[Show abstract][Hide abstract] ABSTRACT: Tailoring the electromagnetic field at the nanoscale has led to artificial materials exhibiting fascinating optical properties unavailable in naturally occurring substances. Besides having fundamental implications for classical and quantum optics, nanoscale metamaterials provide a platform for developing disruptive novel technologies, in which a combination of both the electric and magnetic radiation field components at optical frequencies is relevant to engineer the light-matter interaction. Thus, an experimental investigation of the spatial distribution of the photonic states at the nanoscale for both field components is of crucial importance. Here we experimentally demonstrate a concomitant deep-subwavelength near-field imaging of the electric and magnetic intensities of the optical modes localized in a photonic crystal nanocavity. We take advantage of the “campanile tip”, a plasmonic near-field probe that efficiently combines broadband field enhancement with strong far-field to near-field coupling. By exploiting the electric and magnetic polarizability components of the campanile tip along with the perturbation imaging method, we are able to map in a single measurement both the electric and magnetic localized near-field distributions.
[Show abstract][Hide abstract] ABSTRACT: Arrays of photonic cavities are relevant structures for developing large-scale photonic integrated circuits and for investigating basic quantum electrodynamics phenomena, due to the effective photon hopping between interacting nanoresonators. Here, we investigate, by means of scanning near-field spectroscopy and numerical calculations, the role of different neighboring interactions that give rise to delocalized supermodes in different photonic crystal array configurations. The systems under investigation consist of three nominally identical two-dimensional photonic crystal nanocavities on membrane aligned along the two symmetry axes of the triangular photonic crystal lattice. We find that the nearest and next-nearest-neighbour coupling terms can be of the same relevance. In this case, a non-intuitive picture describes the resonant modes, and the photon hopping between adjacent nano-resonators is strongly affected. Our findings prove that exotic configurations and even post-fabrication engineering of coupled nanoresonators could directly tailor the mode spatial distribution and the group velocity in in coupled resonator optical waveguides.
[Show abstract][Hide abstract] ABSTRACT: We address the photoluminescence emission of individual germanium extrinsic
centers in Al_0.3 Ga_0.7 As epilayers grown on germanium and silicon
substrates. Trough a thorough analysis of micro-photoluminescence experiments
we demonstrate the capability of high temperature emission (70 K) and
multiexcitonic features (neutral exciton X, biexciton XX, positive X+ and
negative X- charged exciton) of these quantum emitters. Finally, we investigate
the energy renormalization of each energy level showing a large and systematic
change of the binding energy of XX and X+ from positive to negative values
(from +5 meV up to -7 meV covering about 70 meV of the emission energy) with
increasing quantum confinement. This light sources exhibiting energy-degenerate
X and XX energy levels at 1.855 eV (680 nm) are a promising resource for the
generation of entangled photons in the time-reordering scheme on a silicon
Physical Review B 12/2014; 91(20). DOI:10.1103/PhysRevB.91.205316 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
Nature Material 05/2014; 13(7). DOI:10.1038/nmat3966 · 36.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We propose a way to engineer the design of photonic molecules, realized by coupling two photonic crystal cavities, that allows an accurate control of the parity of their ground states. The spatial distribution of the fundamental mode of photonic molecules can be tuned from a bonding to an antibonding character by a local and continuous modification of the dielectric environment in between the two coupled cavities. In the systems that we investigate the transition could be experimentally accomplished by post-fabrication methods in either a reversible or an irreversible way. We notably find that the mode parity exchange is tightly related to a dramatic variation of the far field emission pattern, leading to the possibility to exploit these systems and techniques for future applications in optoelectronics.
[Show abstract][Hide abstract] ABSTRACT: The multiexciton properties of extrinsic centers from AlGaAs layers on Ge and Si substrates are addressed. The two photon cascade is found both in steady state and in time resolved experiments. Polarization analysis of the photoluminescence provides clearcut attribution to neutral biexciton complexes. Our findings demonstrate the prospect of exploiting extrinsic centers for generating entangled photon pairs on a Si based device.
[Show abstract][Hide abstract] ABSTRACT: We describe a method for the direct epitaxial
growth of a single photon emitter, based on GaAs quantum dots fabricated by droplet
epitaxy, working at liquid nitrogen temperatures on Si substrates. The achievement of quantum photon statistics up to T=80 K is directly proved by antibunching in the second order correlation function as measured with a H anbury Brown and Twiss interferometer.
31st International Conference on the Physics of Semiconductors (ICPS); 12/2013
[Show abstract][Hide abstract] ABSTRACT: We show that the epitaxial growth of height-controlled GaAs quantum dots, leading to the reduction of the inhomogeneous emission bandwidth, produces individual nanostructures of peculiar morphology. Besides the height controlled quantum dots, we observe nanodisks formation. Exploiting time resolved and spatially resolved photoluminescence we establish the decoupling between quantum dots and nanodisks and demonstrate the high optical properties of the individual quantum dots, despite the processing steps needed for height control.
[Show abstract][Hide abstract] ABSTRACT: In this work we show the possibility to increase the optical quality of quantum dots grown by droplet epitaxy on Ge-on-Si substrates in terms of single dot emission linewidth, decay time and efficiency by operating on the As pressure during the crystallization step without increasing the thermal budget. (c) 2013 Elsevier B.V. All rights reserved.
