M. De Vittorio

Metal-enhanced fluorescence of colloidal nanocrystals with nanoscale control

Engineering the spectral properties of fluorophores, such as the enhancement of luminescence intensity, can be achieved through coupling with surface plasmons in metallic nanostructures1,2,3,4,5,6,7,8,9,10,11. This process, referred to as metal-enhanced fluorescence, offers promise for a range of applications, including LEDs, sensor technology, microarrays and single-molecule studies. It becomes even more appealing when applied to colloidal semiconductor nanocrystals, which exhibit size-dependent optical properties, have high photochemical stability, and are characterized by broad excitation spectra and narrow emission bands12. Other approaches have relied upon the coupling of fluorophores (typically organic dyes) to random distributions of metallic nanoparticles or nanoscale roughness in metallic films1,2,3,4,6,8. Here, we develop a new strategy based on the highly reproducible fabrication of ordered arrays of gold nanostructures coupled to CdSe/ZnS nanocrystals dispersed in a polymer blend. We demonstrate the possibility of obtaining precise control and a high spatial selectivity of the fluorescence enhancement process.

Multicolor oligothiophene-based light-emitting diodes

We demonstrate wide tunability, from green to near infrared, of the electroluminescence emission of substituted oligothiophene compounds. The compounds are characterized by high chemical stability, electron affinities up to 3.1 eV and photoluminescence efficiencies up to 70%. These characteristics make these materials excellent candidates for application in light-emitting diodes. We obtain low turn-on voltage devices with electroluminescence efficiency up to 0.2%, more than one order of magnitude larger than the values reported for unsubstituted oligothiophene compounds.

Graphene-based absorber exploiting guided mode resonances in one-dimensional gratings

A one-dimensional dielectric grating, based on a simple geometry, is proposed and investigated to enhance light absorption in a monolayer graphene exploiting guided mode resonances. Numerical findings reveal that the optimized configuration is able to absorb up to 60% of the impinging light at normal incidence for both TE and TM polarizations resulting in a theoretical enhancement factor of about 26 with respect to the monolayer graphene absorption (≈2.3%). Experimental results confirm this behavior showing CVD graphene absorbance peaks up to about 40% over narrow bands of a few nanometers. The simple and flexible design points to a way to realize innovative, scalable and easy-to-fabricate graphene-based optical absorbers.

Capture and thermal re-emission of carriers in long-wavelength InGaAs/GaAs quantum dots

We investigate the ultrafast carrier dynamics in metalorganic chemical vapor deposition-grown InGaAs/GaAs quantum dots emitting at 1.3 μm. Time-resolved photoluminescence upconversion measurements show that the carriers photoexcited in the barriers relax to the quantum-dot ground state within a few picoseconds. At low temperatures and high carrier densities, the relaxation dynamics is dominated by carrier–carrier scattering. In contrast, at room temperature, the dominant relaxation process for electrons is scattering between quantum-dot levels via multiple longitudinal optical (LO)-phonon emission. The reverse process, i.e., multiple LO-phonon absorption, governs the thermal re-emission of electrons from the quantum-dot ground state.

Wavelength control from 1.25 to 1.4 μm in InxGa1−xAs quantum dot structures grown by metal organic chemical vapor deposition

This letter reports on the realization of long-wavelength InGaAs quantum dots (QDs) fabricated by metal organic chemical vapor deposition. By controlling the In incorporation in the QD layers and/or in the barrier embedding the QDs, we are able to tune the wavelength emission continuously from 1.25 to 1.4 μm at room temperature. Efficient stacking of dots emitting at 1.3 μm is also demonstrated.

