No abstract

In high energy physics, unknown particles are identified by determining their mass from the Cherenkov radiation cone that is emitted as they pass through the detector apparatus. However, at higher particle momentum, the angle of the Cherenkov cone saturates to a value independent of the mass of the generating particle, making it difficult to effectively distinguish between different particles. Here, we show how the geometric formalism of transformation optics can be applied to describe the Cherenkov cone in an arbitrary anisotropic medium. On the basis of these results, we propose a specific anisotropic metamaterial to control Cherenkov radiation, leading to enhanced sensitivity for particle identification at higher momentum. DOI: 10.1103/PhysRevLett.113.167402 PACS numbers: 78.67.Pt, 41.20.Jb, 41.60.Bq Cherenkov radiation, experimentally discovered by Pavel Cherenkov [1] and theoretically formalized by Ilya Frank and Igor Tamm [2], is a peculiar form of electromagnetic radiation that arises when charged particles travel through a medium at a velocity greater than the phase velocity of light in that medium [3]. This effect has proven useful in applied and experimental physics [4], e.g., for the detection of cosmic rays [5], novel electromagnetic sources [6][7][8], localized sensing in biological systems [9], spectroscopy of complex nanostructures [10], and identification of elementary particles [11]. Recently, there has been significant interest in Cherenkov radiation inside or in the vicinity of electromagnetic structured media [12][13][14][15][16][17][18][19][20][21][22][23][24]. This interest is fueled by the prediction that the direction of the Cherenkov cone can be reversed, such that the Cherenkov radiation and the emitting particle travel in opposite directions [25]. Aside from this experimentally observed phenomenon-called reversed Cherenkov radiation [12][13][14][15][16][17]20,21]-other unusual phenomena, such as Cherenkov radiation without a velocity threshold [24], have been predicted when charged particles travel through metamaterials and other electromagnetic structured systems [26][27][28][29][30][31][32].Here, we demonstrate how the geometric techniques of transformation optics [33][34][35][36][37] can be used to understand the Cherenkov radiation emitted in arbitrary anisotropic media. We start by calculating the Cherenkov radiation emitted by a particle traveling along a principal axis of an anisotropic medium and we show how the resulting Cherenkov cone can be described from the threedimensional coordinate transformation in the underlying electromagnetic space. Subsequently, we discuss the physics of Cherenkov radiation in uniaxially transformed media, and we highlight the fundamental difference between transformations in directions parallel and perpendicular to the velocity of the charged particle. Finally, we demonstrate how the geometric reality of these media offers an elegant recipe for designing ring imaging Cherenkov (RICH) detectors with better resolution.To this end, we calculate t...

We demonstrate how the optical gradient force between two waveguides can be enhanced using transformation optics. A thin layer of double-negative or single-negative metamaterial can shrink the interwaveguide distance perceived by light, resulting in a more than tenfold enhancement of the optical force. This process is remarkably robust to the dissipative loss normally observed in metamaterials. Our results provide an alternative way to boost optical gradient forces in nanophotonic actuation systems and may be combined with existing resonator-based enhancement methods to produce optical forces with an unprecedented amplitude.

Transformation Optics asks Maxwell's equations what kind of electromagnetic medium recreate some smooth deformation of space. The guiding principle is Einstein's principle of covariance: that any physical theory must take the same form in any coordinate system. This requirement fixes very precisely the required electromagnetic medium. The impact of this insight cannot be overestimated. Many practitioners were used to thinking that only a few analytic solutions to Maxwell's equations existed, such as the monochromatic plane wave in a homogeneous, isotropic medium. At a stroke, Transformation Optics increases that landscape from 'few' to 'infinity', and to each of the infinitude of analytic solutions dreamt up by the researcher, corresponds an electromagnetic medium capable of reproducing that solution precisely. The most striking example is the electromagnetic cloak, thought to be an unreachable dream of science fiction writers, but realised in the laboratory a few months after the papers proposing the possibility were published. But the practical challenges are considerable, requiring meta-media that are at once electrically and magnetically inhomogeneous and anisotropic. How far have we come since the first demonstrations over a decade ago? And what does the future hold? If the wizardry of perfect macroscopic optical invisibility still eludes us in practice, then what compromises still enable us to create interesting, useful, devices? While 3D cloaking remains a significant technical challenge, much progress has been made in 2dimensions. Carpet cloaking, wherein an object is hidden under a surface that appears optically flat, relaxes the constraints of extreme electromagnetic parameters. Surface wave cloaking guides subwavelength surface waves, making uneven surfaces appear flat. Two dimensions is also the setting in which conformal and complex coordinate transformations are realisable, and the possibilities in this restricted domain do not appear to have been exhausted yet. Beyond cloaking, the enhanced electromagnetic landscape provided by Transformation Optics has shown how fully analytic solutions can be found to a number of physical scenarios such as plasmonic systems used in electron energy loss spectroscopy (EELS) and cathodoluminescence (CL). Are there further fields to be enriched? A new twist to Transformation Optics was the extension to the space-time domain. By applying transformations to space-time, rather than just space, it was shown that events rather than objects could be hidden from view; Transformation Optics had provided a means of effectively redacting events from history. The hype quickly settled into serious nonlinear optical experiments that demonstrated the soundness of the idea, and it is now possible to consider the practical implications, particularly in optical signal processing, of having an 'interrupt-without-interrupt' facility that the socalled temporal cloak provides. Inevitable issues of dispersion in actual systems have only begun to be addressed. Now that time is included in ...