[1] [1] Franken PA, Hill AE, Peters CW, Weinreich G. Generation of optical harmonics. Phys Rev Lett. 1961;7(4):118–9.10.1103/PhysRevLett.7.118

[2] [2] Shen YR. The principles of nonlinear optics. New York: Wiley; 1984.

[3] [3] Boyd RW. Nonlinear optics. 3rd ed. New York: Academic; 2008.

[4] [4] Fejer MM, Magel GA, Jundt DH, Byer RL. Quasi-phase-matched 2nd harmonic-generation-tuning and tolerances. IEEE J Quantum Electron. 1992;28(11):2631–54.10.1109/3.161322

[5] [5] Yamada M, Nada N, Saitoh M, Watanabe K. First-order quasi-phase matched LiNbO3 waveguide periodically poled by applying an external field for efficient blue second-harmonic generation. Appl Phys Lett. 1993;62(5):435–6.10.1063/1.108925

[6] [6] Myers LE, Eckardt RC, Fejer MM, Byer RL, Bosenberg WR, Pierce JW. Quasi-phase-matched optical parametric oscillators in bulk periodically poled LiNbO3. J Opt Soc Am B-Opt Phys. 1995;12(11):2102–16.10.1364/JOSAB.12.002102

[7] [7] Armstrong JA, Bloembergen N, Ducuing J, Pershan PS. Interactions between light waves in a nonlinear dielectric. Phys Rev. 1962;127(6):1918–39. .10.1103/PhysRev.127.1918

[8] [8] Blanchard F, Sharma G, Razzari L, Ropagnol X, Bandulet H-C, Vidal F, Morandotti R, Kieffer JC, Ozaki T, Tiedje H, Haugen H, Reid M, Hegmann F. Generation of intense terahertz radiation via optical methods. IEEE J Selected Topics Quantum Electron. 2011;17(1):5–16. .10.1109/JSTQE.2010.2047715

[9] [9] Yan H, Chen J. Narrowband polarization entangled paired photons with controllable temporal length. Sci China-Phys Mech Astronomy. 2015;58(7):1–10.

[10] [10] Couteau C. Spontaneous parametric down-conversion. Contemp Phys. 2018;59(3):291–304.10.1080/00107514.2018.1488463

[11] [11] He R, Lin ZS, Zheng T, Huang H, Chen CT. Energy band gap engineering in borate ultraviolet nonlinear optical crystals: ab initio studies. J Phys-Condens Matter. 2012;24(14):145503.

[12] [12] Wu M, Ghimire S, Reis DA, Schafer KJ, Gaarde MB. High-harmonic generation from Bloch electrons in solids. Phys Rev A. 2015;91(4):043839.

[13] [13] Ghimire S, Reis DA. High-harmonic generation from solids. Nat Phys. 2019;15(1):10–6.10.1038/s41567-018-0315-5

[14] [14] Tani M, Fukasawa R, Abe H, Matsuura S, Sakai K, Nakashima S. Terahertz radiation from coherent phonons excited in semiconductors. J Appl Phys. 1998;83(5):2473–7.10.1063/1.367007

[15] [15] Klein MW, Enkrich C, Wegener M, Linden S. Second-harmonic generation from magnetic metamaterials. Science. 2006;313(5786):502.10.1126/science.1129198

[16] [16] Aouani H, Rahmani M, Navarro-Cia M, Maier SA. Third-harmonic-upconversion enhancement from a single semiconductor nanoparticle coupled to a plasmonic antenna. Nat Nanotechnol. 2014;9(4):290–4.10.1038/nnano.2014.27

[17] [17] Celebrano M, Wu X, Baselli M, Grossmann S, Biagioni P, Locatelli A, et al. Mode matching in multiresonant plasmonic nanoantennas for enhanced second harmonic generation. Nat Nanotechnol. 2015;10(5):412–7. .10.1038/nnano.2015.69

[18] [18] O'Brien K, Suchowski H, Rho J, Salandrino A, Kante B, Yin XB, et al. Predicting nonlinear properties of metamaterials from the linear response. Nat Mater. 2015;14(4):379–83.10.1038/nmat4214

[19] [19] Segal N, Keren-Zur S, Hendler N, Ellenbogen T. Controlling light with metamaterial-based nonlinear photonic crystals. Nat Photonics. 2015;9(3):180–4.10.1038/nphoton.2015.17

[20] [20] Palomba S, Zhang S, Park Y, Bartal G, Yin XB, Zhang X. Optical negative refraction by four-wave mixing in thin metallic nanostructures. Nat Mater. 2012;11(1):34–8.10.1038/nmat3148

[21] [21] Ren MX, Plum E, Xu JJ, Zheludev NI. Giant nonlinear optical activity in a plasmonic metamaterial. Nat Commun. 2012;3:833.

[22] [22] Lee J, Tymchenko M, Argyropoulos C, Chen PY, Lu F, Demmerle F, et al. Giant nonlinear response from plasmonic metasurfaces coupled to intersubband transitions. Nature. 2014;511(7507):65–9.10.1038/nature13455

[23] [23] Chong KE, Staude I, James A, Dominguez J, Liu S, Campione S, et al. Polarization-independent silicon metadevices for efficient optical wavefront control. Nano Lett. 2015;15(8):5369–74.10.1021/acs.nanolett.5b01752

[24] [24] Li GX, Chen SM, Pholchai N, Reineke B, Wong PWH, Pun EYB, et al. Continuous control of the nonlinearity phase for harmonic generations. Nat Mater. 2015;14(6):607–12.10.1038/nmat4267

[25] [25] Almeida E, Bitton O, Prior Y. Nonlinear metamaterials for holography. Nat Commun. 2016;7:12533.

