M. Białek, J. Zhang, H. Yu, J. -Ph. Ansermet
Strong coupling of electromagnetic cavity fields with antiferromagnetic spin waves in hematite ($α$-Fe$_2$O$_3$) was achieved above room temperature. A cube of hematite was placed in a metallic tube and transmission was measured, using a continuous-wave THz spectrometer. Spectra, collected as a function of temperature, reveal the formation of magnetic polaritons.
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An important vision of modern magnetic research is to use antiferromagnets as controllable and active ultrafast components in spintronic devices. Hematite is a promising model material in this respect because its pronounced Dzyaloshinskii-Moriya interaction leads to the coexistence of antiferromagnetism and weak ferromagnetism. Here, we use femtosecond laser pulses to drive terahertz spin currents from hematite into an adjacent Pt layer. We find two contributions to the generation of the spin current with distinctly different dynamics: the impulsive stimulated Raman scatting that relies on the…
Antiferromagnetic materials have been proposed as new types of narrowband THz spintronic devices owing to their ultrafast spin dynamics. Manipulating coherently their spin dynamics, however, remains a key challenge that is envisioned to be accomplished by spin-orbit torques or direct optical excitations. Here, we demonstrate the combined generation of broadband THz (incoherent) magnons and narrowband (coherent) magnons at 1 THz in low damping thin films of NiO/Pt. We evidence, experimentally and through modelling, two excitation processes of magnetization dynamics in NiO, an off-resonant insta…
The realization of THz sources is a fundamental aspect for a wide range of applications. Over different approaches, compact THz oscillators can be realized taking advantage of dynamics in antiferromagnetic (AFMs) thin films driven by spin-Hall effect. Here we perform a systematic study of these THz oscillators within a full micromagnetic solver based on the numerical solution of two coupled Landau-Lifshitz-Gilbert-Slonczewski equations, for the case of ultra-thin films, i.e. when the Néel temperature of an AFM is substantially reduced. We have found two different dynamical modes depending on t…
Signatures of Topological Magnon Edge States in THz Spectroscopy and Cavity Response
研究利用全光路径检测铁磁拓扑磁振子绝缘体中边缘模式，通过磁电耦合机制实现参量放大，提出太赫兹泵浦-探测光谱协议及利用太赫兹腔的方案，为探测拓扑磁体和研究磁量子材料中的拓扑现象开辟新途径。
Excitation of coherent high-frequency magnons (quanta of spin waves) is critical to the development of high-speed magnonic devices. Here we computationally demonstrate the excitation of coherent sub-terahertz (THz) magnons in ferromagnetic (FM) and antiferromagnetic (AFM) thin films by a photoinduced picosecond acoustic pulse. Analytical calculations are also performed to reveal the magnon excitation mechanism. Through spin pumping and spin-charge conversion, these magnons can inject sub-THz charge current into an adjacent heavy-metal film which in turn emits electromagnetic (EM) waves. Using …
The development of compact and tunable room temperature sources of coherent THz-frequency signals would open a way for numerous new applications. The existing approaches to THz-frequency generation based on superconductor Josephson junctions (JJ), free electron lasers, and quantum cascades require cryogenic temperatures or/and complex setups, preventing the miniaturization and wide use of these devices. We demonstrate theoretically that a bi-layer of a heavy metal (Pt) and a bi-axial antiferromagnetic (AFM) dielectric (NiO) can be a source of a coherent THz signal. A spin-current flowing from …
Hybrid magnon-phonon cavity for large-amplitude terahertz spin-wave excitation
本文提出一种自支撑金属/磁性绝缘体/电介质多层结构，利用腔增强磁振子-声子相互作用实现大振幅太赫兹自旋波激发，振幅比衬底支撑结构大十倍以上，并展示了非线性磁振子相互作用及太赫兹光电子换能应用。
Cavity-mediated coupling of antiferromagnetic spin waves
本文通过赤铁矿薄片的太赫兹反射光谱实验与理论建模，证明晶体自身构成的腔可在毫米距离、室温以上介导两个反铁磁振荡器耦合并形成磁振子–极化激元杂化模式。
