二次谐波产生为何只在非中心对称晶体中发生，并满足怎样的相位匹配条件？
Why does second-harmonic generation occur only in non-centrosymmetric crystals, and what phase-matching condition must be satisfied?
二次谐波来自二阶非线性极化 P^(2) = ε0 χ^(2) E²，χ^(2) 在非中心对称介质中非零。要高效产生倍频光，基频与倍频波的波矢须满足相位匹配 k(2ω) = 2k(ω)，通常通过双折射温度或角度调谐实现。
Second-harmonic generation arises from the second-order polarization P^(2) = ε0 χ^(2) E², and χ^(2) is non-zero only in non-centrosymmetric media. Efficient frequency doubling requires phase matching k(2ω) = 2k(ω), typically achieved by birefringent temperature or angle tuning.
难度 专业 · 适合 高中(15-18)
CC BY 4.0 · 已核验 · Verified 2026-11-03
来源 · Source
Boyd, Nonlinear Optics（第3版, Academic Press, 2008）
R. W. Boyd, Nonlinear Optics (3rd ed., Academic Press, 2008)
独立核验依据 · Independent verification: 核验依据 1 OPENLIBRARY 标识 /works/OL4079855W：Nonlinear optics（首版 199
#非线性光学 #二次谐波 #χ² #相位匹配
光子晶体如何像半导体能带那样出现「光子带隙」，并借此控制光传播？
How do photonic crystals produce a photonic band gap analogous to semiconductor bands and thereby control light propagation?
光子晶体是介电常数在空间周期性调制的微结构，麦克斯韦方程在周期性介质中给出 Bloch 光子本征模，某些频率区间因布拉格散射相消干涉而被禁止传播，形成光子带隙。带隙内光无法传输，可用于波导、反射镜与慢光器件。
A photonic crystal is a microstructure with periodically modulated dielectric constant; solving Maxwell's equations in the periodic medium yields Bloch photonic eigenmodes, and certain frequency ranges are forbidden from propagating due to destructive Bragg scattering, forming a photonic band gap. Light in the gap cannot propagate and can be used for waveguides, mirrors, and slow-light devices.
难度 专业 · 适合 高中(15-18)
CC BY 4.0 · 已核验 · Verified 2026-11-03
来源 · Source
Joannopoulos, Johnson, Winn & Meade, Photonic Crystals: Molding the Flow of Light（第2版, 普林斯顿大学出版社, 2008）
J. D. Joannopoulos, S. G. Johnson, J. N. Winn & R. D. Meade, Photonic Crystals: Molding the Flow of Light (2nd ed., Princeton Univ. Press, 2008)
独立核验依据 · Independent verification: 核验依据 1 ARCHIVE 标识 photoniccrystals0000joan：Photonic crystals :
#光子晶体 #光子带隙 #布拉格散射 #布洛赫模
受激发射损耗（STED）显微镜如何突破衍射极限实现纳米分辨率？
How does stimulated-emission-depletion (STED) microscopy overcome the diffraction limit to reach nanometer resolution?
STED用一束激发光使荧光团发光，再用一束环形耗散光通过受激发射把激发区外围的荧光熄灭，只留下中心远小于衍射斑的发光点。有效分辨率与耗尽光强平方根成反比，可达数十纳米，突破了阿贝衍射极限。
STED uses an excitation beam to make fluorophores emit, then a donut-shaped depletion beam that forces stimulated emission to quench fluorescence everywhere except a central spot much smaller than the diffraction spot. The effective resolution scales inversely with the square root of depletion intensity, reaching tens of nanometers and breaking the Abbe diffraction limit.
难度 专业 · 适合 高中(15-18)
CC BY 4.0 · 已核验 · Verified 2026-11-04
来源 · Source
Hell & Wichmann,《Breaking the diffraction resolution limit by stimulated emission: stimulated-emission-depletion fluorescence microscopy》,Optics Letters,1994 (DOI: 10.1364/ol.19.000780)
Hell & Wichmann, 'Breaking the diffraction resolution limit by stimulated emission: stimulated-emission-depletion fluorescence microscopy', Optics Letters, 1994, DOI: 10.1364/ol.19.000780
独立核验依据 · Independent verification: 核验依据 1 Crossref DOI 10.1364/ol.19.000780
#STED #超分辨 #衍射极限 #受激发射
光子晶体光纤仅靠规则排列的空气孔便能导光，其中一类甚至能让光在空气芯里传输。实芯与空芯两类光子晶体光纤分别依靠什么机制束缚光？
Photonic-crystal fibres guide light using nothing but a regular lattice of air holes, and one class even transmits light through a hollow air-filled core. What mechanism confines light in solid-core fibres, and what different mechanism operates in hollow-core ones?