[26] [26] Li G, Zentgraf T, Zhang S. Rotational doppler effect in nonlinear optics. Nat Phys. 2016;12(8):736–40.10.1038/nphys3699

[27] [27] Ellenbogen T, Voloch-Bloch N, Ganany-Padowicz A, Arie A. Nonlinear generation and manipulation of airy beams. Nat Photonics. 2009;3(7):395–8.10.1038/nphoton.2009.95

[28] [28] Kildishev AV, Boltasseva A, Shalaev VM. Planar photonics with metasurfaces. Science. 2013;339(6125):1232009.

[29] [29] Meinzer N, Barnes WL, Hooper IR. Plasmonic meta-atoms and metasurfaces. Nat Photonics. 2014;8(12):889–98.10.1038/nphoton.2014.247

[30] [30] Yu N, Capasso F. Flat optics with designer metasurfaces. Nat Mater. 2014;13(2):139–50.10.1038/nmat3839

[31] [31] Linden S, Niesler FBP, Forstner J, Grynko Y, Meier T, Wegener M. Collective effects in second-harmonic generation from split-ring-resonator arrays. Phys Rev Lett. 2012;109(1):6488–92.

[32] [32] Shcherbakov MR, Neshev DN, Hopkins B, Shorokhov AS, Staude I, Melik-Gaykazyan EV, Decker M, Ezhov AA, Miroshnichenko AE, Brener I, Fedyanin AA, Kivshar YS. Enhanced third-harmonic generation in silicon nanoparticles driven by magnetic response. Nano Lett. 2014;14(11):6488–92. .10.1021/nl503029j

[33] [33] Kruk S, Weismann M, Bykov AY, Mamonov EA, Kolmychek IA, Murzina T, et al. Enhanced magnetic second-harmonic generation from resonant metasurfaces. Acs Photonics. 2015;2(8):1007–12.10.1021/acsphotonics.5b00215

[34] [34] Yang Y, Wang W, Boulesbaa A, Kravchenko BDP II, Puretzky A, et al. Nonlinear Fano-resonant dielectric metasurfaces. Nano Lett. 2015;15(11):7388–93. .10.1021/acs.nanolett.5b02802

[35] [35] Konishi K, Higuchi T, Li J, Larsson J, Ishii S, Kuwata-Gonokami M. Polarization-controlled circular second-harmonic generation from metal hole arrays with threefold rotational symmetry. Phys Rev Lett. 2014;112(13):135502.10.1103/PhysRevLett.112.135502

[36] [36] Bomzon Z, Biener G, Kleiner V, Hasman E. Space-variant Pancharatnam-Berry phase optical elements with computer-generated subwavelength gratings. Opt Lett. 2002;27(13):1141–3.10.1364/OL.27.001141

[37] [37] Tymchenko M, Gomez-Diaz JS, Lee J, Nookala N, Belkin MA, Alu A. Gradient nonlinear pancharatnam-berry metasurfaces. Phys Rev Lett. 2015;115(20):207403.

[38] [38] Silveirinha M, Engheta N. Tunneling of electromagnetic energy through subwavelength channels and bends using epsilon-near-zero materials. Phys Rev Lett. 2006;97(15):157403.

[39] [39] Alu A, Silveirinha MG, Salandrino A, Engheta N. Epsilon-near-zero metamaterials and electromagnetic sources: Tailoring the radiation phase pattern. Phys Rev B. 2007;75(15):155410.

[40] [40] Edwards B, Alu A, Young ME, Silveirinha M, Engheta N. Experimental verification of epsilon-near-zero metamaterial coupling and energy squeezing using a microwave waveguide. Phys Rev Lett. 2008;100(3):033903.

[41] [41] Molesky S, Dewalt CJ, Jacob Z. High temperature epsilon-near-zero and epsilon-near-pole metamaterial emitters for thermophotovoltaics. Opt Express. 2013;21(1):A96–A110.10.1364/OE.21.000A96

[42] [42] Niu XX, Hu XY, Chu SS, Gong QH. Epsilon-near-zero photonics: a new platform for integrated devices. Adv Opt Mater. 2018;6(10):1701292.