All-optical magnetometric characterization of the antiferromagnetic exchange-spring system Mn$_2$Au|Py by terahertz spin-torques
本研究通过太赫兹泵浦磁光探针实验，利用场类Néel自旋轨道力矩控制Mn2Au|NiFe交换弹簧系统，校准了AFM的光学双折射并提取了NSOTs效率，揭示了自旋主导的光学响应及畴分布，为太赫兹超快自旋相干翻转奠定基础。
We develop the microscopic theory of a terahertz (THz) laser based on the effects of resonant tunneling in a double quantum well heterostructure embedded in both optical and THz cavities. In the strong coupling regime the system hosts dipolaritons, hybrid quasiparticles formed by the direct exciton, indirect exciton and optical photon, which possess large dipole moments in the growth direction. Their radiative coupling to the mode of a THz cavity combined with strong non-linearities provided by exciton-exciton interactions allows for stable emission of THz radiation in the regime of the contin…
Resolving self-cavity effects in two-dimensional quantum materials
本研究针对二维范德华异质结太赫兹近场探测中的自腔效应问题，提出结合片上时域太赫兹光谱技术的分析框架，可实现高分辨率的电导率提取与集体模式动力学解析，为相关量子相研究奠定基础。
Strongly coupled ultracompressed self-cavity plasmon polaritons at millikelvin temperatures
本文提出基于超导层自腔与石墨烯集成的平台，在毫开尔文温度下实现亚太赫兹欠阻尼等离激元极化激元，并利用超导电路光谱仪进行探测，为腔-物质混合系统研究开辟新途径。
Artificial cavity photon resonators with ultrastrong light-matter interactions are attracting interest both in semiconductor and superconducting systems, due to the possibility of manipulating the cavity quantum electrodynamic ground state with controllable physical properties. We report here experiments showing ultrastrong light-matter coupling in a terahertz metamaterial where the cyclotron transition of a high mobility two-dimensional electron gas is coupled to the photonic modes of an array of electronic split-ring resonators. We observe a normalized coupling ratio $\fracΩ{ω_c}=0.58$ betwe…
Coupling of spin-waves with electromagnetic cavity field is demonstrated in an antiferromagnet, dysprosium ferrite (DyFeO3). By measuring transmission at 0.2-0.35 THz and sweeping sample temperature, magnon-photon coupling signatures were found at crossings of spin-wave resonances with Fabry-Perot cavity modes formed in samples. The obtained spectra are explained in terms of classical electrodynamics and a microscopic model.
Understanding how fast short-range interactions build up long-range order is one of the most intriguing topics in condensed matter physics. FeRh is a test specimen for studying this problem in magnetism, where the microscopic spin-spin exchange interaction is ultimately responsible for either ferro- or antiferromagnetic macroscopic order. Femtosecond laser excitation can induce ferromagnetism in antiferromagnetic FeRh, but the mechanism and dynamics of this transition are topics of intense debates. Employing double-pump THz emission spectroscopy has enabled us to dramatically increase the temp…
Impulsive Fermi magnon-phonon resonance in antiferromagnetic $CoF_{2}$
本研究利用强窄带太赫兹脉冲和可调磁场，在反铁磁体 $CoF_{2}$ 中实验实现了费米磁振子-声子共振，揭示了非线性自旋-晶格耦合对相干磁振子-声子能量交换的控制机制。
We report experimental observation of electrically-tunable coherent perfect absorption (CPA) of terahertz (THz) radiation in graphene. We develop a reflection-type tunable THz cavity formed by a large-area graphene layer, a metallic reflective electrode and an electrolytic medium in between. Ionic gating in the THz cavity allows us to tune the Fermi energy of graphene up to 1eV and to achieve critical coupling condition at 2.8 THz with absorption of 99%. With the enhanced THz absorption, we were able to measure the Fermi energy dependence of the transport scattering time of highly doped graphe…
Polariton-mediated light-sound interaction is investigated through resonant Brillouin experiments in GaAs/AlAs multiple-quantum wells. Photoelastic coupling enhancement at exciton-polariton resonance reaches 105 at 30 K as compared to a typical bulk solid room temperature transparency value. When applied to GaAs based cavity optomechanical nanodevices, this result opens the path to huge displacement sensitivities and to novel ultrastrong-coupling cavity phenomena with optomechanical couplings g0 in the range of 100 THz.
