO4和TiO6晶格振动造成的光学外振动模.同时也说明THz波对于晶格弱振动非常敏感,对于微弱的光学声子吸收峰的频谱分析更加精准.     

Nonlinear spoof surface plasmon polariton phenomena based on conductor metamaterials

We present nonlinear phenomena produced from spoof surface plasmon polariton (SSPP) modes. Below the THz spectrum, artificially textured conducting metastructures on a subwavelength scale generate surface-bound modes and are called SSPP modes, similar to surface plasmon polariton (SPP) modes in the visible spectrum. Even though nonlinear effects in the THz domain are negligible, subwavelength metallic gap structures are ideal candidates to realize nonlinear behavior in the THz domain because of slow light propagation, strong electromagnetic confinement, and a high quality factor Q. In particular, when SSPP structures are combined with Kerr nonlinear materials, nonlinear-bistable curves can be observed below the THz spectrum.     

Nonclassical behaviour of phonons in a polariton system

We consider a model polariton system and show that the phonon distribution in this system is always super-Poissonian. We furthermore show that both the phonon and photon subsystems can exhibit nonclassical behaviour, the threshold temperature for the photons being higher than that for the phonons. We however observe that the whole polariton complex can become nonclassical at a still higher temperature.     

KDP晶体的太赫兹光谱研究

实验测量了室温下磷酸二氢钾（KDP）晶体0．2～1．6THz的时域光谱,以及50～4000cm叫范围内的远红外光谱,200～2000nm的紫外一可见一红外光谱。KDP晶体的禁带宽度是5．91eV,在测量范围内有一个很宽的声子吸收带,从0．2～205．5THz吸收系数在35～80cm-1,声子吸收的低频端小于0．2THz。最高的纵光学模声子的频率大约是205．5THz,由此求出这支声子的H—O键的力常数为13．13N·cm-1。     

Coherent optical phonons and parametrically coupled magnons induced by femtosecond laser excitation of the Gd(0001) surface

Coherent spin dynamics in the THz domain coupled to a coherent phonon is observed in the time-resolved second harmonic response of the Gd(0001) ferromagnetic metal surface. An LO phonon of 2.9 THz is excited by a transient charge displacement at the surface caused by resonant absorption of a fs laser pulse in the exchange-split surface state. This lattice vibration modulates the interlayer distance inducing a coherent variation of the exchange interaction between spins in adjacent layers. The resulting magnetization dynamics is considered as optical magnon wave packets coupled to the phonon.     

KH_2PO_4晶体的太赫兹-紫外光谱

实验测量了室温下磷酸二氢钾KH2PO4(KDP)晶体0.2～1.6THz的时域光谱,以及50～4000cm-1范围内的远红外光谱,200～2000nm的紫外-可见-红外光谱。KDP晶体的禁带宽度是5.91eV。在测量范围内有一个很宽的声子吸收带。从0.2～205.5THz吸收系数在35～80cm-1,声子吸收的低频端小于0.2THz。最高的纵光学模声子的频率νLO大约是205.5THz,由此求出这支声子的H—O键的力常数为13.13N·cm-1。     

Interaction of terahertz radiation with surface and interface plasmon–phonons in AlGaAs/GaAs and GaN/Al2O3 heterostructures

Surface phonon and plasmon–phonon polariton characteristics of GaAs, Al _x Ga_1?x As/GaAs, and GaN/Al₂O₃ layered structures are investigated by means of terahertz radiation reflection spectroscopy. The strong resonant absorption peaks and selective emission of the THz radiation dependent upon the lattice composition and free electron density in these layered structures are experimentally observed and analyzed.     