[43] [43] Vassant S, Hugonin J-P, Marquier F, Greffet J-J. Berreman mode and epsilon near zero mode. Opt Express. 2012;20(21):23971–7. .10.1364/OE.20.023971

[44] [44] Campione S, Brener I, Marquier F. Theory of epsilon-near-zero modes in ultrathin films. Phys Rev B. 2015;91(12):121408.10.1103/PhysRevB.91.121408

[45] [45] Jia W, Liu M, Lu Y, Feng X, Wang Q, Zhang X, et al. Broadband terahertz wave generation from an epsilon-near-zero material. Light: Sci Appl. 2021;10(1):11.10.1038/s41377-020-00452-y

[46] [46] Liberal I, Engheta N. Near-zero refractive index photonics. Nat Photonics. 2017;11(3):149.10.1038/nphoton.2017.13

[47] [47] Engheta N. Pursuing near-zero response. Science. 2013;340(6130):286.10.1126/science.1235589

[48] [48] Reshef O, De Leon I, Alam MZ, Boyd RW. Nonlinear optical effects in epsilon-near-zero media. Nat Rev Mater. 2019;4(8):535–51.10.1038/s41578-019-0120-5

[49] [49] Kinsey N, DeVault C, Boltasseva A, Shalaev VM. Near-zero-index materials for photonics. Nat Rev Mater. 2019;4(12):742–60.10.1038/s41578-019-0133-0

[50] [50] Tian W, Liang F, Chi S, Li C, Yu H, Zhang H, et al. Highly efficient super-continuum generation on an epsilon-near-zero surface. ACS Omega. 2020;5(5):2458–64.10.1021/acsomega.9b04026

[51] [51] Niu X, Hu X, Sun Q, Lu C, Yang Y, Yang H, et al. Polarization-selected nonlinearity transition in gold dolmens coupled to an epsilon-near-zero material. Nanophotonics. 2020;9(16):4839–51.10.1515/nanoph-2020-0498

[52] [52] Moitra P, Yang Y, Anderson Z, Kravchenko II, Briggs DP, Valentine J. Realization of an all-dielectric zero-index optical metamaterial. Nat Photonics. 2013;7(10):791–5.10.1038/nphoton.2013.214

[53] [53] Capretti A, Wang Y, Engheta N, Dal NL. Comparative study of second-harmonic generation from epsilon-near-zero indium tin oxide and titanium nitride nanolayers excited in the near-infrared spectral range. ACS Photonics. 2015;2(11):1584–91.10.1021/acsphotonics.5b00355

[54] [54] Capretti A, Wang Y, Engheta N, Dal NL. Enhanced third-harmonic generation in Si-compatible epsilon-near-zero indium tin oxide nanolayers. Opt Lett. 2015;40(7):1500–3.10.1364/OL.40.001500

[55] [55] Luk TS, de Ceglia D, Liu S, Keeler GA, Prasankumar RP, Vincenti MA, et al. Enhanced third harmonic generation from the epsilon-near-zero modes of ultrathin films. Appl Phys Lett. 2015;106(15):151103.10.1063/1.4917457

[56] [56] Alam MZ, De Leon I, Boyd RW. Large optical nonlinearity of indium tin oxide in its epsilon-near-zero region. Science. 2016;352(6287):795–7. .10.1126/science.aae0330

[57] [57] Caspani L, Kaipurath RP, Clerici M, Ferrera M, Roger T, Kim J, et al. Enhanced nonlinear refractive index in epsilon-near-zero materials. Phys Rev Lett. 2016;116(23):233901. .10.1103/PhysRevLett.116.233901

[58] [58] Niu X, Hu X, Lu C, Sheng Y, Yang H, Gong Q. Broadband dispersive free, large, and ultrafast nonlinear material platforms for photonics. Nanophotonics. 2020;9(15):4609–18. .10.1515/nanoph-2020-0420

[59] [59] Kuttruff J, Garoli D, Allerbeck J, Krahne R, De Luca A, Brida D, et al. Ultrafast all-optical switching enabled by epsilon-near-zero-tailored absorption in metal-insulator nanocavities. Commun Phys. 2020;3(1):114.10.1038/s42005-020-0379-2

[60] [60] Jiang X, Lu H, Li Q, Zhou H, Zhang S, Zhang H. Epsilon-near-zero medium for optical switches in a monolithic waveguide chip at 1.9 μm. Nanophotonics. 2018;7(11):1835–43. .10.1515/nanoph-2018-0102

[61] [61] Chen P-Y, Argyropoulos C, Alu A. Enhanced nonlinearities using plasmonic nanoantennas. Nanophotonics. 2012;1(3–4):221–33.10.1515/nanoph-2012-0016

[62] [62] Kauranen M, Zayats AV. Nonlinear plasmonics. Nat Photonics. 2012;6(11):737–48. .10.1038/nphoton.2012.244

[63] [63] Hou-Tong C, Antoinette JT, Nanfang Y. A review of metasurfaces: physics and applications. Rep Prog Phys. 2016;79(7):076401.10.1088/0034-4885/79/7/076401

[64] [64] Kuznetsov AI, Miroshnichenko AE, Brongersma ML, Kivshar YS, Luk'yanchuk B. Optically resonant dielectric nanostructures. Science. 2016;354(6314):2472.

[65] [65] Kruk S, Kivshar Y. Functional meta-optics and nanophotonics governed by Mie resonances. ACS Photonics. 2017;4(11):2638–49. .10.1021/acsphotonics.7b01038

[66] [66] Chang S, Guo X, Ni X. Optical metasurfaces: progress and applications. Annu Rev Mater Res. 2018;48(1):279–302.10.1146/annurev-matsci-070616-124220

[67] [67] Krasnok A, Tymchenko M, Alù A. Nonlinear metasurfaces: a paradigm shift in nonlinear optics. Mater Today. 2018;21(1):8–21.10.1016/j.mattod.2017.06.007

[68] [68] Zou C, Sautter J, Setzpfandt F, Staude I. Resonant dielectric metasurfaces: active tuning and nonlinear effects. J Phys D Appl Phys. 2019;52(37):373002.10.1088/1361-6463/ab25ff

[69] [69] Pertsch T, Kivshar Y. Nonlinear optics with resonant metasurfaces. MRS Bull. 2020;45(3):210–20. .10.1557/mrs.2020.65

[70] [70] Terhune RW, Maker PD, Savage CM. Optical harmonic generation in calcite. Phys Rev Lett. 1962;8(10):404.10.1103/PhysRevLett.8.404

[71] [71] Kang L, Cui YH, Lan SF, Rodrigues SP, Brongersma ML, Cai WS. Electrifying photonic metamaterials for tunable nonlinear optics. Nat Commun. 2014;5:4680.

