Fig. 1. Schematic of near-field optics. (a) Comparison between far-field and near-field optics. The point spread function in far-field optics is determined by the diffraction limit, while the spatial resolution in near-field optics is determined by the size of probe.(b) Explanation of breaking the diffraction limit in near-field optics based on uncertainty principle.近场光学成像原理图　(a)近场成像和远场成像的比较: 远场光学中物体的点扩散函数由传统衍射极限决定, 而近场光学中物体的点扩散函数由探针尺寸决定; (b)近场光学突破衍射极限的不确定原理解释

Fig. 2. Comparison of four classical sub-wavelength approaches, including photon emission electron microscopy (PEEM), cathode-luminescence spectroscopy (CL), electron energy loss spectroscopy (EELS), and scanning near-field optical microscopy (SNOM).光发射电子显微镜(PEEM)、阴极荧光光谱(CL)、电子能量损失谱(EELS)、扫描式近场光学显微镜(SNOM)等不同纳米级成像技术之间的对比

Fig. 3. Experimental scheme of SNOM: (a) The illumination scheme of a-SNOM; (b) the illumination scheme of s-SNOM.SNOM实验原理　(a) 孔径型SNOM照射原理; (b) 散射型SNOM照射原理

Fig. 4. The influence of AFM tip in near-field measurement: (a) Topography and near-field amplitude of a gold nanodisk obtained by carbon nanotube (CNT) tip and Pt-coated Si tip^[47], the scale bar is 100 nm; (b) the numerical simulation of local electric field between AFM tip and substrate. 近场光学成像中金属探针和介质探针的比较　(a) 金纳米圆盘的形貌像和光学像, 分别由碳纳米管探针(CNT)和金属探针扫描所得^[47], 标尺为100 nm; (b) 不同探针尖端局域电磁场的数值模拟结果

Fig. 5. Polaritons in low-dimensional materials. Polaritons are collective excitation from coupling photons with other quasiparticles, such as plasmons in electron-rich systems, infrared-active phonons in polar insulators, excitons in semiconductors, cooper-pairs in superconductors, spin resonances in (anti)-ferromagnets and hybrids in heterostructures.低维体系中的极化激元. 极化激元是光子和其他粒子或准粒子耦合后产生的一种玻色子, 包括富电子体系中的等离极化激元、极化晶体中的声子极化激元、半导体中的激子极化激元、超导体中的库珀对极化激元、铁磁体中的磁振子极化激元以及异质结中的杂化极化激元

Fig. 6. Surface plasmon polaritons in monolayer graphene: (a) Near-field spectroscopic measurement and theoretically calculated dispersion of Dirac plasmons in monolayer graphene^[67]; (b) s-SNOM scheme (upper), experimental amplitude of graphene plasmons (middle) and calculated local density of optical states (bottom)^[68], the incident wavelength is 9.7 μm; (c) nano-image of graphene plasmons launched by gold antenna under liquid-nitrogen temperature, the incident wavelength is 11.28 μm^[70]; (d) plasmonic hot-spots inside graphene nanobubbles on boron nitride substrate^[71], the incident frequency is 910 cm^–1; (e) asymmetric plasmonic fringes induced by superposition of propagating and localized modes in graphene nanoribbons^[72], the incident frequency is 1184 cm^–1; (f) experimental (left) and calculated (right) near-field amplitude of graphene rectangle resonators, representing 1D edge mode and 2D sheet mode^[74], the incident wavelength is 11.31 μm; (g) edge plasmons at the top boundary of graphene nanoribbons^[75], the incident frequency is 1160 cm^–1. Scale bars in all panels represent 200 nm, except for 1 μm in (c). 石墨烯中的表面等离极化激元　(a) 单层石墨烯中狄拉克等离激元的近场光谱测量及其色散的理论计算结果^[67]; (b) 石墨烯等离激元的红外近场光学图像^[68], 入射光波长为9.7 μm; (c) 液氮温区下石墨烯等离激元的近场光学图像^[70], 入射光波长为11.28 μm; (d) 石墨烯纳米泡中等离激元局域“热点”^[71], 入射频率为910 cm^–1; (e) 石墨烯纳米带中等离激元传播态和局域态之间耦合产生的近场光学强度非对称现象^[72], 入射频率为1184 cm^–1; (f) 石墨烯方形谐振腔中等离激元一维边界模式和二维模式的近场光学测量及其数值模拟结果^[74], 入射光波长为11.31 μm; (g) 石墨烯纳米条带中等离激元一维边界模式的近场光学成像^[75], 入射光频率为1160 cm^–1(图(c) 中标尺为1 μm, 其他图中标尺均为200 nm)

