Free-electron radiation results from the interaction between swift electrons and the local electromagnetic environment. Recent advances inmaterial technologies provide powerful tools to control light emission from free electrons and may facilitate many intriguing applications of free-electron radiation in particle detections, lasers, quantum information processing, etc. Here, we provide a brief overview on the recent theoretical developments and experimental observations of spontaneous free-electron radiation in various structured environments, including two-dimensional materials, metasurfaces, metamaterials, and photonic crystals. We also report the research progresses on the stimulated free-electron radiation that results from the interaction between free electrons and photonic quasi-particles induced by the external field. Moreover, we provide an outlook of potential research directions for this vigorous realm of free-electron radiation.
1. Čerenkov, P., "Visible light from pure liquids under the impact of γ-rays," Dokl. Akad. Nauk SSSR, Vol. 2, 451-457, 1934.
2. Tamm, I. and I. Frank, "Coherent radiation of fast electrons in a medium," Dokl. Akad. Nauk SSSR, Vol. 14, 107-112, 1937.
3. Ypsilantis, T. and J. Séguinot, "Theory of ring imaging Cherenkov counters," Nucl. Instrum. Meth. A, Vol. 343, 30-51, 1994.
4. Abashian, A., K. Gotow, N. Morgan, L. Piilonen, S. Schrenk, K. Abe, I. Adachi, J. Alexander, K. Aoki, and S. Behari, "The belle detector," Nucl. Instrum. Meth. A, Vol. 479, 117-232, 2002.
5. Adam, I., R. Aleksan, L. Amerman, E. Antokhin, D. Aston, P. Bailly, C. Beigbeder, M. Benkebil, P. Besson, and G. Bonneaud, "The DIRC particle identification system for the BaBar experiment," Nucl. Instrum. Meth. A, Vol. 538, 281-357, 2005.
6. Elder, F., A. Gurewitsch, R. Langmuir, and H. Pollock, "Radiation from electrons in a synchrotron," Phys. Rev., Vol. 71, 829, 1947.
8. Happek, U., A. Sievers, and E. Blum, "Observation of coherent transition radiation," Phys. Rev. Lett., Vol. 67, 2962, 1991.
9. Smith, S. J. and E. Purcell, "Visible light from localized surface charges moving across a grating," Phys. Rev., Vol. 92, 1069, 1953.
10. Koch, H. and J. Motz, "Bremsstrahlung cross-section formulas and related data," Rev. Mod. Phys., Vol. 31, 920, 1959.
11. Tian, H., J. Tice, R. Fei, V. Tran, X. Yan, L. Yang, and H. Wang, "Low-symmetry two-dimensional materials for electronic and photonic applications," Nano Today, Vol. 11, 763-777, 2016.
12. Chen, H., C. T. Chan, and P. Sheng, "Transformation optics and metamaterials," Nat. Mater., Vol. 9, 387-396, 2010.
13. Joannopoulos, J. D., S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 1st Ed., 1995.
14. Khanikaev, A. B., S. H. Mousavi, W.-K. Tse, M. Kargarian, A. H. MacDonald, and G. Shvets, "Photonic topological insulators," Nat. Mater., Vol. 12, 233-239, 2013.
15. Liu, S., P. Zhang, W. Liu, S. Gong, R. Zhong, Y. Zhang, and M. Hu, "Surface polariton Cherenkov light radiation source," Phys. Rev. Lett., Vol. 109, 153902, 2012.
16. Tao, J., Q. J. Wang, J. Zhang, and Y. Luo, "Reverse surface-polariton Cherenkov radiation," Sci. Rep., Vol. 6, 1-8, 2016.
17. Massuda, A., C. Roques-Carmes, Y. Yang, S. E. Kooi, Y. Yang, C. Murdia, K. K. Berggren, I. Kaminer, and M. Soljačić, "Smith-Purcell radiation from low-energy electrons," ACS Photonics, Vol. 5, 3513-3518, 2018.
18. Su, Z., F. Cheng, L. Li, and Y. Liu, "Complete control of Smith-Purcell radiation by graphene metasurfaces," ACS Photonics, Vol. 6, 1947-1954, 2019.
19. Su, Z., B. Xiong, Y. Xu, Z. Cai, J. Yin, R. Peng, and Y. Liu, "Manipulating Cherenkov radiation and Smith-Purcell radiation by artificial structures," Adv. Opt. Mater., Vol. 7, 1801666, 2019.