[72] [72] Lee K-T, Taghinejad M, Yan J, Kim AS, Raju L, Brown DK, et al. Electrically biased silicon metasurfaces with magnetic Mie sesonance for tunable harmonic generation of light. Acs Photonics. 2019;6(11):2663–70.10.1021/acsphotonics.9b01398

[73] [73] Chen S, Li KF, Li G, Cheah KW, Zhang S. Gigantic electric-field-induced second harmonic generation from an organic conjugated polymer enhanced by a band-edge effect. Light: Sci Appl. 2019;8(1):17.

[74] [74] Yap BK, Xia R, Campoy-Quiles M, Stavrinou PN, Bradley DDC. Simultaneous optimization of charge-carrier mobility and optical gain in semiconducting polymer films. Nat Mater. 2008;7(5):376–80. .10.1038/nmat2165

[75] [75] Friberg S, Hong CK, Mandel L. Measurement of time delays in the parametric production of photon pairs. Phys Rev Lett. 1985;54(18):2011–3.10.1103/PhysRevLett.54.2011

[76] [76] Hong CK, Ou ZY, Mandel L. Measurement of subpicosecond time intervals between 2 photons by interference. Phys Rev Lett. 1987;59(18):2044–6.10.1103/PhysRevLett.59.2044

[77] [77] Bocquillon E, Couteau C, Razavi M, Laflamme R, Weihs G. Coherence measures for heralded single-photon sources. Phys Rev A. 2009;79(3):035801.

[78] [78] O'Brien JL, Furusawa A, Vučković J. Photonic quantum technologies. Nat Photonics. 2009;3(12):687–95.10.1038/nphoton.2009.229

[79] [79] Kwiat PG, Mattle K, Weinfurter H, Zeilinger A, Sergienko AV, Shih Y. New high-intensity source of polarization-entangled photon pairs. Phys Rev Lett. 1995;75(24):4337–41.10.1103/PhysRevLett.75.4337

[80] [80] Kwiat PG, Waks E, White AG, Appelbaum I, Eberhard PH. Ultrabright source of polarization-entangled photons. Phys Rev A. 1999;60(2):R773–R6.10.1103/PhysRevA.60.R773

[81] [81] Kim T, Fiorentino M, Wong FNC. Phase-stable source of polarization-entangled photons using a polarization Sagnac interferometer. Phys Rev A. 2006;73(1):012316.

[82] [82] Yin J, Cao Y, Li Y-H, Liao S-K, Zhang L, Ren J-G, et al. Satellite-based entanglement distribution over 1200 kilometers. Science. 2017;356(6343):1180–4.10.1126/science.aan3211

[83] [83] Okoth C, Cavanna A, Santiago-Cruz T, Chekhova MV. Microscale generation of entangled photons without momentum conservation. Phys Rev Lett. 2019;123(26):263602.10.1103/PhysRevLett.123.263602

[84] [84] Marino G, Solntsev AS, Xu L, Gili VF, Carletti L, Poddubny AN, et al. Spontaneous photon-pair generation from a dielectric nanoantenna. Optica. 2019;6(11):1416–22.

[85] [85] Poddubny AN, Iorsh IV, Sukhorukov AA. Generation of photon-plasmon quantum states in nonlinear hyperbolic metamaterials. Phys Rev Lett. 2016;117(12):123901.10.1103/PhysRevLett.117.123901

[86] [86] Petrov MI, Nikolaeva AA, Frizyuk KS, Olekhno NA. Second harmonic generation and spontaneous parametric down-conversion in Mie nanoresonators. J Phys: Conf Ser. 2018;1124:051021.

[87] [87] Song H, Nagatsuma T. Present and future of terahertz communications. IEEE Trans Terahertz Sci Technol. 2011;1(1):256–63.10.1109/TTHZ.2011.2159552

[88] [88] Nagatsuma T, Ducournau G, Renaud CC. Advances in terahertz communications accelerated by photonics. Nat Photonics. 2016;10(6):371–9.10.1038/nphoton.2016.65

[89] [89] Wade CG, Šibalić N, de Melo NR, Kondo JM, Adams CS, Weatherill KJ. Real-time near-field terahertz imaging with atomic optical fluorescence. Nat Photonics. 2017;11(1):40–3.10.1038/nphoton.2016.214

[90] [90] Stantchev RI, Sun B, Hornett SM, Hobson PA, Gibson GM, Padgett MJ, et al. Noninvasive, near-field terahertz imaging of hidden objects using a single-pixel detector. Sci Adv. 2016;2(6):e1600190.10.1126/sciadv.1600190

[91] [91] Mathanker SK, Weckler PR, Wang N. Terahertz (THz) applications in food and agriculture: a review. Trans ASABE. 2013;56(3):1213–26.