实芯光子晶体光纤中，石英-空气周期包层的平均折射率低于纯石英芯，光靠改良的全内反射传播；调节空气孔径与孔间距还能独立塑造波导色散。空芯光子带隙光纤则完全不同：包层周期结构的布拉格散射禁止特定频段的光在其中传播，形成二维光子带隙，光因此被束缚进折射率更低的空气芯，大幅减少与玻璃的相互作用及材料损伤风险。
In solid-core photonic-crystal fibre the periodic silica-air cladding has an average index below that of the pure-silica core, so light is guided by modified total internal reflection, while hole size and pitch can be tuned to shape waveguide dispersion independently. Hollow-core photonic-bandgap fibres work quite differently: Bragg scattering forbids selected frequency bands from propagating in the periodic cladding, creating a two-dimensional bandgap that traps light in a lower-index air core, sharply cutting interaction with the glass and material-damage risk.
难度 专业 · 适合 高中(15-18)
CC BY 4.0 · 已核验 · Verified 2026-11-03
来源 · Source
P. St. J. Russell, Photonic Crystal Fibers, Science 299, 358-362 (2003), DOI: 10.1126/science.1079280
P. St. J. Russell, "Photonic Crystal Fibers", Science 299, 358-362 (2003), DOI: 10.1126/science.1079280
独立核验依据 · Independent verification: 核验依据 1 CROSSREF 标识 10.1126/science.1079280：Photonic Crystal Fib
#光子晶体光纤 #全内反射 #光子带隙 #波导色散
激光器为何能产生高度相干、单色且方向性极强的光束？
Why can a laser produce a highly coherent, monochromatic, and strongly directional beam?
激光利用受激辐射使处于粒子数反转的工作物质中，特定频率的光子在谐振腔内反复放大并选出单一纵模。受激辐射光与入射光同频、同相、同方向，因而输出光束具有极好的时间相干与空间相干。
A laser uses stimulated emission in a population-inverted gain medium, with photons of a specific frequency amplified repeatedly inside a resonant cavity that selects a single longitudinal mode. Stimulated-emission light shares the frequency, phase, and direction of the incident light, giving excellent temporal and spatial coherence.
难度 进阶 · 适合 初中(12-15)
CC BY 4.0 · 已核验 · Verified 2026-10-04
来源 · Source
Svelto, Principles of Lasers（第5版, Springer, 2010）
O. Svelto, Principles of Lasers (5th ed., Springer, 2010)
独立核验依据 · Independent verification: 核验依据 1 ISBN 9781461576716 (2016)
#激光 #受激辐射 #粒子数反转 #谐振腔
光纤通信利用全反射原理传输光信号，其低损耗窗口位于哪个波段？
Optical fiber communication relies on total internal reflection; in which wavelength window is the loss lowest?
光纤纤芯折射率高于包层，光以大于临界角的角度入射时发生全反射而沿纤芯传输。石英光纤在约1550 nm附近因瑞利散射与红外吸收最小化而损耗最低（约0.2 dB/km），成为长途通信主用窗口。
The fiber core has a higher refractive index than the cladding, so light entering beyond the critical angle undergoes total internal reflection and propagates along the core. Silica fiber shows minimum loss near about 1550 nm (around 0.2 dB/km) due to minimized Rayleigh scattering and infrared absorption, the main window for long-haul communication.
难度 进阶 · 适合 大众
CC BY 4.0 · 已核验 · Verified 2026-11-04
来源 · Source
Keck, Maurer & Schultz, Appl. Phys. Lett. 22, 307 (1973)（低损耗石英光纤）；Miya et al., Rev. Electr. Commun. Lab. 25, 579 (1977)（1.55 μm最低损耗）
Keck, Maurer & Schultz, Appl. Phys. Lett. 22, 307 (1973) (low-loss silica fiber); Miya et al., Rev. Electr. Commun. Lab. 25, 579 (1977) (1.55 μm minimum loss)
独立核验依据 · Independent verification: 核验依据 1 Crossref DOI 10.1063/1.1654649（复合著录按段核验：本标识只担保第一段 Keck,
#光纤 #全反射 #低损耗窗口 #1550纳米
杨氏双缝干涉中，为何增大光源空间相干性（如使用窄缝）才能看到清晰干涉条纹？
In Young's double-slit interference, why must the spatial coherence of the source be increased to see clear fringes?
