Mind the Gap Near-field microscopy has benefited from subwavelength near-field plasmonic probes that make use of the field-concentrating properties of gaps. These probes achieve maximum enhancement only in the tip-substrate gap mode, which can yield large near-field signals, but only for a metallic substrate and for very small tip-substrate gap distances. Bao et al. (p. 1317 ) designed a probe that unites broadband field enhancement and confinement with bidirectional coupling between far-field and near-field electromagnetic energy. Their tips primarily rely on the internal gap modes of the tip itself, thereby enabling it to image nonmetallic samples.

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...

We experimentally observe a sizable and reversible spectral tuning of the resonances of a two-dimensional photonic crystal microcavity induced by the introduction of a subwavelength size glass tip. The comparison between experimental near-field data, collected with / 6 spatial resolution, and results of numerical calculations shows that the spectral shift induced by the tip is proportional to the local electric field intensity of the cavity mode. This observation proves that the electromagnetic local density of states in a microcavity can be directly measured by mapping the tip-induced spectral shift with a scanning near-field optical microscope.

Intonti, F.; Emiliani, V.; Lienau, Ch.; Elsaesser, T.; Savona, V.; Runge, E.; Zimmermann, R.; Nötzel, R.; Ploog, K. Document VersionPublisher's PDF, also known as Version of Record (includes final page, issue and volume numbers)Please check the document version of this publication:• A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication Citation for published version (APA):Intonti, F., Emiliani, V., Lienau, C., Elsaesser, T., Savona, V., Runge, E., ... Ploog, K. (2001). Quantum Mechanical Repulsion of Exciton Levels in a Disordered Quantum Well. Physical Review Letters, 87(7), 076801-1/4. [076801]. DOI: 10.1103/PhysRevLett.87.076801 General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Download date: 10. May. 2018 VOLUME 87, NUMBER 7 Spatially resolved photoluminescence spectra of a single quantum well are recorded by near-field spectroscopy. A set of over four hundred spectra displaying sharp emission lines from localized excitons is subject to a statistical analysis of the two-energy autocorrelation function. An accurate comparison with a quantum theory of the exciton center-of-mass motion in a two-dimensional spatially correlated disordered potential reveals clear signatures of quantum mechanical energy level repulsion, giving the spatial and energetic correlations of excitons in disordered quantum systems. P H Y S I C A L R E V I E W L E T T E R S 13 AUGUST 2001 Quantum Mechanical Repulsion of Exciton Levels in a Disordered Quantum Well

The authors present a technique that allows to modify the local characteristics of two-dimensional photonic crystals by controlled microinfiltration of liquids. They demonstrate experimentally that by addressing and infiltrating each pore with a simple liquid, e.g., water, it is possible to write pixel by pixel optical devices of any geometry and shape. Calculations confirm that the obtained structures indeed constitute the desired resonators and waveguide structures.

We report on the observation of Anderson localization of near-visible light in two-dimensional systems. Our structures consist of planar waveguides in which disorder is introduced by randomly placing pores with controlled diameter and density. We show how to design structures in which localization can be observed and describe both the realization of the materials and the actual observation of Anderson localized modes by near-field scanning microscopy.

Microcavities and nanoresonators are characterized by their quality factors Q and mode volumes V. While Q is unambiguously defined, there are still questions on V and in particular on its complexvalued character, whose imaginary part is linked to the non-Hermitian nature of open systems. Helped by cavity perturbation theory and near field experimental data, we clarify the physics captured by the imaginary part of V and show how a mapping of the spatial distribution of both the real and imaginary parts can be directly inferred from perturbation measurements. This result shows that the mathematically abstract complex mode volume in fact is directly observable.This supplementary Material provides complementary results and discussions to the main text, starting with a Section detailing discussing the reliability of the Δ -measurements, followed by a formal comparison of the classical perturbation formula of Eqs. (1) and (2), a study of the accuracy of Eq. (2) for predicting resonance shifts, and by an analytical study of the domain of validity of Eq. (2) that leads to upper bounds for the maximum perturber strength.

The paper demonstrates the nonresonant magnetic interaction at optical frequencies between a photonic crystal microcavity and a metallized near-field microscopy probe. This interaction can be used to map and control the magnetic component of the microcavity modes. The metal coated tip acts as a microscopic conductive ring, which induces a magnetic response opposite to the inducing magnetic field. The resulting shift in resonance frequency can be used to measure the distribution of the magnetic field intensity of the photonic structure and fine-tune its optical response via the magnetic field components.