20. Hu, H., D. Gao, X. Lin, S. Hou, B. Zhang, Q. J. Wang, and Y. Luo, "Directing Cherenkov photons with spatial nonlocality," Nanophotonics, Vol. 9, 3435-3442, 2020.
21. Burlak, G. and E. Martinez-Sanchez, "Change of structure of the Cherenkov emission at modulated source in dispersive metamaterials," Progress In Electromagnetics Research, Vol. 139, 277-288, 2013.
22. Burlak, G. and E. Martinez-Sanchez, "The Cherenkov emission in regular and random photonic crystals," Progress In Electromagnetics Research M, Vol. 47, 77-86, 2016.
23. Liu, Y. and L. Ang, "Motion-induced radiation from electrons moving in Maxwell's fish-eye," Sci. Rep., Vol. 3, 1-7, 2013.
24. Vorobev, V. V. and A. V. Tyukhtin, "Nondivergent Cherenkov radiation in a wire metamaterial," Phys. Rev. Lett., Vol. 108, 184801, 2012.
25. Liu, F., L. Xiao, Y. Ye, M.Wang, K. Cui, X. Feng, W. Zhang, and Y. Huang, "Integrated Cherenkov radiation emitter eliminating the electron velocity threshold," Nat. Photonics, Vol. 11, 289-292, 2017.
26. Chen, H. and M. Chen, "Flipping photons backward: Reversed Cherenkov radiation," Mater. Today, Vol. 14, 34-41, 2011.
27. Rivera, N. and I. Kaminer, "Light-matter interactions with photonic quasiparticles," Nat. Rev. Phys., Vol. 2, 538-561, 2020.
28. García de Abajo, F. J. and V. Di Giulio, "Optical excitations with electron beams: Challenges and opportunities," ACS Photonics, Vol. 8, 945-974, 2021.
29. Xia, F., H. Wang, D. Xiao, M. Dubey, and A. Ramasubramaniam, "Two-dimensional material nanophotonics," Nat. Photonics, Vol. 8, 899-907, 2014.
30. Koppens, F. H., D. E. Chang, and F. J. García de Abajo, "Graphene plasmonics: A platform for strong light-matter interactions," Nano Lett., Vol. 11, 3370-3377, 2011.
31. Lin, X., Y. Yang, N. Rivera, J. J. López, Y. Shen, I. Kaminer, H. Chen, B. Zhang, J. D. Joannopoulos, and M. Soljačić, "All-angle negative refraction of highly squeezed plasmon and phonon polaritons in graphene-boron nitride heterostructures," Proc. Natl. Acad. Sci., Vol. 114, 6717-6721, 2017.
32. Jiang, Y., X. Lin, and H. Chen, "Directional polaritonic excitation of circular, huygens and janus dipoles in graphene-hexagonal boron nitride heterostructures," Progress In Electromagnetics Research, Vol. 170, 169-176, 2021.
33. Wang, C., C. Qian, H. Hu, L. Shen, Z. J. Wang, H. Wang, Z. Xu, B. Zhang, H. Chen, and X. Lin, "Superscattering of light in refractive-index near-zero environments," Progress In Electromagnetics Research, Vol. 168, 15-23, 2020.
34. Garcia de Abajo, F. J., "Graphene plasmonics: Challenges and opportunities," Acs Photonics, Vol. 1, 135-152, 2014.
35. Zhao, T., M. Hu, R. Zhong, S. Gong, C. Zhang, and S. Liu, "Cherenkov terahertz radiation from graphene surface plasmon polaritons excited by an electron beam," Applied Physics Letters, Vol. 110, 231102, 2017.
36. Tao, J., L. Wu, and G. Zheng, "Graphene surface-polariton in-plane Cherenkov radiation," Carbon, Vol. 133, 249-253, 2018.
37. Rosolen, G., L. J. Wong, N. Rivera, B. Maes, M. Soljačić, and I. Kaminer, "Metasurface-based multi-harmonic free-electron light source," Light: Sci. Appl., Vol. 7, 1-12, 2018.
38. Pizzi, A., G. Rosolen, L. J. Wong, R. Ischebeck, M. Soljačić, T. Feurer, and I. Kaminer, "Graphene metamaterials for intense, tunable, and compact extreme ultraviolet and X-ray sources," Adv. Sci., Vol. 7, 1901609, 2020.
39. Zhang, X., M. Hu, Z. Zhang, Y. Wang, T. Zhang, X. Xu, T. Zhao, Z. Wu, R. Zhong, and D. Liu, "High-efficiency threshold-less Cherenkov radiation generation by a graphene hyperbolic grating in the terahertz band," Carbon, Vol. 183, 225-231, 2021.
