Rossella Capasso

Nanostructured PEDOT:PSS film with two-dimensional photonic quasi crystals for efficient white OLED devices

A polymeric PEDOT:PSS film nanostructured with Photonic Quasi Crystals that opens the path towards more efficient white OLEDs is presented. For the first time three different quasi crystal families were fabricated (octagonal, dodecagonal and Thue-Morse) onto a conductive polymeric film combining high-resolution electron beam lithography (EBL) and plasma etching techniques to improve light extraction and to control spectral tunability. The efficiency gain obtained in light extraction holds great promise for the use of quasi crystals as functional components in polymeric based White Organic Light Emitting Diode (WOLED) devices.

Novel organic LED structures based on a highly conductive polymeric photonic crystal electrode

In this work we demonstrate the possibility to realize a novel unconventional ITO-free organic light emitting diode (OLED) utilizing a photonic polymeric electrode. Combining electron beam lithography and a plasma etching process to partially structure the highly conductive poly(3,4 ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) it is possible to realize an embedded photonic crystal (PC) structure. The realized PC-anode drastically reduces the light trapped in the OLED, demonstrating the possibility to eliminate further process stages and making it easier to use this technology even on rollable and flexible substrates.

Fabrication of Novel Two-Dimensional Nanopatterned Conductive PEDOT:PSS Films for Organic Optoelectronic Applications

This paper presents a novel strategy to fabricate two-dimensional poly(3,4 ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) photonic crystals (PCs) combining electron beam lithography (EBL) and plasma etching (PE) processes. The surface morphology of PEDOT:PSS PCs after mild oxygen plasma treatment was investigated by scanning electron microscopy. The effects on light extraction are studied experimentally. Vertical extraction of light was found to be strongly dependent on the geometric parameters of the PCs. By changing the lattice type from triangular to square and the geometrical parameters of the photonic structures, the resonance peak could be tuned from a narrow blue emission at 445 nm up to a green emission at 525 nm with a full width at half-maximum of 20 nm, which is in good agreement with Braggs diffraction theory and free photon band structure. Both finite-difference time-domain and plane wave expansion methods are used to calculate the resonant frequencies and the photonic band structures in the two-dimensional photonic crystals showing a very good agreement with the experiment results. A 2D nanopatterned transparent anode was also fabricated onto a flexible polyethylene terephthalate (PET) substrate and it was integrated into an organic light-emitting diode (OLED). The obtained results fully confirm the feasibility of the developed process of micro/nano patterning PEDOT:PSS. Engineered polymer electrodes prepared by this unique method are useful in a wide variety of high-performance flexible organic optoelectronics.

Plasmonic octagonal quasicrystals for surface enhanced Raman sensing

Abstract Surface enhanced Raman scattering (SERS) on eight-fold quasicrystal arrays with precisely controlled size and spacing fabricated via electron beam lithography was investigated. This SERS substrate shows high efficiency at 785 nm excitation in the detection of p-mercaptoaniline (pMA), and a SERS enhancement factor (EF ) of 107 is achieved. SERS behavior of the realized engineered SERS substrate indicates that the present engineered metamaterial may be used as an ultrasensitive Raman probe and could open up interesting new opportunities in biosensing.

CdSe/CdS nanorods-polymer nanocomposites patternable by e-beam lithography: A novel active 2D photonic quasicrystal simulated, designed, fabricated and characterized

Quasiperiodic crystals (QCs) are a new class of materials that have fascinating optical properties lying somewhere between those of disordered and period structures. Advances in 2D photonic structures are expected in the introduction of active functionality into a 2D photonic QC. Semiconductor nanostructures are a very promising material as an active medium. CdSe/CdS core/shell nanorods (NR) present the appealing characteristics of strong and tunable light emission from green to red, are highly fluorescent and show linearly polarized emission. These characteristics open the way to a new class of hybrid devices based on polymers and colloidal NRs in which the unique optical properties of the inorganic moiety are combined with the processability of the host matrix to develop new high performing optical devices such as organic light-emitting diodes, ultra-low threshold lasers and non-linear devices. In this paper two-dimensional (2D) active new designed PQCs which consist of air rods in a nanocomposite prepa...

Electron beam lithography for nanofabrication of metal induced Bragg reflectors

Electrically Induced Bragg Reflectors can be very attractive to realize programable waveguides networks. Their practical realization is nevertheless intrinsically connected to the capability to make a peculiar comb-structure electrode on the top of the waveguides with typical period of 200 nm (corresponding to the Bragg length) and a tolerance of few nanometers. In this work, the experimental fabrication of these comb-structure electrodes by means of electron beam lithography is described. We fabricated large areas 1D periodic gold structures with nanometer resolution by using a high resolution electron beam lithography (EBL) process and a post-processing technique based on lift-off. These electrodes can be employed as Induced Bragg Reflectors in a multilayer structure for a not permanent periodically modulation of the effective refractive index of the guiding structure. The desired structures are obtained with nanometric resolution by means of EBL, digging furrows of rectangular section in both a polymetilmethacrylate (PMMA) and in α-chloromethacrylate and α-methylstyrene (ZEP) layer spin-coated on silicon, then evaporating a metal layer (Au) on the top and then by lift-off of metal. The EBL technique allows a very accurate control of the dielectric distribution of the exposed area able to produce feasible, high efficiency periodic and photonic band-gap structures. The resulting 1D gratings are made of metal lines 100 nm wide with a period of 200 nm and, 120 nm wide with a period of 250 nm, respectively. Large area structures (up to 1 mm x 6 μm) have been realized with nanometre resolution and they have been characterized by scanning electron microscopy (SEM). These structures will be used in a future work of ours to realize 40 GHz switching speed modulator by inducing a Bragg Reflector with a reverse biased vertical InP/InGaAsP p-i-n diode according to the predictions of the grating reflectivity spectra and of the transient response.

