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• Vol. 3, Issue 5, 056002 (2021)
Changqin Liu1、2、†, Shunjia Wang1, Sheng Zhang1, Qingnan Cai1, Peng Wang1, Chuanshan Tian1, Lei Zhou1、*, Yizheng Wu1、2、*, and Zhensheng Tao1、*
Author Affiliations
• 1Fudan University, Department of Physics and State Key Laboratory of Surface Physics, Shanghai, China
• 2Shanghai Research Center for Quantum Sciences, Shanghai, China
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Fig. 1. Generation of chiral terahertz waves from a stripe-patterned spintronic-metasurface emitter. (a) Schematic of the experimental setup. The femtosecond pulse is focused to excite a stripe-patterned spintronic-metasurface emitter along the $z$ direction. The stripe is aligned along the $x$ direction. An orientated external magnetic field ($H$) is applied in the $x–y$ plane with a field angle of $θH$. A few-cycle chiral terahertz pulse is generated, which can be decomposed into electric-field components parallel ($E//$) and perpendicularly ($E⊥$) to the stripes. The stripe width is $d$, and the spacing between the stripes is $l$. Under laser illumination, spin currents $js$ are driven from the FM layer (yellow) to the NM layer (blue) through the interface. (b) Illustration of the current dynamics and inductive coupling in the $x–y$ plane when $θH=90 deg$. Owing to ISHE, spin currents are converted to charge currents $jc$, which flow along the stripes. (c) Illustration of the charge and current dynamics and the capacitive coupling in the $x–y$ plane when $θH=0 deg⁡$. $jc$ flows perpendicularly to the stripes, which induces the transient charges ($Qi$) and counteractive currents ($ji$), which suppresses the current density $ja⊥$.
Fig. 2. Modulation of terahertz spectrum and phase due to metasurface structure. (a) Terahertz waveforms of $E//$ under different field angles $θH$. The peak-to-peak field amplitude ($Vpp$) is defined as $V1−V2$. (b) Same as (a) for $E⊥$. (c) The peak-to-peak field amplitude ($Vpp$) of $E//$ and $E⊥$ [defined in (a) and (b)] as a function of $θH$. (d) Normalized spectra of $E//$ and $E⊥$ under different field angles $θH$. (e) The relative phase difference between the parallel and the perpendicular components $φ⊥−φ//$ under different field angles $θH$. The colored regions represent the experimental errors.
Fig. 3. Generation and manipulation of chiral terahertz waveforms. (a) Typical time dependence of the electric-field vector for a chiral terahertz waveform generated from a metasurface emitter with $d=l=10 μm$ at $θH=−17 deg$. The simulation result under the same conditions is plotted for comparison. The terahertz wavevector $k→THz$ is labeled. Inset: the illustration of the polarization states under different $θH$. L-EP: left-handed elliptical polarization; R-EP: right-handed elliptical polarization. (b) The parametric plots of $E//(t)$ and $E⊥(t)$ with different $θH$ for $d=l=10 μm$ under different $θH$. H-LP: horizontal linear polarization; V-LP: vertical linear polarization. The direction of $k→THz$ is labeled. (c) The broadband ellipticity $⟨ε⟩$ and the relative intensity $η$ for the chiral terahertz waves generated with different stripe width $d$. (d) The spectrally resolved ellipticity $ε(ω)$ as a function of $d$. The white filled symbols represent the anomaly frequencies shown in Fig. 4(e).
Fig. 4. Spectral anomaly due to coupling over the metasurface structure. (a) The normalized field spectra of $E⊥$ for different $d$ and $l$ on a $SiO2$ substrate measured in experiments. The spectra are normalized by those obtained from homogeneous thin-film emitters ($Ehomo$). The solid triangles label the low-frequency anomaly features, and the open triangles label the high-frequency ones. (b) Same as (a) for $E//$. The solid diamonds label the corresponding low-frequency anomaly features. (c) and (d) Simulation results obtained under the same conditions as in (a) and (b). The dash-dot lines align the corresponding anomaly features shared by $E//$ and $E⊥$ spectra. (e) The summary of the anomaly frequencies under different $d$ and $l$ as a function of the geometrical frequency $fgeo=vc/(d+l)$. The black filled triangles and diamonds represent the low-frequency features of $E⊥$ and $E//$, respectively, for $FF=0.5$ [shown in (a) and (b)]. The black open triangles represent the high-frequency features of $E⊥$ for $FF=0.5$. The colored filled and open symbols are obtained from experiments with $FF≠0.5$, and the half-filled symbols illustrate anomaly frequencies measured from emitters with an $Al2O3$ substrate. The blue solid lines represent the linear fitting of the experimental data. (f) The spatial and frequency distribution of the normalized total current density flowing perpendicularly to the stripes $ja⊥$ (along the $y$ axis) for $d=l=50 μm$. The flowing direction of the currents is labeled at the corner. (g) Same as (f) for $ja//$ (along the $x$ axis).
Fig. 5. Optimizing the terahertz ellipticity of the stripe-patterned metasurface emitter. (a) The broadband ellipticity $⟨ε⟩$ for $d=50 μm$ as a function of the FF. The solid line is the simulation results. The simulation results of the geometrical factor $C$ as a function of FF are shown as the dashed line. (b) $⟨ε⟩$ for $FF=0.5$ as a function of the stripe width $d$. The solid blue line is the simulation results. (c) 2D map of $⟨ε⟩$ under different FFs and $d$. The open symbols label the experimental results in (a) and (b). The dashed line corresponds to $d+l=100 μm$, where the values of anomaly frequency and the terahertz central frequency coincide.
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Changqin Liu, Shunjia Wang, Sheng Zhang, Qingnan Cai, Peng Wang, Chuanshan Tian, Lei Zhou, Yizheng Wu, Zhensheng Tao. Active spintronic-metasurface terahertz emitters with tunable chirality[J]. Advanced Photonics, 2021, 3(5): 056002