40. Lin, X., I. Kaminer, X. Shi, F. Gao, Z. Yang, Z. Gao, H. Buljan, J. D. Joannopoulos, M. Soljačić, and H. Chen, "Splashing transients of 2D plasmons launched by swift electrons," Sci. Adv., Vol. 3, e1601192, 2017.
41. Chen, J., H. Chen, and X. Lin, "Photonic and plasmonic transition radiation from graphene," Journal of Optics, Vol. 23, 034001, 2021.
42. Cox, J. D. and F. J. Garcia de Abajo, "Nonlinear interactions between free electrons and nanographenes," Nano Lett., Vol. 20, 4792-4800, 2020.
43. Kaminer, I., Y. T. Katan, H. Buljan, Y. Shen, O. Ilic, J. J. López, L. J. Wong, J. D. Joannopoulos, and M. Soljačić, "Efficient plasmonic emission by the quantum Čerenkov effect from hot carriers in graphene," Nat. Commun., Vol. 7, 1-9, 2016.
44. Wong, L. J., I. Kaminer, O. Ilic, J. D. Joannopoulos, and M. Soljačić, "Towards graphene plasmon-based free-electron infrared to X-ray sources," Nat. Photonics, Vol. 10, 46-52, 2016.
45. Geim, A. K. and I. V. Grigorieva, "Van der Waals heterostructures," Nature, Vol. 499, 419-425, 2013.
46. Govyadinov, A. A., A. Konečná, A. Chuvilin, S. Vélez, I. Dolado, A. Y. Nikitin, S. Lopatin, F. Casanova, L. E. Hueso, and J. Aizpurua, "Probing low-energy hyperbolic polaritons in van der Waals crystals with an electron microscope," Nat. Commun., Vol. 8, 1-10, 2017.
47. Maciel-Escudero, C., A. Konečná, R. Hillenbrand, and J. Aizpurua, "Probing and steering bulk and surface phonon polaritons in uniaxial materials using fast electrons: Hexagonal boron nitride," Phys. Rev. B, Vol. 102, 115431, 2020.
48. Tao, J., L. Wu, G. Zheng, and S. Yu, "Cherenkov polaritonic radiation in a natural hyperbolic material," Carbon, Vol. 150, 136-141, 2019.
49. Zhang, Y., C. Hu, B. Lyu, H. Li, Z. Ying, L. Wang, A. Deng, X. Luo, Q. Gao, and J. Chen, "Tunable Cherenkov radiation of phonon Polaritons in silver nanowire/hexagonal boron nitride heterostructures," Nano Lett., Vol. 20, 2770-2777, 2020.
50. Xi, S., H. Chen, T. Jiang, L. Ran, J. Huangfu, B.-I. Wu, J. A. Kong, and M. Chen, "Experimental verification of reversed Cherenkov radiation in left-handed metamaterial," Phys. Rev. Lett., Vol. 103, 194801, 2009.
51. Duan, Z., X. Tang, Z. Wang, Y. Zhang, X. Chen, M. Chen, and Y. Gong, "Observation of the reversed Cherenkov radiation," Nat. Commun., Vol. 8, 1-7, 2017.
52. Duan, Z., B.-I. Wu, S. Xi, H. Chen, and M. Chen, "Research progress in reversed Cherenkov radiation in double-negative metamaterials," Progress In Electromagnetics Research, Vol. 90, 75-87, 2009.
53. Zhang, B. and B.-I. Wu, "Electromagnetic detection of a perfect invisibility cloak," Phys. Rev. Lett., Vol. 103, 243901, 2009.
54. Hu, H., X. Lin, J. Zhang, D. Liu, P. Genevet, B. Zhang, and Y. Luo, "Nonlocality induced Cherenkov threshold," Laser Photonics Rev., Vol. 14, 2000149, 2020.
55. Hu, H., J. Zhang, S. A. Maier, and Y. Luo, "Enhancing third-harmonic generation with spatial nonlocality," ACS Photonics, Vol. 5, 592-598, 2018.
56. Luo, C., M. Ibanescu, S. G. Johnson, and J. Joannopoulos, "Cerenkov radiation in photonic crystals," Science, Vol. 299, 368-371, 2003.
57. Adamo, G., K. F. MacDonald, Y. Fu, C. Wang, D. Tsai, F. G. De Abajo, and N. Zheludev, "Light well: A tunable free-electron light source on a chip," Phys. Rev. Lett., Vol. 103, 113901, 2009.