双缝各自作为相干子波源，只有当来自两个缝的光在屏上保持稳定的相位关系时才能叠加出明暗条纹。扩展光源各点产生互相错位的条纹，叠加后对比度下降；用窄缝或单色光源可提升空间与时间相干性，恢复清晰条纹。
Each slit acts as a coherent secondary source, and clear fringes require a stable phase relation between the two beams at the screen. An extended source produces mutually shifted fringe patterns that wash out contrast; a narrow or monochromatic source improves spatial and temporal coherence and restores clear fringes.
难度 进阶 · 适合 大众
CC BY 4.0 · 已核验 · Verified 2026-10-03 · 2 条独立证据
来源 · Source
《费曼物理学讲义》第1卷（光学）
The Feynman Lectures on Physics vol.1 (optics)
独立核验依据 · Independent verification: 核验依据 2 ISBN 978-0-7382-0298-3 (2006 Pearson 版，原始版为1963-65 Addis
#杨氏双缝 #干涉 #空间相干 #时间相干
左手超材料如何实现负折射率，并据此构造「隐身斗篷」的思路是什么？
How do left-handed metamaterials achieve a negative refractive index, and what is the idea behind a metamaterial cloak?
通过亚波长人工结构使有效介电常数与磁导率同时为负，根据 n = ±√(εμ) 可得负折射率，使光线向与常规相反一侧偏折。隐身斗篷利用坐标变换设计各向异性参数，使电磁波绕物体传播后重建，从而不对背景产生散射。
Subwavelength artificial structures can make both effective permittivity and permeability negative, giving a negative refractive index from n = ±√(εμ) and bending light to the opposite side of the normal. A cloak uses transformation optics to design anisotropic parameters so waves flow around an object and re-emerge, producing no scattering.
难度 进阶 · 适合 大众
CC BY 4.0 · 已核验 · Verified 2026-11-04
来源 · Source
Pendry, Schurig & Smith, Science 312, 1780 (2006)（变换光学隐身斗篷）；Veselago, Sov. Phys. Usp. 10, 509 (1968)（负折射）
Pendry, Schurig & Smith, Science 312, 1780 (2006) (transformation-optics cloak); Veselago, Sov. Phys. Usp. 10, 509 (1968) (negative refraction)
独立核验依据 · Independent verification: 核验依据 1 Crossref DOI 10.1126/science.1125907（复合著录按段核验：本标识只担保第一段
#超材料 #负折射率 #变换光学 #隐身斗篷
拉曼散射与瑞利散射在物理机制和光谱特征上有何区别？
How do Raman scattering and Rayleigh scattering differ in mechanism and spectral signature?
瑞利散射是光子与介质弹性碰撞，频率不变；拉曼散射是非弹性过程，光子与分子振转能级交换能量，散射光出现相对入射光偏移±ν的斯托克斯与反斯托克斯谱线。拉曼位移直接反映分子振动指纹，用于物质识别。
Rayleigh scattering is elastic, leaving the photon frequency unchanged, while Raman scattering is inelastic: the photon exchanges energy with molecular vibrational or rotational levels, producing Stokes and anti-Stokes lines shifted by ±ν. The Raman shift reveals molecular vibrational fingerprints for material identification.
难度 进阶 · 适合 初中(12-15)
CC BY 4.0 · 已核验 · Verified 2026-11-04
来源 · Source
RAMAN, KRISHNAN,《A New Type of Secondary Radiation》,Nature,1928 (DOI: 10.1038/121501c0)
RAMAN, KRISHNAN, 'A New Type of Secondary Radiation', Nature, 1928, DOI: 10.1038/121501c0
独立核验依据 · Independent verification: 核验依据 1 Crossref DOI 10.1038/121501c0
#拉曼散射 #瑞利散射 #非弹性 #振动指纹
超材料实现负折射率的条件是什么?负折射会带来哪些反常的电磁现象?
What condition allows a metamaterial to exhibit a negative refractive index, and what anomalous effects follow from negative refraction?
当人工设计的单元结构使材料在某一频段内等效介电常数ε和磁导率μ同时为负,且损耗足够低时,折射率取负值分支,即n=-√(εμ)。此时坡印廷矢量与波矢方向相反,相速度逆着能量流方向传播。由此出现一系列反常现象:斯涅尔定律中折射光线偏向入射点法线同一侧、多普勒频移反转、切伦科夫辐射朝后发射,以及平板负折射透镜可放大倏逝波、突破传统衍射极限成像。
A metamaterial shows a negative refractive index when engineered unit cells make both the effective permittivity ε and permeability μ simultaneously negative over some band with sufficiently low loss, selecting the branch n=-√(εμ). Then the Poynting vector points opposite to the wavevector: phase propagates against the energy flow. This produces several anomalies—the refracted ray in Snell's law lies on the same side of the normal as the incident ray, the Doppler shift reverses sign, Cherenkov radiation is emitted backwards, and a flat negative-index slab can amplify evanescent waves to image beyond the conventional diffraction limit.