[92] [92] Federici JF, Schulkin B, Huang F, Gary D, Barat R, Oliveira F, et al. THz imaging and sensing for security applications—explosives, weapons and drugs. Semicond Sci Technol. 2005;20(7):S266–S80.10.1088/0268-1242/20/7/018

[93] [93] Liu J, Dai J, Chin SL, Zhang XC. Broadband terahertz wave remote sensing using coherent manipulation of fluorescence from asymmetrically ionized gases. Nat Photonics. 2010;4(9):627–31.10.1038/nphoton.2010.165

[94] [94] Xu W, Xie L, Zhu J, Xu X, Ye Z, Wang C, et al. Gold nanoparticle-based terahertz metamaterial sensors: mechanisms and applications. ACS Photonics. 2016;3(12):2308–14.10.1021/acsphotonics.6b00463

[95] [95] Nahata A, Weling AS, Heinz TF. A wideband coherent terahertz spectroscopy system using optical rectification and electro-optic sampling. Appl Phys Lett. 1996;69(16):2321–3.10.1063/1.117511

[96] [96] Wu Q, Litz M, Zhang XC. Broadband detection capability of ZnTe electro-optic field detectors. Appl Phys Lett. 1996;68(21):2924–6.10.1063/1.116356

[97] [97] Hebling J, Almasi G, Kozma IZ, Kuhl J. Velocity matching by pulse front tilting for large-area THz-pulse generation. Opt Express. 2002;10(21):1161–6.10.1364/OE.10.001161

[98] [98] Yeh KL, Hoffmann MC, Hebling J, Nelson KA. Generation of 10 mu J ultrashort terahertz pulses by optical rectification. Appl Phys Lett. 2007;90(17):171121.

[99] [99] Luo L, Chatzakis I, Wang J, Niesler FBP, Wegener M, Koschny T, et al. Broadband terahertz generation from metamaterials. Nat Commun. 2014;5(1):3055.10.1038/ncomms4055

[100] [100] Keren-Zur S, Tal M, Fleischer S, Mittleman DM, Ellenbogen T. Generation of spatiotemporally tailored terahertz wavepackets by nonlinear metasurfaces. Nat Commun. 2019;10(1):1778.10.1038/s41467-019-09811-9

[101] [101] Fang M, Shen NH, Sha WEI, Huang Z, Koschny T, Soukoulis CM. Nonlinearity in the dark: broadband terahertz generation with extremely high efficiency. Phys Rev Lett. 2019;122(2):027401.10.1103/PhysRevLett.122.027401

[102] [102] Okawachi Y, Saha K, Levy JS, Wen YH, Lipson M, Gaeta AL. Octave-spanning frequency comb generation in a silicon nitride chip. Opt Lett. 2011;36(17):3398–400.10.1364/OL.36.003398

[103] [103] Jung H, Xiong C, Fong KY, Zhang X, Tang HX. Optical frequency comb generation from aluminum nitride microring resonator. Opt Lett. 2013;38(15):2810–3.10.1364/OL.38.002810

[104] [104] Shen Y, Harris NC, Skirlo S, Prabhu M, Baehr-Jones T, Hochberg M, et al. Deep learning with coherent nanophotonic circuits. Nat Photonics. 2017;11(7):441–6.10.1038/nphoton.2017.93

[105] [105] Taghinejad M, Cai WS. All-optical control of light in micro- and nanophotonics. Acs Photonics. 2019;6(5):1082–93.10.1021/acsphotonics.9b00013

[106] [106] Almeida VR, Barrios CA, Panepucci RR, Lipson M. All-optical control of light on a silicon chip. Nature. 2004;431(7012):1081–4.10.1038/nature02921

[107] [107] Pelc JS, Rivoire K, Vo S, Santori C, Fattal DA, Beausoleil RG. Picosecond all-optical switching in hydrogenated amorphous silicon microring resonators. Opt Express. 2014;22(4):3797–810.10.1364/OE.22.003797

[108] [108] Shcherbakov MR, Vabishchevich PP, Shorokhov AS, Chong KE, Choi DY, Staude I, Miroshnichenko AE, Neshev DN, Fedyanin AA, Kivshar YS. Ultrafast all-optical switching with magnetic resonances in nonlinear dielectric nanostructures. Nano Lett. 2015;15(10):6985–90. .10.1021/acs.nanolett.5b02989

[109] [109] Wurtz GA, Pollard R, Hendren W, Wiederrecht GP, Gosztola DJ, Podolskiy VA, et al. Designed ultrafast optical nonlinearity in a plasmonic nanorod metamaterial enhanced by nonlocality. Nat Nanotechnol. 2011;6(2):106–10.10.1038/nnano.2010.278

[110] [110] Guo PJ, Schaller RD, Ketterson JB, Chang RPH. Ultrafast switching of tunable infrared plasmons in indium tin oxide nanorod arrays with large absolute amplitude. Nat Photonics. 2016;10(4):267–73. .10.1038/nphoton.2016.14

[111] [111] Clerici M, Kinsey N, DeVault C, Kim J, Carnemolla EG, Caspani L, et al. Controlling hybrid nonlinearities in transparent conducting oxides via two-colour excitation. Nat Commun. 2017;8:15829.