58. Song, Y., N. Jiang, L. Liu, X. Hu, and J. Zi, "Cherenkov radiation from photonic bound states in the continuum: Towards compact free-electron lasers," Phys. Rev. Appl., Vol. 10, 064026, 2018.
59. Yang, Y., A. Massuda, C. Roques-Carmes, S. E. Kooi, T. Christensen, S. G. Johnson, J. D. Joannopoulos, O. D. Miller, I. Kaminer, and M. Soljačić, "Maximal spontaneous photon emission and energy loss from free electrons," Nat. Phys., Vol. 14, 894-899, 2018.
60. Yu, Y., K. Lai, J. Shao, J. Power, M. Conde, W. Liu, S. Doran, C. Jing, E. Wisniewski, and G. Shvets, "Transition radiation in photonic topological crystals: Quasiresonant excitation of robust edge states by a moving charge," Phys. Rev. Lett., Vol. 123, 057402, 2019.
61. Piazza, L., T. Lummen, E. Quinonez, Y. Murooka, B. Reed, B. Barwick, and F. Carbone, "Simultaneous observation of the quantization and the interference pattern of a plasmonic near-field," Nat. Commun., Vol. 6, 1-7, 2015.
62. Kurman, Y., R. Dahan, H. H. Sheinfux, K. Wang, M. Yannai, Y. Adiv, O. Reinhardt, L. H. Tizei, S. Y. Woo, and J. Li, "Spatiotemporal imaging of 2D polariton wave packet dynamics using free electrons," Science, Vol. 372, 1181-1186, 2021.
63. Wang, K., R. Dahan, M. Shentcis, Y. Kauffmann, A. B. Hayun, O. Reinhardt, S. Tsesses, and I. Kaminer, "Coherent interaction between free electrons and a photonic cavity," Nature, Vol. 582, 50-54, 2020.
64. Dahan, R., S. Nehemia, M. Shentcis, O. Reinhardt, Y. Adiv, X. Shi, O. Be'er, M. H. Lynch, Y. Kurman, and K. Wang, "Resonant phase-matching between a light wave and a free-electron wavefunction," Nat. Phys., Vol. 16, 1123-1131, 2020.
65. Vanacore, G. M., G. Berruto, I. Madan, E. Pomarico, P. Biagioni, R. Lamb, D. McGrouther, O. Reinhardt, I. Kaminer, and B. Barwick, "Ultrafast generation and control of an electron vortex beam via chiral plasmonic near fields," Nat. Mater., Vol. 18, 573-579, 2019.
66. Galiffi, E., P. Huidobro, and J. Pendry, "Broadband nonreciprocal amplification in luminal metamaterials," Phys. Rev. Lett., Vol. 123, 206101, 2019.
67. Oue, D., K. Ding, and J. Pendry, "Čerenkov radiation in vacuum from a superluminal grating,", arXiv preprint, Vol. arXiv:.13681, 2021, https://arxiv.org/abs/2105.13681.
68. Sloan, J., N. Rivera, J. D. Joannopoulos, and M. Soljačić, "Two photon emission from superluminal and accelerating index perturbations,", arXiv preprint, Vol. arXiv:.09955, 2021, https://arxiv.org/abs/2103.09955.
69. Rivera, N., I. Kaminer, B. Zhen, J. D. Joannopoulos, and M. Soljačić, "Shrinking light to allow forbidden transitions on the atomic scale," Science, Vol. 353, 263-269, 2016.
70. Ginis, V., J. Danckaert, I. Veretennicoff, and P. Tassin, "Controlling Cherenkov radiation with transformation-optical metamaterials," Phys. Rev. Lett., Vol. 113, 167402, 2014.
71. Lin, X., S. Easo, Y. Shen, H. Chen, B. Zhang, J. D. Joannopoulos, M. Soljačić, and I. Kaminer, "Controlling Cherenkov angles with resonance transition radiation," Nat. Phys., Vol. 14, 816-821, 2018.
72. Lin, X., H. Hu, S. Easo, Y. Yang, Y. Shen, K. Yin, M. P. Blago, I. Kaminer, B. Zhang, H. Chen, J. Joannopoulos, M. Soljačić, and Y. Luo, "A Brewster route to Cherenkov detectors,", arXiv preprint, Vol. arXiv:.11996, 2021, https://arxiv.org/abs/2107.11996.
73. Hu, H., X. Lin, L. J. Wong, Q. Yang, B. Zhang, and Y. Luo, "Surface Dyakonov-Cherenkov radiation,", arXiv preprint, Vol. arXiv:.09533, 2020, https://arxiv.org/abs/2012.09533.