Surface-enhanced Raman spectroscopy on engineered plasmonic metamaterials for “label free” biosensing

The last decade has been characterized by artificial electromagnetic (EM) materials, including photonic crystals (PCs) and photonic quasi-crystals (PQCs), making these very attractive given that there are new possibilities to control the EM field in innovative way. Quasiperiodic crystals (QCs) are a new class of materials that have fascinating optical properties lying somewhere between those of disordered and period structures. With the use of PCs and PQCs, it is possible to synthesize novel artificial structures characterized by selective EM responses, which, in turn, undergo significant frequency shifts, in presence of biological material. In the present work we studied artificial EM nanomaterials to develop innovative plasmonic nanobiosensors based on Surface Enhanced Raman Scattering (SERS) substrates and working in the visible and NIR frequency bands. A fabricated gold PQC in a Thue Morse arrangement is proposed for the engineering of reproducible SERS substrates. Structural characterization of this surface is performed by SEM and AFM. Optical properties of this plasmonic nanostructure are evaluated via UV/ Vis absorption spectroscopy and surface–enhanced Raman spectroscopy (SERS). Using a molecular monolayer of pMA (p-mercaptoaniline) as a Raman reporter, we show that a high value of SERS enhancement factor (measured up to 1.4 x 107) can be achieved in a properly optimized photonic structure, in good agreement with FDTD calculations. SERS enhancement factor is dependent on the plasmon absorption wavelength and laser wavelength used in these experiments.

Gold Photonic Crystals and Photonics Quasi-crystals for Reproducible Surface-Enhanced Raman Substrates

Since the initial discovery of surface-enhanced Raman scattering (SERS), an increased amount of work has been done on the research of substrates for highly efficient Raman scattering enhancement due to their extraordinary potential for trace analysis and biological tags (Jarvis et al., Anal Chem 76:5198–202, 2004). The optical properties of noble metals with nanostructures have attracted enormous attention because of their potential application in optical sensing (Chen et al., Biosens Bioelectron 22:926–32, 2007), biosensor (Liu and Lu, J Am Chem Soc 125:6642–6643, 2003) and cell diagnostics (Huang et al., Nano Lett 7:1591–1597, 2007). Recently, the plasmonic optical responses of metal nanoparticles, based on localized surface plasmon resonances (LSPR) and significant fluorescence enhancement in the visible and near IR region, have been intensively researched. Many groups have demonstrated that the plasmon resonance is closely related to the size and shape of metal nanoparticles and the dielectric properties of the surrounding medium (Huang et al. Adv Mater 21:4880–4910, 2009). The possibility of engineering complex metal nanoparticle arrays with distinctive plasmonic resonances extending across the entire visible spectrum can have a significant impact on the design and fabrication of novel nanodevices based on broadband plasmonic enhancement (Gopinath et al., Opt Express 17:3741, 2009). In the present work we studied artificial electromagnetic (EM) nanomaterials to develop innovative plasmonic nanobiosensors based on SERS and working in the visible frequency band.

Electro-Optical Tuning in Photonic Crystals – Dispersed Liquid Crystalline Metamaterials

Two different metamaterial-liquid crystal structures are fabricated with the metamaterials as liquid crystal (LC) and semiconductor nanorods doped LC alignment layers. E-beam lithography was used to pattern the electron-sensitive polymer one-dimensional photonic crystal (PC) structures. The nanostructured aligning surfaces have been characterized by scanning electron microscopy. The PCs-Dispersed Liquid Crystalline Metamaterials have been investigated through polarized optical microscopy. The threshold voltage and response times for the undoped and doped nematic LCs in the glass cells are measured as a function of the applied external electric field.

Novel hybrid organic/inorganic 2D photonic quasicrystals with 8-fold and 12-fold diffraction symmetries

Quasiperiodic crystals (QCs) are a new class of materials that exhibit long-range aperiodic translational order and high rotational symmetries. Unlike periodically arranged photonic crystals (or photonic band-gaps), PQCs possess unique light localization and transport properties related to their complex, multi-fractal energy spectra. Advances in 2D photonic structures are expected in the introduction of non linear and/or active functionality into a 2D PQC. One-dimensional semiconductor nanostructures are likewise promising materials both in fundamental research and in practical applications. CdSe/CdS rods present the appealing characteristics of strong and tunable light emission from green to red, are highly fluorescent and show linearly polarized emission. These characteristics open the way to a new class of hybrid devices based on polymers and colloidal NRs in which the unique optical properties of the inorganic moiety are combined with the processability of the host matrix to develop new high performing optical devices such as organic light-emitting diodes, ultra-low threshold lasers and non-linear devices. One of the challenges of these applications is the incorporation of inorganic nanoparticles into organic polymer matrices, since this is usually accompanied by phase separation, aggregation of nanoparticles, loss of transparency and luminescence quenching due to exciton energy transfer. In this paper two-dimensional (2D) hybrid PQCs which consist of air rods in a nanocomposite prepared by incorporating CdSe/CdS core/shell nanorods (NR) in a polymer are proposed and experimentally demonstrated. Scanning electron microscopy and far field diffraction are used to characterize the experimental structures.