Fig. 7. Plasmon polaritons in bilayer graphene: (a) Left panel: experimental measurement of voltage-dependent plasmonic wavelength in monolayer (SLG) and bilayer (BLG) graphene. Right panel: Theoretical calculation of voltage- and frequency-dependent imaginary part of the optical conductivity. The double-headed arrows indicate plasmon-off region of bilayer graphene^[77]; (b) near-field study of interaction between plasmons and intrinsic phonons in highly doped double-layer (left) and bilayer graphene (right)^[76]. The dispersed symbols represent experimental data and background color indicates the imaginary part of the calculated Fresnel reflection coefficient. Inset: representative near-field images of graphene plasmons and corresponding symmetry of phonon-induced charge densities. 双层石墨烯中的等离激元　(a) 左: 单层石墨烯和双层石墨烯中等离激元随施加电压的变化趋势; 右: 双层石墨烯中光电导随电压变化趋势的理论计算结果, 图中箭头表示等离激元关闭区域^[77]; (b) 随机堆叠型(左) 和Bernal堆叠型(右) 双层石墨烯中等离激元与声子之间相互耦合作用的近场光学测量^[76]. 散点代表实验数据, 背景色为菲涅耳反射系数虚部的理论计算结果. 内插图为石墨烯等离激元的近场光学图像

Fig. 8. The applications of graphene plasmons: (a) Phase control of infrared light by gate-tunable graphene plasmons^[78]. Upper panel: schematic of experimental configuration. Bottom panel: Theoretical (solid lines) and experimental (dispersed circles) phase shift, which can be changed from 0 to ; (b) hybridized polaritons in graphene/hBN heterostructures^[79]. Upper panel: With monolayer graphene, both amplitude and wavelength of phonon polaritons in pristine hBN increase. Bottom panel: The gate-tunable hyperbolic phonon-plasmon polaritons (HP³) in graphene/hBN and un-tunable hyperbolic phonon polaritons (HP²) in hBN. The incident frequency is 1495 cm^–1. Scale bar, 300 nm. 石墨烯等离激元的应用　(a) 基于石墨烯等离激元的红外光相位调制器^[78], 上图为实验原理图, 下图为 的相位调制, 实线为理论计算结果, 散点为实验数据; (b) 石墨烯/氮化硼中的杂化极化激元^[79]上图为杂化极化激元和氮化硼声子极化激元的光学图像, 下图为杂化极化激元的电压调控 (入射光频率为1495 cm^–1, 标尺为300 nm)