[112] [112] Yang YM, Kelley K, Sachet E, Campione S, Luk TS, Maria JP, et al. Femtosecond optical polarization switching using a cadmium oxide-based perfect absorber. Nat Photonics. 2017;11(6):390–5.10.1038/nphoton.2017.64

[113] [113] Alam MZ, Schulz SA, Upham J, De Leon I, Boyd RW. Large optical nonlinearity of nanoantennas coupled to an epsilon-near-zero material. Nat Photonics. 2018;12(2):79–83.10.1038/s41566-017-0089-9

[114] [114] Voisin C, Del Fatti N, Christofilos D, Vallée F. Ultrafast electron dynamics and optical nonlinearities in metal nanoparticles. J Phys Chem B. 2001;105(12):2264–80.10.1021/jp0038153

[115] [115] Krasavin AV, Ginzburg P, Zayats AV. Free-electron optical nonlinearities in plasmonic nanostructures: a review of the hydrodynamic description. Laser Photonics Rev. 2018;12(1):1700082.

[116] [116] Del Fatti N, Bouffanais R, Vallee F, Flytzanis C. Nonequilibrium electron interactions in metal films. Phys Rev Lett. 1998;81(4):922–5.10.1103/PhysRevLett.81.922

[117] [117] Ren MX, Jia BH, Ou JY, Plum E, Zhang JF, MacDonald KF, et al. Nanostructured plasmonic medium for terahertz bandwidth all-optical switching. Adv Mater. 2011;23(46):5540–4. .10.1002/adma.201103162

[118] [118] Kinsey N, DeVault C, Kim J, Ferrera M, Shalaev VM, Boltasseva A. Epsilon-near-zero Al-doped ZnO for ultrafast switching at telecom wavelengths. Optica. 2015;2(7):616–22. .10.1364/OPTICA.2.000616

[119] [119] Guo QB, Cui YD, Yao YH, Ye YT, Yang Y, Liu XM, et al. A solution-processed ultrafast optical switch based on a nanostructured epsilon-near-zero medium. Adv Mater. 2017;29(27):7.

[120] [120] Wang J, Coillet A, Demichel O, Wang Z, Rego D, Bouhelier A, et al. Saturable plasmonic metasurfaces for laser mode locking. Light Sci Appl. 2020;9:50.10.1038/s41377-020-0291-2

[121] [121] Kartner FX, Jung ID, Keller U. Soliton mode-locking with saturable absorbers. IEEE J Selected Topics Quantum Electron. 1996;2(3):540–56. .10.1109/2944.571754

[122] [122] Keller U, Weingarten KJ, Kartner FX, Kopf D, Braun B, Jung ID, et al. Semiconductor saturable absorber mirrors (SESAM's) for femtosecond to nanosecond pulse generation in solid-state lasers. IEEE J Selected Topics Quantum Electron. 1996;2(3):435–53.10.1109/2944.571743

[123] [123] Spuhler GJ, Paschotta R, Fluck R, Braun B, Moser M, Zhang G, et al. Experimentally confirmed design guidelines for passively Q-switched microchip lasers using semiconductor saturable absorbers. J Opt Soc Am B-Opt Phys. 1999;16(3):376–88. .10.1364/JOSAB.16.000376

[124] [124] Keller U. Recent developments in compact ultrafast lasers. Nature. 2003;424(6950):831–8.10.1038/nature01938

[125] [125] Keller U, Tropper AC. Passively modelocked surface-emitting semiconductor lasers. Phys Rep-Rev Sect Phys Lett. 2006;429(2):67–120.

[126] [126] Bao Q, Zhang H, Wang Y, Ni Z, Yan Y, Shen ZX, et al. Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers. Adv Funct Mater. 2009;19(19):3077–83.10.1002/adfm.200901007

[127] [127] Sun Z, Hasan T, Torrisi F, Popa D, Privitera G, Wang F, et al. Graphene mode-locked ultrafast laser. ACS Nano. 2010;4(2):803–10.10.1021/nn901703e

[128] [128] Sun Z, Hasan T, Ferrari AC. Ultrafast lasers mode-locked by nanotubes and graphene. Physica E-Low-Dimensional Syst Nanostructures. 2012;44(6):1082–91.10.1016/j.physe.2012.01.012

[129] [129] Martinez A, Sun Z. Nanotube and graphene saturable absorbers for fibre lasers. Nat Photonics. 2013;7(11):842–5.10.1038/nphoton.2013.304

[130] [130] Wang K, Wang J, Fan J, Lotya M, O'Neill A, Fox D, et al. Ultrafast saturable absorption of two-dimensional MoS2 nanosheets. ACS Nano. 2013;7(10):9260–7. .10.1021/nn403886t

[131] [131] Woodward RI, Kelleher EJR, Howe RCT, Hu G, Torrisi F, Hasan T, et al. Tunable Q-switched fiber laser based on saturable edge-state absorption in few-layer molybdenum disulfide (MoS2). Opt Express. 2014;22(25):31113–22.10.1364/OE.22.031113

[132] [132] Luo Z, Wu D, Xu B, Xu H, Cai Z, Peng J, Weng J, Xu S, Zhu C, Wang F, Sun Z, Zhang H. Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers. Nanoscale. 2016;8(2):1066–72. .10.1039/C5NR06981E

[133] [133] Li D, Jussila H, Karvonen L, Ye G, Lipsanen H, Chen X, et al. Polarization and thickness dependent absorption properties of black phosphorus: new saturable absorber for ultrafast pulse generation. Sci Rep. 2015;5:15899.