Fabrication of 3D metallic photonic crystals by X-ray lithography

Photonic crystals (3D) represent one of the most important building blocks towards the achievement of a full optics communication technology. So far the largest interest has been attracted by two-dimensional photonic crystals because they are potentially more amenable to fabrication and much closer to application. Straightforward application of the photonic band gap concept is generally thought to require three-dimensional (3D) photonic crystals that, however, represent a challenge from a fabrication point of view. Recent works have shown that 3D metallic PC can be fabricated and that they can be advantageous in the low frequency region where the metals become almost completely reflectors. In this work we show the possibility to fabricate 3D PC structures by X-ray lithography. Gold and nickel 3D photonic crystals with threefold (Yablonovite) and fourfold rotation symmetry have been fabricated with a lattice parameter ranging from 1 µm down to 300 nm. The total thickness of the 3D PC is of the order of 10 µm, a value which should allow to achieve a complete bulk behavior. This is supported by variable angle reflectance measurements, which have shown clear indications for true 3D dimensionality of our samples.

3-D FEM Modeling and Fabrication of Circular Photonic Crystal Microcavity

In this paper, we study an unconventional kind of quasi-three-dimensional (3-D) photonic crystal (PhC) with circular lattice pattern: it consists of air holes in a GaAs material (n=3.408) along circular concentric lines. This particular PhC geometry has peculiar behavior if compared with the traditional square and triangular lattices, but it is difficult to model by using conventional numerical approaches such as wave expansion method. The resonance and the radiation aspects are analyzed by the 3-D finite-element method (FEM). The model, based on a scattering matrix approach, considers the cavity resonance frequency and evaluates the input-output relationship by enclosing the photonic crystal slab (PhCS) in a black box in order to define the responses at different input-output ports. The scattering matrix method gives important information about the frequency responses of the passive 3-D crystal in the 3-D spatial domain. A high sensitivity of the scattering parameters to the variation of the geometrical imperfection is also observed. The model is completed by the quality factor (Q-factor) estimation. We fabricated the designed circular photonic crystal over a slab membrane waveguide embedding InAs/GaAs quantum dots emitting around 1.28 mum. Good agreement between numerical and experimental results was found, thus validating the 3-D FEM full-wave investigation.

Graphene-based perfect optical absorbers harnessing guided mode resonances

We investigate graphene-based optical absorbers that exploit guided mode resonances (GMRs) attaining theoretically perfect absorption over a bandwidth of few nanometers (over the visible and near-infrared ranges) with a 40-fold increase of the monolayer graphene absorption. We analyze the influence of the geometrical parameters on the absorption rate and the angular response for oblique incidence. Finally, we experimentally verify the theoretical predictions in a one-dimensional, dielectric grating by placing it near either a metallic or a dielectric mirror, thus achieving very good agreement between numerical predictions and experimental results.

Recent advances on single photon sources based on single colloidal nanocrystals

Single colloidal quantum dots (QDs) are increasingly exploited as triggered sources of single photons. This review reports on recent results on single photon sources (SPS) based on colloidal quantum dots, whose size, shape and optical properties can be finely tuned by wet chemistry approach. First, we address the optical properties of different colloidal nanocrystals, such as dots, rods and dot in rods and their use as single photon sources will be discussed. Then, we describe different techniques for isolation and positioning single QDs, a major issue for fabrication of single photon sources, and various approaches for the embedding single nanocrystals inside microcavities. The insertion of single colloidal QDs in quantum confined optical systems allows one to improve their overall optical properties and performances in terms of efficiency, directionality, life time, and polarization control. Finally, electrical pumping of colloidal nanocrystals light emitting devices and of NC-based single photon sources is reviewed.

Long wavelength emission in InxGa1−xAs quantum dot structures grown in a GaAs barrier by metalorganic chemical vapor deposition

We demonstrate a method to obtain room temperature long wavelength emission from InGaAs quantum dots (QDs) growth directly into a binary GaAs matrix. The wavelength is tuned from 1.26 up to 1.33 μm by varying the V/III ratio during growth of the GaAs cap layer, without using a seeding layer or InGaAs wells. Strong improvement in terms of line-shape narrowing and efficiency is obtained. In addition to the shift in wavelength we observe an impressive reduction of temperature dependent quenching of the emission efficiency, which decreases only by a factor of 3 between cryogenic temperatures and room temperature, very good for QD structures emitting at 1.3 μm. Photoluminescence spectroscopy and theoretical modeling were combined for interpretation of the results.