The first step toward the design of a bio-photonic cavity is to select the right optical resonator. If a bio-inspired approach has to be followed, commonly used materials for optoelectronics are excluded. In this case, indeed, due to the inability of common bio-inspired and biodegradable materials to behave like metals or dielectric mirrors, commonly used optoelectronic architectures are ruled out. New paradigms have to be explored, such as the so-called resonant meta-surfaces
[1][2][3].
Meta-surfaces are novel nano-architectures whose performance and miniaturization capabilities are challenging the certainties of classic optics
[4]. The prefix “meta” refers to meta-materials, the wider category to which meta-surfaces belong, since their properties go well beyond those of the constituent materials, and are mainly determined by their geometry. The term “surface”, instead, points out that these structures share a planar nature which is in perfect agreement with the use of biopolymers. In other words, meta-surfaces (MS) can be considered as nanostructured surfaces whose optical response can be engineered at will to accomplish a particular optical task. To achieve a specific functionality, meta-surfaces can rely on both metallic or dielectric nano-elements
[5][6][7][8][9][10][11][12][13][14]. For our purposes, dielectric meta-surfaces are the ones of interest. Manifesting high-Q resonances in the visible range is only one of the results achievable with meta-surfaces
[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32].
One way to achieve a high-Q-resonant response by means of a meta-surface is to engineer a so-called “biperiodic” disk lattice
[32]. Such a recently studied configuration relies on a particular lattice arrangement of high-refractive-index dielectric elements in which the unitary cell consists of two disks: a larger one, with a diameter equal to
d + (Δ/2), and a smaller one, with a diameter of
d − (Δ/2) (see
Figure 1a). In the case reported in ref.
[5] d = 92 nm, Δ is a spatial component to be added to
d to obtain the large disk and to be subtracted to
d to obtain the small disk. The asymmetry of the system can be evaluated via an “asymmetry parameter”
α = Δ/
d. In the case in which Δ = 0 nm (
α = 0), corresponding to the mono-periodic lattice, no high-Q resonances are present in the transmittance spectrum (see
Figure 1b, dashed line). When a small asymmetry is introduced between the two disks composing the unitary cell, such that Δ = 9.2 nm (
α = 0.1), two steep minima in the transmittance spectrum arise, constituting the high-Q-factor resonances excited in the bi-periodic meta-surface. These resonances are found to possess a dipole-like character. In particular, the high-wavelength resonance has a magnetic-like character (m
α), while the other has an electric-like character (p
α). The Q-factor of these modes is about 500 and 1000, respectively, but it can be further increased by changing the asymmetry of the unitary cell.
Figure 1. Biperiodic meta-surfaces: (
a) Sketch of a biperiodic meta-surface made of a larger element with a diameter equal to d + (Δ/2) and a smaller one of diameter d-(Δ/2). (
b) Transmittance spectra belonging to two different cases. The first one, in which Δ = 0 (no biperiodicity), and the second one, in which Δ = 9.2 nm. In this latter case, two steep minima in the transmission spectrum of the structure are found, corresponding to the high-Q-factor resonances. Reproduced with permission from reference
[32]. Copyright 2020, American Chemical Society.
Another remarkable example of high-Q-factor meta-surfaces are those in which the insurgence of resonances is governed by the so-called “Bound states In the Continuum” (BIC)
[23][24][25][28][29][30][31][33][34][35][36][37][38]. BICs are common phenomena in many fields of physics, such as optics and acoustics. Let us consider, for example, the simple and general case of a wave propagating in the harmonic regime e−iωt. We also consider a certain generic potential which sustains a set of discrete bound states (see Figure 2). The discrete set of bound states sustained by the potential are the familiar cases of the bound states of an electron in an atom or the modes of a metal/insulator/metal cavity or those sustained by an optical fiber. Outside the “discrete” set, a set of modes exists in the “continuum” as scattered waves. In addition, locally confined waves with a complex frequency ω = ωa − iγ, ωa being the resonant frequency, can arise in the continuum coupled with propagating waves. These waves assume the shape of “leaky resonances”. Apart from these three cases, non-leaky bound states can exist in the continuum. These modes are known as “bound states in the continuum”. BICs can be considered as the ideal (γ = 0) case of a leaky resonance with no losses and an infinite quality factor. It is however true that such a wave would be characterized by a completely non-radiative nature, being by definition decoupled from a radiative leaky wave. As such, it would be impossible to excite BICs with free-space light. In practice, only low-loss BICs or “quasi-BIC” modes can be excited via free-space light illumination. The concept of a bound state in the continuum has been originally introduced in the field of quantum mechanics by Von Neumann and Wigner [39]. However, Fredrich and Wintgen generalized it in terms of a destructive interference between two resonances which would originally be decoupled, but that, as a consequence of the tuning of a particular parameter, can be brought to interact and strongly couple with each other [40].
Figure 2. Bound States in the Continuum: (
a) Sketch of the possible states in the “discrete” and in the “continuum” sets. If a potential is structured in such a way that only a discrete set of modes is allowed, then regular bound states will appear due to the confinement action of the potential itself. If the energy is higher than the height of the potential barrier, a continuum of allowed states is accessed. The simplest and most probable state in the continuum is a regular scattered state. A so-called “leaky resonance” can occur in the continuum due to some kind of destructive interference phenomenon that suppresses some of the radiative channels and forms a bound state. Such a leaky bound state or leaky resonance is also coupled to a scattered (radiative) wave and, therefore, can be accessed by free-space light. In the end, a Bound State in the Continuum (BIC) can occur if the aforementioned mechanism for the leaky resonances suppresses all the radiative channels, thus forming an ideal bound state in the continuum. The illustration is inspired by reference
[41]. (
b) Sketch of the architecture re-drawn from references
[42] and
[30], evidencing the tilt angle
θ as the tuning parameter to access the quasi-BIC visible as steep minima in the transmittance spectrum shown in (
c). Reproduced from reference
[30].
If the parameter that enables the coupling between these two modes intervenes on the symmetry of the system, we are in presence of a “symmetry-protected BIC”. These BICs are of special interest for the purpose of this review, since they occur in structures that can be readily replicated with a biopolymer through a soft-molding technique. Indeed, when a particular system holds a degree of symmetry (a reflection or rotational symmetry, for example), the modes with different symmetry classes are totally decoupled. However, if the symmetry is somehow broken, these modes can be brought to interact
[41]. One noticeable example belonging to this family of symmetry-protected BICs consists in a lattice made of a zigzag array of tilted silicon resonators, as shown in
Figure 2b
[41]. The tuning parameter here is the tilt angle
θ of the long axis of one of the facing elements. At
θ = 0°, such a structure is found to have a symmetry-protected BIC that is impossible to excite through free-space light. However, while tilting
θ, the symmetry is broken, and quasi-BIC modes become accessible. Such modes appear as steep minima in the transmission spectrum, whose Q-factor decreases while increasing
θ and, as a consequence, departing from the perfect symmetry condition.