Fig. 9. Hyperbolic phonon polaritons (HPPs) in boron nitride: (a) Hyperbolic behavior of natural hBN crystal, which gives two separate spectral bands called lower and upper Reststrahlen bands with opposite-signed in-plane ( ) and out-of-plane ( ) dielectric permittivity^[80], the corresponding hyperboloid-type dispersion of polaritons is shown in left (type 1) and right (type 2) panels; (b) nano-infrared images of HPPs in a tapered hBN crystal^[56]. The incident frequency is 1550 cm^–1, scale bar, 800 nm; (c) in-plane hyperbolic phonon polaritons in nano-patterning boron nitride crystal^[88], left panel: near-field image of concave wavefront of phonon polaritons in boron nitride metasurfaces, right panel: schematic of the experiment; (d) volume-confined polaritons (M0) and surface polaritons (SM0) near the edge of hBN crystal^[90], the incident frequency is 1420 cm^–1. Scale bar, 2 μm; (e) manipulation of hyperbolic surface polaritons with corner angle of hBN crystals^[91]. Left panel: representative near-field image with crystal angle of 120°. Right panel: simulated reflected (R), transmitted (T) and scattered (S) fractions of polaritons as a function of crystal angles. Red squares are experimental data. 氮化硼中双曲线型声子极化激元　(a) 天然氮化硼晶体中的双曲线行为, 其等频面为两类双曲面^[80]; (b) 氮化硼晶体中双曲线型声子极化激元的近场光学图像^[56], 入射光频率为1550 cm^–1, 标尺为800 nm; (c) 氮化硼超表面面内双曲线型声子极化激元的近场光学图像^[88]; (d) 氮化硼中表面局域声子极化激元(HSPs) 的近场光学图像^[90], 入射光频率为1420 cm^–1, 标尺为2 μm; (e) 不同角度氮化硼中HSPs的散射行为^[91]

Fig. 10. The applications of hyperbolic phonon polaritons: (a) Near-field imaging and nano-focusing realized by hBN-HPPs^[92]. Upper panel: simulated perfect imaging (ω₀ = 761 cm^–1) and enlarged imaging (ω₀ = 778.2 cm^–1) of gold nanodisk beneath the hBN crystal. Bottom panel: experimental nano-infrared images of gold nanodisk beneath hBN with the broadband incident laser; (b) sub-wavelength focusing of mid-infrared light through an hBN crystal^[93]. Left panel: AFM image of gold disks on SiO₂/Si substrate before hBN transfer. Right panel: near-field amplitude on the top of hBN crystal with incident frequency at 1515 cm^–1. Scale bar, 1 μm; (c) linear hBN dielectric antenna with different lengths^[94], 1327 nm in upper panel and 1713 nm in bottom panel. The incident frequency is 1432 cm^–1. 双曲线型声子极化激元的应用　(a) 基于氮化硼声子极化激元的超分辨成像^[92], 上: 数值模拟; 下: 近场光学测量; (b) 基于氮化硼实现中红外光的纳米聚焦^[93], 标尺为1 μm; (c) 不同长度氮化硼线性天线的近场光学图像^[94], 上: 长度为1327 nm; 下: 长度为1713 nm

Fig. 11. Near-field studies of exciton polaritons in semiconductors: (a) Representative near-field image of a WSe₂ flake, whose edges are marked by white dashed lines^[96]; (b) near-field image of exciton polaritons in planar MoSe₂ waveguide at laser energy of 1.41 eV^[97]. Scale bar is 1 μm. 半导体中激子极化激元的近场光学成像　(a) 二硒化钨中激子极化激元的近场光学图像^[96], 白色虚线为二硒化钨的边界; (b) 二硒化钼中激子极化激元的近场光学图像^[97], 图中标尺为1 μm

Fig. 12. Polaritons in van der Waals heterostructures: (a) Near-field image of low-loss graphene plasmons in hBN/Graphene/hBN heterostructures^[98]. Upper panel: Side-view sketch of near-field measurement of back-gate graphene encapsulated by hBN layers. Bottom panel: representative near-field image with incident wavelength at 10.6 μm. The graphene edge is marked as black dashed line. (b) Hybridized plasmon-phonon polaritons in graphene/hBN heterostructures^[99]. Upper panel: experimentally extracted wavelength of plasmon-phonon polaritons. Bottom panel: representative near-field images of polaritons. The graphene edge is marked by red dashed lines. The incident frequency is 950 cm^-1 and 970 cm^-1, respectively. Scale bar is 500 nm. 范德瓦耳斯异质结中的极化激元　(a) 氮化硼/石墨烯/氮化硼异质结中超低损耗等离激元的近场光学成像^[98], 黑色虚线为石墨烯边界, 入射光波长为10.6 μm; (b) 石墨烯/氮化硼中杂化等离–声子极化激元的近场光学成像^[99], 红色虚线为石墨烯边界 (图中标尺为500 nm)