Cavity spintronics explores light matter interactions at the interface between spintronic and quantum phenomena. Until now, studies have focused on the hybridization between ferromagnets and cavity photons.In this article, we realize antiferromagnetic cavity-magnon polaritons. The collective spin motion in single hematite crystals (α-Fe2O3) hybridizes with 18 - 45 GHz microwave cavity photons with required specific symmetries. We show theoretically and experimentally that the photon-magnon coupling in the collinear phase is mediated by the dynamical Neel vector and the weak magnetic moment in …
Delayed Formation of Landau Polaritons in Phase-Resolved THz Spectroscopy
本研究利用相位分辨太赫兹时域光谱，观测到GaAs/Al₀.₃₆Ga₀.₆₄As二维电子气中朗道极化激元的拉比振荡延迟一个腔往返时间启动，揭示了太赫兹频段杂化光-物质态的动力学特征。
Cavity based sensing of antiferromagnetic canting and nonzero-momentum spin waves in a van der Waals cavity-magnon-polariton system
研究通过磁控腔体探测CrCl3中抗铁磁偏转和非零动量自旋波，发现磁场影响磁子-光子耦合强度非单调变化，揭示了自旋 flop 转变和非耦合传播模式。
Magnons in antiferromagnetic insulators couple strongly to conduction electrons in adjacent metals. We show that this interfacial tie can lead to superconductivity in a tri-layer consisting of a metal sandwiched between two antiferromagnetic insulators. The critical temperature is closely related to the magnon gap, which can be in the THz range. We estimate the critical temperature in MnF$_2$-Au-MnF$_2$ to be on the order of 1 K. The Umklapp scattering at metal-antiferromagnet interfaces leads to a d-wave superconductive pairing, in contrast to the p-wave superconductivity mediated by magnons …
The impulsive generation of two-magnon modes in antiferromagnets by femtosecond optical pulses, so-called femto-nanomagnons, leads to coherent longitudinal oscillations of the antiferromagnetic order parameter that cannot be described by a thermodynamic Landau-Lifshitz approach. We argue that this dynamics is triggered as a result of a laser-induced modification of the exchange interaction. In order to describe the oscillations we have formulated a quantum mechanical description in terms of magnon pair operators and coherent states. Such an approach allowed us to} derive an effective macroscop…
Enhanced Terahertz Photoresponse via Acoustic Plasmon Cavity Resonances in Scalable Graphene
研究通过石墨烯中的声子等离子体腔共振增强太赫兹光电响应，利用天线耦合装置实现对太赫兹场的精确控制和纳米级限制，展示出高效的光电热电信号。
Strong coupling between cavity photons and various excitations in condensed matters boosts the field of light-matter interaction and generates several exciting sub-fields, such as cavity optomechanics and cavity magnon polariton. Axion quasiparticles, emerging in topological insulators, were predicted to strongly couple with the light and generate the so-called axion polariton. Here, we demonstrate that there arises a gapless level attraction in cavity axion polariton of antiferromagnetic topological insulators, which originates from a nonlinear interaction between axion and the odd-order reso…
The Zener-Klein (ZK) interband tunneling in graphene layers (GLs) with the lateral n-i-n and p-i-n junctions results in the nonlinear I-V characteristics that can be used for the rectification and detection of the terahertz (THz) signals. The transit time delay of the tunneling electrons and holes in the depletion regions leads to the phase shift between the THz current and THz voltage causing the negative dynamic conductance in a certain frequency range and resulting in the so-called transit-time (TT) instability. The combination of the ZK tunneling and the TT negative dynamic conductance ena…
A superconducting metasurface operating in the THz range and based on the complementary metamaterial approach is discussed. Experimental measurements as a function of temperature and magnetic field display a modulation of the metasurface with a change in transmission amplitude and frequency of the resonant features. Such a metasurface is successively used as a resonator for a cavity quantum electrodynamic experiment displaying ultrastrong coupling to the cyclotron transition of a 2DEG. A finite element modeling is developed and its results are in good agreement with the experimental data. In t…
Terahertz Slonczewski propagating spin waves and large output voltage in antiferromagnetic spin-Hall nano-oscillators
