Engineering of light confinement in strongly scattering disordered media

Abstract

The ability to mold the flow of light at the wavelength scale has been largely investigated in photonic-crystal-based devices, a class of materials in which the propagation of light is driven by interferences between multiply Bragg scattered waves and whose energy dispersion is described by a photonic band diagram [1]. Light propagation in such structures is defined by Bloch modes, which can be engineered by varying the structural parameters of the material [2][3][4]. In disordered media, both the direction and phase of the propagating waves are randomized in a complex manner, making any attempt to control light propagation particularly challenging. Disordered media are currently investigated in several contexts, ranging from the study of collective multiple scattering phenomena [5,6] to cavity quantum electrodynamics and random lasing [7,8], to the possibility to provide efficient solutions in renewable energy [9], imaging [10], and spectroscopy-based applications [11]. Transport in such systems can be described in terms of photonic modes, or quasi-modes, which exhibit characteristic spatial profiles and spectra [12,13]. In diffusive systems, these modes are spatially and spectrally overlapping while in the regime of Anderson localization, they become spatially and spectrally-isolated [14]. Unlike Bloch modes in periodic systems, the precise formation of photonic modes in a single realization of the disorder is unpredictable.Control over light transport can be obtained by shaping the incident wave to excite only a specific part of the modes available in a given system [15][16][17][18]. For fully exploiting the potential of disordered systems, however, a mode control is needed. It was shown 3 theoretically that isolated modes could be selectively tuned and possibly coupled to each other by a local fine modification of the dielectric structure [19,20].In this Article, we demonstrate experimentally the ability to fully control the spectral properties of an individual photonic mode in a two-dimensional disordered photonic structure [21], in a wavelength range that is relevant for photonic research driven applications. A statistical analysis of individual spatially-isolated random photonic modes is performed by multi-dimensional near-field imaging, leading to a detailed determination of intensity fluctuations, decay lengths and mode volumes. We then demonstrate that individual modes can be fine-tuned either by near-field tip perturbation or by local sub-micrometer-scale oxidation of the semiconductor slab [22]. The resonant frequency of a selected mode is gradually shifted until it is in perfect spectral superposition with the frequency of other two modes, located a few micrometers apart and spatially overlapping with the tuned mode. On spectral resonance, we observe frequency crossing and anti-crossing behaviours, respectively, the latter indicating mode interaction. This provides the experimental proof-of- (e) and (f), respectively). The main difference between the two spectra normalized to the average intensity i...

Near-field speckle imaging of light localization in disordered photonic systems

“…Still due to the large spectral range under investigation, we find that the resonances can be spectrally isolated, assuring that these modes fulfill the Thouless criterion for the localized regime [2]. Other statistical analysis, such as the probability density function gives the same conclusion [15]. Let's stress that our near field approach directly shows that the observed resonances are spatially confined, which is an evident and unambiguous proof that, in the investigated wavelength range, the sample supports localized photonic modes.…”

“…Finally, wet etching of the bottom AlGaAs sacrificial layer is used to release the membrane. We designed the position of the holes by a random sequential addition generator [15], and we imposed a minimum distance (1.3 hole diameters) between the centers of the nearest neighboring holes in order to avoid merging between adjacent holes during the fabrication process. Therefore, even though the sample is not fully random, hereafter we will use the denomination random system/random modes both to stress the maximum practical randomness of our realization and to distinguish our sample from disordered media with higher spatial correlation [23].…”

“…Therefore, even though the sample is not fully random, hereafter we will use the denomination random system/random modes both to stress the maximum practical randomness of our realization and to distinguish our sample from disordered media with higher spatial correlation [23]. This system has previously demonstrated to support localized modes in the telecom wavelength range, which is relevant for photonic applications [15,24]. We use the emission of embedded quantum dots (QDs) to pump the randomly generated modes.…”

Ultra-subwavelength phase-sensitive Fano-imaging of localized photonic modes

“…The high sensitivity of this method is largely due to the high refractive index material probes used to enhance the scattering. However, the disadvantage of this light scattering technique is that the investigation of optical micro-and nano-resonators (e.g., ring resonators, microdisk cavities, and PCNs) excessively perturbs the dielectric environment compared to glass probes 24 , thereby deteriorating the high quality factor (Q) and also detuning/mixing modes localized in photonic molecules or in random cavities 8,25 . Moreover, this technique, as in the case of cathodoluminescence, suffers from being mainly sensitive to the out-of-plane component of the electric LDOS 22 , and this technique is not suited for TE-like localized modes.…”

Spatial steadiness of individual disorder modes upon controlled spectral tuning

“…[22][23][24] The recent realization of artificial coupled states in a disordered system by nano-oxidation represents, therefore, a remarkable achievement in controlling random modes. 25 However, the spatial steadiness of a single random mode size, shape, and position as a function of the spectral tuning is still an open question. The relevance of this issue can be unfold, in a simplified picture, by the parallelism between disordered photonic media and photonic crystal nanocavities.…”