Francesca Intonti

Rewritable photonic circuits

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.

Mapping Local Charge Recombination Heterogeneity by Multidimensional Nanospectroscopic Imaging

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. A near-field optical probe designed to maximize its own signal enhancement can be used to image nonmetallic samples. As materials functionality becomes more dependent on local physical and electronic properties, the importance of optically probing matter with true nanoscale spatial resolution has increased. In this work, we mapped the influence of local trap states within individual nanowires on carrier recombination with deeply subwavelength resolution. This is achieved using multidimensional nanospectroscopic imaging based on a nano-optical device. Placed at the end of a scan probe, the device delivers optimal near-field properties, including highly efficient far-field to near-field coupling, ultralarge field enhancement, nearly background-free imaging, independence from sample requirements, and broadband operation. We performed ~40-nanometer–resolution hyperspectral imaging of indium phosphide nanowires via excitation and collection through the probes, revealing optoelectronic structure along individual nanowires that is not accessible with other methods.

Engineering of light confinement in strongly scattering disordered media

Disordered photonic materials can diffuse and localize light through random multiple scattering, offering opportunities to study mesoscopic phenomena, control light-matter interactions, and provide new strategies for photonic applications. Light transport in such media is governed by photonic modes characterized by resonances with finite spectral width and spatial extent. Considerable steps have been made recently towards control over the transport using wavefront shaping techniques. The selective engineering of individual modes, however, has been addressed only theoretically. Here, we experimentally demonstrate the possibility to engineer the confinement and the mutual interaction of modes in a two-dimensional disordered photonic structure. The strong light confinement is achieved at the fabrication stage by an optimization of the structure, and an accurate and local tuning of the mode resonance frequencies is achieved via post-fabrication processes. To show the versatility of our technique, we selectively control the detuning between overlapping localized modes and observe both frequency crossing and anti-crossing behaviours, thereby paving the way for the creation of open transmission channels in strongly scattering media.

Tuning of photonic crystal cavities by controlled removal of locally infiltrated water

We present a spectral tuning mechanism of photonic crystal microcavities based on microfluidics. The microinfiltration with water of one or few cavity holes and its subsequent controlled evaporation allow us to tune the cavity resonances in a spectral range larger than 20 nm, with subnanometer accuracy, and we also observe that the addition of water in the microcavity region improves its quality factor Q.

Femtosecond near-field spectroscopy of a single GaAs quantum wire

Quasi-two-color femtosecond pump and probe spectroscopy and near-field scanning optical microscopy are combined to study the carrier dynamics in single semiconductor nanostructures. In temporally, spectrally, and spatially resolved measurements with a time resolution of 200 fs and a spatial resolution of 200 nm, the nonlinear change in reflectivity of a single quantum wire is mapped in real space and time. The experiments show that carrier relaxation in a single quantum wire occurs on a 100 fs time scale at room temperature.

Near-field imaging of coupled photonic-crystal microcavities

We report by means of near-field microscopy on the coupling between two adjacent photonic crystal microcavities. Clear-cut experimental evidence of the spatial delocalization of coupled-cavity optical modes is obtained by imaging the electromagnetic local density of states. We also demonstrate that it is possible to design photonic structures with selective coupling between different modes having orthogonal spatial extensions

Anderson localization of near-visible light in two dimensions

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.

Polarization-sensitive near-field investigation of photonic crystal microcavities

We report on polarization sensitive imaging of two-dimensional photonic crystal microcavity modes. By using a near-field scanning optical microscope with a polarization sensitive setup, it is possible to selectively map, with a resolution beyond the diffraction limit, each electric field component in the plane of the sample. In addition, the simultaneous analysis of photoluminescence maps in different polarization channels allowed us to obtain important insight on near-field microscopy detection mechanism. Finite difference time domain simulations confirm the experimental results.

Mode tuning of photonic crystal nanocavities by photoinduced non-thermal oxidation

A method to achieve photoinduced tuning of PhC nanocavity modes is discussed and implemented. It is based on light induced oxidation in air atmosphere with very low thermal budget which produces a local reduction of the GaAs membrane effective thickness and a large blueshift of the nanocavity modes. It is also shown that green light is much more efficient in inducing the micro-oxidation with respect to near infrared light. The observed behaviour is attributed to oxide growth promoted by photoenhanced reactivity.

Nanofluidic control of coupled photonic crystal resonators

A fine control of a photonic molecule is obtained by nanofluidic techniques. The coupling condition between the modes of two photonic crystal nanocavities is modified by spectrally tuning each single resonator. Clear mode anticrossing and transition from localized to delocalized states are observed. The detuning induced by disorder, always present in real device, is experimentally compensated by locally modifying the photonic environment of the cavity.