难度 进阶 · 适合 初中(12-15)
CC BY 4.0 · 来源待核验 · Source pending review
来源 · Source
出处备注：著录含书名，但缺版次 / 出版社 / 年份中的一部分，需编辑补全后才能做独立比对。
崔铁军等《信息超材料》绪论;Veselago, Sov. Phys. Usp. 10, 509 (1968);Shelby, Smith & Schultz, Science 292, 77 (2001)
Veselago, Sov. Phys. Usp. 10, 509 (1968); Shelby, Smith & Schultz, Science 292, 77 (2001)
#负折射率 #等效介电常数
负折射率超材料是如何突破“自然界材料介电常数与磁导率均为正”这一限制的?
How do negative-index metamaterials get around the restriction that natural materials have both positive permittivity and permeability?
超材料用亚波长人工结构单元代替原子或分子作为基本响应单元:金属开口环产生共振磁响应使 μ 在特定频段为负,金属线阵列产生等离子体型响应使 ε 为负,Pendry 于 2000 年指出两者结合可实现 ε 与 μ 同时为负从而 n = -√(εμ)。此时波的群速度与相速度反向,多普勒频移、切伦科夫辐射方向等均反转,并可借负曲率等频面实现超越衍射极限的完美透镜。该方案的关键在于单元尺寸远小于波长,使媒质可用均匀化的有效参数描述。
Metamaterials replace atoms or molecules with subwavelength engineered unit cells as their elementary response units: arrays of metallic split rings give a resonant magnetic response that drives μ negative over a band, while wire grids provide a plasmon-like response making ε negative. Pendry showed in 2000 that combining them yields ε and μ simultaneously negative, hence n = -√(εμ). The group velocity then opposes the phase velocity, reversing Doppler shifts and Cherenkov radiation directions, and the negatively curved isofrequency surface enables a perfect lens beyond the diffraction limit. The scheme hinges on unit cells much smaller than the wavelength so an effective homogeneous description applies.
难度 进阶 · 适合 大众
CC BY 4.0 · 已核验 · Verified 2026-11-04
来源 · Source
Pendry, Phys. Rev. Lett. 85, 3966 (2000);Shelby 等, Science 293, 77 (2001)
J. B. Pendry, Phys. Rev. Lett. 85, 3966 (2000); R. A. Shelby et al., Science 293, 77 (2001)
独立核验依据 · Independent verification: 核验依据 1 Crossref DOI 10.1103/physrevlett.85.3966（复合著录按段核验：本标识只担保
#超材料 #负折射
掺铒光纤放大器为何无需光电转换就能直接放大光信号？
How does an erbium-doped fiber amplifier boost optical signals without any optoelectronic conversion?
光纤纤芯中的铒离子被980或1480纳米的泵浦光抽运到上能级，形成粒子数反转；当1550纳米附近的信号光经过时，激发态离子通过受激辐射释放出与信号同频同相的光子，从而在纯光学域内直接放大信号。其增益波段恰好覆盖石英光纤损耗最低的窗口，使长距离波分复用通信成为现实。
Erbium ions in the fiber core are pumped to an upper energy level by 980 nm or 1480 nm pump light, creating population inversion. When signal light near 1550 nm passes through, the excited ions release photons by stimulated emission—identical in frequency and phase to the signal—so amplification happens entirely in the optical domain. The gain band coincides with the lowest-loss window of silica fiber, making long-haul wavelength-division-multiplexed communication practical.
难度 进阶 · 适合 大众
CC BY 4.0 · 已核验 · Verified 2026-11-04
来源 · Source
Mears, Reekie, Jauncey, Payne,《Low-noise erbium-doped fibre amplifier operating at 1.54μm》,Electronics Letters,1987 (DOI: 10.1049/el:19870719)
Mears, Reekie, Jauncey, Payne, 'Low-noise erbium-doped fibre amplifier operating at 1.54μm', Electronics Letters, 1987, DOI: 10.1049/el:19870719
独立核验依据 · Independent verification: 核验依据 1 Crossref DOI 10.1049/el:19870719
#掺铒光纤放大器 #受激辐射 #光纤通信