Fig. 13. Ultrafast near-field optics. (a) The experimentally extracted propagation of type-1 HPPs in the space-time domain^[100]. The yellow region represents the gold antenna launching polaritons. The inset shows zoom into the fringe patterns. Right panel: the line profiles for different time delays. The black and green solid lines show the envelope of the fringe patterns (group velocity) and intrinsic fringe patterns (phase velocity), respectively. (b) Near-infrared (NIR) pump-induced changes in the near-field amplitude of graphene for different pump-probe time delays^[103]. The pump and probe lasers are 1.56 μm and broadband mid-infrared pulses, respectively. The dark region in near-field images represents SiO₂ substrate. Different optical contrast is caused by different layered graphene. Scale bar, 1 μm. (c) Ultrafast controlling of photo-induced plasmon polaritons in graphene encapsulated by two hBN layers^[104]. Left panel: the schematic of pump-probe s-SNOM set-up. Right panel: the two-dimensional hyperspectral map of photo-induced plasmons in hBN/graphene/hBN device. The black solid line gives the edge of device. The pump laser is at 1.56 μm. The probe beam spans frequencies from 830–1000 cm^–1. 超快近场光学　(a) 实验测量氮化硼声子极化激元的动力学参数^[100], 黄色区域代表金天线, 内插图显示了极化激元波的传播, 右图为不同延迟时间下的极化激元波包, 黑色和绿色实线分别代表群速度和相速度; (b) 石墨烯抽运-探测近场光谱图^[103], 从左到右探测光与抽运光之间的延迟分别为0, 200和400 fs, 标尺为1 μm; (c) 石墨烯中光诱导等离激元的超快光学成像^[104]

Fig. 14. Development in near-field optics. (a) Chemical identification of nanoscale sample contaminations with nano-FTIR, which is combination of s-SNOM and Fourier transform infrared spectrum (FTIR)^[111]. Left panel: Topography image of poly-(methyl methacrylate) thin film (PMMA, marked as A on silicon substrate, with a contaminated particle of polydimethylsiloxane (PDMS, marked as B. Right panel: corresponding absorption spectra of PMMA (taken from spot A) and PDMS (taken from spot B). (b) Near-field imaging of plasmonic wavefront launched by gold antenna, instead of AFM tip^[112]. Upper panel: AFM topography images of fabricated gold antenna. Bottom panel: representative near-field image of plasmonic wavefront with incident wavelength at 11.06 μm. (c) Near-field imaging of wavefront of hBN-HPPs launched by gold antenna^[113]. The brighter region represents gold antenna, encapsulated between hBN and SiO₂ substrate. Scale bar is 1 μm. 近场光学成像的发展　(a) 通过散射型SNOM和红外光谱结合测量纳米区域化学分子的红外光谱^[111]; (b) 共振金天线激发石墨烯等离激元的近场光学图像^[112]; (c) 非共振金天线激发氮化硼声子极化激元的近场光学图像^[113], 图中标尺为1 μm

Fig. 15. The perspective of near-field optics: (a) Chemically fabricated carbon nanotube cup, whose properties can be effectively controlled by chemical component^[114]; (b) split-ring probe is sensitive to both in-plane (H_x or H_y) and out-of-plane (H_z) component of near-field magnetic field^[51], scale bar, 500 nm; (c) the combination of near-field optics and mass spectroscopy for highly chemical resolution and spatial resolution, simultaneously; (d) the developed s-SNOM in extreme environment, including ultralow temperature, strong magnetic field and ultrahigh vacuum. 近场光学前景展望　(a) 化学合成的碳纳米管结构^[114], 其光学性质可以通过化学组分有效调控; (b) 开口环形探针可用于近场磁场面内和面外分量的测量^[51], 图中标尺为500 nm; (c) 散射型SNOM与质谱耦合, 可同时得到纳米级空间分辨率和超高化学分辨率; (d) 极端环境下SNOM的发展, 包括超低温、强磁场和超高真空等