[134] [134] Wang Y, Huang G, Mu H, Lin S, Chen J, Xiao S, et al. Ultrafast recovery time and broadband saturable absorption properties of black phosphorus suspension. Appl Phys Lett. 2015;107(9):091905.

[135] [135] Hu G, Albrow-Owen T, Jin X, Ali A, Hu Y, Howe RCT, et al. Black phosphorus ink formulation for inkjet printing of optoelectronics and photonics. Nat Commun. 2017;8:278.

[136] [136] Krause JL, Schafer KJ, Kulander KC. High-order harmonic generation from atoms and ions in the high intensity regime. Phys Rev Lett. 1992;68(24):3535–8.10.1103/PhysRevLett.68.3535

[137] [137] Schafer KJ, Yang B, DiMauro LF, Kulander KC. Above threshold ionization beyond the high harmonic cutoff. Phys Rev Lett. 1993;70(11):1599–602. .10.1103/PhysRevLett.70.1599

[138] [138] Corkum PB. Plasma perspective on strong field multiphoton ionization. Phys Rev Lett. 1993;71(13):1994–7.10.1103/PhysRevLett.71.1994

[139] [139] Stolow A, Bragg AE, Neumark DM. Femtosecond time-resolved photoelectron spectroscopy. Chem Rev. 2004;104(4):1719–57. .10.1021/cr020683w

[140] [140] Cavalieri AL, Mueller N, Uphues T, Yakovlev VS, Baltuska A, Horvath B, et al. Attosecond spectroscopy in condensed matter. Nature. 2007;449(7165):1029–32.10.1038/nature06229

[141] [141] Corkum PB, Krausz F. Attosecond science. Nat Phys. 2007;3(6):381–7.10.1038/nphys620

[142] [142] Krausz F, Ivanov M. Attosecond Phys. Rev Mod Phys. 2009;81(1):163–234.10.1103/RevModPhys.81.163

[143] [143] Ferray M, Lhuillier A, Li XF, Lompre LA, Mainfray G, Manus C. Multiple-harmonic conversion of 1064-nm radiation in rare-gases. J Phys B-Atom Mol Opt Phys. 1988;21(3):L31–L5.10.1088/0953-4075/21/3/001

[144] [144] Baltuska A, Udem T, Uiberacker M, Hentschel M, Goulielmakis E, Gohle C, et al. Attosecond control of electronic processes by intense light fields. Nature. 2003;421(6923):611–5.10.1038/nature01414

[145] [145] McFarland BK, Farrell JP, Bucksbaum PH, Gühr M. High harmonic generation from multiple orbitals in N2. Science. 2008;322(5905):1232.10.1126/science.1162780

[146] [146] Kim S, Jin J, Kim Y-J, Park I-Y, Kim Y, Kim S-W. High-harmonic generation by resonant plasmon field enhancement. Nature. 2008;453(7196):757–60.10.1038/nature07012

[147] [147] Ghimire S, DiChiara AD, Sistrunk E, Agostini P, DiMauro LF, Reis DA. Observation of high-order harmonic generation in a bulk crystal. Nat Phys. 2011;7(2):138–41.10.1038/nphys1847

[148] [148] Luu TT, Garg M, Kruchinin SY, Moulet A, Hassan MT, Goulielmakis E. Extreme ultraviolet high-harmonic spectroscopy of solids. Nature. 2015;521(7553):498–502.10.1038/nature14456

[149] [149] Schubert O, Hohenleutner M, Langer F, Urbanek B, Lange C, Huttner U, et al. Sub-cycle control of terahertz high-harmonic generation by dynamical Bloch oscillations. Nat Photonics. 2014;8(2):119–23.10.1038/nphoton.2013.349

[150] [150] Han S, Kim H, Kim YW, Kim Y-J, Kim S, Park I-Y, et al. High-harmonic generation by field enhanced femtosecond pulses in metal-sapphire nanostructure. Nat Commun. 2016;7(1):13105.10.1038/ncomms13105

[151] [151] Vampa G, Ghamsari BG, Siadat Mousavi S, Hammond TJ, Olivieri A, Lisicka-Skrek E, et al. Plasmon-enhanced high-harmonic generation from silicon. Nat Phys. 2017;13(7):659–62.10.1038/nphys4087

[152] [152] Liu H, Guo C, Vampa G, Zhang JL, Sarmiento T, Xiao M, et al. Enhanced high-harmonic generation from an all-dielectric metasurface. Nat Phys. 2018;14(10):1006–10.10.1038/s41567-018-0233-6

[153] [153] Yang Y, Lu J, Manjavacas A, Luk TS, Liu H, Kelley K, Maria JP, Runnerstrom EL, Sinclair MB, Ghimire S, Brener I. High-harmonic generation from an epsilon-near-zero material. Nat Phys. 2019;15(10):1022–6. .10.1038/s41567-019-0584-7

