Nanoscale Light Confinement: the Q’s and V’s

Abstract

Microcavities and nanoresonators have the ability to strongly enhance many light−matter-interaction processes used in various applications in nano-optics. This enhancement is due to the resonant excitation of an electromagnetic mode that confines light in space (mode volume) and in time (quality factor). The confinement is not perfect and the modes, even dark ones, always leak some energy and have a finite lifetime. Their non-Hermitian character does significantly more than merely broadening the resonances. In this Perspective, we clarify the main difference between bound modes of Hermitian systems and leaky modes of non-Hermitian resonators, emphasizing the key existence of a spatially dependent phase factor as a signature of nonhermiticity. For decades, the phase factor has often be considered as puzzling or has even be ignored, although it plays a key role in the interpretation of many experiments on nanoscale resonant-mediated light−matter interaction, such as sensing with cavity perturbation, modification of the spontaneous emission rate, optomechanics, strong coupling, and so on. The situation has changed recently, to a point that nowadays a sound non-Hermitian formalism has been developed and freeware packages exist, helping analyze experiments that continuously push back the extraordinary limit offered by large fields for exploring matter with light.

CPA-lasing associated with the quasibound states in the continuum in asymmetric non-Hermitian structures

“…Due to the symmetry of lineshapes outside the immediate vicinity of the transition from the dip to peak, it can be estimated with the simple relation Q = ω 0 /∆ω, where ω 0 is the frequency of the resonance peak or dip and ∆ω is the full width of the resonance. Such a consideration is in accordance with the modern treatment of non-Hermitian resonances using the quasi-normal modes characterized by complex frequencies [58]. Dependence of the Q factor on the parameter α calculated with this expression is shown in Fig.…”

Flexible Manipulation of Lasing Modes in an Erbium-Doped Microcavity via an Add–Drop Configuration

“…The maximum lasing threshold of 414 μW is achieved when the microfiber is at 63 μm away from the bottle center. It is because the Purcell factor is inversely proportional to V m according to the equation: , where λ m is the resonant wavelength, n is the refractive index (RI) of silica, which leads to the higher lasing threshold . (For V m calculations of WGMs in the microbottle cavity, see part S5 in the Supporting Information.…”

“…, where λ m is the resonant wavelength, n is the refractive index (RI) of silica, which leads to the higher lasing threshold. 46 (For V m calculations of WGMs in the microbottle cavity, see part S5 in the Supporting Information.) Microcavity-based add-drop configuration is a functional structure, which is an excellent candidate for multiports resonant filters, 47 optical switching, 48 and second harmonic generation (SHG).…”

Mode mapping Q > 500 000 photonic crystal nanocavities using free carrier absorption

“…They are prime candidates for optical integrated-circuit applications [5][6][7]. One reason for this is the possibility to design a defect on the PhC platform, creating a PhC resonator (PhCR) that typically has small mode volume and high quality factor (Q) [8] which maximizes the optical nonlinearity crucial to active photonic devices. Two-dimensional PhCs have been shown to be suitable for numerous applications such as fast optical switching [9,10], reconfigurable circuits [11], optical memories [12] and optical parametric oscillation [13].…”