基于钽酸锂晶体的太赫兹波参量振荡器运转特性的研究

基于晶格振动模受激电磁耦子散射过程的基本原理,对由LiTaO₃（LT）晶体组成的太赫兹波参量振荡器（TPO）的输出调谐特性、增益和吸收损耗特性,以及基于硅棱镜阵列耦合装置的THz波输出特性等方面进行了详细的理论研究和分析.研究结果表明,基于LiTaO₃晶体A₁对称性晶格振动的特点以及自身优异的非线性光学特性,通过利用短波长抽运光、适当提高抽运能量以及缩短TPO谐振腔腔长等方法,完全可以实现LT-TPO的高性能运转,证明了LiTaO₃晶体是一种性能优良的TPO工作介质.理论计算结果及方法为以后的LT-TPO实验工作提供了详实的理论依据和实验指导. 关键词： 非线性光学 THz辐射 3晶体')" href="#">LiTaO₃晶体 电磁耦子     

Quantum Theory of Surface Polaritons in Semi-Infinite Dielectric Systems

The interaction system of a semi-infinite diatomic dielectric and the electromagnetic field propagating parallelly to the surface is studied. The long-wave optical phonon modes of the semi-infinite dielectric are calculated including the electronic polarizability by a quantum-mechanical theory. Retardation effects are neglected. A quantum field theory method of surface polaritons is developed to calculate the operator describing the interaction between a photon and the phonon polarization fields of the semi-infinite crystal, with explicit inclusion of surface effects. The dispersion relations of the surface phonon-polaritons are derived and discussed. The results are in good agreement with those obtained by the conventional, macroscopic dielectric theory of surface polaritons and experimental ATR (attenuated total reflection) spectra. The dispersion cuwes of polariton leaking modes caused by the interaction between bulk TO mode with surface effect and photons are presented in a graphical form. The k_||-dependence of the electric field strength in each polariton branch is calculated and presented also in a graphical form. We also research the spatial dependence of the field strength in each polariton branch. It is found that these properties are influenced and modulated by the surface phonons because of the interaction between photons and the surface phonon modes of the semi-infinite dielectric system. Hence the divergent nature (diffraction effect) of light propagating along the fib& and waveguides is restrained. We present a method how to use the boundary conditions in the quantum theory, which will be discussed in detail.     

Towards time-resolved THz imaging

We describe a setup that allows for the measurement of high-quality images at frequencies in the far-infrared (THz) regime of the spectrum. By using water-cooled THz emitters that are biased with a 50-kHz, ±400-V square wave, rapid delay scanning, and differential lock-in detection at 50 kHz, we attain shot-noise-limited detection of THz transients. As a result, THz transients with a large dynamic range of 5000 can be measured in 20 ms. We show that the THz imaging setup enables the time-resolved detection of the diffusion of gases. PACS 07.57.-c; 42.30.-d; 95.85.Gn; 36.20.Ng     

Phonon-induced polariton superlattices

We show that the coherent interaction between microcavity polaritons and externally stimulated acoustic phonons forms a tunable polariton superlattice with a folded energy dispersion determined by the phonon population and wavelength. Under high phonon concentration, the strong confinement of the optical and excitonic polariton components in the phonon potential creates weakly coupled polariton wires with a virtually flat energy dispersion.     

Approach to broadband terahertz pulse field detection and recovery with electro-optic sampling measurement

Electro-optic sampling (EOS) measurement of terahertz (THz) pulses inherently leads to distortions, especially for relatively broadband THz pulses. In this paper, we proposed an approach to broadband THz pulse field detection and recovery with EOS measurement. In our approach, we use an EOS measurement utilizing a thick GaP crystal, which has a high transverse optical phonon frequency, to reduce the distortions caused by dispersive propagation, interface reflection and Fabry-Perot effect (multiple reflections within an EO crystal). And a retrieval algorithm is designed to eliminate the distortion of broadband THz pluses after passing through the thick GaP crystal (The distortion in a thick GaP crystal is mainly attributed to phase mismatching between the THz pulse and the sampling laser pulse.), in which preconditioned conjugated gradient method is used.     

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.     

马来酰肼多形态的太赫兹光谱研究

分子的多形态(多晶型)是指化学组成相同但存在不止一种晶体形式的物质.这些多形态广泛存在于自然界中,其中药物的多形态尤其普遍.这些药物多形态虽然具有相同的化学分子组成,但其理化性质却存在差异,最终会导致药物作用功能的不同.近年来,随着太赫兹(THz)辐射源的产生方式成为一种常规技术后,太赫兹时域光谱技术(THz-TDS)...