本文理论研究了单轴反铁磁自旋霍尔纳米振荡器，发现其可在太赫兹频率产生径向传播的 Slonczewski 自旋波，并预测了最小阈值电流及比双轴反铁磁体大三个数量级的交流输出电压，为器件优化和远程太赫兹信号生成提供了依据。
The interaction between intersubband resonances (ISRs) and metamaterial microcavities can form a strongly coupled system where new resonances form that depend on the coupling strength. Here we present experimental evidence of strong coupling between the cavity resonance of a THz metamaterial and the ISR in a high electron mobility transistor structure with a triangular confinement. The device is electrically switched from an uncoupled to a strongly coupled regime by tuning the ISR with epitaxially grown transparent gates. An asymmetric triangular potential in the heterostructure enables ultraw…
Cavity-Enhanced Optical Manipulation of Antiferromagnetic Magnon-Pairs
该研究提出利用驱动光学腔的反作用实现反铁磁中磁子对的压缩稳态，通过调节失谐可控制耗散符号，在红失谐下产生压缩态，在共振和蓝失谐下出现极限环与混沌，为量子信息处理提供新平台。
A method to perform spectrum analysis on low power signals between 0.1 and 10 THz is proposed. It utilizes a nanoscale antiferromagnetic tunnel junction (ATJ) that produces an oscillating tunneling anisotropic magnetoresistance, whose frequency is dependent on the magnitude of an evanescent spin current. It is first shown that the ATJ oscillation frequency can be tuned linearly with time. Then, it is shown that the ATJ output is highly dependent on matching conditions that are highly dependent on the dimensions of the dielectric tunneling barrier. Spectrum analysis can be performed by using an…
Quantized Magneto-Terahertz Effects in the Antiferromagnetic Topological Insulator MnBi$_2$Te$_4$ Thin Films
本文利用太赫兹法拉第与克尔旋转光谱研究分子束外延生长的 MnBi$_2$Te$_4$ 薄膜，观测三种磁态，并在无栅压的 6 SL、7 SL 样品中发现 6 T 以下的量子化霍尔态。
Magnonics is a research field complementary to spintronics, in which the quanta of spin waves (magnons) replace electrons as information carriers, promising less energy dissipation. The development of ultrafast nanoscale magnonic logic circuits calls for new tools and materials to generate coherent spin waves with frequencies as high, and wavelengths as short, as possible. Antiferromagnets can host spin waves at THz frequencies and are therefore seen as a future platform for the fastest and the least dissipative transfer of information. However, the generation of short-wavelength coherent prop…
Dipolaritons are mixed light-matter quasiparticles formed in double quantum wells embedded in microcavities. Due to resonant coupling between direct and indirect excitons via electronic tunnelling, dipolaritons possess large dipole moments. Resonant excitation of the cavity mode by a short pulse of light induces oscillations of the indirect exciton density with a characteristic frequency of Rabi flopping. This results in oscillations of classical Hertz dipoles array which generate supperradiant emission on a terahertz (THz) frequency. Resulting THz signal may be enhanced using the supplementar…
Cavity photon resonators with ultrastrong light-matter interactions are attracting interest both in semiconductor and superconducting systems displaying the capability to manipulate the cavity quantum electrodynamic ground state with controllable physical properties. Here we review a series of experiments aimed at probing the ultrastrong light-matter coupling regime, where the vacuum Rabi splitting $Ω$ is comparable to the bare transition frequency $ω$ . We present a new platform where the inter-Landau level transition of a two-dimensional electron gas (2DEG) is strongly coupled to the fundame…
Magnon and photon blockade in a hybrid antiferromagnet-cavity quantum system
研究了反铁磁绝缘体与腔耦合系统中的磁振子和光子阻塞，发现亮暗模行为不同，调节相互作用或施加磁场可优化阻塞效率，为量子信息技术提供单态生成途径。
Terahertz sources based on intra-cavity difference-frequency generation in mid-infrared quantum cascade lasers (THz DFG-QCLs) have recently emerged as the first monolithic electrically-pumped semiconductor sources capable of operating at room-temperature (RT) across the 1-6 THz range. Despite tremendous progress in power output, that now exceeds 1mW in pulsed and 10 μW in continuous-wave regime at room-temperature, knowledge of the major figure of merits of these devices for high precision spectroscopy, such as spectral purity and absolute frequency tunability, is still lacking. Here, by explo…