[154] [154] Sivis M, Taucer M, Vampa G, Johnston K, Staudte A, Naumov AY, Villeneuve DM, Ropers C, Corkum PB. Tailored semiconductors for high-harmonic optoelectronics. Science. 2017;357(6348):303–6. .10.1126/science.aan2395

[155] [155] Wang J, Bo F, Wan S, Li W, Gao F, Li J, et al. High-Q lithium niobate microdisk resonators on a chip for efficient electro-optic modulation. Opt Express. 2015;23(18):23072–8.10.1364/OE.23.023072

[156] [156] Rao A, Patil A, Rabiei P, Honardoost A, Desalvo R, Paolella A, et al. High-performance and linear thin-film lithium niobate Mach-Zehnder modulators on silicon up to 50 GHz. Opt Lett. 2016;41(24):5700–3.10.1364/OL.41.005700

[157] [157] Liang H, Luo R, He Y, Jiang H, Lin Q. High-quality lithium niobate photonic crystal nanocavities. Optica. 2017;4(10):1251–8.10.1364/OPTICA.4.001251

[158] [158] Zhang M, Wang C, Cheng R, Shams-Ansari A, Loncar M. Monolithic ultra-high-Q lithium niobate microring resonator. Optica. 2017;4(12):1536–7.10.1364/OPTICA.4.001536

[159] [159] Wang C, Zhang M, Chen X, Bertrand M, Shams-Ansari A, Chandrasekhar S, Winzer P, Lončar M. Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages. Nature. 2018;562(7725):101–4. .10.1038/s41586-018-0551-y

[160] [160] Wang C, Zhang M, Stern B, Lipson M, Loncar M. Nanophotonic lithium niobate electro-optic modulators. Opt Express. 2018;26(2):1547–55.10.1364/OE.26.001547

[161] [161] Gao BF, Ren MX, Wu W, Hu H, Cai W, Xu JJ. Lithium niobate metasurfaces. Laser Photonics Rev. 2019;13(5):6.10.1002/lpor.201800312

[162] [162] Vabishchevich PP, Liu S, Sinclair MB, Keeler GA, Peake GM, Brener I. Enhanced second-harmonic generation using broken symmetry III–V semiconductor Fano metasurfaces. ACS Photonics. 2018;5(5):1685–90. .10.1021/acsphotonics.7b01478

[163] [163] Koshelev K, Tang Y, Li K, Choi D-Y, Li G, Kivshar Y. Nonlinear metasurfaces governed by bound states in the continuum. ACS Photonics. 2019;6(7):1639–44.10.1021/acsphotonics.9b00700

[164] [164] Liu Z, Xu Y, Lin Y, Xiang J, Feng T, Cao Q, et al. High-Q quasibound states in the continuum for nonlinear metasurfaces. Phys Rev Lett. 2019;123(25):253901.

[165] [165] Lin Z, Liang X, Loncar M, Johnson SG, Rodriguez AW. Cavity-enhanced second-harmonic generation via nonlinear-overlap optimization. Optica. 2016;3(3):233–8.10.1364/OPTICA.3.000233

[166] [166] Hughes TW, Minkov M, Williamson IAD, Fan S. Adjoint method and inverse design for nonlinear nanophotonic devices. ACS Photonics. 2018;5(12):4781–7.10.1021/acsphotonics.8b01522

[167] [167] Sitawarin C, Jin W, Lin Z, Rodriguez AW. Inverse-designed photonic fibers and metasurfaces for nonlinear frequency conversion invited. Photonics Research. 2018;6(5):B82–B9.10.1364/PRJ.6.000B82

[168] [168] Lei X, Rahmani M, Yixuan M, Smirnova DA, Kamali KZ, Fu D, et al. Enhanced light-matter interactions in dielectric nanostructures via machine-learning approach. Adv Photonics. 2020;2(2):026003.

[169] [169] Shi L, Iwan B, Nicolas R, Ripault Q, Andrade JRC, Han S, et al. Self-optimization of plasmonic nanoantennas in strong femtosecond fields. Optica. 2017;4(9):1038–43.10.1364/OPTICA.4.001038

[170] [170] Zimmermann P, Hötger A, Fernandez N, Nolinder A, Müller K, Finley JJ, et al. Toward plasmonic tunnel gaps for nanoscale photoemission currents by on-chip laser ablation. Nano Lett. 2019;19(2):1172–8.10.1021/acs.nanolett.8b04612

[171] [171] Li Z-Z, Wang L, Fan H, Yu Y-H, Sun H-B, Juodkazis S, et al. O-FIB: far-field-induced near-field breakdown for direct nanowriting in an atmospheric environment. Light-Sci Appl. 2020;9(1):41.

[172] [172] Liu X-Q, Yu L, Yang S-N, Chen Q-D, Wang L, Juodkazis S, et al. Optical nanofabrication of concave microlens arrays. Laser Photonics Rev. 2019;13(5):1800272.

[173] [173] Liu X-Q, Yang S-N, Yu L, Chen Q-D, Zhang Y-L, Sun H-B. Rapid engraving of artificial compound eyes from curved sapphire substrate. Adv Funct Mater. 2019;29(18):1900037.

[174] [174] Wei D, Wang C, Wang H, Hu X, Wei D, Fang X, et al. Experimental demonstration of a three-dimensional lithium niobate nonlinear photonic crystal. Nat Photonics. 2018;12(10):596–600.10.1038/s41566-018-0240-2