Chiral sensing with achiral isotropic metasurfaces

Abstract

Recently, we proposed a metasurface design for chiral sensing that (i) results in enhanced chiroptical signals by more than two orders of magnitude for ultrathin, subwavelength, chiral samples over a uniform and accessible area, (ii) allows for complete measurements of the total chirality (magnitude and sign of both its real and imaginary part), and (iii) offers the possibility for a crucial signal reversal (excitation with reversed polarization) that enables chirality measurements in an absolute manner, i.e., without the need for sample removal. Our design is based on the anisotropic response of the metasurface, rather than the superchirality of the generated near-fields, as in most contemporary nanophotonic-based chiral sensing approaches. Here, we derive analytically, and verify numerically, simple formulas that provide insight to the sensing mechanism and explain how anisotropic metasurfaces, in general, offer additional degrees of freedom with respect to their isotropic counterparts. We provide a detailed discussion of the key functionalities and benefits of our proposed design and we demonstrate practical measurement schemes for the unambiguous determination of an unknown chirality. Last, we provide the design principles towards broadband operation -from near-infrared to near-ultraviolet frequencies -opening the way for highly sensitive nanoscale chiroptical spectroscopy.

Reconfigurable chirality with achiral excitonic materials in the strong-coupling regime

“…As a result, and despite the absence of absorbing chiral regions, asymmetric absorption takes place in the achiral regions of the combined shell-sphere system, mainly the excitonic shell, thus leading to nonzero CD signal. This observation is in accord with theoretical predictions of nonvanishing CD signals in achiral metasurfaces with chiral inclusions characterized by entirely real κ [14]. Additionally, this is the reason why, eventually, the CD signal inherits the behaviour of the absorption in the achiral particle, as presented in Fig.…”

“…This observation is in accord with theoretical predictions of nonvanishing CD signals in achiral metasurfaces with chiral inclusions characterized by entirely real κ. 14 Additionally, this is the reason why, eventually, the CD signal inherits the behaviour of the absorption in the achiral particle, as presented in Fig. 2(c).…”

“…Moreover, nanoparticles (NPs) -most commonly metallic-arranged in specific geometries can demonstrate optical chirality stemming from their structural characteristics [13]. Finally, a chiral response can also be induced externally, either by means of chiral inclusions in achiral metasurfaces [14] or by external magnetic fields [15]. For example, magnetic circular dichroism (MCD) describes a special case of magnetically induced chirality, where an otherwise achiral object becomes optically active in the presence of a static magnetic field, as observed in gyroelectric media [16,17].…”

Absolute Chiral Sensing in Dielectric Metasurfaces Using Signal Reversals

“…In particular, most contemporary nanophotonic chiral-sensing approaches are based on a similar principle of operation, which essentially relies on the generation of superchiral near-fields, i.e., fields with higher chiral optical density compared to circularly polarized light, to lead into enhanced circular dichroism (CD) signals in the presence of a natural optically active substance. − ,, To achieve this, right- and left-circularly polarized waves are used to excite the nanosystem in question and generate the superchiral fields, while also enabling the ability to perform CD measurements in transmission. While several works have attributed the resulting CD signals to be proportional only to the imaginary part of the chirality (Pasteur) parameter κ, i.e., Im­(κ) (see, e.g., refs , , and ), past and recent experimental (see particularly refs , , and ) and theoretical results (ref ) demonstrate that the observed CD signals depend on both the real and the imaginary part of κ [Re­(κ) and Im­(κ), respectively], a result that also explains the origin of enhanced CD signals at spectral regions far from molecular resonances (i.e., in the visible spectrum). As such, these approaches do not allow for quantitative detection of both the real and imaginary part of the chirality parameter (responsible for refraction and absorption, respectively) and the discrimination of their effects, − ,− ,, a crucially sought after aspect for any chiral-sensing technique .…”

Perspective on Meta-Boundaries

“…Figure c shows that the time-modulated metasurface, which consists of metallic patches array and parallel capacitor, may provide another route for the design of biisotropic meta-boundary . Meanwhile, the biisotropic boundary can be achieved by inserting chiral materials (which intrinsically have the isotropic magnetoelectric coupling) into an ultrathin dielectric slab in Figure d . With the help of the biisotropic meta-boundary, the scattering cross section of certain objects can be reduced significantly .…”