A superconductor can interact with magnets through the magnetic field which is induced by the supercurrent. However, this interaction is rather weak and requires relatively large magnets to reach a sizable hybridization of microwave electromagnetic modes with magnons. The size of the device can be drastically reduced by means of the large interfacial exchange interaction between spins in a magnet and a superconductor. In the spin-orbit coupled superconductor this interaction can be transmitted to the s-wave condensate through a mixing of triplet and singlet Cooper pairs. In this paper we theor…
The ability to engineer quantum-cascade-lasers (QCLs) with ultrabroad gain spectra and with a full compensation of the group velocity dispersion, at Terahertz (THz) frequencies, is a fundamental need for devising monolithic and miniaturized optical frequency-comb-synthesizers (FCS) in the far-infrared. In a THz QCL four-wave mixing, driven by the intrinsic third-order susceptibility of the intersubband gain medium, self-lock the optical modes in phase, allowing stable comb operation, albeit over a restricted dynamic range (~ 20% of the laser operational range). Here, we engineer miniaturized T…
Multimode vibrational coupling across the insulator-to-metal transition in 1T-TaS$_{2}$ in THz cavities
本文通过在可调谐太赫兹法布里-珀罗腔中嵌入1T-TaS₂，研究其绝缘体-金属相变过程中的多模振动强集体耦合，发现自由电荷引起的声子屏蔽会抑制该耦合，为调控腔限域关联材料中的相干光-物质相互作用提供了新方向。
We describe the dynamics of an antiferromagnetic nano-oscillator in an external magnetic field of any given time distribution. The oscillator is powered by a spin current originating from spin-orbit effects in a neighboring heavy metal layer, and is capable of emitting a THz signal in the presence of an additional easy-plane anisotropy. We derive an analytical formula describing the interaction between such a system and an external field, which can affect the output signal character. Interactions with magnetic pulses of different shapes, with a sinusoidal magnetic field and with a sequence of …
Currently, there is a growing interest in studying the coherent interaction between magnetic systems and electromagnetic radiation in a cavity, prompted partly by possible applications in hybrid quantum systems. We propose a multimode cavity optomagnonic system based on antiferromagnetic insulators, where optical photons couple coherently to the two homogeneous magnon modes of the antiferromagnet. These have frequencies typically in the THz range, a regime so far mostly unexplored in the realm of coherent interactions, and which makes antiferromagnets attractive for quantum transduction from T…
Terahertz chiral photonic-crystal cavities for Dirac gap engineering in graphene
本文设计了利用轻掺杂半导体磁等离子体实现时间反演对称性破缺的高品质因子太赫兹手性光子晶体腔，并结合第一性原理计算预测其在石墨烯中可诱导约1 meV的狄拉克能隙。
Long-lived Zone-boundary Magnons in an Antiferromagnet
本文在反铁磁绝缘体CoTiO3中发现了长寿命的区界THz磁振子，其寿命长于区中心磁振子，揭示了反铁磁序与交换各向异性等因素对延长磁振子寿命的积极作用，为拓扑磁振子的未来研究提供了新视角。
Magnon-photon coupling has been experimentally realized inside a cavity and the emerging field known as cavity spintronics has attracted significant attention for its potential docking with quantum information science. However, one seldom knows whether this coupling implies an entanglement state among magnons and photons or not, which is crucial for its usage in quantum information. Here we study the entanglement properties among magnons and photons in an antiferromagnet-light system and find that the entanglement between magnon and photon is nearly zero while the magnon-magnon entanglement is…
Chiral TeraHertz surface plasmonics
本文利用高度受限的太赫兹表面等离激元模式的手性特性，设计了在适度磁场下表现出显著可调手性行为的超小型太赫兹腔和超表面，实现了磁场调节腔谐振的极点和零点，为开发多功能太赫兹超表面和超强手性光物质相互作用铺平了道路。
Electrical generation of THz spin waves is theoretically explored in an antiferromangetic nanostrip via the current-induced spin-orbit torque. The analysis based on micromagnetic simulations clearly illustrates that the Neel-vector oscillations excited at one end of the magnetic strip can propagate in the form of a traveling wave when the nanostrip axis aligns with the magnetic easy-axis. A sizable threshold is observed in the driving current density or the torque to overcome the unfavorable anisotropy as expected. The generated spin waves are found to travel over a long distance while the ang…
Strong photon coupling to high-frequency antiferromagnetic magnons via topological surface states
