Lecturer: Ori Katz, Institute of Applied Physics, The Hebrew University of Jerusalem
Abstract:
In the emerging field of non-Hermitian photonics, the use of amplification (gain) and
attenuation (loss) is being explored for new and unconventional ways of manipulating light
fields [1–3]. With gain being the time-reverse of loss, a central insight in this domain is that the
laser can be operated in reverse to realize a highly efficient absorption device known as the
‘coherent perfect absorber’ (CPA). This device, also termed a ‘time-reversed laser’, or an ‘anti-
laser’ [4,5], is realized by simply replacing the gain medium in any laser with a lossy medium
with loss equal to the threshold-lasing gain. While such CPAs open the possibility to perfectly
absorb radiation in weakly lossy media, and to control the absorption process
interferometrically [5], they come with the severe limitation that only a single, suitably adjusted
wavefront, can be perfectly absorbed [6].
In this talk I will present our recent work [7], where we show how to completely eliminate the
limitation on the number of perfectly absorbed modes by time-reversing a unique (but well
known) laser that can lase in an arbitrary mode. Specifically, our solution builds on time-
reversing a degenerate cavity laser [8,9], which we have exploited in the past for rapid
wavefront shaping [10]. The unique laser cavity contains a simple telescopic arrangement that
self-images the incoming field onto itself in each roundtrip. We use this apparatus to
demonstrate absorption of complex dynamically-varying wavefronts consisting of more than
1,000 modes with >94% efficiency, into a glass slab of 15% absorption. These newly shown
characteristics open interesting new possibilities for applications in light-harvesting, energy
delivery, light control, and imaging, as well as potential applications in radio-frequency and
acoustics.

References
[1] R. El-Ganainy, et al., Nature Physics 14, 11 (2018).
[2] S ̧. K. Ozdemir, S. Rotter, F. Nori, L. Yang, Nature materials 18, 783 (2019).
[3] M.-A. Miri, A. Alu, Science 363, eaar7709 (2019).
[4] Y. Chong, L. Ge, H. Cao, A. D. Stone, Physical review letters 105, 053901 (2010).
[5] D. G. Baranov, A. Krasnok, T. Shegai, A. Alu, Y. Chong, ` Nature Reviews Materials 2, 1 (2017).
[6] K. Pichler, et al., Nature 567, 351 (2019).
[7] Y. Slobodkin, et al., Science 377, 995 (2022).
[8] J. Arnaud, Applied optics 8, 189 (1969).
[9] M. Nixon, E. Ronen, A. A. Friesem, N. Davidson, Physical review letters 110, 184102 (2013).
[10] M.Nixon, O.Katz et al., Nature Photonics, 7, pages 919–924 (2013).