[Show abstract][Hide abstract] ABSTRACT: We study the multiexciton properties of GaAs quantum dots self aggregated on Si substrates. Sequential emission of two photons radiative cascade is observed both in continuous wave and in time resolved measurements. Polarization resolved measurements, with high spectral resolution, allow us to attribute the observed photon cascade to positively charged biexciton. Our results highlight the possibility of obtaining systems showing quantum correlations on a Si based device. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790148]
[Show abstract][Hide abstract] ABSTRACT: We present a detailed investigation performed at low temperature (T<50 K) concerning the exciton dynamics in GaN epilayers grown on c-plane sapphire substrates, focusing on the exciton formation and the transition from the nonthermal to the thermal regime. The time-resolved kinetics of longitudinal-optical-phonon replicas is used to address the energy relaxation in the excitonic band. From picosecond time-resolved spectra, we bring evidence for a long lasting nonthermal excitonic distribution, which accounts for the first 50 ps. Such a behavior is confirmed in different experimental conditions when both nonresonant and resonant excitations are used. At low excitation power density, the exciton formation and their subsequent thermalization are dominated by impurity scattering rather than by acoustic phonon scattering. The estimate of the average energy of the excitons as a function of delay after the excitation pulse provides information on the relaxation time, which describes the evolution of the exciton population to the thermal regime.
[Show abstract][Hide abstract] ABSTRACT: The post-fabrication control of evanescent tunnelling in photonic crystal molecules is demonstrated through the combination of selective infiltration and oxidation. By laser non thermal oxidation, we reduce the photonic coupling by more than 30% while by means of water micro-infiltration, we increase it by 28%. Fine-tuning of the photonic coupling is achieved by low-power laser oxidation and forced evaporation, opening the route to post-fabrication control of array of coupled cavities. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4767216]
[Show abstract][Hide abstract] ABSTRACT: We present a detailed investigation concerning the exciton dynamics in GaN
epilayers grown on c-plane sapphire substrates, focussing on the exciton
formation and the transition from the nonthermal to the thermal regime. The
time-resolved kinetics of LO-phonon replicas is used to address the energy
relaxation in the excitonic band. From ps time-resolved spectra we bring
evidence for a long lasting non-thermal excitonic distribution which accounts
for the rst 50 ps. Such a behavior is con rmed in di erent experimental
conditions, both when non-resonant and resonant excitation are used. At low
excitation power density the exciton formation and their subsequent
thermalization is dominated by impurity scattering rather than by acoustic
phonon scattering. The estimate of the average energy of the excitons as a
function of delay after the excitation pulse provides information on the
relaxation time, which describes the evolution of the exciton population to the
[Show abstract][Hide abstract] ABSTRACT: We show that the epitaxial growth of thin layers of AlGaAs on Ge and Si substrates allows to obtain single photon sources by exploiting the sparse and unintentional contamination with acceptors of the AlGaAs. Very bright and sharp single photoluminescence lines are observed in confocal microscopy. These lines behave very much as single excitons in quantum dots, but their implementation is by far much easier, since it does not require 3D nucleation. The photon antibunching is demonstrated by time resolved Hanbury Brown and Twiss measurements.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate that a gentle gas adsorption technique can be used to achieve an optimal covering of silicon-based photonic crystal slabs, leading to an unexpectedly large (up to 42 nm) shift of the resonant modes wavelength, with possibility of fine tuning. Strong enhancement (up to 30 times) of the emission band of the Er3+ ion into such structures is obtained. Finally, we were able to balance the adsorption and desorption processes by controlling the sample temperature, thus yielding a stable mode at the desired wavelength.
Photonics and Nanostructures - Fundamentals and Applications 10/2012; 10(4):547–552. DOI:10.1016/j.photonics.2012.04.008 · 1.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Spectral diffusion in the optical transition of self-assembled GaAs quantum dots is studied using the intensity correlation functions of spectrally filtered photoluminescence signals. Analysis of the bunched photon correlation reveals the statistical nature of frequency drift over six orders of magnitude in time (10−9–10−3 s) with 10 μeV energy resolution. We demonstrate large variation in the characteristic time of spectral diffusion reflecting carrier hopping among limited numbers of trapping sites in the vicinity of a quantum dot.
[Show abstract][Hide abstract] ABSTRACT: We employ a far-field analysis of the angular emission pattern to experimentally assess the symmetry of localized modes in coupled photonic-crystal cavities. We demonstrate that the spatial distribution of localized modes in photonic-crystal nanocavities may change from a bonding to an antibonding orbital, a feature that is unusual in quantum mechanical coupled systems. Experimental data are confirmed by numerical calculations and interpreted in terms of the peculiar oscillatory behavior of the evanescent waves in photonic-crystal band gaps.
[Show abstract][Hide abstract] ABSTRACT: We report on triggered single photon emission from GaAs quantum dots, grown on Si substrates and obtained by means of fabrication protocols compatible with the monolithic integration on Si based microelectronics. Very bright and sharp individual exciton lines are resolved in the spectra and can be followed up to 150 K. The nature of quantum emitters of single photon pulses can be measured up to liquid nitrogen temperature by Hanbury Brown and Twiss interferometric correlations. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4726189]
[Show abstract][Hide abstract] ABSTRACT: A local tuning of the modes of photonic crystal systems both to blue and to red sides of the resonance is implemented by nano-infiltration/evaporation of water and laser micro-oxidation. This technology is used to completely control coupled photonic nanocavities (photonic crystal molecules). Beside the standard condition of zero detuning between identical modes of the two cavities (homoatomic molecule), we are also able to produce coupling between two modes of different polarization and spatial distribution (heteroatomic molecule).
Photonics and Nanostructures - Fundamentals and Applications 06/2012; DOI:10.1016/j.photonics.2011.05.001 · 1.47 Impact Factor