# Light emission based on nanophotonic vacuum forces

## Abstract

The vanishingly small response of matter to light above ultraviolet frequencies makes the manipulation of light emission at such frequencies challenging. As a result, state-of-the-art sources of high-frequency light are typically active, relying on strong external electromagnetic fields. Here, we present a fundamental mechanism of light emission that is fully passive, relying instead on vacuum fluctuations near nanophotonic structures. This mechanism can be used to generate light at any frequency, including high-frequency radiation such as X-rays. The proposed mechanism is equivalent to a quantum optical two-photon process, in which a free electron spontaneously emits a low-energy polariton and a high-energy photon simultaneously. Although two-photon processes are nominally weak, we find that the resulting X-ray radiation can be substantial. The strength of this process is related to the strong Casimir–Polder forces that atoms experience in the nanometre vicinity of materials, with the essential difference being that the fluctuating force here acts on a free electron, rather than a neutral, polarizable atom. The light emission can be shaped by controlling the nanophotonic geometry or the underlying material electromagnetic response at optical or infrared frequencies. Our results reveal ways of applying the tools of nanophotonics even at frequencies where materials have an insubstantial electromagnetic response. The process we study, when scaled up, may also enable new concepts for compact and tunable X-ray radiation.

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## Data availability

The data represented in Figs. 24 are available as Supplementary information files. All other data that support the plots within this paper and other findings of this study are available from the corresponding author on reasonable request.

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## Acknowledgements

We thank T. Christensen and G. Rosolen for helpful discussions. This research was supported by the Binational USA-Israel Science Foundation (BSF). N.R. was supported by Department of Energy Fellowship DE-FG02-97ER25308. L.J.W. was supported by the Advanced Manufacturing and Engineering Young Individual Research Grant (no. A1984c0043) from the Science and Engineering Research Council of the Agency for Science, Technology and Research, Singapore. This work was also partly supported by the Army Research Office through the Institute for Soldier Nanotechnologies under contract no. W911NF-18-2-0048. This work was also supported in part by the MRSEC Program of the National Science Foundation under award number DMR – 1419807. I.K. was also supported by a Starter Grant from the European Research Council and by the Israel Science Foundation.

## Author information

N.R. led the work with substantial input from all other authors.

Correspondence to Nicholas Rivera or Ido Kaminer.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature Physics thanks Frank Koppens and Kimball Milton for their contribution to the peer review of this work.

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## Supplementary information

### Supplementary Information

Supplementary Figs. 1–5, additional discussion and supplementary references.

### Figure 2 data

Source data for Fig. 2.

### Figure 3 data

Source data for Fig. 3.

### Figure 4 data

Source data for Fig. 4.

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