本研究利用拓扑绝缘体表面态介导，通过全量子路径积分方法实现了反铁磁磁振子与微波腔光子的高频强耦合，耦合强度较直接耦合高数个量级，有望推动高频腔磁振子学在量子信息技术中的应用。
Terahertz high-harmonic generation in gapped antiferromagnetic chains
本文结合自旋波、平均场与 Lindblad 方法，研究有隙反铁磁链中线性及二次自旋–声子耦合对太赫兹高次谐波的调控机制与参数影响。
Second-order correlation and squeezing of photons in cavities with ultrastrong magnon-photon interactions
本研究在超强耦合磁子-腔系统中，通过精确求解二阶关联函数，发现反向旋转相互作用可诱导腔模压缩，且各向异性耦合可增强压缩，为量子信息技术中的光子阻塞优化提供途径。
In a collinear antiferromagnet with easy-axis anisotropy, symmetry dictates that the spin wave modes must be doubly degenerate. Theses two modes, distinguished by their opposite polarization and available only in antiferromagnets, give rise to a novel degree of freedom to encode and process information. We show that the spin wave polarization can be manipulated by an electric field induced Dzyaloshinskii-Moriya interaction and magnetic anisotropy. We propose a prototype spin wave field-effect transistor which realizes a gate-tunable magnonic analog of the Faraday effect, and demonstrate its ap…
We consider the nonlinear terahertz emission by the system of cavity polaritons in the regime of polariton lasing. To account for the quantum nature of terahertz-polariton coupling we use the Lindblad master equation approach and demonstrate that quantum microcavities reveal rich variety of the nonlinear phenomena in terahertz range, including bistability, short THz pulse generation and THz switching.
We have theoretically demonstrated the emission of circularly-polarized terahertz (THz) waves from intrinsic Josephson junctions (IJJs) which is locally heated by an external heat source such as the laser irradiation. We focus on a mesa-structured IJJ whose geometry is slightly deviate from a square and find that the local heating make it possible to emit circularly-polarized THz waves. In this mesa, the inhomogeneity of critical current density induced by the local heating excites the electromagnetic cavity modes TM (1,0) and TM (0,1), whose polarizations are orthogonal to each other. The mix…
We analyze the interaction of a nanomagnet with a single photonic mode of a microcavity in a fully quantum-mechanical treatment and find that exceptionally large quantum-coherent magnet-photon coupling can be achieved. Coupling terms in excess of several THz are predicted to be achievable in a spherical cavity of ~1 mm radius with a nanomagnet of ~100 nm radius and ferromagnetic resonance frequency of ~200 GHz. Eigenstates of the magnet-photon system correspond to entangled states of spin orientation and photon number, in which over 10^5 values of each quantum number are represented; conversel…
We present experimental results showing phonon-mediated coupling between two quantum dots embedded inside a photonic crystal microcavity. With only one of the dots being spectrally close to the cavity, we observe both frequency up-conversion and down-conversion of the pump light via a $\sim1.2$ THz phonon. We demonstrate this process for both weak and strong regimes of dot-cavity coupling, and provide a simple theoretical model explaining our observations.
Antiferromagnets naturally exhibit three obvious advantages over ferromagnets for memory device applications: insensitivity to external magnetic fields, much faster spin dynamics (~THz) and higher packing density due to the absence of any stray field. Recently, antiferromagnetic spintronics emerges as a cutting-edge field in the magnetic community. The key mission of this rapidly rising field is to steer the spins or spin axes of antiferromagnets via external stimuli and then realize advanced devices based on their physical property changes. Herein, the state of the art of antiferromagnetic sp…
We develop in this work a simple qualitative quantum electron transport model, in the strong light-matter coupling regime under dipole approximation, able to capture polaritonic signatures in the time-dependent electrical current. The effect of the quantized electromagnetic field in the displacement current of a resonant tunneling diode inside an optical cavity is analyzed. The original peaks of the bare electron transmission coefficient split into two new peaks due to the resonant electron-photon interaction, leading to coherent Rabi oscillations among the polaritonic states that are develope…
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